1: The types of holes are divided into: via holes (Vai), plug-in holes (Pad holes), and non-copper mounting holes (Npth). Via holes (via): It only serves the purpose of electrical conduction and does not require soldering of plug-in devices. Its surface can be opened. Window (exposed pad), cover with oil or plug with oil. Plug-in hole (PadL): The pin hole that needs to be inserted into the device for soldering, and the pad surface must be exposed. No copper mounting holes, (Npth): screw holes, or device plastic fixing feet, which have no electrical properties and serve as positioning and fixing functions.
2: Hole attributes, board factory holes are defined with two attributes, metal and non-metal. Most of the metal holes are device pin holes, and some are metal screw holes, which can allow electrical conduction from top to bottom. Non-metallic holes are holes with no copper on the inner wall that are not connected up and down. They are also called mounting holes. The difference between the attributes of metal holes and non-metal holes is whether "Plated" is checked. If the hole is checked "Plated", the hole attribute is metal. If it is not checked, it is a non-metal hole. Non-metal holes normally have no outer diameter.
3: The spacing of via holes
a: The distance between vias (Via) and vias (Via): the edge spacing of vias in the same network is ≥8mil (0.2mm), and the edge spacing of vias in different networks is ≥12mil (0.3mm).
b: The distance between the plug-in hole and the plug-in: the distance between the hole edges is ≥ 17mil (0.45mm), and the limit is 12mil. When making the plug-in hole PCB, the drilling hole will be pre-drilled by 0.15mm. After drilling, the copper will be sunk to ensure that the hole diameter after sinking the copper is as large as the finished hole in the Pcb design. (hole edge spacing 0.45 = 0.15 hole compensation + 0.1 hole ring + 0.1 hole ring + 0.1 safety distance, unit mm)
c: The impact of near holes on production: If two holes are too close, it will affect the PCB production drilling process. If the two holes are too close, the material on one side will be too thin when drilling the second hole. The force on the drill tip will be uneven and the heat dissipation of the drill tip will be uneven, which will lead to the drill tip breaking, causing the PCB hole to collapse and become unsightly or missing holes. No conduction.
4: Slotted hole (long hole): The knife for drilling slotted holes is different from the knife for drilling round holes, so the minimum width of the metal groove is 0.45mm, and the length of the groove must be >2 times the groove width (<2 times the groove width will If a hole collapses, similar to a near hole, the entire groove will be deformed). The smallest non-metallic groove is 0.8mm wide, and the non-metallic groove is usually milled together with the plate frame.
5. Metal half hole and stamp hole Metal half hole: Metal half hole is the most common name of PCB board factory, and many hardware engineers will call it "stamp hole". The center of the metal half hole needs to be drawn on the center of the outline line, half inside the board and half outside the board. The minimum diameter of the half hole is 0.5mm, and the edge of the hole must be ≥0.5mm from edge to edge. Stamp holes: The so-called stamp holes in the board factory are copper-free holes that serve as bridges and separate boards. Half of them are inside the board and half are outside the board. The size of the stamp holes is generally 0.5mm without copper holes. The edge spacing is 0.3mm (the middle distance is 0.8mm). 5 holes, or a group of 5 or more (add more holes appropriately according to the size of the board and whether there are heavy components). .) According to the size of the board, use stamp holes to connect the impositions. There will be protruding burrs at the position of the holes after dividing the board. If the shape and structure require that there be no protruding burrs, then the stamp holes can be added inward toward the inside of the board.
Should you have any enquiries, please contact us and we will happy to provide any assistance.
My email ID:kitty@pcb-hero.com
Should you have any enquiries, please contact us and we will happy to provide any assistance.
My email ID:kitty@pcb-hero.com
General PCB design drawing inspection items
1) Has the circuit been analyzed? Has the circuit been divided into basic units in order to smooth the signal?
2) Does the circuit allow short or isolated critical leads?
3) Are the places that must be shielded effectively shielded?
4) Have you made full use of basic grid graphics?
5) Is the printed circuit board the optimal size?
6) Are the selected wire widths and spacing used where possible?
7) Are the preferred pad sizes and hole sizes used?
8) Are the photographic plates and sketches appropriate?
9) Are minimal jumper wires used? Do jumper wires pass through components and accessories?
l0) Are the letters visible after assembly? Are their sizes and models correct?
11) In order to prevent blistering, are windows opened on large areas of copper foil?
12) Are there tool positioning holes?
PCB electrical characteristics inspection items:
1) Have the effects of wire resistance, inductance, and capacitance been analyzed? Especially the impact of the key voltage drop phase grounding?
2) Do the spacing and shape of wire accessories meet the insulation requirements?
3) Is the insulation resistance value controlled and specified at key points?
4) Is the polarity fully recognized?
5) From a geometric perspective, has the influence of wire spacing on leakage resistance and voltage been measured?
6) Has the medium used to change the surface coating been identified?
PCB physical characteristics inspection items:
1) Are all pads and their locations suitable for final assembly?
2) Can the assembled printed circuit board meet the impact and vibration conditions?
3) What is the specified spacing between standard components?
4) Are components that are not firmly installed or heavy components fixed?
5) Are the heating components dissipated and cooled correctly? Or are they isolated from the printed circuit board and other heat-sensitive components?
6) Are voltage dividers and other multi-lead components positioned correctly?
7) Is the component arrangement and orientation easy to inspect?
8) Have all possible interferences been eliminated on the PCB and throughout the PCB assembly?
9) Is the size of the positioning hole correct?
10) Are the tolerances complete and reasonable?
11) Have the physical properties of all coating layers been controlled and signed?
12) Is the hole to lead diameter ratio within an acceptable range?
PCB mechanical design factors:
Although the printed circuit board mechanically supports components, it cannot be used as a structural member of the entire device. On the edge of the printing plate, provide a certain amount of support at least every 5 inches. Factors that must be considered when selecting and designing a printed circuit board are as follows;
1) Structure of printed circuit board - size and shape.
2) The type of mechanical accessories and plugs (seats) required.
3) The adaptability of the circuit to other circuits and environmental conditions.
4) Consider mounting the printed circuit board vertically or horizontally based on factors such as heat and dust.
5) Some environmental factors that require special attention, such as heat dissipation, ventilation, shock, vibration, and humidity. Dust, salt spray and radiation.
6) Degree of support.
7) Hold and fix.
8)Easy to take off.
PCB printed circuit board installation requirements:
Support should be within 1 inch of at least three edges of the printed circuit board. According to practical experience, the spacing between support points of printed circuit boards with a thickness of 0.031--0.062 inches should be at least 4 inches; for printed circuit boards with a thickness greater than 0.093 inches, the spacing between support points should be at least 5 inches. Taking this measure improves the rigidity of the printed circuit board and destroys possible resonances in the printed circuit board.
The mounting technology used for a certain type of printed circuit board is usually decided after considering the following factors.
1) Size and shape of printed circuit board.
2) Number of input and output terminals.
3) Available equipment space.
4) Desired ease of loading and unloading.
5) Type of mounting accessories.
6) Required heat dissipation.
7) Required shieldability.
8) Types of circuits and their relationships with other circuits.
Printed circuit board pullout requirements:
1) No printed circuit board area required for mounting components.
2) The influence of plugging and unplugging tools on the installation distance between two printed circuit boards.
3) Mounting holes and slots should be specially prepared in the printed circuit board design.
4) When the plugging tool is to be used in the equipment, its size must be especially considered.
5) A plug-and-pull device is required, usually with rivets to permanently secure it to the printed circuit board assembly.
6) In the mounting rack of printed circuit boards, special designs such as load bearing flanges are required.
7) The adaptability of the plugging and unplugging tools used to the size, shape and thickness of the printed circuit board.
8) The costs involved in using plug-in tools include both the price of the tool and the increased expenditure.
9) In order to tighten and use plug and pull tools, it is required to have access to the inside of the equipment to a certain extent.
PCB mechanical considerations:
Substrate properties that have an important impact on printed circuit assemblies are: water absorption, thermal expansion coefficient, heat resistance, flexural strength, impact strength, tensile strength, shear strength and hardness.
All these characteristics affect both the functionality and the productivity of the printed circuit board structure.
For most applications, the dielectric backing of a printed circuit board is one of the following substrate materials:
1) Phenolic impregnated paper.
2) Acrylic-polyester impregnated randomly arranged glass mat.
3) Epoxy impregnated paper.
4) Epoxy impregnated glass cloth.
Each substrate can be flame retardant or combustible. The above 1, 2 and 3 can be punched. The most commonly used material for metallized hole printed circuit boards is epoxy-glass cloth. Its dimensional stability is suitable for high-density circuit use and can minimize the occurrence of cracks in metallized holes.
One disadvantage of epoxy-glass cloth laminates is that they are difficult to punch within the common thickness ranges of printed circuit boards. For this reason, all the holes are usually drilled and profile milled to form the printed circuit board. The shape of the circuit board.
PCB electrical considerations:
In DC or low-frequency AC applications, the most important electrical properties of insulating substrates are: insulation resistance, arc resistance and printed wire resistance, and breakdown strength.
In high-frequency and microwave applications, they are: dielectric constant, capacitance, and dissipation factors.
In all applications, the current carrying capacity of printed conductors is important.
Wire pattern:
PCB routing path and positioning
Printed wires should take the shortest route between components under the constraints of prescribed wiring rules. Limit coupling between parallel conductors as much as possible. Good design requires the minimum number of wiring layers, the widest wires and the largest pad size corresponding to the required packaging density. Sharp corners and sharp corners in conductors should be avoided because rounded corners and smooth internal corners may avoid some possible electrical and mechanical problems.
PCB width and thickness:
Ampacity of etched copper conductors on rigid printed circuit boards. For 1 oz and 2 oz wires, a 10% reduction from the nominal value (based on load current) is allowed, taking into account etching methods and normal variations in copper foil thickness and temperature differences; for printed circuit board assemblies with protective coatings For components (substrate thickness less than 0.032 inches, copper foil thickness more than 3 ounces), components are reduced by 15%; for dip-soldered printed circuit boards, a reduction of 30% is allowed.
PCB wire spacing:
Minimum spacing of conductors must be established to eliminate voltage breakdown or arcing between adjacent conductors. Spacing is variable and depends primarily on the following factors:
1) Peak voltage between adjacent wires.
2) Atmospheric pressure (maximum working altitude).
3) Coating used.
4) Capacitive coupling parameters.
Critical impedance components or high-frequency components are generally placed close together to reduce critical stage delays. Transformers and inductive components should be isolated to prevent coupling; inductive signal wires should be run orthogonally at right angles; components that generate any electrical noise due to magnetic field motion should be isolated or rigidly mounted to prevent excessive vibration.
PCB wire pattern inspection:
1) Are the wires short and straight without sacrificing functionality?
2) Are the wire width restrictions observed?
3) Is there any minimum wire spacing that must be ensured between wires, between wires and mounting holes, between wires and pads?
4) Have all wires (including component leads) been routed in close parallel to each other?
5) Are sharp angles (90°C or less) avoided in the conductor pattern?
PCB design project checklist:
1) Check the rationality and correctness of the schematic diagram;
2) Check the correctness of the component packaging of the schematic diagram;
3) The distance between strong and weak electricity and the distance between isolation areas;
4) Check the schematic diagram and PCB diagram accordingly to prevent the loss of the network table;
5) Whether the packaging of the components matches the actual product;
6) Whether the components are placed appropriately:
7) Whether the components are easy to install and disassemble;
8) Whether the temperature-sensitive component is too close to the heating component;
9) Whether the distance and direction of the mutual inductance components are appropriate;
10) Whether the placement of connectors is smooth;
11) Easy to plug in and out;
12) Input and output;
13) Strong and weak electricity;
14) Whether digital simulation is interleaved;
15) Arrangement of components on the upwind and leeward sides;
16) Whether the directional components are incorrectly flipped instead of rotated;
17) Whether the mounting holes of the component pins are suitable and whether they can be easily inserted;
18) Check whether the empty pin of each component is normal and whether it is a leaking wire;
19) Check whether there are via holes in the upper and lower layer wiring of the same network table. The pads are connected through the holes to prevent disconnection and ensure the integrity of the line;
20) Check whether the characters on the upper and lower layers are placed correctly and reasonably. Do not put components to cover the characters to make it easier for welding or maintenance personnel to operate;
21) The very important connection between the upper and lower layer lines should not only be connected by the pads of the directly inserted components, but also preferably by via holes;
22) The arrangement of power and signal lines in the socket must ensure signal integrity and anti-interference;
23) Pay attention to the appropriate proportion of soldering pads and soldering holes;
24) Each plug should be placed on the edge of the PCB as much as possible and easy to operate;
25) Check whether the component numbers match the components, and place the components in the same direction as much as possible and neatly;
26) Without violating design rules, power and ground wires should be as thick as possible;
27) Under normal circumstances, the upper layer should have horizontal lines and the lower layer should have vertical lines, and the chamfer should not be less than 90 degrees;
28) Whether the size and distribution of the mounting holes on the PCB are appropriate to reduce the bending stress of the PCB as much as possible;
29) Pay attention to the height distribution of components on the PCB and the shape and size of the PCB to ensure easy assembly.
Should you have any enquiries, please contact us and we will happy to provide any assistance.
My email ID:kitty@pcb-hero.com
PCB (Printed Circuit Board) is quite a core in electronic products that it is applied in almost all appliances of different fields, from small to big, from computers, telecommunications to military hardware. Simply speaking, PCB plays a significant role in implementing functions of electronic products.
Nevertheless, it's never an easy task to design a circuit board and a lot of associations, between layers, components or circuitries, have to be properly dealt with. A bad-thought-out design will possibly bring forward failures or even catastrophes when it is working inside an electronic system. In spite of the difficulty attribute of PCB design in itself, some problems that commonly occur can be summarized so that all PCB designers can get aware of them in advance and learn to deal with them prior to PCB fabrication phase.
NOTE: This article discusses PCB design problems and solutions based on the participation of Altium Designer software.
Problem#1: According to ERC report, there’s no access signal on pins.
Analysis:
a. I/O should be defined on pins when establishing package;
b. When establishing or placing components, the attribute of inconformity may be modified so that pins and lines stay loose;
c. When establishing components, pin suffers from reverse direction.
Problem#2: Components are beyond paper.
Analysis: Files are not created in the center of component library paper.
Problem#3: The created engineering file netlist can only partially access PCB.
Analysis: The item of “global” is not picked up when generating netlist.
Problem#4: Components fail to be rotated.
Analysis: Input method should be switched.
Problem#1: In the process of network loading, report NODE doesn't occur.
Analysis:
a. The components in schematic possibly take advantage of the package that isn't available in component library;
b. The components in schematic use packages that are incompatible with those used in component library;
Problem#2: DRC report network is divided into a couple of sections.
Analysis: This problem demonstrates that this network isn't connected and CONNECTED COPPER can be used to go through the file.
Problem#3: In the process of operation, blue screen should be used as little as possible.
Analysis: Files can be exported many times to generate new DDR file so as to reduce file size. Auto routing isn't suggested when design complex PCB.
Routing is quite a significant step in PCB design and all the steps before it are all its preparations. When it comes to PCB design, routing calls for the most requirement. PCB routing can be classified into single-side routing, double-side routing and multi-side routing. Two routing methods are available: auto routing and interactive routing. Prior to auto routing, interactive routing can be used for relatively complex system in advance. The sidelines at input and output terminals should avoid being parallel to each other so that RF interference can’t be generated. Ground lines should be added when necessary and routing on two adjacent layers should be vertical to each other. Parallel lines tend to generate parasitic coupling. The routing rate of auto routing depends on well-thought-out layout and routing rules can be set in advance. Generally speaking, inquiry-based routing can be first carried out and routing path should be optimized on the whole. Routed lines will then be closed and rerouting will be implemented in order to improve overall effect. As far as the design for component-densed PCB, through holes alone can hardly count with lots of routing channels wasted. Therefore, blind and buried via technology has been created. Not only do they function like through holes, but save many routing channels as well. As a result, routing can be easier, smoother and better.
Interference always occurs to electronic equipment in the process of debugging and application, which derives from a good number of causes. Among all the causes, irrational routing and improper placement of components bring forward most interference apart from interference resulting from environment. Interference will possibly lead electrical equipment to being unable to normally work or even failure. Therefore, possible interference should be restrained in PCB design phase.
Problem#1: Generation and control of ground line interference.
Analysis and Solutions:
If the ground lines indicate zero potential, the relative potential difference of each grounding point in the whole circuit should be also zero. However, it’s almost impossible to ensure potential difference to be absolutely zero and a tiny potential difference will possibly result in interference signals affecting the normal running of the whole circuit after being magnified through amplifying circuit.
To restrain interference, the following methods can be used: a. correct grounding guidelines should be followed through on; b. digital ground lines should be divided from analog ground lines; c. ground lines should be thickened as much as possible; d. grounding should be coated as much as possible.
Problem#2: Power interference and restraint.
Analysis and Solutions: power interference perhaps derives from irrational schematic design, routing or layout. Therefore, AC-DC loop can't be connected with each other during routing and ground lines shouldn’t run in parallel with the big loop. Additionally, power lines and signal lines shouldn’t be too close to each other and can never be parallel. When necessary, filters can be added between power output terminal and appliance.
Problem#3: EMI (Electromagnetic Interference) and its restrain.
Since components are densely placed, if irrational design is implemented, EMI will be aroused such as distribution parameter interference and component EMI. Corresponding measures should be made to defeat different interferences.
Analysis and Solutions:
a. Parasitic coupling between printing circuits. The effect of distribution parameter between two parallel leads with short distance is equivalent to that of inductance and capacitance that are reciprocally coupling. Signals flow through one lead while inductive signals are generated by the other lead. Thus, signal lines can never be designed to be parallel to each other during PCB design or shielding lines can be used to restrain weak interference signals to stop interference.
b. Interference between magnetic parts. Loudspeakers and electromagnets produce constant magnetic field while high-voltage transformers and relays produce alternating magnetic field. Both of the magnetic fields bring forward interference to peripheral components and printing lines and corresponding restrain measures can be made based on different situations:
• The cutting on printed lines brought by magnetic lines should be reduced.
• Positions of two magnetic parts should maintain vertical to each other along two different magnetic directions to reduce coupling between two parts.
• Interference source should receive magnetic shielding and the shielding cover should be well connected to the ground.
Problem#4: Thermal interference and restrain.
Analysis and Solutions: when appliances with high power are working, they usually feature such a high temperature that heat sources are available in circuit, bringing forward interference to printed circuit. Therefore, temperature-sensitive components should be placed far from heat-generating parts during PCB layout design and heat sources should be placed at the air outside board to stop the generated heat from transferring or thermal dissipation from being generated. If necessary, thermal dissipation sheet should be equipped.
This article just covers the most common problems that we usually meet in PCB design and their solutions. In fact, more issues are expected to be found out in your practical design experience.
By use
Pressure-sensitive and force-sensitive sensors, position sensors, liquid level sensors, energy consumption sensors, speed sensors, acceleration sensors, radiation sensors, thermal sensors.
According to principle
Vibration sensors, humidity sensors, magnetic sensors, gas sensors, vacuum sensors, biosensors, etc.
Press output signal
Analog sensor: converts the measured non-electrical quantity into an analog electrical signal.
Digital sensor: converts the measured non-electrical quantity into a digital output signal (including direct and indirect conversion).
Digital sensor: converts the measured signal quantity into a frequency signal or a short-period signal output (including direct or indirect conversion).
Switch sensor: When a measured signal reaches a certain threshold, the sensor outputs a set low-level or high-level signal accordingly.
According to its manufacturing process
Integrated sensors are fabricated using standard process technologies for producing silicon-based semiconductor integrated circuits. Usually some circuits used for preliminary processing of the measured signal are also integrated on the same chip.
Thin film sensors are formed by depositing thin films of corresponding sensitive materials on a dielectric substrate (substrate). When using a hybrid process, part of the circuit can also be fabricated on this substrate.
Thick film sensors are made by coating the slurry of corresponding materials on a ceramic substrate. The substrate is usually made of Al2O3, and then heat treated to form a thick film.
Ceramic sensors are produced using standard ceramic processes or some variant thereof (sol, gel, etc.).
After appropriate preparatory operations, the shaped components are sintered at high temperatures. There are many common characteristics between the two processes of thick film and ceramic sensors. In some respects, the thick film process can be considered a variation of the ceramic process.
Each process technology has its own advantages and disadvantages. Due to the low capital investment required for research, development and production, as well as the high stability of sensor parameters, the use of ceramic and thick film sensors is more reasonable.
According to measurement purpose
Physical sensors are made by utilizing the characteristics of certain physical properties of the measured substance that change significantly.
Chemical sensors are made of sensitive elements that can convert chemical quantities such as the composition and concentration of chemical substances into electrical quantities.
Biosensors are sensors made by utilizing the characteristics of various organisms or biological substances to detect and identify chemical components in organisms.
According to its composition
Basic sensor: It is the most basic single conversion device.
Combined sensor: It is a sensor composed of a combination of different individual transformation devices.
Applied sensor: It is a sensor composed of a basic sensor or a combined sensor and other mechanisms.
According to action form
According to the mode of action, they can be divided into active and passive sensors.
Active sensors are divided into action type and reaction type. This type of sensor can emit a certain detection signal to the object being measured, and can detect the changes caused by the detection signal in the object being measured, or the detection signal can produce some kind of detection signal in the object being measured. effect to form a signal. The method of detecting signal changes is called the action type, and the method of detecting and responding to form a signal is called the reaction type. Radar and radio frequency range detectors are examples of the action type, while photoacoustic effects analyzers and laser analyzers are examples of the reaction type.
Passive sensors only receive signals generated by the measured object itself, such as infrared radiation thermometers, infrared camera devices, etc.
Figure 1: Simplified flyback converter schematic that can operate in DCM or CCM mode
When Q1 turns off during period 1-D, T1's secondary voltage reverses polarity, allowing D1 to conduct current to the load and charge COUT. The current in D1 decreases linearly from its peak value to zero during t2. Once the energy stored in T1 is exhausted, only ringing remains for the remaining time period t3. This ringing is primarily caused by the magnetizing inductance of T1 and the parasitic capacitances of Q1, D1, and T1. This is easily seen by the drain voltage of Q1 during t3, which drops from VIN plus the reflected output voltage back to VIN, since T1 cannot support the voltage once the current is cut off. (Note: Without enough dead time in t3, it is possible to enter CCM operation.) The currents in CIN and COUT are the same as the currents in Q1 and D1, but there is no DC offset.
The shaded areas A and B in Figure 2 highlight the transformer's volt microsecond products during t1 and t2, which must remain balanced to prevent saturation. Area "A" represents (Vin/Nps)×t1, while "B" represents (Vout+Vd)×t2, both with the secondary side as reference. Np/Ns is the turns ratio between the primary side and the secondary side of the transformer.
Figure 2: Key voltage and current switching waveforms for a DCM flyback converter and several key parameters that the designer must specify.
Table 1 details the characteristics of DCM relative to CCM. A key attribute of DCM is that having lower primary inductance reduces the duty cycle regardless of the transformer turns ratio. It allows you to limit the maximum duty cycle of your design. This may be important if you want to use a specific controller or stay within specific on or off time limits. Lower inductance requires lower average energy storage (albeit with higher peak FET current) and generally allows the use of smaller transformers compared to CCM designs.
Another advantage of DCM is that this design eliminates the D1 reverse recovery losses in a standard rectifier since the current is zero at the end of t2. Reverse recovery losses typically manifest themselves as increased dissipation in Q1, so eliminating them reduces the stress on the switching transistor. This advantage becomes more apparent at higher output voltages, since the reverse recovery time of the rectifier also increases with the diode voltage rating.
Figure 1 60 W CCM flyback converter schematic
Additional transformer windings can be added or even stacked on top of other windings for additional output. But the more output is added, the worse it becomes regulated. This is due to imperfect flux (coupling) between the windings and the core and physical isolation of the windings, creating leakage inductance. Leakage inductance is in series with the primary and output windings as stray inductance. This causes an unexpected voltage drop in series with the winding, effectively reducing the output voltage regulation accuracy. A general rule of thumb is to expect the unregulated output to vary by +/- 5% to 10% under cross-load conditions using a properly wound transformer. Additionally, the heavily loaded regulated output may cause the unloaded secondary output voltage to increase significantly through voltage spikes caused by peak detection leakage. In this case, preload or soft clamping can help limit the voltage.
CCM and discontinuous conduction mode (DCM) operation each have their own advantages. By definition, DCM operation occurs when the output rectifier current drops to 0A before the next cycle begins. Operating advantages of DCM include lower primary inductance (generally enabling smaller power transformers), elimination of rectifier reverse recovery losses and FET conduction losses, and the absence of right half-plane zeros. However, these advantages are offset by higher peak currents in the primary and secondary, increased input and output capacitance, increased electromagnetic interference (EMI), and reduced duty cycle at light loads compared to CCM.
Figure 2 Comparison of CCM and DCM flyback FET and rectifier current
Figure 2 illustrates how the current in Q2 and D1 changes at minimum VIN as the load decreases from maximum to about 25% in CCM and DCM. In CCM, the duty cycle is constant for a fixed input voltage and when the load is between its maximum and minimum design levels (about 25%). The current "base" level decreases with decreasing load until it reaches DCM, at which point the duty cycle decreases. In DCM, maximum duty cycle occurs only at minimum VIN and maximum load. The duty cycle is reduced to increase the input voltage or reduce the load.
This reduces the duty cycle at high and minimum loads, so make sure your controller can operate properly during this minimum on-time. DCM operation results in a dead time with a duty cycle below 50% after the rectifier current reaches 0-A. It is characterized by a sinusoidal voltage on the drain of the FET and is set by residual current, parasitic capacitance and leakage inductance, but is usually benign. For this design, CCM operation was chosen because higher efficiency can be achieved by reducing switching and transformer losses.
This design uses a primary bias winding with a reference voltage of 14V to power the controller after the 12V output reaches regulation, reducing losses compared to being powered directly from the input. I chose a two-stage output filter to achieve low ripple voltage. The first stage ceramic capacitor handles the high RMS current from the pulsating current in D1. Filters L1 and C9/C10 reduce their ripple voltage, providing approximately 10x ripple reduction as well as RMS current reduction in C9/C10. If higher output ripple voltage is acceptable, the L/C filter can be omitted, but the output capacitor must be able to handle the full RMS current.
The UCC3809-1 or UCC3809-2 controller is designed to interface directly with the U2 optocoupler for isolation applications. In non-isolated designs, U2 and U3 can be omitted along with the voltage feedback resistor divider connected directly to the controller, such as the UCC3813-x series with internal error amplifier.
The switching voltage across Q2 and D1 creates high-frequency common-mode currents in the transformer windings and component parasitic capacitances. Without EMI capacitor C12 to provide a return path, these currents will flow into the input and/or output, adding noise or possibly making operation unstable.
The Q3/R19/C18/R17 combination provides slope compensation by adding the oscillator's voltage ramp to R18's primary current sense voltage, which is used for current mode control. Slope compensation eliminates subharmonic oscillations, a phenomenon characterized by duty cycle pulses that are very broad and then very narrow. Since this converter is designed to operate at no more than 50% frequency, I added slope compensation to reduce susceptibility to switching jitter. However, an excessive voltage slope may push the control loop into voltage mode control and may cause instability. Finally, an optocoupler transmits the error signal from the secondary side to keep the output voltage stable. The feedback (FB) signal includes current ramp, slope compensation, output error signal and DC offset to reduce overcurrent threshold.
Figure 3 shows the voltage waveforms for Q2 and D1, showing some ringing caused by leakage inductance and diode reverse recovery.
Figure 3 FET and rectifier ringing is limited by the clamping and snubber circuit (57 VIN, 12 V at 5 A).
Flyback is considered standard in applications requiring low-cost isolated converters. This design example covers basic design considerations for CCM flyback design.
]]>When we are processing PCBA boards, some PCB boards are relatively small in size. For production efficiency, they need to be made into pipes connecting the boards. After completing the PCBA processing, the PCBA boards need to be divided into boards. Then during the deboarding process, you need to pay attention to some precautions to prevent damage to the intact PCBA board.
When manually dividing the board, you need to pay attention to the fact that when folding the edge, you must hold the lower edge of the PCBA board with both hands, less than 20mm away from the V-cut, and try to avoid bending deformation, damage to the PCBA power circuit, components, and tin channels.
Requirements for machine PCBA subboarding:
1. Stable support point. Without support, the stress generated may damage the substrate and welding points. Distorting the board, or stressing the assembly during depaneling, can cause hidden or obvious defects.
2. Wear protective tools and make protective preparations before operating. It is necessary to install high-frequency eye protection lighting devices to protect the safety of operators. It is best to bring a pair of glasses to protect your eyes.
3. Frequently wipe the machine spindle and cutting tools with alcohol to remove PCB board dust generated during the de-boarding process and maintain the normal operation of the de-boarding machine.
4. After using it a certain number of times, oil and lubricate the sliding rods and bearings of the splitter and check whether there are loose screws, etc.
5. During the use of the machine, the work surface should be kept clean, and it is best not to place other things to prevent things from falling on the cutter and causing damage to the cutter and items. Although there is an electric eye for protection, you still need to pay attention to maintaining a certain safe distance between your fingers and the tool during use.
When PCBA is depaneled, machine depaneling is more efficient than manual depanning, with high efficiency and low damage rate. However, when performing machine splitting, the process must be strictly followed to reduce human errors.
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2. Reflection
Reflection is an echo on a transmission line. Part of the signal power (voltage and current) travels down the line and reaches the load, but part of it is reflected.
If the source and load have the same impedance, reflections will not occur. The impedance mismatch between the source and the load will cause reflections on the line, and the load will reflect part of the voltage back to the source.
If the load impedance is less than the source impedance, the reflected voltage is negative; conversely, if the load impedance is greater than the source impedance, the reflected voltage is positive.
Variations in cabling geometry, incorrect wire termination, transmission through connectors, and power plane discontinuities can all cause such reflections.
3. Crosstalk
Crosstalk is the coupling between two signal lines. The mutual inductance and mutual capacitance between the signal lines cause noise on the line.
Capacitive coupling induces coupling current, while inductive coupling induces coupling voltage. PCB board layer parameters, signal line spacing, power characteristics of the driving end and receiving end, and line termination pipes all have a certain impact on crosstalk.
4. Characteristic impedance
Let’s first clarify a few concepts. We often see impedance, characteristic impedance, and transient impedance. Strictly speaking, they are different, but they remain the same. They are still the basic definitions of impedance:
4.1 The input impedance at the beginning of the transmission line is referred to as impedance;
4.2 The real-time impedance that a signal encounters at any time is called transient impedance;
4.3 If the transmission line has a constant transient impedance, it is called the characteristic impedance of the transmission line.
4.4 Characteristic impedance describes the transient impedance encountered when a signal propagates along a transmission line. This is a major factor affecting signal integrity in transmission line circuits.
4.5 Unless otherwise specified, characteristic impedance is generally used to collectively refer to transmission line impedance.
PS: For high-speed PCB design, our goal is to keep the impedance of the signal as stable as possible during the transmission process, and this must maintain the stability of the characteristic impedance of the transmission line.
5. Power integrity
Power integrity, referred to as PI, is to confirm whether the voltage and current at the source and destination of the power supply meet the requirements.
Power integrity is very important in today's electronic products. There are several levels of power integrity: chip level, chip package level, circuit board level, and system level.
Among them, power integrity at the circuit board level must meet the following three requirements:
Make the voltage ripple at the chip pin smaller than the specification (for example, the error between the voltage and 1V is less than +/-50 mV)
Controlling ground bounce (also known as synchronous switching noise SSN, synchronous switching output SSO)
Reduce electromagnetic interference (EMI) and maintain electromagnetic compatibility (EMC): The power distribution network (PDN) is a conductor on the circuit board, which is also an antenna that easily emits and receives noise.
6. Power supply noise
Power supply noise is a type of electromagnetic interference, and the spectrum of its conducted noise is roughly 10kHz~30MHz, up to 150MHz.
Power supply noise, especially transient noise interference, has fast rising speed, short duration, high voltage amplitude and strong randomness, which can cause serious interference to microcomputers and digital circuits.
In high-frequency circuits, the noise contained in the power supply has a particularly obvious impact on high-frequency signals. For this reason, the power supply is first required to be low-noise. Here, a clean ground and clean power supply are equally important.
7. Filtering
Wave filtering is the operation of filtering out specific frequency bands in signals. It is an important measure to suppress and prevent interference. Filtering is divided into classic filtering and modern filtering.
8. Parallel bus
A bus is a common physical path for communication between two or more devices. It is a collection of signal lines and a common connection between multiple components. It is used to transmit information between various components.
Depending on the working mode, the bus can be divided into two types: one is the parallel bus and the other is the serial bus.
Parallel bus: It can transmit multiple bits of data at the same time, just like a spacious road that allows multiple cars to drive side by side, and it is also bidirectional and unidirectional.
9. Serial bus
Serial bus: Only one piece of data can be transmitted at the same time, just like a narrow road that only allows one car to walk. The data must be transmitted one after another, and it looks like a long data string, so it is called "serial".
10. Topology
Topology refers to the way in which various sites in the network are connected to each other. Topology in PCB design refers to the connection relationship between chips.
Commonly used topologies include point-to-point, daisy chain, remote cluster, star, etc.
The above shares with you 10 important matters related to high-speed PCB design. I hope it will be helpful to your learning.
IC package type
The general development process of packaging is probably TO → DIP → PLCC → QFP → PGA → BGA → CSP → MCM. The technology is advanced from generation to generation, and reliability has also been improved.
1. DIP dual in-line packaging. DIP refers to an integrated circuit chip packaged in a dual in-line form. Most small and medium-sized integrated circuits (ICs) use this packaging form, and the number of pins generally does not exceed 100. indivual. ICs in DIP packages have two rows of pins and need to be inserted into a chip socket with a DIP structure.
2. QFP/PFP type packaging. The distance between the chip pins of the QFP/PFP package is very small and the pins are very thin. Generally, large-scale or ultra-large integrated circuits use this packaging form. Chips packaged in this form must use SMD (Surface Mount Device Technology) to solder the chip to the motherboard.
3. BGA type packaging. With the development of integrated circuit technology, the packaging requirements for integrated circuits have become more stringent. This is because packaging technology is related to the functionality of the product. When the frequency of the IC exceeds 100MHZ, the traditional packaging pipeline may produce the so-called "CrossTalk" phenomenon, and when the number of pins of the IC is greater than 208 Pin, the traditional packaging pipeline has its difficulty. Therefore, in addition to using QFP packaging pipelines, most of today's high-pin-count chips have switched to using BGA (BALL Grid Array PACKAGE) packaging technology.
4. SO type packaging. SO type packaging includes: SOP (small outline package), TOSP (thin small outline package), SSOP (reduced SOP), VSOP (very small outline package), SOIC (small outline integrated circuit package) A package similar to the QFP form, but only a chip package with pins on both sides. This type of package is one of the surface mount packages, and the pins are led out from both sides of the package in an "L" shape.
5. QFN package type, QFN is a leadless quad flat package, a lead-free package with peripheral terminal pads and a die pad for mechanical and thermal integrity exposure. The package can be square or rectangular. There are electrode contacts on the four sides of the package. Since the lead-free mounting occupies a smaller area than QFP, it is also lower in height than QFP.
IC substrate has been developing with the booming of new types of ICs like BGA (ball grid array) and CSP (chip scale package) which call for new carriers of package. As one type of the most advanced PCB (Printed Circuit Board), IC substrate PCB has exploded in both popularity and applications together with any layer HDI PCB and flex-rigid PCB, now widely applied in telecommunications and electronics updates.
IC substrate is a type of base board used to package bare IC (integrate circuit) chip. Connecting chip and circuit board, IC belongs to an intermediate product with the following functions:
• it captures semiconductor IC chip;
• there’s routing inside to connect chip and PCB;
• it can protect, reinforce and support IC chip, providing thermal dissipation tunnel.
a. Classified by package types
• BGA IC Substrate. This kind of IC Substrate performs well in thermal dissipation and electrical performance and can dramatically increase chip pins. Therefore, it is suitable for IC package with pin count exceeding 300.
• CSP IC Substrate. CSP is a type of single chip package with light weight and miniaturized scale, featuring similarly size with IC. CSP IC substrate is mainly used in memory products, telecommunication products and electronic products with a small number of pins.
• FC IC Substrate. FC (Flip Chip) is a type of package by flipping chip, featuring low signal interference, low circuit loss, well-performed performance and effective thermal dissipation.
• MCM IC Substrate. MCM is an abbreviated form of multi-chip module. This type of IC substrate absorbs chips with different functions into one package. As a result, the product can be an optimal solution due to its attributes including lightness, thinness, shortness and miniaturization. Naturally, since multiple chips are packaged into one package, this type of substrate doesn’t perform so well in signal interference, thermal dissipation, fine routing etc.
b. Classified by material attribute
• Rigid IC Substrate. It is primarily made by epoxy resin, BT resin or ABF resin. Its CTE (coefficient of thermal expansion) is approximately 13 to 17ppm/°C.
• Flex IC Substrate. It is primarily made by PI or PE resin and features CTE 13 to 27ppm/°C
• Ceramic IC Substrate. It is primarily made by ceramic materials such as Aluminium oxide, Aluminium nitride or silicon carbide. It features a relatively low CTE which is approximately 6 to 8ppm/°C
c. Classified by bonding technology
• Wire Bonding
• TAB (Tape Automated Bonding)
• FC Bonding
IC substrate PCBs are mainly applied on electronic products with light weight, thinness and advancing functions, such as smart phones, laptop, tablet PC and network in fields of telecommunications, medical care, industrial control, aerospace and military.
Rigid PCBs have followed through a series of innovations from multilayer PCB, traditional HDI PCBs, SLP (substrate-like PCB) to IC substrate PCBs. SLP is just a type of rigid PCBs with similar fabrication process approximately semiconductor scale.
Compared with standard PCB, IC substrate has to conquer manufacturing difficulties for its implementations of high performance and advanced functions.
a. IC Substrate Manufacturing
IC substrate is thin and easy to be deformed, which is especially protruding when a board is less than 0.2mm thick. To overcome this difficulty, breakthroughs have to be made in terms of board shrinking, lamination parameters and layer positioning system so that substrate warpage and lamination thickness can be effectively controlled.
b. Microvia Manufacturing Technology
Microvia technology consists of the following aspects: conformal mask, laser-drilled micro blind via technology and plated copper filling technology.
• Conformal Mask aims to logically compensate laser-drilled blind via opening and blind via aperture and positions can be directly defined through copper openings.
• Laser-Drilled Microvia fabrication is correlated with the following technological aspects: via shape, aspect ratio, side etching, left gel under via etc.
• Blind Via Copper Plating is correlated with the following technological aspects: via filling capability, blind via openness, sinking, copper plating reliability etc.
c. Patterning and Copper Plating Technology
Patterning and copper plating technology is correlated with the following technological aspects: circuitry compensation technology and control, fine line fabrication technology, copper plating thickness uniformity control.
d. Solder Mask
Solder mask manufacturing for IC substrate PCB consists of via filling technology, solder mask printing technology etc. Up to now, IC substrate PCB allows less than 10um for surface height difference and surface height difference between solder mask and pad should not be over 15 um.
e. Surface Finish
Surface finish for IC substrate PCB should emphasize thickness uniformity and up to now, surface finish that can be accepted by IC substrate PCB includes ENIG/ENEPIG.
f. Inspection Capability and Product Reliability Test Technology
IC substrate PCB calls for inspection equipment that is different from that used for traditional PCB. In addition, engineers have to be available that are capable of mastering inspection skills on the special equipment.
All in all, IC substrate PCB calls for more requirement than standard PCB and PCB manufacturers have to be equipped with advanced manufacturing capabilities and be proficient in mastering them.
As the commonest components integrated platform, multi-layer PCBs connect circuit boards and components together. With electronic products becoming light, thin and small in size, and having high performance, IC components have become highly integrated, leading to the high integrity of PCBs. As a result, heat production has obviously increased and thermal density of PCBs has increasingly gone up especially because of the mass utilization of high-frequency IC components such as A/D or D/A type and moving up of circuit frequency. If massive thermal loss fails to be sent out, the reliability of electronic equipment will be greatly influenced. According to statistics, among the elements leading to the failure of electronic equipment, temperature accounts for as high as 55%, as the top cause. With the temperature increasing, the failure rate of electronic components will increase exponentially. Once the environment temperature increases by 10°C, the failure rate of some electronic components can increase to twice large. For aerospace products, this type of thermal control design can't even be ignored as the inappropriate design method for the all kinds of circuits in special environment will possibly result in the complete failure of the whole system. Therefore, much attention must be paid to thermal design during PCB design.
The analysis should commence with the cause analysis. The direct cause of high temperature of PCBs lie in the existence of power consumption components. Each component has power consumption in different extent that arouses the change of thermal strength. There exist 2 types of temperature increase phenomena: local temperature rising or large area temperature rising and short-term temperature rising or long-term temperature rising. Heat transfer has 3 ways: heat conduction, heat convection and heat radiation. Radiation dissipates heat through electromagnetic wave motion passing through space. Since the radiation dissipation features a relatively low amount of heat, it is usually regarded as an assisted dissipation method. This passage will introduce a solution to PCB heat dissipation in the process of long-term operation in the environment with high temperature based on the heat conduction and heat sink transient heat storage technology with a type of servo PCB as an example.
On this servo PCB, there are 2 power amplifier chips with a power of 2W, 2 R/D conversion chips, 2 CPU chips, 1 EPLD chip and 1 A/D conversion chip. The overall power of this servo PCB is 9W. The servo PCB is installed in an airtight environment with limited air convection. Besides, because of the limited space, cold plate dissipation can't be installed on the servo PCB. In order to ensure the normal operation of servo PCB, only heat conduction and heat sink transient heat storage technology can be utilized to transfer the heat produced from the PCB to the body.
It is a common method to dissipate heat through metal core PCB. First, a metal board with excellent heat conduction is embedded between a multi-layer PCB. Then, heat is dissipated directly from metal board or disjunctive equipment is connected to the metal board to dissipate heat. The operating structure is shown in Figure 1.
The main material of metal core PCB covers aluminum, copper and steel. It can be also used as the ground layer. The upper layer and lower layer of metal core PCB can be interconnected through plated through hole and heat can be transferred on the inner layer and surface of metal core PCB. Heating elements can be directly soldered on the board through the bottom and heat conduction hole. As a result, the heat generated by heating elements is directly transferred to metal core PCB that transmits the heat to the tangent chassis by the heat conduction hole and sends it out. PCBs with such a structure have a wide series of applications but they can also arouse some problems. Metal core PCBs are so thick that deformation tends to take place in the uneven heat dissipation, leading to the loose contact between chips on PCBs and pins. It's easy and quick for metal core PCBs to dissipate heat, which brings about enormous difficulties to chip changing and in the process of chip changing; the local heat attraction of metal core PCBs will lead to the serious deformation of PCBs. It is verified that the larger area a PCB has, the more easily it is deformed.
In order to get the problems above solved, upgrading design must be done to metal core PCBs:
a. 4-layer copper foil with thickness of 0.15mm can be nipped in PCBs so that the thickness of PCBs can increase by 3mm in order to ensure that PCBs aren't deformed easily and the through-hole reliability rises.
b. As to chips with heat generation of 2W, SMT pad can be added to the bottom of chips to transfer the heat to the metal layer of PCB.
c. The chip bottom is capable of transferring heat to the internal copper foil layer by the copper foil with a large area and heat conduction through hole.
d. The insulating layer on both sides of PCB can be milled off to realize the PCB edge metallization. Heat dissipation can be achieved through the contact between bare edge PCB and base. The installation can be finished by 36 screws to increase the heat conduction of PCB and the body.
After the implementation of the measures mentioned above, the upgraded PCB design is shown in Figure 2.
In order to set up simulation modeling and analysis on servo PCB, the software FLoTHERM is used for electronic equipment heat situations. The edge condition of servo PCB is: the environment is 65°C with the operating time of 90 minutes. The components on servo PCB all meet X derating requirement. The allowing body temperature of each component is shown as the following table:
Components | Heat Consumption/W | Max Temperature of X Derating/°C | Max Body Temperature of X Derating/°C |
CPU Chip | 0.6 | 100 | 87 |
R/D Chip | 0.5 | 100 | 87 |
EPLD Chip | 0.5 | 100 | 85 |
Power Amplifier Chip | 2.0 | 100 | 87 |
The main power components on servo PCB include 2 chips (49.76mm*41.4mm) each of which has a heat consumption of 2W. The heat consumption of other components on servo PCB is 5W in all and the heat consumption of the whole PCB is 9W, servo driving components 10W, power supply 40W, and the overall heat consumption of servo and power supply is 59W.
The temperature of servo control chip is shown in Figure 3.
The heat analysis of operating for 90 minutes in the 65°C environment shows: in the process of operating for continuous 30 minutes, the temperature of chip rises quickly, reaching 72°C above; in the process of operating for continuous 50 minutes, the temperature of chip gradually remains stable; in the process of operating for continuous 90 minutes; the body temperature of 2W chip (87°C) is 77.9°C; the body temperature of 0.6W chip (87°C) is 84°C;; the body temperature of 0.5W chip (87°C) is 78.2°C; the body temperature of 0.5W chip (85°C) is 77°C;.
Based on the calculation and simulation heat design operating condition, the servo control chip temperature remains in the reasonable range. In the process of theoretical analysis, there's no space between chips and PCB by default. But in the actual process of installation, there is possibly some space between them and silica gel can be used to fill the space to ensure the heat dissipation effect of PCB.
(1) Circuit schematic design: The circuit schematic design is mainly based on the Advanced Schematic system of PROTEL099 to draw a circuit schematic. In this process, we need to fully utilize the various schematic drawing tools and editing functions provided by PROTEL99 to achieve our goal of obtaining a correct and exquisite circuit schematic.
(2) Generating a network table: A network table is a bridge between circuit schematic design (SCH) and printed circuit board design (PCB), and it is the soul of circuit board automation. The network table can be obtained from the circuit schematic or extracted from the printed circuit board.
(3) Printed Circuit Board Design: The design of printed circuit boards is mainly aimed at another important part of the PCB in PROTEL99. In this process, we use the powerful functions provided by PROTEL99 to achieve the layout design of the circuit board and complete high difficulty tasks.
But in practice, the specific steps are mainly divided into the following steps:
1、 Preliminary work for circuit board design: 1. Use schematic design tools to draw schematic diagrams and generate corresponding network tables. Of course, in some special cases, such as when the circuit board is relatively simple and there is already a network table, it is also possible to enter the PCB design system directly without designing the schematic. In the PCB design system, parts can be directly packaged and a network table can be manually generated. 2. Manually modify the network table to define some components with fixed pins or pads that are not on the schematic diagram to the connected network, and those without any physical connections can be defined to ground or protective ground. Change the pin names of some devices that do not match the schematic and PCB packaging library to match those in the PCB packaging library, especially for diodes and transistors.
2、 Draw your own packaging library for non-standard devices. It is recommended to include all the devices you have drawn in a dedicated PCB library design file that you have created.
3、 Set up the PCB design environment and draw the frame of the printed circuit, including the hollow in the middle.
1. The first step after entering the PCB system is to set up the PCB design environment, including setting grid size and type, cursor type, layout parameters, wiring parameters, and so on. Most parameters can be set with system default values, and after being set, they are in line with personal habits and do not need to be modified in the future.
2. Planning the circuit board mainly involves determining the border of the circuit board, including the size of the circuit board, and so on. Place appropriately sized solder pads where fixed holes need to be placed. For 3mm screws, pads with 6.5~8mm outer diameter and 3.2~3.5mm inner diameter can be used. For standard boards, they can be transferred from other boards or PCWizard. Attention - Before drawing the circuit board border, make sure to set the current layer as the KeepOut layer, which prohibits wiring layers.
4、 After opening all the PCB library files to be used, it is a very important step to call in the network table file and modify the part packaging. The network table is the soul of PCB automatic wiring, and is also the interface between schematic design and impression circuit board design. Only after the network table is loaded can the circuit board be wired. In the process of schematic design, ERC inspection will not involve packaging issues of parts. Therefore, during schematic design, the packaging of parts may be forgotten, and when introducing network tables, the packaging of parts can be modified or supplemented according to the design situation. Of course, a network table can be manually generated within the PCB and part packaging can be specified.
5、 The placement of part packaging, also known as part layout, Protel99 can be automatically or manually laid out. If performing automatic layout, run "AutoPlace" under "Tools" and use this command, you need to have enough patience. The key to wiring is layout, and most designers use manual layout. Select a component with the mouse, hold down the left mouse button, drag the component to the destination, release the left mouse button, and fix the component. Protel99 has added some new techniques in terms of layout. New interactive layout options
Pre-baking precautions for multilayer circuit boards
1. If an oven is used for the circuit board, it must be equipped with air blast and constant temperature control to ensure uniform pre-baking temperature. Moreover, the oven should be clean and free of impurities to avoid falling on the board and damaging the membrane surface.
2. Do not use natural drying, and the drying must be complete, otherwise it will easily stick to the film and cause poor exposure.
3. After circuit board processing and pre-baking, the multi-layer circuit board should be air-cooled or naturally cooled before film alignment exposure.
4. After pre-baking the multi-layer circuit board, the time from coating to development should not exceed two days at most. When the humidity is high, try to expose and develop within half a day.
5. Different models of liquid photoresist have different requirements. You should read the instructions carefully and adjust the process parameters, such as thickness, temperature, time, etc., according to production practice.
It is important to control the pre-baking temperature and time. If the temperature is too high or the time is too long, it will be difficult to develop and remove the film. If the temperature is too low or the time is too short, the drying will be incomplete and the film will be pressure-sensitive, which will easily stick to the film, causing poor exposure and damage to the film. Therefore, if the multi-layer circuit board is processed and pre-baked properly, the development and film removal will be faster and the graphics quality will be good.
Although circuit board baking is only a small part of the SMT part processing process, it plays a vital role in the quality and performance of the product. Only through scientific and reasonable circuit board baking processing can we produce higher quality PCBA products.
Integrated manufacturing and molding
One of the biggest concerns for designers is how to manufacture parts as efficiently as possible. Most parts require a large number of parts to be manufactured using traditional techniques and then assembled later. These operations affect the quality and reliability of the design.
Custom steel brackets made through traditional manufacturing methods start with a CAD model. After the design is completed, manufacturing begins by cutting the steel profiles to size. The profiles are then clamped in place and welded one at a time to form the brackets. Sometimes custom jigs will need to be made to ensure all components are aligned correctly. The welds are then ground to provide a good surface finish. The next step is to drill holes for mounting the brackets to the wall. Finally, the bracket is sandblasted, primed and painted to improve its appearance.
With a 3D printer, construction can be completed in just one step, with no machine operator intervention required during the construction phase. Once the CAD design is complete, it can be uploaded to a machine and printed in one step, within a few hours.
The ability to produce parts in one piece greatly reduces reliance on different manufacturing processes (machining, welding, painting) and gives designers greater control over final product quality.
3D printing process (red) compared to traditional manufacturing process (black) process manufacturing process
spend
Manufacturing costs can be divided into three categories: machine operating costs, material costs and labor costs.
Machine operating costs: Most desktop 3D printers consume the same amount of power as a computer. Industrial 3D printing technology consumes large amounts of electricity and may consume more electricity to produce a single part. However, the ability to generate complex geometries in just one step leads to greater efficiency and turnaround. Machine operating costs are usually the lowest contributor to total manufacturing costs.
Material costs: Material costs for 3D printing vary depending on the technology. Desktop FDM printers use filament coils and cost about $25 per kilogram, while SLA printing requires resin, which retails for about $150 per liter. The range of materials available for 3D printing makes it difficult to quantify comparisons with traditional manufacturing. Nylon powder used in SLS costs about $70 per kilogram, while nylon pellets used in injection molding cost only $2 to $5 per kilogram. Material costs are the largest source of cost for 3D printed parts. This is also an important reason that restricts many companies from purchasing 3D printers.
Labor costs: One of the main advantages of 3D printing is lower labor costs. In addition to post-processing, most 3D printers only require one or two operators. The machine then follows a fully automated process to produce the parts. Compared with traditional manufacturing, which often requires skilled machinists and operators, the labor cost of 3D printers is almost zero.
Compared with traditional manufacturing processes, 3D printing costs for small batches are extremely competitive. For the production of prototypes that verify shape and fit, it is much cheaper than other alternative manufacturing methods (such as injection molding) and is often competitive in manufacturing one-off functional parts. As production volumes increase, traditional manufacturing techniques become more cost-effective, so 3D printing is an important supplement to traditional manufacturing.
Reduce potential risks
Accidentally creating defective prototypes can waste designers time and money. Using traditional mold processing and manufacturing methods, even small changes may result in huge financial expenditures (high mold opening costs).
The ability to verify a design by printing a production-ready prototype before purchasing expensive manufacturing equipment such as molds or tooling and fixtures removes risk from the prototyping process. This helps to verify the feasibility of the project through low-cost trial and error of 3D printing before committing to the large investment required for mass production.
Complexity and design freedom
Traditional manufacturing methods have higher restrictions on the products that can be manufactured. Design requirements such as draft angle, undercut and tool entry are not suitable.
For 3D printing, there may be some limitations on the smallest size features that can be accurately printed, but in most cases the main limitations revolve around how to optimize the printing direction to reduce support dependence and the possibility of printing failure. This gives the designer a great deal of design freedom and allows the creation of very complex geometries with ease.
Customization
Not only does 3D printing offer greater design freedom, it also allows for completely customizable designs. Since current 3D printing technology can only produce a small number of parts at a time, it is very suitable for low-volume customized production. This customization concept has been embraced by the medical and dental industries for the production of customized prostheses, implants, and dental aids, among others. From premium sports gear tailored to fit athletes perfectly to custom sunglasses and fashion accessories, 3D printing can cost-effectively produce custom parts in one go.
sustainable development
Traditional manufacturing methods, such as CNC milling or turning, remove large amounts of excess material from the starting material, resulting in large amounts of scrap. 3D printing, on the other hand, typically uses only the necessary amount of material needed to make the part. Most processes use raw materials that can be recycled and reused in multiple projects. So the 3D printing process produces almost no waste.
]]>PCB baking pipe
1. Large PCBs are mostly placed flat, with a maximum of 30 pieces stacked. Take the PCB out of the oven within 10 minutes after baking is completed, and place it flat at room temperature to cool naturally.
2. Small and medium-sized PCBs are mostly placed flat, with a maximum of 40 pieces stacked. There is no limit to the number of uprights. Remove the PCB from the oven within 10 minutes after baking is completed, and place it flat at room temperature to cool naturally.
PCB baking precautions
1. If an oven is used for the PCB circuit board, it must be equipped with air blast and constant temperature control to ensure uniform pre-baking temperature. Moreover, the oven should be clean and free of impurities to avoid falling on the board and damaging the membrane surface.
2. Do not use natural drying, and the drying must be complete, otherwise it will easily stick to the film and cause poor exposure.
3. After PCB circuit board processing and pre-baking, the multi-layer circuit board should be air-cooled or naturally cooled before film alignment exposure.
4. After pre-baking the multi-layer circuit board, the time from coating to development should not exceed two days at most. When the humidity is high, try to expose and develop within half a day.
5. Different models of liquid photoresist have different requirements. You should read the instructions carefully and adjust the process parameters, such as thickness, temperature, time, etc., according to production practice.
6. It is important to control the pre-baking temperature and time. If the temperature is too high or the time is too long, it will be difficult to develop and remove the film. If the temperature is too low or the time is too short, the drying will be incomplete and the film will be pressure-sensitive, which will easily stick to the film, causing poor exposure and damage to the film. Therefore, if the multi-layer circuit board is processed and pre-baked properly, the development and film removal will be faster and the graphics quality will be good.
The above is the analysis of PCB circuit board baking requirements and precautions introduced by PCB manufacturer Audemars Piguet. I hope it can provide you with a reference.
3. Functional Test
PCBA function detection is the earliest automatic detection principle. It is a basic detection method for a specific PCB or specific unit and can be completed with various detection equipment. There are two main types of functional testing: Final Product Test and Hot Mock-up.
4. Flying-Probe Tester
Flying probe inspection machine, also called probe inspection machine, is also a commonly used inspection method. It has gained widespread popularity over the past few years due to advances in mechanical precision, speed and reliability. In addition, the current requirements for inspection systems with fast changeover and fixture-free capabilities required for prototype manufacturing and low-volume manufacturing make flying probe inspection the best choice. The main advantages of flying probe inspection machines are that it is the fastest time to market tool, automatically generates inspections, has no fixture costs, good diagnostics and easy programming.
5. Manufacturing Defect Analyzer (MDA)
MDA is a great tool for diagnosing only PCBA manufacturing defects in high volume/low mix environments. The main advantages of this detection method are low upfront cost, high output, easy follow-up diagnosis and fast and complete short circuit and open circuit detection. The main disadvantages are that functional testing cannot be performed, there is usually no testing coverage instruction, fixtures must be used, and testing costs are high.
There are three main types of packaging-free technologies: LTCC packaging, LGA packaging and CSP packaging.
LTCC packaging has good electrical and mechanical performance and is suitable for high-frequency and high-speed applications.
LGA packaging uses a flexible substrate to achieve multi-chip packaging and high-density integration.
CSP packaging is a compact package suitable for small mobile devices.
Integrated packaging technology is also an important development direction of MEMS packaging technology. Integrated packaging technology refers to integrating MEMS chips and other electronic devices (such as radio frequency components, power amplifiers, etc.) on the same chip to achieve more powerful and compact packaging.
There are two main types of integrated packaging technologies: OLP packaging and SiP packaging.
OLP packaging uses photolithography technology to create protective films and circuits on a silicon substrate, and then integrates MEMS chips into it. This packaged pipe offers the advantages of small size, high reliability and high operating frequency, making it suitable for wireless communications and sensing applications.
SiP packaging integrates MEMS chips and other chips (such as processors, memories, etc.) into the same package to form a package with multiple functions. SiP packages offer advanced processing capabilities, lower power consumption and smaller size for fast and versatile applications.
MEMS packaging technology has a wide range of applications. Among the most common applications are sensors and actuators. MEMS sensors can measure and detect physical and chemical parameters in the environment, such as temperature, pressure, humidity, acceleration, etc. MEMS actuators can control and operate mechanical and electronic devices. These sensors and actuators are widely used in automobiles, smartphones, medical equipment and other fields, providing solutions with higher precision, higher efficiency and lower power consumption. In addition, MEMS packaging technology is also used in biomedicine, aerospace, industrial automation and other fields, providing important support for the development of these fields.
MEMS packaging
With the continuous development of MEMS technology and the widespread application of MEMS, MEMS packaging technology is also constantly innovating and improving. Package-free technology and integrated packaging technology provide smaller, more precise and higher-performance solutions for MEMS packaging. MEMS packaging technology is also widely used, mainly in fields such as sensors and actuators, and plays an important role in improving product performance and user experience. With the continuous advancement of technology and the continuous expansion of applications, it is believed that MEMS packaging technology will be more widely used and developed.
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2. The location of components must be determined
This benchmark part can also be said to be the main component used at the beginning of schematic drawing. After determining the benchmark parts, drawing based on the pins of these benchmark parts can ensure the accuracy of the schematic diagram to a greater extent.
For engineers, the determination of reference parts is not a very complicated matter. Generally, components that play a major role in the circuit can be selected as reference parts. They are generally larger in size and have more pins, making it easier to draw. Such as integrated circuits, transformers, transistors, etc., can be used as suitable reference components.
3. Reasonable drawing and wiring
For the distinction between ground wires, power wires, and signal wires, engineers also need to have relevant power supply knowledge, circuit connection knowledge, PCB wiring knowledge, etc. The distinction between these lines can be analyzed from the aspects of component connection, line copper foil width, and characteristics of the electronic product itself.
In wiring drawing, in order to avoid crossing and crossing of lines, a large number of grounding symbols can be used for ground wires. Different lines of different colors can be used to ensure clarity for various lines. Special signs can also be used for various components, and even Draw the unit circuits separately and finally combine them.
4. Master the basic framework and learn from similar schematics
Engineers need to be proficient in the framework composition and principle drawing methods of some basic electronic circuits. They must not only be able to directly draw the basic composition forms of some simple and classic unit circuits, but also be able to form the overall framework of electronic circuits.
On the other hand, don’t ignore that electronic products of the same type have certain similarities in schematic diagrams. Engineers can make full use of similar circuit diagrams to infer new product schematics based on accumulated experience.
5. Checking and Adjustment
After the schematic drawing is completed, the reverse design of the PCB schematic diagram is completed after testing and verification. The nominal values of components that are sensitive to PCB distribution parameters need to be checked and optimized. According to the PCB file diagram, the schematic diagram is compared, analyzed and checked to ensure that the schematic diagram and the file diagram are completely consistent.
If the schematic layout is found to be inconsistent with the requirements during the verification, the schematic will be adjusted until it is completely reasonable, standardized, accurate and clear.
PCBA assembly
Causes of PCBA bridging
1. PCB design issues: The larger and heavier components of the PCB are placed on the same side, causing uneven weight distribution of the PCB and causing tilt.
2. The direction of the components is reversed.
3. Lack of redundancy in the space between gaskets.
4. The reflow oven temperature curve setting is unscientific.
5. The patch pressure setting is unreasonable, etc.
PCBA welding solutions
1. PCB circuit board design: Strictly carry out scientific planning at the circuit board design level, reasonably distribute the weight of components on both sides, open air holes and through-holes, adjust the spacing between dense components, and add solder resist layers appropriately, etc. wait.
2. Reflow oven temperature curve: In PCBA assembly, literally speaking, when the liquid solder melts, the activity of the solder at the higher end is higher. If the reflow soldering temperature curve is set unreasonably, it will cause disordered flow of solder paste. Chance of adding new bridge connection.
3. Choose a solder paste printer: A solder paste printer does not need to apply solder paste through a stencil. This will reduce solder paste contact caused by unscientific template opening, stencil warping, and stencil separation. Poor coating.
4. Reasonably control the amount of solder paste: Reasonably control the amount of solder paste applied to reduce the problem of excessive solder paste collapse and high fluidity.
5. Properly set the solder mask: Properly setting the solder mask goes a long way in reducing the risk of solder bridging.
After understanding how to prevent PCBA soldering and PCBA bridging, you can focus on issues related to their processes, PCB design, reflow curves, etc. when reviewing PCBA assembly plants to reduce uncontrollable costs caused by problems.
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Understanding the path and manner in which current returns to ground is key to optimizing mixed-signal circuit board designs. Many design engineers only consider where the signal current flows and ignore the specific path of the current. If the ground layer must be divided, wiring must be carried out through the gap between the partitions. A single point connection can be made between the divided ground layers to form a connecting bridge between the two ground layers, and then the wiring is carried out through the connecting bridge. This provides a DC loop under each signal line, resulting in a small loop area.
Opto-isolating devices or transformers can also be used to span the gap. In the former, the optical signal passes through the gap, while in the case of a transformer, the magnetic field passes through the gap. Another possible approach is to use differential signaling: signals flow in from one line and return from another line, in which case they don't need to be used as return paths.
In order to explore the interference of digital signals to analog signals, we must first understand the characteristics of high-frequency currents. High frequency current always takes the impedance (inductance) path directly beneath the signal, so the return current will flow through an adjacent circuit layer, whether the adjacent layer is a power or ground layer.
In practice, PCBs are usually divided into analog and digital parts. Analog signals are routed in the analog areas of each layer of the circuit board, while digital signals are routed in the digital circuit areas. In this case, the return current of the digital signal does not flow to the ground of the analog signal.
Interference from digital signals to analog signals will occur only when digital signals are wired in the analog part of the circuit board or when analog signals are wired in the digital part of the circuit board. This problem is not due to lack of partitioning, the real reason is improper digital signal routing.
PCB design adopts a unified design. Through the division of digital circuits and analog circuits and appropriate signal routing, some difficult layout and wiring problems can usually be solved without causing potential troubles caused by some ground divisions. In this case, the layout and partitioning of components become key to the design. With proper layout, digital ground current will be limited to the digital portion of the board and will not interfere with analog signals. Such wiring must be carefully checked and inspected to ensure 100% compliance with wiring regulations. Otherwise, a poorly routed signal line can completely ruin a perfectly good circuit board.
When connecting the analog ground and digital ground pins of the A/D converter together, most A/D converter manufacturers will recommend connecting the agnd and DGND pins to the same low impedance ground via short leads (Note: Due to the large Most A/D converter chips do not connect analog and digital ground together, they must be connected via external pins) Any external impedance connected to DGND will couple more digital noise to the analog inside the IC via parasitic capacitance in the circuit. According to this proposal, the A/D converter's agnd and DGND pins need to be connected to analog ground. However, this method may lead to problems such as whether the ground end of the digital signal decoupling capacitor should be connected to analog ground or digital ground.
If the system has only one A/D converter, the above problems can be easily solved. As shown in Figure 3, under the A/D converter, the ground is separated and the analog and digital sections are connected together. When using this method, you need to ensure that the width of the connecting bridge between the two grounds is equal to the width of the IC, and that no signal lines can span the separation gap.
If there are many A/D converters in the system, for example, how to connect 10 A/D converters? If you connect analog and digital grounds together at the bottom of each A/D converter, you will have multiple connections and isolation between analog and digital grounds will be meaningless. Failure to connect in this manner violates the manufacturer's requirements.
If you have questions about the unified design of mixed-signal PCBs, we can use the method of dividing ground layers to lay out and route the entire circuit board. In the design, we should try to make the board easy to connect to separate grounds later in the experiment with jumpers spaced less than 1/2 inch apart or with 0 ohm resistors. Pay attention to zoning and routing to ensure that there are no digital signal lines above the analog part of all layers or any analog signal lines above the digital part. Additionally, there are no signal lines to span the ground gap or divide the gap between power supplies. Test the functionality and EMC performance of the circuit board, then connect the two grounds via a 0 ohm resistor or jumper wire, and retest the functionality and EMC performance of the circuit board. Comparing the test results,
This method can be used in the following three situations: some medical equipment requires low leakage current between circuits and systems connected to the patient; the output of some industrial process control equipment may be connected to high-noise, high-power electromechanical equipment; another The first situation is that the layout of the PCB is restricted.
In mixed-signal PCBs, there are usually separate digital and analog power supplies, and split power supplies can and should be used. However, signal lines adjacent to the power supply layer cannot span the gap between the power supplies, and all signal lines crossing the gap must be located on large adjacent circuit layers. In some cases, replacing a surface analog power supply design with PCB connections can avoid power-side segmentation issues.
Mixed-signal PCB design is a complex process. The following points should be noted during the design process:
1. Divide the PCB into independent analog parts and digital parts.
2. Correct layout of components.
3. A/D converters are placed across partitions.
4. Don’t divide the land. The circuit board is laid out evenly underneath the analog and digital sections.
5. In all layers of the circuit board, digital signals can only be routed in the digital part of the circuit board.
6. In all layers of the circuit board, analog signals can only be routed in the analog part of the circuit board.
7. Achieve separation of analog and digital power supplies.
8. The wiring must not cross the gap between the split power supply surfaces.
9. Signal lines that must pass through the split power supply gap should be located on large adjacent wiring layers.
10. Analyze the actual flow path and return flow method.
11. Use correct wiring rules.
It's time to delve deeper into the elements of PCB design files. In stages, everything from hand-drawn blueprints is now as simple as physical representations built using laminated or ceramic materials. Use Light Painting PCB.
The content of the PCB design file follows the blueprint of the schematic process flow, but is not the same and is built in many different ways.
When we always project PCB model design files, what we project in 3D is 3D, including circuit boards and design files. Because it can be single layer or fine, but the most common is two layers.
We can observe some differences between PCB drawings and PCB design files.
All components are correctly sized and positioned
If two points shouldn't, then you have to separate yourself or switch to another PCB layer, thus avoiding crossing over each other.
Therefore, we briefly illustrate the actual performance of the PCB design since native verification is a later stage of the final product. Physical requirements.
Nearly all manner of components allow for connector current and temperature related issues near distribution boundaries, and the various traces must be more…
Because physical limitations and requirements mark PCB design files that often look much different than the design on the schematic, the design file includes a screen-printed layer. Use a circuit board.
All the parts need to be assembled onto the PCB and then work as planned. If not, you need to re-render.
The circuit schematic diagram is a representation pipeline used to show the working principle of the equipment's electricity, the role of each electrical component, and the relationship between each other. Using the methods and techniques of electrical schematic diagrams is very beneficial for analyzing power lines and troubleshooting machine tool circuit faults. Electric power schematic diagram generally consists of main circuit, control circuit, protection, distribution circuit and other parts.
Although PCB schematics and PCB design files are often restored, but rarely, making PCB schematics and PCB design refer to two separate processes when making printed circuit boards. A PCB schematic diagram that can draw the process flow must be created first before PCB design can be carried out, and PCB design is an important equipment that determines PCB performance and intellectual property rights.
The design of the circuit board is based on the circuit schematic diagram to achieve the functions required by the circuit designer. The design of the circuit board mainly refers to the layout design, and the layout of external connections needs to be considered. Optimized layout of internal electronic components. Optimized layout of metal connections and vias. Electromagnetic protection. heat dissipation and other factors. Excellent circuit board design can save production costs and achieve good circuit performance and heat dissipation performance. .
Should you have any enquiries, please contact us and we will happy to provide any assistance.
My email ID:kitty@pcb-hero.com
]]>1. Selection of test points: Based on the complexity of the PCB assembly board and the number of collector channels (generally, the collector has 3-12 test channels), select at least three representative temperature test points that can reflect high (hot spot) temperatures. ), medium and low (cold spot) PCB surface assembly boards.
The temperature (hot spot) is generally located in the middle of the furnace, where there are no components or there are few components. The temperature (cold spot) is generally located in large components (such as PLCC), large areas of copper distribution, transmission rails or edges of the furnace hall, and where hot air convection is impossible. Arrival location.
2. Fixed thermocouple: Use high-temperature solder (sn-90pb, solder with a melting point exceeding 289°C) to weld the test ends of multiple thermocouples on the test points (solder points). The solder on the original solder points must be the same as before welding. Remove; or use high-temperature tape to stick the test end of the thermocouple to each temperature test point on the PCB. No matter which method is used to fix the thermocouple, it is necessary to ensure that the thermocouple is welded, glued, and clamped firmly.
3. Insert the other end of the thermocouple into 1, 2.3... on the machine. The location of the jack, or the socket into which the collector is inserted, pay attention to the polarity and do not insert it backwards. Number the thermocouples and record the relative position of each thermocouple on the surface mount board.
4. Place the PCB assembly board on the surface to be measured on the conveyor chain/mesh belt at the entrance of the reflow soldering machine (if a collector is used, place the collector at a distance of more than 200mm behind the surface PCB assembly board), and then start the KIC temperature Curve test procedure.
5. As the PCB is operated, real-time curves are drawn (displayed) on the screen (when the device comes with KIC test software).
6. When the PCB runs through the cooling zone, pull the thermocouple wire to pull the PCB assembly board back. At this time, a test process is completed, and the complete temperature curve and peak temperature/schedule are displayed on the screen (if a temperature curve collector is used, take out the PCB and collector from the reflow oven outlet, and then read the temperature curve and peak temperature table through the software ).
FPC
3. Production process
Here we take an ordinary carrier board as an example to detail the SMT key points of FPC. When using silicone boards or magnetic fixtures, the fixation of FPC is much more convenient and does not require the use of tape. However, the process points of printing, patching, welding and other processes are it's the same.
3.1. Fixing of FPC
Before proceeding with SMT, the FPC first needs to be accurately fixed on the carrier board. Special attention should be paid to the shorter the storage time between the FPC being fixed on the carrier board and printing, mounting and welding, the better. The carrier board is available with or without positioning pins. The carrier board without positioning pins needs to be used with the positioning template with positioning pins. First, put the carrier board on the positioning pins of the template so that the positioning pins are exposed through the positioning holes on the carrier board. Then put the FPC one by one on the template. The exposed positioning pins are then fixed with tape, and then the carrier board is separated from the FPC positioning template for printing, patching and welding. The carrier plate with positioning pins has been fixed with several spring positioning pins about 1.5mm long. You can directly put the FPC piece by piece on the spring positioning pins of the carrier plate and then fix it with tape. During the printing process, the spring positioning pin can be completely pressed into the carrier plate by the steel mesh without affecting the printing effect.
Method 1 (Single-sided tape fixation) Use thin high-temperature resistant single-sided tape to fix the four sides of the FPC on the carrier board to prevent the FPC from shifting and warping. The viscosity of the tape should be moderate, it must be easy to peel off after reflow soldering, and it must be on the FPC No adhesive residue. If you use an automatic tape machine, you can quickly cut tapes of consistent length, which can significantly improve efficiency, save costs, and avoid waste.
Method 2 (fixed with double-sided tape) First use high-temperature resistant double-sided tape to stick to the carrier board. The effect is the same as that of the silicone board. Then stick the FPC to the carrier board. Pay special attention to the viscosity of the tape not being too high, otherwise it will peel off after reflow soldering. When used, it is easy to cause FPC tearing. After repeated passes through the oven, the viscosity of the double-sided tape will gradually become lower. When the viscosity is too low to reliably fix the FPC, it must be replaced immediately. This station is a key station to prevent the FPC from getting dirty, and you need to wear finger gloves when working. Before the carrier board is reused, it needs to be properly cleaned. You can use a non-woven cloth dipped in detergent to scrub it, or you can use an anti-static dust-sticking roller to remove surface dust, tin beads and other foreign matter. Be careful not to use too much force when picking up and placing FPC. FPC is fragile and prone to creases and breaks.
3.2. FPC solder paste printing
FPC does not have special requirements for the composition of solder paste. The size and metal content of solder ball particles are subject to whether there are fine-pitch ICs on FPC. However, FPC has higher requirements for the printing performance of solder paste, and solder paste should have excellent Being thixotropic, the solder paste should be able to be easily printed and demoulded and firmly attached to the FPC surface, without defects such as poor demoulding blocking the stencil leaks or collapse after printing.
Because FPC is loaded on the carrier board, and there is high-temperature resistant tape for positioning on the FPC, which makes the plane inconsistent. Therefore, the printed surface of FPC cannot be as flat and consistent in thickness and hardness as PCB. Therefore, it is not suitable to use a metal scraper, but should use a hardness of 80 -90 degree polyurethane type scraper. It is best for the solder paste printer to have an optical positioning system, otherwise it will have a great impact on the printing quality. Although the FPC is fixed on the carrier board, there will always be some small gaps between the FPC and the carrier board. This is related to the hardness of the PCB. The biggest difference is that the setting of this equipment parameter will also have a great impact on the printing effect.
The printing station is also a key station to prevent FPC from getting dirty. You need to wear finger gloves when working. At the same time, you must keep the station clean and wipe the steel mesh frequently to prevent solder paste from contaminating the gold fingers and gold-plated buttons of the FPC.
3.3. FPC patch
Depending on the characteristics of the product, the number of components and the placement efficiency, medium or high-speed placement machines can be used for placement. Since each FPC has optical MARK marks for positioning, there is not much difference between SMD mounting on FPC and mounting on PCB. It should be noted that although the FPC is fixed on the carrier board, its surface cannot be as flat as a PCB hard board. There will definitely be a local gap between the FPC and the carrier board. Therefore, the drop height of the nozzle, blowing pressure, etc. It needs to be set accurately and the moving speed of the nozzle needs to be reduced.
3.4. FPC reflow soldering
A forced hot air convection infrared reflow oven should be used so that the temperature on the FPC can change more evenly and reduce the occurrence of poor soldering. If you use single-sided tape, because it can only fix the four sides of the FPC, the middle part will be deformed under hot air, and the pad will easily tilt, and the molten tin (liquid tin at high temperature) will flow, resulting in empty soldering, continuous soldering, and Tin beads make the process defective rate higher.
3.4.1. Temperature curve test method
Due to the different heat absorption properties of the carrier board and the different types of components on the FPC, their temperatures rise at different rates after being heated during the reflow soldering process, and the heat they absorb is also different. Therefore, carefully set the temperature curve of the reflow soldering furnace to ensure Welding quality has a great impact. A more reliable method is to place two carrier boards with FPC in front and behind the test board according to the actual production carrier board spacing. At the same time, components are mounted on the FPC of the test carrier board, and the test temperature is removed with high-temperature solder wire. The probe is soldered on the test point, and the probe wire is fixed on the carrier board with high temperature resistant tape. Note that the test points cannot be covered with high temperature resistant tape. The test points should be selected near the solder joints and QFP pins on all sides of the carrier board, so that the test results can better reflect the real situation.
3.4.2. Setting of temperature curve
In the furnace temperature debugging, because the temperature uniformity of FPC is not good, it is best to use a temperature curve pipeline of heating/insulation/reflow, so that the parameters of each temperature zone are easier to control, and the impact of thermal shock on FPC and components is small. Some. According to experience, it is best to adjust the furnace temperature to the lower limit of the solder paste technology requirements. The wind speed of the reflow furnace is generally the lowest wind speed that the furnace can use. The reflow furnace chain should have good stability and should not be shaken.
3.5. Inspection, testing and subboarding of FPC
Since the carrier plate absorbs heat in the furnace, especially the aluminum carrier plate, the temperature is higher when it comes out of the oven, so it is best to add a forced cooling fan at the outlet to help cool down quickly. At the same time, workers need to wear heat-insulating gloves to avoid being burned by the high-temperature carrier plate. When picking up the welded FPC from the carrier board, the force must be even and do not use brute force to prevent the FPC from being torn or creased.
The removed FPC is visually inspected under a magnifying glass of more than 5 times, focusing on the surface glue residue, discoloration, gold fingers stained with tin, tin beads, IC pin air soldering, continuous soldering and other issues. Since the surface of FPC cannot be very flat, the misjudgment rate of AOI is very high, so FPC is generally not suitable for AOI inspection. However, by using special test fixtures, FPC can complete ICT and FCT tests.
Since most FPCs are connected boards, it may be necessary to separate boards before testing ICT and FCT. Although tools such as blades and scissors can also be used to complete the board separation work, the work efficiency and work quality are low. If it is mass production of special-shaped FPC, a special FPC stamping and splitting mold can be made for stamping and splitting, which can greatly improve work efficiency. At the same time, the edges of the punched FPC are neat and beautiful, and the internal stress generated during stamping and cutting is very low. It can effectively prevent solder joints from cracking.
In the assembly and welding process of PCBA flexible electronics, the precise positioning and fixation of FPC is the focus. The key to the quality of fixation is to make a suitable carrier board. Followed by FPC pre-baking, printing, patching and reflow soldering. Obviously, the SMT process of FPC is much more difficult than that of PCB hard board, so it is necessary to accurately set the process parameters. At the same time, strict production process management is equally important. It is necessary to ensure that workers strictly implement every regulation on the SOP and follow the line. Engineers and IPQC should strengthen inspections, promptly discover abnormalities in the production line, analyze the causes and take necessary measures to control the defective rate of the FPCSMT production line within dozens of PPM.
4. PCBA production equipment
The basic equipment required for PCBA production includes solder paste printers, placement machines, reflow soldering, AOI detectors, component clippers, wave soldering, tin ovens, plate washers, ICT test fixtures, FCT test fixtures, Aging test rack, etc.
4.1. Solder paste printing machine
Modern solder paste printing machines generally consist of mechanisms such as plate mounting, solder paste addition, imprinting, and circuit board delivery. Its working principle is to first fix the circuit board to be printed on the printing positioning table, and then use the left and right scrapers of the printing machine to leak the solder paste or red glue through the steel mesh onto the corresponding pads. For PCBs with uniform leakage, pass through the transmission The machine is input to the placement machine for automatic placement.
4.2. SMT machine
The placement machine is also called "mounting machine" or "Surface Mount System" (Surface Mount System). In the production line, it is configured after the solder paste printing machine. It accurately places surface mount components on the PCB by moving the placement head. A device on the disk, divided into two types: manual and fully automatic.
4.3. Reflow soldering
There is a heating circuit inside the reflow soldering, which heats the air or nitrogen to a high enough temperature and then blows it to the circuit board where the components have been attached, allowing the solder on both sides of the components to melt and bond with the motherboard. The advantage of this process is that the temperature is easy to control, oxidation can be avoided during the welding process, and the manufacturing cost is also easier to control.
4.4. AOI detector
The full name of AOI (Automatic Optic Inspection) is automatic optical inspection. It is a device based on optical principles to detect common defects encountered in welding production. The machine automatically scans the PCB through the monitor, collects images, and compares the tested solder joints with the qualified parameters in the database. After image processing, defects on the PCB are detected, and the defects are displayed/marked through the display or automatic signs. Maintenance crew repairs.
4.5. Component shearing machine
Used for trimming and deforming pin components.
4.6. Wave soldering
Wave soldering is to make the soldering surface of the plug-in board directly contact with the high-temperature liquid tin for the purpose of soldering. The high-temperature liquid tin maintains a slope, and a special device makes the liquid tin form a wave-like phenomenon, so it is called "wave soldering". The main material is solder bar.
4.7. Tin furnace
Generally speaking, a tin furnace refers to a soldering tool used in electronic welding. For discrete component circuit boards, the welding consistency is good, the operation is convenient and fast, and the work efficiency is high.
4.8. Plate washer
Used to clean PCBA boards and remove residues from the boards after welding.
4.9. ICT test fixture
ICTTest mainly uses the test probe to contact the test points coming out of the PCBlayout to detect open circuits, short circuits, and the welding conditions of all parts of the PCBA.
4.10. FCT test fixture
FCT (Functional Test) refers to providing an analog operating environment (excitation and load) to the test target board (UUTUnitUnderTest), allowing it to work in various design states, thereby obtaining the parameters of each state to verify the functionality of the UUT. test methods. Simply put, it is to load the appropriate stimulus to the UUT and measure whether the output response meets the requirements.
4.11. Aging test stand
The aging test rack can test PCBA boards in batches and test out problematic PCBA boards by comparing the operations used by users for a long time.
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1.PCB circuit board classification
PCB substrate data is divided into two categories:
1. Organic material substrates, including phenolic resin, fiberglass/epoxy resin (FR4), polyimide, BT/epoxy resin, etc.
2. Inorganic material substrates, including inorganic metal substrates (copper substrates, aluminum substrates, etc.) and ceramic substrates.
2. Analysis of advantages of PCB circuit board materials
1. Metal bottom plate: 0.3mm-2. Omm thick metal plates, such as aluminum, iron, copper, epoxy resin prepreg and copper foil, are pressed by thermal lamination. This metal plate can be used for large-area patch processing and has the following performance characteristics:
(1) Good mechanical performance: The metal substrate has good mechanical strength and toughness. Solve the brittleness problem of printed circuit boards based on inorganic materials. Suitable for large-area patch processing and can withstand the installation of heavy-duty components. In addition, the dimensional stability and smoothness of metal substrates are its main advantages.
(2) Good heat dissipation: Since the metal substrate and prepreg are in direct contact, it has excellent heat dissipation performance. When a metal substrate is used for patch processing, the heat generated during patch processing can be well dispersed. The heat dissipation capacity of the circuit board depends on the thickness of the metal substrate and the thickness of the resin layer. Of course, the design should also consider the effects of electrical performance, such as resistance.
(3) Can shield electromagnetic waves: In high-frequency circuits, designers are mainly concerned with preventing electromagnetic wave radiation. Metal substrates can form a natural protective layer to achieve the purpose of shielding electromagnetic waves.
3. PCB ceramic substrate
Ceramic substrate: Ceramic substrate is widely used in power electronic circuits and has the following advantages:
(1) Ceramic substrate has good electrical insulation performance, which is the basic performance of the substrate;
(2) The ceramic substrate also has high thermal conductivity and can transfer the heat generated by the circuit well.
(3) Ceramic substrates also have excellent welding performance, high adhesion strength and high current carrying capacity.
PCB circuit boards come in a variety of substrate materials, each with its own advantages. Appropriate substrate data should be selected based on actual use and processing conditions to ensure the molding quality of the final product. IPCB will also continue to improve its processes to provide customers with better PCB board processing services.
The following are some of the causes of PCB board deformation and warpage:
1. PCB design is unreasonable
PCB board design is an important factor affecting its deformation and warping. If the design is unreasonable, it will cause the PCB board to deform and warp during processing and use. For example, if the length-to-width ratio of the board is unreasonable, or the layout of components on the board is unreasonable, it will cause the PCB board to deform and warp.
2. Improper data selection
The choice of PCB board material is also an important factor affecting its deformation and warping. Different materials have different characteristics, such as thermal expansion coefficient, density, etc. If the selected data does not match the application environment of the PCB board, it will cause the PCB board to deform and warp.
3. Improper PCB processing technology
The processing technology of the PCB board is also an important factor affecting its deformation and warping. If the processing technology is improper, it will cause the PCB board to deform and warp. For example, if the welding temperature is too high or the time is too long, it will cause the PCB board to deform and warp. If the drilling depth is too large or the hole diameter is too small, it will cause the PCB board to deform and warp.
4. Improper use environment
The usage environment of the PCB board is also an important factor affecting its deformation and warping. If the use environment is improper, it will cause the PCB board to deform and warp. For example, if the ambient temperature is too high or the humidity is too high, it will cause the PCB board to deform and warp. If used in a high-frequency electromagnetic field environment, it will cause the PCB board to deform and warp.
5. The weight of the PCB board itself will cause the board to dent, deform and warp.
Generally, reflow ovens use chains to drive the PCB board forward in the reflow oven. If there are overweight parts on the board, or the size of the board is too large, the center of the board will be dented due to its own weight, causing the board to be dented. bend.
6. The depth of V-Cut and the connecting strip will affect the amount of panel deformation and warping.
V-Cut is to cut grooves in the original large sheet, so the place where V-Cut appears is prone to deformation and warping.
7. Deformation and warping caused by PCB board processing
The causes of deformation and warping during PCB board processing are very complex and can be divided into two types of stress: thermal stress and mechanical stress. Thermal stress is mainly generated during the lamination process, and mechanical stress is mainly generated during the stacking, handling, and baking processes of panels.
The following is a brief discussion in process order.
Incoming copper clad laminate materials: The size of the copper clad laminate press is large, and there are temperature differences in different areas of the hot plate, which will lead to slight differences in the resin curing speed and degree in different areas during the lamination process, and will also produce local stress, which will be gradually released during future processing. Deformation board warping.
Pressing: The PCB board pressing process is the main process that generates thermal stress, which is released during subsequent drilling, shaping or grilling processes, causing the board to deform and warp.
Baking process of solder mask, characters, etc.: Since the solder mask ink cannot be stacked on top of each other when curing, the PCB boards will be placed vertically in the rack for curing. The boards are easily deformed and warped under their own weight or the strong wind of the oven.
Hot air solder leveling: The entire hot air solder leveling process is a sudden heating and cooling process, which will inevitably cause thermal stress, resulting in microscopic strain and overall deformation of the board warping area.
Storage: PCB boards are usually stored in a shelf during the semi-finished product stage. Improper adjustment of the shelf tension, or stacking of boards during storage, etc. will cause mechanical deformation of the board and warping of the board.
In addition to the above factors, there are many factors that affect the deformation and warping of the PCB board. Once you find that the PCB board is deformed and warped, you need to immediately check the cause and make improvements.
Preventive measures for PCB board deformation and warping
1. Reasonable design of PCB board
When selecting boards, you need to select appropriate boards according to the application environment. For example, if the application environment temperature is high, you need to choose a plate with a smaller thermal expansion coefficient. If the application environment has high humidity, you need to choose a board with better moisture-proof performance.
2. Reasonable layout of components
In order to avoid signal interference between components, the distance between components must be reasonable. If the distance between components is too small, signal interference will occur. If the distance between components is too large, the space utilization of the board will be reduced. The height of components should also be reasonable. If the height of the component is too small, it will cause signal interference. If the height of the components is too large, the space utilization of the board will be reduced.
3. Choose the appropriate processing technology
3.1. Select the appropriate welding temperature and time
During the welding process, it is necessary to choose the appropriate welding temperature and time. If the soldering temperature is too high or the soldering time is too long, it will cause the PCB board to deform and warp. If the welding temperature is too low or the time is too short, the welding may not be strong.
3.2. Select the appropriate drilling depth and hole diameter
During the drilling process, it is necessary to choose the appropriate drilling depth and hole diameter. If the drilling depth is too large or the hole diameter is too small, it will cause the PCB board to deform and warp.
In addition to the above factors, the causes of PCB board deformation and warping and the preventive measures for PCB board deformation and warping. There are many factors that affect the deformation and warping of the PCB board. Once you find that the PCB board is deformed and warped, you need to immediately check the cause and take improvement measures.
High-speed PCB design
2. Reflection
Reflection is an echo on a transmission line. Part of the signal power (voltage and current) travels down the line and reaches the load, but part of it is reflected.
If the source and load have the same impedance, reflections will not occur. The impedance mismatch between the source and the load will cause reflections on the line, and the load will reflect part of the voltage back to the source.
If the load impedance is less than the source impedance, the reflected voltage is negative; conversely, if the load impedance is greater than the source impedance, the reflected voltage is positive.
Variations in cabling geometry, incorrect wire termination, transmission through connectors, and power plane discontinuities can all cause such reflections.
3. Crosstalk
Crosstalk is the coupling between two signal lines. The mutual inductance and mutual capacitance between the signal lines cause noise on the line.
Capacitive coupling induces coupling current, while inductive coupling induces coupling voltage. PCB board layer parameters, signal line spacing, power characteristics of the driving end and receiving end, and line termination pipes all have a certain impact on crosstalk.
4. Characteristic impedance
Let’s first clarify a few concepts. We often see impedance, characteristic impedance, and transient impedance. Strictly speaking, they are different, but they remain the same. They are still the basic definitions of impedance:
4.1 The input impedance at the beginning of the transmission line is referred to as impedance;
4.2 The real-time impedance that a signal encounters at any time is called transient impedance;
4.3 If the transmission line has a constant transient impedance, it is called the characteristic impedance of the transmission line.
4.4 Characteristic impedance describes the transient impedance encountered when a signal propagates along a transmission line. This is a major factor affecting signal integrity in transmission line circuits.
4.5 Unless otherwise specified, characteristic impedance is generally used to collectively refer to transmission line impedance.
PS: For high-speed PCB design, our goal is to keep the impedance of the signal as stable as possible during the transmission process, and this must maintain the stability of the characteristic impedance of the transmission line.
5. Power integrity
Power integrity, referred to as PI, is to confirm whether the voltage and current at the source and destination of the power supply meet the requirements.
Power integrity is very important in today's electronic products. There are several levels of power integrity: chip level, chip package level, circuit board level, and system level.
Among them, power integrity at the circuit board level must meet the following three requirements:
Make the voltage ripple at the chip pin smaller than the specification (for example, the error between the voltage and 1V is less than +/-50 mV)
Controlling ground bounce (also known as synchronous switching noise SSN, synchronous switching output SSO)
Reduce electromagnetic interference (EMI) and maintain electromagnetic compatibility (EMC): The power distribution network (PDN) is a conductor on the circuit board, which is also an antenna that easily emits and receives noise.
6. Power supply noise
Power supply noise is a type of electromagnetic interference, and the spectrum of its conducted noise is roughly 10kHz~30MHz, up to 150MHz.
Power supply noise, especially transient noise interference, has fast rising speed, short duration, high voltage amplitude and strong randomness, which can easily cause serious interference to microcomputers and digital circuits.
In high-frequency circuits, the noise contained in the power supply has a particularly obvious impact on high-frequency signals. For this reason, the power supply is first required to be low-noise. Here, a clean ground and clean power supply are equally important.
7. Filtering
Wave filtering is the operation of filtering out specific frequency bands in signals. It is an important measure to suppress and prevent interference. Filtering is divided into classic filtering and modern filtering.
8. Parallel bus
A bus is a common physical path for communication between two or more devices. It is a collection of signal lines and a common connection between multiple components. It is used to transmit information between various components.
Depending on the working mode, the bus can be divided into two types: one is the parallel bus and the other is the serial bus.
Parallel bus: It can transmit multiple bits of data at the same time, just like a spacious road that allows multiple cars to drive side by side, and it is also bidirectional and unidirectional.
9. Serial bus
Serial bus: Only one piece of data can be transmitted at the same time, just like a narrow road that only allows one car to walk. The data must be transmitted one after another, and it looks like a long data string, so it is called "serial".
10. Topology
Topology refers to the way in which various sites in the network are connected to each other. Topology in PCB design refers to the connection relationship between chips.
Commonly used topologies include point-to-point, daisy chain, remote cluster, star, etc.
The above is iPCB sharing with you 10 important matters related to high-speed PCB design. I hope it will be helpful to your learning.
PCBA processing and production
The main reasons and solutions for throwing materials
1. Vacuum problem,
Insufficient air pressure is usually caused by the vacuum air pipe channel being blocked or leaking. When the air pressure is insufficient, the material cannot be picked up normally. It is usually impossible to pick it up or it will fall off on the way to apply.
Solution: Adjust the air pressure steep slope to the air pressure value required by the equipment (such as 0.5~0.6Mpa – YAMAHA chip placement machine), clean the air pressure pipeline, and repair the leaking air path.
2. Programming problem,
The parameter settings of the components in the edited program are incorrect, and they do not match the size, brightness and other parameters of the incoming material, resulting in failed recognition and being discarded. Countermeasures: Modify component parameters and search for optimal parameter settings for components.
Solution: Modify the component parameters and search for the best parameter settings for the component.
3. Problems with incoming materials,
The incoming materials are irregular and are substandard products such as pin oxidation. Countermeasures: IQC does a good job in inspecting incoming materials, detecting defective components in a timely manner, and keeping defective products out of the production line;
Solution: IQC conducts inspection of incoming materials and contacts component suppliers.
4. Feeder problem,
The feeder position is deformed, the feeder feeds poorly (the feeder ratchet gear is damaged, the material belt hole is not stuck on the feeder ratchet gear, there is foreign matter under the feeder, the spring is aging, or the power is poor), The result is that the material cannot be retrieved or is poorly retrieved and the material is thrown away, and the feeder is damaged.
Solution: Adjust the feeder, clean the feeder platform, and replace damaged parts or feeders.
5. Nozzle problem
The suction nozzle of the placement machine is deformed, clogged, etc., which leads to insufficient air pressure, air leakage, etc., which in turn results in the inability to properly absorb materials and incorrect material picking, and the phenomenon of material throwing due to failure in identification.
Solution: Clean and replace the nozzle of the placement machine.
6. Identify system problems
The identification of the placement machine or the laser lens of the part processing is contaminated, there are debris interfering with the identification, the light source is improperly selected and the intensity and grayscale are not enough, or the identification system is damaged.
Solution: Clean the surface of the recognition system, keep the monitor clean and free of debris, adjust the intensity and grayscale of the light source, and replace damaged parts if the recognition system is damaged.
7. Location problem
The material is not taken at the center of the material, and the height of the material is incorrect (generally, the pressure is 0.05MM after touching the part), resulting in offset bits. The material is not taken correctly or offset, and the identification does not match the corresponding data parameters and is detected. The identification system discards it as invalid material.
Solution: Adjust the picking position.
8. PCB quality issues
The thickness of the PCB, the degree of warpage of the PCB board exceeds the allowable error of the equipment, and there is a problem with the placement of the PCB board support pins. When the PCB is mounted on both sides, the support pins on both sides press against the bottom components of the PCB, causing the PCB to warp upward.
Solution: Choose a PCB suitable for mounting
Solving these parts throwing problems requires careful inspection of the part production line and determining the root cause of the problem. It is usually necessary to adjust the quantity and quality of solder paste, ensure that the viscosity and temperature are correct, check whether the placement head is flat and intact, remove dirt and grease on the PCB board, ensure that the size and quality of electronic components meet the requirements, and ensure that the placement The program settings are correct.
1. How to solve the contradiction between manual wiring and automatic wiring of high-speed signals?
Answer: Most of the automatic routers of the current powerful wiring software have set constraints to control the winding method and the number of vias. The winding engine capabilities and constraint setting items of various EDA companies are sometimes very different. For example, are there enough constraints to control the way the serpentine lines meander, can they control the trace spacing of differential pairs, etc. This will affect whether the routing pattern produced by automatic routing can conform to the designer's ideas. In addition, the difficulty of manually adjusting wiring is also absolutely related to the capabilities of the winding engine. For example, the pushing ability of traces, the pushing ability of vias, and even the pushing ability of traces on copper coating, etc. Therefore, choosing a router with a strong winding engine is the solution.
2. Can a ground wire be added in the middle of the differential signal line?
Generally, a ground wire cannot be added in the middle of differential signals. Because the most important point in the application principle of differential signals is to take advantage of the benefits brought by mutual coupling between differential signals, such as flux cancellation, noise immunity, etc. If a ground wire is added in the middle, the coupling effect will be destroyed.
3. What issues should be paid attention to when wiring high-frequency signals?
Answer: 1. Impedance matching of signal lines; 2. Spatial isolation from other signal lines; 3. For digital high-frequency signals, differential lines will have better effects;
4. In what situation is snake-shaped wiring suitable for high speed? Are there any disadvantages? For example, for differential wiring, the two sets of signals are required to be orthogonal.
Answer: Snake wiring has different functions depending on the application:
(1) If the serpentine trace appears in the computer board, it mainly plays the role of a filter inductor and impedance matching, improving the anti-interference ability of the circuit. The serpentine traces in computer motherboards are mainly used in some clock signals, such as PCI-Clk, AGPCIK, IDE, DIMM and other signal lines.
(2) If used in an ordinary PCB board, in addition to functioning as a filter inductor, it can also be used as an inductor coil for a radio antenna, etc. For example, it is used as an inductor in 2.4G walkie-talkies.
(3) Some signal wiring lengths must be strictly equal. The equal length of high-speed digital PCB boards is to keep the delay difference of each signal within a range and ensure the validity of the data read by the system in the same cycle ( If the delay difference exceeds one clock cycle, the data of the next cycle will be read incorrectly). For example, there are 13 HUBLinks in the INTELHUB architecture. They use a frequency of 233MHz and must be strictly equal in length to eliminate hidden dangers caused by time lag. Winding is the only solution. It is generally required that the delay difference does not exceed 1/4 clock cycle. The line delay difference per unit length is also fixed. The delay is related to the line width, line length, copper thickness, and board structure. However, if the line is too long, the distributed capacitance and distributed inductance will increase. , causing the signal quality to decrease. Therefore, the clock IC pins are generally connected to "" termination, but the serpentine trace does not function as an inductor. On the contrary, the inductor will phase shift the higher harmonics in the rising edge of the signal, causing the signal quality to deteriorate, so The spacing between serpentine lines is required to be at least twice the line width. The smaller the rise time of the signal, the more susceptible it is to distributed capacitance and distributed inductance.
(4) Snake wiring plays the role of a distributed parameter LC filter in some special circuits.
5. How to choose the thickness of power traces in PCB board design? Are there any rules?
Answer: You can refer to: 0.15×line width (mm)=A, you also need to consider the copper thickness
6. Are data lines wired in parallel to interfere with each other?
Answer: When running lines in parallel, pay attention to the distance between lines to prevent crosstalk.
7. When designing with 6 layers, what are the layer allocation techniques? Which wiring should go through the middle layer?
Answer: It depends on your design. The principle is to ensure that the analog signal line and the analog ground have two separate layers.
8. What frequency of crystal oscillator should we consider the wiring method between MCU and crystal oscillator?
Answer: The crystal oscillator and MCU should be as close as possible and connected with the shortest straight line.
9. For high-speed PCB, how to avoid via holes during wiring? Do you have any good suggestions?
Answer: For high-speed PCB, it is best to drill fewer vias and add signal layers to solve the need for more vias.
10. How to avoid noise introduced during wiring?
Answer: The digital ground and analog ground must be grounded at a single point, otherwise the digital ground return flow will flow through the analog ground and cause interference to the analog circuit.
11. When bending PCB wiring, there are two types: 45-degree angle and arc bending. What are the advantages and disadvantages? How to choose?
Answer: From the perspective of impedance matching, both lines can be made into matching corners. But rounded corners may be difficult to process.
12. When there are high-speed logic devices in the circuit, what is the maximum wiring length?
Answer: We are not afraid of long wiring, but we are afraid of asymmetry or relatively large differences, which can easily cause wrong logic due to time delay.
Knowledge expansion: basic requirements for PCB wiring
1) Give priority to laying out key signal lines or signal lines with rules, and check the rules. Key signal lines with rules are required to meet the corresponding requirements.
Constraint rules; secondly, check the overall rules for non-critical signal line routing, requiring non-critical signal lines to meet ordinary design requirements; comprehensively use wiring strategies to resolve conflicts.
Post-process the wiring to improve signal quality and facilitate processing.
2) Rule priority: If there are rules, the signal lines required by the rules will be laid out first, and then the non-critical signal lines will be laid out.
3) Prioritize key signal lines: Key signals such as power supply, analog signals, high-speed signals, clock signals, differential signals and synchronization signals should be routed first.
4) Density priority: Start wiring from the devices with the most complex connection relationships on the board, and start wiring from the areas with the densest connections on the board.
5) Avoid using extreme values for vias, line widths, and safe spacing.
6) The distance from the trace to the edge of the board usually needs to be ≥ 2mm. If the conditions cannot be met, it must be at least no less than 20mil;
7) Try to provide dedicated wiring layers for key signals such as clock signals, high-frequency signals, sensitive signals, etc., and ensure the minimum loop area. Use methods such as shielding and increasing safety distances to ensure signal quality.
8) The EMC environment between the power layer and the ground layer is poor, so signal lines that are sensitive to interference should be avoided.
9) Follow the ground loop rules, crosstalk control rules, shielding protection, chamfering, device decoupling, 3W, 20H and other rules.