Before explaining the inspection work after the PCB wiring is completed, I will first introduce three special PCB wiring techniques. The routing of PCB LAYOUT will be explained from three aspects: right-angle routing, differential routing, and serpentine routing:

1. Right-angle wiring (three aspects)
The impact of right-angle wiring on signals is mainly reflected in three aspects: first, the corners can be equivalent to capacitive loads on the transmission line, slowing down the rise time; second, impedance discontinuity will cause signal reflection; third, right-angle tips produce In the field of EMI and RF design above 10GHz, these small right angles may become the focus of high-speed problems.

2. Differential routing ("equal length, equal distance, reference plane")
What is a differential signal (Differential Signal)? In layman's terms, the driving end sends two equal and opposite signals, and the receiving end determines the logic state "0" or "1" by comparing the difference between the two voltages. The pair of traces that carry differential signals are called differential traces. Compared with ordinary single-ended signal wiring, the most obvious advantages of differential signals are reflected in the following three aspects:

1) Strong anti-interference ability, because the coupling between the two differential traces is very good. When there is noise interference from the outside, it is coupled to the two lines almost at the same time, and the receiving end only cares about the difference between the two signals. Therefore, external common mode noise can be completely offset.

2) It can effectively suppress EMI. In the same way, since the polarity of the two signals is opposite, the electromagnetic fields radiated by them can cancel each other out. The closer the coupling, the less electromagnetic energy released to the outside world.

3) Timing positioning is accurate. Since the switching change of the differential signal is at the intersection of the two signals, unlike ordinary single-ended signals that rely on high and low threshold voltages to judge, it is less affected by process and temperature, and can reduce timing errors. , and it is also more suitable for circuits with low amplitude signals. The currently popular LVDS (low voltage differential signaling) refers to this small amplitude differential signaling technology.

3. Snake line (adjust delay)
Snake lines are a type of wiring method often used in Layout. Its main purpose is to adjust the delay and meet the system timing design requirements. The two most critical parameters are parallel coupling length (Lp) and coupling distance (S). Obviously, when signals are transmitted on a serpentine line, coupling will occur between parallel line segments in the form of differential mode, S The smaller it is, the larger Lp is, and the greater the degree of coupling. It may lead to a reduction in transmission delay and a significant reduction in signal quality due to crosstalk. The mechanism can be referred to the analysis of common mode and differential mode crosstalk. Here are some suggestions for Layout engineers when dealing with serpentine lines:

1) Try to increase the distance (S) between parallel line segments, at least greater than 3H. H refers to the distance from the signal trace to the reference plane. In layman's terms, it means routing the wire around a big bend. As long as S is large enough, mutual coupling effects can be almost completely avoided.

2) Reduce the coupling length Lp. When twice the Lp delay approaches or exceeds the signal rise time, the crosstalk generated will reach saturation.

3) The signal transmission delay caused by the serpentine line of Strip-Line or Embedded Micro-strip is smaller than that of Micro-strip. Theoretically, stripline will not affect the transmission rate due to differential mode crosstalk.

4) For high-speed signal lines and those with strict timing requirements, try not to use serpentine lines, especially in small areas.

5) Snake-shaped wiring at any angle can often be used, which can effectively reduce mutual coupling.

6) In high-speed PCB design, serpentine lines have no so-called filtering or anti-interference capabilities and can only reduce signal quality, so they are only used for timing matching and have no other purpose.

7) Sometimes spiral wiring can be considered for winding. Simulation shows that its effect is better than normal serpentine wiring.

Now that we have finished talking about PCB wiring, is it done after laying out the wiring? Obviously, no! Inspection after PCB wiring is also necessary, so how to check the wiring in PCB design and pave the way for subsequent design? Please see below!

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.

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