Routing is one of the most important aspects of PCB design. Next, let’s take a look at the problems engineers often encounter when routing PCBs.
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.