50 Common Questions in PCB Layout Design

1. What issues should be considered when routing high-frequency signals?

Impedance matching of signal traces and spatial isolation from other signals are very important. For digital high-frequency signals, using differential pairs yields better results.

2. In board layout, if traces are too dense, it can affect the board's electrical performance. How can the electrical performance of a single board be improved?

For low-frequency signals, vias have little impact. However, for high-frequency signals, the number of vias should be minimized. If routing space is truly insufficient, consider increasing the number of layers (using a multilayer board).

3. Is it better to add more decoupling capacitors on the board?

It's not about quantity, but about placing decoupling capacitors with "appropriate capacitance values" in suitable locations. For example, place them near the power pins of analog components, and use capacitors with different values to filter noise at different frequencies.

4. How significant is the impact of through-holes versus blind vias on signals? What is the principle for their application?

Using blind or buried vias is an effective method to increase multilayer board density, reduce layer count and board size, and decrease the number of plated through-holes. However, comparatively, through-holes are easier to manufacture and lower in cost, so they are still commonly used in general designs.

5. What are the standards for PCB Layout?

Reasonable component placement, sufficient margin in power trace widths, consideration of characteristic impedance for high-frequency signals, and clean and simple routing for low-frequency signals.

6. For mixed-signal systems (analog/digital), some suggest splitting the ground plane and copper pour. Others suggest splitting power and ground planes, connecting different grounds at the power end. How does one choose the appropriate method for a specific application?

If you have a high-frequency (>20MHz) signal line, with significant length and quantity, then this analog high-frequency signal requires at least two layers: one signal layer and one large-area ground layer, and the signal layer needs sufficient vias connected to the ground layer. The purpose is: This provides a complete transmission medium and impedance matching; the ground layer isolates the analog signal from other digital signals; because of the many vias and the large ground plane, the return loop can be made sufficiently small.

7. On a circuit board, the signal input connector is on the left edge, and the MCU is on the right. During layout, should the voltage regulator chip be placed close to the connector or on the right side near the center?

First, clarify if the signal input connector is for analog signals. If so, it's recommended that the power supply layout minimizes its impact on the signal integrity of the analog section. Consider the following points:

• Is the voltage regulator chip a clean, low-ripple power source?
• The analog section has higher requirements for power supply.
• Does the analog section share the same power supply as the MCU? In advanced circuit design, it is recommended to separate analog and digital power supplies.
• Consider the power supply to the digital section, minimizing its impact on the analog circuit section.

8. In high-speed signal chain applications with multiple ASICs having analog and digital grounds, should the grounds be split or not?

Generally, refer directly to the chip's datasheet. Manuals for mixed-signal chips from companies like ADI usually recommend grounding schemes. Some suggest common grounding, others suggest isolation, depending on the chip's internal design.

9. When should trace length matching be considered? If length matching is required, what is the maximum allowable length difference between two signal lines? How is it calculated?

The calculation approach for differential pairs: If transmitting a sinusoidal signal, a length difference equal to half its transmission wavelength results in a 180-degree phase shift, causing the two signals to cancel completely. This length difference is the limit; the actual length difference must be strictly less than this value.

10. In what situations is serpentine routing suitable? Are there any drawbacks?

Serpentine routing serves different functions depending on the application:

• Computer Motherboards: Primarily acts as a filter inductor and for impedance matching, improving anti-interference capability (e.g., for clock signals like PCI-Clk).
• General PCBs: Can serve as an inductive coil for RF antennas.
• High-speed Digital PCBs: Used to keep the delay difference between various signals within a specific range, ensuring valid data capture within the same cycle (e.g., strict length matching required in INTEL HUB architectures).
• Drawbacks: Excessively long traces increase distributed capacitance and inductance. Inductance can cause phase shift in higher-order harmonics of the signal rise edge, degrading signal quality. Therefore, the spacing between serpentine segments must be greater than twice the trace width. Smaller signal rise times are more susceptible to these effects.

11. How should EMC/EMI be considered during PCB design? What measures can be taken?

Good EMI/EMC design must consider component placement, stack-up arrangement, critical routing, and component selection early in the layout phase. For example: Clock generators should not be placed near external connectors. High-speed signals should be routed on inner layers whenever possible. Pay attention to characteristic impedance matching and the continuity of the reference plane to minimize reflections. Consider the frequency response when selecting decoupling capacitors. Pay attention to the return path of high-frequency signals, keeping the loop area as small as possible to reduce radiation. Splitting the ground plane can also help control the range of high-frequency noise.

12. What should be considered when designing PCB transmission lines for RF broadband circuits?

Impedance matching is generally designed based on information provided by the manufacturer. This requires considering many factors, such as various parameters of the PCB material, transmission line models built from these parameters, component specifications, etc.

13. When digital and analog circuits share a power supply with the same voltage, what are the techniques for routing and ferrite bead placement?

Directly sharing is generally not recommended. Debugging such a system can be complex and difficult.

14. In high-speed multilayer PCB design, what is the main basis for selecting resistor and capacitor packages?

0402 is common in mobile phones; 0603 is common in general high-speed signal modules. The principle is: smaller packages have smaller parasitic parameters. Of course, the high-frequency performance of the same package can vary greatly between manufacturers; it is recommended to use high-frequency specific components in critical locations.

15. In general double-layer board design, should signal traces or ground traces be routed first?

This should be considered comprehensively. Typically, routing is done after ensuring the component placement is reasonable.

16. What should be noted in high-speed multilayer PCB design?

The focus is on "stack-up planning"—how to assign signal, power, and ground layers. The general principle is: Analog signals and analog ground should at least have a dedicated layer, and a dedicated power layer is also recommended.

17. When should 2-layer, 4-layer, or 6-layer boards be used? Is CPU frequency the deciding factor?

Using multilayer boards first provides a "solid ground plane," and also increases signal layers for easier routing. For applications where the CPU controls external memory, the interaction frequency should be considered. If the frequency is high, a solid ground plane must be ensured, and signal traces must maintain equal length.

18. How can one distinguish whether noise introduced during signal transmission is caused by "routing" or by the "operational amplifier"?

This is difficult to distinguish in practice. We can only start from the PCB routing side, minimizing additional noise introduced by the routing as much as possible.

19. For a high-speed multilayer PCB, what trace width is appropriate? For example, for a 300MHz operating frequency?

For a 300MHz signal, impedance simulation software must be used to calculate the trace width and the distance from the trace to the ground layer. The width of power traces is determined by the current magnitude. In mixed-signal PCBs, the ground is typically not "traced," but uses an entire solid plane (copper pour).

20. What kind of layout achieves good heat dissipation?

Heat dissipation is primarily achieved through "reducing heat generation" and "accelerating heat dissipation." There are three main heat sources in a PCB: heat from electronic components, heat from the PCB itself, and externally transferred heat. Component heating is the primary cause.

21. Explain the relationship between trace width and via size ratio?

It's difficult to state a simple proportional relationship because their transmission models differ (one is surface transmission, one is vertical transmission). It is recommended to use impedance calculation software to make the via impedance as consistent as possible with the transmission line impedance.

22. In a general MCU control board, is it better to route a ring of ground traces around the entire board edge?

Generally, a properly implemented internal ground plane is sufficient.

23. If a board has multiple ADCs and multiplexers, how should analog and digital ground be handled?

Try to place multiple ADCs together and connect analog and digital ground at a "single point" under the ADCs. Regarding the multiplexers (MUX), it depends on the switching speed of the MUX and ADC. Typically, ADCs are faster, so the connection point is recommended to be under the ADC. For safety, a ferrite bead footprint can be预留, allowing for flexible selection during debugging.

24. In traditional network circuit design, multiple grounds are sometimes connected together. Why?

Mixed-signal systems certainly have multiple types of grounds, and ultimately they are usually connected at a single point. The purpose is to achieve "equipotentiality," providing a common ground reference level for all signals.

25. How to effectively handle the analog and digital sections, and analog and digital ground, within a PCB?

Analog and digital circuits should be placed in different areas, ensuring that the return current for analog circuits stays in the analog area, and digital return current stays in the digital area. The core principle is: Prevent digital signal return current from flowing through the analog ground, avoiding mutual interference.

26. What are the differences in ground trace design between analog and digital circuits?

The main requirements for the ground in analog circuits are: integrity, small loop area, and impedance matching. For digital signals, if the speed is not high, the requirements are lower. However, for high-speed digital signals, impedance matching and ground integrity must also be considered.

27. Decoupling capacitors often come in values like 0.1uF and 10uF. If space is tight, how should they be placed? Is the backside acceptable?

This must be designed based on the specific application and the chip's datasheet recommendations. Typically, small-value (0.1uF) high-frequency decoupling capacitors need to be placed closest to the chip's power pins.

28. In RF circuits, there are often two IQ signals. Do the lengths of these two traces need to be equal?

Yes, they need to be equal in length, and the same approach is used in RF circuits.

29. What is the difference between designing high-frequency signal circuits and ordinary circuit design?

High-frequency circuit design needs to consider the impact of many parasitic parameters. At high frequencies, many parameters that can be ignored in ordinary circuits can no longer be ignored, and "transmission line effects" must be considered.

30. For high-speed PCBs, how can one avoid using too many vias during routing?

The most fundamental solution is to increase the number of signal layers in the PCB, providing more routing channels and thus reducing the need for layer changes via vias.

31. How to choose the trace width for power lines in PCB design?

A rule of thumb: 0.15 × trace width (mm) ≈ allowable current (A). The board manufacturer's copper weight specifications must also be considered.

32. When digital and analog circuits are on the same multilayer board, should analog and digital ground be placed on different layers?

This is not necessary, but analog and digital circuits should be placed in "separate zones."

33. How many vias are generally acceptable for digital signal transmission? (For signals below 120MHz)

As a rule of thumb: On the same signal line, try not to exceed "two" vias.

34. In circuits containing both analog and digital parts, how can mutual interference be avoided?

Digital circuits, due to their complex frequency components, are definitely interference sources. Common solutions include: reasonable component placement/zoning, thorough power decoupling, and independent PCB layers. If interference is significant or the analog part is extremely sensitive, adding a metal shielding cover is recommended.

35. For high-speed circuit boards, parasitic parameters may exist everywhere. How should performance be balanced?

Analyze the impact of parasitic parameters on circuit performance. If the impact is significant enough to be ignored, it must be addressed and eliminated.

36. What precautions should be taken during multilayer board layout?

Since power and ground layers are internal, ensure there are no "floating" ground or power planes. Verify that ground vias are properly connected to the ground layer. Additionally, important signals should have预留 test points for convenient measurement during debugging.

37. How to avoid crosstalk in high-speed signals?

Maintain sufficient spacing between signal traces, avoid parallel routing, or isolate them through grounding, adding shielding, etc.

38. Is it necessary to design a power layer in a double-layer board?

It is difficult to achieve because in double-layer board layouts, various signal traces almost occupy all routing space.

39. Does the PCB thickness affect the circuit?

Board thickness is very important for "impedance matching." When a PCB manufacturer calculates impedance for you, they need the specified board thickness and stack-up information, and then fabricate according to your requirements.

40. A ground plane can reduce the signal loop area, but it also creates parasitic capacitance with signal traces. How to decide?

This depends on whether the parasitic capacitance has a non-negligible impact on the signal. If it cannot be ignored, the layout must be re-evaluated.

41. Should an LDO output be used for digital power or analog power?

If a single LDO must supply both digital and analog circuits, it is recommended to connect it to the "analog power" first. Then, after LC filtering the analog power, supply it to the "digital power."

42. Should a ferrite bead be used between analog Vcc and digital Vcc, or between analog ground and digital ground?

Analog Vcc is filtered (e.g., via LC) to obtain digital Vcc. A ferrite bead is typically used between analog ground and digital ground.

43. How should differential signal lines like LVDS be routed?

The most core principle is: All routing (including surrounding components) and the underlying ground plane must maintain "symmetry."

44. What measures should be taken in a circuit to prevent external electromagnetic interference (EMI)?

The best physical method is adding a shielding cover. In circuit design, for example, adding an RFI filter at the front end of an instrumentation amplifier (INA) can effectively filter out radio frequency interference.

45. How to address transmission line effects when using high-clock-frequency fast chips?

For digital chips, it generally depends on the situation. For analog chips, it depends on whether the transmission line effect is significant enough to affect the chip's performance.

46. In multilayer board design, if inner layers have solid planes, is copper pour still needed on the top and bottom layers?

If there are already solid ground and power planes internally, the top and bottom layers typically do not need large-area copper pours, as this could cause antenna effects.

47. When designing high-speed multilayer PCBs, how is impedance simulation generally performed, and what software is used?

There are many professional simulation software packages available. Software like Multisim can also be used to辅助 simulate the effects of resistors and capacitors.

48. Some component pins are thin, but the PCB traces are wider. Could this cause impedance mismatch after connection?

This depends on the component specifications. Usually, the impedance of component pins is provided in the datasheet and generally has little relation to the width of the external traces.

49. Differential pairs generally need length matching. If this is difficult to achieve in Layout, are there any other remedies?

Length matching issues usually must be solved by using serpentine traces. Most modern PCB Layout software has automatic length matching functions, making the operation very convenient.

50. When measuring the interface between a chip's analog and digital ground with a multimeter, aren't they connected at multiple points internally?

The ground pins inside the chip are usually all connected together. However, the ideal single-point grounding strategy is: understand the location of the boundary between the chip's analog and digital sections, and on the corresponding boundary on the PCB, design a dedicated "single-point connection" location.

Conclusion

The above is the "50 Common Questions in PCB Layout" recompiled by the eCloud team. Layout and routing are an art combining circuit theory and practical experience. We hope this list can assist engineers during actual design and prototyping.

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