PCB Layout FAQs

1. [Question] What problems should be paid attention to when wiring high-frequency signals?

[Answer] The impedance matching of the signal line;

Space isolation from other signal lines;

For digital high-frequency signals, the effect of differential lines will be better.

2. [Question] In the layout of the board, if the wires are dense, there may be more vias, which of course will affect the electrical performance of the board. How can I improve the electrical performance of the board?

 [Answer] For low-frequency signals, vias do not matter. For high-frequency signals, minimize vias. If there are many lines, consider multi-layer boards.

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

[Answer] The decoupling capacitor needs to be added with a suitable value at a suitable position. For example, add it to the power supply port of your analog device, and you need to use different capacitance values to filter out spurious signals of different frequencies.

4. [Question] What are the criteria for a good board?

[Answer] The layout is reasonable, the power redundancy of the power line is sufficient, and the high-frequency impedance and the low-frequency wiring are simple.

5. [Question] How much influence do the through-hole and blind hole have on the signal difference? What are the principles applied?

[Answer] The use of blind vias or buried vias is an effective method to increase the density of multilayer boards, reduce the number of layers and board size, and greatly reduce the number of plated through holes. However, in comparison, through-holes are easy to implement in-process and low cost, so through-holes are generally used in the design.

6. [Question] When it comes to analog-digital hybrid systems, some people suggest that the electrical layer should be divided, and the ground plane should be copper-clad. Others suggest that the electrical ground should be divided, and different grounds should be connected to the power source terminal. The return path is far away. How do choose a suitable method for specific applications?

[Answer] If you have a high-frequency signal line> 20MHz, and the length and quantity are relatively large, then you need at least two layers for this analog high-frequency signal. A layer of signal lines, a layer of large area ground, and the signal line layer need to punch enough vias to the ground.

The purpose of this is to provide a complete transmission medium and impedance matching for analog signals;

The ground plane isolates analog signals from other digital signals;

The ground loop is small enough, because you have made a lot of vias, and the ground is a large plane.

7. [Question] In the circuit board, the signal input plug-in is on the left edge of the PCB, and the MCU is on the right. Then the stabilized power supply chip is placed close to the plug-in during the layout (the power supply IC outputs 5V after a relatively long path To reach the MCU), or place the power supply IC is to the right of the center (the 5V output line of the power supply IC is relatively short to reach the MCU, but the input power line passes through a relatively long PCB board)? Or is there a better layout?

[Answer] First of all, is your so-called signal input plug-in an analog device? If it is an analog device, it is recommended that your power supply layout should not affect the signal integrity of the analog part as much as possible.

Therefore, there are several considerations:

First of all, whether your regulated power supply chip is a relatively clean power supply with a low ripple. For the power supply of the analog part, the requirements for the power supply are relatively high;

Whether the analog part and your MCU are the same power supply, in the design of a high circuit, it is recommended to separate the power supply of the analog part and the digital part;

The power supply to the digital part needs to be considered to minimize the impact on the analog circuit part.

8. [Question] In the application of a high-speed signal chain, there are analog and digital grounds for multiple ASICs. Should the ground be divided or not? What are the existing guidelines? Which is better?

[Answer] So far, there is no conclusion. Under normal circumstances, you can refer to the manual of the chip. The manuals of all ADI hybrid chips recommend you a grounding scheme, some are recommended for common ground, and some are recommended for isolation. It depends on the chip design.

9. [Question] When should the equal length of the line be considered? If you want to consider using equal-length wires, how much can the difference between the lengths of the two signal wires not exceed? How to calculate?

[Answer] Differential line calculation idea: If you transmit a sinusoidal signal, your length difference is equal to half of its transmission wavelength, and the phase difference is 180 degrees. At this time, the two signals are completely canceled. So the difference in length at this time is the value. By analogy, the signal line difference must be less than this value.

10. [Question] Which kind of situation is suitable for serpentine routing at high speed? Are there any disadvantages, for example, for differential wiring, the two sets of signals are required to be orthogonal.

 [Answer] Serpentine routing has different functions due to different applications.

If the serpentine trace appears on the computer board, it mainly plays the role of filter inductance and impedance matching to improve the anti-interference ability of the circuit. The serpentine traces in the computer motherboard are mainly used in some clock signals, such as PCI-Clk, AGPCIK, IDE, DIMM, and other signal lines.

If it is used in a general PCB board, in addition to the filter inductance, it can also be used as the inductance coil of the radio antenna and so on. For example, it is used as an inductor in 2.4G walkie-talkies.

The wiring length requirements for some signals must be strictly equal. The equal line length of high-speed digital PCB boards is to keep the delay difference of each signal within a range to ensure the validity of the data read by the system in the same cycle (the delay difference exceeds In one clock cycle, the data of the next cycle will be read incorrectly).

For example, there are 13 HUBLinks in the INTEL HUB architecture, which use a frequency of 233MHz. They must be strictly equal in length to eliminate hidden dangers caused by the time lag. Winding is the only solution.

Generally, it is required that the delay difference does not exceed 1/4 clock cycle, and the line delay difference per unit length is also fixed. The delay is related to the line width, line length, copper thickness, and layer structure, but excessively long lines will increase distributed capacitance and distributed inductance so that the signal quality has decreased.

Therefore, the clock IC pins are generally terminated, but the serpentine trace does not act as an inductance.

On the contrary, the inductance will shift the phase shift of the higher harmonics in the rising edge of the signal, causing deterioration of the signal quality, so the serpentine line spacing is required to be less than twice the line width. The smaller the rise time of the signal, the more susceptible to the influence of distributed capacitance and distributed inductance.

The serpentine trace acts as a distributed parameter LC filter in some special circuits.

11. [Question] When designing PCB, how to consider electromagnetic compatibility EMC/EMI, and what aspects need to be considered? What measures are taken?

[Answer] A good EMI/EMC design must take into account the location of the device, the arrangement of the PCB stack, the routing of important connections, and the selection of the device at the beginning of the layout.

For example, the location of the clock generator should not be as close to the external connector as possible. High-speed signals should go to the inner layer as much as possible. Pay attention to the characteristic impedance matching and the continuity of the reference layer to reduce reflections. 

The slew rate of the signal pushed by the device should be as small as possible to reduce the height. Frequency components, when choosing decoupling/bypass capacitors, pay attention to whether their frequency response meets the requirements to reduce noise on the power plane.

In addition, pay attention to the return path of the high-frequency signal current to make the loop area as small as possible (that is, the loop impedance as small as possible) to reduce radiation. You can also divide the ground layer to control the range of high-frequency noise. Choose the PCB and Ground point of the enclosure.

12. [Question] What should I pay attention to in the design of the transmission line of the RF broadband circuit PCB? How to set up the ground hole of the transmission line is more appropriate, do you need to design the impedance matching yourself or cooperate with the PCB processing manufacturer?

[Answer] There are many factors to consider in this question. For example, various parameters of PCB materials, transmission line models established based on these parameters, device parameters, etc. Impedance matching is generally designed according to the information provided by the manufacturer.

13. [Question] When analog circuits and digital circuits coexist, for example, one half is the FPGA or the digital circuit part of the single-chip microcomputer, and the other half is the analog circuit part of the DAC and related amplifiers. There are many power supplies of various voltage values. When encountering power supplies of voltage values that are used in both digital and analog circuits, can a common power supply be used? What skills are there in wiring and magnetic bead layout?

[Answer] This is generally not recommended. Such use will be more complicated and difficult to debug.

14. [Question] Hello, what is the main basis for the selection of the packaging of resistors and capacitors when designing high-speed multilayer PCBs? Which packages are commonly used, can you give me some examples?

[Answer] 0402 is commonly used in mobile phones; 0603 is commonly used in general high-speed signal modules; the basis is that the smaller the package, the smaller the parasitic parameters. Of course, the same package from different manufacturers has great differences in high-frequency performance. It is recommended that you use high-frequency special components in key locations.

15. [Question] Generally, in the design of the double panel, should the signal line or the ground line be taken first?

[Answer] This should be considered comprehensively. In the case of first considering the layout, consider routing.

16. [Question] What should be paid attention to when designing high-speed multilayer PCB? Can you make a detailed solution to the problem?

[Answer] What you should pay attention to is the design of your layers, that is, how you divide the signal lines, power lines, ground, and control lines on each layer. The general principle is that the analog signal and analog signal ground must be at least a separate layer. It is also recommended to use a separate layer for the power supply.

17. [Question] When to use 2-layer, 4-layer, and 6-layer boards, are there any strict technical restrictions? (Excluding volume reasons) Is the frequency of the CPU or the frequency of data interaction with external devices the standard?

[Answer] The use of a multilayer board can provide a complete ground plane first, and more signal layers can be provided to facilitate wiring. For applications where the CPU needs to control external storage devices, the frequency of interaction should be considered. If the frequency is high, a complete ground plane must be guaranteed. In addition, the signal lines must be kept the same length.

18. [Question] How to analyze the influence of PCB wiring on analog signal transmission, and how to distinguish whether the noise introduced in the signal transmission process is caused by wiring or op-amp devices.

[Answer] This is difficult to distinguish, only PCB wiring can be used to minimize the additional noise introduced by wiring.

19. [Question] I recently studied PCB design. For high-speed multilayer PCBs, what are the appropriate line width settings for power lines, ground lines, and signal lines? What are the common settings? Can you give me an example? For example, how to set the working frequency at 300Mhz?

[Answer] For 300MHz signals, impedance simulation must be done to calculate the line width and the distance between the line and the ground; the power line needs to determine the line width according to the size of the current. When the ground is mixed-signal PCB, the line is generally not used, but the entire plane is used. In this way, the loop resistance can be guaranteed, and there is a complete plane under the signal line.

20. [Question] What kind of layout can achieve the heat dissipation effect?

[Answer] There are three main aspects of heat in PCB as follows.

Heating of electronic components;

Heat generation of PCB itself;

The heat from other parts.

Among the three heat sources, the heat generated by the components is the main heat source, followed by the heat generated by the PCB board. The heat transferred from the outside depends on the overall thermal design of the system and is not considered for the time being. 

The purpose of thermal design is to take appropriate measures and methods to reduce the temperature of the components and the temperature of the PCB board so that the system can work normally at a suitable temperature. It is mainly achieved by reducing heat generation and accelerating heat dissipation.