Electronic Design - 105 golden rules of PCB design must-see

In the design of electronic products, PCB layout and routing is the most important step. The quality of PCB layout and routing will directly affect the performance of the circuit. Now, although there are many softwares that can realize PCB automatic layout and routing, as the signal frequency continues to increase, many times, engineers need to understand the most basic principles and techniques of PCB layout and routing, so that they can make their designs perfect.PCB (Printed Circuit Board) Layout 100 Questions" covers the relevant basic principles and design skills of PCB layout and routing, and answers difficult questions about PCB layout in the form of question and answer. It is a very rare practical reading for PCB designers., Welcome everyone to add content and improve on this basis.

105 golden rules for PCB design!

1[Ask] What problems should be paid attention to when high-frequency signal wiring?

Answer 1. The impedance matching of the signal line;

2. Space isolation from other signal lines;

3. 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. Multi-layer boards can be considered if there are more lines;

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

Answer The decoupling capacitor needs to add the appropriate value at the appropriate location. 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[Q] What is the standard for a good board?

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

5[Q] How much influence does the through hole and blind hole have on the signal difference? What are the principles applied?

Answer The use of blind holes or buried holes 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 is divided, and the ground plane should be copper-clad, and some people suggest that the electrical ground layer is divided, and different grounds are connected at the power source terminal, but this will return the signal The path is far, how to choose the appropriate 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 needs to punch enough vias to the ground. The purpose of this is:

1. For analog signals, this provides a complete transmission medium and impedance matching;

2. The ground plane isolates analog signals from other digital signals;

3. 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 leftmost edge of the PCB, and the MCU is on the right side. Then the stabilized power supply chip is placed close to the plug-in when the layout is made (the power supply IC outputs 5V after a relatively long path). To reach the MCU), or place the power supply IC 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 points to consider (1) First of all, whether your regulated power supply chip is a relatively clean power supply with a small ripple. For the power supply of the analog part, the requirements for the power supply are relatively high. (2) Whether the analog part and your MCU are the same power supply, in the design of high-precision circuits, it is recommended to separate the analog part and the digital part of the power supply. (3) The power supply for the digital part needs to be considered to minimize the impact on the analog circuit part.

8 [Question] In the application of high-speed signal chain, there are analog ground and digital ground for multiple ASICs. Is the ground divided or not divided? What are the existing guidelines? Which one 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[Q] When should the equal length of the line be considered? If you want to consider the use of equal-length cables, what is the maximum difference between the lengths of the two signal lines? How to calculate?

Answer the calculation idea of the differential line: 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. Therefore, the length difference at this time is the maximum value. By analogy, the signal line difference must be less than this value.

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

Answer The snake-shaped wiring has different functions because of different applications:

(1) If the serpentine trace appears in the computer board, it mainly acts as a 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.

(2) 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.

(3) 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 ( When 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, which use a frequency of 233MHz. They must be strictly equal in length to eliminate hidden dangers caused by 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., The signal quality has decreased. Therefore, the clock IC pins are generally connected; "terminated, but the serpentine trace does not play the role of inductance. On the contrary, the inductance will cause the phase shift of the higher harmonics in the rising edge of the signal, causing the signal quality to deteriorate, so The serpentine line spacing is required to be at least twice the line width. The smaller the signal rise time, the more susceptible to distributed capacitance and distributed inductance.

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

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

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 position 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 its 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. Finally, make an appropriate choice PCB and chassis ground.

12 [Ask] 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?

There are many factors to consider when answering this question. For example, the various parameters of PCB materials, the transmission line model finally established according to these parameters, the parameters of the device, and so on. Impedance matching is generally designed according to the information provided by the manufacturer

13 [Question] When analog circuits and digital circuits coexist, for example, half of them are FPGA or microcontroller digital circuits, and the other half are analog circuits of 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[Ask] 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 [Ask] What is the most important issue to pay attention to when designing high-speed multi-layer PCBs? Can you make a detailed solution to the problem?

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

17 [Ask] Excuse me, 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 as the standard?

Answer The use of a multilayer board can first provide a complete ground plane, and in addition, it can provide more signal layers 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 should be kept the same length.

18 [Ask] 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 by op amp devices.

Answer This is difficult to distinguish. The only way to minimize the extra noise introduced by the wiring can be through the PCB 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 300MHz signal must do impedance simulation 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. Generally, the "line" is not used in the mixed-signal PCB, but the entire plane is used. In order to ensure that the loop resistance is minimal, and there is a complete plane under the signal line

20 [Ask] What kind of layout can achieve the best heat dissipation effect?

Answer There are three main sources of heat in PCB: (1) the heat of electronic components; (2) the heat of PC B itself; (3) the heat transferred from other parts. Among the three heat sources, the components generate the largest amount of heat and are 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. Then 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.

21 [Question] Can you explain the relationship between the line width and the size of the matching via?

Answering this question is very good, it is difficult to say that there is a simple proportional relationship, because the simulations of the two are different. One is surface transmission and the other is ring transmission. You can find a via's impedance calculation software on the Internet, and then keep the via's impedance the same as the impedance of the transmission line.

22 [Question] In an ordinary PCB circuit board controlled by an MCU, but there is no high-current high-speed signal and other requirements are not very high, then is there a layer of ground wire on the outermost edge of the PCB to wrap the entire circuit board? Would be better?

Answer Generally speaking, just lay a complete ground.

23 [Question] 1. I know that the analog ground and digital ground should be connected at a single point under the AD conversion chip, but what should I do if there are multiple AD conversion chips on the board? 2. In the multi-layer circuit board, when the multiplexer switches the analog quantity sampling, is it necessary to separate the analog part and the digital part like the AD conversion chip?

Answer 1. Put several ADCs together as much as possible, and connect the analog and digital grounds at a single point under the ADC; 2. Depending on the switching speed of MUX and ADC, the speed of ADC is generally higher than that of MUX, so it is recommended to place it under ADC. Of course, to be on the safe side, a magnetic bead package can also be placed under the MUX, and the single-point connection can be selected according to the specific situation during debugging.

24 [Question] In the conventional network circuit design, some uses connect several grounds together. Is it such a usage? Why? Thanks!

The answer is not very clear about your question. There are definitely several types of grounds for hybrid systems, and they will eventually be connected together at one point. The purpose of this is to equipotential. Everyone needs a common ground level as a reference.

25 [Ask] How to deal with the analog part and digital part, analog ground and digital ground effectively in PCB, thank you!

Answer The analog circuit and the digital circuit should be placed in separate areas, so that the return flow of the analog circuit is in the analog circuit area, and the digital circuit is in the digital area, so that the digital will not affect the analog. The starting point of analog ground and digital ground processing is similar, and the backflow of digital signals cannot be allowed to flow to the analog ground.

26 [Ask] What are the differences in the design of the ground wire between the analog circuit and the digital circuit in the PCB board design? What issues should be paid attention to?

Answer The main requirements of analog circuits to ground are completeness, small loops, and impedance matching. If the digital signal has no special requirements for low frequency; if the speed is high, impedance matching and ground integrity also need to be considered.

27 [Ask] There are generally two decoupling capacitors, 0.1 and 10. If the area is relatively tight, how to place the two capacitors, which one is better on the back?

The answer should be designed according to the specific application and for what chip

28 [Ask] I would like to ask the teacher, there are often two IQ signals in RF circuits. Do the lengths of the two wires need to be the same?

Answer Try to use the same in the RF circuit

29 [Question] Is there any difference between the design of high frequency signal circuit and ordinary circuit design? Can you briefly explain it by taking the wiring design as an example?

Answer The design of high-frequency circuits should consider the influence of many parameters. Under high-frequency signals, many common circuits can be ignored.

The omitted parameters cannot be ignored, so transmission line effects may have to be considered.

30 [Ask] High-speed PCB, how to deal with the avoidance of vias in the wiring process, any good suggestions?

Answer For high-speed PCBs, it is best to punch fewer vias and increase the signal layer to solve the need to increase vias.

31 [Ask] How to choose the thickness of the power trace in PCB board design? Are there any rules?

Answer can refer to: 0.15 * line width (mm) = A, also need to consider the copper thickness

32 [Question] When the digital circuit and the analog circuit are on the same multilayer board, should the analog ground and digital ground be arranged on different layers?

Answer It is not necessary to do this, but the analog circuit and the digital circuit should be placed separately.

33 [Question] How many vias are more suitable for general digital signal transmission? (Signals below 120Mhz)

Answer It is best not to exceed two vias.

34 [Question] In a circuit with both analog and digital circuits, how to avoid mutual interference when designing the PCB board?

Answer If the analog circuit is matched with reasonable radiation, it is generally interfered. The sources of interference come from devices, power supplies, space, and PCB; digital circuits must be sources of interference because of their many frequency components. The solution is generally, reasonable device layout, power supply decoupling, PCB layering, if the interference characteristics are large or the analog part is very sensitive, you can consider using a shielding cover.

35 [Question] For high-speed circuit boards, there may be parasitic parameters everywhere. In the face of these parasitic parameters, do we correct various parameters and then eliminate them, or use empirical methods to solve them? How should this issue of efficiency and performance be balanced?

Answer Generally speaking, the influence of parasitic parameters on circuit performance should be analyzed. If the impact cannot be ignored, it must be resolved and eliminated.

36 [Ask] What matters should be paid attention to when laying out multi-layer boards?

Answer When the multilayer board is laid out, because the power supply and the ground layer are in the inner layer, be careful not to have a floating ground plane or power plane. In addition, make sure that the vias that hit the ground are actually connected to the ground plane. Finally, it is necessary for some important Add some test points to the signal to facilitate the measurement when debugging.

37 [Q] How to avoid crosstalk of high-speed signals?

Answer: You can keep the signal lines farther away, avoid parallel lines, shield them by laying ground or adding protection, and so on.

38 [Ask] May I ask the power plane often used in the multi-layer board design, but do I need to design the power plane in the double-layer board?

The answer is difficult, because your various signal lines are almost in the double-layer layout

39 [Question] Does the thickness of the PCB have any effect on the circuit? How is it generally selected?

Answer Thickness is more important for impedance matching. The PCB manufacturer will ask what the thickness of the board is when the impedance matching is calculated, and the PCB manufacturer will make it according to your requirements.

40 [Question] The ground plane can minimize the signal loop, but it will also generate parasitic capacitance with the signal line. How should I choose this?

The answer depends on whether the parasitic capacitance has a non-negligible effect on the signal. If it can’t be ignored, then reconsider

41 [Question] Is the LDO output used as a digital power supply or an analog power supply?

Answer If you want to use an LDO to provide power for digital and analog, it is recommended to connect the analog power first. After the analog power is filtered by LC, it becomes the digital power.

42 [Question] Should I use magnetic beads between analog Vcc and digital Vcc, or should I use magnetic beads between analog ground and digital ground?

Answer The analog VCC is filtered by LC to obtain the digital VCC, and a magnetic bead is used between the analog ground and the digital ground.

43 [Ask] How to wire differential signal lines such as LVDS?

Answer Generally need to pay attention: all wiring including the surrounding devices and the ground plane need to be symmetrical.

44 [Ask] A good PCB design needs to be able to emit as little electromagnetic radiation as possible, and also to prevent external electromagnetic radiation from interfering with itself. What measures should the circuit take to prevent external electromagnetic interference?

Answer The best way is to shield to prevent external interference from entering. On the circuit, for example, when there is an INA, an RFI filter needs to be added before the INA to filter out RF interference.

45[Question] How to solve the problem of transmission line effect in PCB board design with fast integrated circuit chip circuit with high clock frequency?

Answer What kind of chip is this fast integrated circuit chip? If it is a digital chip, it is generally not considered. If it is an analog chip, it depends on whether the transmission line effect is large enough to affect the performance of the chip.

46 [Question] In a multi-layer PCB design, do you still need to pour copper? If it is copper-clad, which layer should it be connected to?

Answer If there is a complete ground plane and power plane inside, the top and bottom layers do not need to be coated with copper.

47 [Question] In the high-speed multi-layer PCB design, how to conduct impedance simulation and what software is used? Are there any issues that should be paid special attention to?

Answer You can use Multisim software to simulate the effect of resistance and capacitance.

48 [Question] Some devices have thinner pins but thicker traces on the PCB. Will it cause impedance mismatch after connection? If so, how to solve it?

The answer depends on what device it is. Moreover, the impedance of the device is generally given in the data sheet, and is generally not related to the thickness of the pin.

49 [Question] Differential lines generally need equal length. If it is difficult to achieve in LAYOUT, are there other remedies?

Answer The problem of equal length can be solved by taking the serpentine line. Now most PCB software can automatically take the equal length, which is very convenient.

50 [Question] When measuring the analog ground and digital ground interface of the chip with a multimeter, it is turned on. Isn't the analog ground digital ground connected to multiple points?

Answer The ground pins inside the chip are all connected together. But it still needs to be connected on the PCB board. The most ideal single-point grounding should be to understand the position of the connection point between the analog and digital parts of the chip, and then design the single-point connection position on the PCB board at the analog and digital boundary of the chip.