Now, although there is a lot of software that can realize PCB automatic placement and routing. However, as the signal frequency continues to increase, in many cases, engineers need to understand the basic principles and techniques of PCB layout and routing in order to make their designs perfect.

PCB layout

The following covers the relevant basic principles and design skills of PCB layout and answers difficult questions about PCB layout in the form of Q&A.

Related basic principles and design skills of PCB layout

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

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. In the layout of the board, if the wires are dense, which will affect the electrical performance of the board. How can I improve the electrical performance of the board?

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

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

The decoupling capacitor needs to be added with 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. What are the criteria for a good board?

Reasonable layout, sufficient power line power redundancy, high-frequency impedance, and simple low-frequency wiring.

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

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 the 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. 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 layer should be divided, and different grounds should be connected at the end of the power source. How to choose a suitable method for specific applications?

If you have a high frequency> 20MHz signal line, 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:

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

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. In the circuit board, the signal input plug-in is on the left edge of the PCB, and the MCU is on the right side, so in the layout, the stabilized power supply chip is placed close to the plug-in (the power IC output 5V goes through a relatively long path to reach the MCU), Or place the power supply IC to the right of the center (the output 5V 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?

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:

Whether your regulated power supply chip is a relatively clean power supply with 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. In the application of high-speed signal chains, there are analog and digital grounds for multiple ASICs. Should the ground be divided or not? What are the existing guidelines? Which effect is better?

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. When should the equal length of the line be considered? If you want to consider the use of equal length cables, how much can the difference between the lengths of the two signal lines not exceed? How to calculate?

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. What 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?

Serpentine routing has different functions due to different applications:

If the serpentine trace appears in 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 INTELHUB 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.

Therefore, the clock IC pins are generally connected, 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 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. When designing PCB, how to consider electromagnetic compatibility EMC/EMI, and what specific aspects need to be considered? What measures are taken?

A good EMI/EMC design must take into account the position 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 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 to reduce radiation. You can also divide the ground layer to control the range of high-frequency noise, and choose the PCB and Ground point of the enclosure.

12. What should I pay attention to in the design of the transmission line of the RF broadband circuit PCB? How to set 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 in this issue. 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. When analog circuits and digital circuits coexist, for example, one half is the FPGA or single-chip digital circuit part, 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?

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

14. 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?

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

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

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

It should be noted that the design of your layers 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 analog signal ground must be at least a separate layer. It is also recommended to use a separate layer for the power supply.

17. When to use 2-layer board, 4-layer board, and 6-layer board, 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?

The use of multi-layer boards can first provide a complete ground plane, and in addition, 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 must be kept the same length.

18. 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?

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

19. 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?

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 in a mixed-signal PCB, the "line" is generally not used, but the entire plane is used. In order to ensure the loop resistance, and there is a complete plane under the signal line.

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

There are three main aspects of heat in PCB:

• Heating of electronic components;

• 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.

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. Can you explain the relationship between the line width and the size ratio of the matching vias?

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-hole impedance calculation software on the Internet, and then keep the impedance of the via hole consistent with the impedance of the transmission line.

22. 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 it will be better if a layer of the ground wire is laid on the edge of the PCB to wrap the entire circuit board?

Generally speaking, just lay a complete ground.

23.1) I know that there is a single-point connection between the analog ground and the digital ground 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, does the analog part and the digital part need to be separated like the AD conversion chip?

Put several ADCs together as much as possible, and connect the analog and digital grounds at a single point under the ADC;

It depends on the switching speed of MUX and ADC. Generally, the speed of ADC will be higher than that of MUX, so it is recommended to place it below 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. In the conventional network circuit design, some connect several grounds together. Is there such a usage? Why?

There are definitely several types of grounds for hybrid systems, and they will always be connected at one point. The purpose of this is to equipotential. Everyone needs a common ground level as a reference.

25. How to deal with the analog part and digital part, analog ground and digital ground effectively in PCB?

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. 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?

The main requirements of analog circuits to the 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. There are generally two decoupling capacitors, 0.1 and 10. If the area is tight, how to place the two capacitors and which one is better on the back?

It should be designed according to the specific application and for what chip.

28. 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?

Try to use the same in the RF circuit.

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

The design of high-frequency circuits needs to consider the influence of many parameters. Under high-frequency signals, many parameters that can be ignored in ordinary circuits cannot be ignored, so transmission line effects may be considered.

30. For high-speed PCBs, how to deal with the avoidance of vias in the wiring process? What are some good suggestions?

High-speed PCB, less via holes, by increasing the signal layer to solve the need to increase the need for via holes.

31. How to choose the thickness of power traces in PCB board design? Are there any rules?

You can refer to: 0.15×line width (mm)=A, and copper thickness also needs to be considered.

32. 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?

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

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

Do not exceed two vias.

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

If the analog circuit is reasonably matched, the radiation is very small, and it generally interferes. Interference sources come from devices, power supplies, space, and PCB; digital circuits must be interference sources due to 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. For high-speed circuit boards, there may be parasitic parameters everywhere. In the face of these parasitic parameters, do we eliminate various parameters and then use empirical methods to solve them? How should this issue of efficiency and performance be balanced?

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

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

In the multi-layer board layout, 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. This is to add some important signals. Some test points are convenient for measuring when debugging.

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

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

38. May I often use the power plane in the multi-layer board design, but do I need to design the power plane in the double-layer board?

 It is difficult because your various signal lines are almost the same in the double-layer layout.

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

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

It depends on whether the parasitic capacitance has a non-negligible effect on the signal. If it is not negligible, then it must be reconsidered.

41. Is the LDO output used as a digital power supply or an analog power supply?

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. Should I use magnetic beads between analog Vcc and digital Vcc, or should I use magnetic beads between analog ground and digital ground?

After the analog VCC is filtered by LC, the digital VCC is obtained, and a magnetic bead is used between the analog ground and the digital ground.

43. How to route differential signal lines such as LVDS?

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

44. A good PCB design requires as little as possible its own emission of electromagnetic radiation, and also to prevent external electromagnetic radiation from interfering with itself. What measures should the circuit take to prevent external electromagnetic interference?

The method 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. How to solve the problem of transmission line effect in PCB board design using fast integrated circuit chip circuits with high clock frequency?

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. 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?

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

47. When designing high-speed multi-layer PCBs, how is impedance simulation generally performed and what software is used? Are there any issues that should be paid special attention to?

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

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

It depends on the device. Moreover, the impedance of the device is generally given in the datasheet and is generally not related to the thickness of the pin.

49. Differential lines generally need equal length. If it is difficult to achieve in LAYOUT, are there other remedial measures?

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

50. 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?

The ground pins inside the chip are all connected together. But it still needs to be connected to the PCB board.

The 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 at the analog and digital boundary of the chip.