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PCB electronic circuit board layout and wiring 20 FQA

Mar 03 2020

In the design of electronic products, PCB ELECTRONIC CIRCUIT BOARD layout is the most important step. The quality of PCB ELECTRONIC CIRCUIT BOARD layout will directly affect the performance of the circuit.

Now, although there is a lot of software that can implement PCB ELECTRONIC CIRCUIT BOARD automatic layout. However, as the signal frequency continues to increase, many times, engineers need to understand the most basic principles and techniques of PCB ELECTRONIC CIRCUIT BOARD layout in order to make their design perfect.

The following covers the basic principles and design skills of PCB ELECTRONIC CIRCUIT BOARD layout, and answers questions about PCB ELECTRONIC CIRCUIT BOARD layout.

1. [Q] What should be paid attention to when wiring high-frequency signals?
[Answer] 1. Impedance matching of signal line;
2. Space isolation from other signal lines;
3. For digital high-frequency signals, the effect of differential lines will be better;

2. [Q] When wiring the board, if the wires are dense, there may be more vias, of course, it will affect the electrical performance of the board. How to improve the electrical performance of the board?
[Answer] For low-frequency signals, the vias don’t matter. The high-frequency signals should minimize vias. If there are more wires, consider multi-layer boards;

3. [Ask] Is it better to add more decoupling capacitors to the board?
[Answer] Decoupling capacitors need to be added with appropriate values ​​at the appropriate locations. For example, add it at the power supply port of your analog device, and need to use different capacitor values ​​to filter out stray signals of different frequencies;

4. [Ask] What is the standard of a good board?
[Answer] The layout is reasonable, the power line power redundancy is sufficient, the high-frequency impedance impedance, and the low-frequency wiring are simple.

5. [Q] How much does the difference between the via and the blind via affect the signal? What are the principles applied?
[Answer] Using blind 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 and cost-effective, so they are generally used in 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 a whole piece of copper. Some people also suggested that the electrical layer should be divided, and different grounds should be connected at the power source terminals. The return path is far away. How to choose the appropriate method for specific applications?
[Answer] If you have a high-frequency> 20MHz signal line, and both the length and number are large, then you need at least two layers for this analog high-frequency signal. A layer of signal line, a large area of ​​ground, and the signal line layer needs to punch enough vias to ground. The purpose is to:

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 make a lot of vias, and the ground is a large plane.

7. [Q] In the circuit board, the signal input plug is on the leftmost edge of the PCB ELECTRONIC CIRCUIT BOARD, and the MCU is on the right. Then, when the layout is made, the voltage stabilized power chip is placed near the connector (the power IC output is 5V after a comparison The long path to the MCU), or the power IC is placed to the right to the center (the output 5V line of the power IC arrives at the MCU is short, but the input power line goes through a longer PCB ELECTRONIC CIRCUIT BOARD board)? Or is there a better layout?
[Answer] 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. So there are several points to consider:

(1) First of all, whether your power supply chip is relatively clean and has low 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 a power supply. In the design of high-precision circuits, it is recommended to separate the power supply of the analog part and the digital part.
(3) The power to the digital part needs to be considered to minimize the impact on the analog circuit part.

8. [Q] 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 criteria? Which effect is better?
[Answer] So far, no conclusion. In general, you can consult the manual of the chip. In the manuals of all ADI hybrid chips, you are recommended to use a grounding scheme. Some recommend a common ground and some recommend an isolated ground. It depends on the chip design.

9. [Q] When should I consider the line length? If you want to consider the use of isometric lines, the maximum difference between the length of the two signal lines can not exceed how much? How to calculate?
[Answer] The calculation idea of ​​the differential line: If you pass 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 cancelled. So the length difference at this time is the maximum. By analogy, the signal line difference must be smaller than this value.

10. [Q] What kind of situation is the serpentine wiring at high speed suitable for? Are there any disadvantages, for example, for differential traces, and the two sets of signals are required to be orthogonal?
[Answer] Serpentine wiring has different functions because of different applications:

(1) If the serpentine trace appears in the computer board, it mainly plays a role of filtering inductance and impedance matching, improving the anti-interference ability of the circuit. 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 it is in the ordinary ordinary PCB ELECTRONIC CIRCUIT BOARD board, besides having the function of filtering inductance, it can also be used as the inductance coil of the radio antenna and so on. Such as 2.4G radios are used as inductors.

(3) The length requirements of some signal wiring must be strictly equal. The isoline length of the high-speed digital PCB ELECTRONIC CIRCUIT BOARD board is to keep the delay difference of each signal within a range and ensure the data read by the system in the same cycle. Validity (when the delay difference exceeds one clock cycle, the data in the next cycle will be misread).

For example, there are 13 HUBLinks in the INTELHUB architecture. A frequency of 233 MHz is required. The length must be strictly equal to eliminate the hidden danger caused by time delay. Winding is the only solution. Generally, the delay difference is not more than 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 layer structure. To reduce the signal quality. Therefore, the clock IC pins are generally connected; “terminated, but the serpentine trace does not function as an inductor. On the contrary, the inductance will cause a higher-order harmonic phase shift in the rising edge of the signal, causing signal quality to deteriorate, It is required that the distance between the serpentine lines is at least twice the line width. The smaller the rise time of the signal, the more susceptible it is to the influence of distributed capacitance and distributed inductance.

(4) The serpentine trace plays the role of an LC filter with distributed parameters in some special circuits.

11. [Q] How to consider electromagnetic compatibility EMC / EMI when designing PCB ELECTRONIC CIRCUIT BOARD, what aspects need to be considered specifically? What measures are taken?
[Answer] A good EMI / EMC design must consider the position of the device, the layout of the PCB ELECTRONIC CIRCUIT BOARD stack, the important online method, and the device selection.

For example, the position of the clock generator should not be as close to the external connector as possible. The high-speed signal should go as far as possible to the inner layer and pay attention to the continuous matching of the characteristic impedance and the reference layer to reduce the reflection. Frequency component. When choosing a decoupling / bypass capacitor, pay attention to whether its frequency response meets the requirements to reduce power plane noise. In addition, pay attention to the return path of the high-frequency signal current so that the loop area is as small as possible (that is, the loop impedance is as small as possible) to reduce radiation. You can also divide the stratum to control the range of high-frequency noise. Finally, an appropriate choice Ground point of PCB ELECTRONIC CIRCUIT BOARD and enclosure

12. [Q] Is there anything that needs attention in the design of the transmission line of the RF broadband circuit PCB ELECTRONIC CIRCUIT BOARD? How to set the ground hole of the transmission line is more appropriate. Does impedance matching need to be designed by yourself or in cooperation with PCB ELECTRONIC CIRCUIT BOARD processing manufacturers?
[Answer] There are many factors to consider in this question. For example, various parameters of PCB ELECTRONIC CIRCUIT BOARD material, the transmission line model and device parameters finally established according to these parameters. Impedance matching is generally designed based on the information provided by the manufacturer.

13. [Q] When analog circuits and digital circuits co-exist, half of them are part of the digital circuit of FPGA or single-chip microcomputer, and the other half are part of the analog circuit of DAC and related amplifier. There are many power supplies with various voltage values. Can I use a common power supply when encountering voltage supplies that are required by both digital and analog circuits? What are the techniques for wiring and bead arrangement?
[Answer] This is generally not recommended. This will be more complicated to use and difficult to debug.

14. [Q] Hello, what is the main basis for the selection of the packaging of resistors and capacitors when designing high-speed multilayer PCB ELECTRONIC CIRCUIT BOARD? Those commonly used packages, can you give a few examples.
[Answer] 0402 is commonly used in mobile phones; 0603 is commonly used in 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 dedicated components in critical locations.

15. [Q] Generally, in the design, do the double panels go through the signal line or the ground line first?
[Answer] This has to be considered comprehensively. With the layout first considered, consider routing.

16. [Q] What should you pay attention to when designing high-speed multilayer PCB ELECTRONIC CIRCUIT BOARD? Can you do a detailed explanation of the problem.
[Answer] The most important thing to pay attention to is the design of your layer, which is how you divide the signal line, power line, ground, and control line into each layer. The general principle is that at least a separate layer must be guaranteed for the analog signal and the analog signal ground. A separate layer is recommended for the power supply.

17. [Q] Is there any strict technical limitation on when to use 2-layer board, 4-layer board, and 6-layer board? (Except for volume reasons) based on the frequency of the CPU or the frequency of data interaction with external devices?
[Answer] Using a multilayer board can first provide a complete ground plane, and in addition, it can provide more signal layers to facilitate routing. For the application of the CPU to control external storage devices, the frequency of interaction should be considered. If the frequency is high, the complete ground plane must be guaranteed. In addition, the signal line should be kept the same length.

18. [Q] How to analyze the influence of PCB ELECTRONIC CIRCUIT BOARD wiring on the transmission of analog signals, and how to distinguish whether the noise introduced during signal transmission is caused by wiring or op amp devices.
[Answer] This is difficult to distinguish. You can only use PCB ELECTRONIC CIRCUIT BOARD wiring to minimize the introduction of extra noise.

19. [Q] I recently studied the design of PCB ELECTRONIC CIRCUIT BOARD. For high-speed multi-layer PCB ELECTRONIC CIRCUIT BOARD, how much is the line width of the power, ground, and signal lines? What are the common settings? Can you give an example? For example, how to set the working frequency at 300Mhz?
[Answer] The impedance of 300MHz signal must be calculated by the impedance 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 magnitude of the current. In mixed signal PCB ELECTRONIC CIRCUIT BOARD, generally “wire” is not used, but Use the entire plane to ensure minimum loop resistance and a complete plane under the signal line.

20. [Q] What kind of layout can I achieve the best heat dissipation effect?
[Answer] There are three main sources of heat in PCB ELECTRONIC CIRCUIT BOARD:
(1) Heating of electronic components;
(2) fever of P c B itself;
(3) Heat from other parts.

Among these three heat sources, the component generates the largest amount of heat, which is the main heat source, followed by the heat generated by the PCB ELECTRONIC CIRCUIT BOARD board. The heat transferred from the outside depends on the overall thermal design of the system and will not be considered for the time being. Then the purpose of thermal design is to take appropriate measures and methods to reduce the temperature of components and the temperature of the PCB ELECTRONIC CIRCUIT BOARD board, so that the system works normally at the appropriate temperature. This is mainly achieved by reducing heat generation and accelerating heat dissipation.