20 Questions about PCB Board Design and Wiring

PCSPCB is a large, medium and small batch circuit board manufacturer, more than 10 years focused on high-precision through hole (1-8 layer) FR4 single-sided PCB board, double-sided circuit board, multi-layer circuit board, high-frequency board, blind hole board, high TG thick copper plate, special plate production and manufacturing, Jiangxi factory covers an area of about 50,000 square meters. The total number of employees is more than 300 people (including more than 100 professionals with more than 10 years), the minimum line: 3mil, the maximum hole: 0.1mm, the top number can reach 8 layers.

1. high frequency signal wiring should pay attention to what problems?

Impedance matching of signal lines; Spatial isolation from other signal lines; For digital high frequency signals, differential lines work better.

2. in the board, if the line is dense, the hole may be more, of course, it will affect the electrical performance of the board, how to improve the electrical performance of the board?

For low-frequency signals, the hole does not matter, high-frequency signals to minimize the hole. If there are many lines, you can consider multi-layer boards.

3. is it not the more decoupling capacitors added to the board, the better?

The decoupling capacitor needs to be at the right place with the right value. For example, the power supply port of the simulator needs to be added, and different capacitance values need to be used to filter out stray signals of different frequencies.

4. What is the standard of a good board?

Reasonable layout, power cable power redundancy, high frequency impedance, low frequency routing simple.

5. through holes and blind holes on the difference in signal impact how much? What are the principles applied?

The use of blind holes or buried holes is an effective way to increase the density of multilayer plates, reduce the number of layers and surface size, and greatly reduce the number of plating through holes. However, in comparison, the through hole is better realized in the process and the cost is lower, so the general design uses the through hole.

6. when it comes to analog digital hybrid systems, some people suggest that the electric layer segmentation, the ground plane to take the whole piece of copper, some people suggest that the electric layer segmentation, different ground at the power endpoint connection, but so that the signal return path is far away, the specific application should be how to choose the appropriate method?

If there is a high frequency >20MHz signal line, and the length and number are relatively large, then at least two layers are needed to give this analog high frequency signal. One layer of signal line, one layer of large area, and the signal line layer needs to be drilled enough holes 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 a lot of holes, and the ground is a big plane.

7. in the circuit board, the signal input plug-in is on the left side of the PCB, and the MCU is on the right side, then in the layout is the voltage regulator power chip placed near the source connector (power IC output 5V after a long path to reach the MCU), or the power IC placed to the middle right (power IC output 5V line to reach the MCU is relatively short, But the input power segment line passes a relatively long section of PCB board)? Or a better layout?

First, is the signal input plug-in an emulator? If it is a simulator, it is recommended that the power supply layout should not affect the signal integrity of the analog part as much as possible. Therefore, there are several points to consider: Is the voltage regulator chip a relatively clean power supply with small ripple? Analog part of the power supply, the power supply requirements are relatively high; Whether the analog part and MCU are a 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; The power supply to the digital part needs to be considered to minimize the impact on the analog part of the circuit.

8. In the application of high-speed signal chain, there are analog and digital ground for multiple ASics, is it to use ground segmentation, or unsegmentation? What are the established norms? Which works better?

So far the jury is out. In general, you can consult the manual of the chip. The manual for all ADI hybrid chips recommends a ground solution, some common ground, some isolated ground, depending on the chip design.

9. When to consider the isometric length of a line? If you want to consider the use of equal length lines, the maximum difference between the length of the two signal lines can not exceed what? How to calculate?

Difference line calculation idea: If a sinusoidal signal is transmitted, the length difference is equal to half of its transmission wavelength, and the phase difference is 180 degrees, then the two signals will completely cancel out. So the length difference here is the maximum. By analogy, the signal line difference must be less than this value.

10. high speed in the snake line, suitable for what kind of situation? Are there any disadvantages? For example, for differential routing, the two sets of signals are required to be orthogonal.

Serpentine routing has different functions because of different applications: if the serpentine routing appears in the computer board, it mainly plays a role of filtering inductance and impedance matching, which is used to improve the anti-interference ability of the circuit. Serpentine wiring in the computer host board is mainly used in some clock signals, such as PCI-Clk, AGPCIK, IDE, DIMM and other signal lines. If in the general ordinary PCB board, in addition to the role of filtering inductance, it can also be used as the inductor of the radio antenna and so on. For example, 2.4G walkie-talkies are used as inductors.

The length of some signal wiring requirements must be strictly equal, and the equal length of high-speed digital PCB board is to keep the delay difference of each signal within a range to ensure the effectiveness of the data read by the system in the same cycle (the delay difference exceeds one clock cycle will misread the data of the next cycle). For example, HUBLink in the INTELHUB architecture, a total of 13 roots, the use of 233MHz frequency, the requirement must be strictly equal in length, in order to eliminate the hidden dangers caused by time delay, winding is the only solution. Generally, the delay difference is not more than 1/4 clock cycle, and the line delay difference per unit length is also fixed, and the delay is related to the line width, line length, copper thickness, and plate structure, but too long the line will increase the distributed capacitance and distributed inductance, and the signal quality will be reduced. Therefore, the clock IC pin is generally connected to the end, but the snake wire does not play the role of inductance. On the contrary, the inductance will shift the higher-order harmonic phase 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 rise time of the signal, the more susceptible it is to distributed capacitance and distributed inductance. The serpentine circuit acts as an LC filter with distributed parameters in some special circuits.

11. in the design of PCB, how to consider electromagnetic compatibility EMC/EMI, what specific aspects need to be considered? What measures will be taken?

EMI/EMC design must take into account the location of the device, the arrangement of PCB stacks, the movement of important connections, the selection of devices, etc., at the beginning of the layout. For example, the position of the clock generator should not be close to the external connector as much as possible, the high-speed signal should go to the inner layer and pay attention to the continuity of the characteristic impedance matching and the reference layer to reduce reflection, and the slope of the signal pushed by the device (slew rate) should be as small as possible to reduce the high-frequency component. When selecting the decoupling/bypass capacitor, pay attention to whether the frequency response meets the requirements to reduce the noise at the power layer. 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. The range of high frequency noise can also be controlled by dividing the strata. Finally, the appropriate selection of PCB and shell ground (chassis ground).

12. What should we pay attention to in the transmission line design of RF broadband circuit PCB? How to set the ground hole of the transmission line is more appropriate, and whether the impedance matching needs to be designed by itself or to cooperate with PCB processing manufacturers?

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

13. when the analog circuit and digital circuit coexist, such as half of the FPGA or MCU digital circuit part, the other half is the analog circuit part of the DAC and related amplifier. There are more power supplies of various voltage values, and the voltage value of the power supply used by both digital and analog circuits can be used in common power supplies, and what skills are available in wiring and magnetic bead arrangement?

It is generally not recommended to use it this way, it will be more complex and difficult to debug.

14. in the high-speed multi-layer PCB design, what is the main basis for the selection of packaging devices such as resistors and capacitors? What kinds of packages are commonly used? Can you give some examples?

0402 is commonly used mobile phones; 0603 is the general high-speed signal module commonly used; Based on 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 components in critical locations.

15. Generally in the design of the double panel is the signal line or the ground line first?

This has to be taken into consideration. In the case of thinking about layout first, consider routing.

16. in the high-speed multi-layer PCB design, what should be the most attention to the problem? Can you explain the solution of the problem in detail?

The most important thing to pay attention to is the design, that is, how the signal lines, power lines, ground, and control lines are divided in each layer. The general principle is to ensure that the analog signal and the analog signal ground are at least one separate layer. A separate layer is also recommended for power supply.

17. when to use 2 layers of board, 4 layers of board, 6 layers of board? Are there strict technical limits (aside from volume reasons)? Is it the frequency of the CPU or the frequency of its data interaction with external devices?

The use of multi-layer board can first provide a complete ground plane, in addition to provide more signal layers, convenient wiring. For the application of the CPU to control the external memory device, the frequency of interaction should be considered, if the frequency is high, the complete ground plane must be guaranteed, in addition, the signal line is best to maintain the same length.

18. how to analyze the impact of PCB wiring on analog signal transmission, how to distinguish the noise introduced in the signal transmission process is caused by wiring or operation amplifier device?

This is difficult to distinguish, only through PCB wiring to try to avoid the introduction of additional noise.

19. for high-speed multi-layer PCB, the line width of the power cord, ground wire and signal line is set to how much is appropriate, what is the common setting, can you give an example? For example, how to set the operating frequency at 300Mhz?

For 300MHz signal, 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, and the "line" is generally not used to indicate the ground when the mixed signal PCB is used, but the whole plane is used, so as to ensure the minimum circuit resistance, and there is a complete plane below the signal line.

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

There are three main sources of heat in PCB: the heating of electronic components; Heating of PCB itself; Heat from other parts.

Among these three heat sources, the heat generated by components is the largest, which is the main heat source, followed by the heat generated by the PCB board, and the external heat incoming depends on the overall thermal design of the system, which is not considered for the time being.

So 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 works normally at the right temperature. It is mainly achieved by reducing heat and accelerating heat dissipation.