PCB Technical - 12 questions about PCB circuit board wiring

1. What problems should be paid attention to when PCB wiring?

The impedance matching of the signal line; the spatial isolation from other signal lines; for digital high-frequency signals, the effect of the differential line will be better.

2. In the layout of the board, if the wires are dense, there may be more holes, which of course will affect the electrical performance of the board. How to 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, it needs to be added to the power supply port of the analog device, and different capacitance values need to be used to filter out spurious signals of different frequencies.

4. What are the criteria for a good board?

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

5. How much influence does 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 the PCB process and cost less, 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 power terminal, but this way the signal return path is far away. How to choose the appropriate method for specific applications?

If there are high-frequency> 20MHz signal lines, and the length and quantity are relatively large, then at least two layers are required for this analog high-frequency signal. One layer of signal line, one layer of large area ground, and the signal line layer needs 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 leftmost edge of the PCB, and the MCU is on the right, so in the layout, the stabilized power supply chip is placed near the connector (the power supply IC outputs 5V after a relatively long path) MCU), or place the power IC to the right of the center (the output 5V line of the power IC reaches the MCU is relatively short, but the input power segment line passes through a relatively long PCB board)? Or is there a better layout?

First, is the signal input plug-in an analog device? If it is an analog device, 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 considerations:

First of all, is the regulated power supply chip a relatively clean, low-ripple power supply? The power supply of the analog part has relatively high requirements for the power supply;

Whether the analog part and MCU are the same 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;

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 chain, 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?

There is no conclusion so far. 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, depending 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, what is the maximum difference between the lengths of the two signal lines? How to calculate?

Differential 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. 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. What kind of situation is suitable for serpentine routing in 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 because of different applications:

If the serpentine trace appears in the computer board, it mainly functions 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.

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 time lag. 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. The delay is related to the line width, line length, copper thickness, and layer structure, but too long a line will increase the distributed capacitance and distributed inductance., 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 the signal quality to deteriorate. Therefore, 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 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 aspects need to be considered in detail? What measures are taken?

EMI/EMC design must consider 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 close to the external connector. 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 a decoupling/bypass capacitor, 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. The ground can also be divided to control the range of high-frequency noise.

Finally, properly choose the chassis ground between the PCB and the housing.

12. 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 in this issue. For example, the various parameters of PCB materials, the transmission line model finally established according to these parameters, the parameters of the device, etc. Impedance matching is generally designed according to the information provided by the manufacturer.

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.