PCB Tech - Understand PCB layout

PCB design is physical work in the eyes of many people. However, for a long time, the early stage of a plan has always been personally placed and routed. Only after the finalization, some modifications are handed over to colleagues, but they will also be explained to them one by one. Wiring like this. The PCB boards designed by colleagues often comment and point out the missing parts, so that colleagues have greatly improved their PCB design.

A stepper motor driver board that my colleague was in charge of fabricating a year ago, the performance indicators are always not up to the performance mentioned in the document, although it can be used, the high current is lost, the high-speed is not up, the waveform is poor, and after in-depth analysis, it is found that it violates Some of the basic principles of PCB wiring are very good after modification. This makes me feel the importance of PCB wiring again, especially for high-power power supplies and sensors that have extremely strict requirements for PCB wiring.

In the msOS group in the last few days, netizens "buzzed" and raised the problem of PCB wiring. The problems caused by the wiring of stepper motors before. Use common sense to understand the PCB wiring, easy to understand, avoid circuit loops, electromagnetic field transmission lines and other high-level complexities., The more you talk about things that are more unclear, let everyone understand what is going on fundamentally, not be restricted by some professional terms, and get the approval of netizens in the group.

PCB wiring is to lay the roads for energizing signals to connect various devices. This is like building roads and connecting various cities to cars. It is exactly the same thing.

Road construction requires two lines once and one back. PCB wiring is the same. A two-line loop needs to be formed. For low-frequency circuits, it is a loop. For high-speed electromagnetic fields, it is a transmission line. The most common one is a differential signal. String. Such as USB, network cable, etc. For the impedance characteristics of transmission lines, etc., this article will not explain further, please refer to the article "Understandable Electromagnetic Field Theory".

It can be said that the differential signal line is an ideal model for connecting device signals. The higher the signal requirements, the closer to the differential signal line.

When there are many components on a board, if they are all arranged according to differential lines, one is that the area of the PCB is too large, and the other is to lay out 2N lines, which is too much work and difficult, so people put forward a multilayer PCB for actual needs. The most typical concept is the double-sided PCB board. The bottom layer is used as a common reference circuit, so that only N+1 wires are needed for wiring, and the PCB layout is greatly reduced.

The common reference circuit, which is commonly referred to as the reference ground, for most embedded industries, because the signal quality is not very high after digitization, the use of a whole-layer reference ground can reduce the board size and improve efficiency., It greatly saves time, and is loved by everyone. In fact, reducing the board size means shortening the length of the signal line, which can also partially offset the signal quality degradation caused by the reference ground. Therefore, in practice, the PCB wiring effect of introducing the reference ground is basically close to the ideal model of the differential line. Today, everyone is accustomed to this method. It seems that PCB wiring requires a layer of reference ground. There is no reason.

In the double-panel design, because there are often crossover wires, it is necessary to jumper the wire to the ground layer for crossover wire exchange. This needs to be pointed out that the jumper should not be too long. If it is too long, it is easy to divide the reference ground, especially for some For lines requiring high signal quality, the reference ground at the bottom cannot be divided. Otherwise, the signal circuit is completely destroyed, and the reference ground loses its meaning. Therefore, in general, the reference ground layer is only suitable for short jumpers of signal lines, and the signal lines should be placed on the top layer as much as possible, or more layers of PCB boards should be introduced.

If the road is too close to the road, it is easy to have an impact. For example, when taking a high-speed rail, you can feel the influence of the train from the opposite side on the train you are on. The same is true for the signal lines. They should not be too close. If the signal lines are parallel to the signal lines, a certain distance must be maintained. This is subject to experiment, and there must be a good reference ground at the bottom. Under low frequency and small signal, generally the influence is not very big, high frequency strong signal needs attention.

For high-frequency, high-current PCB wiring, such as switching power supplies, the most taboo is that the driving signal is interfered by the output strong current and strong voltage. The driving signal of the MOS tube is easily affected by the strong output current. Keep a certain distance between the two and do not get too close. In the era of analog audio, if the amplifier magnification is too high, the self-excitation effect will appear. The reason is the same as that of the MOS tube.

The carrier of PCB wiring is the PCB board. Generally, the reference ground is 1mm away from the edge of the PCB board, and the signal line is around 1mm away from the edge of the reference ground. In this way, all signals are confined in the PCB board and EMC radiation can be reduced.

When you don’t have a concept of PCB design, think about the daily path. The two are exactly the same.