PCB Tech - Understand PCB printed circuit board wiring

PCB design is manual work in the eyes of many people. However, I have always placed and routed myself in the early stages of a plan. Only after the finalization of some modifications, I will be responsible for my colleagues, but I will explain to them why. To be wired like this. I often comment on the PCB boards designed by my colleagues and point out the missing areas, so that my 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 when we often make 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 issue of PCB wiring. I felt the problem caused by the wiring of the stepper motor before, and use common sense to understand this PCB wiring. It is easy to understand, avoiding circuit loops and electromagnetic fields. The transmission line is high and complex, and the more you talk about it, the more unclear things you talk about. Fundamentally, let everyone understand what's going on, without being restricted by some professional terms, and gaining the approval of the 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 again. The same is true for PCB wiring. 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 a 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, we are all accustomed to this method, it seems that PCB wiring, is to have 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.

The closeness between the road and the road is prone to influence. 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 drive 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 analog audio era, if the amplifier magnification is too high, the self-excitation effect will occur, and 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 approximately 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 there is no concept of PCB design, think about our daily roads. The two are completely the same.