PCB Tech - In-depth and simple talk about multilayer panel wiring skills

In-depth and simple talk about multilayer panel wiring skills
The layer settings of the multilayer board bothered me for several days, but I didn't understand the difference between plane and layer. I didn’t understand other people’s multi-layer boards, thinking that the ground plane and power plane in the middle of the multi-layer boards should be copper-clad like double-sided boards, but there is no large copper-clad in other people’s multi-layer boards. After setting up the layers like others, there is no way to deposit copper on the middle 2 layers, and the PCB layout book does not mention the layer setting of multi-layer boards. Later, after using plane and layer as keywords, I searched for an article on Baidu, and understood the difference between positive and negative films of PCB. I wanted to reprint that article, but I couldn't find it. I still write it myself, so that I can do a multilayer board for the first time, and someone who is just as confused can search it.
The production of PCB is divided into positive and negative films. Positive films are what we usually understand. There is copper in the place where the line is drawn, and there is no line where there is no line. The negative film has no copper where the lines are drawn, and there is no copper where the lines are drawn.

Both the bottom and top layers of the double panel are made of positive films. In the multilayer board, for the large copper layer such as the ground plane and the power plane, the negative film is generally used for production, and the data volume of the negative film is small, and only a certain amount of cutting is required for the entire plane. The positive film is the layer, and the negative film is the plane. In Protel's layer settings, there are two commands for creating new layers: add layer and add plane. The positive film can be routed, copper-clad, vias and components can be placed, and the plane can only be cut by drawing a line on the negative film. Each part of the cut can be individually set with net, and the negative film cannot be routed or copper-plated. Of course, you can also use the positive film to add copper to realize the ground plane and the power plane, but undoubtedly the negative film is more suitable, the amount of data is smaller, the PCB factory is also convenient for processing, and there is no need to rebuild after adding vias. Every change in the copper coating needs to be rebuild, which makes the software run very slowly.
Having said so much in a mess, to sum it up in one sentence, the power layer and ground layer of the multilayer board use plane, and the signal layer uses layer.
Choosing a four-layer board is not only a problem of power and ground. High-speed digital circuits have requirements for the impedance of the trace. The two-layer board is not easy to control the impedance. 33R resistance is generally added to the driver end, which also plays a role in impedance matching; when wiring, you must first lay out the data address line and the high-speed line that needs to be guaranteed;
At high frequencies, the traces on the PCB board must be regarded as transmission lines. Transmission line has its own characteristic impedance. Those who have studied transmission line theory know that when there is a sudden impedance change (mismatch) somewhere on the transmission line, reflection will occur when the signal passes through, and the reflection will cause interference to the original signal, which will affect the normality of the circuit in severe cases. Work. When a four-layer board is used, signal lines are usually routed on the outer layer, and the middle two layers are power and ground planes. This isolates the two signal layers on the one hand, and more importantly, the outer layer traces and the plane they are close to form a balance. For the transmission line of "microstrip", its impedance is relatively fixed and can be calculated. It is more difficult to do this for two-layer boards. The impedance of this kind of transmission line is mainly related to the width of the trace, the distance to the reference plane, the thickness of the copper and the characteristics of the dielectric material. There are many ready-made formulas and procedures for calculation.
The 33R resistor is usually connected in series at one end of the driver (in fact, it is not necessarily 33 ohms, from a few ohms to five or sixty ohms, depending on the specific situation of the circuit). Its function is to connect with the output impedance of the transmitter and route it in series. Impedance matching, so that the reflected signal (assuming that the impedance of the de-receiving end is not matched) will not be reflected back (absorbed) again, so that the signal at the receiving end will not be affected. The receiving end can also be used for matching, such as using resistors in parallel, but it is less used in digital systems, because it is more troublesome, and in many cases it is one-transmitting and multi-receiving, such as address bus, which is not as easy to do as the source-end matching.
The high frequency mentioned here is not necessarily a circuit with a very high clock frequency. Whether it is high frequency depends not only on the frequency, but more importantly, on the rise and fall time of the signal. Usually the rise (or fall) time can be used to estimate the frequency of the circuit, generally half of the reciprocal of the rise time, for example, if the rise time is 1ns, then its reciprocal is 1000MHz, which means that the design of the circuit should be based on the 500MHz frequency band. . Sometimes it is necessary to deliberately slow down the edge time, and the output slope of the driver of many high-speed ICs is adjustable.
Take the four-layer board design as an example to explain the matters that should be paid attention to when wiring the multilayer board. 1. Connect the wires above 3 points. Try to let the wires pass through each point in turn for easy testing and keep the wire length as short as possible.
2. Try not to put wires between the pins, especially between and around the pins of integrated circuits.
3. The lines between different layers should not be parallel as much as possible, so as not to form actual capacitance.
4. The wiring should be as straight as possible, or a 45-degree broken line, to avoid electromagnetic radiation.
5. The ground wire and power wire should be at least 10-15mil or more (for logic circuits).
6. Try to connect the grounding polylines together to increase the grounding area. Try to be as neat as possible between lines.
7. Pay attention to the uniform discharge of components to facilitate installation, plug-in, and welding operations. The text is arranged in the current character layer, the position is reasonable, pay attention to the orientation, avoid being blocked, and facilitate production.
8. Consider the structure of component placement. The positive and negative poles of SMD components should be marked at the package and at the end to avoid space conflicts.
9. At present, the printed circuit board can be used for 4-5mil wiring, but it is usually 6mil line width, 8mil line spacing, and 12/20mil pad. The wiring should consider the influence of sink current, etc.
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