How to design a two-layer PCB? How is the three-layer PCB designed?
https://blog.csdn.net/qq_42053636/article/details/89577815
Incurable diseases from the general group specializing in PCB incurable diseases (four groups of friends have exceeded 1800 people, add Mr. Yang's WeChat account Johnnyyang206, can be added to the group): Mr. Yang, how to design the three-layer board? I want to learn three-layer board first, and then continue to learn later, to learn ten-layer board?
After some simple analysis, Mr. Yang asked him to learn a two-layer board first, and then learn an eight-layer board. If these two are mastered, regular PCB design can be done. Because the two-layer board is special, I wrote an article on how to design the two-layer board.
An outstanding talent from a PCB design company, the title hangs: once designed an 18-layer board, the highest rate of 25G was a one-time success, and HDI has been done, etc. A series of glorious history, is it not good to design a simple two-layer Board?
Teacher Yang replied: This is not necessarily true. The reason why we rarely talk about two-layer board design is because it is really simple, the signal rate is not high, and the requirements are not strict. As long as it can be connected and meet DFM, DFA requirements can basically pass. For boards that are a little more complicated and uncertain, they will switch to a multi-layer board design. In a real situation, for example, there is a DDR3 two-layer board design. People who have been with ZTE Huawei should know that it is still relatively common. For example, if a more complicated four-layer board is changed to a two-layer board design, can you still ensure that there is no problem with the two-layer board design?
Teacher Yang believes that for those who can truly understand the two-layer board, the multi-layer board is basically fine. People who can do multi-layer boards may not be able to do two-layer boards. How to design a two-layer board, and where are the key points and difficulties?
Difficulty 1: Cannot understand the reflow problem well and apply it in the design.
We know a fact: current is the flow of electric charge, and current can only flow in a closed loop; then we understand the signal return path, that is, return current. The signal flow is from the driver to the load along the PCB transmission line, and then from the load back to the driver through the shortest path along the ground or power supply. Each signal has a return path to form a loop. The signals in the circuit can be summarized as high-frequency signals and low-frequency signals, and their return paths are different: low-frequency signal return selects the path with the lowest impedance, and high-frequency signal return selects the path with the lowest inductance.
When designing a multilayer board, we are more concerned about impedance, reflection, crosstalk, timing, etc., and basically do not mention the return path. Does everyone have a consensus that multi-layer boards do not mention the return path. Is that true? In fact, we will definitely mention the reference plane when designing the multilayer board, and this reference plane is basically a plane with a complete GND. This complete plane provides a good signal return path for low frequency or high frequency, and there is no need for additional consideration at all. But precisely for a two-layer board, without a complete GND reference plane, the return path becomes an extra key.
Next, the problem that Mr. Yang analyzed is what problems will the signal return path cause if it is too long or not handled properly? In fact, the core theory of PCB is still based on the electromagnetic field. Each wire and its loop on the PCB form a current loop. According to the principle of electromagnetic radiation, when a sudden current flows through the wire loop in the circuit, it will be in space. Generate electromagnetic fields and affect other wires. This is what we usually call radiation, which leads to signal integrity problems and EMC problems.
Difficulty 2: I don’t know how to improve or control the signal integrity of the two-layer board?
Isn't the two-layer board a low-speed board, why should you consider signal integrity? This problem arises because everyone encounters a slightly high-frequency or complex board with a multilayer board design.
Many experts will be embarrassed when some or customers require that only two-layer board design is used. For example, the two-layer board of HDMI interface, such as the two-layer board of DDR3, and so on.
The signal integrity problem of the two-layer board, Mr. Yang wants to analyze it with actual problems. Look at the picture below
This clock line, the driving end is transmitted to the receiving end at a frequency of 88.5MHZ. Because it is a two-layer board, traces and power are also passed through the bottom layer, which causes the clock signal to cross and divide five times. In this way, let's take a look at the true return path of this clock line.
As shown in the figure above, the return path of the clock signal goes around a long way to reach the drive end, which brings about signal integrity problems.
In addition, the longer the transmission line, the longer the current return path. As the signal is continuously divided, the return path becomes longer. The current direction of the conductor of the transmission line is opposite to that of the return path, forming a differential mode coupling. The effect is that the magnetic flux cancels out. If the return path increases, the inductance value increases, and the constant cross-segmentation reduces the differential mode coupling effect, which is prone to EMI problems.
Now that Teacher Yang has this problem, how can it be improved?
If there is really no space, based on this wiring situation, you can add a GND-clad ground wire, and add ground holes at both ends of the split to improve the return path of the clock signal. Students who can simulate can try to compare the effects of the two, it is very obvious. As for the improvement of EMI, some measures may need to be taken from the circuit. This can be discussed with EMC teacher Wang.
As for other signal integrity issues such as crosstalk, jitter, ISI, etc., if they are also encountered in a two-layer board, this must be analyzed in detail, so I won't talk about it here.
The real two-layer board design requires a combination of signal integrity knowledge, power integrity knowledge, EMC knowledge, and specific simulation. So the two-layer board design is not simple. It is based on a strong design theory basis.
Difficulty 3: Wiring is difficult and cannot be taken into account at the same time
Two-layer board design Many people still have a misunderstanding of multi-layer board design. Don't worry about GND, connect the wiring first, and then directly lay the ground plane on the top side and bot side, and then adjust it to complete the connection of all signals. There is also a relatively poor foundation, completely at the expense of the ground, to complete the signal connection first. In fact, this kind of thinking is bad or problematic. For more complex projects, the wiring of the two-layer board is indeed difficult, but GND cannot be sacrificed. Once the return path is not handled properly, the PCB is likely to have problems. Especially boards with high-frequency signals.
What is the wiring idea for the two-layer board?
The complex area is simple, the signals are all out including the ground, and the key signals are given priority first, so you are not afraid of punching to ensure backflow. This is basically the idea. The specifics are not important. Everyone has different habits, but the end point is the same. Design an excellent two-layer board.
Difficulty 4: I don't know how to evaluate a two-layer board design or a multi-layer board design?
This is a tricky question. For Mr. Yang, it is easier to evaluate how many layers to use to design the project, but it may be difficult for novices or less professional hardware engineers. In short, the suggestions given are as follows Bar:
1. First, take a look at an article written by Teacher Yang before about high speed? high frequency? Multilayer board design articles
2. When you are not sure, such as worrying that the wiring will not work, such as worrying about the quality of the two-layer board design.
3. It is impossible to send it to our group for consultation.
Pay attention to the WeChat public account: specialize in PCB intractable diseases (PCBDoctor) to solve various PCB intractable diseases encountered.
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Original link: https://blog.csdn.net/qq_42053636/article/details/89577815