What are the applications of serpentine lines in PCBA circuit board design

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• Date: 2022-07-10

In planning pcb circuit board drawings, it is often seen that people are asking questions about serpentine lines. Usually, the places where we can see serpentine lines are mostly high-speed high-density boards. For example, the boards with serpentine lines are more high-end. If you can draw serpentine lines, you are a master. There are also many articles on the serpentine line on the Internet. I always feel that the content of some posts will mislead newbies, cause people trouble, and artificially create some obstacles. So let's see what effect the serpentine line has in practice.

To understand the serpentine line, let's talk about the PCB trace first. It seems that this concept does not need to be introduced. Is it inconvenient for hardware engineers to do wiring work every day? Every trace on the PCB is drawn by hardware engineers one by one. What is there to say? In fact, this simple trace also contains many knowledge points that we usually overlook. For example, the concept of microstrip and stripline. Briefly speaking, the microstrip line is the line that goes on the surface layer of the PCB board, and the strip line is the line that goes on the inner layer of the PCB. What is the difference between these two lines? The reference plane of the microstrip line is the ground plane of the inner layer of the PCB, and the other side of the trace is exposed to the air, so the dielectric constant around the trace is not the same. For example, the dielectric constant of our commonly used FR4 substrate is about 4.2, and the dielectric constant of air is 1. There are reference planes on both sides of the strip line, the entire trace is embedded in the substrate of the PCB circuit board, and the dielectric constant around the trace is the same. This also constitutes the TEM wave transmitted on the stripline, and the quasi-TEM wave transmitted on the microstrip line. Why quasi-TEM waves? That's due to a phase mismatch at the interface between the air and the PCB substrate. What is a TEM wave?... If you dig deeper into this issue, it will take ten days and a half to talk about it. Long story short, whether it's a microstrip line or a stripline, their effect is nothing more than to carry signals, whether digital or analog. These signals travel from one end to the other in the form of electromagnetic waves in the trace. Since it is a wave, it must have speed. What is the speed of the signal on the PCB trace? Depending on the difference in dielectric constant, the speed is not the same. The speed of electromagnetic waves in air is known as the speed of light. The propagation speed in other media is calculated by the following formula:

What are the applications of serpentine lines in pcba circuit board design?

V=C/Er0.5

Among them, V is the speed of propagation in the medium, C is the speed of light, and Er is the dielectric constant of the medium. Through this formula, we can easily calculate the transmission speed of the signal on the PCB trace. For example, we simply take the dielectric constant of the FR4 substrate to 4 to bring it into the formula for calculation, that is, the transmission speed of the signal in the FR4 substrate is half the speed of light. However, the microstrip line with surface traces, because half is in the air and half is in the substrate, the dielectric constant will be slightly reduced, so the transmission speed will be slightly faster than that of the strip line. The commonly used empirical data is that the trace delay of the microstrip line is about 140ps/inch, and the trace delay of the stripline is about 166ps/inch.

As mentioned above, there is only one purpose, that is, the transmission of the signal on the PCB is delayed! That is to say, the signal is not transmitted to another pin through the trace immediately after it is sent out. . Although the signal transmission speed is fast, as long as the trace length is long enough, it will still affect the signal transmission. For example, for a 1GHz signal, the period is 1ns, and the time of the rising edge or the falling edge is about one-tenth of the period, so it is 100ps. If our trace length exceeds 1 inch (about 2.54 cm), then the delay of transmission is more than one rising edge. If the trace exceeds 8 inches (about 20 cm), then the delay can be a full cycle! It turns out that the influence of the PCB is so great that it is very common for the traces on our board to exceed 1 inch. So will the delay affect the normal operation of the board? Looking at the actual system, if only one signal does not want to be turned off with other signals, then the delay does not seem to have any effect. However, in a high-speed system, this delay will actually take effect.

For example, our common memory particles are connected in the form of a bus, with data lines, address lines, clocks, and control lines. Look at our video interface, HDMI or DVI no matter how many channels, will include data channel and clock channel. Maybe some bus protocols, all data and clock are transmitted synchronously. Then, in the actual high-speed system, these clock signals and data signals are sent from the main chip synchronously. If our PCB layout is very poor, and the lengths of the clock signal and the data signal are very different, then it is very difficult to It is easy to constitute wrong sampling of data, then the whole system will not work properly. How to solve this problem? Naturally, we will think of lengthening the traces with short lengths, so that the traces in the same group are about the same length, then the delay will be the same? Then how to lengthen the traces? !Bingo! It's not easy to get back on topic at last. This is the main effect of the serpentine line in high-speed systems. Winding, equal length. It's that simple. The serpentine line is used to wrap the same length. By drawing the serpentine line, we can make the same group of signals achieve the same length, so that after the receiving chip receives the signal, it will not be caused by different delays on the PCB circuit board. Mistake of the constituent data. The serpentine line is the same as the other PCB traces. It is used to connect the signal. It is only longer, without it. So the serpentine line is not too deep and not too complicated.

Since it is the same as other traces, some common wiring rules are also applicable to serpentine wires. At the same time, due to the special structure of serpentine wires, attention should be paid to the wiring. For example, try to keep the parts of the serpentine wires parallel to each other as far as possible. Shorter, that is, as the saying goes, around the big bend, don't go too close or too small in a small area. This all helps reduce signal interference. Since the serpentine line is artificially increased in line length, it must have a bad influence on the signal, so as long as it can meet the timing requirements in the system, it should not be used if it can not be used. Some engineers use DDR or high-speed signals to make the whole group of equal length, and the serpentine lines are flying all over the board. It seems that this is a better wiring. In fact, this is a manifestation of being idle and irresponsible. Many places that do not need to be wound are wound, which wastes the area of ​​​​the board and reduces the signal quality. We should calculate the redundancy of delay according to the actual signal speed requirements, and then determine the wiring rules of the board.

In addition to the isometric effect, several other effects of the serpentine line are often mentioned in articles on the Internet, and I will briefly describe them here.

1. A statement that is often seen is the effect of impedance matching. This statement is very strange. The impedance of the PCB trace is related to the line width, the dielectric constant, and the distance of the reference plane. When is it related to the serpentine line? ? When did the shape of the trace affect the impedance? I don't know where the source of this statement came from.

2. It is also said that it is the effect of filtering. This effect cannot be said to be absent, but there should be no filtering effect in the digital circuit. It may be said that we do not need to use this function in the digital circuit. In the RF circuit, the serpentine trace can form an LC circuit. If it has a filtering effect on the signal of a certain frequency, it is still in the past.

3. Inductance, this is possible. All traces on the original PCB have parasitic inductance. Doing some PCB inductance can be done.

4. Accept the antenna, this is possible. We can see this effect on some mobile phones or radios. Some antennas are made with PCB traces.

5. Fuse, this effect makes me puzzled. How does the short and narrow serpentine wire work as a fuse? It will blow when the current is high? The board is not useless, the price of this fuse is too high, I really don't know what kind of application it will be used in .

After the above introduction, we can clearly see that in analog or radio frequency circuits, the serpentine line has some special effects, which is determined by the characteristics of the microstrip line. In digital circuit design, the serpentine line is used to achieve the effect of equal length timing matching. In addition, the serpentine line will have an impact on the signal quality, so the system requirements should be clearly defined in the system, the redundancy of the system should be calculated according to practical requirements, and the serpentine line should be used cautiously.