PCB Technical - Be cautious: Talking about the role of serpentine thread

You can often see people asking the snake-shaped question. Usually we can see the serpentine lines in places where most of them are high-speed high-density boards. It seems that the boards with serpentine lines are more advanced. If you can draw serpentine lines, you are a master. There are also many articles about snake-shaped lines on the Internet, and I always feel that the content of some posts will mislead novices, cause confusion to people, and create some artificial obstacles. So let's take a look at what the serpentine line does in practical applications.

To understand the serpentine line, let's talk about PCB routing first. This concept doesn't seem to need to be introduced. Isn't the hardware engineer doing wiring work every day? Every trace on the PCB is drawn out one by one by the hardware engineer. What can be said? In fact, this simple trace also contains a lot of knowledge points that we usually ignore. For example, the concept of microstrip line and stripline. Simply put, the microstrip line is the trace that runs on the surface of the PCB board, and the stripline is the trace that runs 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 PCB inner layer, and the other side of the trace is exposed to the air, which causes the dielectric constant around the trace to be inconsistent, such as 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 the upper and lower sides of the strip line, the entire trace is embedded in the PCB substrate, and the dielectric constant around the trace is the same. This also causes the TEM wave to be transmitted on the strip line, while the quasi-TEM wave is transmitted on the microstrip line. Why is it a quasi-TEM wave? It is caused by the phase mismatch at the interface between the air and the PCB substrate. What is the TEM wave?............ If you dig deeper on this issue, you will not be able to finish it in ten and a half months. To make a long story short, whether it is a microstrip line or a stripline, their role is nothing more than to carry signals, whether digital signals or analog signals. These signals are transmitted in the form of electromagnetic waves from one end to the other in the trace. Since it is a wave, there must be speed. What is the speed of the signal on the PCB trace? According to the difference in dielectric constant, the speed is also different. The propagation speed of electromagnetic waves in the air is the well-known speed of light. The propagation velocity in other media must be calculated by the following formula: V=C/Er0.5

Among them, V is the propagation speed 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 base material into the formula to calculate it, that is, the transmission speed of the signal in the FR4 base material is half the speed of light. However, because half of the microstrip line traced on the surface is in the air and half 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 wiring in an instant after one pin is sent. Although the signal transmission speed is very 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 or falling edge is about one-tenth of the period, then it is 100ps. If the length of our trace exceeds 1 inch (about 2.54 cm), then the transmission delay will be more than a rising edge. If the trace exceeds 8 inches (about 20 cm), then the delay will be a full cycle! It turns out that PCB has such a big influence, it is very common for our board to have more than 1inch traces. So does the delay affect the normal operation of the board? Look at the actual system, if it is just a signal, and other signals do not want to be turned off, 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. Take a look at our video interface. No matter how many channels are HDMI or DVI, it will contain data channels and clock channels. Or some bus protocols, all of which are synchronous transmission of data and clock. Then, in an actual high-speed system, these clock signals and data signals are synchronously sent from the main chip. If our PCB trace design is poor, the length of the clock signal and the data signal is very different. It is easy to cause wrong sampling of data, and then the whole system will not work normally. What should we do to solve this problem? Naturally, we would think of lengthening the short-length traces so that the length of the traces in the same group is the same, then the delay will be the same? Then how to lengthen the traces? !Bingo! Finally, it's not easy to return to the topic. This is the main function of the serpentine line in the high-speed system. Winding, equal length. It's that simple. The serpentine line is used to wind the equal length. By drawing the serpentine line, we can make the same group of signals have the same length, so that after the receiving chip receives the signal, the data will not be caused by the different delays on the PCB trace. Wrong pick. The serpentine line is the same as the traces on other PCB boards. They are used to connect signals, but they are longer and don't have it. So the serpentine line is not deep and not too complicated. Since it is the same as other wiring, some commonly used wiring rules are also applicable to serpentine lines. At the same time, due to the special structure of serpentine lines, you should pay attention to it when wiring, for example, try to keep the serpentine lines parallel to each other as far away as possible. Shorter, that is, go around a big bend as the saying goes, don't go too dense and too small in a small area. This all helps to reduce signal interference. The serpentine line will have a bad influence on the signal due to the artificial increase of the line length, so as long as it can meet the timing requirements in the system, don't use it. Some engineers use DDR or high-speed signals to make the entire group equal length. The snake-shaped line is flying all over the board. It seems that this is a better wiring. In fact, this is lazy and irresponsible. Many places that do not need to be wound are wound, which wastes the area of the board, and also reduces the signal quality. We should calculate the delay redundancy according to the actual signal speed requirements to determine the wiring rules of the board.

In addition to the function of equal length, I see several other functions of the serpentine line that are often mentioned in articles on the Internet, and I will also briefly talk about it here.

1. An argument that is often seen is the role 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 does the shape of the trace affect the impedance? I don’t know. Where did this statement come from?

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

3. The receiving antenna. This can be. We can see this effect on some mobile phones or radios. Some antennas are made with PCB traces.

4. Inductance. This can be. All traces on the PCB originally have parasitic inductances. It is achievable to make some PCB inductors.

5. Fuse. This effect makes me puzzled. How does the short and narrow serpentine wire play the role of a fuse? If the current is large, it will blow? The board is not useless. The price of this fuse is too high. I really don’t understand what kind of application it will be used in. .

Through the above introduction, we can clarify that in analog or radio frequency circuits, serpentine lines have some special effects, which are determined by the characteristics of microstrip lines. In digital circuit design, the serpentine line is used for equal length to achieve timing matching. In addition, the serpentine line will affect the signal quality, so the system requirements should be clarified in the system, the system redundancy should be calculated according to the actual requirements, and the serpentine line should be used with caution.