PCB News - The function of serpentine routing and its processing skills

I have been engaged in high-speed PCB design for more than ten years, and many PCB traces involve timing and equal length requirements. I also discussed the role of serpentine traces with some experts in the relevant PCB Layout industry. The details are as follows.
Any trace on the PCB will cause a time delay to the signal when it passes a high-frequency signal. The main function of the serpentine trace is to compensate for the less delayed parts of the "same group of related" signal lines. These parts Usually no or less than other signals through other logic processing; the most typical is the clock line, usually it does not need to go through any other logic processing, so its delay will be less than other related signals.

The equal line length of the high-speed digital PCB board is to keep the delay difference of each signal within a range to ensure the validity of the data read by the system in the same cycle (when the delay difference exceeds one clock cycle, the next cycle will be read incorrectly. Data), generally requires that the delay difference does not exceed 1/4 clock cycle, the line delay difference per unit length is also fixed, the delay is related to the line width, line length, copper thickness, and layer structure, but the line length will increase the distributed capacitance And distributed inductance, so that the signal quality, so the clock IC pins are generally connected to RC termination, but the serpentine trace does not play the role of inductance, on the contrary, the inductance will make the higher harmonic phase shift in the rising element of the signal., Causing the signal quality to deteriorate, so the serpentine line spacing is required to be at least twice the line width.The smaller the signal rise time, the more susceptible to the distributed capacitance and distributed inductance.

Because different applications have different functions, if the serpentine trace appears in the computer board, it mainly acts as a filter inductor to improve the anti-interference ability of the circuit. The serpentine trace in the computer motherboard is mainly used in Some clock signals, such as PCIClk, AGPClk, have two functions: 1. Impedance matching 2. Filtering inductance. For some important signals, such as the HUBLink in the INTEL HUB architecture, there are 13 cables running at 233MHz. The length must be strictly equal to eliminate the hidden dangers caused by time lag. Winding is the only solution. Generally speaking, the line spacing of serpentine traces is >=2 times the line width. The snake line on the PCI board is to meet the line length requirements of the PCI 33MHzClock. If it is a distributed parameter LC filter in an ordinary PCB board, it can also be used as the inductance coil of the radio antenna, and the short and narrow serpentine trace can be used as a fuse, etc.

Supplement 1:

The most important thing to reduce the line-to-line crosstalk is to increase the line spacing. It has nothing to do with the snaking. The snaking line will bring the crosstalk problem of the wire itself. The main board of the computer has very strict timing requirements for some signals, so it must be for each signal The length is matched to meet sufficient setup and hold time. The snaking line is only related to the timing design, and has nothing to do with the integrity of the high-frequency signal. I have read many foreign books on signal integrity, as well as the Guildline of the chipset manufacturer, which did not require the designer to adopt a snake-line routing method. Of course, there will be trace length requirements, but this is only in compliance with timing specifications.

Supplement 2:

The use of serpentine lines does help to improve the stability of the motherboard and graphics card, helps eliminate the inductance caused by the long straight wiring when the current passes, and reduces the crosstalk between the lines. This is especially true at high frequencies. obvious. Of course, you can also achieve the same effect by reducing the density of wiring.

Those with conditions can observe the motherboard at hand. CPU socket-->Northbridge chip, Northbridge-->AGP slot, the back of the frequency generator, and near the memory DIMM slot, these are places where snakes are used intensively. The reason is that these are all working at high frequencies, and a stable current signal is also required.

In PROTEL, generally draw the lines by hand first, and then set all the lines to be set as a CLASS, and select Tools/Equalize net lengths.

Supplement three, when space conditions permit, the spacing of the serpentine wiring itself is best to comply with the 3W principle, and 2W is not acceptable. In short, the larger the distance, the better.

Supplement four, it is best to avoid right-angle routing for serpentine routing, preferably arc routing.