What is the role of CB board serpentine routing?

- May 22, 2018 -

Any trace on the PCB will cause a time delay for the signal in the case of passing a high frequency signal. The main function of the serpentine trace is to compensate for the less time-delayed part of the same set of related signal lines. Usually there is no or less than other signals through additional logic processing; the most typical is the clock line, usually it does not need to go through any other logic, so its delay will be smaller than other related signals.


The constant line length of the high-speed digital PCB is to keep the delay difference of each signal within a range and ensure the validity of the data read by the system in the same cycle (when the delay difference exceeds one clock cycle, it will misread the next cycle. Data), the general requirement is that the delay difference should not exceed 1/4 clock cycle, and 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 if the line length is too long, the distributed capacitance will increase. And distributed inductance, so that the signal quality, so the clock IC pin are generally connected to the RC termination, but the serpentine trace is not acting as an inductor, on the contrary, the inductor will make the signal in the rising element of the higher harmonic phase shift As a result, the signal quality deteriorates. Therefore, it is required that the serpentine line spacing is at least twice the line width. The smaller the signal rise time is, the more susceptible it is to the distributed capacitance and distributed inductance.


Because the application of different occasions have different roles, if the serpentine trace appears in the computer board, it mainly plays a role of a filter inductor, improve the circuit's anti-jamming capability, the serpentine trace in the computer motherboard,


Mainly used in some clock signals, such as CIClk, AGPClk, its role has two points:

1, impedance matching

2, filter inductors.


For some important signals, such as HUBLink in the INTEL HUB architecture, a total of 13 runs 233MHz, the requirements must be strictly equal to eliminate the hidden dangers caused by time lag, winding is the only solution. In general, the serpentine traces have a line spacing >= 2 times the line width. The snake board line on the PCI board is to meet the line length requirement of the PCI 33MHz Clock. If it is an ordinary LC board, it is an LC filter with distributed parameters. It can also be used as the inductor of a radio antenna. Short and narrow serpentine traces can be used as fuses.


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