On a high-frequency PCB, any trace can introduce signal delay. The primary purpose of a serpentine trace is to balance the timing differences between signals that belong to the same group or are related. These signals often have less logic processing compared to others, and the most common example is the clock line. Since clock signals typically don’t go through complex logic, their delay is usually shorter than other related signals.
In high-speed digital PCB design, equal length traces are essential to ensure that the delay difference between signals stays within a controlled range. This helps maintain data integrity during the same clock cycle. If the delay difference exceeds one clock cycle, it could lead to misreading in the next cycle. To avoid this, the maximum allowable delay difference is generally limited to 1/4 of a clock cycle. The delay per unit length is consistent, but it depends on factors like line width, copper thickness, and layer structure. However, longer traces increase distributed capacitance and inductance, which can degrade signal quality. For this reason, clock IC pins are often connected with RC termination, while serpentine traces do not act as inductors. Instead, inductors can cause phase shifts in high-frequency signals, further reducing signal integrity. Therefore, the minimum spacing between serpentine lines should be at least twice the line width. Signals with faster rise times are more sensitive to these effects.
Depending on the application, serpentine traces may serve different purposes. On computer boards, they often act as inductors to improve noise immunity. In motherboards, they are commonly used for clock signals like CIClk and AGPClk, providing two main functions: impedance matching and filtering. For critical signals such as HUBLink in Intel’s HUB architecture, which operates at 233MHz and has 13 lanes, strict equal-length requirements are necessary to eliminate timing-related issues. Serpentine routing is often the only solution. In general, the spacing between serpentine lines should be at least twice the line width. On PCI boards, serpentine traces are designed to meet the length requirements of the 33MHz clock signal. In standard PCBs, they can function as LC filters with distributed parameters or even as inductors for radio antennas. Short and narrow serpentine traces can also be used as fuses in certain applications.
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