PCB Tech - Nine high-speed PCB board signal routing rules

Rule 1 High-speed signal wiring shielding rule

In high-speed PCB board design, key high-speed signal lines such as clocks need to be shielded. If they are not shielded or only partially shielded, they will cause EMI leakage. It is recommended that the shielded wire be grounded with a hole per 1000 mil.

Rule 2: Closed-loop routing rules for high-speed signals

Due to the increasing density of PCB boards, many PCB LAYOUT engineers are prone to a mistake in the process of routing, that is, high-speed signal networks such as clock signals, which produce closed-loop results when routing multi-layer PCBs. As a result of such a closed loop, a loop antenna will be produced, which will increase the radiated intensity of EMI.

Rule 3 Open-loop routing rules for high-speed signals

Rule 2 mentions that the closed loop of high-speed signals will cause EMI radiation, but the open loop will also cause EMI radiation. For high-speed signal networks such as clock signals, once an open-loop result occurs when a multilayer PCB is routed, a linear antenna will be generated, which will increase the EMI radiation intensity.

Rule 4 Characteristic impedance continuity rule of high-speed signal

For high-speed signals, the characteristic impedance must be continuity when switching between layers, otherwise it will increase EMI radiation. In other words, the width of the wiring of the same layer must be continuous, and the impedance of the wiring of different layers must be continuous.

Rule 5 Wiring direction rules for high-speed PCB design

The wiring between two adjacent layers must follow the principle of vertical wiring, otherwise it will cause crosstalk between the lines and increase EMI radiation. In short, adjacent wiring layers follow the horizontal and vertical wiring directions, and vertical wiring can suppress crosstalk between lines.

Rule 6 Topological structure rules in high-speed PCB design

In high-speed PCB design, the control of the characteristic impedance of the circuit board and the design of the topological structure under multi-load conditions directly determine the success or failure of the product. The figure shows a daisy-chain topology, which is generally beneficial in the case of a few Mhz. It is recommended to use a star-shaped symmetrical structure on the back end in high-speed PCB design.

Rule 7: Resonance rule for trace length

Check whether the length of the signal line and the frequency of the signal constitute resonance, that is, when the length of the wiring is an integer multiple of the signal wavelength 1/4, the wiring will resonate, and the resonance will radiate electromagnetic waves and cause interference.

Rule 8 Return path rules

All high-speed signals must have a good return path. As much as possible to ensure that the return path of high-speed signals such as clocks is minimized. Otherwise it will greatly increase the radiation, and the size of the radiation is proportional to the area enclosed by the signal path and the return path.

Rule 9 Decoupling capacitor placement rules for devices

The placement of the decoupling capacitor is very important. Unreasonable placement will not have the effect of decoupling at all. The principle is: close to the pins of the power supply, and the area enclosed by the power traces and ground wires of the capacitor is the smallest.