PCB Blog - High-speed PCB board signal routing rules

Rule 1: high-speed signal routing shielding rules

In the design of the high-speed PCB board, key high-speed signal lines and traces such as clocks need to be shielded. If there is no shielding or only part of the shielding, it will cause EMI leakage. It is recommended that the shielded wire be grounded with holes per 1000mil.

Rule 2: Routing closed-loop rules for high-speed signals

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

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

Rule 2 mentions that the closed loop of the high-speed signal 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 is generated when routing multi-layer PCB boards, a linear antenna will be generated, increasing the radiation intensity of EMI.

Rule 4: Characteristic Impedance Continuity Rule for High Speed Signals

For high-speed signals, the continuity of characteristic impedance must be ensured when switching between layers, otherwise EMI radiation will be increased. That is to say, the width of the wiring on the same layer must be continuous, and the wiring impedance of different layers must be continuous.

Rule 5: Wiring direction rules for high-speed PCB board design

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

Rule 6: Topology Rules in High Speed PCB Design

In the design of high-speed PCB boards, the control of the characteristic impedance of the circuit board and the design of the topology structure under multiple loads directly determine the success or failure of the product. The illustration shows a daisy-chain topology, which is generally beneficial for a few Mhz. It is recommended to use the rear star symmetrical structure in the design of high-speed PCB boards.

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 1/4 of the signal wavelength, the wiring will generate resonance, and the resonance will radiate electromagnetic waves and cause interference.

Rule 8: Return Path Rules

All high-speed signals must have good return paths. Keep the return paths of high-speed signals such as clocks as small as possible. Otherwise, radiation will increase, and the magnitude of the radiation is proportional to the area enclosed by the signal and return paths.

Rule 9: Placement rules for decoupling capacitors of devices

The placement of the decoupling capacitors is very important. Unreasonable placement will not achieve the effect of decoupling at all. The principle is: close to the pin of the power supply, and the area enclosed by the power supply trace and the ground wire of the capacitor is small on PCB board.