Nine rules for high-speed PCB signal routing

- Dec 30, 2019-

Nine rules for high-speed PCB signal routing

Rule 1: Shielding rules for high-speed signal traces


In high-speed PCB design, the clock and other key high-speed signal lines need to be shielded. If there is no shielding or only a part of the shielding, it will cause EMI leakage. It is recommended that the shielded wire be punched to ground every 1000mil.


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 during the routing process, that is, high-speed signal networks such as clock signals, which produce closed-loop results when multilayer PCB routing As a result, such a closed loop will produce a loop antenna and increase the EMI radiation intensity.

Rule 3: Open-loop 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.

High-speed signal networks such as clock signals, once an open-loop result occurs when multilayer PCBs are routed, a linear antenna will be generated, which will increase the EMI radiation intensity.


Rule 4: Continuous Impedance Rules for High-Speed Signals


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


Rule 5: Rules for Wiring Direction of High-Speed PCB Design


 The traces between two adjacent layers must follow the principle of vertical traces, otherwise crosstalk between the lines will be caused and EMI radiation will be increased.


In short, adjacent wiring layers follow horizontal and vertical wiring directions, and vertical wiring can suppress crosstalk between lines.



Rule 6: Topological rules in high-speed PCB design

In high-speed PCB design, the control of the circuit board's characteristic impedance and the design of the topology under multiple 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-symmetric structure at the back end in high-speed PCB design.



Rule 7: Resonance Rule of Trace Length


Check whether the length of the signal line and the frequency of the signal constitute resonance, that is, when the wiring length is an integer multiple of the signal wavelength, this 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 a good return path. Minimize the return path of high-speed signals such as clocks as much as possible. Otherwise, the radiation will be greatly increased, and the size of the radiation is proportional to the area enclosed by the signal path and the return path.


Rule 9: Rules for placing decoupling capacitors


The position of the decoupling capacitor is very important. Improper placement does not have the effect of decoupling at all. The principle is: close to the pins of the power supply, and the area surrounded by the power supply traces and ground of the capacitor is the smallest.


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