Discussion on EMI rules in high-speed PCB design

With the decrease of signal rise time and the increase of signal frequency, the EMI problem of electronic products has attracted more and more attention from electronic engineers. The success of high-speed PCB design has made its contribution to EMI more and more valued. Almost 60% of EMI problems can be controlled and solved by high-speed PCB.

Rule 1: High-speed signal routing shielding rules

As shown in the figure above:

In high-speed PCB design, key high-speed signal lines such as clocks need to be shielded. If they are not shielded or only partially shielded, EMI leakage will occur.

It is recommended to use shielded cables and drill ground holes every 1000 mils.

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

As the density of PCB boards increases, many PCB LAYOUT engineers are prone to making mistakes during the routing process, as shown in the following figure:

High-speed signal networks such as clock signals produce a closed loop when routing on a multi-layer PCB. Such a closed loop will produce a loop antenna, increasing the radiation intensity of EMI.

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

Rule 2 mentions that a closed loop of a high-speed signal will cause EMI radiation. The same open loop will also cause EMI radiation, as shown in the following figure:

High-speed signal networks such as clock signals will produce open loops when routing on multi-layer PCBs. Such open loops will generate linear antennas and increase the radiation intensity of EMI. We should also avoid this in design.

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 the radiation of EMI will increase, as shown in the following figure:

That is: the width of the wiring on the same layer must be continuous, and the routing impedance of different layers must be continuous.