Background Theory > PCB Design > PCB Design Guidelines for EMI and EMC > PCB routing guidelines to reduce EMU and EMC

Electromagnetic Interference (EMI) and Electromagnetic Compatibility (EMC) are critical considerations for Printed Circuit Board (PCB) design. EMI is the interference caused by electromagnetic radiation from electronic devices, and EMC is the ability of a device to function correctly within its electromagnetic environment without introducing intolerable EMI.

Proper routing of PCB traces plays a significant role in controlling EMI and ensuring EMC. Here are some routing guidelines:

Keep Paths Short

High-speed signal paths should be kept as short and direct as possible. The longer the trace, the higher the chance of it acting as an antenna and radiating or picking up interference.

Avoid 90-degree Corners

Instead of 90-degree corners, use 45-degree corners or curves. 90-degree corners have higher inductance and can cause reflections and signal integrity problems.

Maintain Trace Impedance

For high-speed signals, controlled impedance lines should be used. This requires maintaining the trace width, thickness, and distance to the ground plane consistently along the signal path.

Separation of Traces

Keep digital, power, and analog traces separate. Avoid running sensitive signal traces parallel to high-speed or noisy traces.

Avoid Board Edges

High-speed traces should be kept away from the edges of the board, where they are more likely to radiate EMI.

Respect Ground Planes

Never route high-speed signals across gaps or splits in the ground plane. This can cause the return current path to be long and circuitous, increasing inductance, radiation, and susceptibility to interference.

Reduce Use of Vias

Vias cause impedance discontinuities and should be minimized in high-speed signal paths. If vias are necessary, make sure to have nearby ground vias to provide a path for the return current.

Differential Pair Routing

When routing differential pairs, maintain the same length and parallelism for both traces to ensure signal integrity.

Terminate High-Speed Traces Properly

Terminations are necessary to prevent signal reflections, which can degrade signal integrity and cause EMI. Methods can include series termination, parallel termination, Thevenin termination, etc.

Follow Clock Signal Routing Guidelines

Clock signals are often sources of EMI due to their high switching frequencies. They should be routed directly to their destinations (no daisy-chaining), and traces should be shielded by ground planes or traces.

Remember, these are general guidelines. For specific applications, other factors may come into play, and more detailed analysis may be needed. Using EMC simulation software can help predict potential issues, but physical testing is always recommended to verify EMC compliance.