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EMI Reduction Techniques in PCBs

Author Cadence System Analysis

Key Takeaways

  • Reducing EMI is critical, and EMI reduction techniques should be considered at the very beginning of the circuit design process. 

  • Keep traces wider to lower inductance as well as EMI.

  • The proper placement of capacitors in circuits forms low-pass filters, which can bypass high-frequency noises.

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EMI reduction is critical in sensitive circuit boards

All electronic products emit electromagnetic waves, and unfortunately, it is virtually impossible to eliminate all electromagnetic interference (EMI). However, electromagnetic interference can be mitigated by using designs and techniques that reduce EMI waves or shield circuits from interference. The limit to which conducted and radiated EMI has to be reduced is set by regulatory bodies such as the IEC or FCC. Depending on the circuit and the environment in which it is used, the stringency of the acceptable limits varies. To reduce EMI to the acceptable limit, designers should use EMI reduction techniques in PCBs, especially when designing sensitive circuits.

Basic Considerations for EMI Reduction 

There are several EMI reduction techniques in PCBs: filtering, isolation, proper layout, shielding, etc. However, before implementing any of these protective measures, there are a few things to consider:

  1. Reworking a layout to improve EMI immunity in the latter stages of a project takes a lot of time, money, and effort. That is why EMI reduction techniques should be considered from the beginning of the circuit design process, and the design itself should be made electromagnetically compatible.
  2. Active EMI sources need to be deactivated in and around the source itself. They should not be allowed to travel to other parts of the circuit. Redirecting EMI from sources to the ground is a good starting method for EMI reduction. 
  3. Sensitive and critical circuits should be protected from active EMI sources and other EMI disturbances. Sensitive circuits should be placed farthest from EMI sources on the PCB. 

EMI Reduction Techniques in PCBs

Modify the Layout

  • Segregate PCB board sections according to their functions. Keep the control circuit away from the power circuit, the analog circuit away from the digital circuit, and the low-frequency sections away from the high-frequency sections. 
  • Use filters wherever EMI susceptibility needs to be reduced. When placing the filter, keep it closer to the circuit to protect it from EMI. 
  • Reduce parts that are susceptible to EMI. Make such circuits separate to increase EMC in the board.

Consider Routing and Traces

  • Keep the signal traces as short as possible.
  • Keep the trace wider to lower inductance as well as EMI.
  • Replace right-angled bends with 45 degree rounded or curved bends.
  • Reduce the number of vias.
  • Reduce the loop area between the trace pairs.
  • Isolate the analog and digital circuits by ground.

Reduce Common Mode Interference

  • Use FR4 as PCB substrate material and provide up to 6 mil spacing between PCB layers.
  • Include inductors of low value in the power plane. 

Electromagnetic Interference Shielding

  • Enclose sensitive circuits using EMI shields.
  • Apply EMI suppression coating to enclosures or components to reflect or absorb EMI.

Use EMI Reduction Components

The EMI in circuit boards can be reduced using EMI reduction components such as:

  1. Varistors - Varistors behave similarly to Zener diodes, protecting circuits from transients.
  2. Suppressor diodes - Excellent at limiting fast transients of low voltages.
  3. Ferrites - Structures of ferrous material capable of attenuating fast voltage spikes and high-frequency transients.
  4. Capacitors - Properly placing capacitors in circuits forms low-pass filters, which can bypass high-frequency noises.
  5. Series resistors - Low-cost EMI suppression is possible by properly placing series resistors.

By following EMI reduction techniques in PCBs, you can design electronics that are highly immune to EMI. Cadence’s suite of PCB design and analysis tools can help in designing circuits that are electromagnetically compatible.

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