Interesting Question about plastic enclosures and EMC

Interesting Question about plastic enclosures and EMC image #1

28 Mar 2022

My recent discussion with Robert Ferenec about designing PCBs, especially with regard to EMC, see it here, resulted in lots of questions!

One of the more interesting was:

'What if the chassis is not metallic (e.g plastic). Do we need to take care of this structure the same way as pcb to metallic bonding e.g resonance/damping'

My Answer

If your chassis or other enclosure is made entirely from plain, non-conducting plastic, then it cannot create an electromagnetically-resonating cavity with the copper plane (or planes) in your PCB.

In this case, you don’t need to provide RF-bonds between the plastic enclosure and the PCB’s reference plane.

 However, if products that are made with plain, non-conducting plastic chassis or enclosures fail to pass EMC tests, or cause EMI problems in real life, it can sometimes be a ‘quick fix’ to stick metal foil all over the inside of their plain plastic chassis or enclosure, and multi-point RF-bond that metal foil to the PCB’s reference plane as if it was a metal chassis like I described in Robert’s video.

Suitable metal foils can be supplied by many manufacturers of shielding materials, such as:

To avoid the delays of re-spinning the PCB to fit the multi-point RF chassis bonds to the PCB, and perhaps finding that this is not sufficient as a fix on its own, I recommend making provision for these Reference Plane to Chassis RF-bonds when the PCB is first laid out – because it is much easier and quicker to add them at this stage of the project, than later on when failing EMC tests.

My video with Robert was under 2 hours long, but we were discussing a training course that takes a full day so a lot of material was not covered. The answer to your question is described in the full course, entitled: “Essential PCB design/layout techniques for good EMC”, which you can download here.

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