You did it! You designed some fantastic hardware, you passed all your EMC testing, and you got it launched. It’s been out in the field for some time now, and it’s time to make the next generation. Here’s the question: when do you need to re-test it for EMC?
This isn’t addressing the regulatory aspects of when you might be required to re-test a design. Instead, I’m addressing the technical standpoint: what technical changes might impact the EMC performance of a unit. I’ve had several conversations in my career that went something like this:
Manager: “We’re using legacy hardware for this unit, so we don’t need to redo EMC testing.”
Me: “Great! So the [conductive] housing has the same dimensions as the original?”
Manager: “No, we had to change the housing to fit the new installation.”
Me: “Well, the PCB layout and stackup are identical, right?”
Manager: “No, of course we had to change the layout when we changed the enclosure. We also took advantage of market changes to source some new components as well.”
Me: “I’ve got bad news for you… You should redo a lot of EMC testing.”
Manager: Glares because they didn’t budget for that
The customer likely thought that, since nothing about the schematic design changed, they could skip any new testing. But schematics tell only a fraction of the overall EMC story. Let’s talk about some of the most common changes that might occur in a hardware revision (without changing schematics) and what testing those might involve. This list is far from exhaustive!
- Installation. If you’re taking the same unit and installing it in a new platform, you should consider re-evaluating radiated emissions and radiated immunity/susceptibility testing. For instance, it was in the wing area of one aircraft, and now it’s going into the cargo area of a new aircraft. Are the assumptions made to set the radiated environmental testing conditions still valid? Are there new victims or threats that the original testing may not have considered?
- Critical component sources, software, and manufacturing location. You may need to redo most immunity testing, including ESD. New parts, or new factories with different processes, or new software revisions can all have a significant impact on how hardware tolerates or recovers from different transients or interruptions.
- PCB layout, stackup, conductive housing dimensions, connectors, pin assignments, interconnecting cabling, shielding, and shield terminations. Consider redoing all radiated tests (including bulk current injection, which is often a radiated immunity test in disguise). Changes in any of these aspects can drastically change the radiated performance of a unit. Small, well-contained loops in one layout could become large radiators in another. Different housing designs can have apertures that radiate at different frequencies. Changes to pin assignments can change crosstalk pathways. Changes to shield terminations can make the difference between a solid shield and an accidental antenna that looks like a shield.
We all want to avoid unnecessary testing. But skipping re-testing after you’ve made hardware/software changes that have EMC implications, even if the schematic is the same, can lead to missing problems that might lead to failures or recalls. Keep an eye on new hardware revisions to stay on top of everything that can impact EMC success or failure.
