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Exposed GND ring around edge of PCB
I/Os on the mezzanine connector
Switch the mezzanine receptacle and plug? When not connected, the controller board has a strip of exposed contacts, so to reduce the chance of ESD from touching the controller boards, maybe the shrouded plug should be on the controller boards instead of the carrier boards.
Series resistor arrays? Probably the cheapest option, but the cutoff frequency will need to be higher than the highest frequency signals (probably SPI@18Mbit/s).
TVS diodes? More effective than resistors, but will be more expensive and require a LOT of board space.
These connectors might be convenient for programming (although a bit expensive): https://www.tag-connect.com/product/tc2030-idc-6-pin-tag-connect-plug-of-nails-spring-pin-cable-with-legs
What has been researched so far:
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