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A field PCB got returned due to not accepting a firmware update. Upon inspection, the board has a small area between some ICs and resistors that has a large buildup of dust while the rest of the board is clean.

What could cause the dust to stick to only this spot? These PCBs are manufactured and conformal coated if that helps. The dust is near a keepout region of the conformal coating, but is mostly on parts that are coated. I can't post a picture due to company rules, but the dust looks like your standard gray/brown fuzz.

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    \$\begingroup\$ Picture of it? Residual flux? \$\endgroup\$
    – winny
    Commented Jan 31, 2023 at 17:12
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    \$\begingroup\$ What kind of enclosure does the board live in? I've seen some really amazing and even artistic dust patterns from air flow. \$\endgroup\$
    – TypeIA
    Commented Jan 31, 2023 at 17:19
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    \$\begingroup\$ Is there any high voltage in that area of the PCB? High voltage will attract dust well enough that they use it for air filtration. \$\endgroup\$
    – GodJihyo
    Commented Jan 31, 2023 at 18:11
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    \$\begingroup\$ Ah now that you mention it, it looks like it is due to the enclosure. The outside is solid metal so I assumed it wasn't that, but on the inside there is a rubber seal that opens above that spot to let the wires out! This is normally then mounted onto wood or metal but there could be some airflow around the edges \$\endgroup\$
    – orange1234
    Commented Jan 31, 2023 at 18:33

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