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I'm considering putting a circuit board in a ventilated NEMA 4 enclosure outdoors. I'm a little concerned about humidity and condensation inside the enclosure. Do I need to conformal coat my circuit boards, or take any other precautions?

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Any time you have the possibility of condensation forming on your board, you need to take precautions. It may not be that you would get a short, but corrosion instead. I'm not a huge fan of conformal coating unless I am sure I will never need to work on that board again. I'd rather go for a separate, water tight enclosure just for the board, so long as it's not going to cause a cost, space, or thermal issue. That being said, coating it will probably be easier and safer.

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    \$\begingroup\$ Also, outdoors ventilated enclosures get bugs. \$\endgroup\$ Oct 8, 2015 at 19:38
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There are many types of conformal coating out there to deal with parts with or without substantial thermal expansion, allowing easy rework, viscosity, etc. Conformal coating can be silicon-based, polymer-based or even oil-based. You have to research much more to select the right product. That being said we use polymer-based coating from Kontakt or Peters in Europe with excellent results. We use lots of very small SMT parts and need coating to run under parts before setting. So this works for us, keeping insects, water, condensation, etc away. I would never use uncoated boards outside, We have seen at coastal regions even with IP68 boxes, just with the salt in the air that is trapped in the box before closing it, vias corroded through in weeks. The polymer-coating can either be soldered through as if there is no coating, or you can use pain stripper (for sure) for 5min to remove coating and then re-coat it afterwards. Otherwise you can buy a stripper for your coating for reworking. It all depends on whether you have fine pitch SMT, or 5kA contactors you are trying to protect.

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