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Some of my projects need to live outside for long periods. Sometimes, moisture gets into the enclosures. That means that a circuit board might be sitting around in contact with water for hours on end at upwards of 100F.

I've noticed that solder joints can start to corrode, and recently moisture made it under the solder mask on a board and actually ate through one of the traces, breaking the circuit. I know the ideal answer is to make a perfectly sealed enclosure, but I'd like to toughen the boards up a bit as a second level of defense.

Can anyone suggest some tips and tricks for making PCBs more corrosion resistant? Does scrubbing off excess flux help? What about spray acrylic sealant? Special solder?

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    \$\begingroup\$ How about using a multi layer pcb ? You atleast prevent damage to board tracks \$\endgroup\$
    – user80825
    Commented Jul 31, 2015 at 20:17

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Sounds like your boards are in a very tough environment, but anyway, it is common practise (and an accepted industry standard) to use so-called Conformal Coating.

I know from experience at one of my previous companies that it does help.

Material varies, but the article in the above link explains a lot and internet searches for "Conformal Coating" will yield many products. I have used brushes to apply the coating after reworking PCBs, but in volume production, spray or dipping are used.

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    \$\begingroup\$ Conformal Coat will damn near protect your board from a nuclear strike. It's nasty, nasty stuff, but when I worked in industrial motor controls it was the only thing we could do for some customers. If you only need protection from moisture, there is something called a humi-seal that is much lighter, easier to apply and helps protect the system against high humidity environments. Both Conformal Coat and humi-seal make boards a real treat to try and rework/repair though, so don't count on being able to easily fix problems that come back from the field. \$\endgroup\$
    – akohlsmith
    Commented Oct 11, 2010 at 15:49
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    \$\begingroup\$ Conformal coating can actually have very very serious issues with high humidity. Most conformal coatings are permeable to water moisture. if there are even small pockets inside you can get condensation under the conformal coating. \$\endgroup\$
    – Kortuk
    Commented Oct 12, 2010 at 1:58
  • \$\begingroup\$ I am not saying I know this for every conformal coating, just most I have read about and all that I have used. \$\endgroup\$
    – Kortuk
    Commented Oct 12, 2010 at 1:59
  • \$\begingroup\$ I'm accepting this - it seems to be the most solid conventional wisdom. Having the right keyword to start searching gets me on the right path. John Lopez's tip down below is probably where I'll start, though. \$\endgroup\$
    – edebill
    Commented Oct 14, 2010 at 0:29
  • \$\begingroup\$ Note that a vacuum can be applied to remove voids. It is commonly done to transformers to lock everything in place (reduce vibratory noise.) \$\endgroup\$
    – rdtsc
    Commented Jul 31, 2015 at 21:52
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The coating zebonaut mentioned is the best solution.

Another thing to pay attention to is avoiding DC voltage gradients wherever possible.

What I mean is that when your choosing which pins to use on parts, don't choose 2 adjacent pins such that one is almost always say, +5V, and the other is always 0v (GND).

The constant voltage gradient between the two pins will drive electro-chemical reactions, corrosion and the like.

Try to separate pins when possible, use the lowest voltage possible, and as a last option if the signal levels can be modulated, even if it serves no purpose but to modulate them, such as pulsing an LED or a switch you don't really need to, do it to avoid the constant gradient.

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In re conformal coating: a garage shop I talked to once admitted that for conformally coating boards, they just dipped them in urethane deck varnish. They claimed this worked well enough that they used it for PCBs for salt-water marine installs.

I actually got them to do a dead board for me. Cosmetically it wasn't nice (ever seen an epoxy-coated board? Like that, big drips and all) but coverage was 100%. I chickened out of trusting them with the real thing though.

For a hobby project or a garage shop, I think it's worth trying. The big problem with any conformal coating is masking - keeping coating out of where it shouldn't be. Given that most coatings are transparent and thus noninspectable (my experience is that UV dyes don't homogenize and so are worse than useless), process is everything. Do it wrong and you're dead. Dipping has the advantage that you can keep connectors clear if they are all on one side. If not, you can use an acid brush to paint around them.

In summary: haven't tried it myself, but "Home Depot Brand" coating is what I'll try if I ever do my own outdoors project.

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    \$\begingroup\$ This sounds really tempting. This is all personal project stuff, and saving a little money would be really nice. \$\endgroup\$
    – edebill
    Commented Oct 14, 2010 at 0:28
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Conformal coating (something silicone-based) is the less extreme solution, but if your part is small enough (or environment harsh enough) you might consider potting it. However, when potted, boards will be extremely difficult to rework if needed, while conformal coat is somewhat easy (if annoying) to scrape off.

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I used to "paint" the ready PCB with rosin soluted in alcohol. When the alcohol evaporates, it leaves a air-sealing rosin coating on the PCB. However I don't know how it behaves in outdoor conditions maybe it melts in hot.

Alternatively there are special PCB protecting sprays, here is an overview: http://www.acc-silicones.com/products/conformal-coatings-from-acc.ashx

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  • \$\begingroup\$ I believe this is the same Rosin used as a solder flux, and consists of various organic acids (not necessarily bad), primarily abietic acid. Rosin does melt in heat. "The practical melting point varies with different specimens, some being semi-fluid at the temperature of boiling water, others melting at 100 °C to 120 °C. It is very flammable, burning with a smoky flame, ..." Its alcohol solubility and "fluxability" may make rework less expensive. The flammability and low melting point should probably be carefully evaluated before using. \$\endgroup\$
    – MicroservicesOnDDD
    Commented Sep 29, 2020 at 19:56
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I can tell you the conformal coating things are worth crap in the marine industry it has to be completely encased in potting

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We are using a technique called low pressure PCB overmolding. A plastic coating is injection molded around the PCB. The injection process forces out air pockets resulting in a very uniform and non-permeable environmental protection.

https://cavist.com/

Typical tooling for such a process is prohibitively expensive for small one-off boards. But you could do it with a 3D printer and a benchtop injection molding machine.

https://www.techkits.com/

Injection molded PCB

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