I most often make circuits on perfboard. Below is an example of a circuit, both bottom and top sides:

top of perfboard

bottom of perfboard

This circuit will be in a case but outdoors for many months. In case of some rain leakage or humidity or rust ect, I want to protect this circuit by isolating it.

I saw some people use an epoxy like thing in both sides. I couldn't find any detailed information in my search.

I'm totally ignorant about this. What kind of material should I use and should I pour and cover it both sides?

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    \$\begingroup\$ "Conformal coating" \$\endgroup\$ – Bort Sep 12 '17 at 11:59
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    \$\begingroup\$ @PlasmaHH Besides convincing people, I have the problem of time for that milling PCB. That takes ages man to send to a PCB company ect ect. But you are right thats the best way unfortunately I have to go for perfboards \$\endgroup\$ – user1245 Sep 12 '17 at 12:01
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    \$\begingroup\$ A project box with a drain hole in the bottom will take care of most water related issues. Making a box fully watertight will just make it wick in water though any pinholes you couldn't seal off properly. \$\endgroup\$ – ratchet freak Sep 12 '17 at 12:09
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    \$\begingroup\$ When using sealants such as silicone, only use electronics grade ones, not general construction because they give off acetic acid when they cure which eats copper. \$\endgroup\$ – Martin Sep 12 '17 at 12:10
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    \$\begingroup\$ Doing a "proper" job with a PCB and SMDs has a considerably greater opportunity cost. Perf/stripboard and through hole components is perfectly fine for many projects that are one-offs. \$\endgroup\$ – Ian Bland Sep 12 '17 at 12:34

As in the comments the most common solution is conformal coating.

This is similar to a varnish, they can be dipped or sprayed on and are intended to form a waterproof seal without pinholes so they are often formulated with very little solvent to minimise final porosity. They may also contain silicone which is hydrophobic in nature and further repels water from breaks and edges of the sealant. Some of these require a baking step to work as advetised.

Your alternatives are a non-acid cure silicone sealant, these types are available in automotive and electrical grades. The household and construction grades most commonly make use of acetic acid which is corrosive.

You can also use a potting compound, again there are special types for potting electronics that may be polyester resins, epoxy resins or even silicone elastomers. Some products can have catalysts that are corrosive in nature and these should be avoided.

In some situations you could just use a moisture barrier that is not a solid such as grease/vaseline or even submerge your circuit in oil. These are messy but oil is commonly used in industrial and x-ray transformers due to the self healing nature of its insulation characteristics from high voltage breakdown.

An alternative in your case as you mention a specific time that the unit will be deployed in the field is to use a moisture scavenger/desccicant. If you seal your case from direct water entry and there are only a few small holes that air can enter you could keep the air inside very dry with something like silica-gel, anhydrous calcium chloride or a molecular sieve material. These will all eventually saturate but if the desccicant is large in proportion to the air exchange it should easily last for months outside of a steam bath or car wash.

Using strip board as opposed to a production PCB does somewhat increase your problem as the substrate material is usually the phenolic paper laminate instead of the less water absorbing epoxy glass laminate. However once you use one of the moisture mitigation technologies there is no real difference.

My first choice would be to seal the box lid and openings with electrical silicon sealant and leave a generous desccicant pack inside.

My second choice would be to dip it into an electrical conformal coating compound if you can locate it.

Your choices are also affected by how much or how often you expect to have to service the circuitry.

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    \$\begingroup\$ Thanks for the answer. I found this one tme.eu/en/details/dca-200ml/protection-screen-coatings/… Would this be adequate? \$\endgroup\$ – user1245 Sep 12 '17 at 12:55
  • \$\begingroup\$ That looks like a nice product, when examining the datashet you can see that heat curing after drying makes it stand up to solvents and elevated temperatures, if you don't expect those then you just need to let it air dry, it warns not to shake can before spraying and other useful information. - tme.eu/en/Document/6fd5499f49fc7bf1fd1cab8d31bea983/… \$\endgroup\$ – KalleMP Sep 12 '17 at 13:05
  • \$\begingroup\$ Thanks a lot you know a lot about the subject. I will use it as spray. I should only apply to the solder/bottom side where the strips are right? Nothing to the top? \$\endgroup\$ – user1245 Sep 12 '17 at 13:07
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    \$\begingroup\$ Absolutely do not use conformal coat on socketed parts. There is always the possiblity that capillary action will wick CC down into the socket and insulate the pin from the socket. \$\endgroup\$ – WhatRoughBeast Sep 12 '17 at 14:55
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    \$\begingroup\$ @user134429 - Yes. \$\endgroup\$ – WhatRoughBeast Sep 13 '17 at 17:51

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