Homebrew PCBs

Alan,

There are two issues here. The first is oxidation of the copper, and the 
second is operation out of doors. If your only issue is oxidation of 
copper, then don't worry about it. It looks ugly, but as far as I know, 
it is self limiting.

A larger problem, and the reason why military and automotive hardware 
uses conformal coat on their boards is condensation.  In some ways, the 
better the ventilation in the box, the less of an issue this is. Small 
drain holes on the bottom of the box would be a good idea, unless the 
box is subjected to spray.  Condensation on your circuit board will 
cause upset of circuit operation in many cases.  Certainly, in analog 
circuits, the unexpected conductive paths caused by condensation will 
alter performance, possibly leading to catastrophic failure.  Digital 
circuits are less sensitive, but will ultimately be affected, 
particularly if the signals are very low level.

Conformal coats come in a variety of types, Acrylic, Polyurethane and 
Silicones, to mention the most popular types.

Krylon is an acrylic material. I have not used it because I worry about 
how long it will adhere to the board under the outdoor conditions. I 
have used Polyurethane successfully.  It was not a material designed for 
use as a conformal coat, as they are very expensive, and hard to get in 
small quantities. I used a spray material that is suitable for touch up 
on furniture finishes. At this point, I can't remember the name. 
Silicones fall into an entirely different category. They are more 
difficult to use. They usually need a primer, which is another step in 
the process.

In terms of repairability, the acrylics have it over the others. They 
are easily chemically removed, and another spray after repair finishes 
the job. Polyurethane is a killer to remove. Chances of doing damage are 
very high.  The method of removal is physical, with a good chance to 
damage surrounding circuit elements. Silicones can be removed by cutting 
and scraping. Don't be tempted to use the silicone material they sell at 
your local home supply store.  Silicone sealers, not made for the 
specific purpose of contact with electronic circuits use an acetic acid 
cure. This is why they smell like vinegar.  These sealers will 
eventually attack the copper on your circuit board.

All that said, for me, I would skip the Krylon and go with the 
Polyurethane furniture spray. A word of caution: Keep your coating thin 
and if your circuit board uses glass diodes, make sure they are lifted a 
bit off the board, so that the coating does not stick the part to the 
board. I have seen diodes crack because the coefficient of thermal 
expansion of the board is much greater than that  of the glass component.

Harvey


On 7/30/2016 8:34 AM, alan00463@... [Homebrew_PCBs] wrote:
>
> In searching for a vendor of Liquid-Tin I ran across
> this waterproofing spray:
> http://bamtech.com/product/acrylic-coating/
>
> Since I live in the RustBelt and know what oxidized
> copper looks like, I wonder if I should use this on
> my PCB to keep the copper from contacting moist air?
>
> My PCB will be used outside in a plastic box.   The
> plastic box will be waterproof, but not airtight.
> I am guessing the Liquid Tin and leaded solder  ap-
> plied to the traces will prevent oxidation.   I just
> want to know if anybody has had experience with this
> product or, if my thinking is wrong.
>
> Has anybody used this product on a copper PCB used
> outdoors?  If yes, would you recommend it?   Or
> would something cheaper like this
> http://www.sherwin-williams.com/home-builders/products/catalog/krylon-indooroutdoor-lacquer-spray-paint/?referringCategory=interior-paint-coatings/aerosols/&N=1568189400
> work just as well?
>
>