| previous by date | index | next by date |
| previous in topic | topic list | next in topic |
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
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?