Re: gEDA-user: pcb anti-environment coating?

[Bill Gatliff <bgat@xxxxxxxxxxxxxxx>]

DJ Delorie wrote:
> What's a good thing to coat my built boards with, to protect them from
> the environment?  I've got a temperature/humidity sensor outdoors on a
> small pcb, but I just got done cleaning some corrosion off it.  Epoxy?
> Spray paint?

The product term I've heard is called "conformal coating".  Don't know 
precisely what the fundamental stuff is, but there are lots of 
different kinds based on what you need protection from.

As I understand things, you have to be careful what you use because 
some potentially useful materials don't expand/contract with the board 
and components, so over time you get cracks, the coating pulls the 
components off the board, and so forth.  Others are electrically 
conductive, corrosive, etc.

The material selection it isn't intuitive, unless you're a chem-e. 
Things like dissimilar metals being in contact with each other, and 
other blah blah that I hear makes me want to leave the problem to 
someone else.  :)

I am aware of a few situations where they put the board into an 
enclosure and then flushed the inside with nitrogen before sealing it 
up (have a valve on either end of the box, not unlike what you might 
see on an air-inflated bicycle tire).  That's a pretty safe bet, works 
particularly well in very cold temperatures at least until you lose 
the seals on the box.  And unless you maintain a positive pressure 
over temperature, Schrader-type valves will actually promote moisture 
intrusion as the environment warms back up.

If you have a board or two to spare, I've always wanted to "brick" 
something in the clear epoxy you often find at arts/crafts stores for 
preserving insects, stones, and such.  That would definitely keep 
stuff out, especially if you put a pliable grommet around the wires 
and/or the goo would bond to the insulation.  Let me know how it turns 
out.  :)  :)

As long as we're experimenting with disposable hardware, you might 
consider silicone bathroom caulk, too.  Petroleum jelly?  I've seen 
the former used in a pool installation, but there was no expectation 
that the thing would live because of all the chlorine (which is 
extremely corrosive to metals, not just lungs).  Don't know if the 
latter would dissolve packages, or not.

I once asked an auto mechanic about putting distilled water into a 
radiator (we live in an area with very hard water).  He said that as 
long as you aren't continuously adding water, the minerals themselves 
aren't a concern--- they build up, then redissolve and move on.  Based 
on that logic, maybe just putting the board into an enclosure that 
does a reasonably good job of not facilitating air movement would be 
good enough--- if you were careful not to seal it up on a really humid 
day!


b.g.
-- 
Bill Gatliff
bgat@xxxxxxxxxxxxxxx


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