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

[Bob Paddock <bob.paddock@xxxxxxxxx>]

On Wednesday 31 December 2008 07:38:36 pm John Griessen wrote:
> Bob Paddock wrote:
> >    A very common misconception is that Conformal Coating is a Hermetic
> >    Seal.
>
> >    You never notice it in a low impedance digital circuit, but unless
> >    debugging is an obsession don't let it get near a RF tuning circuit or
> >    a high impedance circuit.
>
> So, when your sensor network RF node in a rainproof, but sometimes high
> humidity box corrodes, what do you do?

If it has already corroded your out of luck.

 My context with Conformal Coating is Coal Mine equipment.  It was getting 
hosed down with a high pressure house to clean off the box, then that water
would get to the circuit board via wiking into the RF antenna jack. 
Even the Stainless Steel box corroded in this environment.  After about six 
months the receiver would fail, it would be sent back with a failure report
of being off frequency, but when it got back to us it worked fine.
After a long bit of hair pulling trying to figure out what was going,
we discovered in the four week trip from the Mine to us the unit
dried out enough that it was back on frequency.  When someone
finally went to the mine and carried one back over night the water
as obvious when we pealed the coating off.  We ultimately went to a welded box 
with a gasket to solve that problem.

In data sheets for some ultra high impedance OpAmps, Electrostatic Class like 
the AD515 or AD549, it tells you to leave the traces exposed to air.  The
coating issue is why.

Not directly related to coating but I once took a shower from 7/8" Hard Line
on the second floor when I took the antenna connector off some equipment.
Several gallons of water had worked its way into the Hard Line and wicked
its way up the cable from the ground floor.  Water can work its way into boxes 
in ways that you'd never expect.

> How is polyurethane transformer insulation spray paint for that?  What else?

You have to be careful that the different chemicals don't interact, I have 
insufficient experiences in this area to comment, beyond as was already 
pointed out acidic compounds around electronics are *bad*.
We used a lot of Black RTV which is rated for electronic use.

Conformal Coating is good to use, as long as you understand that
it is not hermetic; you don't want to expect the water to lay on the coating. 

For outside application I'd coat the boards, mount
them so that water could not lay on them by putting them vertically.
I'd put them in a sealed box, with a Gore-Tex Vent so that the enclosure
can 'breath' but not pass water.  Heaters and electronic rated
desiccants might also have to be considered depending on the 
application/location.


-- 
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