[sdiy] pcb rework and conformal coat

[Tony Clark]

> Since we're talking PCB's, I've got two questions:
> What's a good conformal coating (clear coat)
> and how can I remove conformal coat in order to solder on a purchaced module?

   Cheap and good conformal coating:

   Clear Polyurethane paint

   We've been using this stuff for years and it works VERY well.

   As for removing conformal coatings, are you referring to an existing 
coating, or what you'd use to remove the polyurethane?  If it's the poly, 
you can actually just use a soldering iron to burn it off, kinda stinks, 
but it can be done.  Otherwise, a small amount of some form of thinnner 
would probably work.
   Now if you're talking about industrial conformal coating, a really 
thick and waxy substance, then you're talking about RTV.  As I recall, 
Xylene thins that stuff down.

   As for why you'd need to use conformal coating, typically you don't.  
The only reason that I can think of is the same reason why we use it here 
at work:  humidity

   The circuits we build are very high-gain, and just breathing on the 
circuits causes gain errors!  So a light coat of the polyurethane seals 
everything up and keeps the circuits behaving like they should.
   At Square D where I used to work, they used RTV to seal up boards.  
These were medical-use smart circuit protection units and needed to be 
environmentally rugged.  Nasty stuff, we'd get out a huge bucket, fill a 
squeeze bottle with the RTV, and then just slather it on until every nook 
and cranny was well gooped.  What dripped into the bucket got reused 
until the whole thing was too thick to get out of the bottle.  :)
   Needless to say, RTV didn't go on those boards until they went through 
thermal shock, 24-hour burn-in, and at least 3 testing cycles!  You 
definately didn't want to repair these things after they got sealed up!

   Cheers,

   Tony

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