In a message dated 98-01-28 07:48:22 EST, you write:
>
> I'm working for a big repair lab of cellular phones. For a long time we
> experience returning of many cellular phones due to corrosion, specially
> after repairing.
I would ask about the chemicals you are using to strip the conformal coating,
what cored wire solder are you using, are you doing spot cleaning after hand
soldering and if so, what are you using?.
If you are seeing corrosion on field returns before repair operations, then
there are high levels of halides on the phones from the fabrication or
assembly process, or possibly from the end-use environment. If you see more
corrosion on returns after you repair them, then you are using a high-halide
cored wire solder or are adding a corrosive element in the rework process. If
you can give me a better idea of the materials and processes used, I can give
you a better answer.
> I thought about using a conformal coating to protect the fresh solder
> from humidity and moisture.
In my opinion, this is only a band-aid that may buy you a little time, but not
solve the problem. Many people erroneously believe that conformal coating
will protect against corrosion and other moisture induced failures. Not so.
All organic polymers are water permeable to some degree. You may delay the
onset of corrosion, but not eliminate it.
In many cases, when one of our clients have assemblies diagnosed with high
levels of halides, the first response is to conformally coat, rather than
attack the problem at its source. This seldom works and we counsel against
it.
> Does anyone has got an experience in this kind of work and could
> recommend easily removable coating (we do not seek solvent protection),
> the application method and curing mode (since there are expected many
> shaded points, UV or any radiation curing does not seem feasible).
As far as solvent resistance goes, any acrylic (type AR) would be better than
any of the other coating types (urethane, epoxy, silicone, paralene). More
solvents will remove acrylics and the other coatings are more solvent
resistant. Graham Naisbitt of Concoat, a contributor to TechNet, has more
information on coatings and solvent resistance.
Hope this helps.
Doug Pauls
Technical Director
CSL
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