Hello Rod,

There is a characteristic of some silicone formulations (and there are a
bunch of formulations) known as 'reversion' where the material sort of
de-polymerizes and turns into a jelly-mush.  I'm no polymer chemist, but it
has to do with the catalyst type (tin,platinum,or whatever) and the
environment the silicone is exposed to.  Assuming your raw material was not
too old and the board surface was clean, I'd say you are seeing silicone
reversion, likely due to the coating chemistry.  The military has always
been sensitive to this, and their specs require coatings to withstand three
months exposure to 85C/95% RH with no degradation.  You might want to go to
one of these materials to eliminate this reversion possibility.  If you find
this exciting, get hold of the technical folks at Dow Corning or G.E.
Silicones for more reversion theory.

Ralph Vaughan
Boeing-Atlanta
770 497-5222

> ----------
> From:         Rod Lafond[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;Rod Lafond
> Sent:         Wednesday, March 03, 1999 8:24 PM
> To:   [log in to unmask]
> Subject:      [TN] Silicone Conformal Coating
>
> Technetters, I have just come across a problem that maybe someone in more
> tropical climes is familiar with.  A circuit board has been returned to us
> from a client in Florida.   This board was spray coated with M.G.
> Chemicals
> 422A Silicone Conformal Coating and packaged in a die cast housing, with
> an
> apparently leaky O-ring, and a small drain hole in the bottom and mounted
> on
> the roof of a truck. Sometime in the last four months water got into the
> housing, and with the heat, it must have been like a steam bath in there.
> The conformal coating seems to have become spongy and absorbed the
> moisture.
> Now the board appears white and under magnification the coating looks
> foamy.
> It is also soft and flakes off the board very easily. Any idea why this
> should happen? Shouldn't silicone be waterproof? What do I tell my
> customer?
>
>      Regards Rod Lafond
>      Manufacturing Engineering
>      Aimtronics Corporation
>
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