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March 1999

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Subject:
From:
Mike Barmuta <[log in to unmask]>
Reply To:
Date:
Thu, 4 Mar 1999 11:20:43 PST
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TEXT/PLAIN
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TEXT/PLAIN (75 lines)
        Rod: It sounds like you have a classic case of polymer reversion. When a
polymer, in this case silicone is subjected to heat and moisture over time it
can "revert". That is it starts to change back to it's origanal state of a
liquid. The exposure to heat and moisture begins breaking down the polymeric
structure and bonds of the coating turning it soft and causing it to loose
physical and electrical properties. A materials ability to withstand this
degradation is called reversion resistance or hydrolytic stability. Tests to
evaluate a materials reversion resistance are usually done using an autoclave
and/or a pressure cooker using heat/pressure/moisture/time.
 Typically thinner or low solids coatings are more permeable to moisture and can
revert faster than thicker ones of the same polymer composition. From your
posting it sounds like you are using a one component system. You may want to try
multiple coatings to build up the thickness also make sure the product is still
within it's shelflife. Although not as user friendly, a two component silicone
could possibly provide better resistance.
You may wnat to consider other types of conformal coatings such as paralyne,
urethane, acrylic and epoxy to meet your environmental exposure needs.

                                                Regards
                                                        Michael Barmuta
                                                        Staff Engineer
                                                        Fluke Corp.
                                                        Everett Wa.
                                                        425-356-6076

On Wed, 3 Mar 1999 17:24:47 -0800 Rod Lafond wrote:

> From: Rod Lafond <[log in to unmask]>
> Date: Wed, 3 Mar 1999 17:24:47 -0800
> Subject: [TN] Silicone Conformal Coating
> To: [log in to unmask]
>
> 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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