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From: Phil Hobbs
Subject: Re: Side effects of potting compound !
Date: Mon, 04 Nov 2002 11:09:29 -0500
Organization: IBM T. J. Watson Research Center
References: <email@example.com> <firstname.lastname@example.org>
NNTP-Posting-Date: 4 Nov 2002 16:09:31 GMT
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Tom Del Rosso wrote:
> "Tom Del Rosso" wrote in
> > "Tony Williams" wrote in
> > message news:email@example.com...
> > >
> > > Note; I still don't know whether the shrinkage in the
> > > epoxy puts a pressure or a vacuum on a component.
> > He can find out by putting some epoxy into a rigid container and
> > plastic wrap over the open top. The plastic will either bow in or
> Oops, I just realized that shrinkage is a given. Whether it pulls or
> pushes depends on the layout or component shape, but I think it would
> probably be vacuum like you said.
If it's uniform shrinkage, the holes want to shrink just like the solid
parts. For example, you heat an interference-fit bearing to get it to
loosen from a shaft--if the material wanted to expand into the hole, it
would tighten instead of loosen when heated. Thus it isn't at all
impossible to have shrinking glue pull away from the outer edges and
squash things inside.
The comment about the shear forces generated when a large object such as
a circuit board is potted in hard epoxy that then shrinks is also well
taken-some of the component leads are probably carrying a lot of shear
stress. That's probably most of what the silicone rubber does.
IBM T. J. Watson Research Center
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