Hi Susan and All -

Thick conformal coatings of the acrylic, urethane and epoxy persuasion have
a fairly well documented history of causing solder joint cracks, breakage of
glass/ceramic and other brittle components.  This is a product of dissimilar
expansion and fairly high surface bond strengths of the materials.
Application in the thicknesses specified in J-STD-001 are safe, but
thicknesses greater than .010" commence to be a problem.

The old techniques of using sleeving and buffer materials (re: MIL-STD-275
and others) were for the specific purpose of eliminating this problem when
using thicker coatings.

Some of the softer urethanes in the Solithane and Uralane series are less
succeptible to these problems.  The silicone resins are generally free of
the problem.

Regards - Kelly



-----Original Message-----
From: [log in to unmask] <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Friday, April 28, 2000 10:17 AM
Subject: [TN] conf coating used for potting


>What are the ramifications/advisability  of applying an acrylic conf
coating
>to a thickness of 20 mils over an assembly.  The various components have
this
>coating under as well as over the components so that it appears "potted"
>instead of coated.
>
>During temp cycling the solder joints on surface mount are popping.
>
>Susan Mansilla
>Robisan Lab
>
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