I'll weigh in on this issue since I've been wrestling with it quite a bit
in recent weeks.

The DC3140 silicone needs a primer to get the full adhesion on the data
sheets.  It is a self leveling material so expect it to flow away from
where you put it.  The DC3145 material is thicker (stays put) and needs no
primer.  Both of these materials evolve alcohol when they cure, as opposed
to acetic acid.  The silicones that evolve acetic acid are not suitable for
electronics.

Both of these materials are moisture cure materials.  The thicker the
material, the longer it takes to cure.  It also cures from the outside in,
so the surface skins over first.  Cured silicone resists moisture flowing
through it, so it takes progressively longer to cure the farthest portions
of the silicone mass.  For thicknesses in the 0-0.2" range, figure on 24
hours of ambient cure time at a minimum.  You can accelerated the cure
using a humidity chamber, but you can sometimes get bubbles forming in the
silicone mass when the alcohol cannot get through the skinned over surface.
Consequently, you have a silicone mass that has lower tensile strength and
lower dielectric properties.  We used these materials on some of our RF
components and the bubbles in the silicone lead to arcing and corona in
high power applications.

Because of the long cure times, we are currently investigating thermally
cured epoxy and urethane materials that cure in 30 - 60 minutes and which
don't have the incompatibilities with acrylic conformal coat that silicones
do.

Doug Pauls
Rockwell Collins

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