From: email@example.com (Bob Wilson)
Subject: Re: Potting/Encapsulating - Epoxy ontop of Silicone?
Date: Mon, 11 Nov 2002 04:51:30 -0000
Organization: Your Organization
X-Newsreader: WinVN 0.99.9 (Released Version) (x86 32bit)
In article ,
>I'm thinking of encapsulating a small circuit board with Silicone
>(Neutral cure) because it has surface mount parts. But I wondered if
>I could then encapsulate that assembly with epoxy to give me a hard
>outer shell with a gooey centre ;). Has anyone tried this? Any tips?
> I wondered about how well the epoxy would bond with the outer surface
>of the silicone, if at all, and under temperature cycles if the
>silicone would 'peel' off the inside surface of the epoxy shell.
This doesn't work like you think it does. The silicone is soft, only because
it can be displaced when you squeeze it. Just like Jello, you apply pressure
at one point, and it bulges out somewhere else. But like most materials,
silicone is essentiall incompressible. If it is completely encapsulated in
epoxy, it has "nowhere to go" when pressure is applied.
The epoxy will obviously not bond with silicone. Essentially NOTHING will.
That is why silicone is used as mould release in plastic moulding. Coating
with silicone, followed by potting, will nto make things worse, but don't
expect it to work the way you are thinking.
By the way, as was explained in a recent series of posts, there is no reason
to spend 3 times as much money on neutral (i.e. alcohol) cure RTV. Ordinary
acetic acid cure RTV will not damage your circuit in any way, providing you
allow plenty of time for it ti completely cure before potting in epoxy
(which you MUST do no matter WNAT type of RTV you use). Without repeating
the last week's string of post's on this subject, Acetic acid is not an
"acid" until it is mixed with water, and further, it is only gerentaed by
the curing reaction. Once the cure is complete, the stuff is gone.