From: firstname.lastname@example.org (Jumbaliah)
Subject: Re: Potting/Encapsulating - Epoxy ontop of Silicone?
Date: 12 Nov 2002 10:00:16 -0800
NNTP-Posting-Date: 12 Nov 2002 18:00:16 GMT
email@example.com (Bob Wilson) wrote in message news:...
> In article ,
> firstname.lastname@example.org says...
> >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.
How about this? If the first mould used for silicone created
impressions (say 1cm dia by 5mm or more in depth) on one side of the
'cube' , one could place the siliconed part in a second mould for the
epoxy coating with the impressions face down. While the epoxy is
poured into the mould it would trap air in the impression and leave a
place for the silicone to expand/deform.
> 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.
Thanks for the tip, I do like to save money. How long would the
silicone take to cure? A few days? I guess it depends on the depth I
use. I see that your email is '.ca', I'm in Ontario. Do you know of
something I could get at a Canadian Tire or Home Depot that would
work? I checked out Canadian Tire and only found 1part silicone for
the usual stuff like Kitchen/bath, windows, automotive. They were all
in 300ml tubes and I imagine they don't flow very well.
I also was looking for epoxy but didn't see anything but the 5min
stuff in the tooth paste tubes. I was searching on leevalley.com and
found that they sell Industrial Formulators' G1 and G2 epoxy as well
as a West System.