Steve:

What you point out is true - but even worse if the minute gap is sealed
to form a closed void on the surfaces, the void will collect the
moisture with time (from diffusion transmission). I was responsible for
medical implantable electronic designs while doing research at Stanford
University. I used a vaccum to take out trapped voids before the
encapsulant set. The surfaces were slightly etched with acetone and
tetrachlorofluorcarbon to give better adhesion. Narrow-molecular-weight
wax was the encapsulant with a thin silicone overcoat for body rejection
reasons. The water transmission, absorption and adsorption were all near
zero for years of direct exposure to a 3% NaCl solution. We were
building micropower high-impedance circuits and IC, sensitive to very
small amounts of moisture.

- Wayne Foletta
BMI - Saratoga, CA

> ----------
> From:         Steve Baldwin[SMTP:steveb@KCBBS.GEN.NZ]
> Reply To:     pic microcontroller discussion list
> Sent:         Saturday, January 31, 1998 3:45 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: Encasing Pic in Epoxy
>
> This is obviously a subject where the collective wisdom has plenty of
> experience, so I won't reiterate.
> One 'gotcha' that I haven't seen mentioned is ensuring that your
> encapsulant will actually bond to the surfaces you are potting. For
> example, PCBs with flux remaining and plastic mouldings (including
> looms
> with moulded plugs) that still have release agent on them.
> If there is a minute gap between the potting compound and the surface,
> it
> will draw water in by capillary action.
> For the same reason, don't use the potting as strain relief.
>
> Steve.
>
> ======================================================
>   Very funny Scotty.  Now beam down my clothes.
> ======================================================
> Steve Baldwin                Electronic Product Design
> TLA Microsystems Ltd         Microcontroller Specialists
> PO Box 15-680                email: steveb@kcbbs.gen.nz
> New Lynn, Auckland           ph  +64 9 820-2221
> New Zealand                  fax +64 9 820-1929
> ======================================================
>