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From: Joseph Legris
X-Mailer: Mozilla 4.77 [en]C-SYMPA (Win95; U)
Subject: Re: Side effects of potting compound !
Date: Thu, 31 Oct 2002 09:58:43 -0500
NNTP-Posting-Date: Thu, 31 Oct 2002 09:58:57 EST
Organization: Bell Sympatico
> We've just finished a small run of a project for a client. Everything
> completed. At the last minute the client said "can we have them potted please
> I replied that I saw no reason why not but would check a few to make sure.
> Potted 3 yesterday. Came to test them today. Calibration is miles out, by
> almost exactly the same error in each unit (it's a calibrate once only unit).
> Checked the compound.... fine, compound specs.. fine. I spent all this morning
> breaking open the compound to try to find out what had gone wrong. Had it
> affected some caps ? Had it formed some crazy dissimilar metal junction
> somewhere ? Had it absorbed moisture in the curing process ? Had it not cured
> completely and was slightly conductive ? Checked all sorts of ideas.
> There's a voltage reference feeding a resistor divider with a trimmer at the
> junction for fine trimming. A multi turn preset. Voltage at both sides of the
> trimmer are as they should be. Voltage at the wiper is now 6% higher (which is
> actually about 50% of the trimmer range). Overall resistance of trimmer is
> normal. Ratio has changed.
> Potting the PCB in the box using epoxy potting compound has actually moved the
> Anyone come across this before ? Anyone got any ideas how to prevent it ?
Unless the trimmer is extremely touchy, which is a design issue with or
without potting, I am guessing that the epoxy got inside and mucked up
the contact between the wiper and the resistance element.
Even though it appears quite viscous, epoxy has an incredible ability to
flow into the tiniest holes and crevices when it is liquid. Don't get it
on any electromechanical parts such as switches, pots, connectors or IC
sockets unless they are sealed. You may be able to solve your problem by
completely sealing around the pot with hot glue (or conformal coating as
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