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From: Tony Williams
Subject: Re: Side effects of potting compound !
Date: Thu, 31 Oct 2002 15:03:20 +0000 (GMT)
NNTP-Posting-Date: Thu, 31 Oct 2002 15:11:03 +0000 (UTC)
User-Agent: Pluto/1.14i (RISC-OS/3.60)
In article <email@example.com>,
> Potting the PCB in the box using epoxy potting compound
> has actually moved the wiper.
We used to pot things for a living.
Try temperature-cycling the thing to ease
the stresses in the epoxy.... several cycles
of low-high-low-high temperatures, etc.
Did you use a room-temperature curing epoxy?
Were you tempted to put it in a warm place
to "send it off quickly"? Did you by any
chance use an excess of catalyst? Did you
use a large amount of epoxy in one go?
Any of those will cause an excess shrinkage
as it cures, putting quite large stresses
on components. You can (vaguely) estimate
the shrinkage by looking at the walls of
the potting box... are they still straight
or have they bowed-in slightly?
What we used to do with RTC epoxy was to
avoid the ones with aggressively short curing
times (and/or pot life).... look for 24 hour
types. Pot a little (100 gram) at a time if
possible. Pour the epoxy, and then let it
quietly stand at a (lowish) room temperature
overnight, then give it whiff of warmth to
actually send it off the next day.
Actually we found that silicone rubbers were
just as good for encapsulation.
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