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From: firstname.lastname@example.org (Gibbo)
Date: 31 Oct 2002 17:34:30 GMT
Organization: AOL, http://www.aol.co.uk
Subject: Re: Side effects of potting compound !
Tony Williams email@example.com wrote:
>In article <firstname.lastname@example.org>,
> Gibbo wrote:
>> 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"?
No, if anything it was well below comfortable room temperature.
> Did you by any
> chance use an excess of catalyst?
Nope. Was one of those 2 part bags
> Did you
> use a large amount of epoxy in one go?
Well.... er...... all things are relative. About 250CCs
> 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?
That was my first thought. The sides are perfectly straight. Not bowed in even
a tiny amount.
> 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.
The 2 part ones or the normal ones ?
Tomorrow's attempt is to completely cover the preset with silicon rubber. Check
it still works properly then pot it (in a softer one this time - polyeurethane
instead of epoxy)
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