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From: firstname.lastname@example.org (Bob Wilson)
Subject: Re: Side effects of potting compound !
Date: Fri, 01 Nov 2002 05:35:35 -0000
Organization: Your Organization
X-Newsreader: WinVN 0.99.9 (Released Version) (x86 32bit)
In article <email@example.com>,
>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
>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
>Checked the compound.... fine, compound specs.. fine. I spent all this
>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
>completely and was slightly conductive ? Checked all sorts of ideas.
>There's a voltage reference feeding a resistor divider with a trimmer at
>junction for fine trimming. A multi turn preset. Voltage at both sides of
>trimmer are as they should be. Voltage at the wiper is now 6% higher (which
>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
>Anyone come across this before ? Anyone got any ideas how to prevent it ?
Sure. Potting can be a bloody disaster. Some potting compounds are so
brittle, they actually break solder bonds during thermal expansion. It is
unlikely that the stuff has managed to turn the wiper enough to make its
position change as much as you mention. But there is another problem that is
not often recognized. Most epoxies used for this process are amide epoxies
(aka "nylon" epoxies, since nylon is a polyamide). This common formulation
absorbs significant amounts of water, and in spite of its very common
useage, it is absolutely the wrong material to use.
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