Re: Potting horror...
- From: MooseFET <kensmith@xxxxxxxxx>
- Date: Thu, 12 Feb 2009 17:20:24 -0800 (PST)
On Feb 12, 1:10 pm, n...@xxxxxxxxxxx (Nico Coesel) wrote:
A customer of mine has a problem with a PCB potted in polyurethane.
The circuit consists of some inputs (clamped & filtered), a
microcontroller with internal RAM / flash and some power electronics
to switch loads (up to 40V). 10 PCBs where produced and tested. All
worked fine. 4 of those PCBs where potted and they started to show
erratic behaviour like RAM corruption, software hang-ups. I already
milled one of the modules to expose the microcontroller to check
whether it is hot, but it stays cold. So its not a heat problem.
The polyurethane used is non-conductive and shouldn't shrink, the
potting was done by a company that specializes in potting electronics;
they know about ESD precautions.
Any ideas where to start looking?
Was this a two part polyurethane that has big trouble if there is any
water about? I assume it is.
Never apply power to the board until the reaction is totally finished.
Did someone make the mistake of using "no clean" flux? If they did
abandon all hope of potting the boards in question. Make some more
with washed flux and wash them. Beware of the water from washing
them.
Is this polyurethane colored or is it the sort of clear stuff? The
sort of clear works better than the stuff with pigments.
Is the failure heat related?
What silly fool decided to pot them? Perhaps you can delete the
potting and et on with production.
.
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