From: email@example.com (Bob Wilson)
Subject: Re: Side effects of potting compound !
Date: Tue, 05 Nov 2002 03:32:47 -0000
Organization: Your Organization
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In article ,
>In article ,
>a (Bob Wilson) wrote:
>>Like I said in an earlier post, silicone (or other compliant) conformal
>>coating is not compliant at all when it is fully encapsulated, and hence
>>"nowhere to go" when subjected to stress internally.
>And you said it again, but it is still wrong. Silicone rubbers have a bulk
>modulus in the 100,000 to 200,000 PSI range at moderate pressures, so they
>will compress 1% or 1/2% (linear all directions) respectively with a mere
>PSI pressure. A few air bubbles will dramatically reduce the bulk modulus.
>The manufacturer will provide the exact bulk modulus for your specific
>compound (air bubble free) if you ask.
1.2% or 1% is insignificant. Especially when it takes 1000psi to do it. It
takes a LOT less than that to snap off a lead on an electrolytic or other
device with a reasonable cross sectional area), and ESPECIALLY since the
suggestion was to use a layer of silicone conformal coating (i.e. a thin
layer, by definition). 1% of a very thin layer is 1% of nearly nothing.
The presence of air bubbles most certainly will have a very beneficial
effect on the bulk modulus, but they can defeat the entire purpose of
potting since, as water diffuses into the potting compound, it can condense
in the voids and cause the very sort of electrical problems that potting is
intended to avoid. All polymers are permeable to varying degress to water
vapour (with silicones generally being the worst in this regard).