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From: email@example.com (Gibbo)
Date: 06 Nov 2002 09:47:01 GMT
Organization: AOL, http://www.aol.co.uk
Subject: Re: Side effects of potting compound !
firstname.lastname@example.org (Bob Wilson) wrote:
>In article ,
>>In article ,
>>a (Bob Wilson) wrote:
>>>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
>>>>modulus in the 100,000 to 200,000 PSI range at moderate pressures, so
>>>>will compress 1% or 1/2% (linear all directions) respectively with a mere
>>>>PSI pressure. A few air bubbles will dramatically reduce the bulk
>>>>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
>>>in the voids and cause the very sort of electrical problems that potting
>>>intended to avoid. All polymers are permeable to varying degress to water
>>>vapour (with silicones generally being the worst in this regard).
>>I agree with you completely that the compressibility of silicone is
>>insignificant in this application if the epoxy shrinkage is anywhere near
>>(in which case the OP should consider a different compound). I just could
>>agree with the "not compliant at all" part, when I am in the habit of
>>calculating the resulting stress/strain from actual measured compliance in
>>The vendor may not always have a number for bulk modulus, but will almost
>>always have Youngs Modulus (often just called the modulus of elasticity)
>>Poisson's Ratio (ratio of lateral strain to axial strain when loaded
>>in a standart test machine).
>>Bulk Modulus = Youngs Modulus / 3(1 - 2(Poisson's Ratio)).
>>A material with a Poisson's Ratio of 0.5 would be completely
>>but no such material exists. Silicone Rubbers have Poisson's Ratio in the
>>to .49 range. If you can only get Youngs Modulus, estimating bulk modulus
>>with Poisson's Ratio of .49 would get you in the ball-park.
>>I hope the OP was not using RTV to isolate his trimmer from stress, since
>>acetic acid could be expected to cause long term corrosion. And I still
>>that there is a good chance that even an extremely low shrinkage potting
>>compound, which would not cause any strain problems from shrinkage, could
>>into the trimmer and move the wiper by surface tension. Slitting one open
>>should be able to reveal this, and if it is the case, sealing is all that
>The "acetic acid" problem with RTV is mostly irrelivant. Yes, standard RTV
>liberated acetic acid as part of the condensation curing reaction. But one
>must keep in mind the following:
>1. Pure acetic acid it totally non-corrosive. Like most common acids, it
>MUST be mixed with water to actually be acidic. Another example is
>Hydrochloric Acid: HCl is a gas and without moisture is not corrosive at
>all. The water is required to allow ionization to occur. In this case,
>where's the water?
>2. The liberation of acetic acid continues for only as long as the RTV is
>curing. It does NOT continue forever. The monomer absorbs water from the
>air, and in the process of polymerizing liberates acetic acid. Once the
>stuff has polymerized, no further acetic acid is liberated. This the acetic
>acid "danger" can only occur is one is negligent enough to put the thing
>before the RTV has fully cured (which will not only trap the gaseous acetic
>acid, it will cut off the supply of water vapour and thus stop the curing
>reaction before polymerization is complete).
>Some years ago I ran tests to see if copper wires soldered to a PCB, when
>encapulated in standard acetic-acid-cure-RTV, would result in corrosion.
>There was none at all (mind you, I left the tests out in free air until they
>were completely cured, and did not confine them in any way).
>The alternative is to use alcohol-cure RTV, but this is 3 times the price
>has rather poor adhesion compared to the acetic cure stuff, and takes longer
I also ran some tests many years ago on acetic acid RTV (at Heart Interface). I
had heard many reports of corrosion caused by it and Heart were using it on one
of their main PCBs. The only way I could produce any corrosion on copper (wire
and PCB tracks) was to slow the curing process right down by almost sealing it
in a bag with very little moisture ingress. After several days (as in about 10)
*very* slight corrosion started to appear.
Allowing it to cure in free air produced *no* corrosion whatsoever so matter
what else was done.
We also tried the same thing on aluminium and a few other metals.
The conclusion was that perhaps it *was* possible to corrode copper with RTV
(and hence the scare mongering about it when it first started being used) but
that one had to do something really stupid in order to achieve it.
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