From: firstname.lastname@example.org (Glen Walpert)
Subject: Re: Side effects of potting compound !
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Date: Wed, 06 Nov 2002 13:49:03 GMT
NNTP-Posting-Date: Wed, 06 Nov 2002 08:49:03 EST
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In article , email@example.com (Bob Wilson) wrote:
>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?
Atmospheric water is everywhere, and no epoxy is completely impermeable to
water, the manufacturers usually publish a percent water absorption figure
from a standard immersion test. And ask anyone who deals in high vacuum how
much atmospheric water accumulates on any surface exposed to the atmosphere,
within seconds. By the time your RTV cures, the layer of water inside the
trimpot will be saturated with acetic acid.
>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
Good to know that the acetic acid corrosion problem is generally not a real
problem in practice. But I would still tend to be very cautious with
trimpots, because I have seen so many old trimpots in the field where the
wiper contact resistance has gone high due to corrosion. Potting, perhaps
even potting in acetic acid cure RTV, might well reduce the wiper corrosion
problem by protecting from the atmosphere, which is full of all kinds of
corrosive chemicals these days, along with plenty of water. But I have not
seen any test results to confirm this, and it is certainly not practical to
seal the trimpot with no water inside.
Note that the crevice between the element and wiper of a trimpot, like all
crevices, is an ideal location for corrosion, and it does not take much
corrosion at all to separate the two, far less than that required to affect a
PCB trace. And relating any short term observation to an effect which usually
takes years to manifest itself is of questionable utility.
All of this is a moot point for any product which can tolerate the dismal long
term reliability of unpotted trimpots, however, and now that I have typed it I
can't imagine why I have raised these irrelevant long-term reliability