[ARC5] Questions about silver-mica capacitors

[Dennis Monticelli]

Regarding tin whiskers:  Yes, they grow just fine without the application
of voltage.  We experienced them in early IC packaging where the chips were
stored for lengthy periods and moisture was present.

Regarding silver migration:  The following excerpt from a scholarly paper
that studied circuit board leakages sums it up nicely.

*Silver is considered likely to cause ion migration and thus is not used
for important circuits or is surface-treated or alloyed prior to use.
Silver is thus a risky metal requiring special care. As a result of our
comparison with gold and others with regard to the extent of a leakage
current, it was found that silver has very high likelihood of migration and
that, when it is used where the creeping distance is small, the user must
take special care. In particular, silver reacts with water to be
electrolyzed, turning into silver ions. The silver ions react with oxygen
to develop AgO, which is unstable and likely to be deoxidized metal silver,
which diffuses in the shape of the leaf vein, resulting in a creeping
short. Thus it is important to prevent silver from absorbing moisture and
turning into metal ions as a result. The phenomenon described above is
typically referred to as the "migration" of silver. *

Dennis AE6C

On Wed, Sep 9, 2015 at 10:43 AM, Bruce Long via ARC5 <arc5 at mailman.qth.net>
wrote:

> I know the dreaded tin whiskers grow with the component out of circuit.  i
> suspect at least potentially the same is true for silver migration although
> I think I remember reading, and it makes heretical sense to me as well that
> silver migration is also accelerated by applied voltage.
>
>
>       From: Jim Wiley <jwiley at gci.net>
>  To: DrakeRadio at yahoogroups.com; glowbugs <tetrode at googlegroups.com>; "
> boatanchors at mailman.qth.net" <boatanchors at mailman.qth.net>; ARC-5 List <
> arc5 at mailman.qth.net>; regenrx at yahoogroups.com
>  Sent: Wednesday, September 9, 2015 7:14 AM
>  Subject: [ARC5] Questions about silver-mica capacitors
>
>
> A question for the various group(s) :
>
>
> We have all heard reports of silver-mica capacitors failing due to
> "whiskers" forming on the metallic surfaces and subsequently causing
> shorts.  Does anyone know if this problem is dependent on whether or not
> the capacitor is in use or not?
>
> In other words, does the problem occur even it the part is simply stored
> in a box somewhere, or does it require the application of operating
> voltages (either DC or RF) for the problem to manifest itself?  Or it
> it a combination effect, with the rate of failure being faster (or
> slower) when in use, but the problem will still happen at a faster (or
> slower) rate than when the capacitor is simply sitting in dry storage?
>
> Next, does this problem occur in all types of packages or just some,
> such as the common "epoxy dipped" vs. "postage stamp" (or other type)
> configurations?
>
> Finally, is there a way to reverse the effect?
>
> Inquiring minds want to know.
>
> - Jim, KL7CC
>
>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>
>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>