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August 2000

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Subject:
From:
Mike Fenner <[log in to unmask]>
Reply To:
Mike Fenner <[log in to unmask]>
Date:
Fri, 4 Aug 2000 16:06:46 +0100
Content-Type:
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text/plain (86 lines)
Don't know from knowledge but intuitively my hypothesis:

There are a number of dissimilar metals involved here in lead frame,
silver loaded polymer
as well as the graphite/manganese/carbon/silver being at other
possible
potentials.
IN these devices the capacitance is generated across a very thin film
and they do not have infinite resistance, so small currents may flow.

So now add damp and a few volts or possibly even just damp, and leave
for
a year or three or four or.... You now have a little battery or
corrosion cell The dissimilar metals will be in galvanic corrosion
resulting in erosion and/or change in internal volumes from reaction
products, eventually disrupting the insulating film. As soon as that
happens but not before, device turns into a heating element. Be
interesting to see what leakage volts are on similar age units?

Mike

----- Original Message -----
From: "Alan Peters" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, August 04, 2000 9:24 AM
Subject: [TN] Solid tantalum capacitor failures


> I have been asked to investigate two failures of 10u 25v  solid
tantalum capacitors.
>
> They are rectangular plastic/resin encapsulated, sintered tantalum/
manganese
> dioxide/ graphite-silver types.
> These items are "cremated" beyond investigation!
>
>   I am lead to believe (hmm!) all correct operating conditions have
been met/not
> exceeded i.e. excessive voltage, ripple and reverse voltage.
> The capacitors  have seen service for at least 5 years in a very
humid environment.
> I am not aware if they are from the same batch. One was fitted
across a +15v rail
> in one unit and the other was fitted across a -15 rail in another
unit.
>
> Question? Does anyone know of an ageing condition, or what the
effects of humidity
> are that can lead to failure of these capacitors?
>
> Alan Peters
> Failure Analysis Engineer
> Materials & Components Labs
>
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