We also saw a similar thing at Nortel Networks back in the mid nineties.  In that case there was no conformal coating and there was no cleaning (the process was no-clean).  The aluminum electrolytic capacitors (about as big around as your baby finger) came from two reputable suppliers, neither one is mentioned in the articles that we have been alerted to by Safavi-Bayat.  So, Graham, it is looking more and more like it is not your coating or how your customers are applying it, but the caps themselves.
regards,
Bev

-----Original Message-----
From: Paul Signorelli [mailto:[log in to unmask]]
Sent: July 1, 2003 2:37 PM
To: [log in to unmask]
Subject: Re: [TN] Leaky capacitors


i have seen that before,
could be electrolyte leakage or possibly precipitate from cleaning with
halogenated solvents.
Aluminum Electrolytics are corroded by Halogenated (like Cl) solvents,
what kind of solvent did it see ?
Solvent infiltrates capacitor through rubber seal and Chlorine reacts
with aluminum inside cap, causes latent failures.
It could also be surface dendrites from trapped moisture or electrolyte
leakage.
High heat in area or high ripple current will cause the reaction to
become faster.
Paul Signorelli


>>> [log in to unmask] 07/01/03 11:30AM >>>
Good evening Techies

We have a customer who has discovered a "black deposit" underneath an
electrolytic capacitor that has caused a circuit failure. They
conformally
coat the assembly using a classic AR coating and claim that the
coating
softens the seal on the capacitor. We are awaiting the results of a
surface
analysis they are running to determine what the "black deposit" might
be.

We believe that the solvents in the coating might be attacking the
capacitor
seal causing the leakage, but cannot understand why this should happen
on
components from a reputable supplier. The customer has changed the
component
supplier, but the problem seems even more severe.

The end product is safety critical Class 3 - folks may die if the
product
fails.

Can you good folks maybe shed some light on what might be done to
overcome
the problem?

Change the seal on the capacitor and if so, what type of seal should
be
used?

Anyone with similar experience?

--
Regards Graham Naisbitt

[log in to unmask]

Cell: 079 6858 2121
Office: +44 (0)1252 813706

Concoat Limited - Engineering Reliability in Electronics

Web: www.concoat.co.uk  and  www.concoatsystems.com

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