we don't love tantalum caps for HiRel applications. For new design, in many cases, they are forbidden. Because of poor yield. I have written a lot for internal company use, explaining what the designer has to make sure: enough cooling if there is a lot rippling and dissipated heat, avoid inductances in series with caps that have to take a high inrush current, follow the cap maker's recommended in-series resistance, format the cap if it's an old lot, make correct parallel and series connections, protect from transients, derate, derate, derate etc...so, in many cases it is a question of what good engineering you have. The tiny oxide in tantalum caps is very sensitive to rough handling...On the other hand, if you do exactly as the cap maker says (which most of our engineers do), the caps may live forever. So, we have to admit, that some unexperienced engineers are sometimes too close to the red zone....
E.g.
we took some hundreds caps, two lots from two well known cap makers. Used 1 kOhm in series on each cap, connected to maximum voltage for room temperature and let them go for 1,000 hs with regular current monitoring. Manufacturer A had a steady few microamps throughout the whole period, while the other had several caps that started increase leakage current from microamps, to milliamps, and without the series resistance, they had finally gone to shortage. The manufacturer B is not a bad mark, but they had same trouble as they all have: without knowing, a whole batch can get impurities in the oxide, with increasing leakage as a result. Our experience is, that this can happen to even the best.
Interesting to see someone who dares tell openly.
Ingemar Hernefjord
Ericsson Microwave Systems
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]
Sent: den 28 april 2005 16:27
To: [log in to unmask]
Subject: [TN] Capacitor Failures
Has any one had any problems with Vishay Capacitor P/N 595D226X9050R2T failing?
From our electrical engineers statement the most common failure is during
in-circuit test. This is conducted at ambient temperature & humidity. These
capacitors are used to decouple an input voltage, which is set to increasing
values. The failures have been occurring when a steady-state input voltage of
around 38 VDC is used. When the voltage is left on, the chances of failure
increases. For example, two different capacitors were blown when the input
voltage was left on for about 2 minutes.
--
Gary Bremer
Senior Manufacturing Engineer
Curtiss-Wright Controls Inc.
Embedded Computing -Subsystem
28965 Avenue Penn
Santa Clarita, CA 91355-4185
Main: 661.257.4430 extension 2170
Fax: 661.257.4782
e-mail: [log in to unmask]
Web: www.cwcembedded.com
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