[SI-LIST] Re: Failed Tantalum capacitors - package hermeticity compromised

I wouldn't get scared away from using tantalum capacitors.  They are the
predominate bulk capacitor used in hi-rel space applications and
designed to last years on orbit without failure and have a well
establish reliability.  Their reliability is partially driven by
sufficient derating (there are other factors, but this is one the
designer has control over).  I'm not sure 16V is sufficient for a 12V
nominal application.  Inrush conditions could briefly stress the
capacitor, causing latent failures.  While this is probably overkill for
your application, MIL-HDBK-1547A provides some guidance on derating
components.  Typically, solid tantalum chip capacitor derating starts at
around 70% and drops down linearly to about 50% at high temperature
extremes.  So, a 16V capacitor, following these guidelines, would only
be used up to 11.2V under nominal conditions.  Again, the use of this
particular handbook is probably overkill for your application, but it
reinforces my point that these particular parts can be used with
confidence in their reliability.

I would also look into the source of your components.

Regards,
Tim Nash
Lockheed Martin Space Systems Company



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Ravinder.Ajmani@xxxxxxxxxxxxxx
Sent: Monday, January 26, 2009 7:11 PM
To: Salkow, Steven
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Failed Tantalum capacitors - package hermeticity
compromised

Hi Steve,
We have stopped using Tantalum capacitors for some years now.  When we 
used them in the past, we were using 3X voltage derating for Solid 
Tantalum capacitors, and 2X derating for Low ESR Organic Tantalum 
capacitors.

Regards, 
Ravinder Ajmani
Server PCB Development
Hitachi Global Storage Technologies


Email: Ravinder.Ajmani@xxxxxxxxxxxxxx



"Salkow, Steven" <steven.salkow@xxxxxxxx> 
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01/26/2009 05:40 PM

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Recently we have had some tantalum capacitors rated at 16V fail on turn
of in a 12V circuit. While some percentage of low ESR capacitors
experience a high surge infant mortality on first turn on unless
serrated by 50% or more, these capacitor were sent for additional
electrical screening at an independent test house.
 
Just as a precaution we decided to also sample test the lot for package
hermeticity by conducting a bubble test. 25 samples were picked at
random from the lot of 450 and 100% of these failed bubble test.
 
Has anyone had this problem before? Has any had counterfeit capacitors?
 
Steven Salkow
Lockheed IS&S
3130 Zanker Rd, San Jose
Ca. 95134
(408) 473-4058
H: (925) 462-1075
steven.salkow@xxxxxxxx
salkow@xxxxxxxxxxxx
 
 



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