Ken,
We use 20 tantalum caps in parallel in a military radio and have seen a
much higher fallout in our factory testing in the last few years. I
believe the cell phone industry placed a major burden on the major
capacitor suppliers, who could barely keep up. It appears the quality
levels have dipped slightly because of the extra demand. This makes proper
derating and series resistance even more critical. ESR comes into play as
well when these caps are in parallel. The good caps will also dump their
energy into the leaky one, as well as your supply voltage, to burn them up.
I agree with John, AVX & Kemet have valuable technical information
abstracts on surge failures and construction. They also supply a more
surge robust, low ESR, capacitor that can take more abuse if your
application requires it.
Good luck
Jim Carlson
Rockwell Collins Reliability Engineer
John Maxwell <[log in to unmask]>@ipc.org> on 03/07/2002 02:12:04 PM
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
to John Maxwell <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
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cc:
Subject: Re: [TN] Tantalum Capacitor Failure
Ken,
As a side note, tantalum is like most other valve metals and burns when
localized heating gets too high. There is plenty of oxygen (Ta2O5) in the
dielectric and extra provided by the manganese dioxide cathode (MnO2) so
once failure starts the part consumes itself. Unfortunately one looses the
evidence. Over voltage, unrestricted voltage ramp up (that's why vendors
recommend a series resistor), reflow profile especially if the part has a
polymer cathode or if there is too rapid a cool down after reflow and of
course weak parts all are on the list for potential root cause. I suggest
getting their recommended application notes along with those from Kemet and
AVX and see if thermal or electrical conditions are outside manufacturers
recommendations. That analysis should get you down the trail of determining
what is happening.
John Maxwell
At 12:57 PM 3/7/02 -0600, you wrote:
>I am wondering if anybody has had similiar issues or can help resolve
>a failure we are experiencing. On about 4% of our units at first time
>power up 1 of two capacitors that are in parallel are burning up. Upon
>replacement the capacitor it to will work fine.
>The cap we are using is Vishay Sprague 594D336X9035R2. Since the other
>capacitor in parallel is unaffected, I am assuming that it is a Vishay
>issue.
>
>Per Vishay:
>The root cause of failure could not be identified. It is requested that
the
>failure circumstances be reviewed. Failures can occur due to thermal
stress
>during board mounting, strenuous application conditions, or because of
>reduced capacitor capability. There was no specific evidence of reduced
>capacitor capability identified. Please be aware, the thermal profile used
>for board mounting, and the application conditions (even transient
>conditions such as may occur with unregulated power supplies) can affect
>product performance.
>
>Has anyone else experienced a similiar failure or have an opinion? I wish
I
>could pre-test the caps prior to them burning to supply an intact
>capacitor to Vishay for their analysis.
>
>Please note: When allowed to play its course the cap turned red hot,
>unsoldered itself from the PCB and burned a whole in the lab bench!
>
>
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