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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