November 12, 2005

Folks, we are experiencing large number of failures of tantalum surface-mount chip capacitors (CWR06 style).  The capacitors are a modified version of MIL-C55365 parts to give an extended CV range.  The parts are modified 35 Volt rated parts to yield 22 microfarad, 50 volt rated parts.  Reverse peak voltage for these parts is 2.5V at 85C and 5V at 25C.  The failed parts were installed correctly.  We checked voltage ratings, ripple current, etc. and all are OK for the application.

There is a problem with board layout since the vendor hand soldered the parts to the board by applying soldering iron directly to the top surface of the capacitor rather than to the pad area (the capacitor manufacturer indicated this is not recommended).  This is because the layout was flawed since the land area was not larger than the part so the pad could not be heated via the soldering iron.  The vendor subsequently changed method of soldering to implement pre-heat and reflow soldering and no longer uses hand soldering.  However, even with these changes, the parts are still failing.

This part is a commercial version of a MIL-C-55365 part.  The vendor does no burn-in electrical screening testing on the parts so we suspect our failures may be due to the normal bath-tub curve in that we are not weeding-out premature failures.

Does anyone have reliability data showing what failure rates can be expected for these type capacitors for both non-electrically screened tested vice electrical screened tested parts.  Also, what are the most likely failure modes for this type part and what are the failure 
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mechanisms?