to:       [log in to unmask]
Subj:     Long term capacitor failure
Date:     98-22-23
Reply To: John Oliver
From:     [log in to unmask] (Mary Sugden)

Hi John
One explanation for the capacitor shorting problem you describe
could be excessive solder.  When solder solidifies and cools,
it shrinks.  The shrinkage places a constricting force on the
capacitor.  Over time, the residual compression forces cause
the elements within the capacitor to cold flow (move under
pressure). Eventually, the squeezing force wins and causes the
capacitor plates to touch and short out.

Some companies have experienced this failure mode for assemblies
that sit in the stock room for months before they are used.
Just after assembly, the boards tested just fine. They were
bagged, tagged and placed in the stock room. While sitting in
the stock room, the residual compression forces went
to work and caused the capacitors to fail.

Capacitors fail for a variety of reasons such as mechanical
abuse, electrical abuse and environmental effects however, the
failure mode under these circumstances is usually an "open".
Since the failure mode you describe is a "short" after a long
period of time (9 months), I would suspect excessive solder as
the culprit.

I am guessing that the capacitor in question is wave soldered
rather than reflowed.  With reflow soldering, you have very good
control over the amount of solder that is deposited.  With wave
soldering, the amount of solder on the joint is totally dependent
on the geometries of the component and footprint.  During wave
soldering, the entire part is emersed in the molten solder and
the amount of solder that remains on the footprint depends on the
area of metalization on the component terminals and the size and
shape of the footprint.

A different footprint is required for wave soldering than is for
reflow soldering.  The wave solder footprint must be narrower to
minimize the amount of solder that is retained.  For wave soldering,
the width of the footprint should only be 2/3 of the width of the
part.  During wavesoldering, the solder should form a nice concave
and feathered joint at each end of the component.  The solder joint
should not be bulged nor should it build up over the top of the
termination.

I hope this scenario proves helpful in finding the cause and cure
for the problem.


Good Hunting, Mary

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