>I do not know which 'thermal shock issue' you investigated; heating too fast
>or cooling too fast? Chip components, unless already defective, do not crack
>on fast heating, but can do so on fast cooling. That can happen in
>combination with pad designs that allow exessively large solder fillets. When
>these large solder volumes cool, they shrink at ~25 ppm/C and the ceramic
>chip components at only ~6 ppm/C. When you cool too fast the solder can not
>creep fast enough to relax the stresses and component cracking can result.
>
>Werner Engelmaier

To those that have experienced ceramic capacitor cracking.

First, barium titanate capacitors have coefficients of expansion ranging
from about 9 ppm/degree C for NPO glassy ceramics to about 11-12 ppm/degree
C for X7R and Z5U type materials. Alumina based chips like resistors have a
CTE of approximately 6 ppm/degree C. With the rare exception of very large
solder joints rapid cooling has not been a problem in capacitor cracking.
Large fillet size coupled with excessive glass diffusion during termination
firing is the usual culprit.

The crack shape tells all and was documented in a paper I wrote in 1987
while working for AVX which was titled "Cracks: The Hidden Defect". It was
later made into an application note and may still be available at their web
site located at avxcorp.com. If not contact me off the forum and I will
send you a copy. Capacitor cracking can have many surprising sources so
check the crack signature first to get a better idea on where to look for
the root cause.

Regards,


John Maxwell

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