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October 1997

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Date:
Fri, 3 Oct 1997 09:31:33 -0400
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In a message dated 10/02/97 20:20:47, you wrote:

>>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

Hi John,
Essentially we are in agreement.  While I do not have any direct experience
with the effects of excessive glass diffusion during termination firing, I
certainly do with varying solder joint fillet sizes and different cooling
rates from reflow.
Larger fillet sizes and faster cooling rates tend to increased cracking rates
for chip capacitors, particularly in combination.
I would appreciate a copy of your paper "Cracks: The Hidden Defect"
Thanks,

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 973-543-2747
E-mail: [log in to unmask]

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