I agree with Werner- there are no less than 5 different ceramic matrices
used by chip cap manufacturers, with barium titanate and calcium titante
being the most prevalent in X7R and Z5U composites. Neodymium and dysprosium
also are used in low tolerance (electrical tolerance) matrices, further
adding complexity to the thermal arena. Each of these variants affects
porosity during the ceramic matrix manufacture, and ultimately the
ductility/brittleness of the ceramic. These determine the ultimate stress
fracture point of failure. The time to failure electrically, is dependent
upon cap size, application and particular electrode spacing ( determined by
the cap value, size and specific manufacturer(.
Cumulative stresses occur and are 'trapped' in the matix; these are
generally mechanical stresses, with some part being thermally induced. A
single cycle of say 10C/sec vs. multiple cycles of 7C/sec have very
different effects. I my experience a 0805 can withstand 10C/sec (less than
10 cycles), whereas a 0402 cap of the same ceramic can withstand only half
that. I theorize that the samller cap has less volumetric and planar area
over which to spread the thermallly induced stresses, thereby making any
point on the matrix at a higher stress level than a larger part.
Hope this helps...
Bill Davis, Ph.D.
Senior Scientist
(408) 325-7868/321-4681
-----Original Message-----
From: Werner Engelmaier [mailto:[log in to unmask]]
Sent: Friday, August 20, 1999 7:30 PM
To: [log in to unmask]
Subject: Re: [TN] Thermal stress in reflow
Hi Greg,
While fast heating can set up stresses due to thermal gradients in chip
capacitors (CCs), there is evidence that the cooling rate after the
solidification of the solder may be more critical. That is particularly so
in
the presence of large solder fillets. I would think the actual rates would
be
size and construction dependent.
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com
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