Hi Scott,
On cooling the CC is in overall compression, but there also is a bending
moment that could the CC layers farthest away from the PCB in tension. As the
solder creeps the overall compression is reduced, and on rewarming you get
tension in the CC. The surface copper layers have clearly have an impact on
the local expansion mismatch, but the global expansion ismatch is dominated by
the PCB material combination.

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]

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