Hi Ian,
You write: "...but this failure mode isn't anything to do with assembly
stresses or thermal shock during the soldering operation."
On the contrary, it got everything to do with assembly stresses or thermal
shock during the soldering operation. What makes these stresses exceed the
capability of the CCs to withstand them is the fact that you have too much
solder present [bulbous SJs are not necessary].
You need to reduce SJ size; it is easiest to do this be reducing pad size.
There is no negative side to this as far as reliability or functionality iis
concerned.
One other thing that helps somewhat is to increase the CC stand-off from the
pads [more solder between CC and PCB].

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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