Hi Frank,
The coefficient of thermal expansion in independent of size, however the
magnitude of the thermal expansion is directly dependent on size. Thus, the
strain on the solder joints is proportionately reduced with size. Many people
erroneously belief that the failures of solder joints are
stress-dependent--they are not. Fatigue failures of solder joints are
strain-driven; while the strains from the thermal expansion mismatches do
initially produce stresses, these stresses are reduced to near zero in
relatively short times due to temperature-dependent creep. Stress does become
an issue at lower temperatures (-20 C and below) and for solder joints where
little solder remains due to the formation of intermetallic compounds (IMCs).

Fillets serve the purpose to prolong life due to longer distances through
which a crack has to propagate to cause electrical failure, but they do not
affect the number of cycles necessary for crack initiation. I certainly would
not worry about the reliability of 0402 solder joints as long as proper
wetting has taken place.

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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