Hi Bev,
A crack is a crack is a crack--even a micro-crack.
So what does that mean?
As I said: "Solder exhibits micro-voids at grain boundary intersections after
about 25 to 30% of the fatigue life having been consumed, and micro-cracks
along grain boundaries after about 40%. That means about 60% of life is still
available when 'observable' micro-cracks are present." These micro-cracks are
only 'observable' in cross-sections under the microscope. Prior to the
micro-voids, grain coarsening is 'observed.' As the fatigue loading
continues, the micro-cracks grow, and coalesce, becoming macro-cracks until
a continuous fracture makes the SJ separable. In all this time the SJ will be
electrically conductive [in the later stages high frequency signals may
experience rise-time degradation, however]. However, the strength  of the SJ
will continuously decline during the process of accumulating fatigue damage.
"How big of a crack in thermally cycled solder joints is too big of a
crack?"--you must make that determination for yourself depending on your
application.

Werner Engelmaier

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