a1 Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan 48824–1226
Single shear lap specimens were subjected to creep, isothermal aging, and thermomechanical fatigue (TMF). Scanning electron microscopy micrographs of previously polished specimens revealed changes in surface morphology. Orientation imaging microscopy was carried out on the same specimens to study the microstructural evolution and crystal orientation changes. As-fabricated joints consistently show a preferred crystal orientation with a few minority orientations with highly preferred misorientations. Alloy additions caused an increase in the number of statistically significant crystal orientations and misorientations. The solidification microstructure was unchanged due to room-temperature creep. Aging caused development and motion of well-defined subgrain boundaries and removal of most minority orientations. TMF causes heterogeneous refinement of the microstructure that accounts for the localized grain boundary sliding in regions of high strain concentration. This study implies that the lead-free solder joints are not polycrystals, but multicrystals, so that deformation is very heterogeneous and sensitive to strain and temperature history.
(Received February 07 2002)
(Accepted June 05 2002)