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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.

I. INTRODUCTION

The reliability of a solder joint depends upon a number of mechanical and microstructural evolutionary processes that occur interactively during its service lifetime. About 70% of failure in electronic circuitry is solder related. In lead–tin solders, coarsening of the lamellar microstructure that forms upon solidification leads to longer slip lengths that facilitate nucleation of fatigue cracks.1,2 In contrast to a two-phase microstructure with comparable volume fractions of ductile material present in eutectic Sn–Pb solders, tin–silver-based solders have a small volume fraction of hard submicron-size Ag3Sn intermetallic particles that provide strengthening and result in significantly different interactions among stress, thermal history, and microstructural evolution. These intermetallic particles are known to coarsen with time and temperature.3 Models for grain growth limited by Zener drag of particles that grow by Ostwald ripening have been developed for particle strengthened high temperature deformation.4–8 However, a significant number of observations documenting grain boundary sliding phenomena in fine-grained lead-free Sn–Ag solders have been reported in recent literature.9–11 Grain boundary sliding is a sign of superplastic deformation phenomena, but it depends on grain size and grain boundary misorientation in the tin part of the microstructure. The vast majority of research on the microstructure of solder joints has focused on the distribution of the intermetallic 2294

http://journals.cambridge.org

J. Mater. Res., Vol. 17, No. 9, Sep 2002 Downloaded: 14 Mar 2015

and not on the tin phase, with only a few exceptions.12,13 Since failure of solder joints occurs by deformation and fracture of tin, there is a need to identify a suitable methodology for measuring microstructural evolution in tin before modeling strategies can be developed to predict the reli