Electromigration-induced β-Sn grain rotation in lead-free flip chip solder bumps

The electromigration-induced beta-Sn grain rotation behavior in Ni/ Sn-3.0Ag-0.5Cu/ENEPIG(OSP) lead-free flip chip solder joints under a current density of 1x10(4) A/cm(2) at 150 degrees C were investigated. The occurrence of beta-Sn grain rotation resulted in an obvious morphological evolution on the surface of solder joints with both ENEPIG and OSP finishes. The mechanism on beta-Sn grain rotation was proposed to explain the experimental results. The beta-Sn grains with different orientations in the solder bumps led to different vacancy flux through different grains and the saturation situation of vacancies at the grain boundaries. No grinding-polishing process on the surface of the solder bump during in-situ experiment makes it stress-free and a good sink/source of vacancy, where there is an equilibrium vacancy concentration. Redundant Sn atoms diffused inward or outward the surface, resulting in the floating or sinking of the beta-Sn grains. For real flip chip solder joints, both the cracks and the size of Sn grains have a significant effect on the beta-Sn grain rotation. The present work is expected to provide a theoretical guidance for the electromigration (EM) life prediction of solder joints.