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Electromigration-Induced β-Sn Grain Rotation in Lead-Free Flip Chip Solder Bumps

The electromigration-induced β-Sn grain rotation behavior in Ni/ Sn-3.0Ag-0.5Cu/ENEPIG(OSP) lead-free flip chip solder joints under a current density of 1×104 A/cm2 at 150 oC were investigated. The occurrence of β-Sn grain rotation resulted in an obvious morphological evolution on the surface of solder joints with both ENEPIG and OSP finishes. The mechanism on β-Sn grain rotation was proposed to explain the experimental results. The β-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 β-Sn grains. For real flip chip solder joints, both the cracks and the size of Sn grains have a significant effect on the β-Sn grain rotation. The present work is expected to provide a theoretical guidance for the electromigration (EM) life prediction of solder joints.