Interaction Between β-Sn Grain Orientation and Electromigration Behavior in Flip-Chip Lead-Free Solder Bumps

With the increasing demands for miniaturization, the electromigration (EM)- induced failure by diffusion anisotropy in beta-Sn is expected to be more serious than that induced by local current crowding effect, especially with the downsizing of solder bumps. In this work, the effects of Sn grain orientation on intermetallic compound (IMC) precipitation, dissolution of Ni under bump metallurgy (UBM) at the cathode, EM failure mechanism as well as the EM-induced beta-Sn grain rotation in Ni/Sn-3.0Ag-0.5Cu/Ni-P flip-chip interconnects undergoing solid-solid EM under a current density of 1.0x10(4) A/cm(2) at 150 degrees C were in situ studied. (Ni, Cu)(3)Sn-4-etype IMCs precipitated in these beta-Sn grains with a small angle theta (between the c-axis of Sn grain and electron flow direction), i.e., along the o-axis of beta-Sn grains. Stress relaxation, squeezing beta-Sn whiskers near the anode, was also observed during EM. A mathematical model on the relationship between the dissolution of Ni UBM and beta-Sn grain orientation was proposed: when the c-axis of beta-Sn grain is parallel to the electron flow direction, excessive dissolution of the cathode Ni UBM occurred due to the large diffusivity of Ni along the c-axis; when the c-axis of beta-Sn grain is perpendicular to the electron flow direction, no evident dissolution of cathode Ni UBM occurred. The proposed model agreed well with the experimental results. EM-induced beta-Sn grain rotation was attributed to the different vacancy fluxes caused by EM between adjacent grains of various grain orientation, when vacancies reached supersaturation and undersaturation at the interfaces of the anode and the cathode, respectively. Vacancy fluxes went through free surface along the interface, resulting in a normal vacancy concentration gradient. Accordingly, stress gradient produces a torque to rotate the beta-Sn grain.