Electromigration and stress migration behaviors in flip-chip solder joints
Electromigration (EM) has been an important reliability issue in flip-chip solder joints. Thus, to investigate the current density and temperature distribution in a solder joint is of great significance. In addition, we first reported stress migration in solder joints and it induced large cracks. In this study, we studied the EM and stress migration experimentally and adopted finite element analysis and theoretical analysis to provide further understanding of the two issues. Influence of Cu column under-bump-metallizations (UBMs) on current crowding and Joule heating effects of electromigration in flip-chip solder joints have been investigated. A three-dimensional simulation of the current density distribution was performed to provide a better understanding of the current crowding behavior, which was found to account for the different failure modes for the two kinds of solder bumps. One more important finding is, as confirmed by infrared (IR) microscopy, that the alleviation of current crowding by Cu column UBMs also helped decrease Joule heating effect in solder bumps during current stressing. Therefore, the measured failure time for the solder joints with Cu column UBMs appears to be much longer than that of the ones with the 2-μm Ni UBMs. In addition, void formation and UBM consumption failure mechanisms occurred respectively at the stressing conditions both without a thermal gradient in the SnAg solder joints with 5-μm-Cu/3-μm-Ni UBM measured by IR microscopy. We proposed a model considering the flux divergence at the intermetallic compound/solder interface to calculate the Sn EM fluxes toward the anode side and the chemical potential-driven fluxes toward the cathode side. UBM consumption is responsible for the failure when the Sn chemical potential flux surpasses the EM flux. Yet, voids formed at the interface when the trend reverses. This model successfully explains the experimental results. On the other hand, stress migration results in large cracks in solder joints. After 500 cycles of temperature cycling tests (TCTs) between -55 and 125 °C in SnPb composite solder joints, the Sn grains coarsened and developed anisotropic stripes close to the necking site in the solder joint because of stress-induced atomic migration. Then, cracks triggered by thermal stress were observed to propagate along the Sn stripe interfaces. After a prolonged 14410 cycles of TCT, the cracks expanded across the entire solder joint.
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