Interface microstructure and mechanical fatigue behavior of Sn₆₃Pb₃₇ on electroplated Cu and Ni

Electroplated Cu and Ni are commonly used as under bump metallurgy (UBM) in flip chip bumping due to their good wetting to common solders. The objective of this study was to understand interface microstructure and mechanical fatigue property of eutectic SnPb solder on electroplated Cu and Ni. The study focused on the intermetallic compound morphology, its distribution and the impact of such on interfacial fatigue life under controlled cyclic load. Flexural peel technique was used to study the fatigue crack propagation along the interface of solder and intermetallics. Interface microstructure, fatigue crack profile and crack surface were analyzed by both optical and scanning electron microscopies. The experimental results showed the solder joint fatigue life depends strongly on under bump metallurgy materials and the interfacial microstructure, especially the intermetallic phase and their distribution at the interfaces. For Cu, a continuous coarse cell-shape Cu₆Sn₅ intermetallic formed at the Cu/solder interface and the fatigue crack propagated along this interface under relatively small load. For Ni, it was clearly seen that Ni effectively limited the isotropic growth of intermetallics at the interface. Moreover, the needle-shape Ni₃Sn₄ intermetallic compound tends to grow into the bulk of solder which results in a zigzag intermetallics and solder interface. This zigzag interface is able to deflect the crack tip into solder and divert the crack propagation direction. Consequently, a relatively high resistance to fatigue crack was observed at this interface. The better mechanical fatigue life of Ni makes it a better choice for flip chip UBM selection.