Kinetic study of the intermetallic compound formation between eutectic Sn-3.5Ag alloys and electroplated Ni metallization in flip-chip solder joints

摘要

Ni-based under-bump metallization (UBM) has attracted wide attention due to its low reaction rate with Sn, compared with Cu and Cu alloy. In this study, the interfacial reactions between eutectic Sn-3.5Ag solder and Ni-based UBM, including electroplated Ni (EP-Ni) and electroless Ni (EL-Ni) are investigated. Morphology and growth kinetics of Ni_3Sn_4 intermetallic compounds are studied at different reflow temperatures and durations. The growth rate and the growth activation energy of Ni_3Sn_4 were measured for the two sets of samples. The activation energies are measured to be 25 kJ/mol and 38 kJ/mol for the Ni_3Sn_4 growth on EP-Ni and EL-Ni, respectively. The Ni_3Sn_4 on EP-Ni UBMs shows a slower growth rate and the Ni_3Sn_4/solder interface is void free even after 20-min reflow at 240 ℃. On the other hand, the interface of Ni_3Sn_4/EL-Ni has a lot of microvoids after reflowing at 240 ℃ for 20 min.