Phase evolution at the interface of SnAgCu/FeNi solder joint during solid state aging

Chip scale packaging (CSP) samples with Fe-Ni UBM barrier layer were successfully fabricated by actual process. Solid interfacial reactions between SAC105 and three kinds of Fe-Ni UBM barrier layers of different composition had been investigated to clarify the phase evolution of Fe-Ni/SAC solder joints. Microstructural investigation revealed that after reflow only a thin FeSn₂ layer was formed on Fe-30Ni and Fe-50Ni substrate, while granular-like CuNiSn IMC was observed on FeSn₂ layer for Fe-75Ni solder joint. During solid-state aging, Fe-75Ni films exhibited an obvious IMC growth, while Fe-50Ni and Fe-30Ni films could endure longer time. the outer CuNiSn IMC will transform into (Cu, Ni)₆Sn₅ compound during thermal aging, which could be accelerated by the prolonging of aging time, the increasing of aging temperature, as well as the decreasing of Fe content in the film.