Morphological and microstructural evolution of Sn-patch in SnAgCu solder with Ni(V)/Cu under bump metallization

In flip chip technology, the Ni(V)/Cu multi-metallic thin-films is a widely used under bump metallization (UBM), for doping 7 wt.% V into the Ni target can eliminate the magnetism of Ni during sputtering. It was noted that V in the Ni(V) layer did not react with solders and intermetallic compounds (IMC) during reflow and aging process, yet a Sn-rich phase, as the so-called "Sn-patch", would form in the Ni(V) layer. The possible reason of Sn-patch formation may be the fast Sn diffusion from the solder matrix to the Ni(V) layer. However, the formation mechanism of Sn-patch and the detailed composition variation and structure evolution in Sn-patch were not fully discussed yet. In this study, Sn-patch would be analyzed by a field emission electron probe X-ray microanalyzer (FE-EPMA) and a transmission electron microscope (TEM) to elucidate the composition redistribution and the microstructure evolution, respectively. There existed concentration redistribution of the constituent element in Sn-patch, and its microstructure also varied with the aging time. On the basic of the detailed characterization by FE-EPMA and TEM, it was revealed that Sn-patch was consisted of crystalline Ni and amorphous Sn-rich phase after reflow, while V₂Sn₃ formed with amorphous Sn-rich phase during aging. A possible formation mechanism of Sn-patch was proposed, which could be employed to explain the corresponding composition variation and structure evolution associated with the Sn-patch formation.