Wettability of electroplated Ni-P in under bump metallurgy with Sn-Ag-Cu solder

Nickel plating has been used as the under bump metallurgy (UBM) in the microelectronics industry. In this study, the electroplating process was demonstrated to be a good alternative approach to produce the Ni-P layer as UBM. The wettability of several commercial solder pastes, such as Sn-3.5Ag, Sn-37Pb, and Sn-3Ag-0.5Cu solder, on electroplated Ni-P with various phosphorous contents (7 wt.%, 10 wt.%, and 13 wt.%) was investigated. The role of phosphorus in the wettability was probed. The surface morphology and surface roughness in electroplated Ni-P was observed with the aid of both field emission scanning electron microscope (SEM) and atomic force microscope (AFM). The correlation between wettability and phosphorus contents in electroplated Ni-P was evaluated. As the phosphorous contents increased, the surface morphology of the Ni-P deposit was smoother and surface roughness of Ni-P became smaller. The improvement of surface morphology and surface roughness enhanced the wettability of electroplated Ni-P. The interfacial reaction between lead-free solder and electroplating Ni-P UBM was also investigated.