Solid-state interaction between Ni and Cu for ternary compound growth in Ni/Sn/Cu diffusion couple

The solid-state interaction between Ni and Cu in an Ni/Sn/Cu diffusion couple, which served as a representative of the BGA solder joints of the Ni/Au finished component and the OSP finished board, was investigated during isothermal aging at 200 °C for aging times of up to 100 h. The Ni/Sn/Cu diffusion couple was prepared by sequentially depositing Sn (60 μm) and Ni (2 or 4 μm) on a Cu plate using the electroplating method. During the solid-state aging process, two distinct layers composed of (Cu _1-xNi_x)₆Sn₅ and (Cu _1-yNi_y)₆Sn₅, respectively, which differed in their Ni concentrations, were formed at the Ni/Sn interface, and the ternary IMC grew rapidly by incorporating the Cu originating from the Cu plate. However, no detectable Ni diffusion occurred across the Sn layer. The results of this study show that the dominant Cu source for the solid-state growth of the ternary IMC at the Ni/Au finished component side in the BGA solder joints was the Cu bonding pad of the PCB, rather than the Cu dissolved into the solder during the reflow process.