Intermetallic phase formation and growth kinetics at the interface between molten solder and Ni-containing under bump metallization

Direct Chip Attach (DCA) of Si chips to an organic substrate requires modification to the conventional Cu based Under Bump Metallization (UBM). Ni-containing UBM's have proven to be able to effectively reduce the intermetallic compound growth rate, which in turn reduces the consumption of the wetting metals in the UBM during the reaction between the solder and the UBM. In this research, the microstructure evolution of the interface between molten eutectic Pb/Sn solder and several alloy-foils with different Cu/Ni ratios is studied. The experimental results show that the low diffusivity and dissolution rate of Ni play important roles in restricting the growth of the intermetallic and changing the interface morphology. A kinetic model is proposed for the Intermetallic Compound (IMC) growth based on the Cu-Ni/solder reaction, and is applied to analyze the data from UBM/solder experiments.