Reaction characteristics of the Au-Sn solder with under-bump metallurgy layers in optoelectronic packages

The reaction characteristics of molten Au/Sn eutectic solder with potential diffusion-barrier materials of optoelectronic packages were investigated. The characteristics were studied by reflowing the solder on Pt, Ni, and Co plates, respectively, and by measuring the thickness of the reaction product. In addition, the dissolution rate of Pt into the solder was measured. The results indicated that Pt, which is commonly used as the diffusion-barrier layer in the under-bump metallurgy (UBM) of optoelectronic packages, reacts readily with the molten solder, resulting in discontinuous reaction products at the solder/Pt interface. Cobalt, on the other hand, reacted with the solder at an order of magnitude slower rate than that of Pt and provided an effective barrier against the reaction with the solder.