The metallurgical reaction between the eutectic PbSn solder and the Au/Ni surface finish in advanced microelectronic packages was investigated. In this study, reflowed packages were subjected to aging at 160degreesC and then analyzed using an optical microscope, scanning electron microscope (SEM), electron probe microanalyzer (EPMA), and X-ray diffractometer (XRD). After reflow, the entire Au layer had left the interface, forming many (Au1-xNix)Sn-4 particles distributed throughout the solder joint. At the same time, a thin layer of Ni3Sn4 developed at the interface. After aging for a few hundred hours, most of the (Au1-xNix)Sn-4 articles bad regrouped phenomenon at the interface as a continuous (Au0.45Ni0.55)Sn-4 layer over the Ni3Sn4 layer. Using the Au-Ni-Sn ternary isotherm, we rationalized the driving force for the regrouping. The Au-Ni-Sn ternary isotherm also showed that Ni3Sn4 could dissolve a substantial amount of Au, but the experimental results indicated that the amount of Au in the Ni3Sn4 depended strongly on the locations of the Ni3Sn4. This observation was also explained by the Au-Ni-Sn ternary isotherm.