Interfacial reactions between Sn-57Bi-1Ag and electroless Ni-P/immersion Au were investigated following isothermal aging at 85 and 130 degrees C. A long-term aging study confirmed two intermetallics at the solder/substrate interface. With scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDX) analysis, the metastable NiSn4 phase was detected coexisting with the stable Ni3Sn4 phase. The average thicknesses of both intermetallic compounds (IMCs) were graphically plotted versus the square root of aging time. The EDX showed that Ni3Sn4 nucleated first. However, after nucleation, the IMC of the NiSn4 phase grew faster than the one of Ni3Sn4 between 85 and 130 degrees C. For both temperatures, the growth constants were calculated and the corresponding activation energies were approximated. A high volume of Kirkendall voids appeared along the Ni3Sn4/NiSn4 interface at 130 degrees C, resulting in dramatic shear strength decline.