The interfacial reactions of near-eutectic Sn-Ag-Cu solder with Ni(P)Au metal finishes on printed wiring boards as well as in component under bump metallizations have been investigated in this work. With the help of the scanning electron microscopy and transmission electron microscopy it was discovered that the first phase to form was the metastable nanocrystalline ternary NiSnP layer that provided the substrate for the subsequent formation of (Cu,Ni) 6 Sn5. During cooling or in the subsequent reflows the metastable NiSnP layer partially transformed into the columnar Ni3 P. In this transformation Sn atoms and impurities, which do not dissolve into the Ni3 P, diffuse towards the remaining NiSnP layer. When the specimens are further annealed in solid state at 170 °C the Ni3 P phase transforms into Ni5 P2 implying that some more Ni has diffused towards the solder. However, when Ni(P)Au finishes with a higher P content were used, only the ternary NiSnP layer was observed. Based on the mass balance and available thermodynamic data on the Sn-P-Ni system the evolution of the observed microstructures in the reactions between the solder and Ni(P)Au finishes is discussed.