The interfacial microstructure of Sn–0.7Cu solder with ENIG (electroless Ni/immersion Au) was studied using scanning electron microscopy (SEM), electron probe micro analyzer (EPMA) and transmission electron microscopy (TEM). (Cu,Ni)₆Sn₅ intermetallic compound (IMC) layer was formed at the interface between the solder and Ni–P under bump metallurgy (UBM) upon reflow. The thickness of the (Cu,Ni)₆Sn₅ IMC layer increased with isothermal aging time. Two distinctive layers, P-rich and Ni–Sn–P, were additionally found from TEM observation. Analytical studies using energy dispersive spectrometer (EDS) equipped in TEM revealed that the composition of the P-rich layer is close to that of a mixture of Ni₃P and Ni, while that of the Ni–Sn–P layer is analogous to the P-rich layer but containing a small amount of Sn in it. The shear force of the joints decreased during isothermal aging. The decrease of the shear force should be mainly due to the microstructural degradation within the solder.