Abstract The interfacial microstructure of Sn–37Pb solder with immersion Au/electroless Ni–P under bump metallization (UBM) was studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A Ni 3Sn 4 intermetallic compound (IMC) layer was formed at the interface between the solder and Ni–P UBM upon reflow. However, after isothermal aging, AuSn 4 containing a certain amount of Ni dissolved in it, i.e. (Au,Ni)Sn 4, appeared above the Ni 3Sn 4 layer. Two distinctive layers, P-rich and Ni–Sn–P, were additionally found from the TEM observation. Analytical studies using energy dispersive spectrometer (EDS) equipped in TEM revealed that the averaged composition of the P-rich layer is more close to that of a mixture of Ni 3P 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 growth of the Ni 3Sn 4 IMC layer thickness obeyed a parabolic growth law which implies that the interfacial IMC growth is mainly controlled by volume diffusion of the elements to the reaction layer. The shear force of the joints decreased during isothermal aging at 373 K. The decrease of the shear force could be mainly due to the coarsening effect of the microstructure within the solder. The failure behavior could be well explained with the results of finite element modeling (FEM) analyses.