The effect of Cu content on the evolution of intermetallic compounds (IMCs) in Ni/Sn-xCu/Ni (x= 0.3, 0.7, 1.5, mass fraction, %) micro solder joints during soldering at 240 ℃ under a temperature gradient of 1045 ℃/cm was investigated. Asymmetrical growth and transformation of interfacial IMCs and asymmetrical dissolution of Ni substrate were clearly observed. In Ni/Sn-0.3Cu/Ni micro solder joints, though the interfacial IMC remained as the initial (Ni, Cu)₃Sn₄, asymmetrical IMC growth between cold and hot ends occurred, i.e., the (Ni, Cu)₃Sn₄ IMC at the cold end was obviously thicker than that at the hot end. In Ni/Sn-0.7Cu/Ni and Ni/Sn-1.5Cu/Ni micro solder joints, the interfacial IMC gradually transformed from the initial (Cu, Ni)₆Sn₅ into (Ni,Cu)₃Sn₄. Meanwhile, the transformation at the cold end lagged behind the hot end, namely asymmetrical transformation phenomenon occurred. Moreover, the transformations at the cold and hot ends in the Ni/Sn-1.5Cu/Ni micro solder joints both lagged behind those in the Ni/Sn-0.7Cu/Ni micro solder joints. Based on the analysis of the Cu and Ni atomic fluxes for the IMC growth at both cold and hot ends, the thermomigration (TM) direction was confirmed to be from the hot end towards the cold end. The Cu concentration in the micro solder joints had a significant effect on the main TM element, and thus affected the growth and transformation behavior of the interfacial IMCs at the two ends. In addition, TM promoted the diffusion of Ni atoms into solder at the hot end, which accelerated the dissolution of the hot end Ni substrate. Most of the dissolved Ni atoms migrated to the cold end and participated in interfacial reaction locally. On the contrary, TM inhibited the diffusion of Ni atoms at the hot end, resulting in no obvious dissolution of the cold end Ni substrate.