Metallurgical reactions in Ni/ SnAg2.3 solder/ Cu system are investigated by varying the solder thickness from 40μm to 10μm. We found that the growth rate of the interfacial intermetallic compounds (IMCs) strongly depend on the solder thickness. In the Ni/ 40-μm solder/ Cu samples, the (Cu, Ni) 6Sn 5 IMCs on the Ni side grew slightly faster than those on the Cu side. However, the trend reverses as the solder thickness decreases below 20μm. The (Cu, Ni) 6Sn 5 on the Ni side even stop growing after 4-min reflow at 260°C in the Ni/10μm solder/ Cu samples. Yet, the IMCs on the Cu side grew thicker than that in Ni/ 40μm solder/ Cu samples. Compositional analysis reveals that the Cu and Ni concentrations in the solder increases with the decreasing in solder thickness. The changes in the Ni and Cu concentration in the solder plays crucial role on the growth rates at the IMCs in the Ni/ SnAg solder/ Cu system.