We investigate the cross-interaction in Cu/SnAg/Ni microbumps with a reduced solder thickness of 30 and 10 μm. The concentration of Ni atoms at the opposite site increased with the decrease in solder-height. A considerable concentration gradient of Ni was detected in 10-μm microbumps sample, which strongly triggers the diffusion of Ni atoms to the Cu side. The diffused Ni atoms at Cu side form the ternary intermetallic compounds of (Cu,Ni) 6Sn5, which possesses a lower free energy than Cu 6Sn5 does. Eventually, the growth of the Cu3Sn was inhibited due to the formation of the thermodynamically stable (Cu,Ni) 6Sn5.