Concentration gradient of Ni in reduced SnAg thickness in Ni/SnAg/Cu microbumps during solid-state aging

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.