Effect of Cu additions on mechanical properties of Ni3Sn4-based intermetallic compounds: First-principles calculations and nano-indentation measurements

The (Ni,Cu)(3)Sn-4 phase as intermetallic compounds (IMCs) plays an important role in evaluating reliability of solder joints. The first-principles calculations were performed to investigate the phase stability, mechanical properties and electronic structures of Ni3Sn4 and Ni2.5Cu0.5Sn4 IMCs. The results shows that the doping of Cu atom not only reduced the phase stability but also weakened the mechanical properties of Ni3Sn4 crystal structure. Meanwhile, all studied Ni3Sn4-based structures were structural stability. Moreover, the anisotropy of Ni2.5Cu0.5Sn4 (2a site) was strongest while that of Ni2.5Cu0.5Sn4 (4i site) was the weakest. Based on analysis of electronic structures, it indicated that the ionic bonding of Ni-Cu in Ni2.5Cu0.5Sn4 structure was weaker than covalent bonding of nearest-neighbor Ni in Ni3Sn4, which led to an energy reduction with the formation of banding states. By means of nano-indentation measurements, the young's modulus and hardness were measured to be 135.3 +/- 8.4 GPa and 5.0 +/- 0.63 GPa for Ni3Sn4 while 126.3 +/- 7.6 GPa and 4.7 +/- 0.72 GPa for (Ni,Cu)(3)Sn-4, respectively, which were close to the value of calculated results.