Microstructure of ternary Sn-Bi-xCu alloy on mechanical properties, current endurance and corrosion morphology via cycling corrosion test

Low melting temperature of Sn-58 wt.% Bi (SB) solders adding 0.5 wt.% Cu (SB05C) and 1.7 wt.% Cu (SB17C) concentrations were evaluated for the signal transmission in the 3D IC package. Microstructure combined with phase analysis and Pandat simulation were together with to explain the relationship among the mechanical property, electrical endurance and corrosion resistance. By Cu decoration, the presence Cu₆Sn₅ intermetallic compound (IMC) acts as heterogeneous nucleation sites to reduce surface free energy and generate fine Bi grain that resulted in higher hardness value. The power-law was used to explain the mechanism of interfacial IMC growth, both Cu₆Sn₅ and Cu₃Sn formation were speculated to be volume diffusion control and surface reaction domination, respectively. The fracture morphology by shear testing indicates that earliest plastic deformation accompanied with brittle rupture were major factors due to the intergranular fracture and Cu₆Sn₅ compound induced bulk fracture. By cycling corrosion test (CCT), the lump shape of SnO₂ by-product was found among solders at three cycling. After seven cycling, Sn phase has been completely corroded and Cu₆Sn₅ IMC began to be corroded forming circle shape of Cu₂O but Bi phase still not be corroded among solders. In the electrical endurance, dense Cu₆Sn₅ dispersed in solder bulk and fine grain size were feasible to induce current crowding and joule heating, thus Cu deteriorated SB solders is easily failed.