The formation and growth of intermetallic compounds and shear strength at Sn-Zn solder/Au-Ni-Cu interfaces

The microstructures and shear strength of the interface between Sn-Zn lead-free solders and Au/Ni/Cu interface under thermal aging conditions was investigated. The intermetallic compounds (IMCs) at the interface between Sn-Zn solders and Au/Ni/Cu interface were analyzed by field emission scanning electron microscopy and transmission electron microscopy. The results showed the decrease in the shear strength of the interface with aging time and temperature. The solder ball with highly activated flux had about 8.2% increased shear strength than that with BGA/CSP flux. Imperfect wetting and many voids were observed in the fracture surface of the latter flux. The decreased shear strength was influenced by IMC growth and Zn grain coarsening. In the solder layer, Zn reacted with Au and then was transformed to the β-AuZn compound. Although AuZn grew first, three diffusion layers of γ-Ni5Zn21 compounds were formed after aging for 600 h at 150 °C. The layers divided by Ni5Zn21 (1), (2), and (3) were formed with the thickness of ∼0.7 μm, ∼4 μm, and ∼2 μm, respectively.