Interfacial microstructure and shear strength of Ag nano particle doped Sn-9Zn solder in ball grid array packages

Sn-9Zn solder joints containing Ag nano particles were prepared by mechanically mixing Ag nano particles (0.3, 0.5 and 1 wt%) with Sn-9Zn solder paste. In the monolithic Sn-Zn solder joints, scallop-shaped AuZn3intermetallic compound layers were found at their interfaces. However, after the addition of Ag nano particles, an additional uniform AgZn3intermetallic compound layer well adhered to the top surface of the AuZn3intermetallic compound layer was found. In addition, in the solder ball region, fine spherical-shaped AgZn3intermetallic compound particles were observed as well as Zn-rich and β-Sn phases. With the addition of Ag nano particles, the shear strengths consistently increased with an increase in the Ag nano particle content due to the uniform distribution of fine AgZn3intermetallic compound particles. The shear strength of monolithic Sn-Zn and 1 wt% Ag nano particle content Sn-Zn solder joints after one reflow cycle were about 42.1 MPa and 48.9 MPa, respectively, while their shear strengths after eight reflow cycles were about 39.0 MPa and 48.4 MPa, respectively. The AgZn3IMCs were found to be uniformly distributed in the β-Sn phase for Ag particle doped Sn-9Zn composite solder joints, which result in an increase in the tensile strength, due to a second phase dispersion strengthening mechanism. The fracture surface of monolithic Sn-Zn solder exhibited a brittle fracture mode with a smooth surface while Sn-Zn solder joints containing Ag nano particles showed a typical ductile failure with very rough dimpled surfaces.