Effect of small Sn–3.5Ag–0.5Cu additions on the structure and properties of Sn–9Zn solder in ball grid array packages

Abstract

Sn–9Zn with various additions of Sn–3.5Ag–0.5Cu powder was prepared by mechanically dispersing different weight percentages (1, 3, 5 and 7) of Sn–Ag–Cu powder into Sn–9Zn solder paste. In the Sn–Zn solder, scallop-shaped AuZn 3 intermetallic compound was found at the interfaces. On the other hand, in the Sn–3.5Ag–0.5Cu content solders, an additional ε-AgZn 3 intermetallic compound layer was found to be well adhered on the top surface of the AuZn 3 layer and the ε-AgZn 3 layer thickness increased with the number of reflow cycles. In addition, fine spherical-shaped ε-AgZn 3 intermetallic compound particles as well as an acicular-shaped Zn-rich phase was clearly observed in the β-Sn matrix. On increasing the Sn–Ag–Cu content, the shear load was increased from 1.80 to 2.03 kg after one reflow cycle. In the Sn–3.5Ag–0.5Cu content solders, the fracture surfaces exhibited typical ductile behavior with very rough dimpled surfaces while the fracture surface in the Sn–Zn solder gave fractures with a brittle appearance. In the fracture surface of the Sn–3.5Ag–0.5Cu content solders, some dimples were clearly observed associated with the formation of spherical-shaped ε-AgZn 3 intermetallic compound particles.