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

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 AuZn3 intermetallic compound was found at the interfaces. On the other hand. in the Sn-3.5Ag-0.5Cu content solders. an additional epsilon-AgZn3 intermetallic compound layer was found to be well adhered on the top surface of the AuZn3 layer and the epsilon-AgZn3 layer thickness increased with the number of reflow cycles. In addition, fine spherical-shaped epsilon-AgZn3 intermetallic compound particles as well as an acicular-shaped Zn-rich phase was clearly observed in the beta-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 epsilon-AgZn3 intermetallic compound particles. (C) 2009 Elsevier B.V. All rights reserved.