Effect of small Sn-Ag-Cu additions on structure and properties of Sn-Zn-Bi solder/BGA during as-soldered and as-aged conditions

Sn-8Zn-3Bi solder balls with different weight percentages of Sn-Ag-Cu addition were bonded to Au/Ni metallized Cu pads, and the effect of multiple reflow and aging on impact reliability was investigated. In the Sn-Ag-Cu content Sn-Zn-Bi solder joints, the AgZn₃ intermetallic compound layer was clearly observed at the interfaces and its thickness increased with an increase the number of reflow cycles and aging time. In the solder ball region, fine acicular-shaped Zn-rich phase and spherical-shaped AgZn₃ compound particles were found in szlig-Sn matrix. The shear strength of monolithic Sn-Zn-Bi and 7 wt% Sn-Ag-Cu content Sn-Zn-Bi solder joints after one reflow cycle were about 44.5 MPa and 53.1 MPa, respectively while their shear strengths after eight reflow cycles were about 43.4 MPa and 51.6 MPa, respectively. The fracture surface of monolithic Sn-Zn-Bi solder exhibited a semi ductile fracture mode with a smooth surface while Sn-Zn-Bi solder joints containing Sn-Ag-Cu particles showed a typical ductile failure with very rough dimpled surfaces.