IMPROVED MECHANICAL-PROPERTIES IN NEW, PB-FREE SOLDER ALLOYS

The mechanical properties of solders benefit from uniform dispersion of fine precipitates and small effective grain sizes. Metallurgical methods of attaining such a beneficial microstructure have been investigated in two new, near-eutectic, Pb-free solder alloys systems-Sn-Zn-In (m.p. approximately 188-degrees-C) and Sn-Ag-Zn (m.p. approximately 217-degrees-C). It has been found that small alloying additions of Ag dramatically improve the mechanical properties of the ternary Sn-8Zn-5In alloy. The improvement is attributed to the elimination of the coarse and nonuniform distribution of plate-like dendrites and refining the effective grain size in the solidified microstructure. Also, small amounts of Cu dramatically improve the ductility in the ternary Sn-3.5Ag-1Zn alloy. The quaternary Sn-3.5Ag-1Zn-0.5Cu has better mechanical proper-ties than the binary Sn-3.5Ag alloy because it has a uniform fine dispersion of precipitates and a small effective grain size. The combination of high mechanical strength and high ductility is likely to yield improved fatigue resistance properties in the interconnection of electronic components.