Microstructure and mechanical behavior of novel rare earth-containing Pb-free solders

Sn-rich solders have been shown to have superior mechanical properties when compared to the Pb-Sn system. Much work remains to be done in developing these materials for electronic packaging. In this paper, we report on the microstructure and mechanical properties of La-containing Sn-3.9Ag-0.7Cu alloys. The addition of small amounts of La (up to 0.5 wt.%) to Sn-Ag-Cu refined the microstructure by decreasing the length and spacing of the Sn dendrites and decreased the thickness of the Cu6Sn5 intermetallic layer at the Cu/solder interface. As a result of the change in the microstructure, Sn-Ag-Cu alloys with La additions exhibited a small decrease in ultimate shear strength but significantly higher elongations compared with Sn-Ag-Cu. The influence of LaSn3 intermetallics on microstructural refinement and damage evolution in these novel solders is discussed. Our results have profound implications for improving the mechanical shock resistance of Pb-free solders.