Influence of Lanthanum Additions on the Microstructure and Microhardness of Sn-3.5Ag Solder

This study evaluates the effects of different amounts of lanthanum (La) additions on the microstructure and microhardness of Sn-3.5Ag solders. Sn-3.5Ag-xLa ternary solders were prepared by adding 0 wt.% to 1.0 wt.% La to Sn-3.5Ag alloy. Copper substrates were then dipped in the molten solders and these samples aged at 150°C for up to 625 h. The microstructure and microhardness of the as-solidified solder and the aged solder/copper samples were investigated. The Sn-3.5Ag-xLa solders comprised β-Sn, Ag₃Sn, and LaSn₃ phases, and their microstructure was refined by La additions. As-cast, the addition of La increased the microhardness of the Sn-Ag solder due to the refining effect of Ag₃Sn particles and increased formation of LaSn₃ compounds. As the aging time was increased, the microhardness of the solders decreased and the Ag₃Sn compounds coarsened. However, the coarsening of Ag₃Sn compounds was retarded by La, and the size and amount of LaSn₃ compounds did not change perceptibly with aging time. Therefore, La additions can improve the microhardness and thermal resistance of solder joints.