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, Ag 3Sn, and LaSn 3 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 Ag 3Sn particles and increased formation of LaSn 3 compounds. As the aging time was increased, the microhardness of the solders decreased and the Ag 3Sn compounds coarsened. However, the coarsening of Ag 3Sn compounds was retarded by La, and the size and amount of LaSn 3 compounds did not change perceptibly with aging time. Therefore, La additions can improve the microhardness and thermal resistance of solder joints.
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