Microstructural evolution of Sn-3.5Ag solder with lanthanum addition

Abstract

The goal of this research is to evaluate the effects of La addition on the microstructure and microhardness of Sn-Ag based solders. Sn-3.5Ag-xLa ternary alloy solders were prepared by adding 0-1.0wt% La into a Sn-3.5Ag alloy. Copper substrates were then dipped into these solders and aged at 150°C for up to 625 hours. The microstructure and microhardness of the as solidified solder and the aged solder/copper couples were investigated. Experimental results show that Sn-3.5Ag-xLa solders are composed of β-Sn, Ag 3 Sn, and LaSn 3 phases, and that their microstructure is refined with La addition. After isothermal storage, the Ag 3 Sn and IMC layer can be effectively depressed by adding a small amount of La element, and the size and amount of LaSn 3 compounds does not change perceptibly with storage time. For as-cast solder, the addition of La increased the microhardness of the Sn-Ag solder due to the refining effect of Ag 3 Sn particles in the eutectic zone and increased formation of LaSn 3 compound. After isothermal storage, the microhardness of solders was decreased with the increasing coarsening of Ag 3 Sn compounds as aging time was increased. However, coarsening of Ag 3 Sn compounds was retarded by La addition. Therefore, La addition helped to improve the microhardness and thermal resistance of the solder joints.