Influence of lanthanum addition on microstructure and properties of Sn-3.5Ag solder system

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 solder 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 their microstructure is refined with La addition. After isothermal aging, 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 did not change perceptibly with storage time. The addition of La increased the microhardness of the Sn-Ag solder due to the refining effect of Ag
3
Sn particles in eutectic 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 as revealed in this study. Therefore, La addition helped to improve the microhardness and thermal resistance of the solder joints.