High reliable solder joints using Sn-Pb-La solder alloy

Reliability of solder joints is essential to electronic packaging since they provide both the mechanical and electrical connections at PCB level assembly. Several efforts have been made to improve the mechanical properties of traditional Sn60-Pb40 solder alloys by adding small amounts of alloy elements, such as Ag, Sb, Cu, Pd, etc. (Vaynman et al., 1998; Takemoto et al., 1997; Devore, 1982). Although the provided experimental data illustrated the effect of improvements, the corresponding modification mechanism had been little discussed. In this work, a small amount of rare earth element La was added to Sn60-Pb40 solder in order to improve the reliability of solder joints without increasing the melting point. High temperature tensile tests of solder alloys and thermal fatigue tests of solder joints were conducted to validate the improvement. SEM observation further clarified the corresponding microstructure modification. Finally, the effect of adding small amount of La was deeply studied based upon thermodynamic models and eutectic growth kinetics.