Nucleation and Growth of Tin in Pb-Free Solder Joints

The solidification of Pb-free solder joints is overviewed with a focus on the formation of the beta Sn grain structure and grain orientations. Three solders commonly used in electronics manufacturing, Sn-3Ag-0.5Cu, Sn-3.5Ag, and Sn-0.7Cu-0.05Ni, are used as case studies to demonstrate that (I) growth competition between primary dendrites and eutectic fronts during growth in undercooled melts is important in Pb-free solders and (II) a metastable eutectic containing NiSn4 forms in Sn-3.5Ag/Ni joints. Additionally, it is shown that the substrate (metallization) has a strong influence on the nucleation and growth of tin. We identify Co, Pd, and Pt substrates as having the potential to control solidification and microstructure formation. In the case of Pd and Pt substrates, beta Sn is shown to nucleate on the PtSn4 or PdSn4 intermetallic compound (IMC) reaction layer at relatively low undercooling of similar to 4 K, even for small solder ball diameters down to < 200 mu m.