Solidification processes in the Sn-rich part of the SnCu system

In this study SnCu solder spheres (Ø 270 μm, CR ~ 1 K/s) were investigated in order to verify the solidified microstructure according to the Sn-rich part of the SnCu phase diagram. The investigated alloys are Sn99.9, SnCu0.25, SnCu0.5, SnCu0.7, SnCu0.9, SnCu1.2, SnCu1.5, and SnCu3.0. In order to understand the solidification process, such aspects as morphology, grain structure and undercooling were analysed. The microstructure was investigated by optical microscopy, SEM and EDX. The undercooling was measured by DSC. It will be shown that small SnCu solder spheres solidify not only with commonly known β-Sn dendrites and fine Cu₆Sn₅ IMCs in the interdendritic spacing, but with specific and systematic changes in morphology, which depend on composition. The successive morphology transitions were found: from 1) fine Cu₆Sn₅ IMCs in β-Sn to 2) small β-Sn cells to 3) β-Sn cellular/dendritic to 4) fine Cu₆Sn₅ IMCs in β-Sn or undirected β-Sn cells. The area fraction of these different morphologies and the number of grain orientations were estimated from the cross-sections of about 20 solder spheres per composition. This allows a quantitative description of the microstructure and its compositional dependency. The results also show that the formation of large Cu₆Sn₅ IMCs provokes more grain orientations compared to SnCu solders solidified without large intermetallic phases.