Heterogeneous Nucleation of Bulk Cu6Sn5 in Sn-Ag-Cu-Al and Sn-Cu-Al Solders

Cu6Sn5 is a common intermetallic in Pb-free soldering, and the volume fraction, size and shape of Cu6Sn5 particles in the bulk solder plays an important role in the reliability of solder joints. We have previously shown that Al additions can significantly reduce the size of primary Cu6Sn5 crystals in Sn-Cu-Al alloys, due to epitaxial nucleation of Cu6Sn5 on either Cu33Al17 or Cu9Al4 particles [1]. In this paper, we explore the extent to which dilute aluminium additions can control the size of large Cu6Sn5 hexagonal rods that can form during soldering of Sn-0.7Cu and Sn-3.0Ag-0.5Cu (SAC305) BGAs on Cu substrates. We find that Al additions cause significant grain refinement of primary Cu6Sn5 in all test samples. For example, a 0.05wt% Al addition in Sn-0.7Cu/Cu joints increased the number of primary Cu6Sn5 per mm(2) by a factor of similar to 7 and decreased the mean 3D length of primary Cu6Sn5 rods by a factor of similar to 4. The average size of Cu6Sn5 rods also decreased significantly after adding 0.05wt% Al to SAC305/Cu joints. A Cu9Al4 or Cu33Al17 particle was found near the centre of many primary Cu6Sn5 rods and reproducible crystallographic orientation relationships (ORs) were measured by electron backscatter diffraction (EBSD). The grain refinement effect after adding Al is due to the heterogeneous nucleation of Cu6Sn5 on gamma(1)-Cu9Al4 or delta-Cu33Al17 particles. The size of active nuclei was 2-5 mu m in Sn4Cu-0.2Al solders, and <1 mu m in Sn-0.7Cu-0.05Al/Cu and SAC305+0.05Al/Cu joints.