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 mm² by a factor of ~7 and decreased the mean 3D length of primary Cu₆Sn₅ rods by a factor of ~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 γ₁-Cu₉Al₄ or δ-Cu₃₃Al₁₇ particles. The size of active nuclei was 2-5μm in Sn-4Cu-0.2Al solders, and <;1μm in Sn-0.7Cu-0.05Al/Cu and SAC305+0.05Al/Cu joints.