Heterogeneous nucleation of bulk Cu<inf>6</inf>Sn<inf>5</inf> in Sn-Ag-Cu-Al and Sn-Cu-Al solders

Abstract

Cu 6 Sn 5 is a common intermetallic in Pb-free soldering, and the volume fraction, size and shape of Cu 6 Sn 5 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 Cu 6 Sn 5 crystals in Sn-Cu-Al alloys, due to epitaxial nucleation of Cu 6 Sn 5 on either Cu 33 Al 17 or Cu 9 Al 4 particles [1]. In this paper, we explore the extent to which dilute aluminium additions can control the size of large Cu 6 Sn 5 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 Cu 6 Sn 5 in all test samples. For example, a 0.05wt% Al addition in Sn-0.7Cu/Cu joints increased the number of primary Cu 6 Sn 5 per mm2 by a factor of ∼7 and decreased the mean 3D length of primary Cu 6 Sn 5 rods by a factor of ∼4. The average size of Cu 6 Sn 5 rods also decreased significantly after adding 0.05wt%Al to SAC305/Cu joints. A Cu 9 Al 4 or Cu 33 Al 17 particle was found near the centre of many primary Cu 6 Sn 5 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 Cu 6 Sn 5 on γ 1 -Cu 9 Al 4 or δ-Cu 33 Al 17 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.