Controlling Bulk Cu6Sn5 Nucleation in Sn0.7Cu/Cu Joints with Al Micro-alloying

We show that dilute Al additions can control the size of primary Cu6Sn5 rods in Sn-0.7Cu/Cu ball grid array joints. In Sn-0.7Cu-0.05Al/Cu joints, the number of primary Cu6Sn5 per mm(2) is 7 times higher and the mean three-dimensional length of rods is 4 times smaller than in Al-free Sn-0.7Cu/Cu joints, while the area fraction of primary Cu6Sn5 is similar. It is shown that epitaxial nucleation of primary Cu6Sn5 occurs on delta-Cu33Al17 or gamma (1)-Cu9Al4 particles, which are stablea in pound the Sn-0.7Cu-0.05Al melt during holding at 250A degrees C. The observed facet relationships agree well with previously determined orientation relationships between delta-Cu33Al17 and Cu6Sn5 in hypereutectic Sn-Cu-Al alloys and result in a good lattice match with < 2.5% lattice mismatch on two different interfacial planes.