Effect of Ag and Cu Concentrations on Creep of Sn-Based Solders

The creep behavior of Sn1Ag0.5Cu, Sn2.5Ag1Cu and Sn4Ag0.5Cu ball grid array (BGA) solder balls and 99.99% pure polycrystalline Sn bulk was studied using impression creep. The microstructures of the as-reflowed solders was characterized. It was found that SnAgCu solders consist of primary dendrites/grains of β-Sn, and a eutectic microconstituent comprising fine Ag₃Sn and Cu₆Sn₅ particles in β. With increasing concentrations of Ag and Cu in the alloy, the proportion of the eutectic microconstituent in relation to the primary β phase increases. In pure Sn and Sn-1Ag-0.5Cu, the β grains form the continuous matrix, whereas in Sn2.5Ag1Cu and Sn4Ag0.5Cu, the eutectic microconstituent forms a continuous network around the β grains, which form isolated islands within the eutectic. The steady state creep behavior of the alloys was dominated by the response of the continuous microstructural constituent (β-Sn or solid solution β for pure Sn and Sn1Ag0.5Cu, and the eutectic microconstituent for Sn2.5Ag0.5Cu and Sn4Ag0.5Cu). In general, the steady-state creep rate decreased with increased alloy content, and in particular, the volume fraction of Ag₃Sn and Cu₆Sn₅ precipitates. The rate-limiting creep mechanism in all the materials investigated here was core diffusion controlled dislocation climb. However, subtle changes in the stress exponent n and activation energy Q were observed. Pure Sn shows n = 5, Q = 42kJ/mole, Sn1Ag0.5Cu shows n = 5, Q = 61kJ/mole, whereas both Sn2.5Ag1Cu and Sn4Ag0.5Cu show n = 6 and Q = 61kJ/mole. Rationalizations for the observed changes of n and Q are provided, based on the influence of the microstructure and the solute concentrations.