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.