Constitutive model development for lead free solder alloys at multiple specimen scales

A fundamental study of thermal-mechanical response of Sn3.9Ag0.6Cu at different specimen scales was conducted. The investigation includes aging effects on microstructure and tensile property. It also includes tensile creep behavior and microstructure changes. At all stages, we compared our Sn3.9Ag0.6Cu measurements with the well known 63Sn37Pb lead-tin eutectic. The constitutive models were then developed based on the experimental data.; This work led to some important conclusions, which indicate that (i) the thin cast material exhibited a much finer as-quenched microstructure than the bulk material with the IMC phase restricted to a thin network. Both the bulk and thin cast materials continually softened during room temperature aging, while both materials initially softened and then subsequently hardened when aged at 120°C and 180°C. The thin cast material was in all cases significantly softer than the bulk material, and responded to aging as if it were bulk material aged at a higher temperature, (ii) the Sn3.9Ag0.6Cu alloy showed much lower absolute creep rates than the 63SnPb37. The power law defined stress exponent significantly increases with increasing stress in both the 63Sn37Pb and Sn3.9Ag0.6Cu alloys, therefore the Dorn model is unsuitable for these materials over large stress and temperature ranges. Both sets of experimental data were successfully fit with the present power law stress dependant energy barrier model and the Garofalo model, and (iii) the thin cast material is less creep-resistant than the bulk material. In the bulk material the relevant climb process occurs within a finely dispersed IMC eutectic which covers broad areas within the material. In the thin cast material the relevant climb process occurs primarily in the beta-Sn grains which continuously surround isolated, coarse IMC particles. This resulted in the activation energy of the bulk material being larger than that for the thin cast material. The strength deficiency of the thin cast material is persistent, once the material is cast in thin cast form it will remain weak in comparison to the bulk material.