Creep behaviour of the ternary lead-free solder alloy : Sn-3.8wt.%Ag-0.7wt.%Cu

Cooper, Shellene (2002). Creep behaviour of the ternary lead-free solder alloy : Sn-3.8wt.%Ag-0.7wt.%Cu. PhD thesis The Open University.



Electronic equipment is facing the challenge of both miniaturisation and the need to replace lead in interconnections. In service, interconnections generally fail by thermomechanical fatigue, and this behaviour is strongly affected by the creep process. This thesis examines the creep behaviour of a popular lead-free replacement alloy, Sn-3.8wt.%Ag-0.7wt%Cu (Sn-Ag-Cu), in joint and bulk form. Experimental work involved the determination of the creep properties of this alloy at various temperatures, over a range of stresses. Over the regions tested, the creep behaviour is best described by the Norton power law constitutive equation. The stress exponent for bulk Sn-Ag-Cu ranged between 10 and 18 (at 125 to -lOoC respectively) and indicates that a dispersion-strengthened mechanism is dominant in the creep process. The activation energy for creep in the bulk Sn-Ag-Cu is approximately 120kJ/moi and falls in the region similar to that observed for the self-diffusion of tin. In joint form the stress exponent is greater than 10 at high stresses but a change in mechanism is indicated at lower stress where the creep exponent falls to 3. The activation energy for creep in Sn-Ag-Cu when used in joint form is approximately 70kJ/moi and falls in the region similar to that observed for the short-circuit diffusion of tin. Results obtained from the ternary alloy were directly compared to those from Sn- 37wt.%Pb (Sn-Pb) and other prospective lead-free alloys in bulk form. The creep resistance of the ternary lead-free alloy at 75°C is superior to the conventional Sn-Pb alloy and the possible replacement alloys (tin-copper and tin-silver). This superiority is retained when tested at similar homologous temperatures. However, the Sn-3.8Ag- O.7Cu alloy is less ductile but generally possesses strains to failure above 10 percent in comparison to the 25 to 50 percent ductility of Sn-Pb.

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