Shear deformation of indium solder joints

Indium solder joint deformation was investigated under conditions of shear loading and stress relaxation at room temperature. The deformation behavior was characterized as a function of strain, strain rate, joint thickness, and hold time. Work hardening during loading was found to increase with strain rate in the range of 10/sup -4/ to 10/sup -2/ s/sup -1/ and decrease with joint thickness in the range of 200-600 mu m. The latter suggests that a thick layer of indium will reduce thermal stresses in die attachment applications. The work hardening behavior was also dependent on the amount of recovery that occurred in the prior stress relaxation cycle. For relaxation times of 2 to 13 h, nearly complete recovery occurs if the plastic strain is less than about 0.02. For shorter relaxation times of larger strains, the material was left in a work hardened state. The stress-strain rate characteristics during relaxation were found to be a function of the initial strain level and the spring constant of the assembly. It was demonstrated how a material constitutive relation can be used with the assembly spring constant to calculate stress reduction with time. This procedure can be applied to any electronic assembly which undergoes stress relaxation during a constant temperature hold.<
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