Temperature Dependent Relaxation Behavior of Ag-Sintered Copper Joints

In the field of power electronics device function leads to high operating temperatures. High stresses and strains are induced as a result of mismatch in Coefficient of Thermal Expansion (CTE) leading to plastic deformation in constrained joints. The thermomechanical stresses stimulate mechanisms such as dislocation glide or creep. Especially when the homologous temperature increases, time dependent creep failure becomes a dominant issue in the joint reliability. This work is devoted to the development of a temperature dependent material model for pressure assisted silver sintered joints. This model is validated on the basis of stress relaxation experiments aside from measurements of shear strength and mechanical fatigue. For this purpose, Ag-sintered copper joints were manufactured (0-hour) and heat treated at 250 °C for 250 hours in air and in protective atmosphere. Investigations of microstructure were made by scanning electron microscopy (SEM). Shear and stress relaxation tests were conducted in a tensile machine with thermal chamber at temperatures of 25 °C, 130 °C and 200 °C. For lifetime estimation of the samples Weibull probability plots for low cycle fatigue were determined. Further, Norton power law was employed to determine material parameters such as stress exponent n and creep activation energy Q. In conclusion, a unified model of plasticity and creep was established.