Creep behavior of Cu/Sn-3.0Ag-0.5Cu/Cu solder joints under tensile stress coupled with DC current stressing

Creep behavior of microscale Cu/Sn-3.0Ag-0.5Cu/Cu joints with different thicknesses under electric current stressing was studied in comparison with those without current stressing. The effect of current stressing on creep mechanism was characterized by calculation of the stress exponent (n) of the steady-state creep rate and fractographic analysis of fractured joints. Results show that creep curves of solder joints under current stressing consist of three distinct stages, namely the primary, secondary and tertiary stages. With increasing current density, the steady-state creep rate increases significantly while the creep lifetime decreases. The higher the electric current density is, the higher the steady-state creep rate is and the lower the creep lifetime is. The decrease of joint thickness leads to decrease in steady-state creep rate and increase in creep lifetime under current stressing. The steady-state creep rate of solder joints with a large thickness is more sensitive to the change of the applied tensile stress than that with a small thickness. The value of the stress exponent (n) varies with the current density and joint thickness. Decrease in joint thickness brings about the change of the fracture location from the middle of the solder matrix to the transition region between the solder/Cu₆Sn₅ interface and solder matrix, and correspondingly the fracture mode tends to transform from ductile to a mixed ductile-brittle mode of fracture.