Size effect on creep deformation and fracture behavior of micro-scale Cu/Sn-3.0Ag-O.5Cu/Cu solder joints

Creep of solder joints usually takes place in serving electronics due to the high homologous temperature of lead-free solders. With scaling down the dimension of electronics, the size of solder joints shrinks sharply, which may influence significantly the mechanical performance of solder joints, including creep behavior. In this study, the creep behavior of micro-scale Cu/Sn-3.0Ag-0.5Cu/Cu joints with a diameter of 300 mu m and decreasing thicknesses of 200, 100 and 50 mu m was investigated. Results show that the typical "three-stage" feature of the creep curve remains regardless of the change in solder joint thickness. On the other hand, there is an obvious size effect on creep deformation and fracture behavior of solder joints, i.e., when the joint thickness decreases, the steady-state creep rate decreases and the creep lifetime increases, and the fracture mode changes from ductile to the mixed mode of ductile and brittle. However, the creep mechanism is independent of joint thickness. The creep stress exponents are 4.2, 3.8 and 3.1 for solder joints with thickness of 200, 100 and 50 mu m respectively, and the corresponding creep activation energies are 90.0, 62.0 and 66.0 kJ/mol respectively, indicating that the creep behavior of all the solder joints is dominated by lattice diffusion mechanism.