Abnormal creep behavior of micro-scale Cu/Sn-3.0Ag-0.5Cu/Cu joints with different joint thicknesses under electro-thermo-mechanical coupled loads

Creep deformation and fracture behavior of solder joints with a constant diameter (300 mu m) and the decreasing joint thickness (from 300 to 25 mu m) under electro-thermo-mechanical coupled loads were characterized using a dynamic mechanical analyzer assembled with a DC power source. The results show that the feature of creep curves of solder joints remains unchanged with the decreasing joint thickness, which consists of the primary, secondary and tertiary stages. The steady-state creep rate increases with increasing stress level and temperature, while hardly presenting a monotonical decrease with the decreasing joint thickness, compared with that of joints without current stressing. When the joint thickness decreases from 300 to 25 mu m, the steady-state creep rate first decreases and then increases with an alternate change feature, showing an abnormal creep behavior. Moreover, fracture position shifts from the solder matrix to the interface between the solder matrix and interfacial CU6Sn5 layer in solder joints with thicknesses of 300 and 200 mu m when the temperature is increased, while the creep fracture occurs preferentially only at the solder/Cu6Sn5 interface of all the solder joints with thicknesses of 150, 100, 75, 50 and 25 mu m. Both the abnormal variation of the steady-state creep rate and the fracture behavior are influenced by Joule heating, constraint effect, electromigration, electromigration-induced back stress, grain size of interfacial CU6Sn5 layer, the number and orientation of grains in the solder matrix.