Study of accelerated shear creep behavior and fracture process of micro-scale ball grid array (BGA) structure Cu/Sn–3.0Ag–0.5Cu/Cu joints under coupled electro-thermo-mechanical loads

Shear creep deformation and fracture behavior of micro-scale ball grid array (BGA) structure Cu/Sn–3.0Ag–0.5Cu/Cu joints under electro-thermo-mechanical coupled loads with high-density current stressing (6.0 × 10 3 A/cm 2 ) at different temperatures (40, 60, 80, 100 and 120 °C) and various shear stress levels (12.5, 15, 17.5, 20 and 22.5 MPa) were studied intensively. The results show that electric current stressing can significantly accelerate the creep deformation and fracture process of BGA solder joints in terms of increasing steady-state creep stain rate and shortening creep lifetime of the joints. Garofalo hyperbolic-sine function has shown to be a suitable creep constitutive equation, which can be well used to formulate the creep deformation behavior of micro-scale Cu/Sn–3.0Ag–0.5Cu/Cu joints under electro-thermo-mechanical coupled loads, and the equation has the increased stress exponent of 8.6 and creep activation energy of 90.8 kJ/mol, respectively. Combined theoretical analysis and experimental characterization suggest that electric current stressing leads to the change of the dominant steady-state creep deformation mechanism from dislocation climb to lattice diffusion. The interruption creep tests demonstrate that crack initiates at the interface between the solder and intermetallic compounds (IMC) near the position of electrons entering or exiting the solder matrix when the steady-state creep stage approaches the limit, and then the crack first propagates along the solder/IMC interface and gradually turns to the solder matrix under combined shear and tensile stresses. The joint fractographies exhibit a mixed brittle-ductile fracture mode with one part in the interface and the other part in the solder matrix.