In situ observations on shear and creep-fatigue fracture behaviors of SnBi/Cu solder joints

Shear and creep-fatigue fracture behaviors of the SnBi/Cu solder joints were investigated in this study. The deformation and fracture morphologies were in situ observed by scanning electron microscope, and the fracture mechanisms were discussed based on the observation results. It is found that the SnBi solder in the solder joint shows good ductility under shear stress, there is serious deformation mismatch between the Sn and Bi phases in micro-scale but no macro-scale cracking occurs inside the solder, and the shear fracture occurs along the Cu/solder interface. Under creep-fatigue loadings, the strain of the solder joints increases rapidly during the initial few cycles, but the increase rate decreases due to strain hardening. After the strain hardening becomes saturated, the strain increases exponentially with increasing cycles and the damage inside the solder keeps developing, final fracture occurs inside the solder near the joint interface. As the plastic deformation of the SnBi solder concentrates at the grain boundary, it is predicated that grain-boundary sliding is the major creep deformation mechanism. The influencing factors on creep-fatigue resistance include the stress range, holding time and grain size of the solder. Based on the understandings, techniques to enhance the creep-fatigue resistance were proposed.