Shear performance of microscale BGA structure Cu/Sn-3.0Ag-0.5Cu/Cu joints with shrinking volume at low and cryogenic temperatures

Shear deformation and fracture behavior of microscale BGA structure Cu/Sn-3.0Ag-0.5Cu/Cu joints with a pad diameter 320 μm and a joint height 220 μm and different solder ball diameters 600, 500 and 400 μm (i.e., different solder volumes) were investigated at a temperature from 25 to -120 ºC in this study. Results show that, when the solder ball diameter of the solder joint is 600 μm, the shear strength increases with the descending temperature; while, as the solder ball diameter of the solder joint decreases to 500 and 400 μm, a peak shear strength appeared at -100 and -80 ºC, respectively. Moreover, the fracture behavior of all the joints is temperature dependent. Regardless of the solder ball diameter, the joint fracture position keeps in the solder matrix at a higher temperature, which shifts to the interface between the solder and the interfacial Cu6Sn5 layer at a lower temperature, and the fracture mode shows a ductile-to-brittle transition with the descending temperature. Moreover, the temperature of the ductile-to-brittle transition is joint size dependent, which is lowered as the solder ball diameter decreases.