Fatigue fracture mechanisms of Cu/lead-free solders interfaces

The fatigue fracture behaviors of a series of Cu/lead-free solder joints deformed under different directions of loadings were investigated in this study. Observation results showed that fatigue cracks generally initiate around the IMC/solder interface when the loading axis is vertical to the interface. For all the solder joints, the interfacial deformations are resulting from strain localization induced by the stain mismatch. Fracture surface observations reveal that the crack propagation path and fatigue resistance of the solder joints are affected by the yield strength and mechanical property of the solder. When the copper/solder interface is parallel to the loading axis, the interfacial IMC layer failed approximately perpendicular to the interface under the action of slip bands, and then the cracks propagated to the IMC/solder interface, leading to the fracture along the interface.