Mixed Mode I/II Fracture and Fatigue Crack Growth Along 63Sn-37Pb Solder/Brass Interface

Abstract

Solder joints are extensively used in electronic packaging. They provide critical electrical and mechanical connections. Single edge notched sandwich specimens, which were made of two blocks of brass joined with a 63Sn-37Pb solder layer, were prepared for fatigue and fracture study of the joint under mixed mode loading. Mode I and mixed mode I/II fracture toughness, fatigue crack thresholds, and fatigue crack growth rates (FCGR) were measured at room temperature using a four point bending test setup. It was found that the fracture toughness of the joint increased and FCGR decreased upon the introduction of mode II component. The interface fracture toughness was higher than that of reported for pure solder. The data of FCGR correlated well with the power law relation of da / dN = C* (ΔG)m. It was also observed that both fracture toughness and FCGR were a function of thickness of solder layer. When the solder layer thickness increased from 0.1mm to 1.0mm, the fracture toughness decreased substantially and FCGR increased slightly. For mode I loading, fatigue crack propagated inside the solder layer. However, for mixed mode loading, once a crack initiated, it changed its direction toward the interface and then propagated along the interface. These observations were related to local mode I and mode II stress fields. Fracture surface showed sign of rubbing under mixed mode loading with elongated cavities at the crack tip. However, under mode I loading, fracture surface was covered with equi-ax voids.