Mechanical Failure of Cu-Sn Solder Joints

The effects of the microstructure on the fracture mechanisms of Cu-Sn intermetallic compounds (IMC) in solder joints under tensile load are studied with numerical simulations. When solder joints are exposed to elevated temperature environments Cu\(_6\)Sn\(_5\) and Cu\(_3\)Sn nucleate and grow due to the reaction of Cu and Sn, and the fracture strength is degraded. During early stages of the thermal treatment, the Cu\(_3\)Sn layer of the IMC is thin and composed of a single stack of grains, and the simulations indicate that failure occurs at the Cu\(_6\)Sn\(_5\)/Cu\(_3\)Sn interface. After hundreds of hours at temperatures over 150\(^\circ \) the Cu\(_3\)Sn layer thickness grows and is composed of a stack of several grains. Then, failure switches to intergranular fracture inside the Cu\(_3\)Sn layer. Therefore, the reduction of the fracture strength is a consequence of a transition in the Cu\(_3\)Sn layer from a single stack of grains to a multi-stacked structure. This structural transition occurs by the reduction of grain size and the growth of the Cu\(_3\)Sn layer thickness.