The Microstructural Aspect of the Ductile-to-Brittle Transition of Tin-Based Lead-Free Solders

This work focuses on specific aspects of the ductile-to-brittle transition in the fracture behavior of tin-based lead-free solders. This transition is essentially associated with the crystal structure of ß-Sn, which is the main constituent of these solders. Moreover, the transition is affected by many factors, including the ambient temperature, the applied strain rate, the mechanical constraint, and certain solder microstructural features such as the shape, size, and spatial distribution of intermetallic particles. Since the mechanical constraint in the solder is related with the specimen dimensions, this work compares the fracture behavior of two different sizes of specimens made of tin-based solders: Rectangular beams and solder joints. Both types of specimens were tested in impact, while the produced fracture surfaces were studied using scanning electron microscopy. The detailed fractography analysis allowed the correlation of the overall solder fracture behavior with certain features in the solder microstructure. This study used also the additional insight into the embrittlement mechanism of tin-based solders to explain previous results from the thermal cycling of eutectic tin-lead solder joints.