Formation of intermetallics in lead-free systems

Formation and growth of intermetallics are one of the most important problem in the search for reliable lead-free alloys. Since the majority of the commercially available lead-free alloys contain high tin concentration, there is a long-term reliability concern due to tendency of tin to form intermetallics. A prolonged exposure to higher temperatures of lead-free alloys results in the continuous growth of brittle intermetallic layer that is prone to facture thus leading to mechanical and electrical failure of joint. In this work, the formation and growth of intermetallics between lead-free alloys and contact materials such as copper, tin-plated and silver-plated copper materials were studied. For this purpose bimetallic couples formed between commercially available lead free alloys and selected contact materials were subjected to diffusion annealing using thermal gradients and heating by electrical current. Following diffusion annealing, the contact interfaces were subjected to a detailed metallographic, scanning electron microscope (SEM), energy dispersive X-ray (EDX) analyses. In addition, electrical resistance was used for electrical characterization of the intermetallics formed at the contact interfaces. The results of the work enabled to make a comparative assessment of the susceptibility of the lead free alloys to the formation of intermetallics.