Pb-free solder: New materials considerations for microelectronics processing

A single printed circuit board includes thousands, sometimes even hundreds of thousands, of solder joints. The failure of even a single solder joint is usually enough to compromise the functionality of an electronic device or system. PbSn solder had been the standard material for these joints until various regulations around the world began to limit Pb use. SnAgCu and related alloys are quickly replacing PbSn, but much still needs to be understood and controlled. None of the paradigms for understanding the mechanical response of PbSn alloys is applicable to lead-free alloys. Much of the surprising behavior of SnAgCu solder arises from the complex and fascinating nature of its solidification behavior. In this article, the impact of solidification on the microstructure and therefore the mechanical properties of these solder joints will be addressed in the context of microelectronics processing. The need for better simulations of SnAgCu solder behavior will also be examined. Notably, modelers will have to account for a variety of new parameter dependencies not previously considered.