The life of lead free solder joints under even the simplest long-term service conditions cannot be predicted by following common accelerated test protocols and/or just modifying parameters in current models or expressions for eutectic tin-lead solder (SnPb). Quantitative predictions of long-term life under conditions of vibration, shock, and/or thermal excursions may be off by orders of magnitude. This is particularly critical for the long-term life required of typical military and aerospace applications. The objective of this project was to develop a quantitative knowledge and understanding of lead free solder joints, as well as lead free joints mixed with SnPb.

A systematic study of the formation and evolution of realistic lead free solder microstructure and the associated deformation properties led to a general materials science based understanding and the development of constitutive laws for selected alloys. A range of different testing of both assemblies and individual solder joints provided for correlations with the microstructure and the definition of different damage functions for isothermal and thermal cycling. Experiments were conducted to ensure the applicability of these functions under typical service conditions.

This effort led to the development of a quantitative knowledge and understanding of lead free solder joints, as well as lead free joints mixed with SnPb. This is seriously complicated by the ongoing evolution of solder properties over the entire life. It was shown that the properties of lead free solder joints under any kind of loading are completely determined by their composition and evolving microstructure, and a quantitative materials science based understanding of these was established. Coupled constitutive relations are microstructurally adaptive so that they can self-adjust with dynamic changes in solder microstructure. Deliverables include test protocols and guidelines for the interpretation of test results in terms of life in long-term service and constitutive relations for selected lead free alloys.

Applications of this research include accounting for effects of design, assembly processes, and subsequent thermomechanical history in finite element modeling (FEM), as well as the definition of simpler estimates of acceleration factors. It also allows for the definition of appropriate accelerated tests protocols. Aside from the development and optimization of FEM models using the new constitutive relations, there is also a need for further research to improve predictions. An ongoing National Science Foundation (NSF)-sponsored research effort is leading to an understanding of the effect of amplitude variations on the properties of solder in cycling. Follow-up research will be required to translate this into improved constitutive relations.