Constitutive Relations for Finite Element Modeling of SnAgCu in Thermal Cycling -- How Wrong We Were!

The constitutive relations commonly used to model the behavior of SnAgCu solder joints in thermal cycling predict strain rates that are wrong and may be extremely misleading, particularly regarding their dependence on stress. Previous work has already shown cycling induced coarsening to lead to order of magnitude changes in strain rates, and it appears that cycling leads to a change in the dominant deformation mechanism, and thus the dependence on stress, as well. Evidence is presented that the steady state creep rate at stresses and temperatures of relevance tends to be proportional to the stress, rather than varying with the stress to a power of 6 or more as usually assumed.