Modeling the deformation behavior of a Sn-Pb solder alloy using the simplified viscoplasticity theory based on overstress (VBO)

Stress-strain diagrams showing the influence of strain rate at five different high homologous temperatures for a Sn-Pb solder alloy are used to determine the material parameters in a minimal version of the viscoplasticity theory based on overstress (VBO), accounting for static recovery effects. VBO is a "unified" state variable theory that does not use a yield surface and has three state variables with appropriate growth laws. It is shown that the effects of strain rate can be modeled well by this theory that requires eight material parameters to describe the inelastic behavior. The behavior in tensile and cyclic strain-controlled loadings, ratcheting, and creep is predicted. The results compare very well with the experiments for tensile and cyclic loading. For the other tests no comparisons were made due to the lack of proper experimental data. The model is formulated in tensorial form and can be used for inelastic stress analyses of solder joints.