A Viscoplastic Constitutive Model and Estimation Method of Material Constants for Lead-Free Solder Alloys

A viscoplastic constitutive model for lead-free solder alloys was proposed. The proposed model is the elasto-plastic-creep type which comprises the sum of the plastic, transient creep, and steady-state creep strains. To determine the material constants employed by the proposed constitutive model, a simple method to estimate the constants was also proposed. The method was applied to the lead-free solder alloy of Sn-3.0Ag-0.5Cu. Then, all the material constants for Sn-3.0Ag-0.5Cu employed by the constitutive model were estimated only by three tensile tests and one subsequent stress relaxation test. Using the constitutive model with the estimated constants, the ratcheting deformations of Sn-3.0Ag-0.5Cu under different strain rates were simulated. As a result, the simulations could predict the strain rate effect on the ratcheting deformation well. Therefore, the FEM analysis employing the constitutive model with the estimation method for material constants must be useful for the faster selection of the optimum lead-free solder alloys for electronic devices mounting because the material constants in the model can be estimated without conducting the long-time creep tests.