Primary Creep in Sn-3.8Ag-0.7Cu Solder, Part II: Constitutive Creep Model Development and Finite Element Analysis

A constitutive creep model is presented for Sn-3.8Ag-0.7Cu that incorporates both transient and steady-state creep to provide agreement for both creep and stress relaxation data with a single set of eight coefficients. The model utilizes both temperature-compensated time and strain rate to normalize minimum strain rate and saturated transient creep strain, thereby establishing equivalence between decreased temperature and increased strain rate. The apparent activation energy of steady-state creep, 83.6 kJ/mol, was indicative of both dislocation core and bulk lattice diffusion. A saturation threshold was defined that distinguishes whether transient or steady-state creep is dominant under either static or variable loading.