An advanced constitutive model for SnPb and SnAg solder materials

Solder joint integrity is recognized as a key issue in the reliability of electronic devices. In the past decades, SnAgCu eutectic based solders are replacing SnPb eutectic solders in the electronics industry. Although the Pb-free solder is usually regarded as having a higher melting temperature compared with traditional Pb-Sn eutectic solder, solder joints were generally operated at a high homologous temperature, and exhibit complex rate-dependent behaviors. An advanced constitutive model for SnPb and SnAg solder materials is developed in this paper. A unified creep-plasticity constitutive law is adopted to simulate the complex inelastic behaviors of SnPb and SnAg solders. Damage during fatigue loading is predicted using continuum mechanics based method. The hardening of solders is considered combining both isotropic and nonlinear kinematic hardening. The developed model was incorporated into a finite element code. The mechanical properties and damage accumulation of both SnPb and SnAg solders were analyzed. Fatigue life under fully reversed cycling is investigated by monitoring the changing of peak stresses, which will decrease continuously due to damage accumulation. The numerical results were compared with experiment data and shows that the developed model can predict the stress-strain relationship and damage accumulation process of solder materials with reasonable accuracy.