Constitutive and damage model for a lead-free solder

A unified creep plasticity theory with damage is presented for a lead-free solder. The damage is caused by microcracking both inside grains and along grain boundaries and increases with cyclic loading and creep. The Mura theory of microcrack nucleation was used to model the microcrack formation, while the percolation theory was used to characterize the damage such microcracking caused to the mechanical performance of the solder. The model is materials science based and is capable of application to solder joints of different sizes. It is also cast within the framework of phenomenological damage mechanics and is therefore convenient for implementation into commercially available computational software package. The theory was used to model isothermal experimental data for a eutectic solder 96.5Sn-3.5Ag, and good agreement was achieved.