Modelling of Ag3Sn coarsening and its effect on creep of Sn–Ag eutectics

A new constitutive model, which can account for the solderʹs microstructure and its evolution, is proposed to describe the creep behaviour of the Sn–Ag eutectic phase. In this model, the threshold stress, being a function of the particle size, volume fraction and distribution of Ag3Sn intermetallic compound (IMC), is introduced to build the relationship between the creep behaviour of the Sn–Ag solder and its microstructure. Evolution of the eutectic phaseʹs microstructure is accounted for in terms of the coarsening model. Both the creep strain rate and hydrostatic stressʹs influence are taken into account in the IMC coarsening model. The proposed model is implemented into the commercial finite element code ABAQUS. The creep deformation due to the applied stress and IMC coarsening are discussed in the case of a flip chip solder joint. The obtained results show that the shape of the solder joint influences the particle distribution caused by heterogeneous coarsening. The solder joint is softened due to microstructure evolution over a long range of time.