Damage mechanics characterization on the fatigue behaviour of a solder joint material

This paper presents the first part of a comprehensive mechanics approach ca pable of predicting the integrity and reliability of solder joint material under fatigue loading without viscoplastic damage considerations. A separat e report will be made to present the comprehensive damage model describing life prediction of the solder material under thermomechanical fatigue (TMF) loading. The method is based on the theory of damage mechanics, which make s possible a macroscopic description of the successive material deteriorati on caused by the presence of microcracks/voids in engineering materials. A damage mechanics model based on the thermodynamic theory of irreversible pr ocesses with internal state variables is proposed and used to provide a uni fied approach in characterizing the cyclic behaviour of a typical solder ma terial. With the introduction of a damage effect tensor, the constitutive e quations are derived to enable the formulation of a fatigue damage dissipat ive potential function and a fatigue damage criterion. The fatigue evolutio n is subsequently developed on the basis of the hypothesis that the overall damage is induced by the accumulation of fatigue and plastic damage. This damage mechanics approach offers a systematic and versatile means that is e ffective in modelling the entire process of material failure, ranging from damage initiation and propagation leading eventually to macrocrack initiati on and growth. As the model takes into account the load history effect and the interaction between plasticity damage and fatigue damage, with the aid of a modified general-purpose finite element program, the method can readil y be applied to estimate the fatigue life of solder joints under different loading conditions.