Phase-field Method Study Of Micro Solder Joint Reliability Uder Thermo-mechano-electrical Coupling

The continuous miniaturization of electronic products has brought huge challenges to packaging technology,which has led to a multiplied increase in the electrical,thermal and mechanical load bore by solder joints in electronic components.The failure of one solder joint will lead to failure of the whole electronic components.Therefore,the research on the reliability of solder joints under the thermo-mechano-electrical coupling is one of the most concerned fields in packaging technology.The reliability of solder joints can be divided into two categories:on the one hand,the continuous growth of intermetallic compounds（IMCs）in solder joints leads to the degradation of mechanical properties;on the other hand,under the action of electromigration,the void will form and propagate at the cathode,which will eventually lead to the electrical failure and void initiation failure of solder joint,and these two failure mechanisms compete with each other.In this work,the growth of IMCs and failure mechanism in Cu/Sn/Cu solder joints under the thermo-mechano-electrical coupling were studied in detail.（1）A strong coupling thermo-mechano-electro-diffusional phase field model is presented and used in investigating the reliability of solder joint during service.（2）The curves of diffusion coefficient and effective charge number of Cu and Sn with concentration were fitted respectively to characterize the inter-diffusion behavior of Cu and Sn in four different materials（Cu,Sn,Cu₃Sn and Cu₆Sn₅）of solder joint based on the experimental data.（3）By comparing the classical mass flux equation with the mass flux equation in the strong coupling thermo-mechano-electro-diffusional theory in solder joint,the equivalent relationship between back stress and volume strain is obtained.（4）The stress intensity factor（SIF）and energy release rate（ERR）are calculated by stress-based extrapolation method,and the mixed mode fracture toughness is introduced as the critical value to judge the void initiation failure of solder joint,which provides the basis for clarifying the competitive failure mechanism.The simulation results show that the growth curves are well consistent with the experiment values at low current densities when the coupled phase field model is adopted.The reasons for the poor coincidence at high current density are investigated and explained.In terms of thermodynamic driving force,the evolution rule of total driving force is consist with IMCs growth rate.The back stress will accelerate the growth of IMCs,while the temperature gradient driving force has little effect on the growth of IMCs.Based on the equivalent relationship between back stress and volume strain,the evolution of back stress in the growth process of IMCs is represented,and the definition of back stress proves that the equivalent relationship is valid.The inhomogeneous distribution of electromigration and back stress driving forces leads to the inhomogeneous growth of IMCs,and the concentration coefficienta^F_totalis defined to represent the inhomogeneous of the total driving force distribution.The electromigration-induced void propagation occurs at the interface between the solder and IMCs,and leads to thermal,mechanical and electrical concentration effect at the void tip and electrical failure eventually.The coupling coefficienta_C is discussed through parameterization to obtain that the strain gradient driving force can hinder the electromigration.The criterion for determining the electrical failure is a voltage increase of20%.However,in the process of void propagation,the ERR of the void tip is larger than the critical value will directly lead to the void initiation failure.Among them,the ERR at the void tip is obtained by stress-based extrapolation method,where the mixed mode fracture toughness is introduced as the critical value.Based on the relationship between the two failure mechanisms induced by the void propagation,the safe load area of the external loads are obtained,that is,no void initiation failure of the solder joint occurs in the safe load area.