| Solder joints are generally regarded as the weakest part in packaging systems andelectronic assemblies. With the global trend of electronic products and systems movingtoward miniaturized, lightweight and multifunctional, the dimension of solder joints andpitches has been scaling down continuously, bringing a serious challenge to the reliability ofsolder joints. In this study, both experimental characterization and three-dimentional (3D)finite element (FE) analysis based on the framework of linear elastic mechanics, visco-plasticmechanics, fracture mechanics and damage mechanics were performed to study the size andconstraint effects on the mechanical behavior of ball grid array (BGA) structureCu/Sn-3.0Ag-0.5Cu/Cu joints under shear load. Meanwhile, to understand the difference inmechanical performance and the size effect of solder joints between shear and tensile loadingmodes, the mechanical behavior of microscale Ni(Cu)/Sn-3.0Ag-0.5Cu/Ni(Cu)sandwich-structured line-type joints with different solder thicknesses was studied undertensile load. Moreover, the low cycle fatigue performance of microscale BGACu/Sn-3.0Ag-0.5Cu/Cu joints under cyclic shear load was investigated and a fatigue lifeprediction model was proposed in the framework of continuum damage mechanics (CDM).According to the proposed fatigue life prediction model, the thermal fatigue behavior ofmicrobump joints in through silicon via (TSV) structure3D chip stacking packages wasstudied and the fatigue life of the dangerous joint (the joint with the shortest fatigue life) wasevaluated and predicted. Furthermore, the influence of configuration and microstructureinhomogeneity of Cu/Sn-58Bi/Cu joints on the electromigration (EM) behavior of the jointswas studied by cellular automaton (CA) modeling embedded with FE simulation, and themicrostructure evolution in the solder joint under current stressing was simulated.Potential energy and geometry of BGA structure solder joints during reflow solderingwere calculated and simulated by energy-based method Surface Evolver. Results show that,when the diameter of Sn-3.0Ag-0.5Cu solder balls is smaller than760μm, the proportion ofgravitational potential energy in the total energy system of the molten solder is less than0.513%. Moreover, the proportion in the energy system will decrease further with decreasingsolder volume. Thus, the influence of gravity on the geometry of microscale BGA joints isvery limited.Results of the size effect on the mechanical behavior of microscale BGA structureCu/Sn-3.0Ag-0.5Cu/Cu joints under shear load show that, the decrease in standoff height (or the ratio of standoff height to pad diameter) leads to the decrease of softening coefficient ofstress state and torque, resulting in an obvious increase in the stiffness of BGA solder jointsand a very limited effect on stress triaxiality. Moreover, the decrease of standoff height canalleviate concentration of stress and strain energy and then increase the shear strength of BGAsolder joints, and consequently the fracture location of BGA solder joints changes from nearthe interface to the middle of the solder matrix. Furthermore, under the same shear load thecack driving forces (KI, KIIand strain energy release rate) in the intermetallic compound (IMC)layer and at the solder/IMC interface increase obviously with decreasing standoff height,hence the brittle fracture is more likely to occur in the joints with a large standoff height aftera long time thermal aging.Results of the size effect on the mechanical behavior of microscale line-typeNi/Sn-3.0Ag-0.5Cu/Ni joints under tensile load show that the joints have much higher tensilestrengths than that of Cui/Sn-3.0Ag-0.5Cu/Cu joints owing to the strengthening effect of fineAg3Sn particles with a uniform dispersion in the solder matrix. Stress triaxiality depends onthe loading mode and increases dramatically with decreasing solder height (or joint thickness)under tensile load, which is not sensitive to the change of solder height under shear loadingmode. Moreover, with decreasing joint thickness, both stiffness and tensile strength of solderjoints increase obviously under tensile loading mode. Furthermore, being different from thetrend of solder joints under shear load, under tensile load the crack driving forces in the IMClayer and at the interface increase significantly with decreasing joint thickness, hence, thefracture mode of solder joints changes from ductile to brittle.The study on the low cycle fatigue behavior of BGA structure Cu/Sn-3.0Ag-0.5Cu/Cujoints under cyclic shear displacement loading shows that cycle numbers of crack initiationand propagation have power function relationship with the plastic strain energy densitygenerated in each fatigue cycle. Accordingly, a fatigue life prediction model based on plasticstrain energy density is proposed to evaluate the initiation and propagation of fatigue crack,and the relationship between crack growth correlation constants in the proposed model andcontinuum damage mechanics was clarified. Furthermore, results show that crack growthcorrelation constants identified in this study could be well used to predict fatigue life of solderjoints with different geometries and volumes, and the influence of concentraton of plasticstrain energy due to change of joint sizes and configuration can be effectively eliminated.Simulation results of the thermal fatigue behavior of microbump joints in TSV structure3D chip stacking packages show that the outmost microbump joint in the lower array is themost dangerous one during the thermal cyclic load. The decrease in size of microbump joints leads to decrease of fatigue life of the joints, and the fatigue life of the joints with the scale attens of microns or less is mainly determined by the maximum plastic strain energy density.The study on the electromigration behavior of microscale Cu/Sn-58Bi/Cu joints withdifferent configurations shows that both configuration and microstructure inhomogeneity havesignificant influence on magnitude and distribution of current density. However, themicrostructure inhomogeneity exhibits more obvious influence on current density for thejoints with small scale at tens of microns or less. Moreover, the current density in the Sn-richphase is much higher than that in the Bi-rich phase, and Bi atoms in the Sn-rich phase aremore prone to migrating to the anode. Consequently, the damage in the Sn-rich phase couldusually be observed in the experiment. Furthermore, simulation results of separation ofSn-rich and Bi-rich phases by employing the criterion of EM induced atomic flux of Bi in FEanalysis under CA rules are consistent well with the experimental observation under currentstressing. |
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