Phase Field Study of the Microstructure Evolution and Its Influence on Reliability of Microscale Interconnects under Electro-thermo-mechanical Loads

Author:Liang Shui Bao

Supervisor:zhang xin ping


Degree Year:2019





With the rapid development of integrated circuit package and integration technologies,there is continuous decrease in the size of solder joints and their spacing.Currently,the dimension of solder joints and Cu-filled through silicon via(TSV)structures popularly used in three-dimensional package and integration is usually in micron scale.Then,the evolution of microstructure and its inhomogeneity in solder joints and Cu-filled TSV structures have a more significant effect on performance and reliability of the two microscale interconnect structures under electro-thermo-mechanical loads.In this dissertation,the phase field method is used to study the microstructure evolution and reliability issues in the two types of microscale interconnect structures under electro-thermo-mechanical loads from the perspective of“applied physical fields–microstructure evolution–macroscopic performance and reliability”.Firstly,the nucleation and growth behavior of Kirkendall voids in microscale solder joints is preliminarily studied by phase field crystal model.The phase field method is used to study the evolution of microvoids in solder joints under electric field and temperature gradient,and the effects of the migration and interaction effects of multiple voids on the voltage across the system and the coarsening velocity of voids are deeply discussed.Secondly,a phase field model incorporating the electrostatic free energy and the grain orientation effect is developed and employed to study the grain boundary migration and grain growth behavior ofβ-Sn which is widely used in microscale solder joints,with a focus on clarifying the mechanisms behind the behavior of grain boundary directional migration,preferential grain growth.Then,the microstructural evolution and change in physical and electrical properties of BGA structure Cu/Sn-58Bi/Cu joints under electric field are studied,and the correlation among current density distribution,stress distribution and microstructure characteristics in the solder joints is clarified.Furthermore,the microstructure evolution and change of the heat transfer property in line-type Cu/Sn-58Bi/Cu joints under temperature gradient are studied,and the influence of inhomogeneous microstructure on local heat transfer property of the solder is deeply analyzed.Finally,the phase field method is used to quantitatively study the interaction between Cu grain morphology evolution and thermo-mechanical behavior of the Cu-filled TSV and to reveal the underlying physical mechanism.The results show that the evolution process of Kirkendall voids shows four stages,i.e.,incubation,nucleation,growth and healing stages.The increase in misorientation at the interface will promote nucleation of the voids and lead to the increasing number of Kirkendall voids.The study also manifests that the microvoid migrates to the low potential side of the solder joint under the electric field,and under a weak electric field the migration velocity of the microvoid is constant.The microvoid migration velocity is inversely proportional to the radius of the void,while increasing linearly with the magnitude of the applied electric field.Under a strong electric field,the circular microvoid migrates directionally and changes its shape into narrow slit-like.Under temperature gradient,the microvoids migrate to the cold end,and the migration velocity increases with the increase of temperature gradient.When the temperature gradient is high enough,the microvoid evolution becomes unstable.The uneven temperature distribution in the solder joint leads to the microvoids coalescing into a narrow slit-like microvoid near the low temperature region.Under the electric field the multiple voids migrate to the cathode side,resulting in increase of the voltage in the solder joint,and the variation in the voltage of the solder joint under an electric field with lower magnitude will be larger when the open circuit failure of the solder joint occurs.The electric field can accelerate the coarsening of the voids.The results of the directional migration of grain boundaries and preferential grain growth inβ-Sn under electric field show that grain boundaries in the tricrystal system migrate to the anode side,and the grain grows preferentially when its c-axis is perpendicular to the current direction,which induces the decrease in the voltage across the system.The shrinkage/growth rate of the grain caused by preferential growth of grains is proportional to current density.The circular grains in the bicrystal system migrate toward the anode side,and the shrinkage velocity of the circular grains is lower when the grains’c-axis is perpendicular to the current direction.The high-density electric current can induce the instability of grain morphology evolution,and the voltage across the bicrystal system changes with increasing time.The high-density electric current can induce the competitive growth of grains with different orientations in homogeneous polycrystals;in the inhomogeneous polycrystals,the growth velocity of grains with a small angle between the c-axis and the current direction is decreased,and high-density electric current(1×105 A/cm2)induces the serious instability of grain morphology evolution and grains tend to show nonpolygonal shape.The results of the microstructural evolution and change in physical properties of BGA structure Cu/Sn-58Bi/Cu joints under electric field show that Bi atoms migrate along the electron flow direction,resulting in the formation of Sn-rich and the Bi-rich phase layers respectively on the cathode and the anode sides,and so-induced microstructure is extremely inhomogeneous,leading to the uneven distribution of current density in the solder,and meanwhile there is a tendency for electric current to transfer through the Sn-rich phase and detour the Bi-rich phase.The resistance of the solder joint increases with time when the electric current stressing is applied,and the maximum current density of the solder gradually decreases.The microstructural inhomogeneity results in uneven distribution of von Mises stress,the stress in the Sn-rich phase is higher than that in the Bi-rich phase.The redistribution of Bi-rich and Sn-rich phases leads to decrease of the average von Mises stress in the solder with time.Moreover,the Bi-rich phase exhibits much faster coarsening rate under electric field than isothermal aging.The results of the microstructure evolution and change in heat transfer property in line-type Cu/Sn-58Bi/Cu joints under temperature gradient show that,Bi atoms migrate along the heat flux direction,leading to formation of a Bi-rich phase layer on the cold end and a Sn-rich phase area left on the hot end.Coarsening of the Bi-rich phase takes place faster under temperature gradient than isothermal aging.The heat flux transfers preferentially through the Sn-rich phase while bypassing the Bi-rich phase,which produces an obviously localized temperature gradient between two phases in the solder interconnect.The thermomigration induced segregation of the Bi-rich phase under temperature gradient leads to severe decrease of thermal conductivity of the Sn-58Bi solder interconnect.The results also show that,with increasing temperature,the Cu filler in the TSV involves elastic,elasto-plastic and plastic deformation stages.The grains with different orientations in the Cu-filled TSV make distinct contributions to the elastic and plastic deformation behavior of the TSV,which leads to uneven distribution of the stress and strain in the Cu filler.The average von Mises stress and equivalent plastic strain decrease with increase of the size of grains in the Cu filler.However,the average von Mises stress increases with increase of the average size of grains in the Cu filler when a limited number of grains exist in the Cu filler.With the increase of grain size in the Cu-filled TSV,some Cu grains protrude out from the TSV and the protrusion height decreases firstly and then increases when relatively less Cu grains are in the Cu filler.The delamination of the Cu/SiO2 interface can greatly promote Cu protrusion.For the TSV under service conditions,anisotropic mechanical properties of Cu grains in the TSV significantly affect the thermal stress distribution,and the thermal stress drives the preferential growth of Cu grains with low Young’s modulus in the TSV,leading to decrease of the average von Mises stress and elastic strain energy density in the Cu filler.