Study On Mechanical Properties And Fracture Behavior As Well As Their Size Effects Of Micro-scale BGA Structure Solder Joints Under Coupled Electro-mechanical Loads

Ball grid array(BGA)solder interconnect(joint)and its derivative forms of ?BGA and Tiny BGA as well as other similar drum-like joints have long been widely used in different levels of electronic packaging.Micro-scale solder joints are very important yet usually the weakest part of electronic packages,failure of the joints may often cause loss of function even overall failure of electronic products and equipment.Under real working conditions,micro-scale solder joints are often subjected to simultaneous action of electric,thermal and mechanical loads.With the continuous miniaturization and multi-functionalization of electronic products,the size of solder joints decreases gradually,resulting in supra-normal electric current,temperature and mechanical loads,which may bring about a series of reliability problems for solder joints and packaging structures.This thesis study aims to investigate systematically the mechanical behavior and properties,fracture behavior and failure mechanism of micro-scale BGA structure solder joints under coupled electro-mechanical loads.Firstly,the influence of different loading rates on the shear performance and fracture behavior of BGA structure solder joints is studied;then,the effects of electric current on the shear performance and fracture behavior of BGA structure solder joints are explored comprehensively by means of experimental characterization,theoretical formulation and finite element simulation.The size effects on microstructure,shear performance and fracture behavior of BGA structure solder joints are clarified.The creep performance and fracture behavior of micro-scale solder joints subjected to coupled electromechanical loads and the relevant mechanism are studied deeply.Based on the conclusion that cracks tend to initiate and propagate at the solder/IMC interface under coupled electromechanical loads,as derived from the above study,the theory of linear elastic fracture mechanics is applied to calculation of fracture mechanics parameters of the solder/IMC interface in micro-scale solder joints under coupled electro-mechanical loads;and further,the phase field method is used to study and reveal the initiation and propagation process of microcrack at the interface of the joint under coupled electro-mechanical loads.The experimental results show that the electric current does not change the trend of the shear performance of the BGA structure joint varying with the loading rate.The mechanical properties of micro-scale solder joints decrease with the increase of current density.Finite element simulation results also confirm that there are two regions with high dissipative energy density dominated by shear stress in the solder matrix near the solder/IMC interface of the joints.In addition,at high current density and low loading rate,fracture mainly occurs at the solder/IMC interface of the joints with brittle fracture mode;at low current density and high loading rate,fracture tends to occur in the solder matrix of the joints with ductile fracture mode.Experiments were designed to measure actual temperatures of micro-scale BGA structure joints at different current densities.The results obtained from finite element analysis are almost the same as the experiment measurement results.The finite element simulation results also reveal two types of current crowding effects in micro-scale BGA structure joints,that is,the current crowding occurs at the solder/IMC interface as well as at the electron entrance and exit,and the degree of current crowding increases with the increase of current density,which is the reason why the shear performance decreases with the increase of current density.The crosssection morphologies of the joints after shear tests show that there are three types of fracture modes in the joints under coupled electro-mechanical loads depending on the current density.At low current density,the ductile fracture occurs in the solder matrix near the solder/IMC interface;at moderate current density,ductile-brittle mixed fracture takes place first in the solder matrix and then develops to the solder/IMC interface;at high current density,brittle fracture happens dominantly at the solder/IMC interface.The results of interruption experiments show that the crack tends to initiate at the solder/IMC interface of the joints under coupled electromechanical loads.The microstructures of micro-scale BGA structure joints with different sizes are obviously distinct,which may influence the shear performance of the joints subjected to coupled electromechanical loads.Combined with the analysis of cross-sectional morphologies of the joints after the interruption experiment,apparently,the joints with varying sizes show significantly different fracture modes.For small size joints,crack initiates at the solder/IMC interface,in either the middle or end of the interface,and then grows along the solder/IMC interface first and then turns to propagate along the solder matrix,and the fracture occurs by a ductile-brittle mixed mode.While for large size joints,crack initiates at the end of the older/IMC interface,on either cathode side or anode side,and almost propagates along the solder/IMC interface,and the fracture happens dominantly by brittle mode.It is worth pointing out that for large size joints,when the crack initiates and starts to grow at the solder/IMC interface of one side,the new crack often initiates and propagates at the other solder/IMC interface of the joint,and finally the fracture takes place on both interfaces simultaneously.In-depth study on creep performance of micro-scale BGA structure solder joints under coupled electro-mechanical loads shows that the imposed electric current stressing does not change the three-stage characteristics of creep strain-time curve,which is dominated by the shear load.Definitely,electric current can accelerate the steady-state creep rate by changing the motion speed of atoms and dislocations etc.,thereby promoting creep deformation process and reducing the creep life.A comprehensive analysis of creep activation energy and stress index indicates that upon applying electric current,the dominant steady-state creep deformation mechanism changes from dislocation climbing to lattice diffusion.The study also shows that compared with the Norton power law,the Garofalo hyperbolic-sine creep constitutive relationship is more suitable for describing the steady-state creep deformation of BGA solder joints under coupled electro-mechanical loads.Further,according to experimental creep data of the joints under different current densities,a modified Garofalo hyperbolic-sine creep constitutive equation is obtained,which takes the contribution of current density into account.Based on the theory of fracture mechanics,electrical and thermal theories,the fracture mechanics parameters(i.e.,stress intensity factor and strain energy release rate)have been calculated to characterize crack propagation driving force at the solder/IMC interface of microscale solder joints under coupled electro-mechanical loads.Moreover,combined with the studies in Chapters 2 to 5,fracture mechanic parameters at the interface of joints under different loading conditions are calculated,and the performance of the joints is evaluated,and finally the evaluation results are compared with the experimental results in Chapters 2 to 5.The calculated results and experimental data are very self-consistent,indicating that the method proposed in this study is reliable to accurately evaluate the fracture mechanics parameters of the interfacial crack in the actual solder joint under coupled electro-mechanical loads.Finally,phase field method has also been developed and used to study the initiation and propagation process of micro-cracks in solder joints under coupled electro-mechanical loads,as a supplementary part and in-depth analysis of the above studies.In this section,the electric field energy was added successfully to the free energy density function of the quasi-static crack phase field model,which broadens the application range of the quasi-static crack phase field model,making it capable of being used in the coupled electro-mechanical condition.Then,the phase field models without voids,with one void and multiple voids are established,and the initiation and dynamic propagation process of micro-cracks in micro-scale solder joints under the corresponding conditions are realized by using the optimized quasi-static crack phase field method.This is the first successful attempt in revealing visually the dynamic process of microcrack initiation and propagation under coupled electro-mechanical loads,which is very meaningful and may be a very important reference for future work.