Influence Of Mechanical Properties,Anisotropy And Morphology Of Intermetallic Compounds On Tensile Mechanical Behavior Of Microscale Solder Joints

In electronic packaging,solder joints have the function of mechanical fixation and electrical interconnection.The failure of solder joints will directly lead to the failure of electronic products.During the soldering process,the molten solder will react with the Cu pad to form intermetallic compound（IMC）,which is also a necessary condition for the formation of good welded joints.However,with the development of high-density packaging technology,the size of micro solder joints has reached tens of microns.The proportion of IMC in the solder layer of micro solder joints cannot be ignored,and the formed IMC also has obvious morphological characteristics.Secondly,due to the wide use of high-power electronic equipment and the third-generation semiconductor power devices,the high temperature service environment accelerates the growth of IMC.In addition,the solder body of a single micro solder joint contains only a few or even one grain,and the solder material exhibits obvious anisotropic mechanical properties,which seriously affects the mechanical reliability of micro solder joints.Therefore,under anisotropic conditions,it is of great significance to study the influence of IMC ratio and morphology on the mechanical behavior of micro solder joints to improve the reliability of micro solder joints.In this paper,Cu/Cu₃Sn/Cu₆Sn₅/Cu₃Sn/Cu solder joints were prepared by reflow soldering with pure Sn of 99.99%purity and 50μm thickness.The load-displacement curves,elastic modulus and hardness of Cu₆Sn₅ and Cu₃Sn phases in the joints were obtained by nanoindentation test.The strain strengthening index and yield strength of Cu₆Sn₅ and Cu₃Sn were extracted from the load-displacement curve by using finite element method,dimensional analysis method and inversion analysis method.The elastic-plastic constitutive equations of Cu₆Sn₅ and Cu₃Sn were obtained as isotropic material properties for subsequent finite element analysis.The elastic constant stiffness matrix[C_ij]of Cu,Sn,η-Cu₆Sn₅,η′-Cu₆Sn₅ and Cu₃Sn single crystals was calculated by the first-principles method based on the density functional theory,which was used as the anisotropic material properties of the subsequent finite element analysis.The anisotropy of elastic modulus,shear modulus,bulk modulus and Poisson’s ratio of five materials was analyzed.It was found that only the bulk modulus of Cu was isotropic,and the rest showed obvious anisotropy.The anisotropy of Poisson’s ratio is significantly higher than that of elastic modulus,shear modulus and bulk modulus.Considering the material mechanical anisotropy,the finite element model of Cu/IMC/Sn/IMC/Cu linear micro-solder joint with IMC morphology was simulated,and the effects of material mechanical anisotropy,IMC ratio and IMC morphology on the stress magnitude,distribution and stress concentration in the solder layer of micro-solder joint were studied.The results show that the strength of micro solder joints increases with the increase of IMC proportion in the solder layer.However,the formation of IMC morphology reduces the strength of micro solder joints;secondly,the mechanical anisotropy of materials can significantly change the magnitude and distribution of stress in the solder layer,and easily lead to stress concentration.