Effect Of Anisotropic Diffusivity In ?-Sn Grains On Electromigration Behavior In Cu/Sn/Ni Interconnects

With the rapid development of 3D electronic packaging,the diameter of solder bumps is downsizing sharply.The number of?-Sn grains in solder bumps decrease to several or even one.The solid Sn exists in the?-type possesses a body-centered tetragonal?BCT?structure,which shows a high anisotropic diffusivity.Compared with Cu substrate,the Ni substrate has a lower reaction rate in reflow process.So it is widely applied in electronic packaging industry.Thus,it is vital to understand the effect of anisotropic diffusivity of?-Sn grain on the electromigration?EM?behavior of Cu/Sn/Ni solder interconnect.In this work,the EM behaviors of Cu/Sn/Ni interconnects under a current density of 1.0×10⁴ A/cm² at 150°C have been in-situ studied and compared with Cu/SAC305/Cu interconnects.Modified the dissolution model of Ni substrate based on the fact that?Cu,Ni?₆Sn₅ can prevent the EM-induced diffusion of Ni atoms.The behavior of?Cu,Ni?₆Sn₅ hindering Ni atomic EM induced diffusion was studied.The effect of Sn diffusion anisotropy on the EM behavior of solder joints was excluded by EBSD.?Cu,Ni?₆Sn₅ layer thickness was controlled by using different aging process.The main conclusions are as follows:In this work,the EM behaviors of Cu/Sn/Ni interconnects under a current density of1.0×10⁴ A/cm² at 150°C have been in-situ studied and compared with Cu/SAC305/Cu interconnects.Studies indicated that?1?The consuming thickness of Ni substrate decreases with the increase of angle??between the c-axis of Sn grain and electron flow direction?and follows a linear relationship with cos²?.?2?When electrons flowed from a small angle??-Sn grain to a large angle??-Sn grain,excessive dissolution of cathode Ni occurred and massive precipitation of?Ni,Cu?₃Sn₄ IMCs selectively precipitated in the small angle??-Sn grain.When electrons flowed in the opposite direction,limited dissolution of cathode Ni and precipitation of IMCs occurred.There's void formation and sunken crack propagation at the cathode interface.?3?The EM behaviors of the solder interconnects with different cathode substrate?Ni and Cu?were compared under the same?-Sn grain orientation,and it was found that there was a significant difference in the failure mode.The difference is well explained in viewpoint of the ratio of Ni/Cu diffusion flux and volume shrinkage due to the formation of IMC.?4?Explain the EM-induced diffusion prevention mechanism of?Cu,Ni?6Sn5 in the view point of grain size,grain boundary diffusion flux,thermodynamic stability.The dissolution of Ni substrate is mainly controlled by grain boundary diffusion flux of Ni atoms in IMC.The increase in thickness of IMC will increase the grain size of IMC,resulting in the decrease of grain boundary area of IMC.Therefore,the flux of Ni along grain boundary will decrease.With the content of Ni increase in?Cu,Ni?6Sn5,the thermodynamic stability of?Cu,Ni?6Sn5continuously increase,which means to be more difficult to dissolve during the EM.The slower rate of dissolution suppress the increase of grain boundary diffusion flux,showing the diffusion prevention of Ni atoms.