Interaction Between Anisotropic Diffusivity In β-Sn Grain And Electronmigration Behavior In Flip Chip Lead-free Solder Bumps

Electromigration（EM）,one of the most serious issues for solder interconnects,results in a reliability issue on the failure of bumps due to the demands of continuous miniaturization as well as high-performance.With downsizing,the number ofβ-Sn grains in micro-bumps sharply decreases,while theβ-Sn grain exhibits a high anisotropy,leading to a different EM behavior.Therefore,it is vital to understand the interaction ofβ-Sn grain orientation and the EM behavior of the bumps.In the present work,Ni/Sn-3.0Ag-0.5Cu/Ni-P（ENPIG）and Ni/Sn-3.0Ag-0.5Cu/Cu（OSP）flip chip solder joints were carried out to investigate the interaction ofβ-Sn grain orientation and EM behavior,including the dominant effect of anisotropy inβ-Sn grain on the EM behavior and mechanism onβ-Sn grain rotation.Different failure modes and IMC distribution in the inner of joints were compared.A mathematical model of the relationship between the dissolution of Ni UBM andβ-Sn grain orientation agrees well with experimental results.The main conclusions are drawn as follows:（1）For Ni/Sn-3.0Ag-0.5Cu/Ni-P（EINPIG）flip chip solder joints:When Ni UBM acts as the cathode,failure modes at the cathode interface depend on the angleθbetween the c-axis of Sn grain and electron flow direction.Whenθis 0°,Ni UBM dissolves to fail;whenθis 90°,void and crack propagate to fail at the cathode interface.A relationship model betweenβ-Sn grain orientation and Ni UBM dissolution is established.When Ni-P finish acts as the cathode,the failure mode is void and crack propagation to fail asθis nearly 90°,with no obvious dissolution of Ni-P finish.（Ni,Cu）₃Sn₄-type IMCs precipitate inβ-Sn grains with smallθor along the c-axis ofβ-Sn grains.（2）For Ni/Sn-3.0Ag-0.5Cu/Cu（OSP）flip chip solder joints:When Cu acts as the cathode,failure modes at the cathode interface depend on the angleθ,the same as the Ni/Sn-3.0Ag-0.5Cu/Ni-P（EINPIG）joints.When Ni UBM acts as the cathode,voids form and propagate to fail.Cu₆Sn₅-type IMCs selectively precipitate inβ-Sn grains with smallθor along the c-axis ofβ-Sn grain（3）Grain rotation in flip chip joints under EM stressing is detected and illustrated,which is attributed to different vacancy fluxes caused by EM in between adjacent grains of various grain orientations,when vacancies reach supersaturation and undersaturation at the interfaces of the anode and the cathode,respectively.Vacancy fluxes go through free surface along the interface,resulting in a normal vacancy concentration gradient.Accordingly,stress gradient produces a torque to rotate theβ-Sn grain.