Deformation And Fracture Behavior Of Microscale Lead-free Solder Joints Under Electro-thermo-mechanical Coupled Loads And Their Size Effects

Solder joints?interconnects?are generally regarded as the weakest part in electronic systems and assemblies.Failure of solder joints is the main cause of electronic products and systems.When solder joints are in service,they are usually subjected to electro-thermo-mechanical?ETM?coupled loads.With the global trend of electronic products and systems moving toward miniaturized and multifunctional,the dimension of solder joints and pitches have been continuously scaling down,which results in more severe reliability issues than before due to the higher current density,temperature and mechanical load stressed on the solder joints.In this thesis study,in order to study and evaluate the deformation and fracture behavior and the reliability of solder joints under service conditions,both experimental characterization and finite element?FE?analysis were performed to investigate the tensile and creep deformation,fracture behavior,mechanical performance and their size-dependence of line-type Cu/Sn-3.0Ag-0.5Cu/Cu?or Ni?microscale joints under ETM coupled loads.First,the tensile deformation,fracture behavior and tensile performance of Cu/Sn-3.0Ag-0.5Cu/Cu joints under different electro-mechanical coupled loading modes in room temperature were studied.Furthermore,the influences of joint height?or thickness?,current density and testing temperature on the tensile deformation,fracture behavior and tensile performance of Cu/Sn-3.0Ag-0.5Cu/Cu joints under ETM coupled loads were discussed.Meanwhile,a comparative study of the size effect on the tensile deformation,fracture behavior and tensile performance of solder joints under loading condition without current stressing was conducted.Moreover,the influecnes of current density and joint height on the creep deformation and fracture behavior of Cu/Sn-3.0Ag-0.5Cu/Cu joints under ETM coupled loads were investigated,comparison to the creep deformation and fracture behavior of the solder joints with different sizes under loading conditions without current stressing.Meantime,the fracture behavior and mechanism of Cu/Sn-3.0Ag-0.5Cu/Cu and Cu/Sn-3.0Ag-0.5Cu/Ni joints under electro-thermal coupled loads with the increasing temperature were studied,and the synthesis mechanism of the obtained product?SnO₂?on the fracture surface of the joint was discussed.The tensile deformation,fracture behavior and tensile performance of Cu/Sn-3.0Ag-0.5Cu/Cu joints under three loading conditions,i.e.,tensile,electro-tensile and electro-tensile following electromigration were investigated.Results show that the stress-strain curves of solder joints under electro-tensile coupled loads consist of three stages,i.e.,the fast deformation stage,the linear deformation stage and the accelerating fracture stage,whereas only the last two stages appeared in the stress-strain curves of solder joints under tensile loading and electro-tensile coupled loads following electromigration.In the sequence of the above three loading conditions,the tensile fracture strength of solder joints decreases,and the equivalent modulus increases.Nevertheless,the fracture happens in the solder matrix with a ductile mode regardless of loading conditions.Moreover,the orientation of the?-Sn phase tends to rearrange along the direction of electric current and tensile load.Results on the influences of joint height,current density and testing temperature on the tensile deformation,fracture behavior and tensile performance of Cu/Sn-3.0Ag-0.5Cu/Cu joints under ETM coupled loads show that,the tensile fracture strength of solder joints is severely lowered under ETM coupled loads when comparing with that under the loading condition without current stressing,and the tensile fracture strength increases with the decreasing joint height,showing an obvious size effect.Besides,as the joint height decreases,the fracture position changes from the site in the solder matrix to the site partial in the solder matrix and partial at the interface between the solder matrix and the interfacial intermetallic compound?IMC?layer,exhibiting a fracture mode transition from ductile mode to mixed ductile and brittle mode.The tensile fracture strength of solder joints decreases with the increasing current density and testing temperature.The fracture position remains in the solder matrix of joints with increasing current density from 7.0×10³ to 1.1×10⁴ A/cm²,while it shifts from the site in the solder matrix to the solder/IMC interface as the testing temperature is increased,and the fracture mode of all joint is in ductile.The study on the creep deformation and fracture behavior of Cu/Sn-3.0Ag-0.5Cu/Cu joints under ETM coupled loads with the increasing current density shows that the features of creep curves of solder joints,which consist of the primary,secondary and tertiary stages,remain unchanged;and electric current is proved to be another factor,in addition to stress and temperature,influencing the creep deformation of solder joints under service conditions.With increasing current density,tensile stress level and testing temperature,the steady-state creep rate of solder joints increases and the creep lifetime decreases,and the creep deformation mechanism is only dominated by lattice diffusion.With the increase of current density and testing temperature,the fracture position of solder joints changes from the site in the solder matrix to the solder/IMC interface,and fracture occurs in all solder joints by ductile mode.At different temperatures,when the tensile stress level is lower than the tensile fracture strength,the fracture happens in the solder matrix;when the tensile stress is higher than the tensile fracture strength,the fracture takes place at the solder/IMC interface.Notably,the interfacial fracture happens at either the cathode side or the anode side.Results on the creep deformation and fracture behavior of Cu/Sn-3.0Ag-0.5Cu/Cu joints with the decreasing joint height under ETM coupled loads show that creep curve features keep unchanged.Comparing with the solder joints under loading condition without current stressing,under the same tensile stress and at the same testing temperature,the steady-state creep rate of solder joints under ETM coupled loads is much higher,which also increases with the increasing stress and testing temperature under both loading conditions.Different from that under loading conditions without current stressing,the steady-state creep rate of solder joints under ETM coupled loads does not decrease with the decreasing joint height,while it increasing first and then decreasing with an alternate change.Consequently,the creep deformation mechanism of solder joints with different sizes under loading conditions without current stressing can be determined to be lattice diffusion,while it is unclear for that under ETM coupled loads.The fracture behavior of solder joints with the decreasing joint height is also different under loading conditions with and without current stressing.In solder joints without current stressing,the fracture position changes from the solder matrix with a ductile mode to partial in the solder matrix and partial at the solder/IMC interface with a mixed ductile and brittle mode.However,in solder joints under ETM coupled loads,the fracture behavior varies with tensile stress and testing temperature,and tends to happen at the solder/IMC interface.Results on the fracture behavior of Cu/Sn-3.0Ag-0.5Cu/Cu and Cu/Sn-3.0Ag-0.5Cu/Ni joints under electro-thermal coupled loads indicate that there is a fracture position transition from the solder/IMC interface to the solder matrix as the testing temperature is increased.The interfacial fracture happens at either the cathode or the anode side in Cu/Sn-3.0Ag-0.5Cu/Cu joints,while it occuring only at the Ni side in Cu/Sn-3.0Ag-0.5Cu/Ni joints.The interfacial fracture is triggered by localized-melting of the solder induced by current crowding at grooves of the interfacial IMC grains and driven by the strain mismatch between the solder and the interfacial IMC layer-substrate.Fracture in the solder matrix is caused by matrix bulk melting induced by Joule heating and the constraint effect from the interfacial IMC layer-substrates.After fracture happens in the solder matrix,the resultant product on fracture surface of the joint is confirmed as SnO₂ which exhibits various morphologies with size varying from micro-to nano-scale,is synthesized by the oxidation of Sn in the tin-content alloy utilizing the high temperature and ionized oxygen induced by flashing spark occurring at the fracture moment of the joint specimen.The various morphologies of SnO₂ result from fast flowing and spraying of the molten Sn and the oriented growth of SnO₂.