Research On The Interfacial Behaviors Of Sn-58Bi Solder Joints In Coupling Thermo-electric Stress

With the adverse effects of low-melting-point Sn-Pb solder on human and ecological environment,the eutectic Sn-Bi solder has become one of the most promising alternatives to lead-containing solder due to its low melting point and high performance.Therefore,the reliability of Sn-58Bi solder has attracted considerable attentions by many researchers.Because electromigration and thermomigratio are the key parts of reliability research,this paper mainly studies the interfacial behaviors of solder joints under the effect of electromigration and thermomigration in coupling thermo-electric stress.Under the effect of individual electric field,the solid-solid electromigration occurred in Cu/Sn-58Bi/Cu solder joints when the current density was less than or equal to 1.0×10⁴A/cm².With the increase of stressing time,Bi atoms will migrate towards the anode and form a clear Bi-rich layer.Meantime,the Bi phase in the solder matrix will undergo a significant coarsening phenomenon.The interface at the anode would form an IMC-Bi layer-IMC three-stage structure in the early stage when the current density was 1.5×10⁴A/cm².However,if the current density was greater than 1.5×10⁴ A/cm²,a large number of Cu-Sn compounds will generated at the anode while the Cu substrate will be seriously consumed.In the process of solid-solid electromigration,when the applied current changed from low current density to high current density,the mode of Bi diffusion in the solder matrix would have a significant change in which bulk diffusion was the main diffusion mode.The thermomigration of Cu/Sn-58Bi/Cu solder joints under individual thermal field showed the asymmetry growth of IMCs at both ends,which was the biggest change on the interface.The main reason was contributed to the migration of Cu atoms from hot end to cold end under the effect of thermomigration.When the thermal field interacted with the electric field,the microstructure of Cu/Sn-58Bi/Cu solder joints changed significantly.In the process of solid-solid electromigration,the migration speed of Bi atoms increased with the increase of current and temperature provided by the thermal field,so was the product of the diffusivity and effective charge number DZ*.At the same time,the Bi phase coarsening phenomenon will be different under different thermal fields and electric fields.During the liquid-solid electromigration process,the effective charge constant Z*in the solder joint played a key role in the migration of Bi atoms.At high temperature,the effective charge constant of Bi atoms would change from negative to positive,which lead the migration direction of Bi atoms change.When the solder joint was at a specific temperature,it required a threshold current density to promote electromigration.When the ambient temperature was 30,50,70°C,the estimated threshold current was 0.57,0.46,0.33×10⁴ A/cm²,respectively.When the thermal field provided a temperature gradient that can trigger thermomigration,the coupling phenomenon of thermomigration and electromigration occurred in the solder joints.Under a low current density,a Bi-rich layer formed at the anode of the solder joint and there was a large amount of Cu₆Sn₅ compounds in the solder when the anode and the cold end were at the same end.When the anode and the hot end were at the same side,the Bi-rich layer phenomenon disappeared while a large number of Bi-blocks and Cu₆Sn₅ compounds could be found in the solder joints.Under the effect of high current density,there will be a large number of dispersive IMCs and Bi-blocks at the cold side when the anode and the cold side were at the same end.When the anode was at the hot end,the thickness of the IMCs at the cathode will increase as the stressing time prolonged.In the initial melting stage of liquid-solid electromigration,Bi atoms in Sn-58Bi solder will rapidly diffuse and accumulate at the anode,and then began to reversely dissolve into the solder.The driving force attracted to the additional temperature gradient played a role in the reverse dissolution of the Bi atoms.With the applied current density increases,its effect will weaken.