Interfacial Behavior And Electromigration Reliability Of Sn-10Bi-xZn Lead-free Solder Joints

Sn-10Bi solder has high strength due to the solution strengthening of Bi atoms,and its melting point is close to Sn-Ag-Cu solder,so Sn-10Bi solder is taken as the research object in this paper.At the same time,it is found that the addition of Zn can effectively inhibit the growth of Cu-Sn intermetallic compounds,but excessive Zn will affect the mechanical properties of solder joints.Therefore,in this paper,trace Zn was added to Sn-10Bi solder,and the effects of Zn addition on the interfacial reaction,shear properties and electromigration behavior of Sn-10Bi solder joints were studied.With 0.2wt.%Zn addition,the intermetallic compounds of Sn-10Bi-0.2Zn/Cu changed to Cu₆(Sn,Zn)₅,the growth rate of intermetallic compound decreased,the shear force of unaged solder joint decreased,the shear fracture occurred more in intermetallic compound layer,and the shear force of solder joint after thermal aging increased compared with Sn-10Bi/Cu,and the probability of shear fracture occurring in solder matrix increased.In addition,the intermetallic compounds of Sn-10Bi-0.2Zn/Ni changes to Ni₃(Sn,Zn)₄,and the Zn atoms as a diffusion barrier layer inhibits the diffusion of Ni atoms,resulting in the reduction of Ni rich phase in the intermetallic compound layer.Compared with Sn-10Bi/Ni,the shear force of Sn-10Bi-0.2Zn/Ni decreased,and the shear fracture occurred more in the intermetallic compound layer.With the addition of 0.5wt.%Zn,Cu₅Zn₈ is formed at the interface,and then Cu₆(Sn,Zn)₅-Cu₅Zn₈-Cu₆(Sn,Zn)₅ structure is formed at the interface of Sn-10Bi-0.5Zn/Cu.The Cu₆(Sn,Zn)₅ grown on Cu₅Zn₈ is easy to fall off and diffuse into the solder matrix due to the instability of Cu₅Zn₈,resulting in the decrease of the thickness of intermetallic compound layer.At the later stage of aging,Cu₅Zn₈ completely decomposes and the intermetallic compound layer is composed of Cu₆(Sn,Zn)₅.At the same time,with the addition of 0.5wt.%Zn,the shear force of solder joint increases greatly,and the probability of shear fracture in solder matrix increases.After thermal aging,the shear force decreases gradually,and the shear fracture occurs more in the intermetallic compound layer.Ni₃(Sn,Zn)₄ was also formed at the interface of Sn-10Bi-0.5Zn/Ni.A large number of Zn atoms in the intermetallic compound layer act as diffusion barrier layers,which significantly inhibit the growth of intermetallic compounds.The results show that the shear stress of Sn-10Bi-0.5Zn/Ni increases greatly in the unaged and the early aging stage,and the shear fracture occurs more in the solder matrix.The shear stress decreases with the thermal aging,and the shear fracture occurs more in the intermetallic compound layer.When the Cu/Sn-10Bi/Cu solder joint is electrified at room temperature and current density of1.0×10⁴A/cm²,Bi atoms diffuse to the anode continuously.The distribution of Bi phase in?-Sn gradually changes from dispersion to linear distribution.There is no Bi rich layer on anode side,and the growth rate of intermetallic compounds on anode side is much higher than that on cathode side.The thickness of intermetallics layer on cathode decreases gradually during the electrifying process Moreover,Cu substrate of cathode dissolves slightly.After adding 0.2wt.%Zn,the atomic diffusion at the interface is inhibited,the growth of the anode intermetallic compounds and the decrease of the cathode intermetallic compounds are greatly reduced.The addition of 0.2wt.%Zn inhibits the consumption of the Cu substrate on the cathode,and also greatly slows down the excessive growth of the intermetallic compounds on the anode side.