Study On The Properties Of Sn58Bi-GNSs Composite Lead-free Solder And Its Intermetallic Compound Ni₃Sn₄ Alloys

In modern microelectronic industry,Sn58Bi low-temperature lead-free solder is widely used in the field of low temperature sensitive device packaging.Graphene nannosheets（GNSs）is considered to be one of the best reinforcing phases because of its high mechanical strength,excellent electrical and thermal properties.With the miniaturization and integration of electronic products,the intermetallic compound Ni₃Sn₄,which is produced by the reaction of the Sn58Bi lead-free solder and Ni substrate at the chip end,has gradually occupied a larger volume proportion in the solder joint and attracted wide attention.In this paper,Sn58Bi-GNSs composite solder was prepared by adding graphene as an enhanced phase into Sn58Bi solder.The microstructure,wettability and corrosion resistance of the composite solder were tested.The mechanical properties of its tensile and creep resistance,as well as the microstructure and mechanical properties under the condition of 100oC solid-state aging were studied.Meanwhile,the structure,elastic properties,electronic structure and the effects of Cu elements addition on the stability and properties of Ni₃Sn₄ are systematically studied by the method of first principle calculation.The results show that the addition of GNSs refines the microstructure and improves the wettability and corrosion resistance of Sn58Bi solder.Tensile test and hardness results show that the addition of GNSs makes the ultimate tensile strength and hardness increase up to 14%and 38%,and the ductility increases by 49%after adding 0.01 wt.%GNSs,which brings the transition of brittle to ductile fracture.The nanoindentation tests show that the changed creep mechanism improves the creep resistance of Sn58Bi-GNSs composite solder.In terms of comprehensive performance,the addition of 0.01 wt.%GNSs has the best enhanced properties.After 360 h solid state aging,the addition of GNSs inhibits the roughening and reduction of the structure and mechanical properties in Sn58Bi-GNSs composite solder,and improves the tensile strength and the elastic modulus by 15%and 24%with 0.03 wt.%GNSs addition,respectively.Moreover the addition of GNSs effectively inhibits the crack formation and propagation at the fracture surface.In addition,hardness and tensile strength display a linear relationship,indicating that the mechanical stability of solder is mainly controlled by microstructure.The structural,elastic and electronic properties of ternary intermetallic compound Ni_3-xCu_xSn₄（x=0,0.5,1 and 1.5）are studied by theoretical calculation.The results show that Cu atoms prefer to occupy the Ni 4i site to form a thermodynamicly stable intermetallic compound and the variation of lattice constants indicates the directional change of the atomic binding force in the system.The elastic modulus obtained from single-crystal elastic constants shows a downward trend with the incorporation of Cu atoms,but the anisotropy of Young’s modulus and shear modulus is effectively weakened.The Zener anisotropic index indicates that the addition of21.4 at.%Cu atoms reduces the anisotropy by 13.4%and improves its reliability in the circuit.The density of states and charge density distribution provide that the change of Cu-Ni,Ni-Sn and Cu-Sn bonding affects all the properties in the system.