Effect Of Ni On Interface Reaction And Joint Properties Of Sn58Bi Brazed Joint

The melting point of Sn-Bi lead-free solder is low(the melting point of eutectic composition is 139?),which can reduce the thermal damage of electronic components and substrates in the packaging process.The Sn-Bi lead-free solder has important application value in the field of low temperature lead-free soldering.However,the stability of Aging Microstructure of Sn-58 Bi solder is poor,and in the service environment of 80? to 130?,the Bi will be abnormal coarsening,which leads to the weak impact resistance of the joint.At the same time,the segregation of Bi phase will appear in the Cu3Sn/Cu interface of the brazed joint,it greatly reduces the strength and reliability of the solder joint,which makes the application of Sn-Bi lead-free solder be greatly limited.So in this paper,in constant temperature conditions(120?),the effect of Ni on the interface reaction and properties of Sn-58 Bi brazed joints was studied under the three conditions by Ni particle reinforced solder matrix,surface modification of Cu substrate by Ni,and the combination of the above two ways.The main results show that:The addition of Ni particles into Sn-58 Bi solder can effectively refine the local microstructure of the solder alloy,at the same time,the IMC(intermetallic compound)formed at the interface of the solder joint was Cu(Ni)6Sn5,which promoted the diffusion equilibrium of Cu and Sn,and inhibited the segregation of Bi phase in the IMC/Cu interface.In the aging reaction stage,the addition of Ni particles can inhibit the segregation of Bi phase in the solder,at the same time,the IMC barrier layer at the interface of the solder joint suppresses the diffusion rate of Cu atoms,and indirectly suppresses the segregation of Bi phase in the interface of the solder joint.It not only improves the microstructure stability of Sn58Bi-Ni solder joint in low temperature service environment,but also improves the mechanical properties of brazed joints in service environment.When the Cu substrate was modified by Ni on the surface of the substrate,the Ni element with high melting point in the coating promotes the refinement of the internal structure of the solder,the segregation of Bi phase in the microstructure of solder alloy was delayed,and the Ni3Sn4 structure formed at the interface was stable and not easy to change,which hindered the segregation of Bi phase at the interface.In the aging reaction stage,under the combined action of the IMC layer and the Ni-P barrier layer,the diffusion rate of Cu element is greatly reduced,and the segregation of Bi at the interface is effectively suppressed.The overall stability of Sn-58Bi/Cu(Ni)brazed joint at low temperature and the mechanical properties of brazed joints were improved.The treatment of Ni elements by both of the two ways not only solves the problem that the thickness of IMC layer in the interface of the solder joint is too thick,which results in the decrease of the mechanical properties of the brazed joint,when the Ni particles are added into the Sn-58 Bi solder.It also solves the problem that the growth of IMC layer is restrained,which results in the poor performance of the mechanical properties of the brazed joint,when the Ni is modified on the surface of Cu substrate.And in the aging reaction stage,the diffusion rate of Cu element decreased sharply,the growth rate of IMC layer was the lowest at the three process conditions,and the segregation of Bi at IMC/Cu(Ni)interface was further suppressed,so the microstructure stability of the interface was further improved.The mechanical properties of Sn58Bi-Ni/Cu(Ni)brazed joints have been greatly improved.The influence of three kinds of process conditions on the growth rate constant of IMC layer at the interface of brazed joints is known: Under the condition of combined treatment of Ni elements,the growth rate of IMC layer at the interface of Sn58Bi-0.5Ni/Cu(Ni)brazed joint is the lowest,and it was decreased by 96.19%compared with IMC layer at the interface of Sn58Bi-0.5Cu brazed joint.It shows that the growth rate of IMC layer can be restrained effectively under the condition of Ni joint treatment,and the service life of brazed joint is prolonged.The influence of three kinds of process conditions on the mechanical properties of brazed joints is known: Under the condition of combined treatment of Ni elements,the tensile strength of Sn58Bi-0.5Ni/Cu(Ni)joint is the maximum under the three conditions,and after 9 days of aging,the tensile shear strength of it is still 40.81 MPa,which is higher than the tensile shear strength of Sn-58Bi/Cu joints by 25.67 MPa.