The Influence Of Ag And Ce On The Structure And Properties Of SnBiCu Solder Matrix And Interface

With the implementation of lead-free in the world,most of the electronic solders have changed from lead to lead-free.Sn30Ag0.5Cu?SAC3005?solder alloy has good solderability and thermal fatigue resistance,making it one of the candidates to replace SnPb37 eutectic alloy.However,its melting point is much higher than SnPb eutectic,and the high cost of silver limits its wide application.In this paper,SnBiCu solder alloys with great potential for development are studied from three aspects:performance,cost and impact on environment and human health.SnBi?10-25%?Cu?0.3-0.7%?solder alloys and solder/Cu solder joints were prepared by smelting method.On the basis of this optimum formula,trace Ag and Ce elements were added to improve the properties.Thermo-Calc software,XRD,SEM and metallographic microscope were used to analyze the phase precipitation,microstructure and intermetallic compound morphology and thickness of the solder.DSC,wetting balance and universal material testing machine were used to analyze the properties of the solder.The effects of Bi and Cu on SnBiCu base alloys and Ag and Ce on the properties,structure and interface structure of Sn20Bi0.7Cu solder alloys were studied respectively.The internal relationship between the structure and macro-properties of solder was emphatically analyzed.The formula of SnBiCuAgCe solder alloy which can replace SnPb eutectic is expected through research and analysis.The results show that:1.Bi element can effectively reduce the melting point of solder,but at the same time increase the melting range and produce more defects in solder matrix during solidification process.The increase of Bi content will form a barrier layer between intermetallic compound layer and solder,which will reduce the growth rate of the compound to a certain extent.However,because Bi is a brittle phase,it is easy to cause fracture,so the content of Bi should not exceed 20%.The increase of Cu content will change the precipitation sequence during solidification.When the content of Cu exceeds 0.5%,the first precipitation in liquid phase is Cu₆Sn₅,which plays the role of second phase strengthening and grain refinement.However,excessive Cu content can cause the aggregation and growth of Cu₆Sn₅ phase in the aging process,resulting in microcracks.2.The precipitation order of Sn20Bi0.7CuAg solder alloy added with Ag during solidification is Cu₆Sn₅,?-Sn,Ag₃Sn and Bi.The surface adsorption effect of Ag₃Sn can refine the matrix structure,effectively prevent the growth of intermetallic compounds and inhibit the formation of voids and microcracks at the interface,and the size of SnBi eutectic segregation decreases.Ag can effectively reduce the melting point of the solder,but has little effect on the melting process.With the increase of Ag content,the tensile strength,elongation,yield strength and fracture strength of the solder become higher and higher,indicating that the mechanical properties of the solder become better and better.When Ag content is 1.0%,the mechanical properties of the solder reach the best.3.After adding rare earth element Ce to Sn20Bi0.7Cu1.0Ag solder alloy,the?-Sn phase is refined and the distance between SnBi eutectic region and Sn-rich phase decreases.During aging,segregation of intermetallic compounds into SnBi eutectic is inhibited,and the structure distribution is uniform.Therefore,the elastic modulus,yield strength and elongation of solder can be greatly improved.Ce element can reduce the melting point and melting process of the solder,and 0.05%Ce can greatly improve the wettability and spreadability of the solder alloy.In summary,by analyzing the morphology,thickness and comprehensive properties of intermetallic compounds in the interface structure of solder joints,and by adjusting the content of Bi,Cu,Ag and Ce in solder alloys,the optimum formula of solder is SnBi20Cu0.7Ag1.0Ce0.05.