Research On The Microstructure And Properties Of Sn58Bi-X Lead-free Solder

With the development requirements of miniaturization,thinning and low cost of electronic products,the risk of cracking,bridging and pillow-head effec failure caused by the warpage and deformation caused by the different thermal expansion coefficients of the materials is increasing,Reducing brazing temperature becomes an effective way to solve the failure problem.In addition,since environmental protection has become a hot issue in the world,the application of lead-free solder has become an inevitable trend.Therefore,the development of lead-free solders with lower soldering temperature and higher soldering reliability has become an urgent research topic in the field of electronic packaging.In this paper,Sn58Bi solder with low melting point is taken as the research object,and multi-element alloying method is adopted to improve its microstructure and properties.Analyze and discuss the influence of alloy elements on the microstructure,wetting properties and mechanical properties of solder and solder joints,and the following conclusions are obtained:(1)The Sn58Bi-Ag-Zn-Sb-Ge alloy system was first proposed.Compared with Sn58Bi solder,when Ag,Zn,and Sb are added together,AgsZns and Sb2(Sn,Zn)3 compound phases are generated in the solder,and the total amount of P-Sn and dendrite content increase significantly,the second The dendrite arm spacing and eutectic layer spacing decreased,and the microstructure was further refined by the addition of trace Ge.(2)An index,the spread area coefficient,which is suitable for evaluating the wettability of the solder,is proposed,and the results of the spread area coefficient are in better agreement with the wettability balance test results of the solder.The wettability of Sn58BilAg solder can be improved obviously when adding 1 wt.%Zn or Sb to the filler metal.The addition of Ag,Zn,Sb and Ge can obviously promote the wetting of the solder.(3)The relationship between the equiaxed eutectic Vickers hardness of Sn58Bi-X solder and its Bi phase content and the eutectic layer spacing was established.With the addition of Ag and Zn solder,the Bi phase content is the main influencing factor of equiaxed eutectic hardness;with the addition of Ag and Sb solder,the Bi phase content is the main influencing factor of the welded equiaxed eutectic hardness,while the aged equiaxed eutectic hardness is jointly affected by the Bi phase content and the eutectic layer spacing.With the addition of Ag,Sb and Zn solders,the equiaxed eutectic hardness is lower than that of Sn58Bi solder,while the addition of trace Ge can increase the aging equiaxed eutectic hardness.(4)The addition of Ag and Zn changes the Sn58Bi/Cu solder joint IMC from Cu6Sn5 to Cu8Zn5+Ag5Zn8.When Ag,Zn and Sb are added at 1wt.%each,the as welded IMC is still Cu8Zn5+Ag5Zn8 After aging,the IMC becomes Cu6Sn5+Cu8Zn5+Ag5Zn8,and the trace Ge addition will produce Cu6Sn5 as welded condition.The growth rate of IMC of Sn58Bi-X/Cu solder joint from low to high is Sn58Bi-lAg-Zn<Sn58Bi-1Ag1Sb1Zn<Sn58Bi-1Ag1Sb1Zn0.008Ge<Sn58Bi-lAg-Sb<Sn58Bi.(5)Compared with Sn58Bi solder,Sn58Bi-lAg1Sb1Zn0.008Ge solder has fine microstructure,improved wettability,reduced IMC growth rate and increased solder joint shear strength by 7%,solder is refined,the wettability is improved,the IMC growth rate of the solder joint is reduced,the shear strength of the solder joint is increased by 7%,showing good comprehensive performance.