Thermodynamic Optimization And Composition Design Of Sn-Based Lead-Free Solder Multiple System

Solder plays a vital role in the electronics packaging process and tin-lead(Sn-Pb)alloys are widely used in micro-connections for electronics due to their stability,low melting point,good fluidity and low price.However,due to the excessive use of lead-containing solders,which has caused heavy metal pollution and even more serious environmental crises,Pb-containing solders have been restricted and standards for lead-free solders have been proposed.Therefore,this project intends to design and prepare a new type of lead-free solder based on the element Sn,with the addition of some other elements,which can meet the characteristics of good wettability,low soldering temperature,low cost and environmental protection.This project takes Sn-based lead-free solder as the research object and carries out the optimisation of the liquid phase parameters of each binary subsystem in the Sn-Ag-Bi-Cu-Ni multiple system,the optimization of the given thermodynamic parameters of each ternary system,the optimisation of some of the thermodynamic parameters,and the calculation of the isothermal cross section and vertical cross section of the ternary system.Based on literature research and evaluation of the thermodynamic parameters of each system,the phase diagram calculation technique(CALPHAD)is used to construct an integrated Sn-Ag-Bi-Cu-Ni multivariate thermodynamic database,which is used for the composition design of lead-free solders.In this work,the binary and ternary thermodynamic parameters and phase equilibrium data were evaluated through literature research and the CALPHAD method was used to optimise the binary and ternary thermodynamic parameters.The data on thermodynamic properties such as activity,solid solubility and phase equilibrium data of the binary systems Ag-Bi,Ag-Cu,Ag-Ni,Bi-Cu,Bi-Ni and Cu-Sn were critically evaluated,and the liquid phase of each binary subsystem was optimised using the MQMPA model,and the data on thermodynamic properties such as phase diagram,activity and enthalpy of mixing of the liquid phase of each binary system were calculated using the optimised parameters The results were compared with the experimental data assessed in the literature,and the calculated results achieved reasonable agreement with the experimental results,validating the thermodynamic optimisation.The phase equilibria of isothermal cross sections,longitudinal cross sections and DSC measurements for the ternary systems Ag-Bi-Cu,Ag-Bi-Ni,Ag-Cu-Ni and Bi-Cu-Ni were also evaluated,and the thermodynamic parameters of each ternary were optimised based on the already optimised binary parameters,combined with the phase equilibrium data of the ternary systems.Based on the optimised thermodynamic parameters,the isothermal and vertical cross sections of the ternary system were calculated and compared with the phase equilibrium data and the DSC data,and it was found that the calculated isothermal cross sections of the ternary system matched well with the phase equilibrium data of the ternary system and the vertical cross sections with the DSC data,which also verified the accuracy of the optimised thermodynamic parameters.The thermodynamic parameters of the binary and ternary systems were then synthesised and summarised to construct an integrated Sn-Ag-Bi-Cu-Ni multivariate thermodynamic database,which was used to design the composition of the lead-free solder and then to prepare the lead-free solder according to the composition points.The composition of the designed lead-free solder is:Mg(0.3-0.4)wt.%,Bi(1.5-2)wt.%,Zn(6.5-8)wt.%,the rest is Sn.The research results of this work provide a theoretical basis and technical support for the design and development of new lead-free solders with low melting point,environmental protection and low cost.