Thermodynamic And Kinetic Investigations Of Some Lead-free Solders Systems

Solders play an important role in the electronic packaging.However,lead (Pb) inthe traditional Sn-Pb based solders is hurtful to the environment and human health.This problem has been paid the attention by all over the world,and many rules werecarried out in USA,Japan,European Unit,and China.Many investigations havefocused on the development of Pb-free solders.According to the recent researches,themost promising candidates should be the multi-component alloys.Thethermodynamic and kinetic calculations have been recognized as an important tool inthe design of materials and investigation on interfacial reaction,because itsignificantly decreases the amount of experimental work.The purpose of this work is to develop an extensive thermodynamic database anda basic kinetic database.In this work,the phase diagrams in some ternary systemswere determined by EPMA (electron probe micro-analyzer) using equilibrium alloysand diffusion couples,and transformation temperatures of alloys were determined byDSC (differential scanning calorimetry).Based on the experimental data determinedin this work and previous literature,some binary and ternary systems werethermodynamically assessed using the CALPHAD (calculation of phase diagrams)method.The kinetic parameters of pure elements and binary systems were alsooptimized in this work.The details of the results are described as follows:(1) Based on the experimental method,(a) the isothermal sections at 300℃,400℃,and 500℃of the Sn-Ag-Au system and the transformation temperatures of alloyswere determined;(b) the isothermal sections at 200℃,250℃,300℃,and 400℃ofthe Sn-Au-Bi system and the transformation temperatures of alloys were determined;(c) the isothermal section at 500℃of the Sn-Cu-Ni and Sn-Au-Cu systems weredetermined;(d) the isothermal sections at 300℃,400℃,and 500℃of the Ag-Bi-Niand Cu-Bi-Ni systems were firstly determined;(e) the isothermal sections at 300℃,500℃,and 700℃of the Bi-Ni-Zn system were determined;(f) the phase equilibriainformation on the Sn-Ni and Sn-Bi-Ni systems was determined,which shows that the Ni3Sn4 phase has a large solubility region.A new intermetallic compound ((?)) wasfound in the Sn-Au-Bi system.The space group of the (?) phase is P63/mmc and itscrystal structure is similar with that of the AuSn phase.(2) Based on the experimental data determined in this work and reported in theprevious literature,(a) the Sn-Ni,Sn-Ag-Au,Sn-Ag-Ni,Sn-Au-Bi,and Cu-Bi-Nisystems were thermodynamically assessed using the CALPHAD method;(b) theSn-Cu-Ni and Sn-Au-Cu systems were basically assessed,where there were slightdeviations between calculated and experimental results,and the thermodynamicparameters should be re-optimized based on further experimental data;(c) theSn-Bi-Ni and Bi-Ni-Zn systems were thermodynamically calculated by extrapolatingthe thermodynamic parameters of the sub-binary systems,and a good agreementbetween calculated and experimental results was obtained;(d) the Ag-Cu-Ni,Ag-Au-Cu,Ag-Au-Ni,and Au-Cu-Ni systems were also thermodynamically assessedas well,and agreement between calculated and experimental results was obtained.(3) Based on the experimental data,the slef-diffusion coefficients of Sn,Ag,Cu,Ni,Au,In,Sb,Zn,and Pb in their stable structures were optimized in the frame ofDICTRA.The self-diffusion coefficients of the pure elements in the Fcc_A 1,Bcc_A2,and Hcp A3 structures were also calculated.Based on the experimental data,kineticparameters in the sub-binary systems of the Sn-Ag-Cu-Au system were optimized,and a good agreement between calculated and experimental results was obtained.Onthe basis of the kinetic parameters of the pure elements and binary systems,thekinetic database was basically developed.(4) The present thermodynamic and kinetic databases were applied in the design oflead-free solders,research of interfacial reactions between lead-free solders andsubstrates,and simulation of movement of interface in the soldering process.TheBi-Ag-Ni solder was made on the basis of the calculation in this work.The calculatedresults illustrate that the CALPHAD method is a powerful tool to design lead-freesolders.The kinetic database was applied in the simulation of diffusion couple and themovement of interface in the soldering process.