Research On The Phase Equilibria And Interfacial Reactions Of Sn-Zn-Bi-Cr/Co Solder Alloy System

In recent years,due to the health and environmental concerns associated with the use of Pb,legislation in the European Union and elsewhere has moved to restrict its use.Thus,the development of alternative lead-free solder alloys is indispensable.Sn-Zn solder is considered to be a candidate of lead-free solder to replace Sn-Pb solder because of its good mechanical properties,economic and safety.However,some shortcomings such as poor wettability and corrosion resistance of Sn-Zn solder have restricted its industrial production applications.Current researches have shown that Bi and Cr can improve the wettability and corrosion resistance of Sn-Zn solder,respectively.Co can be used as a diffusion barrier for the under-bump metallization layer.Therefore,a systematic study of the phase relationship of Sn-Zn-Bi-Cr-Co system and the interface reaction mechanism of Sn-Zn-Bi-Cr/Co will help develop high-performance Sn-Zn alloys and promote application of lead-free solders.In present work,Sn-Zn-Bi-Cr-Co alloy system was selected as the research object.The main conclusions are as follows:1.The 250,450 and 650°C isothermal sections of Sn-Zn-Co ternary system were experimentally investigated,and the existence of ternary compouds T₁,T₂ and T₃ was confirmed.The results from this work showed that the T₂ phase is stable through these temperatures,while T₁ and T₃ phase are stable at 250 and 650°C,respectively.Combining our results with the reported experimental data in the literature,the existence of six three-phase regions at 650°C isothermal section was determined.Taking the adjacent phase relationship into account,three-phase regions can be derived from the experimental data.At 450°C isothermal section,eight three-phase regions are identified.The isothermal section at 250°C includes seven three-phase regions.A thermodynamic description of the Sn-Zn-Co system was obtained using the CALPHAD approach.In conclusion,the agreement between the calculated and experimental results was satisfactory,demonstrating the reliability of the present thermodynamic assessment.2.The isothermal sections of the Cr-Sn-Zn ternary system at 250°C and 450°C were determined.Four three-phase regions were found at 250°C isothermal section,while only one three-phase region can be found at 450°C isothermal section.The existence of the ternary compound T has been confirmed.The experimental results show that the T phase is stable at 250°C and dissolved at 450°C.The investigation of the primary solid phases in the as-solidified samples was performed.Most samples can be found in the solid solution(Cr)primary solidification field,and only one in the(Sn)first solid field.The vertical section of Cr-Sn₅₀Zn₅₀ was measured by DSC,and the solidification process of alloy A9 was analyzed according to the DSC results.The thermodynamic assessment of Cr-Sn-Zn ternary system was performed with the aid of CALPHAD on the basis of both the thermodynamic parameters for the constitutive binary systems and the experimental data emerging from this study.The calculated isothermal sections were compared with the experimental data determined in this work,and reasonable agreement was reached.3.Two isothermal sections at 250 and 450°C of the Sn-Bi-Co system are constructed.Five three-phase regions have been identified in the Sn-Bi-Co ternary system at 250°C,while three three-phase regions have been determined in the 450°C isothermal section.The experimental results show that there is almost no solubility of Bi in the(Co)phase,and the maximum solubility of Bi in Co₃Sn₂,Co Sn,Co Sn₂ and?Co Sn₃are all less than 1 at.%.4.Two isothermal sections of Co-Cr-Zn ternary system at 450 and 600°C were determined.Seven three-phase regions determined in the 450°C isothermal section,five three-phase regions are found in the 600°C isothermal section.It is found that?Co,?Co,?₁ and?have a large solubility range in these two sections,and no ternary compound phase was observed in both cross sections.5.By changing the Cr content,the study found that the addition of Cr element can refine the needle-like Zn phase in the Sn-8Zn-3Bi solder.When the Cr content is further increased,the Cr-Sn phase will appear in the solder.By changing the aging time,it was found that the intermetallic compound layer at the interface becomes thicker with the extension of the aging time,and at the same aging time,increasing the Cr content can inhibit the growth of the intermetallic compound layer,when the Cr content is 0.3wt.%,The compound layer grows the slowest.