Study On Microstructure And Properties Of Solder Alloy Of Sn-Bi-Ag-(Ni,Si) Photovoltaic Ribbon

In order to solve the energy crisis,since the end of the 20th century,the solar photovoltaic industry has continued to develop at a high rate of 30～40%.The role of photovoltaic ribbon is to connect the cells within the solar PV module as well as the conductive convergence.Sn₆₀Pb₄₀ solder has been widely used in the production of photovoltaic ribbon because of its good weldability and suitable price.However,due to the toxic and environmental pollution of Pb.Moreover,photovoltaic modules are also developing towards thin and miniaturization.The thinner the battery plate is,the lower the welding temperature of the photovoltaic ribbon is.Sn-Bi solder is recognized as an ideal choice for low-temperature lead-free solder due to its low melting point and low cost.But Bi element has poor wettability and is easy to coarsen at high temperature,which deteriorates plasticity and ductility.Therefore,researchers often add a small amount of Ag to Sn-Bi solder to improve its performance.At present,the research on Sn-Bi-Ag solder mainly focuses on the eutectic part of Sn-58Bi and the Sn-10Bi.However,the mechanical properties of eutectic solder are poor and the melting point of Sn-10Bi solder is high,which cannot meet the needs of industrial production.In this work,scanning electron microscopy,energy spectrometer and X-ray diffraction,differential scanning calorimetry,electronic universal testing machine and wetting angle meter were used to determine the effect of Bi or Ag content on microstructure,thermal performance and mechanical properties of Sn-Bi-Ag solders,as well as to explore the effect of adding trace Ni or Si elements on the microstructure and properties of Sn-Bi-Ag solders.Furthermore,the effect of variations in Ag content on the properties of Sn-Bi-Ag-0.5Si solders were also investigated.Main conclusions are as follows:（1）The microstructure of Sn-Bi-Ag solder was mainly composed of（Sn）,（Bi）and Ag₃Sn phases.The tensile strength of the solder decreased with the increase of Bi content,and increased with the increase of Ag content.The wettability of the solder can be improved with the increase of Ag or Bi content.The results show that when the Bi content was 35wt.%or 37wt.%,the melting point and the melting range of Sn-Bi-Ag solder decreased with the increase of Bi content,and the melting point of the solder increased slightly with the increase of Ag content,while the melting range decreased;when the Bi content was 45wt.%or 47wt.%,the melting point and the melting range of the solder decreased with the increase of Bi content,but with the increase of Ag content,the melting point and the melting range of the solder increased slightly.（2）The addition of 0.5 wt.%Ni to Sn-Bi-1Ag solder resulted in the formation of Ni₃Sn₄ phase in its microstructure,which improved the wetting properties of the solder,but reduced the tensile strength of the solder.The results show that there was no significant change in the main peak starting point temperature,the main peak temperature and the secondary peak temperature of the solder after the addition of trace Ni.However,when the Bi content was 45wt.%or 47 wt.%,the addition of trace Ni expanded the melting range of the solder.The thickness of（Cu,Ni）₆Sn₅ layer at Sn-Bi-Ag-Ni/Cu interface was thinner and more continuous and dense than that of the Cu₆Sn₅ layer.What’s more,after 0.5 wt.%Si was added to the Sn-Bi-1Ag solder,the microstructure was refined,and the melting point and melting range of the solder did not change significantly.However,the tensile strength and wettability were improved,and the thickness of the diffusion layer formed with the Cu substrate was also significantly reduced.（3）With the increase of Ag content in Sn-Bi-Ag-0.5Si alloy,the dark black Ag₃Sn phase tended to coarsen.The tensile strength of the solder alloy was further increased,the wettability was further improved and the thickness of the Cu₆Sn₅ layer was further reduced.When the Bi content is 45 wt.%or 47 wt.%,the addition of trace Si enlarged the melting range of the solder,but the melting range was still small.After comprehensive comparison,it is found that Sn-45Bi-3Ag-0.5Si and Sn-47Bi-3Ag-0.5Si solder alloys have more development prospects.This work is beneficial to the development of low-temperature lead-free solder alloy for photovoltaic industry.