Microstructure And Mechanical Properties Of Sn58Bi/Sn3.0Ag0.5Cu/Cu Solder Joints

With the implementation of the ban on the Pb in the microelectronics industry,lead-free solder have ushered in a good opportunities for development.In order to reduce the manufacturing costs and improve product yield,the manufacturers usually adopts low-temperature brazing in actual production.Due to the advantages of low cost,good wettability and creep resistance,Sn58Bi(SnBi)eutectic alloy is the most widely-used lead-free solder material in lowtemperature packaging.SnBi alloys are the most commonly used low temperature solders due to their low price,good wettability and high yield strength.However,a critical disadvantage of SnBi solder joint is its brittleness induced from the inherent brittleness of Bi element in the as-soldered SnBi/Cu joints,which degrades the overall performances and reliability of the solder joints.Therefore,it is signifcant to suppress the interfacial brittleness of the SnBi/Cu joints and optimization of microstructure for its further industrial applications.In order to reduce the hardness and brittleness of SnBi solder,the advantages of SnBi and Sn3.0Ag0.5Cu(SAC)solders were combined.SnBi/SAC/Cu composite solder joints were fabricated by two-step soldering process to obtain solder joint with superposition structure.To improve the microstructure and comprehensive mechanical properties of the SnBi/SAC/Cu solder joint by the solid-liquid diffusion between the solid SAC and the molten SnBi paste during the second soldering process.The microstructure and hardness of SnBi/SAC/Cu joints were investigated in comparison with SnBi/Cu and SAC/Cu solder joints.It is found that the addition of SAC bulks makes the SnBi/SAC/Cu composite solder joint with obvious superposition structure.Compared with the solder bulks of SnBi/Cu joints,the solid-liquid element diffusion significantly increased the relative areasof ?-Sn in the SnBi region of the SnBi/SAC/Cu solder joints.The hardness of solder bulks and the average thickness of the(Intermetallic Compounds)IMCs are always in the medium position among the three solder joints.In addition,the SnBi/SAC/Cu joints presented a better plastic deformation than the SnBi/Cu joints and a higher shear resistance than the SAC/Cu joints during the shearing process.The failure mode of SnBi/SAC/Cu joints is brittle-ductile failure after shearing.The three kinds of solder joints were also aged for 600 h at 80°C.The atomic diffusion during aging led to the volume reduction of SAC bulk in theSnBi/SAC/Cu joint,the microstructure coarsening occurred in both SnBi and SAC regions in SnBi/SAC/Cu solder joints.Additionally,the SnBi/SAC/Cu solder joints transformed its failure mode from brittle-ductile failure to brittle failure as SnBi/Cu joints after aging.The aging process reduces the plastic deformation ability of SnBi/SAC/Cu composite solder joints,but the hardness and shear resistance of the solder bulks of SnBi/SAC/Cu joints have no obvious decreasing trend before and after aging.