Swirl-like Cu-sn Phases Formation And The Effects On The Ultrasonic Spot Brazed Joint Of Sn-coated Cu Plates

Previous studies have shown that sharp physical and chemical changes may induce the formation of swirl-like reaction phases in the ultrasonic spot welding(USW)of different materials.However,the mechanisms have not been well understood.In particular,swirl-like structures have seldom been discovered or discussed in the USW of Cu/Sn.In this study,the most basic Cu/Sn system metal material in the field of electronic packaging is selected for ultrasonic spot welding experiment.Firstly,the Cu/Sn foil/Cu sandwich structure was used for ultrasonic welding,and the optimal welding parameters were determined as 100-400 J,and the joint with good bonding ability was obtained;the mechanical properties and interfacial microstructure of the joint were analyzed,and the critical welding energy value of swirling phases were determined to be 300 J.Subsequently,the process parameters were adjusted to electroplate a pure Sn layer with a thickness of about 50?m on the surface of the copper substrate,and the Sn-plated layer was directly used as the interlayer solder for ultrasonic welding experiments.The ultrasonic welded joint was obtained under the same welding parameters and compared with the Cu/Sn foil/Cu welded joint.When the welding energy was over 300 J,the swirl-like phases with Cu₆Sn₅ and Cu₃Sn as the main components were also found.Finally,an electroless Sn plating process is used to plate a pure Sn layer with a thickness of about 10?m on the surface of the Cu substrate.Swirl-like phases were observed in the USW of the Sn-coated T2 Cu plate with different welding energies(300-400 J).The evolution of the microstructure from the flat region to the swirl region was studied,the diffusion coefficient of Cu in Sn and the peak temperature of the interface during welding were theoretically calculated and experimentally verified,the formation mechanism of the swirl-like phases was discussed,and the relationship between the joint properties and welding energies was investigated.The results show that the content of Cu₃Sn increases,while that of Cu₆Sn₅ decreases gradually from the flat region to the swirl region and that grain refinement of Cu and IMCs occurs near the swirl regions.The greatly enhanced diffusivity shows that intense atomic diffusion takes place during the welding process.When the welding energy exceeds 300 J,the experimentally measured peak temperatures in the welding processes are higher than the theoretical values.The formation of the swirl-like phase is attributed to the uneven distribution of interfacial stress and the collapse of cavitation bubbles.When the welding energy reaches 400 J,the peak lap shear load of the welded specimen reaches 1008 N,and the associated fractures indicate the combined characteristics of ductility and brittleness.