| At present,the soldering of Sn-based lead-free solder and Cu base metal has been widely used in the electronic packaging industry.How to ensure the quality of solder joints is still an important research topic.In order to solve this problem,scholars have proposed many methods,for example,by applying physical fields such as electric fields,magnetic fields,sound fields,and the combined fields of composite interactions to improve the reliability of solder joints.Among them,ultrasonic-assisted connection is considered to be a green,efficient,and promising connection method.However,little research has been done on the dissolution mechanism and mass transfer mechanism of metallic base materials under the action of sound fields.Therefore,in this paper,the Cu/Sn system is used as the research object,and the dissolution mechanism of solid metal in liquid metal and the mechanism of solid-liquid mass transfer under ultrasonic action are systematically studied through experiments combined with finite element simulation.This paper first uses the multiphysics coupling numerical calculation software COMSOL Multiphysics 5.4 to simulate the sound pressure distribution in the molten Sn solder under the power ultrasonic of different parameters,to obtain the possible cavitation area in Sn liquid,that is,the cavitation active area.Then derive the sound pressure at the Cu/Sn solid-liquid interface,and use the sound pressure here to calculate the effective temperature and pressure of the Cu/Sn solid-liquid interface when the cavitation bubbles collapse.And the flow field distribution caused by ultrasonic vibration in the molten pool is simulated,and the experimental results are theoretically analyzed using the above data.The immersion method was used to investigate the dissolution behavior of Cu/Sn system with or without ultrasonic action at temperatures of 523 K,553 K and 573 K.The experiment found that the dissolution rate of Cu wire in molten Sn under the action of ultrasound was 6.790 to 24.106 times that without ultrasound.The results show that the collapse of the cavitation bubble will cause a high temperature of about1500 K locally at the Cu/Sn interface,which not only increases the solid solubility limit of Cu in the Sn solution,but also causes Cu in the "micro-dot" region to melt;The high pressure generated by the microjet effect is 85 MPa,which is enough to reduce the thickness of the intermetallic compound layer(IMCs)and change its morphology,and increase the channel for atom diffusion;The acoustic flow effect willproduce agitation,which will continuously push the solute Cu atoms at the leading edge of the Cu/Sn solid-liquid interface into the Sn liquid,so that the solute atom solubility is always lower than the saturated solubility.Therefore,the dissolution amount and dissolution rate of solid Cu in Sn solution increase significantly under the action of ultrasound.Although ultrasound can promote dissolution,greatly shorten the brazing time and improve efficiency.However,it was found through experiments that the dissolution was uneven along the height of the Cu wire,and the fastest dissolving part of the brazed joint may have a hidden failure.By calculating the dissolution rate constant K? 1.26762 without ultrasonic effect at 573 K,and K?2.45173 with ultrasonic effect,modeling and analysis of the reason why K becomes larger from the viewpoint of mass transfer,It is expected to provide theoretical basis and technical guidance for ultrasonic-assisted brazing. |
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