The Research On The Liquid-solid Interfacial Behavior Of Solder Joint Under The Action Of Ultrasonic Wave

Flux assisted soldering is commonly used in the field of electronic packaging to joining integrated circuit?IC?or other components.However,the flux is easy to produce toxic gas during the heating process and difficult to remove after soldering.The residual flux is easy to corrode the joint,resulting in the decline of the mechanical properties of the joint and reducing the reliability of soldering.Ultrasonic assisted soldering is a kind of joining method which can be directly carried out in atmospheric environment without flux.The reliable connection of the joint can be realized by ultrasonic activation to avoid the corrosion of the joint caused by flux residue after soldering.In order to find out the mechanism of ultrasonic effect on mechanical properties of joints,it is necessary to study the physical mechanisms of ultrasound and the relationship between evolution law of interface and wettability.Therefore,in my study,aiming at the common materials of electronic packaging Al and Ni as the substrate,pure Sn solder as the research object,the interfacial dissolution and wetting behavior of solid solution Al/Sn system and interface reactive Ni/Sn system under the action of ultrasound and the growth mechanism of interfacial intermetallic compound?IMC?are systematically studied.Firstly,the dissolution amount of Al and Ni substrate in molten Sn solder were measured by immersion experiment.The dissolution rate and activation energy treated with and without ultrasonic vibration?USV?were calculation.Based on the finite element analysis?FEA?results of acoustic pressure,ultrasonic streaming distribution and cavitation heat effect in the molten pool,as well as the microstructure analysis,the mechanism of the dissolution promoting by USV is proved.Secondly,by using the method of wetting balance to measure the change of wetting force in Al/Sn and Ni/Sn systems after ultrasonic action,and combining with the method of microstructure analysis,the mechanism of ultrasound enhancing the wetting force of two different systems were proved.Finally,the changes of interface and microstructure of Ni/Sn/Ni joint with or without USV were compared and analyzed,and the changes of Ni₃Sn₄particle morphology of Ni/Sn interface after ultrasonic action were observed and analyzed by selective corrosion and other technologies,and the influencing factors of ultrasonic action on mechanical properties of joint were explored.Through the above experiments and analysis,the main results of this study are as follows:?1?The results of FEA show that the value of acoustic pressure in the molten Sn solder reaches the cavitation threshold,which can cause cavitation bubbles form at the substrate interface.When the cavitation bubble bursts,the temperature of the interfacial micro-region rises rapidly.At the same time,the strong ultrasonic streaming promotes the heat and mass transfer in molten Sn solder.?2?The dissolution of Al and Ni substrate in liquid Sn solder is greatly promoted by USV.The mechanism of the dissolution promoting by USV is mainly the increase of saturated solubility of solder caused by the thermal effect of ultrasonic cavitation,and the acceleration of mass transfer by ultrasonic streaming,which makes the solute at the interface maintain a large chemical potential difference,thus promoting the dissolution rate.For Al/Sn system,the thermal effect of ultrasonic cavitation makes the local micro area of Al wire melt and dissolved,which greatly promotes the dissolution rate.For Ni/Sn system,ultrasonic cavitation is beneficial to the formation,fragmentation and redissolution of Ni₃Sn₄ at the interface,which ensures that Ni substrate can always contact with Sn solder and dissolve into molten Sn solder at a certain speed.?3?The wettability of Al/Sn system and Ni/Sn system were improved by USV.For Al/Sn system,ultrasonic cavitation accelerated the removal of oxide film on the Al substrate,and gradually changed the wetting state of solid Al interface,which increase the wetting force.For Ni/Sn system,the ultrasonic cavitation accelerated the removal of the oxide film on the Ni substrate,promoted the dissolution and diffusion of Ni into Sn solder,and improved interface reactivity.Therefore,the solid-liquid interface tension was reduced,which improve wettability.?4?Ultrasonic vibration is helpful to reduce the thickness of intermetallic compound at Ni/Sn interface,change the morphology of Ni₃Sn₄ grain at Ni/Sn interface,and produce a large number of fine Ni₃Sn₄ particles dispersed in the joint,so as to improve the shear strength of the joint.This study not only contributions to enrich the understanding of the dissolution,wettability,and interfacial IMC growth mechanism at the interface of liquid/solid treated with ultrasonic vibration,but also provides useful guidance for the application of ultrasonic-assisted soldering in electronic packaging and the improvement of joint quality.