Study On The Dissolution And Wetting Behavior Of Sn/Cu System With Ultrasonic And Direct Current

With the development of electronic packaging in the direction of light weight,miniaturization,and high integration,higher requirements are put forward for the reliability of micro solder joints that are interconnected through soldering technology.Therefore,the soldering technology also put forward higher requirements.The application of ultrasound and direct current as a new type of auxiliary material processing and preparation technology has been widely used in the field of soldering.Compared with traditional soldering technology,ultrasonic and current-assisted soldering technology can reduce soldering time and improve efficiency.Meanwhile,no flux needs to be added during the brazing process,which reduces the environmental pollution caused by the volatilization of the flux and the corrosion of the solder joint by the residue after soldering.However,the mechanism of ultrasonic and direct current acting on brazing technology is not very clear,and further research is needed.The dissolution of the base metal and the wettability of the solder on the surface of the base metal are the main factors affecting the reliability of soldering joints.Therefore,Cu and Sn which the most commonly used in the electronics field were selected as the substrate and the solder in this study.The immersion method was used to measure the dissolution of the base metal in the liquid solder and the activation energy of the base metal's dissolution in the liquid solder under different conditions was calculated;The wetting balance method and the sessile drop method were used to measure the liquid solder in the base metal and wetting force and equilibrium wetting angle during surface wetting were measured.combined with finite element simulation and microstructure analysis to systematically studie the dissolution and the wetting behavior with ultrasound and direct current.To explore the influence law of dissolution and wetting under the action of ultrasound and direct current,and lay a theoretical foundation for ultrasonic and direct current assisted soldering.The main findings are as follows:(1)The ultrasonic propagation characteristics on the Cu substrate surface and inside the liquid Sn solder are studied by the finite element simulation software,and found that the distribution of the vibration field on the surface of the Cu substrate and the the sound pressure field inside the liquid solder caused by ultrasound were uneven,but have a certain regularity.The distribution of the vibration field and the sound pressure field is affected by the ultrasonic amplitude,ultrasonic frequency,substrate thickness and the ultrasonic time.Ultrasound causes the maximum sound pressure inside the liquid solder to exceed the cavitation threshold,so the cavitation effect can occur inside the solder,which accelerates the heat and mass transfer inside the liquid Sn.The temperature field of the direct current in the liquid solder was simulated and found that the direct current caused a temperature gradient inside the liquid solder,which led to Marangoni convection.The larger the temperature gradient,the stronger the convection effect.(2)The dissolution behavior of Cu substrate in liquid Sn solder under the action of ultrasonic and direct current was studied by the immersion method.The study found that the dissolution rate with ultrasonic was 7 to 8 times that without ultrasonic.The dissolution rate increased linearly with ultrasound and a parabolic growth without ultrasound.The dissolution rate with direct current was 4 to 5 times that of without direct current.The dissolution rate increased in a parabolic growth with current.The dissolution activation energy of Cu substrate in liquid Sn solder were reduced with ultrasound and current.The application of ultrasound and direct current also has a significant effect on the intermetallic compound layer precipitated at the interface during the dissolution process.The thickness of the intermetallic compound layer decreased with ultrasonic,mainly because the precipitated intermetallic compound was broken and enterd the liquid Sn solder.However,due to the Joule heating effect and electro migration,the intermetallic compound precipitated at the interface Layer increased with direct current.The effective charge number of Cu substrate dissolved in liquid Sn solder was calculated to be 0.69?1.48 at the experimental temperature,and the electro migration force promoted the diffusion of Cu atoms.(3)The wetting behavior of liquid Sn solder on the surface of Cu substrate with ultrasonic and direct current was studied by the wetting balance method and the traditional sessile drop method.The studies have found that the effects of ultrasonic and direct current promote the wettability of the Sn/Cu system.With the ultrasonic time increased,the equilibrium wetting force increased and the zero-cross time decreased.With the ultrasonic amplitude increasee,the wetting angle of the Sn droplets on the Cu substrate surface decreased,and the normalized contact radius increased.As the current intensity increased,the equilibrium wetting force increased,the wetting angle of Sn droplets on the Cu surface decreased,and the normalized contact radius increased.Ultrasonic and direct current have a significant effect on the intermetallic compounds precipitated at the interface during the wetting process.During the spreading of Sn droplets on the Cu substrate,with the increased of the ultrasonic amplitude,the thickness of the Cu6Sn5 layer precipitated at the interface decreased,and the thickness of the Cu3Sn layer increased.There are a large amount of broken Cu6Sn5 phase in the Sn solder.As the current intensity increased,the thickness of the intermetallic compound precipitated at the interface also increased.The improvement of the wettability of the Sn/Cu system with ultrasound and direct current is mainly due to the acceleration of the interface reaction by the action of ultrasonic and direct current,which increases the chemical driving force provided by the precipitation of Cu6Sn5 on the interface.Meanwhile,the action of ultrasonic and direct current accelerates the convection inside the liquid Sn solder,and changes the balance of force in the position of the triple junction,thereby promoting wettability.