Research On Ultrasonic Soldering Technology And Joint Reliability Of Pyrolytic Graphite

Pyrolytic graphite is a new type of carbon material.It has high density(2.20g/cm³),high thermal conductivity,high purity,and anisotropy of electrical,thermal,mechanical,and magnetic properties.With the advent of the 5G era,applying pyrolytic graphite with high thermal conductivity to the soaking plate can effectively improve the heat dissipation efficiency of high-power equipment and ensure the normal operation of the equipment.However,pyrolytic graphite is difficult to be wetted by Sn-based solder,making it difficult to braze under low temperature conditions.In this paper,the method of ultrasonic-assisted brazing is used to realize the low-temperature connection of pyrolytic graphite,and the changes in the microstructure and mechanical properties of brazed joints under different process parameters are studied.Abaqus is used to simulate the residual stress and thermal cycle of the test piece,evaluate the reliability of the brazed test piece and predict its fatigue life.The sound pressure distribution of the solder pool in the hot dip test is simulated.The sound pressure in the solder pool is relatively large at the center position,and relatively small at both sides.The sound pressure isosurface usually presents an arc shape parallel to the bottom surface of the liquid solder layer,and the sound pressure value decreases as the distance from the bottom surface increases.Increasing the ultrasonic power can increase the sound pressure value.Using the ultrasonic power when the ultrasonic amplitude is 8?m,the pyrolytic graphite is hot-dipped in the center of the solder pool.When the ultrasonic action time is short,the graphite surface layer appears delamination.As the ultrasonic time increases,the delamination is caused by The direct layering on the graphite side of the interface gradually transforms into a graphite-soldier-graphite layer and is significantly reduced,and finally the solder is completely wetted on the surface of the base material.The pyrolytic graphite after hot dipping was welded together.The microstructure of the brazed joint includes?-Sn phase,Ag₃Sn phase,Sn₃Ti₂,Sn₅Ti₆.When the hot-dip ultrasonic time is longer,a small amount of flake pyrolytic graphite appears in the weld.Increasing the time of hot dipping ultrasonic helps to improve the bonding degree of the brazed joint interface.The shear strength of the joint increases with the increase of the ultrasonic time of hot-dip.When the hot-dip ultrasonic action time is longer,the shear strength of the joint can reach the level of the base material.The residual stress of the pyrolytic graphite brazing specimen was simulated and the fatigue under thermal cycling load was analyzed.It was found that the residual stress level of the solder layer(about 54.7 MPa)was much higher than that of the base material,and the plastic deformation of the solder layer was higher.Small,no toughness damage will occur to the brazing test piece,and it has high reliability.Thermal cycling can alleviate the stress level of the specimen.The increase in the number of thermal cycles will result in the gradual accumulation of the strain energy density and ductile damage coefficient of the solder layer,which will eventually lead to fatigue damage of the specimen.