Study On The Evolution Of Aging Microstructure Of Transient Liquid Phase Lead-free Brazing Joint With High Temperature Resistance

The lead-free solder has been studied for many years,and gradually entered the actual production and manufacturing.In recent years,some researchers have explored alternative methods of high-lead brazing from the connection technology.Among them,the instantaneous liquid phase connection is to use a mixed powder of a high melting point metal and a low melting point metal to mix into a composite solder.During the connection process,the low melting point metal powder is melted to form a liquid phase to realize the connection,and the solid-liquid mutual diffusion with the high melting point metal powder The reaction forms a new high-melting point connection layer,thereby realizing the low-temperature connection of the power chip and resisting high-temperature service.The compound solder was prepared by adding Ag particles to Sn4.7Ag1.7Cu lead-free solder by studying the interconnection process of transient liquid phase connection.Using SEM to observe the structure of the joint under constant temperature and aging,combined with EDS spectrum analysis,analyze and compare the joint structure of the sample under different processes,and compare and analyze their performance.The experimental results show that the structure of the transient liquid phase diffusion welding composite solder joint is compared with the Sn4.7Ag1.7Cu/Cu joint structure.As the aging progresses,the residual Ag particles in the solder continue to melt and are generated in the near area of the joint interface.A large amount of Ag₃Sn compound,and a large amount of Ag₃Sn compound can effectively suppress the diffusion of Sn element in the solder to the Cu substrate,so as to achieve the purpose of inhibiting the growth of Cu₃Sn.As the aging progresses,the mechanical properties of the composite solder joint first rise,then decline and then gradually stabilize.Under the same service conditions,the mechanical properties of the composite solder joint are always better than the Sn4.7Ag1.7Cu/Cu joint.In order to further analyze the influence of Ag particle content and particle size on the structural stability and performance of the composite solder joint.By changing a single variable（Ag particle content or size）and comparing and analyzing the microstructure morphology and performance parameters of the joint under various parameters,it is concluded that as the content of Ag particles inside the solder increases,the wettability of the composite solder Continuous decline.When the content of added Ag particles is less than 25 wt%,as the content of Ag particles increases,the service stability of the composite solder joint structure continues to increase during the constant temperature aging process;when the content of added Ag particles is greater than 25 wt%,with the Ag As the content of particles increases,the service stability of the composite solder joint structure continues to decrease during the constant temperature aging process.Among all the samples,the sample with the Ag content of 25 wt%has the best service stability,and it has not been broken for 15days at 83℃higher than the melting point of the base solder（217℃）,and the joint tensile strength is 25.74 MPa,which achieves the purpose of low temperature welding and high temperature service;As the mesh size of the Ag particles inside the solder increases,the wettability of the composite solder continues to increase,and when the mesh size of the added Ag particles is less than 625 mesh,the wetting angle of the sample is greater than 20°（industry standard requirements）does not meet Claim..In the process of constant temperature aging,when the particle size of Ag is not less than625 mesh,as the mesh size of Ag is added,the service performance of the joint of the composite solder is worse;when the particle size of Ag is not less than 800 mesh,the sample It has not been broken after 15 days of service at 83℃higher than the melting point of the base solder（217℃）.The tensile strength of the sample with 800 mesh of Ag particles added is 22.31 MPa,and the tensile strength of the sample with Ag particles added to 1250 mesh is 24.76 MPa All meet the strength required for service,and achieve the purpose of low temperature welding and high temperature service.The structure of the composite solder joint after constant temperature aging and thermal cycling was observed by SEM,and the structure change of the joint during constant temperature aging and thermal cycling was studied by EDS energy spectrum analysis.The results show that after transient liquid phase diffusion welding,Cu₆Sn₅compounds with higher enthalpy change are preferentially formed at the joint interface and distributed in a bamboo shoot shape;as the constant temperature aging progresses,Cu atoms in the joint diffuse from the copper base material to the solder,The Cu₆Sn₅compound gradually grows and extends,and tends to be flat.The Cu₃Sn compound gradually forms and grows;the Ag₃Sn inside the solder will wrap the unmelted Ag particles and grow,and the thermal effect helps the Ag₃Sn particle size grow.The growing Ag₃Sn compound in the domain helps to improve the service stability of Cu-Sn compounds.