Research on Interfacial Bonding Mechanism and Process of Ultrasonic Assisted Active Soldering SiC Ceramic

Author:Wu Bing Zhi

Supervisor:zuo jiu chun


Degree Year:2019





SiC ceramic is widely used in the aerospace industry as a structural material due to its excellent performance.Meanwhile,due to its excellent characteristics of low thermal expansion coefficient and high thermal conductivity,the joining of SiC ceramic and Cu is also widely applied in the field of electronic packaging.Conventional active brazing requires high temperature and high vacuum conditions,and there is a problem of large residual stress in the joint.For the electronic packaging application,the joining of ceramic at low temperature is necessary.Because ceramic materials are difficult to be wetted at low temperature,traditional ceramic packaging at low temperature can only be achieved by indirect soldering.This paper focuses on overcoming the difficulty of thermal stress of joints and the low temperature wetting in filler metal/ceramic system.In this thesis,the high reliability direct bonding of SiC ceramic itself and Cu was realized by ultrasonic-assisted active soldering.The interfacial reaction mechanism of the soldered joint was clarified.The main controlling factors of rapid interfacial reaction caused by ultrasound was revealed.The interfacial bonding structure and mechanical properties of joints were optimized by adjusting the process parameters.The SiC ceramic was soldered with Zn-based filler metal containing active element Al.The effect of ultrasonic action time on the microstructures and properties of the joint was studied.The typical interfacial microstructure was determined and the strengthening mechanism of the joint was proposed.For the ZnAl solder,the shear strength of the soldered joints was only 102 MPa when the ultrasonic action time was shorter,and fractures occureed in the brittle lamellar eutectic phases in the center of the bond layer.With increasing ultrasonic action time,the lamellar eutectic phase in the bond layer of SiC joints could be completely transformed to a fine non-lamellar eutectic structure.Meanwhile,the grains in the bond layer were obviously refined.Those results led to the remarkable enhancement of the shear strength of the joints(138 MPa)using the ZnAl solder,which had approached that enhancement using the ZnAlCu solder.The enchanced mechanical properties of the joints were attributed to the significant refinement of the grains and the change in the eutectic structure in the bond layer.Prolonged enhanced heterogeneous nucleation triggered by ultrasonic cavitation was the predominant refinement mechanism of the bond metals of the SiC joints.To determine the interfacial bonding mechanism of the ZnAl/SiC system,the interface microstructure was characterized by FIB/TEM.Experimental results indicatd that a nano-thickness amorphous Al2O3 transition layer was formed at the interface.The effect of ultrasonic action time on interface microstructure was also studied.The formation of Al2O3 could be summarized into two stages.The first stage was the displacement reaction between the active Al in the filler metal and the SiO2 which was the oxide of SiC ceramic;the second stage was the deposition reaction between active Al and dissolved oxygen in the melt.The characteristics of the interface reaction with and without ultrasound were compared.The acceleration effect of ultrasound on the interfacial reaction was confirmed based on the thermodynamic analysis.The sound pressure distribution in the narrow gap melt and its influencing factors during ultrasonic soldering were studied by finite element simulation.The sound pressure field in the melt was alternately distributed in the positive and negative pressure.The change of ultrasonic amplitude did not affect the distribution characteristics of sound pressure field,but the amplitude of sound pressure increased as the amplitude increased.When the overlap length of the base metal was changed,the distribution of sound pressure field changed obviously.The growth characteristics of bubble in the melt under ultrasonic excitation and the influencing factors of ultrasonic cavitation phenomenon were studied by solving the Keller-Miksis equation.When the driving sound pressure was low,steady-state cavitation occured.The cavitation bubbles growth exhibited nonlinear oscillation,and the oscillation can extend in multiple sonic cycles.When the driving sound pressure exceeded the cavitation threshold,the bubble underwent a process from explosive growth to violent rupture in one sound cycle.In each cycle,the radius of bubble would be changed by several orders of magnitude.The buble will eventually collapse rapidly under the positive acoustic pressure,namely transient cavitation.nder certain sound pressure conditions,the initial bubble which was close to the resonance size was conducive to the generation of violent transient cavitation.The steady-state cavitation had little effect on temperature and pressure in the bubble.The cavitation rupture process during transient cavitation produced extremely high temperature and pressure effects.Based on the above results,it could be concluded that a strong acoustic cavitation effect was generated in the narrow gap melt under the experimental conditions.Then the effect of ultrasonic cavitation on solid/liquid interface was also studied and it is determined that the high pressure effect induced by acoustic cavitation was the main controlling factor of rapid interfacial reaction caused by ultrasound.Finally,the interfacial reaction and bonding mechanism of ZnAl/SiC system was proposed.In order to further reduce the soldering temperature,the Sn-based solder was used for soldering SiC ceramics.The evolution of the interfacial microstructure under ultrasonic action was studied.For the SnZnAl solder with the soldering temperature of 230°C,the interface microstructure tranformed from SiC/SiO2/SnZnAl to SiC/Al2O3/SnZnAl as the ultrasonic duration time increased.When the ultrasonic time was 4 s,the shear strength of joints could reach up to the highest value of 44 MPa.Based on the high reactivity of Ti,an ultrafast interfacial bond between SiC ceramics and SnAgTi active solder has been succesfully achieved at the temperature of 250°C.When the ultrasonic time is only 0.1 s,the joint strength can reach up to about 28 MPa,which could satisfy the requirements of applications in electronic package.The Ti is easily adsorbed at the solder/ceramic interface and then react with SiO2 on the surface of SiC ceramic.The formation of an amorphous TiO2 at the interface,which creates an exceptional interfacial interfacial structure and facilitates bonding betwee the two dissimilar crystals.SiC ceramic and copper were soldered using a ZnAl solder.The results showed that microcracks appeared in CuZn4 and Cu5Zn8 which were the reaction layer at the copper side,and this led to a reduction in joint strength.This was mainly attributed to the higher residual stress in the joint and the formation of brittle intermetallic compounds near the Cu side.The SnZnAl solder was used to soldering SiC ceramics and Cu.The interfacial bonding mechanism was determined,and the corresponding relationship between the interface microstructures and the mechanical properties was established.The highest strength of joints could reach up to 38 MPa.The residual stress of the joint was evaluated by finite element simulation.It was found that the maximum residual stress of SiC/SnZnAl/Cu joint was reduced by 330 MPa compared with that of SiC/ZnAl/Cu joint.The reduction of soldering temperature and the excellent plastic deformation ability of the Sn-based solder were the main favorable factors for the reduction of the residual stress.