Study Of Soldering Mechanism Between Active Solder Sn3.5Ag4Ti(Ce,Ga) And SiC Substrate At Low Temperature

Silicon carbide（SiC）is suitable for the growth of high quality Ga N heteroepitaxial materials due to its small lattice mismatch,low dislocation density and high thermal conductivity.It is the main substrate material for the fabrication of high-frequency and high-power Ga N HEMT devices.However,the high temperature and chemical treatment during the packaging process may damage the device structure and performance.Therefore,the study of active brazing technology to realize the reliable connection of SiC substrate at low temperature,effectively improve the reliability of the device,reduce packaging steps,and reduce packaging costs,has important significance for the improvement of SiC material packaging process.In this thesis,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,energy spectrum analysis（EDS）and other characterization methods were used to observe and analyze the microstructure,element distribution and interfacial reaction products of the brazing interface of SiC/Sn3.5Ag4Ti（Ce,Ga）/SiC at 250℃and 420℃for different holding time.The brazing strength was evaluated by shear experiment.Results show that with the increase of holding time,the active element Ti will form segregation phenomenon on the solid substrate under the action of substrate adsorption force,and the higher the brazing temperature,the shorter the time of formation of segregation.The active element Ti reacts with SiC substrate at the interface under proposed temperatures,and the reaction products are determined to be Ti C,Ti₅Si₃ and Ti Si₂.The brazed joints with proposed temperatures and holding times all meet the shear strength requirements of MIL-STD-883G-2006.The low temperature brazing mechanism of Sn3.5Ag4Ti（Ce,Ga）and SiC was studied by thermodynamic analysis,brazing kinetics analysis and wetting mechanism.The low temperature brazing process of Sn3.5Ag4Ti（Ce,Ga）to SiC has three stages:Ti element diffusion,interfacial adsorption and interfacial reaction.The diffusion coefficient of Ti element is proportional to the square of temperature.The higher the temperature is,the more intense the diffusion is.The activity of Ti increases exponentially with the increase of temperature.The higher the temperature,the more active the chemical properties of Ti atoms.The dynamic source of interfacial adsorption is the adsorption energy W=1755m J?m^-2between Ti and SiC ceramics,and the adsorption force of Ti on the surface of SiC ceramics makes Ti element form segregation at the interface.The chemical reaction of Ti element at the interface and the formation of continuous interfacial reaction layer are the important reasons for the reliable low temperature brazing.The wetting model of Sn3.5Ag4Ti（Ce,Ga）/SiC shows that the wetting process is driven by adsorption of active elements and chemical reaction.