Surface Metallization And Interfacial Reaction Mechanism Of Si₃N₄ Ceramic Substrate

With the development of power devices,higher requirements for heat dissipation and mechanical properties of insulating substrate were put forward.Silicon nitride ceramic was an excellent insulating substrate material,which had the advantages of high hardness,high strength,small thermal expansion coefficient and high thermal conductivity.At present,the commercial silicon nitride ceramic substrate was connected with copper by active metal brazing.This method need ed to be bonded in vacuum and high temperature environment,which was cumbersome and costly.In view of some problems existing in active metal brazing,this project explored the surface metallization of silicon nitride ceramic substrate by thick film technology.During the course of the project,a glass of Bi-B-Zn component that can react with silicon nitride was prepared.The glass,silver powder and organic solvent were evenly mixed in a certain proportion to prepare thick film silver paste that was convenient for screen printing.Through the research on the sintering performance of silver paste,it was determined that the sintering temperature range was 450 to600℃,the heating rate was 5℃/s,and the holding time was 5 min.When the glass content was 15%and the sintering temperature was 500℃,the resistivity and shear strength of the sintered silver film on the surface of silicon nitride ceramic substrate were 3.87μΩ·cm and 21.84 MPa,respectively.Based on the determination of the sintering process,the effects of glass content,sintering temperature,and thermal cycle on the properties and microstructure of the silver film were studied.With the increase of sintering temperature,the density of silver film increased.As a result,the number of conductive paths increased,the length of conductive paths shortened and the resistivity of silver films decreased.As the glass content increased,the proportion of the non-conductive phase inside the silver film increased.This would reduce the number of conductive paths,increase the length of the conductive path,and weaken the conductive performance of the silver film.After the silver film underwent a shear test,the fracture position was located at the bonding interface between the silver film and the interface glass layer.The interface of glass and silicon nitride ceramic substrate had higher bonding strength.When the sintering temperature increased from 450°C to 500°C or the glass content increased from 10%to 15%,the contact area between the silver film and the interface glass layer can be effectively increased.At the same time,the proportion of brittle fracture inside the glass would increase at the fracture interface.Therefore,the shear strength of the silver film was improved.However,continuing to increase the sintering temperature and glass content would not have a significant impact on the shear strength of the silver film.After 1000 thermal cycles,silver recrystallization appeared on the surface of the silver film.The resistivity and shear strength of the silver film did not change significantly.The silver film prepared in this paper had good performance stability.The silver film and the silicon nitride ceramic substrate was combined through interface glass layer.It was found that the chemical combination between glass and silicon nitride ceramic was achieved by the diffusion of Si element.In addition,glass entered the silicon nitride ceramic substrate through the intercrystalline phase to further enhance the bonding strength.After Si element diffused into the glass,the composition of glass would be changed.The new composition glass precipitated Bi₂O₂Si O₃ crystals.