Research On Active Brazing Of Aluminum Nitride Ceramics And Copper

Ceramic substrates have incomparable performance advantages over traditional substrate materials and have been gradually replaced by the latter in recent years for a wide range of applications in new energy vehicles,power locomotives,aerospace and other fields,such as for building high-power IGBT（insulated gate bipolar transistor）modules.However,the substrate material must be surface coated with Cu before it can be used as an electrical and thermal conductor for interconnect devices.Therefore,the connection technology between Al N ceramics and Cu has become one of the key challenges that restrict the ceramic substrate to large-scale applications.In view of the advantages of high bond strength and good reliability of cold and heat cycles,the conventional Ag-Cu-Ti active braze was firstly used to join Al N ceramics and Cu.On this basis,in order to solve the problem of residual stresses between Al N ceramics and Cu due to the difference of thermal expansion coefficients,two kinds of composite braze fillers,Ag-Cu-Ti+MXene and Ag-Cu-Ti+Al N,were designed in this paper.The wettability of the three brazing materials on the surface of Al N ceramics,the mechanism of joint microstructure evolution,and the effects of brazing material composition and joining process on the joint microstructure and joint mechanical properties were studied in depth.During the vacuum brazing of Al N and Cu using Ag-Cu-Ti brazing material,the active Ti elements in the brazing material diffused toward the Al N ceramic side and enriched in the interface,and the Ti elements were able to react with the Al N ceramic and Cu elements to generate Ti N layer and Cu Ti phase at the interface.At the same time,a large amount of base material Cu dissolves into the brazing material,which leads to the formation of large Cu-based solid solution in the brazing seam and reduces the content of Ag-Cu eutectic organization in the brazing seam.The experimental results of the mechanical properties showed that the flexural strength of the Al N/Cu joints increased and then decreased with the increase of brazing temperature and holding time.The maximum flexural strength of the joints reached 31 MPa when the brazing temperature was 830℃and the holding time was 10 min.The Ti₃Al C₂ precursor was successfully etched into accordion-like MXene phase by using hydrofluoric acid（HF）etching method.When the Ag-Cu-Ti+MXene composite brazing filler was used to join Al N and Cu,the Ti element in MXene was able to react with the Cu element to form a fine and diffusely distributed Cu Ti compound in the brazing seam.With the increase of MXene content,the flexural strength of the joint first increases and then decreases.When the MXene content is 1 wt.%,the maximum flexural strength of the joint reaches 40 MPa.MXene and Cu Ti compounds strengthen the brazing joint on the one hand,and reduce the thermal expansion coefficient of the joint on the other hand,so that the thermal expansion coefficient of the joint is in gradient transition.When vacuum brazing Al N and Cu with Ag-Cu-Ti+Al N composite brazing filler,the active elements in the brazing material react with Al N particles to form Ti N,with Cu in the brazing material to form Cu Ti compounds,and with the Al N ceramic matrix to form Ti N interfacial reaction layer,respectively.As the Al N content in the composite brazing material increases,the bending strength of the joint first increases and then decreases.The maximum flexural strength of the joint reaches 38 MPa when the Al N content is 1 wt.%.The introduction of Al N particles in the brazing seam,as well as the reaction formation of Cu Ti and Ti N can reduce the thermal expansion coefficient of the joint,which in turn improves the mechanical properties of the joint.