Preparation And Characterization Of Ag-Cu-Ti Active Paste For Brazing

Due to its convenient coating and precise control,Ag-Cu-Ti brazing paste is widely used in the manufacture of high-voltage and high-power devices such as IGBT(Insulated Gate Bipolar Transistor).However,domestic production has not been possible due to the difficulty in preparation of Ag-Cu-Ti ternary alloy powders.Besides,the imported Ag-Cu-Ti brazing paste is extremely expensive.As a result,the development of power devices such as IGBT is hindered.Ag68.8-Cu26.7-Ti4.5 powder with a particle size of 48~150 ?m was used to develop a kind of Ag-Cu-Ti active paste in this thesis.The effect of each component contained in the brazing paste on the appearance,physical stability,viscosity and thixotropy of the brazing paste was explored.Thereafter,the best formulation of Ag-Cu-Ti brazing paste was determined.The comprehensive performance of the brazing paste was tested.The spreading performance of the Ag-Cu-Ti brazing paste on aluminum nitride and oxygen-free copper was analyzed and compared with a kind of mature commercial brazing paste abroad.Finally,the effects of brazing parameters on the microstructure and mechanical properties of the brazed joints were investigated.Thus,the optimal brazing parameters for the joining of AlN ceramic and oxygen-free copper were determined.The brazing paste consists of 88 wt.% Ag-Cu-Ti powder and 12 wt.% adhesive.The results indicated that the best formulation for adhesive was 61 wt.% dissolvent(terpineol: diethylene glycol monobutyl ether=3:2),12 wt.% PEG600,5 wt.% stearic acid,6 wt.% polyamide wax,11 wt.% oxalic acid,5 wt.% paraffin oil.As solvents,terpineol and diethylene glycol monobutyl ether can dissolve other components of the adhesive.As the main binder,PEG600 can mix the brazing powder into paste with a suitable viscosity.As a dispersant,stearic acid can prevent the agglomeration of the brazing powder and improve the dispersibility of the brazing powder.As a thixotropic agent,polyamide wax can adjust the viscosity and thixotropic properties of the brazing paste.As an antioxidant,oxalic acid can prevent oxidation of the brazing paste in the preservation process.The comprehensive performance of self-made brazing paste was tested.The appearance of self-made brazing paste is a light yellow paste,without delamination or agglomeration.It provides moderate viscosity and excellent thixotropic property.Besides,the adhesive in the brazing paste is volatilized when the temperature reaches 380 ?.The spreading area of self-made brazing paste on oxygen-free copper surface is smaller than that of commercial Ag-Cu-Ti brazing paste,but the performance of the spread is more regular.The spreading area on aluminum nitride ceramic surface is obviously higher than that of the commercial Ag-Cu-Ti brazing paste.Brazing of aluminum nitride ceramic and oxygen-free copper was achieved using self-made Ag-Cu-Ti brazing paste.The interfacial microstructure was AlN/TiN/Ag(s,s)+Cu(s,s)/Cu.During brazing,active element Ti from the liquid filler metal diffused to the aluminum nitride ceramic surface and reacted with it to form a continuous TiN reaction layer.The brazing seam mainly consisted of an Ag-based solid solution and a Cu-based solid solution.On the oxygen-free copper base metal side,Cu atoms in the base metal diffused into the liquid filler metal to form a Cu-based solid solution.The effects of brazing parameters on the microstructure and properties of brazed joints were studied.It is found that when the brazing temperature is low or the holding time is short,the reaction between active element Ti and aluminum nitride substrate is insufficient and the interface reaction layer formed is very thin and discontinuous,resulting in a relatively low shear strength.When the brazing temperature is too high or the holding time is too long,the reaction layer at the interface is too thick,which deteriorates the shear strength of the joint.When the brazing temperature is 930 ? and holding time is 10 min,a maximum shear strength of 75.18 MPa is obtained,which is better than the joint brazed with commercial Ag-Cu-Ti brazing paste(68.84 MPa).Small and uniform dimples were observed in the brazed joints,showing ductile fracture characteristics.