Preparation method for copper plate-covered silicon nitride ceramic substrate

A preparation method for a copper plate-covered silicon nitride ceramic substrate, wherein a structure of the copper plate-covered silicon nitride ceramic substrate comprises a silicon nitride ceramic substrate, copper sheets disposed on an upper side and a lower side of the silicon nitride ceramic substrate and soldering layers disposed between the copper sheets and the silicon nitride ceramic substrate;

• a composition of the silicon nitride ceramic substrate comprises a silicon nitride phase and a grain boundary phase;
• a content of the silicon nitride phase is more than or equal to 95 wt %;
• the grain boundary phase is a mixture containing at least three elements (Y, Mg and O), a composition and a content of the grain boundary phase are controlled by a two-step sintering process, so that the content of the grain boundary phase is less than or equal to 5 wt %, and a content of a crystalline phase in the grain boundary phase is more than or equal to 40 vol %;
• sintering aids for preparation of the silicon nitride ceramic substrate are Y2O3 and MgO, a molar ratio of Y2O3 to MgO is (1.0 to 1.4):(2.5 to 2.9), and the two-step sintering process comprises:
• in a nitrogen atmosphere with a atmospheric pressure of 0.5 MPa to 10 MPa, performing low-temperature heat treatment at 1600° C. to 1800° C. and high-temperature heat treatment performed at 1800° C. to 2000° C. in sequence;
• a thickness of the silicon nitride ceramic substrate is 0.2 mm to 2.0 mm;
• the composition of each of the soldering layer is AgCuTi, wherein a weight ratio of Ag:Cu:Ti is x:y:z, where x is equal to 0.60 to 0.65, y is equal to 0.33 to 0.37, z is equal to 0.01 to 0.04, x+y+z=1, and a thickness of each of the soldering layer is 20 to 60 microns;
• a thickness of the copper sheet is 0.1 mm to 1.5 mm;
• the copper sheets, solder foils forming the soldering layers and the silicon nitride ceramic substrate are stacked according to the structure of the copper plate-covered silicon nitride ceramic substrate, and after debinding in a protective atmosphere, vacuum soldering is performed at 860° C. to 920° C. kept for 5 to 20 minutes to obtain the copper plate-covered silicon nitride ceramic substrate.