1. Field of the Invention
The present invention relates to a metal-ceramic circuit board and a manufacturing method thereof and, more particularly, relates to a metal-ceramic circuit substrate board having a high heat-cycle resistance, which is suitable for the installation of high power electronic parts, such as power modules, and a manufacturing method thereof.
2. Description of the Prior Art
Recently, high power modules have been used to control a large electric current of electric automobiles, electric cars, tooling machines or the like. The power modules have mainly a plurality of semiconductor tips. A high electric insulation performance is required for a substrate board to which the semiconductor tips are fixed, in order to obtain a large current from a front surface and a back surface of each semiconductor tip. Further, a temperature of the semiconductor tip is elevated by heat generated when a large current is passed through the semiconductor tip. Accordingly, a good heat conductivity has been required for the substrate board on which the semiconductor tips are fixed, and parts surrounding the board.
FIG. 5 shows a conventional power module in section. The power module has semiconductor tips 1, brazing material layers 4, a metal layer 3, a ceramic substrate board 2 as an insulating substrate board, a metal layer 5, a brazing material layer 6, and a metal base plate 7 piled in this order. Reference numeral 8 denotes plating layers formed on the metal layers 3 and 5, and the metal base plate 7. Wirings between the semiconductor tips 1 are omitted in FIG. 5.
Heretofore, various methods have been proposed to bond an aluminum plate and a ceramic substrate board as shown in Japanese Unexamined Utility Model Publication No. 57945/1991 and Japanese Unexamined Utility Model Publication No. 68448/1990. Among these methods, an aluminum plate is bonded to an aluminum nitride board or an alumina board by using a brazing material of Al--Si series or Al--Ge series. U.S. Pat No. 3,994,430, published in 1976, shows the use of silicone as an aluminum binding assistant.
However, such conventional power modules have following problems because the ceramic substrate board 2 is fixed to the metal base plate 7 through the metal layer 5 and the brazing material layer 6.
(1) The construction of the power module is complicated because between the ceramic substrate board 2 and the metal base plate 7, the metal layer 5, the plating layer 8, the brazing material layer 6 and the plating layer 8 are arranged in this order. Accordingly, each of the components is heated and cooled repeatedly due to the repetition of the start and stop of the electrical conduction, so that cracks are generated on the contacting surfaces of the components depending on the difference in thermal expansion coefficient between the components.
(2) The heat conductivity and the heat radiation ability are reduced because the brazing material layer 6 exists between the ceramic substrate board 2 and the metal base plate 7.
(3) The lead brazing material has been used in spite of the fact that the maker of the electric parts wants to reduce the quantity of use of the lead brazing material.
(4) The surface treatment such as plating or brazing is required in order to improve the adhesivity of the brazing material layer 6 to the ceramic substrate board 2 and the metal base plate 7, so that the cost becomes high.
(5) A copper base plate has been used as a metal base plate. However, the thermal expansion coefficient of the copper is larger than that of the ceramics. Accordingly, cracks are formed easily in the ceramics at a portion where the ceramics is contacted with the copper base plate and the reliability of the power modules becomes low when the heat and cool are repeated. Further, the base plate such as a copper molybdenum alloy or aluminum silicon carbide complex is low in thermal conductivity and high in cost.