Ceramics circuit board

Abstract: This invention provides a ceramics circuit board comprising: a ceramics base board (1); a metal circuit plate (7A to 7E) integrally bonded onto a surface of the ceramics base board (1); a terminal connecting port (3A to 3E) formed by bending a part of the matal circuit plate for connecting a terminal (4) of a module, the terminal connecting port being formed so that the terminal connecting port being raised from a surface of the ceramics base board (1), and a curvature radius (R₁, R₂) of the bended portion provided on the terminal connecting port is set to 0.2 mm or more. A communication hole such as groove or through hole may also be formed at a bonding surface between the metal circuit plate and the ceramics base board.

Description: TECHNICAL BACKGROUD OF THE INVENTION The present invention relates to a ceramics circuit board, more particularly to a ceramics circuit board enabling to reduce an influence of stress caused by heat cycles during the operation of the circuit board, and to increase durability and reliability of the ceramics circuit board. In recent years, various type of ceramics circuit boards are widely used as a circuit board for a hi-power transistor module or a high-speed, high-power swithing power source modules. The ceramic circuit board comprises a ceramics base board composed of seramics sintered body and a metal plate such as copper plate integrally bonded onto a surface of the ceramics base board, the metal plate having high-thermal conductivity and high-electrical conductivity. Further, as a material for the ceramics base board, aluminium nitride, silicon nitride, silicon carbonate or the like are paid attentions because of their excellence in electric insulation property and thermal conductivity. The ceramics circuit board having a circuit composed of the conductive metal plate such as copper plate is generally manufactured in accordance with DBC method (Direct Bonding Copper method) wherein the metal plates are directly bonded onto the ceramics base board, or active metal brazing method wherein the metal plates are bonded onto the surface of the ceramics board through a brazing material containing active metals such as 4A family elements and/or 5A family elements listed in the periodic table. As a concrete method of forming the circuit, following methods are known: i.e., the methods may include one wherein a metal plate having a predetermined circuit pattern previously formed by press working or etching treatment are used to be bonded onto the caramics base board; or a method wherein the circuit patterns are formed after a raw material of the metal plate is bonded onto the ceramics base board. FIG. 27 shows a construction of the seramics circuit board manufactured in accodance with the DBC method described above. The ceramics circuit board is manufactured by the following processes. That is, a raw copper material is punched out to form a plurality of copper circuit plates 2A, 2B and 2C each having a predetermined shape (circuit patterns). The copper circuit plates 2A, 2B and 2C are, then, arranged and contacted to both surface sides of the ceramics base board 1 composed of alumina (Al 2 O 3 ) or aluminium nitride (AlN) sintered body or the like having a thickness of about 0.3 to 3 mm to form a board assembly. Then, the board assembly is heated up to a predetermined temperature to generate an eutectic liquid phase thereby to impart wettability to the surface of the ceramics base board 1. Subsequently, the board assembly is cooled and the liquid phase is solidified whereby the copper circuit plates 2A, 2B and 2C are directly bonded onto the ceramics base board 1. Further, among the plurality of the copper circuit plates 2A, 2B and 2C, at an edge portion of the copper circuit plate 2A is provided with a terminal connecting port 3 to which a terminal of a LSI circuit element or module to be loaded onto the ceramics base board 1. As shown in FIG. 28, the terminal connecting port 3 is previously formed by punching out the copper circuit plate 2A by means of a press-punching machine, simultaneously, by bending the edge portion so as to have a U shaped in section. After the copper circuit plate 2A is integrally bonded onto the surface of the ceramics base board 1 by employing the DBC method, a free edge side of the terminal connecting port 3 is retained so as to be raised from the surface of the ceramics base board 1. A termanal 4 of a module (not shown) is connected onto an upper surface of the termanal connecting port 3 through a solder 5. In order to improve the solder wettability with respect to terminal connecting port 3 composed of copper, and to increase a bonding strength between the terminal 4 and the terminal connecting port 3, a nickel metallizing layer 6 having a thickness of about 2 to 6 µm is formed on the each surface of the copper circuit plates 2, 2A to 2C. As a result, even in a case where the terminal 4 is moved and deformed vertically due to a thermal expansion or the like as indicated by an arrow in FIG. 28, the deformation or replacement of the terminal connecting port 3 is absorbed by bending of the terminal connecting port 3 around a base portion thereof. Accordingly, it is effectively prevented the stress concentration caused by the thermal expansion from being concentrated to the ceramics base board 1. On the other hand, FIG. 29 shows a construction of another conventional ceramic circuit board manufactured by utilizing the active metal brazing method. The ceramic circuit board shown in FIG. 29 comprises a ceramic base board 11 and a copper circuit plate 12 integrally bonded onto one surface of the ceramics base board 11 through a bonding layer 13 mainly composed of active metals such as titanium (Ti), silver (Ag), copper (Cu) or the like. A backing copper plate 14 for preventing a tharmal deformation of the caramics base board 11 is bonded onto the other surface side of the ceramics base board 11 through a similar bonding layer 13 composed of the active metal. Further, in a module as a product, a semiconductor chip 15 such as Si chip is bonded onto the surface of the copper circuit plate 12 through a solder layer 16, and required wires are provided so as to electrically connect the semiconductor chip 15 to the copper circuit plate 12. Also a heat sink 17 is bonded onto the back surface side of the backing copper plate 14 through a solder layer 16. By the way, the active metal brazing method is utilized as a method of bonding a ceramics base board and a metal circuit plate to each other, the method comprising the steps of: preparing a paste by mixing a binder such as an organic component or the like and a solvent into active metal powder such as Ti, Cu, Ag or the like; pattern printing the paste onto a surface of the ceramics base board 11; arranging the copper circuit plate onto a surface of the ceramics circuit plate 11 in along with the printed pattern to form an assembled body; and heating the assembled body in an inert gas atmospher (i.e., Ar gas or N 2 gas atmosphere or the like) or vacuum thereby to thermally bond the copper circuit plate 12 onto the ceramics base board 11. The backing copper plate 14 is also bonded onto the ceramics base board by utilizing the same method as described above. According to the method described above, Ti reacts with nitrogen (N) or oxygen (O) to generate a TiN or TiO, at the same time, Cu and Ag react with the copper component to generate an eutectic bonding material whereby the copper circuit plate 12 and the backing copper plate 14 are firmly bonded onto the ceramics base board 11 respectively. Anyone of the ceramics circuit boards that are manufactured in accordance with the DBC method or the active metal brazing method has a simple structure, so that there can be provided advantages of enabling to miniaturize the circuit board and to realize high-density loading, furthermore, to simplify the mountiong process, or the like. In particularly, the active metal brazing method is directly applicable to various ceramics materials, simultaneously, has an advantage of imparting an excellent reproducibility of fine circuit pattern in comparison with that of DBC method. Therefore, the active metal brazing method will become widely used in some particular fields. However, the conventional ceramics circuit boards shown in FIGs. 27 and 28, have a nickel plating layers formed on the copper circuit plates so as to enhance the solder wettability, so that there may be posed a problem that the copper circuit plates are hardened and likely to cause breakages. In fact, the copper material is originally soft and rich in elasticity at the stage prior to be provided with the nickel plating layer. However, after forming of the nickel plating layer, the copper circuit plate will become hard due to the hardness of the nickel material per se. Therefore, in particularly, the bended portions P 1 and P 2 of the terminal connecting port 3 become brittle, so that repeated bending strength of the bended portions P 1 and P 2 will be disadvantageously decreased. In the conventional case described above, a radius of curvature R 1 and R 2 of the bended portions P 1 and P 2 is small indeed about 0.1 mm, so that bending stress is concentrated to the bended portions P 1 and P 2 to easily cause cracks resulting in that the terminal connecting port 3 is liable to be broken in a short period of time, thus reducing the reliability of the ceramics circuit board. More particularly, in the ceramics circuit board having such a terminal connecting port, temperature variation caused by ON/OFF switching operation of the power source is repeatedly applied onto the terminal connecting port. Accordingly, there may be posed a problem that the crack or breakage are more liable to be occurred at the terminal connecting port. On the other hand, in the case of the conventional ceramics circuit board manufactured by employing the active metal brazing method, the bonding layer 13 for bonding the ceramics base board 11 and the copper circuit plates or the like (i.e., the copper circuit plate 12 and the backing copper plate 14), or the bonding layer 14 for bonding the copper plates 12, 14 and the semiconductor chip or the like (i.e., the semiconductor chip 15 and the heat sink 17) do not always impart or realize a sufficient bonded state. Namely, in the process where the ceramics base board 11 and the copper circuit plates 12 and 14 or the like are thermally bonded to each other, if dewaxing operation is effected uncompletely, carbon (C) contained in the binder is not completely removed whereby residual carbon is remained within the bonding layer 13. The residual carbon C is liable to react with the active metal such as Ti, resulting to disadvantageously form TiC which interrupts an essential reaction to be required between the Ti component and N or O contained in the ceramics base board 11. Therefore, there may be posed various problems of that: non-bonded portions are formed in the bonding layer 13; or even in a case where the bonding is efffected completely, the bonding strength thereof may be lowered. In some adverse cases, a proportion of the non-bonded area with respect to the an entire bonding surface will reachs almost up to 30 %. In a module assembling process, when a high-teperature solder is placed onto the surfaces of the copper circuit plates 12, 14 and subsequently, the semiconductor chip 15 and the heat sink 17 are arranged onto the surface of the copper circuit plates 12 and 14, gases such as air, atmospheric gas are likely to be involved between the semiconductor chip 15 and the copper circuit plate 12, or between the heat sink 17 and the backing copper plate 14 whereby there may be posed a case where the gases are remained in the solder layer 16. When the gases are remained in the solder layer 16, there may be posed various problems of that: a pore 18, so-called, a solder void may be formed as schematically shown in FIG. 29, thereby to reduce the bonding strength; or thermal resistance of the final product will increased thereby to deteriorate heat releasing capability; or the semiconductor chip 15 may easily be broken, thus reducing the reliability of the module or the like. Furthermore, when the ceramics circuit board having the semiconductor chip 15 thereon, which is manufactured by employing the active metal grazing method is assembled into a power-transistor module or the like, a terminal of the module is soldered to a part of the circuit of the copper circuit plate or the like so as to secure an electric continuity between the circuit and an electric power source. In such a module described above, a temperature difference is necessarily caused in accordance with ON-OFF switching operations of the power source, and when the temperature difference is repeatedly applied onto the ceramics circuit board for a long period of time, as shown in FIG. 29, cracks 19 are liable to be occurred at a side of the ceramics base board 11. Particularly, in a case where the aluminium nitride base board is used as a ceramics base board, the mechanical strength of the aluminium nitride is lower by nature than that of other ceramics, so that the cracks 19 are more easily to be occurred. Further, the terminal is moved and deformed in accordance with the ON-OFF switching operations of the power source, so that mechanical stresses are also applied to the ceramics circuit board. When the thermal stresses due to the temperature difference and the mechanical stresses are applied onto the ceramics circuit board for much longer period of time, there may be posed various problems such that cleavage fractures are disadvantageously occured from the portion of the cracks 19, resulting in peeling-off the copper circuit plate 12 from the ceramics base board 11, and the thermal resistance of the module per se is increased, thus introducing mal-functions of the module or the like. In order to solve the aforementioned problems inherent in the prior arts, an object of the present invention is to provide a ceramics circuit board enabling to increase a durability of the ceramics base board, and to secure a sufficient bonding reliability of the metal circuit plate. SUMMARY OF THE INVENTION According to the first aspect of the present invention, there is provided a ceramics circuit board characterized by comprising a ceramics base board, a metal circuit plate integrally bonded onto a surface of the ceramics base board, and a terminal connecting port formed by bending a part of the matal circuit plate for connecting a terminal of a module, the terminal connecting port being formed so that the terminal connecting port being raised from a surface of the ceramics base board, and a carvature radius of the bended portion provided on the terminal connecting port being set to 0.2 mm or more. According to the second aspect of the present invention, there is provided a ceramics circuit board characterized by comprising: a ceramics base board; a metal circuit plate integrally bonded onto a surface of the ceramics base board through a bonding layer containing an active metal; and a semiconductor chip bonded onto a surface of the metal circuit plate, the metal circuit plate being provided with a communication hole at a bonding surface between the metal circuit plate and the ceramics base board so as to communicate the bonding surface with an outside of the ceramics base board. According to the third aspect of the present invention, there is provided a ceramics circuit board characterized by comprising: a ceramics base board; a metal circuit plate integrally bonded onto at least one surface of the ceramics base board through a bonding layer containing an active metal; and a step portion formed along with a peripheral edge portion of the metal circuit plate so that a thickness of the step portion being lower than that of central portion of the metal circuit plate. According to the fourth aspect of the present invention, there is provided a ceramics circuit board characterized by comprising: a ceramics base board; a metal circuit plate integrally bonded onto a surface of the ceramics base board through a bonding layer containing an active metal, the metal circuit plate having a terminal connecting port for connecting a terminal of a module; and a gap partially formed between the surface of the ceramics base board and the metal circuit plate. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: FIG. 1 is a partial sectional view showing first to fifth embodiments of a ceramics circuit board according to the present invention; FIG. 2 is a graph showing a relationship between carvature radius of a terminal connecting port and a number of bending cycles untill the terminal connecting port occurred breakage; FIG. 3 is a sectional view showing sixth embodiment of a ceramics circuit board according to the present invention; FIG. 4 is a plan view showing sixth embodiment of a ceramics circuit board shown in FIG. 3; FIG. 5 is a graph showing a relationship between groove depth and bonding strength in sixth embodiment of the present invention; FIG. 6 is a graph showing a relationship between groove width and bonding strength in sixth embodiment of the present invention; FIG. 7 is a graph showing a relationship between groove pitch and bonding strength in sixth embodiment of the present invention; FIG. 8 is a sectional view showing a construction of seventh embodiment of a ceramics circuit board according to the present invention; FIG. 9 is a plan view showing a construction of seventh embodiment of a ceramics circuit board shown in FIG. 8; FIG. 10 is a graph showing a relationship between hole diameter and bonding strength in seventh embodiment of the present invention; FIG. 11 is a graph showing a relationship between hole pitch and bonding strength in seventh embodiment of the present invention; FIG. 12 is a sectional view showing a construction of eighth embodiment of a ceramics circuit board according to the present invention; FIG. 13 is a graph showing a relationship between groove depth and occurrence rate of solder void in eighth embodiment of the present invention; FIG. 14 is a graph showing a relationship between groove depth and voltage dropping ratio in eighth embodiment of the present invention; FIG. 15 is a graph showing a relationship between groove width and occurrence ratio of solder void in eighth embodiment of the present invention; FIG. 16 is a graph showing a relationship between groove pitch and occurrence ratio of solder void in eighth embodiment of the present invention; FIG. 17 is a side view showing partially in section a construction of nineth embodiment of a ceramics circuit board according to the present invention; FIG. 18 is a plan view showing nineth embodiment of the ceramics circuit board shown in FIG. 17; FIG. 19 is a graph showing a relationship between thickness of a thinned portion and number of heat cycles untill a crack occurred in nineth embodiment of the present invention; FIG. 20 is a graph showing a relationship between width of a thinned portion and number of heat cycles untill a crack occurred in nineth embodiment of the present invention; FIG. 21 is a side view showing partially in section a construction of tenth embodiment of a ceramics circuit board according to the present invention; FIG. 22 is a plan view showing tenth embodiment of the ceramics circuit board shown in FIG. 21; FIG. 23 is a graph showing a relationship between inclination angle of a slant surface provided on the thinned portion and number of heat cycles untill a crack occurred in tenth embodiment of the present invention; FIG. 24 is a sectional view showing a construction of eleventh embodiment of a ceramics circuit board according to the present invention; FIG. 25 is a partial sectional view showing a power transistor module into which the ceramics circuit board shown in FIG. 24 is adopted; FIG. 26 is a sectional view showing a power transistor module into which a twelfth embodiment of a ceramics circuit board according to the present invention is adopted; FIG. 27 is a perspective view showing a construction of a conventional ceramics circuit board manufactured by employing DBC method; FIG. 28 is a partial sectional view taken along the line X X XIII - X X XIII in FIG. 27; and FIG. 29 is a sectional view showing a construction of another conventional ceramics circuit board manufactured by employing active metal brazing method. DETAILED DESCRIPTION OF THE INVENTION The inventors of this invention have eagely reviewed and developed various countermeasures for enhancing the durability of the base portion of the terminal connecting port without impairing the bonding strength between the terminal and the terminal connecting port. As a result of the reserches, the inventors have found out that the radius of carvature of the bended portion formed at the edge portion of the terminal connecting port has a great influence on the durability of the bended portion. The first invention has been achieved on the basis of the findings. Namely, according to the first aspect of the present invention, there is provided a ceramics circuit board characterized by comprising a ceramics base board, a metal circuit plate integrally bonded onto a surface of the ceramics base board, and a terminal connecting port formed by bending a part of the matal circuit plate for connecting a terminal of a module, the terminal connecting port being formed in a shape of cantilever so that the terminal connecting port being raised from a surface of the ceramics base board. Further, a radius of carvature of the bended portion formed in the terminal connecting port is set to 0.2 mm or more. As a method of bonding the metal circuit plate onto the ceramics circuit board, DBC method or active metal brazing method are preferably be utilized. In the DBC method, the ceramics base board and the metal circuit plate are directly bonded to each other by an eutectic of the metal and the metal oxide. On the other hand, in the active metal grazing method, the ceramics base board and the metal circuit plate are bonded to each other through a bonding layer containing an active metal. The radius of carvature of the bended portion has a great influence on a degree of stress concentration with respect to the base portion of the terminal connecting port. The value of the radius of carvature is set to 0.2 mm or more in this invention. When the radius of carvature is less than 0.2 mm, the stress is liable to be concentrated at the bended portion of the terminal connecting port, so that cracks will be easily caused resulting in occurance of breakage of the bended portion in a short period of time. On the other hand, even though the radius of carvature is set to greater than 0.5 mm, it cannot be expected to realize a remarkable increase of the durability of the bended portion. Therefore, the radius of carvature is preferably set to a range of 0.2 to 0.5 mm. Particularly, in a case where the radius of carvature is set to 0.3 mm or greater, the same bending strength and repeated fatigue strength can be obtained that are equivalent to those of copper circuit plate having no nickel metallizing layer. The radius of carvature of the bended portion formed in the terminal connecting port is arbitrarily set to a predetermined value by controlling a radius of an edge portion of each of male and female pressing molds provided in a press-punching machine, the radiuses thereof being previously worked and finished. As described above, according to the ceramics circuit board of the first invention, the bended portion of the terminal connecting port is formed in a cantilever shape and the radius of carvature of the bended potion is formed greatly so as to be 0.2 mm or greater, so that the stress concentration with respect to the bended portion can be effectively prevented whereby there can be provided a ceramics circuit board having high strength and less occurance of crack or breakage in the terminal connecting port. More particularly, even in a case where the temperature variation caused by ON/OFF switching operation of the power source is repeatedly applied onto the terminal connecting port. it can be effectively prevented the terminal connecting port from causing the crack or breakage. In order to achieve another object, there is provided a ceramics circuit board according to the second aspect of the present invention. The ceramics circuit board is characterized by comprising: a ceramics base board; a metal circuit plate integrally bonded onto a surface of the ceramics base board through a bonding layer containing an active metal; and a semiconductor chip bonded onto a surface of the metal circuit plate, the metal circuit plate being provided with a communication hole at a bonding surface between the metal circuit plate and the ceramics base board so as to communicate the bonding surface with an outside of the ceramics base board. According to ceramics circuit board having such a construction descrived above, when a dewaxing operation is effected in a thermal bonding process wherein the ceramics base board and the copper circuit plate are thermally bonded by employing the active metal brazing method, binder gases are easily passed through the communication hole such as groove or through hole, so that components of the binder will be burned completely. Therefore, the active metal would not react with C contained in the binder but reacts with N or O contained in the ceramics base board, so that strength dispersion caused by non-bonded portion can be effectively eliminated in a bonding layer between the ceramics base board and the copper circuit plate, thus enhancing the bonding strength and enabling to provide a ceramics circuit board having a stable performance. In the present invention, preferred examples of material for the ceramics base board may include: non-oxide type sintered bodies such as aluminium nitride (AlN), silicon carbide (SiC), silicon nitride (Si 3 N 4 ) or the like; and oxide type sintered bodies such as aluminium oxcide (Al 2 O 3 ), beryllium oxide (BeO) or the like. Further, preferred examples of materials for the active metals may include Ti, Ag, Cu or the like, and composition ratios thereof may preferably set to 1 through 10 weight % of Ti, 10 through 50 wt.% of Ag, and 40 through 90 wt.% of Cu, respectively. In the present invention, preferred examples of methods for forming the communication holes such as groove or through hole onto the copper circuit plate may include: mechanical method such as pressing by utilizing a pressing machine; chemical working method such as an etching treatment. If at least one of a size, arranging number and arranging density of the communication holes is so large that a proportion of a non-bonded portion will disadvantageously increased whereby it become difficult to work the communication holes and there may be posed a disadvantage of occurance of cracks. On the other hand, in a case where the size, arranging number, arranging density are so small that the dewaxing operation will be effected uncompletely. Therefore, carbon contained in the binder cannot be removed completely, so that desired effects cannot be obtained. Taking the points descrived above into consideration, shape, dimension or arrangement of the grooves are preferably set to following ranges respectively, i.e., the groove depth ranges from 10 to 200 µm, the groove width ranges from 30 to 500 µm, the groove pitch ranges from 0.5 to 10 mm. It is more preferable to set the groove depth to a range of 30 to 60 µm, the groove width to a range of 50 to 150 µm and the groove pitch to a range of 1.0 to 3.0 mm. By the way, a plurality of grooves may have various arrangements such as in parallel or in lattice manner, also shapes of the grooves may include U-shaped or V-shaped in section. On the other hand, when a through hole is formed as a communication hole in a copper circuit plate or the like, it is preferable to set a diameter of the through hole to a range of 30 to 500 µm. In a case where a plurality of holes are arranged in a form of lattice, the hole pitch is preferably set to a range of 0.5 to 10 mm. It is more preferable to set the hole diameter to a range of 50 to 150 µm, and the hole pitch to a range of 1.0 to 3.0 mm. According to the ceramics circuit board described above, the active metal such as Ti would not react with C contained in the binder but reacts with N or O contained in the ceramics base board thereby to generate TiN, TiO or the like in the bonding layer so that the strength dispersion caused by non-bonded portion can be effectively eliminated at the bonding layer between the ceramics base board and the copper circuit plate, thus enhancing the bonding strength and enabling to provide a ceramics circuit board having a stable performance. According to the present invention, there is provided a ceramics circuit board characterized by comprising: a ceramics base board; a metal circuit plate integrally bonded onto one surface of the ceramics base board through a first bonding layer containing an active metal; a backing metal plate for preventing thermal deformation, which integrally bonded onto another surface of the ceramics base board through a second bonding layer containing an active metal; a semiconductor chip bonded onto a surface of the metal circuit plate through a first solder layer; and a heat sink for releasing heat bonded onto a surface of the backing metal plate through a second solder layer; and the metal circuit plate and the backing metal plate being provided with communication holes respectively at each of the bonding surfaces so as to communicate the each of the bonding surfaces with an outside of the ceramics base board. Namely, the ceramics circuit board according to the invention has a construction wherein the ceramics base board is disposed between the metal circuit plate and the backing metal plate, both the metal circuit plate and the backing metal plate being provided with the comunication holes such as groove or through hole. A.materials of the ceramics base board and shapes of the grooves or the like may be set similarly to the case of those of the invention descrived above. According to the invention, dispersion in bonding strength caused by non-bonding is effectively eliminated in not only in the bonding layer between the ceramics base board and the metal circuit plate but also in the bonding layer between the ceramics base board and the backing metal plate, thus enhancing the bonding strength and enabling to provide a ceramics circuit board having a stable performance. Next, according to the present invention, there is provided a ceramics circuit board characterized by comprising: a ceramics base board; a metal circuit plate integrally bonded onto a surface of the ceramics base board through a bonding layer containing an active metal; and a semiconductor chip bonded onto a surface of the metal circuit plate through a solder layer; and the metal circuit plate being provided with a groove at the bonding surface with respect to the semiconductor chip so as to communicate the bonding surface with an outside of the semiconductor chip. According to the invention wherein the gooves are formed onto the solder layer side of the copper circuit plate, when the semiconductor chip such as Si chip is soldered onto the surface of the metal circuit plate, an atmospheric gas or air are exhausted and removed from the soldering surface to outside of the semiconductor chip through the grooves, so that the solder void due to a gas pocket is merely occurred. Further, it becomes possible to reduce a thermal resistance in the solder layer of the finished product. In the present invention, preferred examples of the method for forming the communication holes such as groove to be formed onto the copper circuit plate may include: mechanical method such as pressing by utilizing a pressing machine; chemical working method such as an etching treatment. If at least one of a size, arranging number and arranging density of the grooves is so large that a large amount of solder is required and the metal circuit plate will become liable to be broken whereby it becomes difficult to work the grooves. On the other hand, in a case where the size, arranging number, arranging density of the grooves are so small that the atmospheric gas or air cannot always be removed completely resulting in causing the solder void and deteriorating the bonding strength due to the solder void, and the thermal resistance is increased in the ceramics circuit board as a final product whereby desired effects cannot be obtained. Taking the points described above into consideration, shape or arrangement of the grooves are preferably set to following ranges respectively, i.e., the groove depth may range from 10 to 300 µm, the groove width ranges from 30 to 1000 µm, the groove pitch ranges from 0.5 to 10 mm. It is more preferable to set the groove depth to a range of 50 to 150 µm, the groove width to a range of 100 to 200 µm and the groove pitch to a range of 1.0 to 3.0 mm. By the way, a plurality of grooves may have various arrangements such as in parallel or lattice manner, also shapes of the grooves may include U-shaped or V-shaped in section. According to the present invention there is provided a ceramics circuit board characterized by comprising: a ceramics base board; a metal circuit plate integrally bonded onto one surface of the ceramics base board through a first bonding layer containing an active metal; a backing metal plate for preventing thermal deformation, the backing metal plate being integrally bonded onto another surface of the ceramics base board through a second bonding layer containing an active metal; a semiconductor chip bonded onto a surface of the metal circuit plate through a first solder layer; and a heat sink for releasing heat bonded onto a surface of the backing metal plate through a second solder layer, the metal circuit plate and the backing metal plate being provided with grooves respectively at each of the bonding surfaces of the first and second solder layer sides so as to communicate the each of the bonding surfaces with an outside of the semiconductor chip. Namely, the ceramics circuit board according to the invention has a construction wherein the ceramics base board is integrally disposed between the metal circuit board and the backing metal plate, both the metal circuit plate and the backing metal plate being provided with the grooves. A materials of the ceramics base board and shapes of the grooves may be set similar to the case of those of the invention described above. According to the invention, when the semiconductor chip and the heat sink are soldered onto the surfaces of the metal circuit plates during the module assembling process, an atmospheric gas or air are exhausted and removed from the soldering surface to outside of the ceramics circuit board through the grooves, so that solder voids due to gas pocket are merely occurred. The inventors of this invention have eagerly reviewed and reserched the cause and effect relationship of occurring the cracks in the ceramics base board. As the result, following facts have been cleared. That is, when a module in which the ceramics circuit board is loaded is operated for a long period of time, a temperature of the module is raised up to 100°C or more. Due to the temperature rise, an entire ceramics circuit board is thermally expanded. At this time, a thermal coefficient of expansion of the copper circuit plate is larger than that of the ceramics base board, so that in a case where the copper circuit plate is bonded onto only one surface of the ceramics base board, a warpage is liable to be occurred. In order to prevent the warpage, the countermeasure such that the backing copper plate for preventing thermal deformation is bonded onto the other surface side of the ceramics base board is adapted. However, even though such a countermeasure is applied, due to the temperature difference (ΔT) caused by ON/OFF switching operation of the power source, there is caused a difference of thermal expansions (α difference). Therefore, heat-cycles caused on the basis of the α difference are repeatedly applied onto the ceramics base board, so that a compressive stress is remained in the ceramics base board. The compressive stress has a maximum value at a boundary portion between a bonded surface and a non-bonded surface of the copper circuit plate with respect to the ceramics base board. Namely, the boundary portion corresponds to a peripheral edge portion of the copper circuit plate to which the ceramics base board is contacted. As the result, as shown in FIG. 29, cracks 19 will be liable to occur at the portion where the edge peripheral portion of the copper circuit plate 12 and the ceramics base board 11 are contacted to each other. Then, a breakage of the ceramics base board 11 will be occurred from the crack 19 as a starting point, thus causing an adverse influence on the module. By the way, a frequency of crack formation and size of the crack are remarkable in a case where a thickness of the copper circuit plate is large, so that it is considered that a stress to be applied in a direction of the bonding surface is synergistically influenced by the thickness of the copper circiut plate. Accordingly, it is considered that the crack formation will effectively prevented by mitigating the stress. Therefore, it may easily happen to be thought of reducing the thickness of the copper circuit plate. However, it is impossible to excessively reduce the thickness of entire copper circuit plate particularly in a case where the ceramics circuit board is used for high-current devices such as a high-power transistor module, switching power source module or the like. The present invention described above has been made on the basis of the above problems. The invention provides a ceramics circuit board characterized in that an edge portion of the copper circuit plate is provided with a step portion having a thinner thickness than that of a center portion of the copper circuit plate. According to the present invention descrived above wherein only the edge portion of the copper circuit plate has a thinner thickness, it is possible to reduce an amount of compressive strain which is caused by the difference of thermal expansion applied from the thinned portion to the ceramics base board, while the thickness of the main portion of the copper circuit plate is retained so as to have a sufficient thickness. As the result, it becomes possible to prevent the crack formation at the boudary portion between the bonded surface and the non-bonded surface of the copper curcuit plate with respect to the ceramics base board. Accordingly, even in a case where the thickness of the entire copper circuit plate is set to a large value for the purpose of high-current supply, it becomes possible to prevent the crack formation with respect to the ceramics base board, thus enhancing the reliability of the ceramics circuit board. Preferable examples of methods of formimng the thinned portion i.e., the step portion may include: a chemical method wherein an outer peripheral portion of the non-bonding surface side of the copper circuit plate is trimmed or chipped by employing the etching method; and a mechanical method wherein outer peripheral portion of the copper circuit plate is collapsed or crashed by employing a press machine. According to the structure of this invention having the step portion between the thicked portion and the thinned portion, a formation of the thickness difference and a dimentional setting of the copper circuit plates can easily be performed. The inventors of this invention have also found the following fact from the result of the various experimental data. Namely, when the thickness of the thinned portion to be formed at the edge portion of the copper circuit plate is larger than two-thirds (2/3) of that of the thicked portion of the copper circuit plate, it cannot always be possible to securely prevent the crack formation of the ceramics base board. Accordingly, the thickness of the thinned portion to be formed at the peripheral portion of the copper circuit plate may preferably be set to two-thirds of that of the thicked portion of the copper circuit plate. Further, when the width of the thinned portion to be formed at the edge portion of the copper circuit plate is less than a half of that of the thicked portion of the copper circuit plate, also it cannot always be possible to securely prevent the crack formation of the ceramics base board. Accordingly, the width of the thinned portion to be formed at the peripheral portion of the copper circuit plate may preferably be set to a half of that of the thicked portion of the copper circuit plate. Next, the present invention provides a ceramics circuit board characterized by comprising a ceramics base board and a metal circuit plate bonded onto the ceramics base board, an edge portion of the metal circuit plate being provided with a slant surface having a small thickness in comparison with that of a main central portion of the metal circuit plate. Also, according to the invention, it is possible to reduce a stress applied from the metal circuit plate to the ceramics base board. Due to the stress relaxation, it becomes possible to prevent the crack formation at the boundary portion between the bonded surface and the non-bonded surface of the metal curcuit plate with respect to the ceramics base board. Accordingly, even in a case where the thickness of the entire copper circuit plate is set to a large value for the purpose of high-current service, it becomes possible to prevent the crack formation with respect to the ceramics base board, thus enhancing the reliability of the ceramics circuit board. Preferred examples of methods of forming the thinned portion i.e., the slant surface may include: a chemical method wherein an outer peripheral portion of the non-bonding surface side of the metal circuit plate is trimmed or chipped by employing the etching method; and a mechanical method wherein outer peripheral portion of the metal circuit plate is collapsed or crashed by employing a press machine. In this case, when an inclination angle of the slant surface of the metal circuit plate is less than 20 degrees, there may be posed disadvantages such that it becomes difficult to work the metal circuit plate, and to secure a area on the metal circuit plate for loading a semiconductor chip thereon. On the other hand, when the inclination angle of the slant surface of the matal circuit plate is larger than 70 degrees, it is not always possible to securely prevent the crack formation. Accordingly, in the present invention, the inclination angle of the slant surface of the metal circuit plate may preferably be set to a range of 20 to 70 degrees. The invention provides a ceramics circuit board characterized by comprising: a ceramics base board; a metal circuit plate integrally bonded onto a surface of the ceramics base board through a bonding layer containing an active metal, the metal circuit plate having a terminal connecting port for connecting a terminal of a module; and a gap partially formed between the surface of the ceramics base board and the metal circuit plate. The ceramics circuit plate according to the present invention has a construction having a gap (non-bonded portion) formed between the terminal connecting port of the metal circuit plate and the ceramics base board. Due to the formation of the gap, the metal circuit plate positioned above the gap may have a spring-like function. Therefore, it is possible to mitigate or relax the mechanical stress applied in accordance with a movement or replacement of the terminal which had been connected to the terminal connecting port of the metal circuit plate in the module assembling process. Simultaneously, it can be effectively prevented that heat-cycle affects from the terminal are directly applied to the bonded portion, thus greatly enhancing the reliability of the terminal connecting port. PREFERRED EMBODIMENTS OF THE INVENTION Hereinbelow, reference will now be made in detail to the preferred embodiment of the present invention, an example of which is illustrated in the accompanying drawings. FIG. 1 is a partial sectional view showing a ceramics circuit board according to a first embodiment of the present invention, wherein the same reference numerals are used to denote the same elements or members as those of the conventional ceramics circuit board explained in connection with FIG. 26. As a each of Samples 1 to 5 (embodiments 1 to 5), various type of the ceramics circuit boards as shown in FIG. 1 were manufactured in accordance with the following manufacturing process. That is, a ceramics base board substantially composed of aluminium nitride containing 5 wt.% of yttrium oxide, 5 wt.% of alumina and 5 wt.% of silicon dioxide and having a thickness of 0.635 mm was prepared. Then, the ceramics base board was heated up to a temperature of 1100 to 1250 °C for 1 to 5 hours in air atmosphere thereby to form an oxidated layer on the surface of the ceramics base board. On the other hand, a tough pitch electrolytic copper plate having a thickness of 0.3 mm was punched out and simultaneously bended by means of a punching press machine thereby to prepare various type of copper circuit plates 7A to 7E each having a width of 3 mm and a terminal connecting port so that the radius of carvature R 1 , R 2 of the bended portions P 3 , P 4 of the terminal connecting port 3A to 3E was either one of 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm and 0.6 mm, respectively. Subsequently, each of the copper circuit plates 7A to 7E was contacted onto the oxidated layer formed on the respective ceramics base board and integrally bonded by employing the DBC method. Namely, each of the copper circuit plates 7A to 7E having the terminal connecting port 3A to 3E was contacted to the ceramics base board 1 to form an assembled body. Then, the assembled body was heated up to a temperature enabling to generate an eutectic liquid phase composed of Cu, copper oxide (Cu 2 O) and O for 10 minutes under a nitrogen gas atmosphere, thereby to integrally bond the copper circuit plate 7A to 7E with the respective ceramics base board 1. The other end of the respective terminal connecting ports 3A to 3E was raised from the upper surface of the ceramics base board 1 so as to form a cantilever. In the next step, a Ni metallizing layer 6 having a thickness of 3 µm was formed on a surface of the respective copper circuit plates 7A to 7E. Then, a terminal 4 of a module was bonded to the terminal connecting port 3A to 3E through the solder 5 whereby ceramics circuit boards according to Samples 1 to 5 as shown in FIG. 1 were manufactured respectively. On the other hand, a ceramics circuit board of Comparative Sample 1 was manufactured in the same manner as in Sample 1 except that the radius of carvature R 1 , R 2 were respectively set to 0.1 mm which is the same value as that of conventional one. Further, a ceramics circuit board of Comparative Sample 2 was manufactured in the same manner as in Sample 1 except that the Ni plating layer was not formed. In order to evaluate thus manufactured ceramics circuit boards according to Samples 1 to 5 and Comparative Samples 1 to 2, a vibration test was conducted. That is, the vibration test was carried out under the conditions that the terminal 4 connected to the respective terminal connecting ports was forcibely vibrated in the direction indicated by an arrow in FIG. 1. The vibration frequency was set to one cycle per second and the amplitude of the vibration was set to 1 mm. Under such conditions, a number of vibrating cycles was measured untill either one of the bended portion P 3 , P 4 of the terminal connecting port caused a breakage. The test results are shown in FIG. 2. As is clear from the results shown in FIG. 2, in the case of the ceramics circuit boards according to Samples 1 to 5 in which the radius of carvature R of the bended portion P 3 , P 4 were set to 0.2 mm or more, the number of vibrating cycles were remarkably increased in comparison with the Comparative Sample 1 in which the radius of carvature was set to 0.1 mm. Therefore, it is confirmed that the ceramics circuit board according to the present invention may have a high durability. In particular, when the radius of carvature R was set to 0.3 mm or more, there could be obtained a number of vibrating cycles equal to or larger than that of the ceramics circuit board according to Comparative Sample 1 having no Ni plating layer. Namely, though the terminal connecting ports were hardened due to the formation of the Ni plating layer in the Samples 1 to 5, the copper circuit plates of the Samples 1 to 5 have the same bending strength and fatigue strength as those of the copper circuit plate having no Ni plating layer, thus enhancing the reliability of the ceramics circuit board. As described above, according to the ceramics circuit board of the first invention, the terminal connecting port is formed in a cantilever shape and the radius of carvature of the bended portion formed in the terminal connecting port is largely set to 0.2 mm or more, so that the stress concentration to the bended portion can effectively be prevented. Accordingly, there can be provided a ceramics circuit board having a high strength and no fear of occurrence of crack or breakage in the metal circuit plate. More particularly, even in a case where the temperature variation caused by ON/OFF switching operation of the power source is repeatedly applied onto the terminal connecting port. it can be effectively prevented the terminal connecting port from causing the crack or breakage. Next, the preferred embodiments according to the second invention will now be explained with reference to the accompanying drawings. [Embodiment 6 (FIGs. 3 to 7)] FIG. 3 is a sectional view showing a construction of the sixth embodiment of the present invention; FIG. 4 is a plan view of FIG. 3; and FIGs. 5 to 7 are graphs showing characteristics of the ceramics circuit board according to the sixth embodiment of the present invention respectively. As shown in FIGs. 3 and 4, a copper circuit plate 112 was bonded onto one surface of a ceramics base board composed of AlN through a bonding layer 113 by employing a active metal brazing method. At the same time, An auxiliary backing copper plate 104 for preventing thermal deformation was bonded onto the other surface side of the ceramics base board 111 through a bonding layer 113 by employing the same brazing method. That is, a powder mixture material composed of 5 wt.% of Ti, 30 wt.% of Ag and 65 wt.% of Cu was added to a binder such as organic compound or the like and solvent. Then, the mixture was well mixed to prepare a paste, and a pattern was printed onto the ceramics base board 111. Subsequently, the copper circuit plate 112 was arranged onto the upper surface of the ceramics base board 111 in along with the pattern formed thereon. In this state, the ceramics base board and the copper circuit plate were heated in argon (Ar) gas atmosphere thereby to be integrally bonded to each other through the bonding layer 113. A backing copper plate 114 was also bonded onto another side of the ceramics base board 111 in the same manner. On the bonding layer 113 side surfaces of the respective copper circuit plates 112 and 114, there were previously formed grooves 115 by means of press working. Each of the grooves 115 has an approxymately V-shape in section and has openings at both edge portions of the copper circuit board. The grooves were arranged in parallel to each other so as to have a predetermined pitch. The dimensions of the groove were set to 30 µm of depth A, 100 µm of width B and 2.0 mm of pitch C. In the assembling process of the module as a final product, as indicated by the imaginary line in FIG. 3, an Si chip 116 was bonded onto the surface of the copper circuit plate 112 through a solder layer 117, while the backing copper plate 114 was bonded onto the heat sink 118 through another solder layer 117. With respect to the ceramics circuit board according to the sixth embodiment, a compound composition of the bonding layer 113 was analyzed and investigated. As the result of the analysis, there was essentially no generation of TiC, and it was confirmed that only TiN and TiO were existed in the bonding layer. Also, the bonding strength of the bonding layer was measured to be 15 kgf/cm expressed as a peeling strength. Many other ceramics circuit boards having the same construction as that of sixth embodiment were manufactured, subsequently analyzed and evaluated the characteristics thereof. As the result, the same or similar results were obtained with respect to the compound composition and the bonding strength. Furthermore, various type of ceramics circuit boards were manufactured by varying the dimensions such as groove depth A, groove width B and groove pitch C, respectively. Then, a relationship between the groove dimensions and the bonding strength of the bonding layer was investigated. The results of the investigation are shown in FIGs. 5 to 7. That is, FIG. 5 is a graph showing a relationship between the groove depth A and the bonding strength; FIG. 6 is a graph showing a relationship between the groove width B and the bonding strength; FIG. 7 is a graph showing a relationship between the groove pitch C and the bonding strength, respectively. As is clear from FIGs. 5 to 7, preferred setting ranges of the dimensions of the groove are as follows, i.e., the groove depth A may preferably be set to a range of 10 to 200 µm, the groove width B of 30 to 500 µm and the groove pitch of 0.5 to 10 mm. More preferably, the groove depth A may be set to a range of 30 to 60 µm, the groove width B of 50 to 150 µm and the groove pitch C of 1.0 to 3.0 mm. [Embodiment 7 (FIGs. 8 to 11)] FIG. 8 is a sectional view showing a construction of the seventh embodiment of the present invention; FIG. 9 is a plan view of FIG. 8; and FIGs. 10 and 11 are graphs showing characteristics of the ceramics circuit board according to the seventh embodiment of the present invention respectively. As shown in FIGs. 8 and 9, the ceramics circuit board according to the seventh embodiment is different from that of the sixth embodiment in a point that a plurality of through holes 125 were formed in the copper circuit plate 112 and the backing copper plate 114 in place of the grooves 115 formed in the sixth embodiment. The through holes 125 were formed so as to pass through along a thickness direction of the copper circuit plate 112 or the backing copper plate 114. In the seventh embodiment as shown in FIG. 9, the respective through hole 125 was set to be a circular hole having a diameter D of 100 µm, and a plurality of the through holes were arranged in a lattice-like manner so as to have a hole pitch E of 2.0 mm. The arrangement of elements or parts of the ceramics circuit board of seventh embodiment shown in FIGs. 8 and 9 other than those described above are substantially the same as those of sixth embodiment shown in FIGs. 3 and 4, so that these elements or parts are not described herein by adding the same reference numerals to the corresponding elements or parts. With respect to the ceramics circuit board according to the seventh embodiment, a compound composition of the bonding layer 113 was analyzed and investigated. As the result of the analysis, there was essentially no generation of TiC, and it was confirmed that only TiN and TiO were existed in the bonding layer 113. Also, the bonding strength of the bonding layer was measured to be a 14 kgf/cm. Many other ceramics circuit boards having the same construction as that of seventh embodiment were manufactured, subsequently analyzed and evaluated the characteristics thereof. As the result, the same or similar results were obtained with respect to the compound composition and the bonding strength. Furthermore, various type of ceramics circuit boards were manufactured by varying the dimensions such as through hole diameter D and through hole pitch E, respectively. Then, a relationship between the through hole dimensions and the bonding strength of the bonding layer was investigated. The results of the investigation are shown in FIGs. 10 and 11. That is, FIG. 10 is a graph showing a relationship between the diameter D of the through hole 125 and the bonding strength; FIG. 11 is a graph showing a relationship between the hole pitch E and the bonding strength, respectively. As is clear from FIGs. 10 and 11, preferable setting ranges of the dimensions of the through hole are as follows, i.e., the diameter D of the through hole may preferably be set to a range of 30 to 500 µm, and the hole pitch of 0.5 to 10 mm in a case where the plurality of through holes are arranged in a lattice-like form. More preferably, the diameter D of the through hole may be set to a range of 50 to 150 µm and the hole pitch E of 1.0 to 3.0 mm in a case where the plurality of through holes are arranged in a lattice-like form. Next, an eighth embodiment according to the present invention will be explained with reference to the accompanying drawings. [Embodiment 8 (FIGs. 12 to 16)] FIG. 12 is a sectional view showing a construction of the eighth embodiment of the present invention; and FIGs. 13 to 16 are graphs showing characteristics of the ceramics circuit board according to the eighth embodiment of the present invention respectively. As shown in FIG. 12, a copper circuit plate 212 was bonded onto one surface of a ceramics base board 211 composed of AlN through a bonding layer 213 by employing the active metal brazing method. At the same time, An auxiliary backing copper plate 204 for preventing thermal deformation was bonded onto the other surface side of the ceramics base board 211 through a bonding layer 213 by employing the same brazing method. By the way, a material for the bonding layer 213 and the bonding method were the same as those adopted in the sixth and seventh embodiments. On the bonding layer 213 sides of the copper circuit plate 212 and the backing copper plate 214, there were provided with grooves (not shown) respectively, which were the same as those formed in sixth embodiment. Furthermore, in the assembling process of the module as a final product, an Si chip 216 was bonded onto the surface of the copper circuit plate 212 through the solder layer 217, while the backing copper plate 214 was bonded onto the heat sink 218 through the solder layer 217. The thickness of the copper circuit plate 212 was set to 0.4 mm, and a plane size of the Si chip was set to 10 mm × 10 mm. Also, the grooves 219 were formed at the solder layer 217 side surface of the copper circuit plate 212 and the backing copper plate 214, respectively. Each of the grooves 219 had an approxymately V-shape in section and had openings at both edge portions of the copper circuit plate 212 and the backing copper plate 214. The grooves 219 were arranged in parallel to each other so as to have a predetermined pitch. The dimensions of the grooves 219 were set to 100 µm of depth F, 300 µm of width G and 2.0 mm of pitch H. These grooves 219 were almost filled with solder which had been flown from the solder layer 217 during a bonding process, while an extremely small passage was remained at a bottom portion of the respective groove 219. With respect to the ceramics circuit board according to the eighth embodiment, a composition of the solder layer 217 was analyzed and investigated. As the result of the investigation, it was confirmed that a occurrence rate of the solder void to be caused in the solder layers 217 was remarkably reduced to about 4 %. Furthermore, various type of ceramics circuit boards were manufactured by varying the dimensions such as groove depth F, respectively. Then, a relationship between the groove depth F and the accurrence rate of the solder void was investigated. The results of the investigation are shown in FIG. 13. With respect to thus manufactured ceramics circuit boards according to the eighth embodiment, voltage build-up ratios (ΔV BE ) at the solder layer 217 were measured under conditions such as voltage of 45 V, current of 1A and time of 0.1 second. The results are shown in FIG. 14. In case of the conventional ceramics circuit board having no grooves, a fraction defective i.e., a ratio of defective product which was out of an allowance defined in a product specification was about 2 %. On the contrary, in case of the eighth embodiment of the present invention, the fraction defective was 0 % due to the formation of the grooves 219. Further, FIG. 15 is a graph showing a relationship between the groove width G and the occurrence ratio of the solder void; FIG. 16 is a graph showing a relationship between the groove pitch H and the occurrence ratio of the solder void, respectively. As is clear from FIGs. 15 and 16, preferred setting ranges of the dimensions of the groove may be as follows: i.e., the groove depth F may preferably be set to a range of 10 to 300 µm, the groove width G of 30 to 1000 µm and the groove pitch of 0.5 to 10 mm. More preferably, the groove depth F may be set to a range of 50 to 150 µm, the groove width G of 100 to 200 µm and the groove pitch H of 1.0 to 3.0 mm. As described above, according to the eighth embodiment of the present invention, the following effects are advantageously obtained: (1) The communication holes such as grooves or through holes for communicating the bonding surface with an outside of the ceramics circuit board are formed on bonding layer side surface of the copper circuit plate or the like to be bonded by employing the active metal brazing method, so that the binder contained in the brazing material is easily and completely removed through the grooves or the through holes whereby a correct and stable bonding structure can be obtained. Therefore, there may be no fear of occurrence of a product having a non-bonded portion or low bonding strength, thus enabling to increase the production yield and to stabilize the quality of the ceramics circuit board; (2) The communication holes such as grooves or through holes for communicating the bonding surface with an outside of the ceramics circuit board are formed on solder layer side surface of the copper circuit plate or the like to be bonded by using a solder, so that the atmospheric gas or air is easily and completely removed through the grooves or the through holes at the time when the semiconductor chip or the like is solder bonded to the copper circuit plate or the like. Therefore, there may be no fear of occurrence of a solder void, thereby to enable to effectively reduce the non-bonded portion, thus enabling to enhance the bonding strength and to improve the reliability of the semiconductor chips. Next, the nineth embodiment of the present invention will be explained with reference to the accompanying drawings. [Embodiment 9 (FIGs. 17 to 20)] FIG. 17 is a side view partially in section showing a construction of the nineth embodiment of the present invention; FIG. 18 is a plan view of FIG. 17; and FIGs. 19 and 20 are graphs showing characteristics of the ceramics circuit board according to the nineth embodiment of the present invention, respectively. As shown in FIGs. 17 and 18, copper circuit plates 312 were bonded onto one surface of a ceramics base board 311 composed of aluminium nitride (AlN) through a bonding layer 313 by employing active metal brazing method. At the same time, an auxiliary backing copper plate 303 for preventing thermal deformation was bonded onto the other surface side of the ceramics base board 311 through a bonding layer 313 by employing the same brazing method. That is, a powder mixture material composed of 3 wt.% of Ti, 37 wt.% of Ag and 65 wt.% of Cu was added to a binder such as organic compound or the like and solvent. Then, the mixture was well mixed to prepare a paste, and a pattern was printed onto the ceramics base board 311. Subsequently, the copper circuit plate 312 was arranged onto the upper surface of the ceramics base board 311 in along with the pattern formed thereon. In this state, the ceramics base board 311 and the copper circuit plate 312 were heated in Ar gas atmosphere thereby to be integrally bonded to each other through a bonding layer. The backing copper plate 313 was also bonded onto the other surface side of the ceramics base board 311 in the same manner. The copper circuit plate 312 to be used in the ceramics circuit board according to this embodiment was previously prepared in such a manner that an etching treatment was effected onto an entire edge portion, i.e., an upper pheripheral portion of a raw material copper plate having a thickness of 0.3 mm, the upper peripheral portion being on one surface side opposing to the bonding suface side of the material copper plate. Thereby, a step potion 314 was formed on the peripheral portion of the material copper plate, so that a thinned portion 315 having a lower thickness than that of main central portion of the material copper plate. Then, various ceramics circuit boards of this embodiment were manufactured by varying the thickness B of the thinned portion 315 within a range of 1/3 to 1/1 of the thickness of the raw copper material plate. With respect to each of thus formed ceramics circuit boards, a thermal shock cycle test (TCT) was conducted. FIG. 19 is a graph showing a state of occurrence of micro-cracks in the ceramics base board after the TCT was conducted. In FIG. 19, the vertical axis represents a number of heating/cooling cycles (temperature cycles) until the crack occurred, and the horizontal axis represents a thickness of the thinned portion 315. The conditions of the TCT were as follows. That is, one heating/cooling cycle comprises the steps of: heating the ceramics circuit board up to a temperature of 40°C and retaining the state for 30 minutes; cooling the board to RT i.e., a room temperature of about 25°C and retaining the state for 15 minutes; heating again the board up to a temperature of 125°C and retaining the state for 30 minutes; and cooling again the board to the RT and retaining the state for 15 minutes. A detection of a crack was conducted in accordance with a method comprising the steps of: melting the copper cicuit plate 312 to be removed away from the ceramics base board 311; submerging the the ceramics base board 311 into a red ink; and observing the red ink bleeded from the crack by means of a microscope or with the naked eye. The heating/cooling cycle was repeated with respect to the ceramics circuit boards, and presence or absence of a crack was checked out or detected at every five heating/cooling cycles. The number of the heating/cooling cycles untill the occurrence of the crack was plotted onto a graph. The results are shown in FIG. 19. As is clear from FIG. 19, it is confirmed such that when the thickness B of the thinned portion 315 to be formed at the periphery edge portion of the copper circuit plate 312 is larger than two-thirds (2/3) of the thickness of the thicked portion of the copper circuit plate 312, it is not always possible to prevent the crack formation completely. Accordingly, the thickness B of the thinned portion 315 to be formed at the peripheral edge portion of the copper circuit plate 312 may preferably be set to two-thirds (2/3) or less of the thickness A of the thicked portion of the copper circuit plate 312. On the other hand, another various ceramics circuit boards of this embodiment were manufactured by varying the width C of the thinned portion 315 within a range of 0 to 1/5 of the thickness of the raw copper material plate. With respect to each of thus formed ceramics circuit boards each having a different width C of the thinned portion 315, the TCT was conducted. FIG. 19 is a graph showing a state of occurrence of micro-cracks in the ceramics base board after the TCT was conducted. In FIG. 19, the vertical axis represents a number of heating/cooling cycles until the crack occurred, and the horizontal axis represents a width C of the thinned portion 315. The conditions of the TCT and a method of checking or detecting of a crack were the same as described hereinbefore. The results of the TCT are shown in FIG. 20. As is clear from FIG. 20, it is confirmed such that when the width C of the thinned portion 315 to be formed at the peripheral edge portion of the copper circuit plate 312 is larger than a half (1/2) of the thickness of the thicked portion of the copper circuit plate 312, it is not always possible to prevent the crack formation completely. Accordingly, the width C of the thinned portion 315 to be formed at the peripheral edge portion of the copper circuit plate 312 may preferably be set to a half (1/2) or less of the thickness A of the thicked portion of the copper circuit plate 312. Next, the tenth embodiment of the present invention will be explained with reference to the accompanying drawings. [Embodiment 10 (FIGs. 21 to 23)] FIG. 21 is a side view partially in section showing a construction of tenth embodiment of the present invention; FIG. 22 is a plan view of FIG. 21; and FIG. 23 is a graph showing characteristics of the ceramics circuit board according to the tenth embodiment of the present invention. As shown in FIGs. 21 and 22, the ceramics circuit board according to this tenth embodiment is different from that of the nineth embodiment in a point that a slant surface 316 is formed on a peripheral edge portion of the copper circuit plate 312 so as to have a thinned portion 315 having a lower thickness than that of a main central porton of the copper circuit plate 312 in place of the step portion 314 desribed in nineth embodiment. The arrangement of elements or parts of the ceramics circuit board of tenth embodiment shown in FIGs. 21 and 22 other than those described above are substantially the same as those of nineth embodiment shown in FIGs. 17 and 18, so that these elements or parts are not described herein by adding the same reference numerals to the corresponding elements or parts. On the other hand, another various ceramics circuit boards of this embodiment were manufactured by varying the inclination angle  of the slant surface 316 formed on the thinned portion 315 within a range of 20 to 50 degrees for this embodiment, while a range of 55 to 90 degrees for Comparative Samples. With respect to each of thus formed ceramics circuit boards each having a different inclination angle  respectively, the TCT was conducted. FIG. 23 is a graph showing a state of an occurrence of micro-cracks in the ceramics base board after the TCT was conducted. In FIG. 23, the vertical axis represents a number of heating/cooling cycles until the crack occurred, and the horizontal axis represents an inclination angle  of the slant surface 316. As is clear from the results shown in FIG. 23, it was confirmed such that when the inclination angle  of the slant surface 316 to be formed at the peripheral edge portion of the thinned portion 315 of the copper circuit plate 312 is larger than 70 degrees, it is not always possible to prevent the crack formation completely. On the contrary, when the inclination angle  of the slant surface 316 provided in the copper cicuit plate 312 is less than 20 degrees, there may be posed problems such that it becomes impossible to work the copper circuit plate 312 disadvantageously, and to secure a plain surface area onto which a semiconductor chip is to be loaded. Accordingly, it is confirmed that the inclination angle  of the slant surface 316 to be formed at the peripheral edge portion of the copper circuit plate 312 may preferably be set to a range of 20 to 70 degrees. By the way, though the respective embodiments were described hereinbefore with reference to the cases wherein the copper circuit plate 312 was bonded onto only one suface side of the ceramics base board 311, the present invention is not limited to those embodiments. Namely, the present invention is, as a matter of course, also appicable to a case wherein the cooper circuit plates are bonded onto both surface sides of the ceramics base board. As described above, according to the ceramics circuit board of the tenth embodiment of this invention, the peripheral edge portion is provided with a step portion or a slant surface so as to have a thinned portion having a lower thickness than that of a main central portion of the copper circuit plate, so that the thermal stress applied from the copper circuit plate onto the ceramics base board can be mitigated in comparison with a conventional one having no step portion or slant surface. Further, it also becomes possible to mitigate a thermal stress caused by loading of the cooling/heating cycles at a state of the ceramics circuit board being assembled into a module, thus enabling to enhance the reliability of the module. Next, the eleventh embodiment of the present invention will now be explained with reference to the accompanying drawings of FIGs. 24 and 25. FIG. 24 is a sectional view showing a construction of an eleventh embodiment of the present invention. In FIG. 24, reference numeral 401 denotes a ceramics base board. A material for the ceramics base board 401 is not particularly restricted, and various type of materials may be applicable. Preferred examples of the materials may include: oxide type sintered bodies such as those composed of aluminium oxide, mullite (3Al 2 O 3 - 2SiO 2 ) or the like; and non-oxide type sintered bodies such as those composed of aluminium nitride, silicon nitride, silicon carbide or the like. However, in a case of modules such as a power transistor module or high-frequency transistor module wherein a semiconductor chip having a high heat release value is loaded, it is preferable to use the sintered bodies such as those composed of aluminium nitride, silicon nitride, silicon carbide or the like that are all excellent in thermal conductivity. Metal circuit plates 401 such as copper plate having predetermined circuit patterns are integrally bonded onto one main surface 401A of a ceramics circuit board 401 through a brazing material layer 403 containing an active metal (hereinafter referred to as "active metal brazing material"). As a material for the metal circuit plate 402, various kinds of metal can be used, however, it is preferable to use a copper or an alloy thereof in consideration of thermal conductivity, electrical characteristics or the like. The active metal brazing material described above is prepared by adding, as an active metal, 4A family elements such as Ti, Zr, Hf, Nb and 5A family elements listed in the periodic table to a brazing material composed of Ag-Cu type eutectic compound, Cu type compound or the like. For example, the metal circuit plate and the ceramics base board are integrally bonded by utilizing the Ag-Cu eutectic compound, or by utilizing a reaction between the the active metal and the component element of the ceramics base board (e.g., a reaction between N and Ti or the like in case of AlN base board). The metal circuit plates 402 are provided with circuit patterns by patterning to form a plurality of circuit portions 404 and 405. Among them, the circuit portion 404 is used as area for loading a semiconductor chip, and the circuit portion 405 is used as a terminal connecting port, respectively. In this regard, on a metal plate to be used as a terminal connecting port 405, there is provided with a recessed portion 406 having a predetermined depth. The recessed portion 406 is formed at a portion corresponding to a terminal connecting position. The depth of the recessed portion may preferably be set to a range of about 1/20 to 18/20, more preferably to a range of about 1/4 to 1/2, of a initial thickness of the metal plate. A surface side to which the recessed portion 406 is formed is used as a bonding surface, and the bonding surface except the recessed portion 406 is integrally bonded onto the ceramics base board through an active metal brazing material layer 403 to form the terminal connecting port 405. That is, among the circuit portions of the metal circuit plate 402, the terminal connecting port 405 has a construction comprising a gap (non-bonded portion) formed between the ceramics base board 401 and the circuit portion, the gap being defined by the recessed portion 406. The terminal connecting port 405 having the gap 406 has a function of mitigating a movement of the terminal to be connected to the port 405 at a module assembling process by the action of spring-like performance of the metal plate 405A located above the gap 406. The movement or replacement of the terminal described above may be caused in accordance with ON/OFF switching operations of the power source. In addition, the terminal connecting port 405 has also a particular function of preventing temperature cycles (heating/cooling cycles) from the terminal from being directly applied onto a bonded portion (i.e., a bonded portion through the active metal brazing material layer 403) by resistance of heat transfer of the gap 406. By the way, the other circuit portions 404 are uniformly bonded onto the ceramics base board 401 through the active metal brazing material layer 403. Further, a metal plate 407 such as copper plate which is to be used as a connecting port or heat sink is also bonded onto another main surface side of the ceramics base board 401 through the active metal brazing material layer 403, thus constructed the ceramics circuit board according to the eleventh embodiment of this invention. The ceramics circuit board having the construction described above may be manufactured, for example, in accordance with the following steps. First, a paste-like active metal brazing material e.g., Ag-Cu-Ti type brazing material having a composition of Ag : Cu : Ti = 70.6 : 27.4 : 2.0 in weight basis is coated onto the ceramics base board 401 e.g., a aluminium nitride base board in accordance with a designed circuit patterns by utilizing a screen printing method or the like. At this time, the Ag-Cu-Ti type brazing material paste shall not be coated onto a portion to be a non-bonded portion including a portion corresponding to the gap 406 of the terminal connecting port 405. On the other hand, the copper circuit plates 402 having a predetermined circuit patterns are prepared by effecting an etching treatment or press working with respect to a raw metal plate such as a copper plate. At this time, the recessed portion 406 having a predetermined shape is simultaneously formed at a portion to be a terminal connecting port 405. In this regard, when a depth of the recessed portion 406 is excessively shallow, there cannot be provided a sufficient spring-like performance and heat diffusing effect. On the contrary, when the depth of the recessed portion 406 is excessively deep, it becomes impossible to secure a sufficient current capacity. Therefore, as stated above, the depth of the recessed portion, though it varys depending on an initial thickness of the raw metal plate, may preferably be set to a range of about 1/20 to 18/20, more preferably to a range of about 1/4 to 1/2 of the initial thickness of the raw metal plate. Subsequently, thus formed copper circuit plates 402 are arranged on an upper surface of the aluminium nitride base board 401 on which the Ag-Cu-Ti type brazing material paste is coated. At the same time, a backing copper plate 407 is also arranged onto a back surface side of the aluminium nitride base board 401 to form an assembled body. Then, the assembled body is heated up to a temperature of about 850°C under a vacuumed condition of about 1 × 10 -4 Torr, and is retained at the temperature for about 10 minutes whereby the copper circuit plate 402 and the backing copper plate 407 are integrally bonded onto the aluminium nitride base board 401, thus providing a desired ceramics circuit board 408. By the way, as a method of forming the pattern, it is also possible to adopt a method comprising the steps of: coating the Ag-Cu-Ti type brazing material paste onto a surface of a copper plate except a portion corresponding to the gap 406 of the terminal connecting port 405; arranging the copper plate having the recessed portion 406 onto the ceramics base board and thermally bonding both the copper plate and the ceramics base board to each other; and thereafter effecting an etching treatment to form the pattern. In addition, the Ag-Cu-Ti type brazing material is not restricted to those in a form of paste, and a brazing material in a form of a foil can also be interposed between the copper plate and the ceramics base board. Next, a power transistor module into which the ceramics circuit board (aluminium nitride circuit board) described above is assembled will now be explained with reference to FIG. 25. FIG. 25 is a partial sectional view showing an essential part of the power transistor module according to this embodiment. A power transistor 409 is bonded and loaded onto a semiconductor chip loading port 404 of the aluminium nitride circuit board 408 through an Au solder 410 or the like. Further, a terminal 411 connected to a power source circuit or the like is bonded onto a terminal connecting port 405 of a copper circuit plate 402, the terminal connecting port being corresponding to an upper position of the gap 406. The semiconductor chip loading port 404 and the terminal connecting port 405 are electrically connected to each other by means of an Al wire 413. Similarly, the power transistor 409 and a circuit of the semiconductor chip loading port 404 are electrically connected to each other by means of another Al wire 413, respectively. As described above, the aluminium nitride circuit board 408 onto which the power transistor 409 is loaded and the terminal 411 is connected is bonded by a solder (solder layer 415) onto a heat sink 414 through a backing copper plate 407 provided on a back surface side of the aluminium nitride base board 401 whereby the essential part of the power transistor module is constructed. In the power transistor module 416 described above, the terminal connecting port 405 constituting a part of the circuit portions of the copper circuit plate 402 has a construction having the gap 406 between the aluminium nitride base board 401 and the terminal connecting port 405 of itself. Therefore, even in a case when the terminal 411 is moved or vibrated, as indicated by an arrow A in FIG. 25, in accordance with ON/OFF switching operations in a circuit of the power source, a mechanical stress caused by the movement of the terminal 411 can be mitigated by the action of a spring-like performance of the metal plate 405A formed above the gap 406. Further, even in a case where the temperature cycles to be caused in accordance with the ON/OFF switching operatins are applied onto the copper cicuit plate 402, it is possible to prevent a heat due to the temperature cycles from being applied directly to the bonded portion between the copper circuit plate 402 and the aluminium nitride base board 401 by the action of a thermal resistance of the gap 406. Thereby, the reliability of the terminal connecting port 405 is remarkably increased, thus enabling to provide various type modules having an excellent reliability. Furthermore, samples of the power transistor module comprising the aluminium nitride base board 401 and the copper circuit plate 402 having a thickness of o.3 mm and a depth of the recessed portion of 0.15 mm were practically manufactured. Then, with respect to each of thus formed power transistor modules, the ON/OFF switching operation was repeatedly effected for 10000 cycles. Thereafter, the respective bonded surface between the terminal connecting port 405 and the aluminium nitride base board 401 was observed. As the result, defects such as crack was not observed at all, and the respective bonded portion was observed to be retained in a sound condition. On the contrary, in a case of the power transistor modules of Comparative Samples that were manufactured by the same manner as in Samples of the eleventh embodiment according to this invention except that the terminal connecting port 405 having no recessed portion was uniformly bonded onto the surface of the aluminium nitride base board, cracks were occurred in the aluminium nitride base board 401 after the ON/OFF operation was repeated for 5000 cycles. In addition, when the ON/OFF operation was repeated for 10000 cycles, the coper circuit plate (terminal connecting port 405) was peeled off from the aluminium nitride base board 401, thus resulting in remarkable increase of a thermal resistance. Next, a twelfth embodiment of the present invention will now be explained with reference to the accompanying drawings of FIG. 26. FIG. 26 is a partial sectional view showing an essential part of a power transistor module into which the ceramics circuit board 421 of the embodiment described above is assembled in the same manner as that of the eleventh embodiment. The ceramics circuit board 421 used in the twelfth embodiment is provided with a non-bonded portion (gap 423). The non-bonded portion is formed in such a manner that an active metal brazing material layer 403 is not coated onto an area ranging from a side edge portion of the terminal connecting port 422 of the metal circuit plate 402 to an edge portion of a terminal connecting area. Namely, the terminal connecting port 422 has a construction having the gap 423 which is formed by providing a non-coated portion of the active metal brazing material between the ceramics base board 401 and the terminal connecting port 422. As stated above, the non-bonded portion i.e., the gap 423 which ranges from the side edge portion of the terminal connecting port 422 to the edge portion of a terminal connecting area, so that the metal plate formed above the gap 406 may show a spring-like performance or function. Therefore, as is the same manner described in the eleventh embodiment, it is possible to mitigate the mechanical stress caused by the movement of the terminal to be connected to the terminal conecting port 422, and also to mitigate the temperature cycles applied from the terminal 411 in accordance with the ON/OFF switching operatins in the circuit of the power source. Thereby, it becomes possible to remarkably increase the reliability of the ceramics circuit board 421 together with the power transistor module or the like. As described above, according to the eleventh and twelfth embodiments of the present invention, even in a case where the mechanical stresses caused by the heat cycles in accordance with ON/OFF switching operations of the power source or caused by the movement of the terminal are applied onto the ceramics cicuit board for a long period of time, it is possible to secure a sufficient bonding reliability of the metal circuit plate, thus enabling to provide various type of modules having an excellent reliability.

Claims: A ceramics circuit board comprising a ceramics base board (1); a metal circuit plate (7A to 7E) integrally bonded onto a surface of the ceramics base board (1); a terminal connecting port (3A to 3E) formed by bending a part of the metal circuit plate (7A to 7E) for connecting a terminal (4) of a module, the terminal connecting port (3A to 3E) being formed so that the terminal connecting port (3A to 3E) is raised from the surface of the ceramics base board (1), and a curvature radius (R 1 , R 2 ) of the bended portion provided on the terminal connecting port (3A to 3E) being set to 0.2 mm or more. The ceramics circuit board according to claim 1, wherein the ceramics base board (1) and the metal circuit plate (7A to 7E) are directly bonded to each other by a eutectic. The ceramics circuit board according to claim 1, wherein the ceramics base board (1) and the metal circuit plate (7A to 7E) are bonded to each other through a bonding layer containing an active metal. The ceramics circuit board according to any of claims 1 to 3, wherein the terminal connecting port (3A to 3E) is formed in a shape of a cantilever. The ceramics circuit board according to any of claims 1 to 4, wherein the metal circuit plate (7A to 7E) is composed of copper.

Inventors: IKEDA KAZUO, KOMORITA HIROSHI, MATSUMURA KAZUO, MIZUNOYA NOBUYUKI, NABA TAKAYUKI, TANAKA TADASHI, IKEDA, KAZUO, KOMORITA, HIROSHI, MATSUMURA, KAZUO, MIZUNOYA, NOBUYUKI, NABA, TAKAYUKI, TANAKA, TADASHI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1992
Main classification: H05K1
IPC codes: H05K1, H05K3, H01L23