Ceramic circuit board

Abstract

To provide a ceramic circuit board in which a copper plate has high electrical conductivity.SOLUTION: A ceramic circuit board W includes a copper plate bonded to one surface of a ceramic substrate S via a brazing material layer, and the ceramic substrate S has pores on its surface with a diameter of 15 μm or less, the copper plate has an average crystal grain size of 100 μm or less as measured by a cutting method in the grain size test method for drawn copper products specified in JIS 0501:1986, and has electrical conductivity of 96% or more and less than 100% of the IACS standard.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a ceramic circuit board used for a power module.

Background Art

[0002] A ceramic circuit board (hereinafter sometimes simply referred to as a circuit board) is disclosed, for example, in Japanese Patent Application Laid-Open No. 2003-110222. The circuit board described in Japanese Patent Application Laid-Open No. 2003-110222 includes a copper plate joined via a brazing material to at least one surface of a ceramic substrate. Further, according to the disclosure of Japanese Patent Application Laid-Open No. 2003-110222, a copper plate is joined via a brazing material to at least one surface of a ceramic substrate, a resist is applied to a predetermined portion of the surface of the copper plate, and unnecessary portions of the copper plate are etched to form a circuit portion. Then, unnecessary brazing material and reaction products of the brazing material and the ceramic substrate are removed while maintaining the resist, and thereafter, the resist is peeled off to form a circuit pattern, which becomes a circuit board.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] For example, when the average crystal grain size of the copper plate constituting the circuit board is excessively large, there is a concern that the joinability with the ceramic substrate via the brazing material may be impaired. Therefore, when a copper plate having a relatively small average crystal grain size was joined to one surface of a ceramic substrate using a brazing material, there occurred a problem that the conductivity of the copper plate constituting the circuit board decreased. Due to such a decrease in the conductivity of the copper plate, there is a concern that the electrical performance of the circuit board may deteriorate.

[0005] An object of the present invention is to provide a ceramic circuit board in which a copper plate joined via a brazing material layer to one surface of a ceramic substrate has a high conductivity.

Means for Solving the Problems

[0006] In view of the above object, as a result of intensive research, it has been found that the conductivity of the copper plate constituting the circuit board decreases due to coarsening of crystal grains of the copper plate when joining to the ceramic substrate via a brazing material. And, by devising the temperature profile when brazing the copper plate and the ceramic substrate, it has been found that a decrease in the conductivity of the copper plate constituting the circuit board is suppressed, and the present invention has been conceived.

[0007] The ceramic circuit board of the present invention is a ceramic circuit board including a copper plate joined via a brazing material layer to one surface of a ceramic substrate, wherein the ceramic substrate has a pore diameter of 15 μm or less present on the surface, and the copper plate has an average crystal grain diameter of 100 μm or less by the cutting method of the crystal grain size test method for electrolytic tough pitch copper defined in JIS H0501:1986, and the conductivity is 96% or more and less than 100% with respect to the IACS standard.

Effects of the Invention

[0008] The present invention can provide a ceramic circuit board including a copper plate having a high conductivity and a method for manufacturing the same.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0010] The ceramic circuit board of the present invention is, for example, the ceramic circuit board W shown in FIGS. 1 and 2. This ceramic circuit board W includes a ceramic substrate S, two brazing material layers C1 and C2 (hereinafter sometimes referred to as the first brazing material layer C1 and the second brazing material layer C2) formed on the upper surface (one surface) of the ceramic substrate S with a gap G therebetween, and two copper plates M1 and M2 (hereinafter sometimes referred to as the first copper plate M1 and the second copper plate substrate M2) that function as circuit boards on which semiconductor elements and the like are mounted and are joined to the upper surface side of the ceramic substrate S via the two brazing material layers C1 and C2, as a basic configuration. A plating layer such as Ni or Au may be formed on the surfaces of the two copper plates M1 and M2 as required. The ceramic circuit board W shown in FIGS. 1 and 2 has a copper plate M3 that functions as a heat sink and is joined to the lower surface (the other surface) of the ceramic substrate S via a brazing material layer C3. Note that the copper plates are not limited to two, M1 and M2, and three or more copper plates may be provided on the upper surface side.

[0011] The type of the ceramic substrate constituting the ceramic circuit board of the present invention is not particularly limited, and alumina, silicon carbide, etc. can be used, but from the viewpoint of having high thermal conductivity, silicon nitride or aluminum nitride is preferable. It is preferable that the maximum diameter of the pores present on the surface of the ceramic substrate is 15 μm. When the maximum diameter of the pores exceeds 15 μm, the strength of the ceramic substrate decreases, for example, deteriorating the reliability of the ceramic circuit board under thermal cycling. It is more preferable to be composed of a silicon nitride-based sintered body that is excellent in mechanical strength such as strength and fracture toughness.

[0012] The silicon nitride sintered body can be produced using raw material powder containing, for example, 90 to 97% by mass of silicon nitride and 0.5 to 10% by mass of a sintering aid (including a Mg compound and at least one compound of Y and other rare earth elements). Specifically, this silicon nitride sintered body is obtained by adding an appropriate amount of an organic binder, a plasticizer, a dispersant, and an organic solvent to the raw material powder, mixing them with a ball mill or the like to form a slurry, shaping this slurry into a thin plate by a doctor blade method or a calendar roll method to obtain a ceramic green sheet. The obtained ceramic green sheet is punched or cut into a desired shape and sintered at a temperature of 1700 to 1900 °C. In this case, when the sintering aid exceeds 10% by mass, the property of joining the ceramic substrate and the copper plate may become insufficient. Also, when the sintering aid is less than 0.5% by mass, the sintering of silicon nitride particles may become insufficient. To obtain high thermal conductivity and high strength, it is preferable that the sintering aid contains 2 to 4% by mass of magnesium (Mg) in terms of magnesium oxide and 2 to 5% by mass of yttrium (Y) in terms of yttrium oxide.

[0013] In this ceramic circuit board of the present invention, the number of copper plates joined via a brazing material to one surface of the ceramic substrate is not particularly limited. The copper plate constituting this ceramic circuit board of the present invention is made of a copper alloy containing more than 0.03% by mass and 0.15% by mass or less of Zr, with the balance being copper and unavoidable impurities. The copper plate is preferable in terms of electrical resistance, stretchability, high thermal conductivity (low thermal resistance), and less migration.

[0014] The manufacturing method of this ceramic circuit board of the present invention is a manufacturing method of a ceramic circuit board having a joining step of joining a copper plate via a brazing material to one surface of a ceramic substrate, and using the copper plate made of a copper alloy containing more than 0.03% by mass and 0.15% by mass or less of Zr, with the balance being copper and unavoidable impurities, and including a heating step of heating at a temperature within the range of 770 to 880 °C, and a temperature lowering step of lowering the temperature to 350 °C over a time of 50 minutes or more after the heating step, as the joining step.

[0015] Hereinafter, a method for manufacturing a ceramic circuit board of the present invention will be described with reference to the drawings as appropriate, taking as an example the case of manufacturing the ceramic circuit board W shown in FIGS. 1 and 2. Here, the method for manufacturing the ceramic circuit board W can be divided into, for example, (a) a preparation step of preparing a ceramic substrate, a brazing material, and a copper plate, (b) a brazing material region formation step of applying the brazing material to the ceramic substrate, (c) a placement step of stacking the ceramic substrate, the brazing material, and the copper plate in this order, (d) a bonding step, (e) a circuit pattern formation step of etching the copper plate, (f) an unnecessary brazing material layer removal step, and (g) a cleaning step. The (d) bonding step includes the "heating step" and the "cooling step" in the method for manufacturing the ceramic circuit board of the present invention.

[0016] (a) Preparation step A ceramic substrate S, a brazing material (a paste containing brazing material powder and an organic binder), and a copper plate M are prepared.

[0017] (b) Brazing material region formation step As shown in FIG. 3, by applying the brazing material to the ceramic substrate S by a method such as screen printing, brazing material regions c1, c2 containing brazing material powder and an organic binder are formed with a gap G therebetween. Examples of the brazing material powder include brazing material powder containing Ag, Cu, etc. in a predetermined composition, and various organic resins can be used as the organic binder.

[0018] (c) Placement step The copper plate M is placed on the side of the ceramic substrate S to which the brazing material is applied and fixed using a jig.

[0019] (d) Bonding step The bonding process includes a "heating process" and a "cooling process". A holding process may be added to the bonding process. As shown in Fig. 4, by heating (i) a ceramic substrate S, (ii) brazing material regions c1, c2 formed on the ceramic substrate and containing brazing material powder and an organic binder, and (iii) a copper plate M placed via the brazing material regions, the ceramic substrate S and the copper plate M are bonded via a brazing material layer to form a bonded body. The heating for bonding is preferably carried out in a vacuum or a reducing atmosphere. In order to remove the organic components in the brazing material paste during the heating-up process, it is preferably held once near the volatilization temperature of the organic binder (for example, around 400 °C) (holding process). Then, it is heated at a brazing temperature of 770 - 880 °C (heating process (brazing process)). The time for continuing this heating process is preferably 10 minutes or more. The brazing temperature is the temperature at which a brazing material layer can be appropriately formed, that is, a temperature equal to or higher than the melting point of the brazing material. The brazing temperature is usually the highest temperature in the heating-up process. After the heating process, the temperature is lowered to 350 °C over a period of 50 minutes or more (cooling process). Then, the temperature is lowered from 350 °C to room temperature.

[0020] In the holding process during the bonding process, if the holding temperature for removing the organic binder is too low, the organic binder components may not be able to volatilize, and there is a risk that residues of the organic binder will remain. Therefore, the holding temperature for removing the organic binder is preferably 300 °C or higher. For example, in the case of an organic binder containing an acrylic resin, this holding temperature is preferably 360 °C or higher. In order to avoid the active metal in the brazing material being oxidized by the oxygen contained in the resin, etc. in the organic binder, the holding temperature for removing the organic binder is set lower than the brazing temperature in the heating process (brazing process).

[0021] The brazing material used in the joining process is preferably, for example, an Ag-Cu based active brazing material with a eutectic composition mainly composed of Ag and Cu, to which active metals such as Ti, Zr, and Hf are added to obtain high strength, high sealing properties, etc. Further, from the perspective of the joining strength between the ceramic substrate S and the copper plates M1 to M3, a ternary Ag-Cu-In based active brazing material in which In is added to the Ag-Cu based active brazing material is more preferable. The joining between the ceramic substrate S and the copper plate is performed using a brazing paste containing the brazing material powder and an organic binder as described above. As the brazing material, for example, an Ag-Cu based active brazing material having a melting point of 770 to 880°C is used, and the brazing temperature is preferably 770 to 880°C. When the temperature is 770°C or higher, the melting of the brazing material becomes sufficient, suppressing the formation of voids. More preferably, the temperature is 790°C or higher. When the temperature is 880°C or lower, the brazing material does not spread excessively. More preferably, the brazing temperature is 830 to 870°C. The holding time at the brazing temperature depends on the amount charged into the joining heating furnace, but considering normal productivity, it is preferably within 5 hours, and more preferably within 2 hours. The holding time at the brazing temperature is appropriately adjusted and set according to the number of samples charged, and also, for example, in the case of a vacuum atmosphere, according to the volume of the joining heating furnace and the exhaust volume of the vacuum pump. It is preferable to keep the state fixed by applying a load during the placement process so that the copper plate and the ceramic are joined without voids and then heated in the joining process.

[0022] When general oxygen-free copper is used for the copper plate, there is a concern that the crystal grain size will grow to a size exceeding 300 μm due to heating at the above brazing temperature. On the other hand, a copper plate using a copper alloy containing Zr exceeding 0.03% by mass and not exceeding 0.15% by mass according to the present invention can suppress the average crystal grain size to 100 μm or less even during heating at the above brazing temperature. Here, when the Zr content is 0.03% by mass or less, the average crystal grain size cannot be suppressed to 100 μm or less by heating at the brazing temperature. Also, when the Zr content exceeds 0.15% by mass, there is a concern that the decrease in conductivity will increase and it will not be possible to ensure 96% or more based on the IACS standard.

[0023] Furthermore, in the state where the copper plate of the present invention is heated to the brazing temperature, most of the Zr is dissolved in the copper matrix phase, and high conductivity cannot be ensured in this state. Therefore, in order to ensure high conductivity, it is slowly cooled from the brazing temperature to 350 °C over a period of 50 minutes or more. As a result, Zr can change from a state of being dissolved in the copper matrix phase in the temperature range of 550 °C to 350 °C to a state of precipitating to form a phase different from the matrix phase. By slowly cooling to ensure sufficient time to pass through the above temperature range, the precipitation of Zr proceeds, the copper matrix phase becomes highly purified, and high conductivity can be obtained. When the cooling time from the brazing temperature to 350 °C is less than 50 minutes, the precipitation of Zr becomes insufficient, and there is a concern that the conductivity of 96% or more based on the IACS standard cannot be ensured.

[0024] In addition, the content of inevitable impurities in the copper plate is preferably suppressed to be equivalent to the content of impurities in oxygen-free copper, specifically, preferably less than 0.04% by mass. The average crystal grain size of the copper plate can preferably be 60 μm or less, more preferably 30 μm or less. The copper plate preferably has a thermal conductivity of 380 W / m·K or more. The copper plate preferably has an average crystal grain size G≦100 μm and a copper plate thickness T≧0.2 mm (T / G≧2.0). For example, when G = 60 μm and T = 0.3 mm, T / G = 5.0; when G = 60 μm and T = 0.5 mm, T / G = 8.3; when G = 60 μm and T = 0.8 mm, T / G = 13.3.

[0025] When an acrylic resin is used as the organic binder contained in the brazing paste, in the heating process, deposits caused by the gasified acrylic resin are particularly likely to adhere to the surface of the ceramic substrate. Therefore, when an acrylic resin is used as the organic binder, examples include polyacrylic acid esters and polymethacrylic acid esters. Preferably, a methacrylic acid ester is used.

[0026] (e) Circuit pattern forming step Regarding the bonded body obtained in the above bonding step, as further shown in FIG. 5, resist films R1 and R2 are formed on the surface of the copper plate M in a pattern along the outer edge of the brazing material layer formed in the bonding step, and the copper plate M is divided by etching to form circuit patterns M1 and M2 as shown in FIG. 6.

[0027] (f) Unnecessary brazing material layer removal step After the above (e) circuit pattern forming step, an unnecessary brazing material layer removal step for removing the unnecessary brazing material layer can be provided. The resist film is preferably formed in a pattern along the outer edge of the brazing material layer formed in the bonding step. The thickness of the resist film formed in the circuit pattern forming step can be made thin, 10 to 80 μm, preferably 30 to 70 μm. The resist film is desirably formed of an ultraviolet curable resist agent.

[0028] For example, when using a chemical solution containing hydrogen peroxide and ammonium acid fluoride as the brazing material removing solution, an aqueous solution containing 10 to 40% by mass (2.9 to 8.8 mol / L) of hydrogen peroxide and 1 to 8% by mass (0.7 to 2.1 mol / L) of ammonium acid fluoride can be used. When the hydrogen peroxide is less than 10% by mass, the ability to remove the brazing material is insufficient, and when it exceeds 40% by mass, the copper plate is excessively corroded and the dimensional accuracy of the copper plate deteriorates. When the ammonium acid fluoride is less than 1% by mass, the ability to remove the reaction layer containing the active metal generated at the bonding interface between the brazing material layer and the ceramic substrate decreases. On the other hand, when it exceeds 8% by mass, the crystal grains constituting the ceramic substrate are dissolved, and the electrical insulation and strength required for the ceramic substrate are reduced.

[0029] (g) Cleaning step In the cleaning process, the bonded body is immersed in a chemical agent for cleaning. For example, if it is not sufficient to reduce oxygen from the atmosphere and the surface of the copper plate is oxidized in the heating process, it may cause a decrease in electrical conductivity and a decrease in solderability, which is not preferable. Therefore, by immersing the bonded body in a chemical agent containing at least one selected from hydrogen peroxide, sulfuric acid, hydrochloric acid, and ammonium chloride for cleaning, the oxide on the surface of the copper plate is removed. It is preferable to use sulfuric acid as the chemical agent.

[0030] After the steps (e) to (g) above, the deposits that reduce the insulation resistance between the copper plates are removed or reduced.

[0031] As another process, after the (g) cleaning process, there may be a (h) plating process of forming a plating layer such as Ni, Au, Ag, etc. on the surface of the copper plate. For example, when performing Ni plating, by immersing in an electroless plating solution (85 °C) mainly composed of nickel (Ni) and adjusted to a phosphorus (P) concentration of 8% by mass for 20 to 30 minutes, a Ni plating layer with a thickness of about 5 μm can be formed on the surface of the copper plate.

[0032] The present invention will be described with the following examples, but the present invention is not limited thereto.

[0033] (Example) The common part of the examples will be described as [(a) Preparation process] to [(g) Cleaning process], and then the copper plates and the bonding process will be described in Examples 1 to 4 and Reference Examples 1 to 5.

[0034] [(a) Preparation process] Prepare a copper plate, a brazing material, and a ceramic substrate. As the ceramic substrate S, use a silicon nitride substrate (when represented on the plane shown in Fig. 2, with a length and width of 30 mm and 40 mm respectively, and a thickness of 0.32 mm) containing 93% by mass of Si3N4, 4% by mass of Mg in terms of oxide conversion, and 3% by mass of Y in terms of oxide conversion in 100 parts by mass of all raw material powders. As the brazing material, for 100 parts by mass of the brazing material powder adjusted to have a composition of 70.6% by mass of Ag, 2.9% by mass of In, 1.9% by mass of Ti, the balance being Cu and trace impurities, use a brazing material paste obtained by mixing 5.3 parts by mass of polyacrylate as an organic binder, 19.1 parts by mass of α-terpineol as an organic solvent, 0.5 parts by mass of polyoxyalkylene alkyl ether and an alkylbenzene sulfonic acid salt as a dispersant.

[0035] Regarding the manufacturing method of the ceramic circuit board S, it will be described with reference to Figs. 3 to 6 which are plan views showing each process. In the manufacturing process of the ceramic circuit board S described below, the content of each process for forming the copper plates M1, M2 which are circuit boards and the copper plate M3 which is a heat sink is basically the same. Therefore, only the copper plates M1, M2 will be described in detail, and the description of the copper plate M3 will be omitted.

[0036] [(b) Brazing material region forming process] As shown in Fig. 3, on the upper surface (one side) of the ceramic substrate S, two brazing material regions c1, c2 each with a thickness of 40 μm are formed by applying the brazing material paste by screen printing in the planar direction with a gap G therebetween. In the plane shown in Fig. 3, the size of the first brazing material region c1 is 27.6 mm in length and 11.6 mm in width, the size of the second brazing material region c2 is 27.6 mm in length and 23.6 mm in width, and the distance between the brazing material regions c1, c2 of the gap G is 1.0 mm.

[0037] [(c) Placement process] After the brazing material area forming step, as shown in FIG. 4, a single copper plate M with a thickness of 0.5 mm that covers the brazing material areas c1 and c2 is placed on the brazing material areas c1 and c2, and the ceramic substrate S, the brazing material areas c1 and c2, and the copper plate M are laminated and fixed using a jig.

[0038] [(d) Bonding step] Insert it into a heating furnace and heat it under a vacuum atmosphere to bond the ceramic substrate S and the copper plate M through the brazing material layers C1 and C2 to form a bonded body. In consideration of the thermal expansion of the copper plate M in the bonding step, the vertical and horizontal sizes of the copper plate M in the plane shown in FIG. 4 are 29.5 mm and 39.5 mm respectively, and a size smaller than that of the ceramic substrate S is used.

[0039] In the bonding step, it is carried out in a temperature pattern having a holding step of holding at 400 °C, which is the removal temperature of the acrylic resin, an organic binder, for 10 hours, a temperature rising step of heating from the holding step at a constant temperature rising rate, a heating step (brazing step) of holding at 770 °C to 880 °C, which is the melting temperature of the brazing material, for 1 hour, a temperature lowering step of lowering the temperature to 350 °C over a time of 50 minutes or more after the heating step, and a step of lowering the temperature from 350 °C to room temperature.

[0040] [(e) Circuit pattern forming step] After the bonding step, as shown in FIG. 5, two resist films R1 and R2 are formed on the surface of the copper plate M constituting the bonded body in a desired pattern, and then an etching treatment is performed to remove unnecessary portions of the copper plate M. As shown in FIG. 6, two copper plates M1 and M2, which are circuit patterns, are formed in a state where a gap G is sandwiched in the plane direction. Specifically, a bonded body obtained by applying an ultraviolet curable etching resist on the surface of the copper plate M by screen printing in a pattern corresponding to the dimensions of the following first copper plate M1 and second copper plate M2 is immersed in an etching solution [ferric chloride (FeCl3) solution (46.5 Be)] at a liquid temperature of 50 °C to form the copper plates M1 and M2. In the plane shown in FIG. 6, the vertical and horizontal sizes of the first copper plate M1 are 28 mm and 12 mm respectively, and the vertical and horizontal sizes of the second copper plate M2 are 28 mm and 24 mm respectively.

[0041] [(f) Flux layer removal process] As shown in Fig. 6, the resist film formed on the surfaces of copper plates M1 and M2 is removed, and the unnecessary flux layer protruding from the outer edges of copper plates M1 and M2 is removed with a flux removal solution containing 7.6 mol / L hydrogen peroxide and ammonium hydrogen fluoride at a liquid temperature of 40 °C for a treatment time of 40 minutes.

[0042] [(g) Cleaning process] The ceramic circuit board is immersed in sulfuric acid with a concentration of 1 mol / L and a temperature of 50 °C for 10 minutes for cleaning, and then the cleaning solution is removed and dried to obtain the circuit board.

[0043] For the copper plate, the average crystal grain size can be measured using the cutting method of the crystal grain size test method for drawn copper products specified in JIS H0501. After polishing the surface of the copper plate with abrasive paper and alumina abrasive grains until it becomes mirror-like, the surface is etched with ammonia water added with hydrogen peroxide to reveal the crystal grain boundaries. After photographing the appeared crystal structure using a microscope, a line segment of a known length is drawn on the photograph, and the number of crystal grains completely cut by the line segment is counted, and the average value of the cutting length is taken as the average crystal grain size.

[0044] For the copper plate, the conductivity can be measured using, for example, the Sigma Test, an eddy current type conductivity meter manufactured by Fischer. An alternating magnetic field is generated in the probe in contact with the surface of the copper plate, and the conductivity is measured by detecting the strength of the eddy current generated in the copper plate due to the change in the magnetic field with the probe. According to the Sigma Test, the measured conductivity value can be obtained in both unit displays of % unit based on the IACS standard and MS / m unit based on the SI unit system.

[0045] Regarding the thermal conductivity of the copper plate, it is difficult to measure the thermal conductivity of only the copper plate part in the state joined to the ceramic substrate. On the other hand, it is known that there is a proportional relationship called the Wiedemann-Franz law between the thermal conductivity and the conductivity. Therefore, the thermal conductivity of the copper plate can be calculated using the following mathematical formula based on the Wiedemann-Franz law from the measured value of the conductivity.

[0046] K = LTσ K: Thermal conductivity (W / m·K) σ: Conductivity (S / m) L: Correction Lorentz number by copper at 20°C, 2.3×10-8 W / S·K2 T: Temperature (K)

[0047] Regarding the Zr content of the copper plate defined in the present invention, the reasons for the defined range will be described with reference to Examples and Comparative Examples. In Examples 1 to 4 where the Zr content satisfies the defined range of the present invention, the average crystal grain size of the copper plate is as small as 100 μm or less, and the bonding property to the ceramic substrate is excellent. Further, the conductivity of the copper plate is 96% or more based on the IACS standard, and the thermal conductivity is 375 W / m·K or more. In Examples 1 to 4, any 120-μm-wide region was observed to be smooth, and no recesses with a width of 20 μm or more were observed.

[0048] · Example 1 [Regarding the configuration described in [(a) Preparation step] to [(g) Cleaning step], a copper plate with a Zr content of 0.04 mass% was used in [(d) Bonding step]. In the soldering process, the atmosphere temperature was adjusted by a soldering device to a soldering temperature of 800°C, and the holding time was set to 1 h (heating process). Thereafter, the atmosphere temperature was lowered from the soldering temperature to 350°C over 50 minutes (cooling process). When the average crystal grain size of the copper plate was measured for the fabricated circuit board, it was 60 μm. Also, regarding the interface between the copper plate and the solder layer, any 120-μm-wide region was observed to be smooth. Further, when the conductivity of the copper plate was measured, it was 98% based on the IACS standard. The thermal conductivity calculated from the measured conductivity was 383 W / m·K.

[0049] · Example 2 [(a) Preparation process] - [(g) Cleaning process], for the structure described, a copper plate with a Zr content of 0.07% by mass was used in [(d) Bonding process]. The soldering process was carried out at a soldering temperature of 800 °C for 1 h (heating process) as in Example 1. Thereafter, the ambient temperature was decreased from the soldering temperature to 350 °C over 50 minutes as in Example 1 (cooling process). When the average crystal grain size of the copper plate was measured for the fabricated circuit board, it was 30 μm. Also, regarding the interface between the copper plate and the solder layer, an arbitrary region with a width of 120 μm was observed and it was smooth. Also, when the conductivity of the copper plate was measured, it was 97% with respect to the IACS standard. The thermal conductivity calculated from the measured conductivity was 379 W / m·K.

[0050] · Example 3 [(a) Preparation process] - [(g) Cleaning process], for the structure described, a copper plate with a Zr content of 0.15% by mass was used in [(d) Bonding process]. The soldering process was carried out at a soldering temperature of 800 °C for 1 h (heating process) as in Example 1. Thereafter, the ambient temperature was decreased from the soldering temperature to 350 °C over 50 minutes as in Example 1 (cooling process). When the average crystal grain size of the copper plate was measured for the fabricated circuit board, it was 15 μm. Also, regarding the interface between the copper plate and the solder layer, an arbitrary region with a width of 120 μm was observed and it was smooth. Also, when the conductivity of the copper plate was measured, it was 96% with respect to the IACS standard. The thermal conductivity calculated from the measured conductivity was 375 W / m·K.

[0051] · Example 4 [(a) Preparation process] - [(g) Cleaning process], for the structure described, in [(d) Bonding process], a copper plate with a Zr content of 0.04 mass% was used. The soldering process had a soldering temperature of 880 °C and a holding time of 1 h (heating process). Then, similar to Example 1, the ambient temperature was decreased from the soldering temperature to 350 °C over 50 minutes (cooling process). When measuring the average crystal grain size of the copper plate for the fabricated circuit board, it was 100 μm. Also, regarding the interface between the copper plate and the solder layer, an arbitrary 120-μm-wide region was observed and it was smooth. Moreover, when measuring the conductivity of the copper plate, it was 98% relative to the IACS standard. The thermal conductivity calculated from the measured conductivity was 383 W / m·K.

[0052] For the above Example, in Comparative Example 1 where the Zr content is less than the specified range of the present invention, the average crystal grain size of the copper plate exceeds 100 μm, and the bonding property of the ceramic substrate deteriorates. Also, in Comparative Example 2 where the Zr content is more than the specified range of the present invention, the conductivity of the copper plate is less than 96% relative to the IACS standard, and the thermal conductivity is also less than 375 W / m·K.

[0053] · Comparative Example 1 [(a) Preparation process] - [(g) Cleaning process], for the structure described, in [(d) Bonding process], a copper plate with a Zr content of 0.03 mass% was used. The soldering process had a soldering temperature of 800 °C and a holding time of 1 h, similar to Example 1 (heating process). Then, similar to Example 1, the ambient temperature was decreased from the soldering temperature to 350 °C over 50 minutes (cooling process). When measuring the average crystal grain size of the copper plate for the fabricated circuit board, it was 110 μm. Also, regarding the interface between the copper plate and the solder layer, an arbitrary 120-μm-wide region was observed, and one concave portion with a width of about 20 μm was found. Moreover, when measuring the conductivity of the copper plate, it was 98% relative to the IACS standard. The thermal conductivity calculated from the measured conductivity was 383 W / m·K. · Comparative Example 2

[0054] [(a) Preparation process] - [(g) Cleaning process], for the structure described above, in [(d) Bonding process], a copper plate with a Zr content of 0.18 mass% was used. The brazing process was carried out at a brazing temperature of 800 °C and a holding time of 1 h, the same as in Example 1 (heating process). Then, the atmosphere temperature was decreased from the brazing temperature to 350 °C over 50 minutes, the same as in Example 1 (cooling process). When measuring the average crystal grain size of the copper plate for the fabricated circuit board, it was 15 μm. Also, regarding the interface between the copper plate and the brazing material layer, an arbitrary region with a width of 120 μm was observed and it was smooth. Moreover, when measuring the conductivity of the copper plate, it was 95% with respect to the IACS standard. The thermal conductivity calculated from the measured conductivity was 371 W / m·K.

[0055] Regarding the temperature range of the heating process and the cooling time of the cooling process in the bonding process defined in the present invention, the reasons for the defined range will be explained with examples of the examples and reference examples. In the above-described Examples 1 to 4, the temperature of the heating process and the cooling time of the cooling process satisfy the defined range of the present invention. In all cases, the average crystal grain size of the copper plate is as small as 100 μm or less, and the bonding property with the ceramic substrate is excellent. Also, the conductivity of the copper plate is 96% or more with respect to the IACS standard, and the thermal conductivity is 375 W / m·K or more.

[0056] Reference Example 3 and Reference Example 4 are examples where the temperature of the heating process is outside the defined range of the present invention. In Reference Example 3 where the temperature is less than 770 °C, the melting of the brazing material becomes insufficient, so the bonding property with the ceramic substrate deteriorates. In Reference Example 4 where the temperature exceeds 880 °C, the brazing material spreads too much by wetting, so the bonding property with the ceramic substrate deteriorates. Also, when the temperature is too high, there is a possibility that the average crystal grain size of the copper plate exceeds 100 μm.

[0057] Reference Example 5 is an example where the cooling time of the cooling process is shorter than the defined time of the present invention. In this case, the conductivity of the copper plate is less than 96% with respect to the IACS standard, and the thermal conductivity is also less than 375 W / m·K.

[0058] · Reference Example 3 [(a) Preparation process] - [(g) Cleaning process], for the structure described, a copper plate with a Zr content of 0.04% by mass was used in [(d) Bonding process]. The soldering process had a soldering temperature of 750 °C and a holding time of 1 h (heating process). Subsequently, the ambient temperature was decreased from the soldering temperature to 350 °C over 50 minutes (cooling process). When measuring the average crystal grain size of the copper plate for the fabricated circuit board, it was 30 μm. Also, regarding the interface between the copper plate and the solder layer, when observing an arbitrary 120-μm-wide region, two recesses with a width of about 20 μm were found. Moreover, when measuring the conductivity of the copper plate, it was 98% with respect to the IACS standard. The thermal conductivity calculated from the measured conductivity was 383 W / m·K.

[0059] · Reference Example 4 [(a) Preparation process] - [(g) Cleaning process], for the structure described, a copper plate with a Zr content of 0.04% by mass was used in [(d) Bonding process]. The soldering process had a soldering temperature of 900 °C and a holding time of 1 h (heating process). Subsequently, the ambient temperature was decreased from the soldering temperature to 350 °C over 50 minutes (cooling process). When measuring the average crystal grain size of the copper plate for the fabricated circuit board, it was 110 μm. Also, regarding the interface between the copper plate and the solder layer, when observing an arbitrary 120-μm-wide region, one recess with a width of about 20 μm was found. Moreover, when measuring the conductivity of the copper plate, it was 98% with respect to the IACS standard. The thermal conductivity calculated from the measured conductivity was 383 W / m·K.

[0060] · Reference Example 5 [(a) Preparation process] - [(g) Cleaning process], for the structure described, in [(d) Bonding process], a copper plate with a Zr content of 0.04% by mass was used. The soldering process was carried out at a soldering temperature of 800 °C for 1 h (heating process), the same as in Example 1. Then, the ambient temperature was decreased from the soldering temperature to 350 °C over 30 minutes (cooling process). When the average crystal grain size of the copper plate was measured for the fabricated circuit board, it was 60 μm. Also, for the interface between the copper plate and the solder layer, an arbitrarily selected region with a width of 120 μm was observed and it was smooth. Moreover, when the conductivity of the copper plate was measured, it was 95% with respect to the IACS standard. The thermal conductivity calculated from the measured conductivity was 371 W / m·K.

Explanation of symbols

[0061] A, B: Spherical electrodes, C1, C2, C3: Solder layers, c1, c2: Solder regions, G: Gap, M1, M2, M3: Metal substrates, M: Metal substrate, R1, R2: Resist films, S: Ceramic substrate, W: Ceramic circuit board

Claims

1. A ceramic circuit board comprising a copper plate bonded to one surface of a ceramic substrate via a brazing material layer, The ceramic substrate has pores on its surface each having a diameter of 15 μm or less, The copper plate has an average crystal grain size of 100 μm or less according to the cutting method of the crystal grain size test method for drawn copper products specified in JIS H0501:1986, and an electrical conductivity of 96% or more but less than 100% of the IACS standard. Ceramic circuit board.

2. The copper plate has a thermal conductivity of 375 W / m·K or more.

2. The ceramic circuit board according to claim 1.

3. The ceramic substrate is a silicon nitride sintered substrate.

3. The ceramic circuit board according to claim 1 or 2.

4. The copper plate has an average crystal grain size of 60 μm or less. The ceramic circuit board according to any one of claims 1 to 3.

5. The copper plate has a thickness of 0.2 mm or more. The ceramic circuit board according to any one of claims 1 to 4.

6. The interface between the copper plate and the brazing material layer does not have a recess with a width of 20 μm or more. The ceramic circuit board according to any one of claims 1 to 5.

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