Circuit board and method for manufacturing the same

Abstract

This circuit board (1) includes a first ceramic layer (2), a second ceramic layer (3) laminated upon the first ceramic layer (2), and a first metal layer (4) disposed between the first ceramic layer (2) and the second ceramic layer (3). The circuit board (1) has: a metal layer area (11) in which the first metal layer (4) is present between the first ceramic layer (2) and the second ceramic layer (3); and a metal layer-free area (12) in which the first metal layer (4) is not present between the first ceramic layer (2) and the second ceramic layer (3). In the metal layer-free area (12), the first ceramic layer (2) and the second ceramic layer (3) are joined, and the second ceramic layer (3) is a porous boron nitride layer in which voids are filled using a cured product of a thermosetting composition. This circuit board production method includes a step in which a second ceramic sheet is disposed upon the first metal layer of a first laminate that includes the first ceramic layer and the first metal layer laminated on the first ceramic layer, and pressurized for a 10–30 minutes at a pressure of 1–20 MPa, while being heated at a temperature of 150–260°C. The second ceramic sheet is a porous boron nitride sheet in which voids are filled using a semi-cured product of a thermosetting composition. As a result of the present invention, a circuit board having good thermal conductivity, excellent withstand voltage, and a built-in electrode, and a production method therefor can be provided.

Description

Technical Field

[0001] The present invention relates to a circuit board and a method for manufacturing the same.

Background Art

[0002] In recent years, with the high performance and miniaturization of electronic devices represented by mobile phones, LED lighting devices, in-vehicle power modules, etc., mounting technologies have advanced rapidly at each level of semiconductor devices, printed wiring board mounting, and device mounting. Therefore, the heat generation density inside electronic devices has been increasing year by year, and how to efficiently dissipate the heat generated during use has become an important issue. And for the thermally conductive insulating adhesive sheet for fixing electronic components, in addition to insulation and adhesiveness, an unprecedentedly high thermal conductivity is required.

[0003] Conventionally, for the above thermally conductive insulating adhesive sheet, ceramic powders with high thermal conductivity such as aluminum oxide, silicon nitride, boron nitride, aluminum nitride, etc. are dispersed in a thermosetting resin in an uncured state (A stage), and then formed into a sheet shape by coating with various coaters, etc., and a thermosetting resin composition in which the thermosetting resin is in a semi-cured state (B stage) by heating has been used.

[0004] After the above thermally conductive insulating adhesive sheet is adhered to an electronic component such as a metal circuit or a metal plate, the thermosetting resin in a semi-cured state (B stage) is melted by heating and allowed to penetrate into the unevenness on the surface of the electronic component, thereby expressing the adhesiveness of the thermally conductive insulating adhesive sheet to the electronic component. Further heating makes the thermosetting resin in a completely cured state (C stage), strengthening the adhesion with the electronic component.

[0005] Since the above thermally conductive insulating adhesive sheet does not need to form an adhesive layer (a thermosetting resin in an uncured state (A stage) or a thermosetting resin in an uncured state (A stage) with ceramic powder dispersed therein) between the electronic component such as a metal circuit or a metal plate, coating work and the introduction of a precise coating device are unnecessary, and the work by the user becomes very simple, so it is widely used.

[0006] Patent Document 1 describes a metal-based circuit board in which a metal foil is placed on a thermally conductive insulating adhesive sheet containing ceramic powder dispersed in a semi-cured (B-stage) thermosetting resin. By curing the thermosetting resin contained in the thermally conductive insulating adhesive sheet to the C-stage, it is possible to obtain a metal-based circuit board with excellent heat dissipation properties in a simple manner.

[0007] However, in the invention described in Patent Document 1, there was a limitation in obtaining high thermal conductivity in the circuit board because a thermosetting resin layer with low thermal conductivity was present between each particle of the ceramic powder. Therefore, there was a problem in terms of heat dissipation in the increasingly difficult thermal design requirements for electronic devices in recent years.

[0008] As a method to improve the thermal conductivity of the metal base substrate described in Patent Document 1, for example, a thermally conductive insulating adhesive sheet (see, for example, Patent Document 2) is used, which is a ceramic resin composite impregnated with a thermosetting resin composition onto a sintered body having a three-dimensionally continuous integrated structure of non-oxide ceramic primary particles, and a metal foil is bonded to one side of the metal base plate. Since this thermally conductive insulating adhesive sheet forms a continuous network of non-oxide ceramics, the thermal conductivity can be further increased. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2009-49062 [Patent Document 2] International Publication 2017 / 155110 Pamphlet [Overview of the project] [Problems that the invention aims to solve]

[0010] In recent years, circuit boards have become increasingly sophisticated, making it desirable to be able to form electrodes inside the circuit board. For example, by combining electrodes formed on the surface of the circuit board with electrodes formed inside the circuit board, more complex circuits can be formed on the circuit board. Furthermore, by forming electrodes inside the circuit board, a noise shield can be formed inside the circuit board to suppress the influence of noise on circuits formed on the surface of the circuit board. In addition, when forming wiring on the surface of the circuit board, the wiring formed on the surface of the circuit board and the wiring formed inside the circuit board can be arranged in close proximity and facing each other with a ceramic layer in between, thereby reducing the inductance of the wiring formed on the surface of the circuit board.

[0011] By laminating a thermally conductive insulating adhesive sheet described in Patent Document 2 onto the surface of a ceramic layer on which electrodes are formed, electrodes can be formed inside the circuit board. In this case, the distance between the electrodes formed on the surface of the circuit board and the electrodes formed inside the circuit board becomes smaller, so the circuit board is required to have even better voltage resistance.

[0012] Therefore, the present invention aims to provide a circuit board with built-in electrodes that has good thermal conductivity and excellent voltage resistance, and a method for manufacturing the same. [Means for solving the problem]

[0013] Conventionally, porous boron nitride sheets, in which voids are filled with a semi-cured thermosetting composition, have poor flexibility. Therefore, it was believed that even when a porous boron nitride sheet was laminated onto the surface of a ceramic layer with electrodes formed on its surface, the step created by the electrodes would result in insufficient bonding between the porous boron nitride sheet and the ceramic layer. However, through diligent research, the inventors discovered that by carefully examining the pressurization conditions when laminating the porous boron nitride sheet, it is possible to adequately bond the porous boron nitride sheet to the surface of a ceramic layer with electrodes formed on its surface. As a result, the dielectric strength of the circuit board containing the electrodes can be improved. This invention is based on the above findings and its gist is as follows. [1] A circuit board comprising a first ceramic layer, a second ceramic layer laminated on the first ceramic layer, and a first metal layer disposed between the first ceramic layer and the second ceramic layer, wherein the circuit board has a metal layer presence region between the first ceramic layer and the second ceramic layer in which the first metal layer exists, and a metal layer absence region between the first ceramic layer and the second ceramic layer in which the first metal layer does not exist, wherein the first ceramic layer and the second ceramic layer are joined together, and the second ceramic layer is a porous boron nitride layer in which the voids are filled with a cured product of a thermosetting composition. [2] The circuit board according to [1] above, wherein the first ceramic layer is a porous boron nitride layer in which the voids are filled with a cured product of a thermosetting composition. [3] The circuit board according to [1] or [2] above, wherein the thickness of the first metal layer is one-quarter or less of the thickness of the second ceramic layer. [4] The circuit board according to any one of [1] to [3] above, wherein the side surface of the first metal layer is covered with the second ceramic layer. [5] The circuit board according to any one of [1] to [4] above, further comprising a second metal layer bonded to the surface of the first ceramic layer opposite to the surface facing the second ceramic layer. [6] The circuit board according to any one of [1] to [5] above, further comprising a third metal layer formed on the side of the second ceramic layer opposite to the side of the first ceramic layer. [7] The circuit board according to any one of [1] to [6] above, wherein the second ceramic layer has an opening, and the first metal layer is exposed at the opening. [8] A method for manufacturing a circuit board, comprising the steps of: placing a second ceramic sheet on the first metal layer of a first laminate comprising a first ceramic layer and a first metal layer laminated on the first ceramic layer; and pressurizing the laminate at a heating temperature of 150 to 260°C with a pressure of 1 to 20 MPa and a pressurizing time of 10 minutes to 30 hours, thereby producing a second laminate comprising the first ceramic layer, a second ceramic layer laminated on the first ceramic layer, and the first metal layer disposed between the first ceramic layer and the second ceramic layer, wherein the second ceramic sheet is a porous boron nitride sheet in which the voids are filled with a semi-cured product of a thermosetting composition. [9] The method for manufacturing a circuit board described in [8] above, wherein the step for manufacturing the second laminate is to heat and pressurize under vacuum.

[10] The method for manufacturing a circuit board according to [8] or [9], further comprising the step of roughening the surface of the first metal layer of the first laminate.

[11] A method for manufacturing a circuit board according to any one of [8] to

[10] above, further comprising the step of placing a first metal foil on a first ceramic sheet and pressurizing it at a heating temperature of 150 to 260°C while applying pressure at a pressure of 1 to 20 MPa and for a pressurizing time of 10 minutes to 30 hours, wherein the first ceramic sheet is a porous boron nitride sheet in which the voids are filled with a semi-cured product of a thermosetting composition.

[12] The method for manufacturing a circuit board according to

[11] above, wherein the step of manufacturing the first laminate is to pressurize the first ceramic sheet on which the first metal foil is arranged while heating it under vacuum.

[13] The method for manufacturing a circuit board according to

[11] or

[12] , wherein the step of manufacturing the first laminate is to place the first ceramic sheet on a second metal foil, place the first metal foil on the first ceramic sheet placed on the second metal foil, and pressurize while heating, thereby manufacturing the first laminate which further includes a second metal layer on which the first ceramic layer is laminated.

[14] The step of manufacturing the second laminate includes arranging a second ceramic sheet on the first metal layer of the first laminate, further arranging a third metal foil on the second ceramic sheet, and manufacturing the second laminate further including a third metal layer laminated on the second ceramic layer by pressing while heating, according to any one of the manufacturing methods of the circuit board described in [8] to

[13] above.

[15] The method for manufacturing a circuit board according to any one of [8] to

[14] above, further including the step of forming an opening in the second ceramic layer of the second laminate so that the first metal layer is exposed. [Advantages of the Invention]

[0014] According to the present invention, it is possible to provide a circuit board with built-in electrodes having good thermal conductivity and excellent withstand voltage, and a method for manufacturing the same. [Brief Description of the Drawings]

[0015] [Figure 1] FIG. 1 is a schematic diagram of a circuit board according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram of a circuit board of a modification of an embodiment of the example. [Figure 3] FIGS. 3(a) to (e) are schematic diagrams for explaining a method for manufacturing a circuit board according to an embodiment of the present invention. [Figure 4] FIG. 4 is a schematic diagram for explaining a modification of a method for manufacturing a circuit board according to an embodiment of the present invention. [Figure 5] FIGS. 5(a) to (d) are schematic diagrams for explaining a modification of a method for manufacturing a circuit board according to an embodiment of the present invention. [Figure 6] FIG. 6 is a schematic diagram for explaining a modification of a method for manufacturing a circuit board according to an embodiment of the present invention. [Figure 7] FIGS. 7(a) and (b) are schematic diagrams for explaining the arrangement of a photoresist in the method for manufacturing a circuit board of the example, and FIG. 7(c) is a schematic diagram for explaining the arrangement of an opening. [Figure 8]Figure 8 is an SEM image of the region near the side surface of the first metal layer in a cross-section of the circuit board 5. [Modes for carrying out the invention]

[0016] [Circuit board] Referring to Figure 1, a circuit board of one embodiment of the present invention will be described. The circuit board 1 of one embodiment of the present invention includes a first ceramic layer 2, a second ceramic layer 3 laminated on the first ceramic layer 2, and a first metal layer 4 disposed between the first ceramic layer 2 and the second ceramic layer 3.

[0017] (Second ceramic layer) The second ceramic layer 3 is a porous boron nitride layer in which the voids are filled with a cured product of a thermosetting composition. This improves the thermal conductivity of the circuit board 1.

[0018] <Porous boron nitride layer>

[0019] The porous boron nitride layer has a structure in which multiple fine pores (hereinafter also referred to as "pores") are formed. At least some of the pores in the porous boron nitride layer may be connected to each other to form continuous pores.

[0020] The porous boron nitride layer is preferably formed from a sintered body of an insulator containing boron nitride, and more preferably from a sintered boron nitride body. The porous boron nitride layer may be formed by sintering primary boron nitride particles together. As the boron nitride in the porous boron nitride layer, either amorphous boron nitride or hexagonal boron nitride can be used. Alternatively, the porous boron nitride layer may be formed by reacting boron-containing compounds such as boric acid, boron oxide, and borax, and nitrogen-containing compounds such as urea and melamine. Furthermore, a porous boron nitride body may be formed by calcining hexagonal boron carbonitride (h-B4CN4).

[0021] The thermal conductivity of the boron nitride constituting the porous boron nitride layer may be, for example, 30 W / (m·K) or higher, 50 W / (m·K) or higher, or 60 W / (m·K) or higher. Since the porous boron nitride layer is composed of boron nitride, which has excellent thermal conductivity, it can reduce the thermal resistance of a porous boron nitride layer in which the voids are filled with a cured product of a thermosetting composition. The thermal conductivity of the boron nitride constituting the porous boron nitride layer is measured by the laser flash method for a sample in which the boron nitride constituting the porous boron nitride layer is formed to a size of 10 mm in length, 10 mm in width, and 1 mm in thickness.

[0022] The average pore diameter of the porous boron nitride layer may be, for example, 0.5 μm or more, and is preferably 0.6 μm or more, more preferably 0.8 μm or more, and even more preferably 1 μm or more, from the viewpoint of suitably filling the pores with a thermosetting composition. From the viewpoint of improving the insulating properties of the porous boron nitride layer, the average pore diameter is preferably 4.0 μm or less, 3.0 μm or less, 2.5 μm or less, 2.0 μm or less, or 1.5 μm or less.

[0023] The average pore size in a porous boron nitride layer is defined as the pore size at which the cumulative pore volume reaches 50% of the total pore volume, as measured using a mercury porosimeter (horizontal axis: pore size, vertical axis: cumulative pore volume). A mercury porosimeter manufactured by Shimadzu Corporation can be used, and measurements are taken by increasing the pressure from 0.03 atmospheres to 4000 atmospheres.

[0024] The porosity of the porous boron nitride layer is preferably 10% by volume or more, 20% by volume or more, or 30% by volume or more, based on the apparent total volume of the porous boron nitride layer, from the viewpoint of suitably improving the strength of the porous boron nitride layer in which the voids are filled with cured thermosetting material. From the viewpoint of improving the insulating properties and thermal conductivity of the porous boron nitride layer in which the voids are filled with cured thermosetting material, it is preferably 70% by volume or less, more preferably 60% by volume or less, and even more preferably 50% by volume or less. This porosity is determined from the bulk density D1 (g / cm³) which can be calculated from the apparent volume and mass of the porous boron nitride constituting the porous boron nitride layer. 3 ) and the theoretical density D2 of boron nitride (2.28 g / cm³) 3 ) and the following formula: Porosity (volume %) = [1 - (D1 / D2)] × 100 It is calculated according to the following.

[0025] <Thermosetting composition> The thermosetting composition is not particularly limited as long as it contains a thermosetting compound, but it is preferably a composition containing an epoxy compound and a cyanate compound.

[0026] Any epoxy compound can be used, provided it has the desired viscosity as a semi-cured product, or the viscosity suitable for impregnation when impregnating porous boron nitride. Examples of epoxy compounds include 1,6-bis(2,3-epoxypropane-1-yloxy)naphthalene, bisphenol A type epoxy resin, bisphenol F type epoxy resin, and dicyclopentadiene type epoxy resin. Of these, 1,6-bis(2,3-epoxypropane-1-yloxy)naphthalene is commercially available, for example, as HP-4032D (manufactured by DIC Corporation, trade name). Other commercially available epoxy compounds include EP-4000L, EP4088L, EP3950 (all manufactured by ADEKA Corporation, trade names), EXA-850CRP (DIC Corporation, trade name), jER807, jER152, YX8000, and YX8800 (all manufactured by Mitsubishi Chemical Corporation, trade names). As epoxy compounds, compounds containing vinyl groups can also be used. Examples of commercially available epoxy compounds containing vinyl groups include TEPIC-FL, TEPIC-VL (both manufactured by Nissan Chemical Corporation, trade names), MA-DGIC, and DA-MGIC (both manufactured by Shikoku Chemicals, Ltd., trade names).

[0027] The epoxy compound content is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, preferably 85% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less, based on the total amount of the thermosetting composition.

[0028] Examples of cyanate compounds include dimethylmethylenebis(1,4-phenylene)biscyanate and bis(4-cyanatephenyl)methane. Dimethylmethylenebis(1,4-phenylene)biscyanate is commercially available, for example, as TA-CN (manufactured by Mitsubishi Gas Chemical Company, Inc., trade name).

[0029] The cyanate compound content is preferably 5% by mass or more, more preferably 8% by mass or more, even more preferably 10% by mass or more, preferably 51% by mass or less, more preferably 40% by mass or less, and even more preferably 30% by mass or less, based on the total amount of the thermosetting composition.

[0030] In the circuit board manufacturing method described later, a porous boron nitride sheet is used in which the voids are filled with a semi-cured product of a thermosetting composition. For this reason, it is preferable that the thermosetting composition can be maintained in a semi-cured state with a desired viscosity. From this viewpoint, the equivalent ratio of the epoxy groups of the epoxy compound to the cyanate groups of the cyanate compound contained in the thermosetting composition (epoxy group equivalent / cyanato group equivalent) is preferably 1.0 or higher. This equivalent ratio is more preferably 1.5 or higher, even more preferably 2.0 or higher, and even more preferably 2.5 or higher. Furthermore, from the viewpoint of facilitating impregnation of the thermosetting composition and improving the heat resistance of the porous boron nitride sheet in which the voids are filled with a semi-cured product of the thermosetting composition, it is preferably 6.0 or less, 5.5 or less, 5.0 or less, 4.5 or less, 4.0 or less, 3.5 or less, or 3.0 or less.

[0031] The thermosetting composition may further contain other thermosetting compounds, excluding epoxy compounds and cyanate compounds.

[0032] From the viewpoint of further facilitating the maintenance of a semi-cured state with a desired viscosity, the thermosetting composition may further contain a curing agent in addition to the epoxy compound and the cyanate compound. In one embodiment, the thermosetting composition contains a curing agent for the epoxy compound. The curing agent for the epoxy compound is a compound that forms a crosslinked structure with the epoxy compound.

[0033] The curing agent for the epoxy compound preferably contains at least one selected from the group consisting of benzoxazine compounds, ester compounds, and phenol compounds.

[0034] Examples of benzoxazine compounds include bisphenol F-type benzoxazine compounds. Bisphenol F-type benzoxazine compounds are commercially available, for example, as Fa-type benzoxazine (manufactured by Shikoku Chemicals, Inc., trade name).

[0035] Examples of ester compounds include diphenyl phthalate and benzyl 2-ethylhexyl phthalate. The ester compound may also be an active ester compound. An active ester compound is a compound that has one or more ester bonds in its structure, and aromatic rings are bonded to both sides of the ester bond.

[0036] Examples of phenolic compounds include phenol, cresol, bisphenol A, bisphenol F, phenol novolac resin, cresol novolac resin, dicyclopentadiene-modified phenolic resin, terpene-modified phenolic resin, triphenolmethane-type resin, phenol aralkyl resin (having a phenylene skeleton, biphenylene skeleton, etc.), naphthol aralkyl resin, and allylphenol resin. These may be used individually or in combination of two or more. Phenolic compounds are commercially available, for example, as TD2131, VH4150 (manufactured by DIC Corporation, trade name), MEHC-7851M, MEHC-7500, MEH8005, and MEH8000H (manufactured by Meiwa Kasei Co., Ltd., trade name).

[0037] When the thermosetting composition contains a curing agent for epoxy compounds, the curing agent content is preferably 0.1% by mass or more, more preferably 5% by mass or more, even more preferably 7% by mass or more, preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 15% by mass or less, based on the total amount of the thermosetting composition.

[0038] The thermosetting composition may further contain a curing accelerator in addition to the compounds described above. The curing accelerator includes a component that functions as a catalyst for the curing reaction (catalytic curing agent). By including a curing accelerator in the thermosetting composition, the reaction between the epoxy compound and the cyanate compound, the self-polymerization reaction of the epoxy compound, and / or the reaction between the epoxy compound and the curing agent for the epoxy compound can be facilitated, as described later, and the semi-cured product can be easily maintained in a semi-cured state with a desired viscosity. Examples of such components include organometallic salts, phosphorus compounds, imidazole derivatives, amine compounds, or cationic polymerization initiators. The curing accelerator may be used individually or in combination of two or more of these.

[0039] Examples of organometallic salts include bis(2,4-pentanedionato)zinc(II), zinc octoate, zinc naphthenate, cobalt naphthenate, copper naphthenate, iron acetylacetone, nickel octoate, and manganese octoate.

[0040] Examples of phosphorus compounds include tetraphenylphosphonium tetra-p-tolylborate, tetraphenylphosphonium tetraphenylborate, triphenylphosphine, tri-p-tolylphosphine, tris(4-chlorophenyl)phosphine, tris(2,6-dimethoxyphenyl)phosphine, triphenylphosphinetriphenylborane, tetraphenylphosphonium dicyanamide, and tetraphenylphosphonium tetra(4-methylphenyl)borate.

[0041] Examples of imidazole derivatives include 1-(1-cyanomethyl)-2-ethyl-4-methyl-1H-imidazole, 2-ethyl-4-methylimidazole, 2-methylimidazole, 2-phenylimidazole, 1-cyanoethyl-2-phenylimidazole, 2-phenyl-4,5-dihydroxymethylimidazole, 2-phenyl-4-methyl-5-hydroxymethylimidazole, and 2,4,5-triphenylimidazole.

[0042] Examples of amine compounds include dicyandiamide, triethylamine, tributylamine, tri-n-octylamine, 1,4-diazabicyclo[2.2.2]octane, 1,8-diazabicyclo[5.4.0]undeca-7-ene, benzyldimethylamine, 4-methyl-N,N-dimethylbenzylamine, 2,4,6-tris(dimethylaminomethyl)phenol, and 4-dimethylaminopyridine.

[0043] Examples of cationic polymerization initiators include benzyl sulfonium salts, benzyl ammonium salts, benzylpyridinium salts, benzylphosphonium salts, hydrazinium salts, carboxylic acid ester compounds, sulfonic acid ester compounds, amine imides, antimony pentachloride-acetyl chloride complexes, diaryliodonium salts-dibenzyloxycopper, and the like.

[0044] The content of the curing accelerator described above may be 0.001 parts by mass or more, 0.01 parts by mass or more, or 0.05 parts by mass or more, and may be 1 part by mass or less, 0.8 parts by mass or less, 0.5 parts by mass or less, 0.3 parts by mass or less, or 0.1 parts by mass or less, based on 100 parts by mass of the total of the epoxy compound, cyanate compound, and optionally the curing agent of the epoxy compound. By setting the content within this range, it is possible to easily maintain the semi-cured product at the desired viscosity.

[0045] <Thickness of the second ceramic layer> The thickness of the second ceramic layer can be varied depending on the required characteristics. For example, if dielectric strength is not very important and thermal resistance is, a thin layer of 0.1 to 0.35 mm can be used, while a thicker layer of 0.35 to 1.0 mm can be used if dielectric strength and partial discharge characteristics are important.

[0046] <Dielectric strength of the second ceramic layer> The withstand voltage of the second ceramic layer 3, measured in accordance with JIS C2110, is preferably AC 5.0kV or higher. A withstand voltage of AC 5.0kV or higher for the second ceramic layer 3 further improves the reliability of the circuit board 1. From this viewpoint, the withstand voltage of the second ceramic layer 2, measured in accordance with JIS C2110, is more preferably AC 10.0kV or higher. For example, the withstand voltage of the second ceramic layer 3 can be made AC 5.0kV or higher by bonding the first ceramic layer 2 and the second ceramic layer 3 in the metal layer-free region 12 described later. Furthermore, the withstand voltage of the second ceramic layer 3 can be further improved by ensuring sufficient bonding between the side surface 41 of the first metal layer 4 and the second ceramic layer 3. While it is most preferable that the first ceramic layer 2 and the second ceramic layer 3 are bonded without gaps in the metal layer-free region, a bonding of 70% or more, 80% or more, 90% or more, or 95% or more is acceptable. The bonding ratio is the ratio of the bonded length to the total length of a single bonding interface (the length of a straight line drawn horizontally across the bottom surface of the metal layer, from one end of the metal layer to the end of the circuit board) in an SEM image as shown in Figure 8. This ratio is measured in five fields of view and calculated by averaging them.

[0047] (First ceramic layer) The first ceramic layer 2 is not particularly limited as long as it is a layer of ceramic material used as a ceramic substrate material. Examples of ceramic materials used in the first ceramic layer 2 include alumina, forsterite, mullite, magnesia, spinel, steatite, beryllia, glass ceramics, aluminum nitride, silicon carbide, silicon nitride, and boron nitride. The ceramic material used in the first ceramic layer 2 may be a dense body or a porous body. If the ceramic material is a porous body, it is preferable that the voids in the porous body are filled with resin. These ceramic materials can be used individually or in combination of two or more. Among these ceramic materials, from the viewpoint of thermal conductivity and bonding with the second ceramic layer 3, the first ceramic layer 2 is preferably a porous boron nitride layer in which the voids are filled with a cured product of a thermosetting composition. The porous boron nitride and thermosetting composition used in the first ceramic layer 2 may be the same as or different from the porous boron nitride and thermosetting composition used in the second ceramic layer 3. However, from the viewpoint of bonding with the second ceramic layer 3, it is preferable that the porous boron nitride and thermosetting composition used in the first ceramic layer 2 are the same as the porous boron nitride and thermosetting composition used in the second ceramic layer 3.

[0048] <Thickness of the first ceramic layer> The thickness of the first ceramic layer 2 can be varied depending on the required characteristics. For example, if dielectric strength is not very important and thermal resistance is important, a thin layer of 0.1 to 0.35 mm can be used, while a thicker layer of 0.35 to 1.0 mm can be used if dielectric strength and partial discharge characteristics are important.

[0049] <Dielectric strength of the first ceramic layer> The dielectric strength of the first ceramic layer 2, measured in accordance with JIS C2110, is preferably AC 5.0kV or higher. A dielectric strength of AC 5.0kV or higher for the first ceramic layer 2 further improves the reliability of the circuit board 1. From this perspective, the dielectric strength of the first ceramic layer 2, measured in accordance with JIS C2110, is more preferably AC 10.0kV or higher.

[0050] <Porous boron nitride layer and thermosetting composition of the first ceramic layer> If the first ceramic layer 2 is a porous boron nitride layer in which the voids are filled with cured material of a thermosetting composition, then the first ceramic layer 2 is the same as the second ceramic layer 3. Therefore, the explanation of the porous boron nitride layer and the thermosetting composition in the first ceramic layer 2 is omitted.

[0051] (First metal layer) The first metal layer 4 is positioned between the first ceramic layer 2 and the second ceramic layer 3. This allows a circuit formed by the first metal layer 4 to be created inside the circuit board 1. Furthermore, when a circuit is formed on the surface of the circuit board 1, a noise shield can be formed inside the circuit board 1 to suppress the influence of noise on the circuit. In addition, when wiring is formed on the surface of the circuit board 1, the wiring formed on the surface of the circuit board 1 and the first metal layer 4 can be arranged in close proximity and facing each other with the second ceramic layer 3 in between, thereby reducing the inductance of the wiring formed on the surface of the circuit board 1.

[0052] The metal used in the first metal layer 4 is not particularly limited, as long as it is a metal with high electrical conductivity. Examples of metals that can be used in the first metal layer 4 include copper, aluminum, nickel, iron, tin, gold, silver, molybdenum, titanium, and stainless steel. These metals can be used individually or in combination of two or more. Among these metals, copper and aluminum are preferred from the viewpoint of electrical conductivity and cost, with copper being more preferred.

[0053] The thickness of the first metal layer 4 is preferably one-quarter or less of the thickness of the second ceramic layer 3. When the thickness of the first metal layer 4 is one-quarter or less of the thickness of the second ceramic layer 3, it becomes easier to deform the second ceramic layer 3 along the step created at the boundary between the region where the first metal layer 4 is present and the region where the first metal layer 4 is not present. From this viewpoint, the thickness of the first metal layer 4 is more preferably one-fifth or less of the thickness of the second ceramic layer 3, and even more preferably one-sixth or less. Furthermore, from the viewpoint of lowering the electrical resistance of the first metal layer 4, the thickness of the first metal layer 4 is preferably 5 μm or more, more preferably 10 μm or more, and even more preferably 20 μm or more.

[0054] It is preferable that the side surface 41 of the first metal layer 4 is covered with the second ceramic layer 3. This further improves the dielectric strength of the circuit board 1. From the viewpoint of the dielectric strength of the circuit board 1, it is preferable that there is no gap between the side surface 41 of the first metal layer 4 and the second ceramic layer 3, and for example, it is preferable that there is no gap that can be confirmed in an SEM image taken at 100x magnification using a scanning electron microscope (SEM).

[0055] Furthermore, since the second ceramic layer 3 is a porous material made of hard boron nitride, a step difference at the boundary between the metal layer present region 11 and the metal layer non-present region 12 in the second ceramic layer 3 may occur, potentially creating a large gap between the first ceramic layer 2 and the second ceramic layer 3 in the metal layer non-present region 12. However, in the circuit board 1 of one embodiment of the present invention, the occurrence of a large gap between the metal layer present region 11 and the metal layer non-present region 12 is prevented by adjusting the pressurization conditions when the first ceramic layer 2 and the second ceramic layer 3 are laminated. As a result, the dielectric strength of the circuit board 1 is improved. In addition, the side surface 41 of the first metal layer 4 is covered by the second ceramic layer 3.

[0056] Preferably, the area of ​​the first metal layer 4 in plan view is smaller than the area of ​​the second ceramic layer 3 in plan view. By spacing out the edges of the first metal layer 4 and the edges of the second ceramic layer 3, and arranging the first metal layer 4 to be located in the center of the second ceramic layer 3, the circuit board 1 can have the metal layer present region 11 and the metal layer absent region 12 described later. For example, preferably, the area of ​​the first metal layer 4 in plan view is 10 to 90% of the area of ​​the second ceramic layer 3 in plan view.

[0057] The ten-point average roughness (Rz) of the surfaces on both sides of the first metal layer 4 is preferably 1.0 to 20 μm. If the ten-point average roughness (Rz) of the surfaces on both sides of the first metal layer 4 is 1.0 μm or more, the bonding between the first metal layer 4 and the first ceramic layer 2 and the second ceramic layer 3 becomes even stronger. Also, if the ten-point average roughness (Rz) of the surfaces on both sides of the first metal layer 4 is 20 μm or less, the occurrence of defects in the first ceramic layer 2 and the second ceramic layer 3 due to the surface roughness of the first metal layer 4 can be suppressed. From this viewpoint, the ten-point average roughness (Rz) of the surfaces on both sides of the first metal layer 4 is more preferably 2.0 to 15 μm, and even more preferably 3.0 to 12 μm. For example, by using copper foil with a ten-point average roughness (Rz) of 1.0 to 20 μm on both surfaces, a first metal layer 4 with a ten-point average roughness (Rz) of 1.0 to 20 μm on both surfaces can be formed on the circuit board 1. Alternatively, when using copper foil where the ten-point average roughness (Rz) of the S side (shiny side) and the M side (matte side) are different, such as electrolytic copper foil, copper foil with a ten-point average roughness (Rz) of 1.0 to 20 μm on the M side may be used for the first metal layer 4, and the S side may be roughened to bring the ten-point average roughness (Rz) of the S side within the range of 1.0 to 20 μm. Note that the ten-point average roughness (Rz) is a value measured in accordance with JIS B0601:1994.

[0058] (Metal layer existing area, metal layer non-existing area) In one embodiment of the present invention, the circuit board 1 has, in a plan view, a metal layer present region 11 in which a first metal layer 4 exists between the first ceramic layer 2 and the second ceramic layer 3, and a metal layer absent region 12 in which the first metal layer 4 does not exist between the first ceramic layer 2 and the second ceramic layer 3, and the first ceramic layer 2 and the second ceramic layer 3 are joined in the metal layer absent region 12. If the first ceramic layer 2 and the second ceramic layer 3 are not joined in the metal layer absent region 12, this will cause dielectric breakdown, worsening the dielectric strength of the second ceramic layer 3, and as a result, the dielectric strength of the circuit board 1 will also worsen. The joining of the first ceramic layer 2 and the second ceramic layer 3 is as described above.

[0059] (A modified example of a circuit board) The circuit board 1 of one embodiment of the present invention can be modified as follows. <Example 1> As shown in Figure 2, a modified circuit board 1A of one embodiment of the present invention may further include a second metal layer 5 bonded to the surface 22 of the first ceramic layer 2 opposite to the surface 21 on the second ceramic layer 3 side. This allows the circuit board 1A to be bonded to the heat sink base plate using solder, thereby improving the thermal conductivity between the circuit board 1A and the heat sink base plate, and also improving the reliability of the bond between the circuit board 1A and the heat sink base plate. Furthermore, the strength of the circuit board 1A can be improved by the second metal layer 5.

[0060] From the viewpoint of the thermal conductivity of the second metal layer 5 and the cost of the second metal layer 5, the second metal layer 5 is preferably a layer of copper or aluminum. Furthermore, from the viewpoint of improving the strength of the circuit board 1A, the thickness of the second metal layer 5 is preferably 0.1 to 5.0 mm, more preferably 0.2 to 3.0 mm, and even more preferably 0.3 to 2.5 mm. Furthermore, from the viewpoint of improving the strength of the circuit board 1A, the second metal layer 5 is preferably rolled copper foil that has been recrystallized by heat treatment.

[0061] <Modification 2> As shown in Figure 2, the modified circuit board 1A of one embodiment of the present invention may further include a third metal layer 6 formed on the surface 32 of the second ceramic layer 3 opposite to the surface 31 on the first ceramic layer 2 side. This allows circuits to be formed on the surface of the circuit board 1A, or wiring to be formed on the circuit board 1A and semiconductor elements to be mounted. Since the modified circuit board 1A of one embodiment of the present invention has excellent heat dissipation, the semiconductor elements mounted on the circuit board 1A are preferably power semiconductor elements. Examples of power semiconductor elements include Si power semiconductor elements, SiC power semiconductor elements, GaN power semiconductor elements, etc. Furthermore, in recent years, in order to enable high-speed operation of power semiconductor elements, it has been required to reduce the inductance of the wiring formed on the surface of the circuit board 1A. As described above, since the circuit board 1A can achieve close-proximity opposing arrangement, the inductance of the wiring formed on the surface of the circuit board 1A can be reduced.

[0062] From the viewpoint of thermal conductivity and cost of the third metal layer 6, the third metal layer 6 is preferably a layer of copper or aluminum, more preferably copper, and even more preferably electrolytic copper foil or rolled copper foil recrystallized by heat treatment. The thickness of the third metal layer 6 is preferably 0.01 to 2.0 mm, more preferably 0.02 to 0.8 mm, and even more preferably 0.035 to 0.5 mm. Furthermore, from the viewpoint of bonding with the second ceramic layer 3 and suppression of defect occurrence in the second ceramic layer 3, the ten-point average roughness (Rz) of the surface of the third metal layer 6 on the second ceramic layer 3 side is preferably 1.0 to 20 μm, and more preferably 3.0 to 17 μm.

[0063] <Variation 3> As shown in Figure 2, in the modified circuit board 1A of one embodiment of the present invention, the second ceramic layer 3 has an opening 33, and the first metal layer 4 may be exposed at the opening 33. This allows the first metal layer 4 and the metal layer formed on the surface of the circuit board 1A to be electrically connected by filling the inside of the opening 33 with metal plating. The shape of the opening 33 in plan view may be circular or rectangular. If the shape of the opening 33 in plan view is circular, the diameter of the opening 33 is preferably 0.3 to 3 mm, and more preferably 0.5 to 2 mm. If the shape of the opening 33 in plan view is rectangular, the length of one side of the opening 33 is preferably 0.5 to 10 mm, and more preferably 1.0 to 5.0 mm.

[0064] Circuit board 1 of one embodiment of the present invention and circuit board 1A of a modified example of one embodiment of the present invention are merely examples of the circuit board of the present invention. Therefore, the circuit board of the present invention is not limited to circuit board 1 of one embodiment of the present invention and circuit board 1A of a modified example of one embodiment of the present invention.

[0065] [Manufacturing method for circuit boards] A method for manufacturing a circuit board according to one embodiment of the present invention will be described with reference to Figure 3. A method for manufacturing a circuit board according to one embodiment of the present invention includes the steps of: (A) creating a first laminate 111 including a first ceramic layer 2 and a first metal layer 4 laminated on the first ceramic layer 2 by placing a first metal foil 104 on a first ceramic sheet 102 and applying pressure while heating; (B) creating a first laminate 112 including a first ceramic layer 2, a second ceramic layer 3 laminated on the first ceramic layer 2 and a first metal layer 4 disposed between the first ceramic layer 2 and the second ceramic layer 3 by placing a second ceramic sheet 103 on the first metal layer 4 of the first laminate 111 and applying pressure while heating. Each step will be described in detail below.

[0066] (Process (A)) In step (A), as shown in Figure 3(a), the first metal foil 104 is placed on the first ceramic sheet 102. Then, the first ceramic sheet 102 on which the first metal foil 104 is placed is heated and pressurized to produce a first laminate 111 including the first ceramic layer 2 and the first metal layer 4 laminated on the first ceramic layer 2.

[0067] <First ceramic sheet> The first ceramic sheet 102 is not particularly limited as long as it is a sheet of ceramic material used as a ceramic substrate material. Examples of ceramic materials used in the first ceramic sheet 102 include the ceramic material used in the first ceramic layer 2 described above. The ceramic material used in the first ceramic sheet 102 may be a dense body or a porous body. When the ceramic material is a porous body, it is preferable that the voids of the porous body are filled with a semi-cured resin. Among these ceramic materials, from the viewpoint of thermal conductivity and bonding with the second ceramic sheet 103 described later, the first ceramic sheet 102 is preferably a porous boron nitride sheet in which the voids are filled with a semi-cured thermosetting composition. The porous boron nitride and thermosetting composition used in the first ceramic sheet 102 may be the same as or different from the porous boron nitride and thermosetting composition used in the second ceramic sheet 103 described later. However, from the viewpoint of bonding with the second ceramic sheet 103, it is preferable that the porous boron nitride and thermosetting composition used in the first ceramic sheet 102 are the same as those used in the second ceramic sheet 103.

[0068] <First Metal Foil> The metal used for the first metal foil 104 is the same as the metal used for the first metal layer 4 described above, so no explanation is given. When copper foil is used as the first metal foil 104, electrolytic copper foil is preferred because it can form a fine pattern. Also, the ten-point mean roughness (Rz) and thickness of the surface of the first ceramic sheet 102 are the same as the ten-point mean roughness (Rz) and thickness of the surface of the first ceramic layer 2 described above.

[0069] <Pressure Conditions> From the viewpoint of strongly bonding the first ceramic sheet 102 and the first metal foil 104, the pressurizing conditions in step (A) are preferably a heating temperature of 150 to 260°C, more preferably 180 to 230°C, a pressure of preferably 0.1 to 5 MPa, more preferably 0.3 to 2 MPa, and a pressurizing time of preferably 10 minutes to 30 hours, more preferably 20 minutes to 15 hours. Furthermore, if the first ceramic sheet 102 is a porous boron nitride sheet in which the voids are filled with a semi-cured material of a thermosetting composition, the above pressurizing conditions can cure the semi-cured material filling the voids of the porous boron nitride sheet.

[0070] From the viewpoint of further strengthening the bonding between the first ceramic sheet 102 and the first metal foil 104, it is preferable to heat and pressurize the first ceramic sheet 102 on which the first metal foil 104 is placed under vacuum. The pressure under vacuum at this time is preferably 5.0 kPa or less, and more preferably 3.0 kPa or less, in absolute pressure.

[0071] (Process (B)) In step (B), as shown in Figure 3(b), the patterned photoresist 71 is placed on the first metal layer 4 of the first laminate 111. For example, a dry film laminator is used to laminate a dry film resist onto the first metal layer 4, an exposure apparatus equipped with a photomask matched to the pattern of the photoresist 71 is used to irradiate with UV light, and a developing apparatus is used to dissolve and remove the unwanted parts of the photoresist, thereby forming the patterned photoresist 71 on the first metal layer 4. Subsequently, the first metal layer 4 is etched using an etching solution to remove the unwanted parts of the first metal layer 4, the photoresist 71 is peeled off, and the first metal layer 4 is patterned as shown in Figure 3(c). When the material of the first metal layer 4 is copper, examples of etching solutions include ammonia alkaline etching solution, cupric chloride etching solution, ferric chloride etching solution, sulfuric acid / hydrogen peroxide etching solution, peroxodisulfate etching solution, chromic acid / sulfuric acid mixture, etc. Among these etching solutions, sulfuric acid / hydrogen peroxide etching solution is preferred.

[0072] (Process (C)) In step (C), as shown in Figure 3(d), the second ceramic sheet 103 is placed on the first metal layer 4 of the first laminate 111. Then, by heating and pressurizing the first laminate 111 on which the second ceramic sheet 103 is placed, a second laminate 112 is produced, as shown in Figure 3(e), which includes the first ceramic layer 2, the second ceramic layer 3 laminated on the first ceramic layer 2, and the first metal layer 4 placed between the first ceramic layer 2 and the second ceramic layer 3.

[0073] <Second ceramic sheet> The second ceramic sheet 103 is a porous boron nitride sheet in which the voids are filled with a semi-cured product of a thermosetting composition. When the thermosetting composition is a compound containing an epoxy compound and a cyanate compound, the porous boron nitride sheet is prepared, for example, as follows.

[0074] The method for producing the second ceramic sheet 103, when the thermosetting composition is a thermosetting composition containing an epoxy compound and a cyanate compound, comprises the steps of impregnating a porous body with the thermosetting composition containing the epoxy compound and the cyanate compound (impregnation step) and heating the porous body impregnated with the thermosetting composition at a temperature T1 at which the cyanate compound reacts (semi-curing step). The form of the thermosetting composition is as described above.

[0075] In the impregnation process, for example, a porous boron nitride is first prepared. The porous boron nitride may be manufactured by sintering raw materials, or a commercially available product may be used. A porous boron nitride can be obtained, for example, by sintering boron nitride powder. Alternatively, a porous boron nitride can be obtained by calcining a mixture of boron-containing compounds such as boric acid, boron oxide, and borax, and nitrogen-containing compounds such as urea and melamine, or by calcining hexagonal boron carbonitride (h-B4CN4).

[0076] Boron nitride porous bodies may be produced by drying a slurry containing boron nitride powder in a spray dryer or the like to produce boron nitride powder granules, then molding the boron nitride powder using these granules and sintering it. For molding, a die press molding method or a cold isostatic pressing (CIP) method may be used.

[0077] A sintering aid may be added to the boron nitride powder. The sintering aid may be, for example, an oxide of a rare earth element such as yttria (yttrium oxide), alumina (aluminum oxide), and magnesia (magnesium oxide), an alkali metal carbonate such as lithium carbonate and sodium carbonate, and boric acid. When a sintering aid is added to the boron nitride powder, the amount of the sintering aid may be, for example, 0.01 parts by mass or more, or 0.1 parts by mass or more, per 100 parts by mass of boron nitride powder. The amount of the sintering aid may be 20 parts by mass or less, 15 parts by mass or less, or 10 parts by mass or less, per 100 parts by mass of boron nitride powder. By setting the amount of the sintering aid within the above range, it becomes easy to adjust the average pore diameter of the boron nitride porous body to within the range of the average pore diameter of the second ceramic layer described above.

[0078] The firing temperature of the molded body of boron nitride powder may be, for example, 1600°C or higher or 1700°C or higher. Alternatively, the firing temperature of the molded body of boron nitride powder may be, for example, 2200°C or lower or 2000°C or lower. The firing time of the molded body of boron nitride powder may be, for example, 1 hour or more, or 30 hours or less. The atmosphere during firing may be an inert gas atmosphere such as nitrogen, helium, and argon.

[0079] For firing molded bodies of boron nitride powder, for example, batch furnaces and continuous furnaces can be used. Examples of batch furnaces include muffle furnaces, tubular furnaces, and atmosphere furnaces. Examples of continuous furnaces include rotary kilns, screw conveyor furnaces, tunnel furnaces, belt furnaces, pusher furnaces, and koto-type continuous furnaces.

[0080] Boron nitride porous bodies obtained by firing molded bodies of boron nitride powder may be processed into sheets by cutting or other means to the desired shape and thickness, as necessary, before the impregnation process. Alternatively, boron nitride porous bodies obtained by firing molded bodies of boron nitride powder may be in the form of sheets.

[0081] In the impregnation process, a solution containing a thermosetting composition is then prepared in an impregnation apparatus, and the boron nitride porous material is immersed in the solution to impregnate the pores of the boron nitride porous material with the thermosetting composition.

[0082] The impregnation process may be carried out under either reduced pressure or increased pressure conditions, or a combination of impregnation under reduced pressure and impregnation under increased pressure conditions may be performed. When the impregnation process is carried out under reduced pressure conditions, the pressure inside the impregnation apparatus may be, for example, 1000 Pa or less, 500 Pa or less, 100 Pa or less, 50 Pa or less, or 20 Pa or less. When the impregnation process is carried out under increased pressure conditions, the pressure inside the impregnation apparatus may be, for example, 1 MPa or more, 3 MPa or more, 10 MPa or more, or 30 MPa or more.

[0083] When impregnating a boron nitride porous material with a thermosetting composition, the solution containing the thermosetting composition may be heated. Heating the solution containing the thermosetting composition adjusts the viscosity of the solution and promotes impregnation into the boron nitride porous material. The temperature at which the solution containing the thermosetting composition is heated for impregnation may be higher than the temperature T1 described later. The upper limit of the temperature at which the solution containing the thermosetting composition is heated may be T1 + 20°C or lower.

[0084] In the impregnation process, the boron nitride porous body is immersed in a solution containing a thermosetting composition and held in that state for a predetermined time. The predetermined time (immersion time) is not particularly limited and may be, for example, 5 minutes or more, 30 minutes or more, 1 hour or more, 5 hours or more, 10 hours or more, 100 hours or more, or 150 hours or more.

[0085] In the semi-curing step, a porous boron nitride impregnated with the thermosetting composition is heated at a temperature T1 at which the cyanate compound reacts. This causes the cyanate compound contained in the thermosetting composition to react, yielding a semi-cured product. At this time, the cyanate compounds may react with each other, or the cyanate compound may react with a portion of the epoxy compound. On the other hand, in the thermosetting composition, as described above, the equivalent ratio of the epoxy groups of the epoxy compound to the cyanate groups of the cyanate compound is 1.0 or more. That is, in the semi-cured product, the epoxy compound is present in excess of the cyanate compound in terms of epoxy equivalents, and these epoxy compounds remain in an uncured state. As a result, a semi-cured product of the thermosetting composition is obtained.

[0086] From the viewpoint of sufficiently impregnating the porous body with the semi-cured material, the temperature T1 is preferably 70°C or higher, more preferably 80°C or higher, and even more preferably 90°C or higher. From the viewpoint of minimizing viscosity change with respect to time, the temperature T1 is preferably 180°C or lower, more preferably 150°C or lower, and even more preferably 120°C or lower. Note that temperature T1 refers to the ambient temperature when heating the boron nitride porous body impregnated with the thermosetting composition.

[0087] The heating time in the semi-curing process may be 1 hour or more, 3 hours or more, or 5 hours or more, and may be 12 hours or less, 10 hours or less, or 8 hours or less.

[0088] In a porous boron nitride sheet (hereinafter sometimes referred to as a semi-cured sheet) in which the resulting voids are filled with a semi-cured product of a thermosetting composition, some compounds (mainly epoxy compounds) remain uncured, resulting in superior bonding properties compared to a fully cured product of the thermosetting composition. Furthermore, in this semi-cured sheet, the uncured state is maintained for a long period unless heated to the temperature at which the uncured compounds cure (details will be described later), making it easy to maintain the desired viscosity with excellent adhesion to the adherend. This allows for the production of a semi-cured sheet with excellent handling properties.

[0089] <Pressure Conditions> When heating and pressurizing the first laminate 111 on which the second ceramic sheet 103 is placed, the heating temperature is preferably 150 to 260°C, more preferably 180 to 230°C, the pressure is preferably 1 to 20 MPa, more preferably 5 to 15 MPa, and the pressurizing time is preferably 10 minutes to 30 hours, more preferably 20 minutes to 15 hours. By laminating the second ceramic sheet 103 under these pressurizing conditions, the second ceramic sheet 103 can be strongly bonded to the first laminate 111 while preventing gaps from forming between the first laminate 111 and the second ceramic sheet 103 due to the step created by the first metal layer 4. Furthermore, the side surface 41 of the first metal layer 4 can be covered by the second ceramic layer 3. Furthermore, since the above heating temperature is the temperature at which the self-polymerization reaction of the epoxy compound (reaction between uncured epoxy compounds) occurs, the above pressurizing conditions cause the semi-cured material filling the voids of porous boron nitride in the second ceramic sheet to become a cured material, and a circuit board 1 of one embodiment of the present invention can be obtained.

[0090] Prior to the above pressurization operation on the first laminate 111, the first laminate 111 on which the second ceramic sheet 103 is placed may be heated at a temperature lower than the heating temperature for the above pressurization while being pressurized. This allows the thickness of the second ceramic sheet 103 to be adjusted. The heating temperature for pre-pressurization before the above pressurization operation is preferably 100 to 200°C, more preferably 120 to 180°C, the pressure is preferably 1 to 20 MPa, more preferably 5 to 15 MPa, and the pressurization time is preferably 10 minutes to 30 hours, more preferably 20 minutes to 15 hours.

[0091] From the viewpoint of further strengthening the bond between the second ceramic sheet 103 and the first laminate 111, it is preferable to heat and pressurize the first laminate 111 on which the second ceramic sheet 103 is placed under vacuum. This further prevents gaps from forming between the first laminate 111 and the second ceramic sheet 103 due to the step created by the first metal layer 4. The vacuum pressure at this time is preferably 5 kPa or less, and more preferably 3 kPa or less, in absolute pressure.

[0092] (Variable example of a circuit board manufacturing method) The method for manufacturing a circuit board according to one embodiment of the present invention can be modified as follows. <Example 1> A method for manufacturing a circuit board according to one embodiment of the present invention may further include a step of roughening the surface of the first metal layer 4 of the first laminate 111. This can further strengthen the bond between the first metal layer 4 and the second ceramic sheet 103. From this viewpoint, the ten-point average roughness (Rz) of the surface of the roughened first metal layer 4 is preferably 1 to 20 μm, more preferably 2 to 15 μm, and even more preferably 3 to 13 μm. Examples of treatments for roughening the surface of the first metal layer 4 include blackening treatment, blackening reduction treatment, micro-etching treatment, NBD treatment, and the like.

[0093] <Modification 2> Step (A) may be carried out by placing the first ceramic sheet 102 on the second metal foil 105, placing the first metal foil 104 on the first ceramic sheet 102 placed on the second metal foil 105, and applying pressure while heating, as shown in Figure 4. As a result, the first laminate 111A further includes a second metal layer 5 on which the first ceramic layer 2 is laminated. This makes it possible to manufacture a circuit board 1A (see Figure 2) that includes a second metal layer 5 bonded to the surface 22 of the first ceramic layer 2 opposite to the surface 21 on the second ceramic layer 3 side.

[0094] <Variation 3> In step (C), as shown in Figure 5(a), a second ceramic sheet 103 is placed on the first metal layer 4 of the first laminate 111A, a third metal foil 106 is further placed on the second ceramic sheet 103, and by heating and pressing, a second laminate 112A is produced, as shown in Figure 5(b), which further includes a third metal layer 6 laminated on the second ceramic layer 3. This makes it possible to produce a circuit board 1A including a third metal layer 6 formed on the surface 32 of the second ceramic layer 3 opposite to the surface 31 on the first ceramic layer 2 side (see Figure 2).

[0095] In this case, the method for manufacturing a circuit board according to one embodiment of the present invention may include the steps of placing a patterned photoresist 72 on the third metal layer 6 of the second laminate 112A, as shown in Figure 5(c), etching the third metal layer 6 using an etching solution to remove unnecessary portions of the third metal layer 6, peeling off the photoresist 72, and patterning the third metal layer 6, as shown in Figure 5(d). This makes it possible to manufacture a circuit board 1A with circuits or wiring formed by the third metal layer 6 on its surface (see Figure 2).

[0096] <Modification 4> A method for manufacturing a circuit board according to one embodiment of the present invention may further include the step of forming an opening 33 in the second ceramic layer 3 of the second laminate 112A so that the first metal layer 4 is exposed, as shown in Figure 6. This makes it possible to manufacture a circuit board 1A in which the second ceramic layer 3 has an opening 33 and the first metal layer 4 is exposed at the opening 33 (see Figure 2). For example, the opening 33 can be formed in the second ceramic layer 3 by a processing method such as drilling, punching, or laser processing. Among these processing methods, laser processing is preferred from the viewpoint of micro-machining and productivity. Examples of laser processing equipment used in laser processing include carbon dioxide laser processing machines, YAG laser processing machines, and excimer laser processing machines. Among these laser processing equipment, carbon dioxide laser processing machines are preferred.

[0097] The present invention provides a method for manufacturing a circuit board, comprising the steps of: placing a second ceramic sheet on the first metal layer of a first laminate comprising a first ceramic layer and a first metal layer laminated on the first ceramic layer; and pressurizing the laminate at a heating temperature of 150 to 260°C while applying pressure at a pressure of 5 to 15 MPa and a pressurizing time of 10 minutes to 30 hours to produce a second laminate comprising a first ceramic layer, a second ceramic layer laminated on the first ceramic layer, and a first metal layer disposed between the first and second ceramic layers, wherein the second ceramic sheet is a porous boron nitride sheet in which the voids are filled with a semi-cured product of a thermosetting composition, and is not limited to the method for manufacturing a circuit board according to one embodiment of the present invention or a modified example of the method for manufacturing a circuit board according to one embodiment of the present invention. [Examples]

[0098] The present invention will be described in detail below with reference to examples. However, the present invention is not limited to the following examples.

[0099] [Preparation of porous boron nitride sheets in which voids are filled with semi-cured thermosetting material] (Fabrication of boron nitride porous materials) In a container, 40.0% by mass of amorphous boron nitride powder (manufactured by Denka Co., Ltd., oxygen content: 1.5%, boron nitride purity: 97.6%, average particle size: 6.0 μm) and 60.0% by mass of hexagonal boron nitride powder (manufactured by Denka Co., Ltd., oxygen content: 0.3%, boron nitride purity: 99.0%, average particle size: 30.0 μm) were measured out. After adding sintering aids (boric acid, calcium carbonate), an organic binder and water were added and mixed, then dried and granulated to prepare a mixed nitride powder.

[0100] The above mixed powder was filled into a mold and press-molded at a pressure of 5 MPa to obtain a molded body. Next, the molded body was compressed at a pressure of 20 to 100 MPa using a cold isotropic pressing (CIP) apparatus (manufactured by Kobe Steel, Ltd., product name: ADW800). The compressed molded body was sintered by holding it at 2000°C for 10 hours in a batch-type high-frequency furnace (manufactured by Fuji Denpa Kogyo Co., Ltd., product name: FTH-300-1H) to produce a porous boron nitride body. The firing was performed by adjusting the furnace to a nitrogen atmosphere by flowing nitrogen into the furnace at a flow rate of 10 L / min under standard conditions. The average pore size of the pores in the obtained porous boron nitride body was 3.6 μm. The porosity of the obtained porous boron nitride body was 44 volume%.

[0101] (Preparation of thermosetting compositions) The following raw materials were used to prepare the thermosetting composition. Epoxy compound: Product name "HP-4032D", manufactured by DIC Corporation. Cyanate compound: Trade name "TA-CN", manufactured by Mitsubishi Gas Chemical Company, Inc. Benzooxazine compound: Trade name "Fa-type benzooxazine", manufactured by Shikoku Chemicals Co., Ltd. Metal-based hardening accelerator: Bis(2,4-pentanedionato)zinc(II), manufactured by Tokyo Chemical Industry Co., Ltd.

[0102] 66 parts by mass of epoxy compound, 23 parts by mass of cyanate compound, and 11 parts by mass of benzoxazine compound or ester compound were placed in a container. Furthermore, 0.01 parts by mass of a metallic curing accelerator was added to the container for every 100 parts by mass of the total epoxy compound, cyanate compound, and benzoxazine compound or ester compound. These raw materials placed in the container were then mixed while heated to approximately 80°C to prepare a thermosetting composition.

[0103] (Filling voids in porous materials with thermosetting compositions) A thermosetting composition was impregnated into a boron nitride porous material by the following method. First, the boron nitride porous material and the thermosetting composition in a container were placed in a vacuum heating impregnation apparatus (product name "G-555AT-R", manufactured by Kyoshin Engineering Co., Ltd.). Next, the apparatus was degassed for 10 minutes under conditions of a degassing temperature of 100°C and a degassing pressure of 15 Pa. After degassing, the boron nitride porous material was immersed in the thermosetting composition for 40 minutes while maintaining the same conditions, thereby impregnating the boron nitride porous material with the thermosetting composition.

[0104] Subsequently, the container containing the boron nitride porous material and the thermosetting composition was removed and placed in a pressurized heating impregnation device (product name "HP-4030AA-H45", manufactured by Kyoshin Engineering Co., Ltd.). Under conditions of an impregnation temperature of 130°C and an impregnation pressure of 3.5 MPa, it was held for 120 minutes to further impregnate the boron nitride porous material with the thermosetting composition. After that, the boron nitride porous material impregnated with the thermosetting composition was removed from the device and heated for a predetermined time under conditions of a heating temperature of 120°C and atmospheric pressure. As a result, the thermosetting composition partially cured, and a boron nitride porous material was produced in which the voids were filled with the semi-cured material of the thermosetting composition.

[0105] Using a wire saw, the obtained porous boron nitride material was sliced ​​to produce a porous boron nitride sheet in which 50 mm × 50 mm × 0.4 mm voids were filled with a semi-cured thermosetting composition.

[0106] [Fabrication of Circuit Board 1] In addition to the porous boron nitride sheet mentioned above, the following materials were prepared. Copper foil 1: Electrolytic copper foil (manufactured by Hitachi Metals NeoMaterial Co., Ltd., size: 50mm x 50mm x 0.035mm, average roughness of 10 points on the M side (Rz): 10.3μm) Copper foil 2: Rolled oxygen-free copper foil with one side roughened (manufactured by Hitachi Metals NeoMaterial Co., Ltd., size: 50mm x 50mm x 0.3mm, average roughness of 10 points on the roughened surface (Rz): 15.4μm) Cushioning material: Single cushioning material (product name "YOM" (registered trademark), fluororubber grade, manufactured by Yamauchi Co., Ltd., size 50mm x 50mm) Release sheet: Teflon® sheet (product name "Nitoflon", manufactured by Nitto Denko Corporation, size: 50mm x 50mm x 0.05mm)

[0107] A manual hydraulic vacuum heating press (model "IMC-1674", manufactured by Imoto Seisakusho Co., Ltd.) was preheated to a set temperature of 200°C. After confirming that the heating plate temperature of the manual hydraulic vacuum heating press was 200°C using a contact thermometer, a laminate 1, consisting of cushioning material / release sheet / copper foil 1 / porous boron nitride sheet / copper foil 2 / release sheet / cushioning material, was placed in the center of the preheated heating plate of the manual hydraulic vacuum heating press. After closing the front door of the manual hydraulic vacuum heating press, the heating plate was manually raised to pressurize the laminate 1 to a pressure of 15 MPa. After the pressure reached 15 MPa, the vacuum pump of the manual hydraulic vacuum heating press was activated and the valve was operated to create a vacuum inside the manual hydraulic vacuum heating press (pressure inside the manual hydraulic vacuum heating press: 2.6 kPa). In this state, the laminate 1 was pressurized for 30 minutes while maintaining a pressure of 15 MPa. Thirty minutes after pressurization began, the hot plate was lowered, the leak valve was opened, and the pressurized laminate 1 was removed from the manual hydraulic vacuum heating press. Then, the cushioning material and release sheet were removed from laminate 1 to obtain laminate 2, which was laminated in the order of copper foil 1 / porous boron nitride sheet / copper foil 2.

[0108] As shown in Figure 7(a), a 40mm x 40mm dry film resist (corresponding to reference numeral 71) was laminated in the center of the copper foil 1 (corresponding to reference numeral 4) of the laminate 2, and then cured by irradiation with UV light. Then, using a sulfuric acid / hydrogen peroxide etching solution, the areas on the copper foil 1 where the dry film resist was not laminated were removed, and the dry film resist was peeled off, making the size of the copper foil 1 of the laminate 2 40mm x 40mm. As a result, the area within 5mm from the outer edge of the laminate 2 was a region where the copper foil 1 was not present. Next, the surface of the copper foil 1 was roughened by grain boundary etching using NBD treatment. The ten-point average roughness (Rz) of the surface of the copper foil 1 after roughening was 3.2 μm.

[0109] The manual hydraulic vacuum heating press was preheated to a set temperature of 125°C. After confirming that the heating plate temperature of the manual hydraulic vacuum heating press was 125°C using a contact thermometer, a laminate 3, which was constructed by stacking cushioning material / copper foil 1 / porous boron nitride sheet / laminated body 2 (copper foil 1 / porous boron nitride sheet / copper foil 2) / cushioning material in that order, was placed in the center of the heating plate of the preheated manual hydraulic vacuum heating press. After closing the front door of the manual hydraulic vacuum heating press, the heating plate was manually raised to pressurize laminate 3 to a pressure of 10 MPa. After the pressure reached 10 MPa, the vacuum pump of the manual hydraulic vacuum heating press was activated and the valve was operated to create a vacuum inside the manual hydraulic vacuum heating press (pressure inside the manual hydraulic vacuum heating press: 2.6 kPa). In this state, laminate 3 was pressurized for 30 minutes while maintaining a pressure of 10 MPa. Thirty minutes after pressurization began, the set temperature of the manual hydraulic vacuum heating press was changed to 200°C. Then, pressurization was continued at a heating temperature of 200°C for another 30 minutes. Thirty minutes after pressurization began at a heating temperature of 200°C, the heating plate was lowered, the leak valve was opened, and the pressurized laminate 3 was removed from the manual hydraulic vacuum heating press. Then, the cushioning material was removed from laminate 3 to obtain laminate 4, which was formed by laminating copper foil 1 / porous boron nitride sheet / laminated 2 (copper foil 1 / porous boron nitride sheet / copper foil 2) in that order.

[0110] As shown in Figure 7(b), four circular dry film resists (1.0 cm in diameter) (corresponding to reference numeral 72) were laminated onto the copper foil 1 (corresponding to reference numeral 6) of the laminate 4, and then the dry film resists were cured by irradiation with UV light. Then, after removing the parts of the copper foil 1 where the dry film resist was not laminated using a sulfuric acid / hydrogen peroxide etching solution, the dry film resists were peeled off to create a laminate 4 having four circular copper foil layers with a diameter of 1.0 cm on its surface.

[0111] As shown in Figure 7(c), a carbon dioxide laser processing machine was used to form two openings (corresponding to reference numeral 33) that are square in shape in plan view (side length: 3 mm) and reach the copper foil 1 inside the laminate 4, thereby fabricating the circuit board 1. The layer structure of the circuit board 1 is copper foil 1 / porous boron nitride sheet / copper foil 1 / porous boron nitride sheet / copper foil 2, which corresponds to the structure of the third metal layer 6 / second ceramic layer 3 / first metal layer 4 / first ceramic layer 2 / second metal layer 5 of the circuit board 1 of one embodiment of the present invention. Hereinafter, the first layer copper foil 1 of the circuit board 1 may be referred to as the third metal layer, the second layer porous boron nitride sheet as the second ceramic layer, the third layer copper foil 1 as the first metal layer, the fourth layer porous boron nitride sheet as the first ceramic layer, and the fifth layer copper foil 2 as the second metal layer.

[0112] [Fabrication of Circuit Board 2] Instead of pressurizing the laminate 3 at a heating temperature of 125°C, the laminate 3 was pressurized at a heating temperature of 150°C. Otherwise, circuit board 2 was fabricated in the same manner as circuit board 1.

[0113] [Fabrication of circuit board 3] Instead of pressurizing the laminate 3 at a heating temperature of 125°C, the laminate 3 was pressurized at a heating temperature of 175°C. Otherwise, circuit board 2 was fabricated in the same manner as circuit board 1.

[0114] [Fabrication of circuit board 4] In the fabrication of circuit board 1, the laminate 3 was subjected to pressurization at a heating temperature of 125°C for 30 minutes, followed by pressurization at a heating temperature of 200°C for 30 minutes. However, in the fabrication of circuit board 4, the laminate 3 was not subjected to pressurization at a heating temperature of 125°C, but was instead subjected to pressurization at a heating temperature of 200°C for 30 minutes. Otherwise, circuit board 4 was fabricated using the same method as circuit board 1.

[0115] [Fabrication of circuit board 5] The configuration of laminate 3 was changed from cushioning material / copper foil 1 / porous boron nitride sheet / laminated 2 / cushioning material to cushioning material / copper foil 2 / porous boron nitride sheet / laminated 2 / cushioning material. Otherwise, circuit board 5 was manufactured using the same method as circuit board 4.

[0116] [Fabrication of circuit board 6] The configuration of laminate 2 was changed from copper foil 1 / porous boron nitride sheet / copper foil 2 to porous boron nitride sheet / copper foil 2. Otherwise, circuit board 6 was fabricated in the same manner as circuit board 4. Circuit board 6 does not have a first metal layer.

[0117] [Fabrication of circuit board 7] The pressure applied to the laminate 3 was changed from 10 MPa to 0.1 MPa. Otherwise, circuit board 7 was manufactured using the same method as circuit board 4.

[0118] The following evaluations were performed on the fabricated circuit boards 1-7. [Measurement of the thickness of the ceramic layer] Using an eddy current thickness gauge (product name "ISOSCOPE EMP10", manufactured by Fischer Instruments Co., Ltd.), the thickness of the first ceramic layer at locations a-d and f-i in Figure 7(c), the thickness of the second ceramic layer at locations b, d-f and h in Figure 7(c), and the sum of the thicknesses of the first and second ceramic layers at locations a-d and f-i in Figure 7(c) were measured. The average of these measured thicknesses was then used as the thickness of the first ceramic layer, the thickness of the second ceramic layer, and the sum of the thicknesses of the first and second ceramic layers on the circuit board.

[0119] [Confirmation of conductivity with the first metal layer of the opening] A tester (product name "Card HiTester," manufactured by HIOKI E.E. CORPORATION) was used to confirm whether electrical conductivity was possible between the opening and the first metal layer. Note that if any resin from the second ceramic layer remains at the opening, electrical conductivity with the first metal layer will be impossible.

[0120] [Insulation resistance] Using a dielectric strength tester (product name "TOS 5101," manufactured by Kikusui Electronics Industry Co., Ltd.), the dielectric strength of the first and second ceramic layers was measured in accordance with JIS C2110. For the dielectric strength of the first ceramic layer, the first and second metal layers were used. For the dielectric strength of the second ceramic layer, the first and third metal layers were used. The measurement conditions are as follows: Step-by-step voltage boosting conditions: Between AC 2.0 and 5.0 kV, the boost rate was 0.2 kV / 20 sec, and between AC 5.0 and 10.0 kV, it was 0.5 kV / 30 sec. Cutoff current value: 100mA

[0121] [Confirmation of defects within the insulating layer] A sample was prepared by cutting a circuit board in the thickness direction, allowing observation of the circuit board's cross-section. After embedding the cut sample in resin, the portion corresponding to the circuit board's cross-section was wet-polished using an automatic polishing machine. After drying the polished sample, the polished surface was coated with osmium. Then, using a scanning electron microscope (SEM) (product name "JCM-6000Plus", manufactured by JEOL Ltd.), the interface between the first and second ceramic layers in the region without a metal layer, as well as the vicinity of the side surface of the first metal layer, were observed at 100x magnification. The presence or absence of bonding between the first and second ceramic layers, the presence or absence of gaps between the side surface of the first metal layer and the second ceramic layer, and the presence or absence of cracks near the side surface of the first metal layer were confirmed.

[0122] [Operational stability of electronic components] A module was fabricated by incorporating three electronic components (p-MOS-FETs (insulated-gate field-effect transistors), product name "2SK2174S," manufactured by Hitachi, Ltd.) at 2mm intervals on a circuit conductor or on a conductive circuit. The module was operated continuously for 96 hours in a 100°C environment with a power consumption of 10W per component, and the presence or absence of malfunctions was evaluated. If no malfunctions occurred, the power consumption was increased by another 10W and the evaluation was repeated. Subsequently, the power consumption was increased in the same manner, and the operational stability of the power electronic components was evaluated based on the power consumption at which a malfunction occurred.

[0123] The evaluation results of the above assessment are shown in Table 1. As an example, Figure 8 shows an SEM image of the vicinity of the side surface of the first metal layer in the cross-section of the circuit board 5.

[0124] [Table 1]

[0125] From the above examples, it was found that a circuit board with electrodes inside, having high thermal conductivity and excellent electrical resistance, can be fabricated using a porous boron nitride sheet, which is a hard, porous ceramic material. Since it is clear that porous boron nitride, in which the voids are filled with a cured product of a thermosetting composition, has high thermal conductivity, it is understood that the circuit boards in the examples also have high thermal conductivity. Furthermore, it was found that even with a porous boron nitride sheet, which is a hard, porous ceramic material, by controlling the pressurizing conditions when laminating the porous boron nitride sheets, it is possible to join the first ceramic layer and the second ceramic layer without creating a gap between them, even when electrodes are embedded.

[0126] From the evaluation results of comparative example circuit board 6, it was found that the absence of the first metal layer on the circuit board reduces the operational stability of the electronic elements mounted on the circuit board. This indicates that the first metal layer of the circuit board plays a role as a noise shield to suppress the influence of noise on the electronic elements. Furthermore, from the evaluation results of comparative example circuit board 7, it was found that the first ceramic layer and the second ceramic layer are not bonded, resulting in poor voltage resistance of the circuit board. [Explanation of Symbols]

[0127] 1.1A Circuit Board 2. First ceramic layer 3. Second ceramic layer 4. First metal layer 5. Second metal layer 6. Third metal layer 11 Metal layer existing region 12 Metal layer-free region 71,72 Photoresist 102 First ceramic sheet 103 Second ceramic sheet 104 First metal foil 105 Second metal foil 106 Third Metal Foil 111,111A,112,112A laminate

Claims

1. The first ceramic layer and A second ceramic layer laminated on the first ceramic layer, The first ceramic layer and the second ceramic layer are disposed between them, The material has a metal layer-existing region in which the first metal layer exists between the first ceramic layer and the second ceramic layer, and a metal layer-absent region in which the first metal layer does not exist between the first ceramic layer and the second ceramic layer. In the region where the metal layer is absent, the first ceramic layer and the second ceramic layer are joined together. A circuit board in which the first ceramic layer and the second ceramic layer are each porous boron nitride layers in which the voids are filled with a cured product of a thermosetting composition.

2. The circuit board according to claim 1, wherein the thickness of the first metal layer is one-quarter or less of the thickness of the second ceramic layer.

3. The circuit board according to claim 1 or 2, wherein the side surface of the first metal layer is covered with the second ceramic layer.

4. The circuit board according to any one of claims 1 to 3, further comprising a second metal layer bonded to the surface of the first ceramic layer opposite to the surface facing the second ceramic layer.

5. The circuit board according to any one of claims 1 to 4, further comprising a third metal layer formed on the side of the second ceramic layer opposite to the side of the first ceramic layer.

6. The second ceramic layer has an opening, The circuit board according to any one of claims 1 to 5, wherein the first metal layer is exposed at the opening.

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