Patent Description:
With an increase in performance of industrial equipment such as robots and motors, amount of heat generated from semiconductor elements mounted on power modules has been increasing. In order to efficiently dissipate the heat, circuit boards including a ceramic substrate having good thermal conductivity are used. In such a circuit board, thermal stress is generated due to heating and cooling processes during joining of the ceramic substrate and a metal plate and due to heat cycles during use. Accordingly, cracks may be generated in the ceramic substrate or the metal plate may be peeled off.

A technique for improving reliability by relaxing thermal stress generated in a ceramic circuit board has been studied. For example, in Patent Document <NUM>, it is studied to improve the heat cycle resistance by setting the average size and the number density of the Cu-rich phase of the brazing layer within predetermined ranges. Patent Document <NUM> proposes forming an Ag-rich phase along a joining interface between a ceramic substrate and a circuit pattern.

Document <CIT> discloses an AIN board that is brazed to a copper plate using a brazing that contains <NUM> mass parts of Ag, <NUM> mass parts of copper powders, <NUM> mass parts of titanium powder and <NUM> mass parts of zirconium powder.

<CIT> discloses a brazing material for bonding between a ceramic substrate and a metal plate, the brazing material being a powder mixture provided by mixing an alloy powder comprising at least <NUM> to <NUM> mass% of Ag, <NUM> to <NUM> mass% of In, and the balance Ct with inevitable impurities. An active metal hydride powder is also added.

<CIT> discloses a Cu/ceramic bonded body formed by bonding a copper member and a ceramic member made of AIN or Al<NUM>O<NUM> using a bonding material containing Ag and Ti.

Circuit boards are required to be sufficiently excellent in reliability depending on the use thereof. For example, in the field of power modules such as drive units of trains and electric vehicles, it is required to maintain excellent reliability even under severe conditions of heat cycles. This is because, for example, when the cooling temperature is lowered, tensile stress generated in the ceramic substrate is increased and cracks are easily generated in the ceramic substrate. Accordingly, the present disclosure provides a circuit board having excellent heat cycle resistance and a producing method thereof. In addition, the present disclosure provides a joined body having excellent heat cycle resistance and a producing method thereof.

The joined body and the method of producing a joined body according to the present invention are defined in the appended claims.

A circuit board according to one aspect of the present disclosure is a circuit board in which a ceramic substrate (in the present case, a silicon nitride substrate) and a metal circuit board (in the present case, a copper circuit board) are joined together with a Ag-Cu-Sn-based brazing layer containing silver, and a mean value of a KAM value of a silver portion in the brazing layer obtained by EBSP method is <NUM>° or less.

Here, the KAM (Kernel Average Misorientation) value indicates an orientation difference between adjacent measurement points in a crystal grain, and is obtained by crystal orientation analysis using EBSP (Electron Back Scattering Pattern) method. It can be said that the smaller the KAM value is, the smaller the strain of the crystal is.

Since the ceramic substrate and the metal circuit board have significantly different linear thermal expansion coefficients, residual stress is generated in a joint between the ceramic substrate and the metal circuit board due to a difference from temperature during joining. The residual stress appears as strain of the crystal structure of the component contained in the brazing layer. When the residual stress becomes large, cracks are likely to be generated in the ceramic substrate, and heat cycle resistance is impaired. In the circuit board of the present disclosure, since the mean value of the KAM value of the silver portion in the brazing layer is sufficiently small, the residual stress in the joint between the ceramic substrate and the metal circuit board is sufficiently reduced. Accordingly, the circuit board of the present disclosure has excellent heat cycle resistance.

The metal circuit board is a copper circuit board. A mean value of a KAM value of copper contained in the copper circuit board obtained by the EBSP method may be <NUM>° or less. Accordingly, the residual stress in the joint of the circuit board can be sufficiently reduced, and heat cycle resistance can be further improved.

The brazing layer of the circuit board may include an exposed part exposed from an edge of the metal circuit board. In this case, the mean value of the length L of the exposed part may be <NUM> or more, and the mean value of the thickness T of the exposed part may be <NUM> to <NUM>. Accordingly, stress concentration in the joint can be sufficiently reduced, and heat cycle resistance can be further improved.

A joined body according to one aspect of the present disclosure is a joined body in which a ceramic substrate (in the present case, a silicon nitride substrate) and a metal plate (in the present case, a copper plate) are joined together with a brazing layer containing silver, and a mean value of a KAM value of a silver portion in the brazing layer obtained by EBSP method is <NUM>° or less.

Since the ceramic substrate and the metal plate have significantly different linear thermal expansion coefficients, residual stress is generated in a joint between the ceramic substrate and the metal plate due to a difference from temperature during joining. The residual stress appears as strain of the crystal structure of the component contained in the brazing layer. When the residual stress becomes large, cracks are likely to appear in the ceramic substrate, and heat cycle resistance is impaired. In the joined body of the present disclosure, since the mean value of the KAM value of the silver portion in the brazing layer is sufficiently small, the residual stress in the joint between the ceramic substrate and the metal plate is sufficiently reduced. Accordingly, the joined body of the present disclosure has excellent heat cycle resistance. By using such a joined body, a circuit board having excellent heat cycle resistance can be obtained.

The metal plate is a copper plate. The mean value of a KAM value of copper contained in the copper plate obtained by the EBSP method may be <NUM>° or less. Accordingly, residual stress in the joint of the joined body can be sufficiently reduced, and heat cycle resistance can be further improved.

The content of silver in the brazing layer of the joined body is <NUM>% or more by mass or more, and the mean value of a thickness T of the brazing layer may be <NUM> to <NUM>. Accordingly, it is possible to further improve heat cycle resistance by sufficiently reducing stress generated in the joint while achieving miniaturization and thinning.

A method for producing a joined body according to an aspect of the present disclosure includes: applying a brazing material containing silver to a main surface of a silicon nitride substrate; laminating the silicon nitride substrate and a copper plate with interposing the brazing material therebetween to obtain a multilayer body; firing the multilayer body at a firing temperature of <NUM> or higher for <NUM> minutes or longer; and annealing the fired multilayer body at a temperature of <NUM> or higher and lower than <NUM> for <NUM> minutes or longer.

In the producing method, the multilayer body is held at a firing temperature of <NUM> or higher for <NUM> minutes or longer to be fired, and then the fired multilayer body is held at <NUM> or higher and lower than <NUM> for <NUM> minutes or more to be annealed. In general, when the ceramic substrate is sintered at a sintering temperature of <NUM> or higher and cooled to room temperature, a large residual stress is generated in the joint of the joined body due to a difference in linear thermal expansion coefficient between the ceramic substrate and the metal plate. However, in the above-described producing method, since annealing is performed at a temperature of <NUM> or higher and lower than <NUM> for <NUM> minutes or longer, the residual stress of the joint can be reduced. Accordingly, residual stress in the joint is sufficiently reduced, and a joined body having excellent heat cycle resistance can be produced.

In the producing method, the multilayer body may be held at a firing temperature of <NUM> or higher for shorter than <NUM> hours, and may be cooled from the firing temperature to less than <NUM>° C at a cooling rate of <NUM>/min or more. Accordingly, producing efficiency can be improved, and diffusion of silver contained in the brazing material into the metal plate can be prevented. Thus, the heat cycle characteristics can be improved.

In the joined body obtained by annealing, a ceramic substrate and a copper plate as the metal plate are joined together with a brazing layer containing silver, a mean value of a KAM value of a silver portion in the brazing layer obtained by the EBSP method is <NUM>° or less, and a mean value of a KAM value of copper contained in the copper plate obtained by the EBSP method may be <NUM>° or less. In such a joined body, since the mean value of the KAM value of the silver portion in the brazing layer and the copper contained in the copper plate are sufficiently small, the residual stress in the joint between the ceramic substrate and the metal plate is sufficiently reduced. Accordingly, the heat cycle resistance is sufficiently excellent.

A producing method of a circuit board according to an aspect of the present disclosure includes removing a part of a metal plate in a joined body obtained by the above-described producing method to form a metal circuit board, thereby obtaining a circuit board. Since the circuit board obtained in this manner uses the joined body described above, residual stress in the joint between the ceramic substrate and the metal circuit board is sufficiently reduced. Therefore, heat cycle resistance is excellent.

The brazing layer of the circuit board obtained by the producing method may have an exposed part exposed from an edge of the metal circuit board. In this case, the mean value of the length L of the exposed part may be <NUM> or more, and the mean value of the thickness T of the brazing layer may be <NUM> to <NUM>. Accordingly, it is possible to further improve heat cycle resistance by sufficiently reducing stress generated in the joint while achieving miniaturization and thinning.

According to the present disclosure, it is possible to provide a circuit board having excellent heat cycle resistance and a producing method thereof. In addition, it is possible to provide a joined body having excellent heat cycle resistance and a producing method thereof. Such a circuit board and a joined body can be suitably used in, for example, a power module.

Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings in some cases. However, the following embodiments are examples for describing the present disclosure, and are not intended to limit the present disclosure to the following contents. In the description, the same elements or elements having the same functions are denoted by the same reference numerals, and redundant description is omitted in some cases. In addition, positional relationships such as up, down, left, and right are based on positional relationships illustrated in the drawings unless otherwise specified. Further, the dimensional ratio of each element is not limited to the illustrated ratio.

<FIG> is a plan view of the circuit board of the present embodiment. A circuit board <NUM> comprises a ceramic substrate <NUM> and three metal circuit boards <NUM> joined to a main surface 10A of the ceramic substrate <NUM>. The ceramic substrate <NUM> and the metal circuit board <NUM> are joined together with a brazing layer <NUM> interposed therebetween. The edge of the brazing layer <NUM> is exposed along the outer edge of the metal circuit board <NUM>.

<FIG> is a cross-sectional view taken along line II-II of <FIG>. <FIG> illustrates a part of a cross section of the circuit board <NUM> when cut along a plane perpendicular to main surfaces 10A and 10B of the ceramic substrate <NUM>. Each joint <NUM> between the ceramic substrate <NUM> and a pair of the metal circuit board <NUM> includes the brazing layer <NUM>.

The material of the ceramic substrate <NUM> is silicon nitride from the viewpoint of mechanical strength and fracture toughness.

The thickness of the ceramic substrate <NUM> may be, for example, <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>. The thickness of the metal circuit board <NUM> may be, for example, <NUM> to <NUM>.

The brazing layer <NUM> is formed of an Ag-Cu-Sn-based brazing material containing silver, copper, tin, and an active metal contained as TiH<NUM>. The content of silver in the brazing layer <NUM> is <NUM>% by mass or more. This makes it possible to improve denseness of the brazing layer <NUM> while sufficiently reducing the residual stress in the joint <NUM>.

The active metal in the brazing layer <NUM> is <NUM> to <NUM> parts by mass with respect to <NUM> parts by mass of the total of silver and copper. By setting the content of the active metals to <NUM> parts by mass or more, the joinability between the ceramic substrate <NUM> and the brazing layer <NUM> can be improved. By setting the content of the active metal to <NUM> parts by mass or less, it is possible to prevent formation of a brittle alloy layer at the joining interface. The active metal is titanium hydride (TiH<NUM>). The content of TiH<NUM> in the brazing layer <NUM> is <NUM> to <NUM> parts by mass with respect to <NUM> parts by mass of the total of silver and copper. Accordingly, the joining strength between the ceramic substrate <NUM> and the metal circuit board <NUM> can be sufficiently increased.

The amount of tin in the brazing layer <NUM> is <NUM> to <NUM> parts by mass with respect to <NUM> parts by mass of the total of silver and copper. Accordingly, both the effect of reducing residual stress and the wettability of the brazing material with respect to the ceramic substrate <NUM> can be achieved at a high level. The content of Cu in the brazing layer <NUM> may be <NUM> to <NUM> parts by mass with respect to <NUM> parts by mass of silver. This makes it possible to improve the denseness of the brazing layer <NUM> while sufficiently reducing the residual stress in the joint <NUM>.

The mean value of a KAM (Kernel Average Misorientation) value of the silver portion in the brazing layer <NUM> is <NUM>° or less. From the viewpoint of further reducing residual stress in the joint <NUM>, the mean value of the KAM value may be <NUM>° or less, and may be <NUM>° or less. The KAM value of the silver portion may be <NUM>° or more, <NUM>° or more, <NUM>° or more, <NUM>° or more, or <NUM>° or more. Accordingly, the annealing time during producing can be shortened.

The KAM value is a value indicating an orientation difference between adjacent measurement points in a crystal grain of a silver portion, and is obtained by EBSP (Electron Back Scattering Pattern) method using a commercially available OIM (Orientation Imaging Microscopy) crystal orientation analysis device. The orientation difference in the crystal grain may be measured by using analysis software attached to the OIM crystal orientation analysis device. The mean value of the KAM values of the silver portion is obtained as an arithmetic mean value of measured values obtained by performing measurement at five points of the brazing layer <NUM>. When a component other than silver is included in each measurement field of view, the silver portion and the portion of the other component are separated by energy dispersive X-ray analysis (EDS), and the mean value of the KAM value of the silver portion is obtained. The "silver portion" in the present disclosure refers to a region of the brazing layer where silver is detected by EDS.

The metal circuit board <NUM> is a copper circuit board from the viewpoint of improving electrical conductivity and heat dissipation. A mean value of the KAM value of copper contained in the copper circuit board may be <NUM>° or less, and may be <NUM>° or less. Accordingly, the residual stress in the joint <NUM> can be further reduced. The mean value of the KAM value of copper may be <NUM>° or more, <NUM>° or more, <NUM>° or more, <NUM>° or more, or <NUM>° or more. Accordingly, annealing time during producing can be shortened. The mean value of the KAM value of copper can be determined in the same manner as the mean value of the KAM value of silver portion in the brazing layer <NUM>.

The thickness of the metal circuit board <NUM> may be <NUM> to <NUM>. From the viewpoint of improving heat dissipation properties, the thickness of the metal circuit board <NUM> may be <NUM>. <NUM> or more, and may be <NUM> or more. A side part <NUM> of the metal circuit board <NUM> may be formed to expand as it approaches the ceramic substrate <NUM> as illustrated in <FIG>. However, the shape of the side part of the metal circuit board <NUM> is not limited to the shape in <FIG>.

The edge of the brazing layer <NUM> is exposed at outer edge portion of the metal circuit board <NUM>. In the present disclosure, the edge of the brazing layer <NUM> that protrudes from the metal circuit board <NUM> without being covered by the metal circuit board <NUM> is referred to as an exposed part <NUM>. It is not essential to have the exposed part <NUM>, but by having the exposed part <NUM>, the concentration of stresses occurring in the joint <NUM> can be reduced. Accordingly, heat cycle resistance may be further improved.

The mean values of the thickness T and the length L of the exposed part <NUM> are arithmetic mean values of measured values of the thickness T and the length L measured at five arbitrarily selected places (five visual fields) of the exposed part <NUM>, respectively. The mean value of the length L of the exposed part <NUM> may be <NUM> or more, and may be <NUM> or more. The residual stress caused by the difference in linear thermal expansion coefficient between the ceramic substrate <NUM> and the metal circuit board <NUM> tends to be concentrated on the outer edge of the metal circuit board <NUM>. Accordingly, when the mean value of the length L of the exposed part <NUM> is within the above-described range, the concentration of residual stress is alleviated, and heat cycle resistance can be further improved. From the viewpoint of reducing the size of the circuit board <NUM>, the mean value of the length L of the exposed part <NUM> may be <NUM> or less, <NUM> or less, <NUM> or less, or <NUM> or less.

The mean value of the thickness T of the exposed part <NUM> may be <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>. This makes it possible to sufficiently increase the joining strength in the joint <NUM>. The mean value of the thickness T of the exposed part <NUM> is the same as the mean value of the thickness of the brazing layer <NUM> between the ceramic substrate <NUM> and the metal circuit board <NUM>. However, in a modification, the mean value of the thickness T of the exposed part <NUM> may be different from the mean value of the thickness of the brazing layer <NUM> between the ceramic substrate <NUM> and the metal circuit board <NUM>. The mean values of these thicknesses may both be within the above-described range (the range of the mean value of the thickness T). In one field of view, when the thickness T differs depending on the position of the exposed part <NUM>, the mean value of the maximum and minimum values of the thicknesses T of the field of view is the thicknesses T of the field of view.

The metal circuit board <NUM> may have a plating film on a surface portion thereof. From the viewpoint of improving weather resistance and solder wettability, the plating film may be a Ni plating film, a Ni alloy plating film, or a gold plating film. The Ni plating film may be an electroless Ni plating film. The electroless Ni plating film may be a nickel-phosphorus plating film containing <NUM> to <NUM>% by mass of nickel. By having such an electroless Ni-plated film, it is possible to provide the circuit board <NUM> having excellent heat cycle resistance and excellent solder wettability. The exposed part <NUM> may also have a similar plating film on its surface portion.

While only one main surface 10A of the ceramic substrate <NUM> is illustrated in <FIG>, as illustrated in <FIG>, in the ceramic substrate <NUM>, another metal circuit board <NUM> is also joined to another main surface 10B with the brazing layer <NUM> as illustrated in <FIG>. In a modification, a heatsink instead of the metal circuit board <NUM> may be joined to one of the main surface 10A and the main surface 10B with a brazing layer. In this case, the metal circuit board <NUM> and the heatsink may be made of the same material or may be made of different materials. The metal circuit board <NUM> has a function of transmitting an electric signal, whereas the heatsink may have a function of transmitting heat. However, the heatsink may have a function of transmitting an electric signal.

The brazing layer that joins the heatsink to the main surface of the ceramic substrate may have the same composition and thickness as those of the brazing layer that joins the metal circuit board. Also, in a similar manner to the brazing layer <NUM>, an edge of the brazing layer may be exposed from an outer edge of the heatsink.

In the circuit board <NUM>, since the mean value of the KAM value of the silver portion in the brazing layer <NUM> is sufficiently small, the residual stresses in the joint <NUM> of the ceramic substrate <NUM> and the metal circuit board <NUM> are sufficiently reduced. Therefore, the circuit board <NUM> has excellent heat cycle resistance. As an example, the crack rate of the circuit board <NUM> after the heat cycle test may be less than <NUM>% by area, less than <NUM>% by area, or less than <NUM>% by area. The heat cycle test referred to herein is a test in which a series of steps of holding at -<NUM> for <NUM> minutes, at <NUM> for <NUM> minutes, at <NUM> for <NUM> minutes, and at <NUM> for <NUM> minutes is taken as one cycle, and this is performed for <NUM> cycles. The crack rate is the ratio of the areas of cracks to the areas of the metal circuit board <NUM>.

<FIG> is a perspective view of a joined body according to one embodiment. A joined body <NUM> includes a pair of metal plates <NUM> disposed to face each other and a ceramic substrate <NUM> disposed between the pair of the metal plates <NUM>.

<FIG> is a cross-sectional view taken along line IV-IV of the joined body <NUM> of <FIG>. The metal plates <NUM> are joined to each of the main surface 10A and the main surface 10B of the ceramic substrate <NUM> with a brazing layer <NUM>. That is, the joined body <NUM> has two joints <NUM>. The joined body <NUM> can be used as a raw material for the circuit board <NUM>. That is, the ceramic substrate <NUM> in the joined body <NUM> may be the same composition (material), shape, and thicknesses as those of the ceramic substrate <NUM> in the circuit board <NUM>. The compositions (materials) and thicknesses of the metal plates <NUM> and the brazing layer <NUM> in the joined body <NUM> may also be the same as those of the metal circuit boards <NUM> and the brazing layer <NUM> in the circuit board <NUM>.

The mean value of the KAM (Kernel Average Misorientation) value of the silver portion in the brazing layer <NUM> constituting the joint <NUM> is also <NUM>° or less, similarly to that in the silver portion in the brazing layer <NUM> of the circuit board <NUM>. From the viewpoint of further reducing residual stress in the joint <NUM>, the mean value of the KAM value may be <NUM>° or less, and may be <NUM>° or less. The mean value of the KAM value of the silver portion in the brazing layer <NUM> may be <NUM>° or more, and may be <NUM>° or more. Accordingly, annealing time during producing can be shortened. The method for obtaining the mean value of the KAM value is the same as that in the case of the silver portion in the brazing layer <NUM> of the circuit board <NUM>.

The metal plate <NUM> is a copper plate from the viewpoint of improving electrical conductivity and heat dissipation. The mean value of the KAM value of copper contained in the copper plate may be <NUM>° or less, and may be <NUM>° or less. Accordingly, the residual stress in the joint <NUM> can be further reduced. The mean value of the KAM value of copper in the copper plate may be <NUM>° or more, and may be <NUM>° or more. Accordingly, annealing time during producing can be shortened. The method of obtaining the mean value of the KAM value of copper in the copper plate is the same as the case of copper included in the copper circuit board.

The metal plate <NUM> may be <NUM> to <NUM> thick. From the viewpoint of improving heat dissipation properties, it may be <NUM> or more, and may be <NUM> or more. It is not essential to provide the metal plates <NUM> on both of the main surface 10A and the main surface 10B of the ceramic substrate <NUM>, and the metal plate <NUM> may be provided on only one main surface. When the metal plates <NUM> are provided on both of the main surface 10A and the main surface 10B of the ceramic substrate <NUM>, the materials, thicknesses, shapes, and the like of the two metal plates <NUM> may be the same or different from each other. The brazing layer <NUM> may be provided entirely between the ceramic substrate <NUM> and the metal plate <NUM>, or may be provided only in a portion where the metal circuit board <NUM> is formed.

Since the mean value of KAM value of silver portion in the brazing layer <NUM> in the joined body <NUM> is sufficiently small, residual stress in the joint <NUM> of the ceramic substrate <NUM> and the metal plate <NUM> is sufficiently reduced. Therefore, the joined body <NUM> has excellent heat cycle resistance. By using such the joined body <NUM>, the circuit board <NUM> having excellent heat cycle resistance can be obtained.

An example of a method for producing a joined body and a circuit board of the present disclosure will be described. First, a step of obtaining a green sheet by molding a slurry containing an inorganic compound powder, a binder resin, a sintering aid, a plasticizer, a dispersant, a solvent, and the like is performed.

Examples of the additional inorganic compound include nitride ceramics such as aluminum nitride, oxide ceramics such as aluminum oxide and zirconium oxide, carbide ceramics such as silicon carbide, and boride ceramics such as lanthanum boride. Examples of the sintering aid include rare earth metals, alkaline earth metals, metal oxides, fluorides, chlorides, nitrates, and sulfates. These may be used alone or in combination of two or more kinds thereof. By using the sintering aid, sintering of the inorganic compound powder can be promoted. Examples of the binder resin include methyl cellulose, ethyl cellulose, polyvinyl alcohol, polyvinyl butyral, and (meth)acrylic resins.

Examples of the plasticizer include phthalic acid-based plasticizers such as purified glycerin, glycerin trioleate, diethylene glycol, and di-n-butyl phthalate; and dibasic acid-based plasticizers such as di-<NUM>-ethylhexyl sebacate. Examples of dispersing agents include poly (meth)acrylates and (meth)acrylic acid-maleate copolymers. Examples of the solvent include organic solvents such as ethanol and toluene.

Examples of the slurry molding method include a doctor blade method and an extrusion molding method. Next, a step of degreasing and sintering the green sheet obtained by molding is performed. The degreasing may be performed, for example, by heating at <NUM> to <NUM> for <NUM> to <NUM> hours. Accordingly, it is possible to reduce the residual amount of organic materials (carbon) while preventing oxidation and deterioration of the inorganic compound. The sintering is performed by heating to <NUM> to <NUM> in a non-oxidizing gas atmosphere such as nitrogen, argon, ammonia, or hydrogen. As a result, the ceramic substrate <NUM> can be obtained. If necessary, the ceramic substrate <NUM> may be trimmed by cutting the edge by laser processing. In addition, scribe lines may be formed in the main surface 10A and/or the main surface 10B of the ceramic substrate <NUM>.

The above-described degreasing and sintering may be performed in a state where a plurality of green sheets are laminated. In the case where degreasing and sintering are performed in laminate state, a release layer made of a release agent may be provided between the green sheets in order to smoothly separate the base material after firing. As the release agent, for example, boron nitride (BN) can be used. The release layer may be formed, for example, by applying a slurry of boron nitride powder by a method such as spraying, brushing, roll coating, or screen printing. The number of green sheets to be laminated may be, for example, <NUM> to <NUM> and may be <NUM> to <NUM>, from the viewpoint of efficiently performing mass production of the ceramic substrate and sufficiently progressing degreasing.

The ceramic substrate <NUM> as shown in <FIG> and <FIG> is obtained in this manner. Subsequently, a step of obtaining a joined body using the ceramic substrate <NUM> and a pair of the metal plate <NUM> is performed. In detail, first, the main surface 10A and the main surface 10B of the ceramic substrate <NUM> are coated with a brazing material, and a pair of the metal plates <NUM> are attached to the main surface 10A and the main surface 10B, respectively. The metal plate <NUM> may have a flat plate shape similar to that of the ceramic substrate <NUM>.

The brazing material is applied to the main surface 10A and the main surface 10B of the ceramic substrate <NUM> by a method such as a roll coater method, a screen printing method, or a transfer method. The brazing material contains, for example, silver powder, copper powder, tin powder, powder of an active metal or a compound (hydride) thereof, an organic solvent, and a binder. The content of the tin powder is <NUM> to <NUM> parts by mass with respect to <NUM> parts by mass of the total of the silver powder and the copper powder. The content of the metal hydride powder is <NUM> to <NUM> parts by mass with respect to <NUM> parts by mass of the total of the silver powder and the copper powder. The viscosity of the brazing material may be, for example, <NUM> to 20Pa· s. The content of the organic solvent in the brazing material may be, for example, <NUM> to <NUM>% by mass, and the content of the binder may be, for example, <NUM> to <NUM>% by mass.

As the silver powder, for example, one having a specific surface area of <NUM> to <NUM><NUM>/g may be used. Accordingly, joinability may be improved, and uniformity of a structure of the Ag-Cu-Sn-based brazing layer may be improved. The specific surface area can be measured by a gas adsorption method. The copper powder has an effect of improving melting property of the Ag-Cu-Sn-based brazing material. The tin powder has an effect of reducing an angle of contact of the brazing material with respect to the ceramic substrate <NUM> and improving wettability of the brazing material.

A multilayer body is obtained by superposing the metal plates <NUM> on the main surface 10A and the main surface 10B of the ceramic substrate <NUM> coated with brazing material. Subsequently, a firing process of firing the multilayer body in a heating furnace is performed. The temperature in the furnace during firing (firing temperature) is <NUM> or higher, may be <NUM> to <NUM>, and may be <NUM> to <NUM>. The time for holding at the firing temperature (firing time) is <NUM> minutes or more, and may be <NUM> minutes or more. The firing time may be less than <NUM> hours or less than <NUM> minutes from the viewpoint of preventing diffusion of silver into the metal plate <NUM>.

The atmosphere in the heating furnace at the time of firing may be an inert gas such as nitride, and the firing may be performed under a reduced pressure (<NUM>×<NUM>-<NUM> Pa or less) lower than the atmosphere pressure or may be performed under vacuum. By firing the multilayer body under such conditions, the ceramic substrate <NUM> and the metal plates <NUM> can be sufficiently joined. The heating furnace may be a continuous type in which a plurality of joined bodies are continuously produced, or may be a type in which one or more joined bodies are produced in a batch manner. The heating may be performed while pressing the multilayer body in the laminating direction.

Next, in an annealing step, annealing is performed by holding the fired multilayer body at a furnace temperature of <NUM> or more and less than <NUM> for <NUM> minutes or more. The temperature in the furnace during annealing (annealing temperature) may be <NUM> or more and <NUM> or less, or may be <NUM> or more and <NUM> or less. The annealing time may be <NUM> minutes or more from the viewpoint of sufficiently reducing residual stress in the joint. The annealing time may be less than <NUM> hours from the viewpoint of improving producing efficiency. The annealing may be performed using the same heating furnace as that used in the firing, or may be performed using a different heating furnace. The annealing may be performed in an inert gas atmosphere such as nitrogen, may be performed under a reduced pressure less than atmospheric pressure, or may be performed under vacuum.

After the firing step, the inside of the furnace may be cooled at a cooling rate of <NUM>/min or more to lower the temperature to the above-described annealing temperature. Thus, the producing efficiency can be improved, and the diffusion of silver contained in the brazing material into the metal plate can be prevented. From the same viewpoint, the cooling rate may be <NUM>/min or more, and may be <NUM>/min or more. From the viewpoint of reducing thermal shock, the cooling rate may be less than <NUM>/min.

Through the above-described steps, it is possible to obtain the joined body <NUM> having the ceramic plate <NUM> and the metal plates <NUM> each joined, with the brazing material <NUM>, to the main surface 10A and the main surface 10B of the ceramic plate <NUM>. Since this producing method has a step of annealing after firing, residual stress in the joint <NUM> can be reduced. Therefore, the joined body <NUM> having excellent heat cycle resistance can be produced.

In the case of producing a circuit board, a step of forming the metal circuit board <NUM> by removing part of the metal plate <NUM> in the joined body <NUM> obtained in the above-described step is performed. This step may be performed by, for example, photolithography. To be more specific, first, a photosensitive resist is printed on the main surface of the joined body <NUM> (the main surface of the metal plate <NUM>). Then, a resist pattern having a predetermined shape is formed using an exposure apparatus. The resist may be a negative type or a positive type. Uncured resist is removed by, for example, washing.

After forming the resist pattern, a portion of the metal plate that is not covered with the resist pattern is removed by etching. As a result, part of the main surface 10A and the main surface 10B of the ceramic substrate <NUM> is exposed in this portion. Thereafter, the circuit board <NUM> is obtained by removing the resist pattern. The shape of the side part <NUM> of the metal circuit board <NUM> and the length L and thickness T of the exposed part <NUM> of the brazing layer <NUM> can be adjusted by changing the etching conditions or the number of times of etching.

The etching solution is not particularly limited, and examples thereof include a ferric chloride solution, a cupric chloride solution, sulfuric acid, and a hydrogen peroxide solution. When the brazing layer or the like remains on the main surface 10A and the main surface 10B of the ceramic substrate <NUM> after etching, these may be removed by using solutions containing at least one selected from the group consisting of ammonium halide aqueous solutions, inorganic acids such as sulfuric acid and nitric acid, and oxygenated water. The method for removing the resist pattern is not particularly limited, and may be, for example, a method of immersion in an alkaline aqueous solution.

Since the circuit board <NUM> obtained in this manner uses the joined body <NUM>, residual stress in the joint <NUM> between the ceramic substrate <NUM> and the metal circuit board <NUM> is sufficiently reduced. That is, the mean value of KAM value of silver portion in the brazing layer <NUM> of the circuit board <NUM> and the mean value of KAM value of silver portion in the brazing layer <NUM> of the joined body <NUM> are equivalent. The mean value of the KAM value of the metal contained in the metal circuit board <NUM> of the circuit board <NUM> and the mean value of the KAM value of the metal contained in the metal plate <NUM> of the joined body <NUM> are also equivalent. Accordingly, the circuit board <NUM> is also excellent in heat cycle resistance, similarly to the joined body <NUM>. Although an example of the producing method of the joined body <NUM> and the circuit board <NUM> has been described above, the producing method is not limited thereto.

Since the circuit board <NUM> has excellent heat cycle resistance, it may be used in, for example, a power module handling a large current.

Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments. For example, as illustrated in <FIG>, the brazing layer <NUM> in the joined body <NUM> may be formed only in a portion that becomes the metal circuit board <NUM> or in the portion and the vicinity thereof. It may be formed so as to cover the whole of the main surface 10A and the main surface 10B of the ceramic substrate <NUM>. In <FIG>, both the main surface 10A and the main surface 10B of the ceramic substrate <NUM> are provided with the brazing layer <NUM> and the metal plate <NUM>, but in another example, only one of them may be provided with the brazing layer <NUM> and the metal plate <NUM>.

The content of the present disclosure will be described in more detail with reference to Examples and Comparative Examples, but the present disclosure is not limited to the following Examples.

In order to prepare a brazing material, the following raw material powders were prepared.

A brazing material was prepared by blending <NUM> part by mass of tin powder and <NUM> parts by mass of titanium hydride powder with respect to <NUM> parts by mass in total of silver powder (<NUM> parts by mass) and copper powder (<NUM> parts by mass), and adding an organic solvent, a binder, and the like. Both main surfaces of a commercially available silicon nitride substrate (<NUM> thick) were coated with the brazing material by a screen printing method so that the coating amount was <NUM>/cm<NUM>.

Copper plates (thickness: <NUM>, purity: <NUM>% oxygen-free copper plates (JIS H <NUM>, C1020)) were laminated on both main surfaces of the silicon nitride substrate to obtain a multilayer body. Using an electric furnace, in a nitrogen atmosphere, the multilayer body was heated at a furnace temperature of <NUM> for <NUM> minutes to melt the brazing material powder and join the ceramic substrate and the metal plate (firing step). Thereafter, the temperature in the furnace was cooled to <NUM> at an average cooling rate of <NUM>/min. Annealing was performed in a nitrogen atmosphere at a furnace temperature of <NUM> for <NUM> minutes (annealing step). After that, heating was stopped, and natural cooling was performed to room temperature in a nitrogen atmosphere. The average rate of temperature decrease in the furnace after the annealing step was <NUM>/min. In this manner, a joined body in which the ceramic substrate and the pair of metal plates were joined together with the brazing layer was produced.

After embedding the joined body in a resin, the joined body was cut along the thickness direction so as to pass through the center of gravity of the joined body using a coater machine. A flat milling treatment was performed on the cross section of the joined body to obtain a sample for EBSD measurement. EBSD measurement of the cut surface was performed using OIM (Orientation Imaging Microscopy) crystal orientation analysis device manufactured by TSL Solutions, Inc. The measurement voltage was <NUM> kV, the working distance was <NUM>, and the sample inclination angle was <NUM>°.

The KAM value was measured using the OIM crystal orientation analysis device at five locations for each of the brazing layer and the copper plate, with the measurement field of view being <NUM> × <NUM> and the inter-measurement distance d = <NUM>. For the brazing layer, the silver portion and the copper portion in the brazing layer were separated by EDS measurement, and the KAM value of the silver portion was obtained. The mean value of the KAM values of the silver portion measured at five locations was as shown in Table <NUM>. The mean value of the KAM values of copper measured at five points on the copper plate was as shown in Table <NUM>.

An etching resist was printed on predetermined portions of both main surfaces of the copper plates in the joined body, a resist pattern having a predetermined shape was formed on the main surfaces of the copper plates using an exposure device, and then etching was performed using a ferric chloride solution. Further, etching was performed using a mixed solution of ammonium fluoride and hydrogen peroxide to remove a portion where the resist pattern was not formed, thereby forming a predetermined circuit pattern and an exposed part of the brazing layer along the outer edge of the circuit pattern.

Subsequently, a pretreatment by degreasing and chemical polishing was performed, and then a rust prevention treatment was performed using a benzotriazole-based compound. In this manner, a circuit board in which a ceramic substrate and a metal circuit board were joined together with a brazing layer containing silver as illustrated in <FIG> was obtained.

The exposed part of the brazing layer on the circuit board was observed using a scanning electron microscope (manufactured by Hitachi High-Technologies Corporation, SU6600 type). Observation of a reflected electron image (magnification: <NUM> times) was performed in five visual fields (each visual field was <NUM> in length × <NUM> in width), and the length and thickness of the exposed part for each visual field were obtained. The thickness of each visual field was set to be a mean value of the maximum thickness and the minimum thickness of the exposed part. The mean values of the length L and the thickness T of five visual fields were obtained. The results are shown in Table <NUM>.

The produced circuit board was subjected to a heat cycle test. Specifically, a series of steps of holding at -<NUM> for <NUM> minutes, at <NUM> for <NUM> minutes, at <NUM> for <NUM> minutes, and at <NUM> for <NUM> minutes was defined as one cycle, and this cycle was repeated <NUM> times. Thereafter, etching was performed using a ferric chloride solution, and then etching was performed using a mixed solution of ammonium fluoride and hydrogen peroxide to remove the circuit pattern and the brazing layer.

An image of the main surface of the ceramic substrate from which the circuit pattern and the brazing layer were removed was acquired at a resolution of 600dpi x 600dpi using a scanner. The image was binarized using image analysis software GIMP2 (threshold value: <NUM>) to calculate the area of cracks. The calculated crack area was divided by the area of the circuit pattern to obtain the crack rate. The results are shown in Table <NUM>.

A multilayer body was obtained by the same procedure as in Example <NUM>. A joined body in which a ceramic substrate and a pair of metal plates were joined together with a brazing layer was produced in the same manner as in Example <NUM> except that the conditions of the firing step and the annealing step were changed as shown in Table <NUM>. The firing step and the annealing step were performed in a vacuum atmosphere (<NUM>×<NUM>-<NUM> Pa or less). After the annealing step, heating was stopped, and natural cooling was performed to room temperature in a vacuum atmosphere. The average cooling rate after the annealing step was <NUM>/min.

In the same procedure as in Example <NUM>, measurement of KAM value, production of circuit board, measurement of length L and thickness T of exposed part, and evaluation of heat cycle characteristics were performed. The results are shown in Table <NUM>.

A multilayer body was obtained by the same procedure as in Example <NUM>. A joined body in which a ceramic substrate and a pair of metal plates were joined together with a brazing layer was produced in the same manner as in Example <NUM> except that the conditions of the firing step and the annealing step were changed as shown in Table <NUM>. After the annealing step, heating was stopped, and natural cooling was performed to room temperature in a vacuum atmosphere. The average rate of temperature decrease in the furnace after the annealing step was <NUM>/min.

A multilayer body was obtained by the same procedure as in Example <NUM> except that the formulation of the brazing material was as shown in Table <NUM>. Then, a joined body in which a ceramic substrate and a pair of metal plates were joined together with the brazing layer was produced in the same manner as in Example <NUM> except that the conditions of the firing step and the annealing step were changed as shown in Table <NUM>. In Example <NUM>, after the annealing step, heating was stopped, and natural cooling was performed to room temperature in a vacuum atmosphere (<NUM>×<NUM>-<NUM> Pa or less). The average rate of temperature decrease in the furnace after the annealing step was <NUM>/min. In Example <NUM>, after the annealing step, heating was stopped, and natural cooling was performed to room temperature in a nitrogen atmosphere. The average rate of temperature decrease in the furnace after the annealing step was <NUM>/min.

A joined body was obtained in the same manner as in Example <NUM> except that the annealing step was not performed in the production of the joined body. After the firing step, the average cooling rate to room temperature was <NUM>/min. In the same procedure as in Example <NUM>, measurement of KAM value, production of circuit board, measurement of length L and thickness T of exposed part, and evaluation of heat cycle characteristics were performed. The results are shown in Table <NUM>.

A joined body was produced by the same procedure as in Comparative Example <NUM> except that the formulation of the brazing material was as shown in Table <NUM>. After the firing step, the average cooling rate to room temperature was <NUM>/min. In the same procedure as in Example <NUM>, measurement of KAM value, production of circuit board, measurement of length L and thickness T of exposed part, and evaluation of heat cycle characteristics were performed. The results are shown in Table <NUM>.

As shown in Table <NUM>, in Examples <NUM> to <NUM> in which the annealing step was performed, the crystal strains of the silver portion and the copper contained in the brazing layer and the copper plate, respectively, were smaller than those in Comparative Examples <NUM> and <NUM>. As a result, it was confirmed that the crack rate could be reduced.

According to the present disclosure, a circuit board having excellent heat cycle resistance and a producing method thereof are provided. In addition, a joined body having excellent heat cycle resistance and a method for producing the joined body are provided.

Claim 1:
A joined body in which a silicon nitride substrate and a copper plate are joined together with a brazing layer formed of an Ag-Cu-Sn-based brazing material containing silver, copper, tin, and an active metal contained as TiH<NUM>,
wherein the content of silver in the brazing layer is <NUM>% by mass or more,
wherein the amount of tin in the brazing layer is <NUM> to <NUM> parts by mass with respect to <NUM> parts by mass of the total of silver and copper, wherein the content of TiH<NUM> in the brazing layer is <NUM> to <NUM> parts by mass with respect to <NUM> parts by mass of the total of silver and copper,
wherein a mean value of a KAM value of a silver portion in the brazing layer obtained by EBSP method, as described in the description, using an orientation imaging microscopy is <NUM>° or less, and
wherein the KAM value is a kernel average misorientation value indicating an orientation difference between adjacent measurement points in a crystal grain, and EBSP is an electron back scattering pattern method.