Patent Application
20190364667
The present invention relates to a method for producing a ceramic circuit board.
For semiconductor devices to be utilized in power modules and the like, ceramic circuit boards including a ceramic substrate such as alumina and a conductive circuit layer framed on the front and back sides of the ceramic substrate has been put to practical use. Copper is generally used as a circuit material for ceramic circuit boards.
In recent years, the amount of heat generated from semiconductor devices tends to increase in association with the miniaturization and advancement of performance of machinery. For this reason, the ceramic substrate for ceramic circuit board is required to exhibit higher thermal conductivity in addition to high electrical insulating properties.
Hence, the application of highly thermal conductive ceramic substrates such as aluminum nitride has been examined. However, in a case in which a copper circuit is provided on a highly thermal conductive ceramic substrate, there is a problem that a crack is easily generated in the vicinity of the bonding portion between the ceramic substrate and the copper circuit by the influence of repeated thermal cycles involved in the operation of semiconductor devices or temperature changes in the operating environment.
In order to avoid this problem, it has been investigated to use aluminum exhibiting lower yield resistance than copper as a circuit material. As a method for forming an aluminum circuit on a ceramic substrate, for example, the following methods have been proposed.
(1) A molten metal method including bringing molten aluminum into contact with a ceramic substrate and cooling these to faun a bonded body thereof, performing mechanical grinding of the formed aluminum layer to adjust the thickness, and etching the aluminum layer (see, for example, Patent Literatures 1 and 2)
(2) A brazing method in which an aluminum foil or aluminum alloy foil is brazed to a ceramic substrate and then etched (see, for example, Patent Literature 3)
Patent Literature 1: Japanese Unexamined Patent Publication No. H7-193358
Patent Literature 2: Japanese Unexamined Patent Publication No. H8-208359
Patent Literature 3: Japanese Unexamined Patent Publication No. 2001-085808
However, the molten metal method requires great cost for the facility and the maintenance thereof. In addition, it is difficult to improve the productivity by the brazing method since the brazing method involves pressurization at a high temperature and the like.
An object of the present invention is to provide a method by which a ceramic circuit board including a metal layer which contains aluminum and/or an aluminum alloy as a main component and exhibits high adhesion to a ceramic substrate can be efficiently produced using a simple facility.
An aspect of the present invention relates to a method for producing a ceramic circuit board which includes a ceramic substrate and a metal layer which is formed on the ceramic substrate and contains aluminum and/or an aluminum alloy. The method includes a step of forming the metal layer in contact with the ceramic substrate by spraying a metal powder after accelerating the metal powder to a velocity of from 250 to 1050 m/s as well as heating the metal powder to from 10° C. to 270° C. and a step of subjecting the ceramic substrate and the metal layer formed on the ceramic substrate to a heat treatment in an inert gas atmosphere. The metal powder contains at least either of aluminum particles or aluminum alloy particles.
According to this method, it is possible to efficiently produce a ceramic circuit board including a metal layer which contains aluminum and/or an aluminum alloy as a main component and exhibits high adhesion to a ceramic substrate using a simple facility. In particular, a method for depositing a metal layer by spraying a metal powder in a solid phase at a high velocity can be referred to as a cold spray method, and it is possible to efficiently deposit a metal layer exhibiting high adhesion to a ceramic substrate using a simple facility according to this method.
According to the method according to an aspect of the present invention, it is possible to produce an aluminum circuit board by bonding a metal layer containing aluminum or an aluminum alloy as a main component with a ceramic substrate without necessarily requiring molten aluminum and a brazing material. In addition, it is possible to form a metal layer having a wiring pattern on a ceramic substrate without requiring etching by using a mask when the metal layer is formed.
Hereinafter, several embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments.
A method according to an embodiment relates to a method for producing a ceramic circuit board including a metal layer containing aluminum and/or an aluminum alloy.
The metal layers 2a and 2b contain at least either of aluminum or an aluminum alloy as a main component. Here, the “main component” means a component contained at a proportion of 90% by mass or more based on the entire mass of the metal layers 2a and 2b. In a case in which the metal layer contains both aluminum and an aluminum alloy, the entire amount thereof may be 90% by mass or more. The proportion of the main component may be 95% by mass or more. The metal layers or metal particles to be described later may contain unavoidable impurities in a trace amount.
The method according to the present embodiment includes a step of forming the metal layer on the ceramic substrate by spraying a metal powder composed of a plurality of metal particles after accelerating the metal powder to a velocity of from 250 to 1050 m/s as well as heating the metal powder to from 10° C. to 270° C. and a step of subjecting the ceramic substrate and the metal layer formed on the ceramic substrate to a heat treatment in an inert gas atmosphere. As aluminum particles and/or aluminum alloy particles are used as the metal particles constituting the metal powder, a metal layer containing these as a main component is formed.
In the cold spray apparatus 3, the high pressure gas cylinders 4 are filled with an inert gas to be used as a working gas at a pressure of, for example, 1 MPa or more. The inert gas can be, for example, a single gas of helium or nitrogen or a mixed gas thereof. The working gas OG supplied from the high pressure gas cylinders 4 is decompressed, for example, from 0.5 to 5 Ma by the second pressure regulator 5b on one way, then heated by the heater 6, and then supplied to the nozzle 10 of the spray gun. The working gas OG is also decompressed to, for example, from 0.5 to 5 Ma by the third pressure regulator 5c on the other way and then supplied to the powder supply apparatus 7. The metal powder for deposition is supplied from the powder supply apparatus 7 to the nozzle 10 of the spray gun together with the working gas OG.
The heating temperature by the heater 6 is typically set to be lower than the melting point or softening point of the metal powder to be deposited. The heater 6 can be arbitrarily selected from usual heating apparatuses.
The working gas supplied to the nozzle 10 of the spray gun is compressed by passing through the convergent part and is accelerated by expanding at a time at the divergent part on the downstream side thereof. The metal powder is accelerated to a predetermined velocity as well as heated to a predetermined temperature and then ejected through the outlet of the nozzle 10. Specifically, the metal powder is accelerated to a velocity of from 250 to 1050 m/s as well as heated to from 10° C. to 270° C. Here, the temperature to which the metal powder is heated means the maximum temperature of the metal powder reached. The temperature of the inert gas at the inlet of the nozzle 10 can also be regarded as the temperature to which the metal powder is heated. Here, the term “heating” in the present specification is used in the meaning including adjusting the temperature to a predetermined temperature equal to or less than room temperature. In addition, the velocity to which the metal powder is accelerated means the maximum velocity which the accelerated metal powder reaches.
The metal powder ejected from the nozzle 10 is sprayed onto the surface of the ceramic substrate 1. By this, the metal powder is deposited on the ceramic substrate 1 while colliding with the surface thereof in a solid state, and the metal layer 2 is thus formed. The temperature of the metal powder can be typically changed in association with the expansion of the working gas in the nozzle 10 and the like.
The metal powder are mainly composed of aluminum particles and/or aluminum alloy particles. When these are heated to a temperature higher than 270° C., the softened aluminum particles or aluminum alloy particles adhere to the inner wall of the nozzle 10, as a result, the nozzle is clogged, and it is difficult to form the metal layer in some cases. In addition, when the temperature of the metal powder is less than 10° C., it is difficult for the metal particles to be sufficiently plastically deformed at the moment at which the metal powder collides with the ceramic substrate, and there is thus a tendency that it is difficult to perform deposition. From the same viewpoint, the temperature to which the metal powder is heated may be 260° C. or less and may be 20° C. or more.
When the velocity which the accelerated metal powder reaches is less than 250 m/s, it is difficult for the metal powder to be sufficiently plastically deformed when the metal powder collides with the ceramic substrate, and there is thus a tendency that it is difficult to perform deposition or the adhesion of the deposited metal layer deteriorates. When the velocity which the accelerated metal powder reaches exceeds 1050 m/s, the metal powder is pulverized and scattered when the metal powder collides with the ceramic substrate and there is thus a tendency that it is difficult to perform deposition. From the same viewpoint, the velocity which the accelerated metal powder reaches may be 300 m/s or more and may be 1000 m/s or less.
In order to suppress the formation of pores in the metal layer, the metal particles in the metal powder may be spherical. In addition, the variation in particle diameter of the metal powder may be small. The (average) particle diameter of the metal particles constituting the metal powder may be from 10 to 70 μm or from 20 to 60 μm. When the particle diameter of the metal particles is less than 10 μm, the metal powder tends to easily clog at the convergent part of the nozzle. When the particle diameter of the metal particles exceeds 70 μm, it tends to be difficult to sufficiently increase the velocity of the metal powder. Here, the “particle diameter” means the maximum width of each particle. The particle diameter of a sufficient number of metal particles can be measured, and the average particle diameter can be determined from the result.
The metal powder may be aluminum particles or aluminum alloy particles containing other metal elements such as aluminum-magnesium alloy particles and aluminum-lithium alloy particles. When aluminum alloy particles containing metal elements such as magnesium and lithium exhibiting higher affinity for oxygen than aluminum, there is a tendency that the metal element such as magnesium or lithium reacts with aluminum and the oxide layer on the surface of the ceramic substrate and these firmly bond with each other at the time of heat treatment after deposition. From the viewpoint of the proper hardness of metal layer and the heat cycle resistance, the content of the metal elements such as magnesium and lithium in the metal powder or metal particles may be 6.0% by mass or less with respect to the mass of the metal powder or metal particles.
A metal layer having a pattern (circuit pattern) may be formed on the ceramic substrate 1 by disposing a mask material covering a part of the surface of the ceramic substrate 1 on the ceramic substrate 1. According to this method, it is possible to easily form a metal layer having a desired pattern without requiring an additional step such as etching after deposition. In this respect as well, the method according to the present embodiment is advantageous over the molten metal method and brazing method which require etching for pattern formation.
The thickness of the metal layer is not particularly limited but may be, for example, from 200 to 600 μm. In a case in which the metal layer has a first metal layer and a second metal layer, the thickness of the second metal layer may be thinner than the thickness of the first metal layer. In particular, in a case in which the first metal layer is formed using aluminum-magnesium alloy particles, the thickness of the first metal layer may be 200 μm or less or 100 μm or less in order to diminish the influence of the stress generated at the interface between the ceramic substrate and the first metal layer on the heat cycle resistance.
In the heat treatment after the formation of the metal layer, the ceramic substrate and the metal layer may be heated to from 400° C. to 600° C. As this heating temperature is 400° C. or more, there is a tendency that the aluminum and the oxide layer on the surface of the ceramic substrate react with each other and thus the ceramic substrate and the metal layer more thinly bond with each other. As the heating temperature is 600° C. or less, the influence of the softening of the metal layer can be diminished.
The ceramic substrate can be selected from those exhibiting proper insulating properties. The ceramic substrate may exhibit high thermal conductivity. Examples of the ceramic substrate include an aluminum nitride (AlN) substrate, a silicon nitride (Si3N4) substrate, and aluminum oxide (Al2O3). The aluminum nitride substrate may exhibit a three-point bending strength of 400 MPa or more and/or a thermal conductivity of 150 W/mK or more. The silicon nitride substrate may exhibit a three-point bending strength of 600 MPa or more and/or a thermal conductivity of 50 W/mK or more. The size of the ceramic substrate 1 is arbitrarily set according to the application. The thickness of the ceramic substrate 1 is not particularly limited but may be, for example, from 0.2 to 1.0 mm. These ceramic substrates can be respectively procured as commercial products.
Hereinafter, the present invention will be more specifically described with reference to Examples. However, the present invention is not limited to these Examples.
Ceramic Substrate
In all Examples and all Comparative Examples, an aluminum nitride (AlN) substrate (size: 10 mm×10 mm×0.635 mm t, three-point bending strength: 500 MPa, thermal conductivity: 150 W/mK, and purity: 95% or more) or a silicon nitride (Si3N4) substrate (size: 10 mm×10 mm×0.635 mm t, three-point bending strength: 700 MPa, thermal conductivity: 70 W/mK, and purity: 92% or more) was used.
A part of the surface of the aluminum nitride substrate was masked with an iron mask material. A metal layer having a length of 3 mm, a width of 3 mm, and a thickness of 300 μm was formed thereon by a cold spray method using aluminum powder (water atomized powder manufactured by Toyo Aluminium K.K., purity: 99.7%, particle size: 45 μm or less). The deposition by the cold spray method was performed using nitrogen as a working gas under the conditions in which the temperature of powder was set to 20° C. and the velocity of powder was set to 300 m/s.
The aluminum nitride substrate on which the metal layer was formed was maintained at 500° C. for 3 hours in a nitrogen atmosphere for a heat treatment.
After the heat treatment, a test piece illustrated in
Test pieces were obtained in the same manner as in Example 1 except that the ceramic substrate and the temperature and velocity of aluminum powder in cold spray were changed as presented in Table 1.
A part of the surface of the silicon nitride substrate was masked with an iron mask material. A first metal layer having a length of 3 mm, a width of 3 mm, and a thickness of 100 μm was formed thereon by a cold spray method using aluminum-magnesium alloy powder (gas atomized powder manufactured by Kojundo Chemical Laboratory Co., Ltd., magnesium content: 3.0% by mass, content of impurities other than aluminum and magnesium: 0.1% by mass or less, particle diameter: 45 μm or less). The deposition by the cold spray method was performed using nitrogen as a working gas under the conditions in which the temperature of powder was set to 20° C. and the velocity of powder was set to 300 m/s.
A second metal layer having a length of 3 mm, a width of 3 mm, and a thickness of 200 μm was formed on the first metal layer using aluminum powder (water atomized powder manufactured by Toyo Aluminium K.K., purity: 99.7%, particle diameter: 45 μm or less) by a cold spray method (working gas: nitrogen, temperature of powder: 20° C., velocity of powder: 300 m/s) under the same conditions as for the first metal layer.
The aluminum nitride substrate on which the first metal layer and the second metal layer were formed was maintained at 500° C. for 3 hours in a nitrogen atmosphere for a heat treatment.
After the heat treatment, a test piece illustrated in
Test pieces for adhesive strength measurement were obtained in the same manner as in Example 7 except that the ceramic substrate, the metal powder for the formation of the first metal layer, and the temperature and velocity of powder in cold spray were changed as presented in Table 1. In Examples 13 to 18, aluminum-magnesium alloy powder (gas atomized powder manufactured by Kojundo Chemical Laboratory Co., Ltd., magnesium content: 6.0% by mass, content of impurities other than aluminum and magnesium: 0.1% by mass or less, particle diameter: 45 μm or less) was used for the formation of the first metal layer.
Test pieces for adhesive strength measurement were fabricated in the same manner as in Example 1 or 7 except that the ceramic substrate, the metal powder for the formation of the first metal layer, and the temperature and velocity of powder in cold spray were changed as presented in Table 2.
However, the aluminum powder was not attached to the aluminum nitride substrate and it was not able to form the first metal layer in Comparative Examples 7, 8, 11, and 12 in which the temperature of powder in the cold spray method for the formation of the first metal layer was set to 0° C. and Comparative Examples 15 to 22 in which the velocity of powder was set to 200 m/s or 1100 m/s. In Comparative Examples 9, 10, 13, and 14 in which the temperature of powder in the cold spray method for the formation of the first metal layer was set to 280° C., clogging of the aluminum powder occurred in the nozzle and thus it was not able to form the first metal layer.
Adhesive Strength Test
Examples 1 to 18 and Comparative Examples 1 to 6 in which test pieces were obtained were subjected to a test to apply a tensile load in the direction X of
Heat Cycle Test
The test pieces of Examples 1 to 18 which exhibited favorable adhesion were subjected to a heat cycle test of 1000 cycles when the test piece was “left to stand in an environment of 180° C. for 30 minutes and then left to stand in an environment of −45° C. for 30 minutes” was taken as one cycle. Even after the heat cycle test, abnormality such as peeling off did not occur in the metal layer.
1: ceramic substrate, 2, 2a, 2b: metal layer, 3: cold spray apparatus, 4: high pressure gas cylinder, 5a: first pressure regulator, 5b: second pressure regulator, 5c: third pressure regulator, 6: heater, 7: powder supply apparatus, 10: nozzle of spray gun, 21, 21a, 21b: single metal layer, 22a, 22b: first metal layer, 23a, 23b: second metal layer, 30: stud pin, 31: epoxy adhesive, 40, 41: test piece.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017-006090 | Jan 2017 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2018/001054 | 1/16/2018 | WO | 00 |
