Method for manufacturing aluminum circuit board

Abstract

A method for manufacturing an aluminum circuit board including a step of spraying a heated metal powder containing aluminum particles and/or aluminum alloy particles to a ceramic base material, and of forming a metal layer on a surface of the ceramic base material. A temperature of at least a part of the metal powder is higher than or equal to a softening temperature of the metal powder and lower than or equal to a melting point of the metal powder at a time point of reaching the surface of the ceramic base material. A velocity of at least a part of the metal powder is greater than or equal to 450 m/s and less than or equal to 1000 m/s at the time point of reaching the surface of the ceramic base material.

Claims

1. A method for manufacturing an aluminum circuit board, comprising: a step of spraying heated metal powder containing aluminum particles and/or aluminum alloy particles to a ceramic base material, and thereby forming a metal layer on a surface of the ceramic base material, wherein the temperature of at least a part of the metal powder is higher than or equal to 450° C. of the metal powder and lower than or equal to the melting point of the metal powder at a time point of reaching the surface of the ceramic base material, and the velocity of at least a part of the metal powder is greater than or equal to 450 m/s and less than or equal to 1000 m/s at the time point of reaching the surface of the ceramic base material.

2. The manufacturing method according to claim 1, wherein the velocity of at least a part of the metal powder is greater than or equal to 750 m/s and less than or equal to 900 m/s at the time point of reaching the surface of the ceramic base material.

3. The manufacturing method according to claim 1, wherein in the particle diameter distribution of the metal powder, the particle diameter D.sub.10, which is the diameter at which 10% of the sample's mass is comprised of particles with a diameter less than this value, is greater than or equal to 10 μm, and the particle diameter D.sub.90, which is the diameter at which 90% of the sample's mass is comprised of particles with a diameter less than this value, is less than or equal to 50 μm.

4. The manufacturing method according to claim 3, wherein in the particle diameter distribution of the metal powder, the particle diameter D.sub.10 is greater than or equal to 20 μm, and the particle diameter D.sub.90 is less than or equal to 45 μm.

5. The manufacturing method according to claim 1, wherein the aluminum alloy particles are particles consisting of: aluminum; magnesium of less than or equal to 7.5 mass % on the basis of the mass of the aluminum alloy particles; and residual inevitable impurities.

6. The manufacturing method according to claim 1, wherein the metal powder contains aluminum alloy particles, and the manufacturing method further comprises: a step of spraying heated second metal powder containing aluminum particles to a surface of the first metal layer that is the metal layer formed on the surface of the ceramic base material, and thereby forming a second metal layer on the surface of the first metal layer.

7. The method for manufacturing the aluminum circuit board according to claim 6, wherein a thickness of the first metal layer is less than or equal to 150 μm.

8. The manufacturing method according to claim 1, wherein said temperature is higher than or equal to 450° C. and lower than or equal to 660° C., and said velocity is greater than or equal to 750 m/s and less than or equal to 900 m/s.

9. The manufacturing method according to claim 1, wherein the metal powder is sprayed with a warm spraying device comprising a tubular body including an outlet nozzle and a barrel portion, the outlet nozzle includes a divergent nozzle, the barrel portion is a straight cylindrical barrel connected to the outlet nozzle in a continuous manner, and the metal powder is fed from a powder feed port provided between the outlet nozzle and the barrel portion to the inlet of the barrel portion.

10. A method for manufacturing an aluminum circuit board, comprising: a step of spraying heated metal powder containing aluminum particles and/or aluminum alloy particles to a ceramic base material, and thereby forming a metal layer on a surface of the ceramic base material, wherein in a case where the metal powder is sprayed to the ceramic base material by being heated and accelerated by the warm spraying method, in the mass-based cumulative particle diameter distribution of the metal powder, the particle diameter D.sub.10 is greater than or equal to 10 μm, and the particle diameter D.sub.90 is less than or equal to 50 μm, in the metal powder that is sprayed to the ceramic base material, the ratio of the metal powder of which temperature at a time point of reaching the surface of the ceramic base material is lower than 450° C. of the metal powder is less than or equal to 10 mass %, and the ratio of the metal powder of which temperature at the time point of reaching the surface of the ceramic base material is higher than the melting point of the metal powder is less than or equal to 10 mass %, and the temperature of the rest of the metal powder at the time point of reaching the surface of the ceramic base material is higher than or equal to 450° C. of the metal powder and lower than or equal to the melting point of the metal powder, and the velocity of at least a part of the metal powder is greater than or equal to 450 m/s and less than or equal to 1000 m/s at the time point of reaching the surface of the ceramic base material.

11. The manufacturing method according to claim 10, wherein the metal powder is sprayed with a warm spraying device comprising a tubular body including an outlet nozzle and a barrel portion, the outlet nozzle includes a divergent nozzle, the barrel portion is a straight cylindrical barrel connected to the outlet nozzle in a continuous manner, and the metal powder is fed from a powder feed port provided between the outlet nozzle and the barrel portion to the inlet of the barrel portion.

12. An aluminum circuit board, comprising: a ceramic base material; and a metal layer formed by performing film formation of metal powder containing aluminum particles and/or an aluminum alloy particles on a surface of the ceramic base material, wherein the temperature of at least a part of the metal powder is higher than or equal to 450° C. and lower than or equal to the melting point of the metal powder at a time point of reaching the surface of the ceramic base material by the warm spraying method, wherein an electric resistivity of the metal layer is less than or equal to 4×10.sup.−8 Ωm.

13. An electronic device, comprising the aluminum circuit board according to claim 12.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a sectional view illustrating an embodiment of an aluminum circuit board.

(2) FIG. 2 is a sectional view illustrating another embodiment of an aluminum circuit board.

(3) FIG. 3 is a schematic view illustrating an embodiment of a method for manufacturing the aluminum circuit board.

(4) FIG. 4 is microscopic photographs of aluminum particles that were sprayed to an aluminum nitride base material, and are attached onto the surface of the base material.

(5) FIG. 5 is a graph showing the relationship between the ratio of removed area from a metal layer and the duration of ultrasonic wave irradiation.

(6) FIG. 6 is a graph showing the relationship between the velocity and the temperature of aluminum particles provided by a warm spray apparatus used to form of a metal layer.

DESCRIPTION OF EMBODIMENTS

(7) Hereinafter, some embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments.

(8) Technical terms used in the present specification will be defined.

(9) A “powder” used in the present specification indicates an aggregate having fluidity that is configured of a plurality of particles having different particle diameters. In the measurement of a “particle diameter distribution of the powder”, a laser diffraction and scattering method is widely used, and the methods of measurement and description of the results thereof are based on JIS Z 8825 “Particle Diameter Analysis-Laser Diffraction and Scattering Method”.

(10) In the mass-based particle diameter distribution of the powder, particle diameter D.sub.10 indicates the diameter at which 10% of the sample's mass is comprised of particles with a diameter less than this value. Similarly, particle diameter D.sub.90 indicates the diameter at which 90% of the sample's mass is comprised of particles with a diameter less than this value. In particular, D.sub.50 as defined as above is referred to as the median diameter.

(11) FIG. 1 is a sectional view illustrating an embodiment of an aluminum circuit board. An aluminum circuit board 50 illustrated in FIG. 1 includes a ceramic base material 1, and metal layers 3a and 3b that are respectively provided with being in contact with both surfaces of the ceramic base material 1. Each of the metal layers 3a and 3b is a layer that is formed by spraying heated metal powder, and usually has a circuit pattern that is connected to a semiconductor element. The details of a method for forming the metal layers 3a and 3b will be described below.

(12) FIG. 2 is also a sectional view illustrating another embodiment of an aluminum circuit board. The aluminum circuit board 100 illustrated in FIG. 2 further includes second metal layers 5a and 5b in addition to the same configuration as that of the aluminum circuit board 50, that are provided on the surfaces of the metal layers 3a and 3b attached to the ceramic base material and used as the first metal layers respectively. The second metal layers 5a and 5b can configure a circuit layer.

(13) The thickness of the metal layers 3a and 3b is not particularly limited, and for example, may be from 200 μm to 600 μm. Similarly, the total thickness of the metal layers 3a and 3b as the first metal layer, and the second metal layers 5a and 5b, for example, may be from 200 μm to 600 μm. The thickness of the metal layers 3a and 3b as the first metal layer may be less than or equal to 150 μm.

(14) The ceramic base material 1 can be selected from ceramic materials having suitable insulating properties. The ceramic base material 1 should also has high thermal conductivity, and examples thereof include aluminum nitride (AlN), silicon nitride (Si.sub.3N.sub.4), and aluminum oxide (Al.sub.2O.sub.3). The thickness of the ceramic base material 1 is not particularly limited, and for example, may be from 0.2 mm to 1.0 mm.

(15) FIG. 3 is a schematic view illustrating an embodiment of a method for manufacturing the aluminum circuit board exemplified in FIG. 1 or FIG. 2. In the method illustrated in FIG. 3, metal powder P consisting of many metal particles is heated, and is accelerated to a predetermined velocity by a warm spraying device 10, and is sprayed to the surface of the ceramic base material 1. The metal particles P may be aluminum particles, aluminum alloy particles, or a combination thereof The aluminum alloy particles may be particles formed of aluminum, magnesium of less than or equal to 7.5 mass % on the basis of the mass of the aluminum alloy particles, and residual inevitable impurities.

(16) The warm spraying device 10 includes a tubular body 20, a fuel inlet 21, an oxygen inlet 22, an ignition plug 23, a cooling water inlet 24, a cooling water outlet 25, an inert gas inlet 26, and a powder feed port 27. The tubular body 20 includes a combustion chamber 11, a mixing chamber 12, an outlet nozzle 13, and a barrel portion 14. An injection port 15 is provided at the end of the barrel portion 14. In the combustion chamber 11, fuel gas FG such as gasified liquid fuel that may be kerosene or the like or propane gas or liquefied petroleum gas is mixed with oxygen gas O.sub.2, and the mixed gas is ignited by the ignition plug 23. As the mixed gas is combusted, the temperature and pressure of the combustion gas increases. In the mixing chamber 12, inert gas IG is fed and mixed into the combustion gas flowing out from the combustion chamber 11 in order to adjust the combustion gas temperature. In the outlet nozzle 13, thus generated high-temperature and high-pressure mixed gas expands through a divergent nozzle, and becomes a high-velocity jet, and thus, is ejected to the barrel portion 14. The barrel portion 14 is a straight cylindrical barrel connected to the outlet nozzle 13 in a continuous manner. The metal particles P, i.e., the metal powder, are fed from the powder feed port 27 provided between the outlet nozzle 13 and the barrel portion 14 to the inlet of the barrel portion 14. The whole body of the warm spraying device is cooled by cooling water.

(17) By the warm spraying device 10 in FIG. 3, the metal particles P are sprayed towards the ceramic base material 1 from the injection port 15 at a high speed, along with the mixed gas of the combustion gas and the inert gas IG, and thus, the metal particles P are deposited on the ceramic base material 1, and the metal layer 3a is formed.

(18) It is preferred that the temperature of at least a part of the metal powder sprayed to the ceramic base material 1 be higher than or equal to a softening temperature of the metal powder and lower than or equal to the melting point of the metal powder when reaching the surface of the ceramic base material 1. Here, the softening temperature is defined as a value of melting point×0.6 in the absolute temperature scale. It is preferred that the velocity of at least a part of the metal powder sprayed to the ceramic base material 1 be greater than or equal to 450 m/s and less than or equal to 1000 m/s when reaching the surface of the ceramic base material 1. For example, the ratio of metal powder of which the temperature at the time point of reaching the surface of the ceramic base material 1 is lower than or equal to the softening temperature of the metal powder may be less than or equal to 10 mass %, the ratio of metal powder of which the temperature at the time point of reaching the surface of the ceramic base material 1 is higher than the melting point of the metal powder may be less than or equal to 10 mass %, and the temperature of the rest of the metal powder at the time point of reaching the surface of the ceramic base material may be higher than or equal to the softening temperature of the metal powder and lower than or equal to the melting point of the metal powder.

(19) The temperature and the velocity of the metal particles P (or the metal powder) at the time point of reaching the surface of the ceramic base material 1 can be adjusted by suitably changing the particle size distribution of the metal powder, the flow rate of the inert gas, the distance between the injection port 15 and the surface of the ceramic base material 1, and the like. In general, the distance between the injection port 15 and the surface of the ceramic base material 1 is set to be in a range of 100 mm to 400 mm.

(20) In a case where a second metal layer is formed on the surface of the first metal layer as shown in FIG. 3, a method for forming the second metal layer is not limited to the method described above, and may be identical to or different from that of the first metal layer. For example, a second metal powder containing aluminum particles as its main component may be fed to the warm spraying method of which the condition is suitably set, to form the second metal layer.

(21) FIG. 4 is microscopic photographs of aluminum particles that are attached onto a surface of an aluminum nitride base material. These samples were prepared by scanning a warm spraying device over the aluminum nitride base material at a high velocity while minimizing the feed rate of the aluminum particles, and thereby spraying a small amount of aluminum particles to the surface of the aluminum nitride base material. While changing the nitrogen flow rate of the warm spraying device in three steps of 1000 SLM, 1500 SLM, and 2000 SLM, the preheating temperature of the aluminum nitride base material for each nitrogen flow rate was changed in three steps of 300K (without preheating, RT), 473K, and 673K. As shown in FIG. 4, a large number of particles were scattered and became fine particles at a nitrogen flow rate of 1000 SLM, and it is considered that this is because the ratio of aluminum particles that have been melted at a time point of reaching the surface of the base material was high. Semi-molten particles of which the center is not melted and the surroundings are melted are commonly observed at a nitrogen flow rate of 1500 SLM. The ratio of molten particles decreased, and a number of trace of large particles that collided and dropped off is observed at a nitrogen flow rate of 2000 SLM. Here, SLM is an abbreviation of standard liter/min, and is a unit indicating a gas flow rate (liter) per 1 minute at 1 atm and 0° C.

(22) Adhesion of the particles was evaluated by dipping the samples obtained by the aforementioned method into water and applying cavitation by an ultrasonic horn, thereafter the amount of the aluminum particles dropped out of the substrate was compared. FIG. 5 is a graph in which the ultrasonic irradiation time is plotted on the horizontal axis, whereas the ratio of dropped-out particles on a silicon nitride substrate at three levels of nitrogen flow rates and three levels of preheating temperatures are plotted on the vertical axis. As a general trend, more than 80% of the particles were dropped out by irradiation of 1 minute at a nitrogen flow rate of 2000 SLM. It was observed that the dropout rate decreased as the nitrogen flow rate decreased, that is, the adhesion of the particles was improved. The influence of the preheating of the base material is less remarkable, but in a case where the base material is not preheated, there is a tendency that the adhesion is slightly weak, and thus, it is considered that preheating to a temperature higher than 200° C. is suitable. Therefore, the preheating temperature of the ceramic base material to which the metal powder is sprayed, for example, may be from 200° C. to 600° C.

(23) FIG. 6 is a graph showing the relationship between the velocity and the temperature of aluminum particles that are generated by the warm spraying device 10 for fabrication of metal layers. The graph of FIG. 6 shows such relationship of velocity and temperature for particles with three different diameters, i.e., 10 μm, 30 μm, and 50 μm, which appear as three curves.

(24) The relationship between the velocity and the temperature of the aluminum particles at the time point of reaching the surface of the ceramic base material is plotted for different flow rates of the inert gas IG (nitrogen gas) such as 500 SLM, 1000 SLM, 1500 SLM, and 2000 SLM in the warm spraying device 10 in FIG. 6. Here, the distance between the injection port 15 of the warm spraying device 10 and the base material surface is set to 200 mm, and the velocity and the temperature of the aluminum particles at the time point of reaching the surface of the ceramic base material was calculated by a numerical simulation. FIG. 6 also shows the melting point (660° C., 933K) and the softening temperature (287° C., 560K) of aluminum particles.

(25) As shown in FIG. 6, for aluminum particles with a particle diameter of less than or equal to 10 μm, their temperature is higher than the melting point, and thus, the aluminum particles are completely melted. Therefore, it is desirable that the ratio of particles having a particle diameter of less than or equal to 10 μm is small from the viewpoint of suppressing disintegration, oxidation, and the like. In addition, as described above, the adhesion of particles having a particle diameter of 50 μm decreases at a nitrogen flow rate of 2000 SLM. Considering these points, a region indicated as X in FIG. 6, that corresponds to a range of temperature of the particles higher than or equal to 450° C. (723K) and lower than or equal to the melting point (660° C., 933K), and of the velocity of the particles greater than or equal to 750 m/s and less than or equal to 900 m/s, is particularly suitable for the metal layer deposition. Even for the aluminum alloy particles, essentially the same range of temperature and velocity should be suitable.

(26) In addition, from FIG. 6, it is preferable that D.sub.10 is greater than or equal to 10 μm, and D.sub.90 is less than or equal to 50 μm for the particle diameter distribution of the metal powder used for the warm spraying device. It is more preferable that D.sub.10 is greater than or equal to 20 μm, and D.sub.90 is less than or equal to 45 μm.

(27) Electrical resistivity of the metal layer (an aluminum film) formed by the warm spraying method at a nitrogen flow rate of 1000 SLM and 1500 SLM was measured, and values of 3.3 to 3.7×10.sup.−8 Ωm were obtained respectively. 2.65×10.sup.−8 Ωm is known as a reported value of electrical resistance of dense solid of pure aluminum. In addition, there is also a report that a resistivity of an aluminum film formed by a cold spraying method is greater than or equal to 10×10.sup.−8 μm (see Non Patent Literature 1). The electrical resistance value of the aluminum film framed by the warm spraying method is obviously lower than that of the aluminum film formed by the cold spraying method. A low electric resistance value indicates that a denser aluminum film is formed. Thus we can state that the electrical resistivity of a metal layer formed by the method according to the embodiment as described above can be less than or equal to 5×10.sup.−8 Ωm.

(28) In the embodiment described above, a case where a board on which the metal layer (the circuit layer) is provided is monolithic ceramic has been exemplified, but the present invention is not limited thereto. The board may be a metal composite board containing a combination of ceramic and metals such as copper, aluminum, silver, and the like, or a resin composite board containing a combination of ceramic and a resin such as an engineering plastic resin. In such case, a metal layer is formed on the surface of the ceramic base material that is usually provided as the outermost layer. The engineering plastic resin to be combined with the ceramic base material, for example, may be polyacetal (POM), polyamide (PA), polycarbonate (PC), modified polyphenylene ether (m-PPE), or polybutylene terephthalate (PBT).

(29) The aluminum circuit board as described above is useful as a member configuring various electronic devices.

EXAMPLES

(30) Hereinafter, the present invention will be described in more detail by using examples. However, the present invention is not limited to the examples.

(31) In all examples and all comparative examples, an aluminum nitride base material (Size: 60 mm×50 mm×0.635 mmt, Three-Point Bending Strength: 500 MPa, Heat Conductivity: 150 W/mK, and Purity: greater than or equal to 95%) and a silicon nitride base material (Size: 60 mm×50 mm×0.635 mint, Three-Point Bending Strength: 700 MPa, Heat Conductivity: 70 W/mK, and Purity: greater than or equal to 92%) were used as the ceramic base material.

(32) Film Formation Test 1

(33) The following film formation test was performed by using a warm spraying device having the same configuration as that illustrated in FIG. 3.

(34) The aluminum nitride base material was covered with an iron mask with an opening. Aluminum powder (manufactured by Fukuda Metal Foil & Powder Co., Ltd., a water atomized powder, Purity: 99.7%, Particle Diameter: less than or equal to 45 μm, and Melting Point: 933K) was subjected to film formation on the surface of the aluminum nitride base material in the opening of the mask by a warm spraying method, and thus, a metal layer having a predetermined pattern (56 mm×46 mm×300 μnit) was formed. Nitrogen was used as inert gas for the warm spraying method. Similarly, a metal layer was also formed on the opposite surface of the aluminum nitride base material. The warm spraying condition was set to realize particle temperatures and particle velocities as shown in Table 1. Here, the particle temperature and the particle velocity are these values of most of the aluminum powder at a time point of reaching the base material (the same applies to Film Formation Test 2).

(35) Further, a film formation test was performed similarly by using the silicon nitride base material instead of the aluminum nitride base material.

(36) Film Formation Test 2

(37) Film formation of a first metal layer using an aluminum-magnesium alloy powder was performed using the warm spraying condition shown in Table 1 in the same manner with Film Formation Test 1. The aluminum-magnesium alloy powder (manufactured by Kojundo Chemical Lab. Co., Ltd., a gas atomized powder, Magnesium Content: 3.0 mass %, Amount of Impurities Other than Aluminum and Magnesium: less than or equal to 0.1 mass %, Particle Diameter: less than or equal to 45 μm, and Melting Point: 913K) was used instead of the aluminum powder.

(38) The results of these film formation tests are shown in Table 1. In the column “Film formation availability” of the table, “Available” indicates that a (first) metal layer was normally formed, and “Unavailable” indicates that metal particles were not attached to the base material, and thus, a metal layer could not be formed.

(39) Aluminum powder (manufactured by Fukuda Metal Foil & Powder Co., Ltd., a water atomized powder, Purity: 99.7%, and Particle Diameter: less than or equal to 45 μm) was subjected to film formation on the surface of the test examples (15, 16, 20, and 23) on which a first metal layer was successfully formed by the warm spraying method, and thus, a second metal layer (56 mm×46 mm×200 μmt) was further formed on the first metal layer. In the film formation for the second layer by the warm spraying method, nitrogen was used as inert gas, and the condition corresponded to Particle Temperature: 500° C. and Particle Velocity: 800 m/s.

(40) TABLE-US-00001 TABLE 1 (First) Metal layer Second metal Particle Particle Film layer Test Base temp. velocity Particle Thickness formation Particle Thickness No. material (K) (m/s) type (μm) availability Type (μm) 1 AlN 530 900 Al 300 Unavailable — — 2 AlN 560 400 Al 300 Unavailable — — 3 AlN 560 500 Al 300 Available — — 4 AlN 930 900 Al 300 Available — — 5 AlN 930 1100 Al 300 Unavailable — — 6 AlN 960 500 Al 300 Unavailable — — 7 Si.sub.3N.sub.4 530 500 Al 300 Unavailable — — 8 Si.sub.3N.sub.4 560 900 Al 300 Available — — 9 Si.sub.3N.sub.4 560 1100 Al 300 Unavailable — — 10 Si.sub.3N.sub.4 930 400 Al 300 Unavailable — — 11 Si.sub.3N.sub.4 930 500 Al 300 Available — — 12 Si.sub.3N.sub.4 960 900 Al 300 Unavailable — — 13 AlN 530 900 Al—Mg 150 Unavailable — — 14 AlN 560 400 Al—Mg 150 Unavailable — — 15 AlN 560 900 Al—Mg 150 Available Al 150 16 AlN 745 500 Al—Mg 150 Available Al 150 17 AlN 745 1100 Al—Mg 150 Unavailable — — 18 AlN 960 500 Al—Mg 150 Unavailable — — 19 Si.sub.3N.sub.4 530 500 Al—Mg 150 Unavailable — — 20 Si.sub.3N.sub.4 560 500 Al—Mg 150 Available Al 150 21 Si.sub.3N.sub.4 560 1100 Al—Mg 150 Unavailable — — 22 Si.sub.3N.sub.4 745 400 Al—Mg 150 Unavailable — — 23 Si.sub.3N.sub.4 745 900 Al—Mg 150 Available Al 150 24 Si.sub.3N.sub.4 960 900 Al—Mg 150 Unavailable — —

(41) Heat Cycle Test

(42) For the film formation conditions (3, 4, 8, 11, 15, 16, 20, and 23) in which a test body was obtained, these test bodies were subjected to a heat cycle test. In the heat cycle test, “at 180° C. for 30 minutes, and then, at −45° C. for 30 minutes” was set as one cycle, and a test of 1000 cycles was performed. After the heat cycle test, abnormality such as peeling did not occur in the metal layers.

INDUSTRIAL APPLICABILITY

(43) The present invention is useful for manufacturing an aluminum circuit board that includes a circuit containing aluminum. Processing conditions such as the amount of metal powder and its heating temperature can be suitably managed to manufacture a circuit board including a resin composite board of ceramic and an engineering plastic resin as well as a metal composite board of ceramic and a metal. The method of the present invention is excellent in mass productivity and general versatility.

REFERENCE SIGNS LIST

(44) 1: ceramic base material, 3a, 3b: (first) metal layer, 5a, 5b: second metal layer (circuit layer), 11: combustion chamber, 12: mixing chamber, 13: outlet nozzle, 14: barrel portion, 15: injection port, 20: tubular body, 21: fuel inlet, 22: oxygen inlet, 23: ignition plug, 24: cooling water inlet, 25: cooling water outlet, 26: inert gas inlet, 27: powder feed port, 50, 100: aluminum circuit board, FG: fuel gas, IG: inert gas, P: metal particles, W: cooling water.