JP6973218B2 - How to manufacture a board for a power module with a heat sink - Google Patents

Description

The present invention relates to a method for manufacturing a substrate for a power module with a heat sink used in a semiconductor device that controls a large current and a high voltage.

As a board for a power module, with a heat sink in which a circuit layer is bonded to one surface of an insulating layer made of a ceramic substrate such as aluminum nitride, and an aluminum-based heat sink is bonded to the other surface via an aluminum plate. Boards for power modules are known.
For example, in the substrate for a power module with a heat sink disclosed in Patent Document 1, a circuit layer made of a pure aluminum plate, an aluminum alloy plate, a pure copper plate, a copper alloy plate, or the like is bonded to one surface of an insulating layer made of a ceramics substrate. A metal layer made of a metal plate of pure aluminum or an aluminum alloy is bonded to the other surface of the insulating layer, and a heat sink made of aluminum or an aluminum alloy is bonded to the metal layer via a copper layer. In this case, the insulating layer and the metal layer are bonded by using a brazing material, and the metal layer and the heat sink are solid-phase diffusion bonded between the copper layer interposed therein.

In such a substrate for a power module with a heat sink, porous silicon carbide disclosed in Patent Document 2 as a material for a heat sink in order to prevent warpage due to joining of members having different coefficients of thermal expansion such as a ceramic substrate and an aluminum plate. It has been proposed to use a composite having a low coefficient of expansion (referred to as an aluminum silicon carbide composite) obtained by impregnating a molded body with a metal containing aluminum as a main component.

Japanese Unexamined Patent Publication No. 2014-60215 Japanese Unexamined Patent Publication No. 2000-281465

By the way, the metal layer of the substrate for a power module is preferably made of relatively high-purity aluminum (particularly high-purity aluminum having a purity of 99.99% by mass or more) for stress relaxation due to thermal expansion and contraction. On the other hand, in the aluminum silicon carbide composite described in Patent Document 2, a relatively low-purity aluminum alloy containing silicon, magnesium, etc. is used in order to obtain a dense and high-strength composite by a high-temperature casting method or the like. The temperature required for solid phase diffusion between the aluminum silicon carbide composite and copper is, for example, about 500 ° C. is appropriate.
However, if an attempt is made to simultaneously bond the metal layer of the substrate for the power module and the aluminum silicon carbide composite via the copper layer at a temperature of about 500 ° C., the metal layer and the copper layer are insufficiently bonded. _{Intermetallic compounds (CuAl 2} , CuAl, etc.) do not sufficiently grow between the metal layer and the copper layer, and bonding defects are likely to occur. In order to solve this, if an attempt is made to raise the bonding temperature, a part of the aluminum silicon carbide composite may be melted.

The present invention has been made in view of such circumstances, and an object of the present invention is to reliably bond a substrate for a power module and a heat sink made of an aluminum silicon carbide composite at a temperature of 500 ° C. or lower.

In the method for manufacturing a substrate for a power module with a heat sink of the present invention, a circuit layer is bonded to one surface of a ceramic substrate, and a metal layer made of aluminum or an aluminum alloy is bonded to the other surface of the ceramic substrate. For the power module, the metal layer in the substrate for the power module and a heat sink made of an aluminum silicon carbide composite formed by impregnating a porous body of silicon carbide with an aluminum alloy are diffusion-bonded via a copper layer. It has a heat sink joining step of joining a substrate and the heat sink, and the heat sink joining step is a metal foil made of an aluminum alloy to which magnesium is added between the metal layer and the copper layer of the power module substrate. The power module substrate and the heat sink are solid-phase diffusion- bonded by heating to 400 ° C. or higher and lower than 500 ° C. in a state of intervening.
In this case, the magnesium concentration of the metal foil is 0.02% by mass or more and 3.0% by mass or less.

Generally, an aluminum oxide film is formed on the surface of a metal layer in a substrate for a power module or the surface of an aluminum silicon carbide composite of a heat sink, which hinders the formation of an intermetal compound between aluminum and copper, resulting in poor bonding. Is the cause of.
According to this manufacturing method, a metal foil made of aluminum or an aluminum alloy to which magnesium is added is interposed between the metal layer and the copper layer of the power module substrate, so that the magnesium solidly dissolved in the metal foil is diffused. reacts with the oxide film of the metal layer and the surface of the heat sink to break the aluminum oxide film, dispersed as magnesium oxide (MgAl ₂ O ₄ and MgO) and. In this case, since the metal foil is an aluminum alloy in which magnesium is solid-dissolved, the magnesium oxide produced by breaking the aluminum oxide film on the aluminum surface is dispersed as fine particles at the interface between aluminum and copper. Will be. Therefore, it is suppressed that this magnesium oxide inhibits the formation of the metal-metal compound between aluminum and copper, and as a result, the growth of the metal-metal compound between aluminum and copper is promoted, and the power is generated even at a low temperature of 500 ° C. or lower. The board for the module and the heat sink can be firmly joined.

In this case, if the concentration of magnesium in the metal foil is less than 0.02% by mass, the magnesium oxide is not sufficiently formed, and there is a possibility that bonding failure may occur. On the other hand, if the concentration of magnesium in the metal foil exceeds 3.0% by mass, rolling becomes difficult.

As a preferred embodiment of the method for manufacturing a substrate for a power module with a heat sink of the present invention, the thickness of the metal foil is preferably 3.0 μm or more and 300.0 μm or less.
In this case, it is difficult to roll the metal foil to a thickness of less than 3.0 μm, and if the thickness exceeds 300.0 μm, the stress generated by the linear expansion difference between the metal foil (rolled foil) and the ceramics is large. It may cause peeling.

In the method for manufacturing a substrate for a power module with a heat sink according to another aspect of the present invention, the metal foil may further contain silicon, and the concentration of the silicon may be 10.5% by mass or less.
Silicon can also diffuse into the metal layer and the aluminum of the aluminum silicon carbide composite to increase the bonding strength. However, if the silicon concentration of the metal foil exceeds 10.5% by mass, rolling becomes difficult.

According to the present invention, the magnesium component of the metal foil breaks the aluminum oxide film on the surface of the metal layer and the aluminum silicon carbide composite and is dispersed as a fine magnesium oxide. Growth is promoted, and the power module substrate and the heat sink can be firmly bonded at a temperature of 500 ° C. or lower.

It is sectional drawing which shows the whole structure of the substrate for the power module with a heat sink which concerns on one Embodiment of this invention. It is sectional drawing which shows the state before joining about the substrate for a power module of FIG. It is sectional drawing which shows the state before joining the heat sink to the substrate for a power module in the manufacturing method of the said embodiment.

Hereinafter, embodiments of the present invention will be described.
FIG. 1 shows a substrate 1 for a power module with a heat sink manufactured by the manufacturing method of the present embodiment. In the power module substrate 1 with a heat sink, the power module substrate 10 and the heat sink 20 are joined to each other in a laminated state via a copper layer 30.

The power module substrate 10 is a ceramic substrate 11, a circuit layer 12 bonded to one surface (front surface) 11a of the ceramic substrate 11, and a metal layer 13 bonded to the other surface (back surface) 11b of the ceramic substrate 11. And have.
The ceramic substrate 11 is an insulating material that prevents electrical connection between the circuit layer 12 and the metal layer 13, and is formed of, for example, AlN (aluminum nitride), silicon nitride Si ₃ N _4, or the like, and its plate thickness is 0. It is 2 mm to 1.5 mm.

Either aluminum or an aluminum alloy can be applied to the circuit layer 12 and the metal layer 13, but for the metal layer 13, pure aluminum having a purity of 99.00% by mass or more or a purity of 99.99% by mass or more is used for stress relaxation. Is particularly preferable. The plate thickness of the circuit layer 12 and the metal layer 13 is preferably 0.1 mm to 1.0 mm. The circuit layer 12 and the metal layer 13 are joined by laminating aluminum plates on both sides of the ceramic substrate 11 via, for example, an Al—Si brazing material, and pressurizing and heating them in the laminating direction.

The heat sink 20 is formed of an aluminum silicon carbide composite formed by impregnating a silicon carbide porous body with an aluminum alloy. The silicon carbide porous body is obtained by mixing silicon carbide powder and a binder, forming a plate, and sintering the mixture. An aluminum silicon carbide composite is produced by impregnating this silicon carbide porous body with a melt of an aluminum alloy containing magnesium or silicon at a high pressure. It has the characteristics of both aluminum and silicon carbide, has good thermal conductivity as a heat sink, has a low coefficient of thermal expansion, and is bonded to the power module substrate 10 so that thermal expansion and contraction can be achieved by the power module substrate. It is possible to suppress the occurrence of warpage and the like in balance with the ceramic substrate 11 of 10.
As the heat sink 20, a flat plate is preferably used, and the thickness thereof is preferably 0.4 mm to 6.0 mm.
A skin layer (not shown) made of an impregnated aluminum alloy is formed on the surface of the heat sink 20, and the skin layer and the copper layer 30 are bonded to each other.
Further, as the impregnated aluminum alloy, for example, A356 (ASTM standard), ADC12, 6063, 3003 and the like (JIS standard) can be used.
The copper layer 30 is not particularly limited, but is preferably made of pure copper in terms of thermal conductivity. For example, it is formed by a rolled plate of oxygen-free copper, and is formed to have a thickness of 0.05 mm or more and 3.0 mm or less.

Next, a method of manufacturing the substrate 1 for a power module with a heat sink according to the present embodiment will be described.
The manufacturing method includes a power module substrate forming step of joining a circuit layer 12 and a metal layer 13 to a ceramics substrate 11 to form a power module substrate 10, and a heat sink joining step of joining a heat sink 20 to a power module substrate 10. It consists of. Hereinafter, this process will be described in order.

(Substrate forming process for power module)
As shown in FIG. 2, an aluminum plate 12A serving as a circuit layer 12 is provided on one surface 11a of the ceramic substrate 11, an aluminum plate 13A serving as a metal layer 13 is provided on the other surface 11b, and an Al—Si brazing material foil 15 is provided. The circuit layer 12 is bonded to one surface 11a of the ceramic substrate 11 and the metal layer 13 is bonded to the other surface 11b by cooling the laminated body in a state of being pressurized in the stacking direction. The power module substrate 10 is formed. The brazing foil 15 is melted by heating and diffuses into the circuit layer 12 and the metal layer 13 to firmly bond them to the ceramic substrate 11.
The joining conditions at this time are not necessarily limited, but in a vacuum atmosphere, the pressing force in the stacking direction is 0.3 MPa to 1.0 MPa, and the heating temperature is 640 ° C. or higher and 650 ° C. or lower for 1 minute or longer and 60. It is preferable to hold it for a minute or less.

(Heat sink joining process)
As shown in FIG. 3, the heat sink 20 is bonded to the metal layer 13 of the power module substrate 10 via the copper layer 30. This bonding is a solid phase diffusion bonding between the aluminum of the metal layer 13 and the heat sink 20 and the copper of the copper layer 30.
Further, at the time of this joining, the metal foil 40 is inserted between the metal layer 13 and the copper layer 30. The metal foil 40 is a metal foil formed of an aluminum alloy to which magnesium is added.
The metal leaf 40 is interposed between the metal layer 13 and the copper layer 30, and the metal layer 13 and the heat sink 20 are joined by diffusion bonding between aluminum and copper by heating in a state of being pressurized in the stacking direction. .. The joining conditions at this time are not necessarily limited, but are maintained in a vacuum atmosphere at a pressure of 0.3 MPa or more and 3.5 MPa or less in the stacking direction and at a heating temperature of 400 ° C. or more and less than 500 ° C. for 5 minutes or more and 240 minutes or less. It is preferable to do so.

As described above, the aluminum oxide film is present on the surfaces of the metal layer 13 and the heat sink 20, which hinders the solid phase diffusion bonding with the copper layer 30. In this joining step, since magnesium atoms are contained in the aluminum alloy in the metal foil 40, the magnesium atoms destroy the aluminum oxide film on the surfaces of the metal layer 13 and the heat sink 20, and the spinel (MgAl ₂ O ₄ ). To produce magnesium oxides such as. Therefore, the surface of the metal layer 13 and the aluminum silicon carbide aluminum from which the aluminum oxide film has been removed and the surface of the copper layer 30 are in contact with each other for diffusion bonding.

Further, since the magnesium oxide generated at this time is formed by the diffusion of magnesium atoms, it is fine particles and is dispersed in the plane direction along the interface between the metal layer 13 and the copper layer 30. It is formed. Therefore, the diffusion bonding between the aluminum of the metal layer 13 or the heat sink 20 and the copper of the copper layer 30 is less likely to be hindered, the diffusion bonding between aluminum and copper is promoted, and the metal layer 13 and the heat sink 20 are firmly bonded. Will be done.

The metal foil 40 is a rolled foil of an Al—Si alloy (aluminum alloy) to which magnesium is added, and the thickness thereof is preferably 3.0 μm or more and 300.0 μm or less. It is difficult to roll the metal foil 40 to a thickness of less than 3.0 μm, and if the thickness exceeds 300.0 μm, the stress generated by the linear expansion difference between the metal foil 40 (rolled foil) and the ceramics becomes large. , May cause peeling.
The metal leaf 40 is made of an Al—Si alloy to which magnesium is added, but the silicon content in the Al—Si alloy is preferably 10.5% by mass or less. If the silicon content in the Al—Si alloy exceeds 10.5% by mass, rolling becomes difficult.
Further, in the above embodiment, the material of the metal foil 40 is an Al—Si alloy to which magnesium is added, but another aluminum alloy to which magnesium is added may be used, or pure aluminum to which magnesium is added may be used. There may be.

The concentration of magnesium in the metal foil 40 is set to a ratio of 0.02% by mass or more and 3.0% by mass or less of magnesium.
If the magnesium concentration is less than 0.02% by mass, magnesium oxide is not sufficiently formed at the interface between aluminum and copper, and as a result, an aluminum oxide film remains and diffusion bonding between aluminum and copper is hindered, resulting in bonding failure. There is a risk.
On the other hand, when the concentration of magnesium increases, the concentration of aluminum decreases relatively, and the metal layer 40 becomes harder, which makes rolling difficult.

In this way, the metal layer 13 of the power module substrate 10 and the heat sink 20 are solid-phase diffusion-bonded via the copper layer 30, thereby manufacturing a power module substrate 1 with a heat sink in which they are integrated. ..

According to this manufacturing method, the metal layer 13 of the substrate 10 for a power module and the heat sink 20 made of an aluminum silicon carbide composite are formed into a copper layer 30 by interposing a metal foil 40 made of an aluminum alloy to which magnesium is added. It becomes possible to perform solid-phase diffusion bonding at a low temperature of 500 ° C. or lower, and both the metal layer 13 and the heat sink 20 made of an aluminum silicon carbide composite can be firmly bonded.

In addition, the detailed configuration is not limited to the configuration of the embodiment, and various changes can be made without departing from the spirit of the present invention.
Further, in the present embodiment, the heat sink 20 is a flat plate, but the shape is not particularly limited, and for example, a liquid-cooled cooler having a water channel inside may be used.

As the substrate for the power module, a ceramic substrate made of an aluminum nitride plate is bonded to a circuit layer made of a pure aluminum plate (4N-Al) and a metal layer on both sides, and copper having a thickness of 1.0 mm is attached to the metal layer. The layers were joined. A 12 μm-thick Al-7.5 mass% Si brazing material foil was used for joining the power module substrate, and the pressure was maintained at a pressure of 0.6 MPa at a temperature of 640 ° C to 660 ° C for 30 minutes.
The table shows the presence / absence of Mg added to the metal foil formed at the time of joining the metal layer and the copper layer of the power module substrate in Examples 1 to 8 and Comparative Example 1, the concentration of silicon and the concentration of magnesium, and the foil thickness. It is as in 1. In the conventional example, the metal foil was not used for connecting the metal layer and the copper layer of the power module substrate.
In the bonding between the metal layer and the copper layer of the power module substrate, the pressure was maintained at 490 ° C. for 150 minutes at a pressing force of 2.1 MPa.

The bonding ratio (DBA / Cu bonding ratio) between the metal layer and the copper layer was evaluated for the obtained sample.
The bonding ratio was obtained by binarizing the ultrasonic flaw detection image of the bonding surface to obtain the bonded area excluding the peeled portion, and dividing this by the area of the interface to be bonded.
Moreover, about the obtained material, the bonding ratio (AIN / back Al bonding ratio) between the ceramic substrate and the metal layer was evaluated as the thermal cycle reliability. The bonding ratio of the ceramic substrate and the metal layer is a numerical value after performing a temperature cycle test in which the temperature is changed 3000 times between −40 ° C. and 150 ° C.
These results are shown in Table 1.

As is clear from Table 1, the magnesium concentration of the magnesium-added aluminum foil or the magnesium-added Al-Si foil is 0.02% by mass or more and 3.0% by mass or less (Examples 1 to 8). ), The bonding ratio (DBA / Cu bonding ratio) between the metal layer and the copper layer exceeds 90%, and a bonding with no practical problem can be obtained even at a temperature of 500 ° C. or lower.
Further, the bonding ratio (bonding ratio of AIN / back Al) between the ceramic substrate composed of aluminum nitride and the metal layer made of pure aluminum in Examples 1 to 8 exceeds 75%, and the thermal cycle reliability is also high. it was high. In particular, in Examples 1 to 7, the bonding ratio of AIN / back Al exceeds 80%, and the foil pressure of the aluminum foil to which magnesium is added or the Al-Si foil to which magnesium is added is 300 μm or more. By setting it to 0.0 μm or less, the reliability of the cooling cycle could be further improved.

1 Substrate for power module with heat sink 10 Substrate for power module 11 Ceramic substrate 12 Circuit layer 13 Metal layer 20 Heat sink 30 Copper layer 40 Metal foil 50 Bonding layer

Claims (3)

The metal layer in the power module substrate in which the circuit layer is bonded to one surface of the ceramic substrate and the metal layer made of aluminum or an aluminum alloy is bonded to the other surface of the ceramic substrate, and the porosity of silicon carbide. It has a heat sink joining step of joining the power module substrate and the heat sink by diffusion-bonding the heat sink made of an aluminum silicon carbide composite formed by impregnating the body with an aluminum alloy via a copper layer. In the heat sink joining step, a metal made of an aluminum alloy in which magnesium is added at a concentration of 0.02% by mass or more and 3.0% by mass or less between the metal layer and the copper layer of the power module substrate. A method for manufacturing a substrate for a power module with a heat sink, which comprises heating the substrate for a power module to a temperature of 400 ° C. or higher and lower than 500 ° C. with a foil interposed therebetween to solid-phase diffusion-bond the substrate for the power module and the heat sink. The method for manufacturing a substrate for a power module with a heat sink according to claim 1, wherein the thickness of the metal foil is 3.0 μm or more and 300.0 μm or less. The method for manufacturing a substrate for a power module with a heat sink according to claim 1 or 2, wherein the metal foil further contains silicon, and the concentration of the silicon is 10.5% by mass or less.

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