JP5707885B2 - Power module substrate, power module substrate with cooler, power module, and method of manufacturing power module substrate - Google Patents

Description

  The present invention relates to a power module substrate including a circuit layer on which a semiconductor element is mounted, a power module substrate with a cooler including the power module substrate, the power module, and a method of manufacturing the power module substrate. Is.

Among semiconductor elements, a power element for supplying power has a relatively high calorific value. For example, an Al (aluminum) metal plate is Al on a ceramic substrate made of AlN (aluminum nitride). A substrate for a power module joined through a Si-based brazing material is widely used.
This metal plate is used as a circuit layer, and a semiconductor element as a power element is mounted on the circuit layer via a solder material. It has been proposed that a metal plate such as Al is joined to the lower surface of the ceramic substrate to form a metal layer for heat dissipation, and a cooler is joined via the metal layer.

Here, since the aluminum oxide film is formed on the surface of the circuit layer made of aluminum, it cannot be satisfactorily bonded to the solder material.
Therefore, conventionally, as disclosed in, for example, Patent Document 1, a Ni plating film is formed on the surface of a circuit layer by electroless plating or the like, and a solder material is disposed on the Ni plating film to provide a semiconductor element. It was joined.
Patent Document 2 proposes a technique for joining semiconductor elements using Ag nanopaste without using a solder material.

JP 2004-172378 A JP 2006-202938 A

By the way, as described in Patent Document 1, in the power module substrate in which the Ni plating film is formed on the surface of the circuit layer, the surface of the Ni plating film is deteriorated by oxidation or the like in the process until the semiconductor element is joined, There is a possibility that the reliability of bonding with a semiconductor element bonded via a solder material may be reduced.
In addition, when a cooler is joined to a power module substrate by brazing, the Ni plating film deteriorates if brazing or the like is performed after forming the Ni plating film on the surface of the circuit layer. And the cooler were brazed to form a power module substrate with a cooler, and then the entire power module substrate with a cooler was immersed in the plating bath. For this reason, the Ni plating film is also formed on portions other than the circuit layer. Here, when the cooler is made of aluminum and an aluminum alloy, there is a possibility that electrolytic corrosion proceeds between the cooler made of aluminum and the Ni plating film. Therefore, in the Ni plating process, it is necessary to perform a masking process so that the Ni plating film is not formed on the cooler portion. As described above, since the plating process is performed after the masking process is performed, a great amount of labor is required to form the Ni plating film on the circuit layer portion, and the manufacturing cost of the power module is greatly increased. There was a problem.

On the other hand, as disclosed in Patent Document 2, when joining a semiconductor element using Ag nano paste without using a solder material, the layer made of Ag nano paste is formed thinner than the solder material. For this reason, the stress at the time of thermal cycle load tends to act on the semiconductor element, and the semiconductor element itself may be damaged.
Further, as described above, an aluminum oxide film is formed on the surface of a circuit layer made of aluminum, and a semiconductor element cannot be bonded using only the Ag nanopaste described in Patent Document 2. After all, it was necessary to form an intervening layer made of Ag or Ni on the aluminum surface (see paragraph number 0014 of Patent Document 2).

  The present invention has been made in view of the circumstances described above, and is a power module substrate and a cooler capable of easily and surely joining a semiconductor element to a circuit layer via a solder material. It is an object of the present invention to provide a power module substrate, a power module, and a method for manufacturing the power module substrate.

In order to solve such problems and achieve the above object, the power module substrate of the present invention is provided with a circuit layer made of aluminum or an aluminum alloy on one surface of the insulating layer. A power module substrate on which a semiconductor element is disposed via a solder layer on a surface of the circuit layer on which the solder layer is formed , from a sintered body of Ag paste containing lead-free glass An Ag fired layer is formed.

According to the power module substrate of this configuration, since the Ag fired layer made of a fired body of an Ag paste containing lead-free glass is formed on the surface of the circuit layer on which the solder layer is formed , aluminum or The oxide film formed on the surface of the circuit layer made of an aluminum alloy can be removed, and a semiconductor element can be joined to the circuit layer made of aluminum or aluminum alloy via a solder material via an Ag fired layer. It becomes possible. That is, an Ag fired layer is formed as an alternative to the Ni plating film, and this Ag fired layer is used as a solder underlayer. Therefore, it is not necessary to immerse the power module substrate in a plating solution or the like, and troublesome work such as masking processing can be reduced. Further, since the solder material is used for joining the semiconductor elements, it is possible to increase the thickness of the solder material, and it is possible to suppress the stress during the thermal cycle load from acting on the semiconductor element, and the semiconductor element itself Breakage can be prevented.

Here, it is preferable that the Ag fired layer includes a lead-free glass layer formed on the circuit layer and an Ag layer formed on the lead-free glass layer.
In this case, the aluminum oxide film formed on the surface of the circuit layer made of aluminum or aluminum alloy can be removed by reacting with the lead-free glass layer, and the circuit layer and the semiconductor element can be securely connected via the solder material. Can be joined.

Moreover, it is preferable that the electrical resistance value P in the thickness direction of the Ag fired layer is set to 0.5Ω or less.
Since the electrical resistance value P in the thickness direction of the Ag fired layer is 0.5Ω or less, it is possible to reliably conduct electricity between the semiconductor element and the circuit layer via the Ag fired layer and the solder material. This makes it possible to configure a highly reliable power module. Furthermore, in order to ensure electrical continuity between the semiconductor element and the circuit layer, the electrical resistance value P in the thickness direction of the Ag fired layer is preferably 0.2Ω or less.
In the present invention, the electrical resistance value P in the thickness direction of the Ag fired layer is the electrical resistance between the upper surface of the Ag fired layer and the upper surface of the circuit layer. This is because the electrical resistance of aluminum constituting the circuit layer is very small compared to the electrical resistance in the thickness direction of the Ag fired layer. The electrical resistance value P is measured by separating the upper surface center point of the Ag fired layer and the distance H from the upper surface center point of the Ag fired layer to the Ag fired layer end by H from the Ag fired layer end. Between the points on the circuit layer.

The glass composition of the lead-free _glass, Bi ₂ O 3: 68 wt% or more 93 wt% or less, ZnO: 1% by weight to 20% by _weight, B _{2 O} 3: 1 wt% to 11 wt% or less, SiO _2: 5 wt% or _less, Al _{2 O} 3: 5 wt% or _less, Fe _{2 O} 3: 5 wt% or less, CuO: 5 wt% or less, _CeO 2: 5 wt% or less, _ZrO 2: 5 wt% or less, It is preferable that alkali metal oxides: 2% by mass or less, alkaline earth metal oxides: 7% by mass or less.
Lead-free glass containing these oxides has a relatively low softening temperature, making it possible to fire the Ag paste at low temperatures, and the circuit layer made of aluminum or aluminum alloy deteriorates due to the heat during firing of the Ag paste. Can be prevented.
Here, the content of the alkali metal oxide is the total amount of alkali metal oxides such as Li ₂ O, Na ₂ O, and K ₂ O.
The content of the alkaline earth metal oxide is the total amount of alkaline earth metal oxides such as MgO, CaO, BaO, and SrO.

Furthermore, it is preferable that the insulating layer is a ceramic substrate selected from AlN, Si ₃ N ₄ or Al ₂ O ₃ .
The ceramic substrate selected from AlN, Si ₃ N ₄ or Al ₂ O ₃ has excellent insulation and strength, and can improve the reliability of the power module substrate. Further, it is possible to easily form a circuit layer by brazing a metal plate made of aluminum or an aluminum alloy on the ceramic substrate.

A power module substrate with a cooler according to the present invention includes the power module substrate described above and a cooler disposed on the other surface side of the insulating layer of the power module substrate. It is a feature.
According to the power module substrate with a cooler having this configuration, since the above-described Ag fired layer is formed on the circuit layer, the semiconductor element can be bonded via the solder material.
Note that the cooler does not need to be directly bonded to the other surface of the insulating layer, but via a metal layer made of aluminum or an aluminum alloy, or a buffer layer made of a composite material containing aluminum, an aluminum alloy, or aluminum (for example, AlSiC). Thus, it is only necessary to be bonded to the other surface side of the insulating layer.

A power module according to the present invention includes the power module substrate described above and a semiconductor element disposed on one surface side of the circuit layer of the power module substrate, and the semiconductor element includes a solder layer. It is characterized by being joined via.
In the power module having this configuration, since the semiconductor element is joined via the solder material, the stress during the heat cycle load is relieved by the solder material, and damage to the semiconductor element can be suppressed. Therefore, thermal cycle reliability can be greatly improved. Further, since it is not necessary to form a Ni plating film, the production cost can be greatly reduced.

In the method for manufacturing a power module substrate according to the present invention, a circuit layer made of aluminum or an aluminum alloy is disposed on one surface of the insulating layer, and a semiconductor element is disposed on the circuit layer via a solder layer. A method for manufacturing a power module substrate comprising: an application step of applying an Ag paste containing a lead-free glass powder containing ZnO to the surface of the circuit layer on which the solder layer is formed; and applying the Ag paste A firing step of firing the Ag paste by heat treatment in a state, and an Ag fired layer made of a fired body of an Ag paste containing lead-free glass on a surface of the circuit layer on which the solder layer is formed It is characterized by forming.

According to the method for manufacturing a power module substrate having this configuration, an application step of applying an Ag paste containing a lead-free glass powder containing ZnO, and a baking in which the Ag paste is fired by heat treatment in a state where the Ag paste is applied. Therefore, an Ag fired layer can be formed on the surface of the circuit layer on which the solder layer is formed . Moreover, since an Ag paste containing ZnO-containing lead-free glass powder is used, it becomes possible to remove the oxide film formed on the surface of the circuit layer made of aluminum or aluminum alloy, and Ag on the circuit layer. A fired layer can be reliably formed.

Here, the firing temperature in the firing step is preferably 350 ° C. or higher and 645 ° C. or lower.
Since the firing temperature in the firing step is 350 ° C. or higher, the Ag paste can be fired to reliably form the Ag fired layer. Moreover, since the firing temperature is 645 ° C. or lower, it is possible to prevent deterioration of the circuit layer made of aluminum or aluminum alloy in the firing step.

  According to the present invention, a power module substrate capable of easily and surely soldering a semiconductor element to an Ag fired layer made of a fired body of an Ag paste containing lead-free glass formed on a circuit layer. A power module substrate with a cooler, a power module, and a method for manufacturing the power module substrate can be provided.

It is a schematic explanatory drawing of the power module using the board | substrate for power modules which is embodiment of this invention. It is explanatory drawing which shows the board | substrate for power modules which is embodiment of this invention. FIG. 3 is an enlarged explanatory diagram of a circuit layer surface in the power module substrate of FIG. 2. In embodiment of this invention, it is upper surface explanatory drawing which shows the measuring method of the electrical resistance value P of the thickness direction of Ag baking layer. In embodiment of this invention, it is side explanatory drawing which shows the measuring method of the electrical resistance value P of the thickness direction of Ag baking layer. It is a flowchart which shows the manufacturing method of Ag paste. It is a flowchart which shows the manufacturing method of the power module of FIG.

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 shows a power module according to an embodiment of the present invention.
The power module 1 includes a power module substrate 10 on which a circuit layer 12 is disposed, a semiconductor chip 3 bonded to the surface of the circuit layer 12 via a solder layer 2, and a cooler 40.

The power module substrate 10 includes a ceramic substrate 11 constituting an insulating layer, a circuit layer 12 disposed on one surface of the ceramic substrate 11 (upper surface in FIG. 1), and the other surface of the ceramic substrate 11 (FIG. 1 and a metal layer 13 disposed on the lower surface.
The ceramic substrate 11 prevents electrical connection between the circuit layer 12 and the metal layer 13, and is made of highly insulating AlN (aluminum nitride). In addition, the thickness of the ceramic substrate 11 is set within a range of 0.2 to 1.5 mm, and in this embodiment is set to 0.635 mm.

  The circuit layer 12 is formed by bonding a conductive metal plate to one surface of the ceramic substrate 11. In the present embodiment, the circuit layer 12 is formed by joining an aluminum plate made of a rolled plate of aluminum (so-called 4N aluminum) having a purity of 99.99% or more to the ceramic substrate 11.

  The metal layer 13 is formed by bonding a metal plate to the other surface of the ceramic substrate 11. In the present embodiment, the metal layer 13 is formed by joining an aluminum plate made of a rolled plate of aluminum (so-called 4N aluminum) having a purity of 99.99% or more, like the circuit layer 12, to the ceramic substrate 11. Has been.

  The cooler 40 is for cooling the power module substrate 10 described above. The top plate portion 41 joined to the power module substrate 10 and the top plate portion 41 are suspended downward. The heat radiation fin 42 and the flow path 43 for distribute | circulating a cooling medium (for example, cooling water) are provided. The cooler 40 (top plate portion 41) is preferably made of a material having good thermal conductivity, and in this embodiment, is made of A6063 (aluminum alloy).

  In the present embodiment, a buffer layer 15 made of aluminum, an aluminum alloy, or a composite material containing aluminum (for example, AlSiC) is provided between the top plate portion 41 of the cooler 40 and the metal layer 13. Yes.

In the power module 1 shown in FIG. 1, an Ag fired layer 30 is formed on the surface of the circuit layer 12 (upper surface in FIG. 1) by firing an Ag paste described later. A semiconductor chip 3 is bonded to the surface of 30 via a solder layer 2. Here, examples of the solder material for forming the solder layer 2 include Sn—Ag, Sn—In, and Sn—Ag—Cu.
As shown in FIG. 1, the Ag fired layer 30 is not formed on the entire surface of the circuit layer 12 but is selectively formed only on a portion where the semiconductor chip 3 is disposed.

2 and 3 show the power module substrate 10 before the semiconductor chip 3 is joined via the solder layer 2.
In the power module substrate 10, the aforementioned Ag fired layer 30 is formed on the surface of the circuit layer 12 (upper surface in FIGS. 2 and 3). As shown in FIG. 3, the Ag fired layer 30 is in a state before the semiconductor chip 3 is bonded via the solder layer 2, and a lead-free glass layer 31 formed on the circuit layer 12 side and the lead-free glass layer 31. And an Ag layer 32 formed thereon. In the lead-free glass layer 31, fine conductive particles 33 having a particle size of about several nanometers are dispersed. The conductive particles 33 are crystalline particles containing at least one of Ag and Al. The conductive particles 33 in the lead-free glass layer 31 are observed by using, for example, a transmission electron microscope (TEM).

  Furthermore, the electrical resistance value P in the thickness direction of the Ag fired layer 30 is set to 0.5Ω or less. Here, in this embodiment, the electrical resistance value P in the thickness direction of the Ag fired layer 30 is an electrical resistance value between the upper surface of the Ag fired layer 30 and the upper surface of the circuit layer 12. This is because the electric resistance of 4N aluminum constituting the circuit layer 12 is very small compared to the electric resistance in the thickness direction of the Ag fired layer 30. In the measurement of the electrical resistance, as shown in FIGS. 4 and 5, the upper surface center point of the Ag fired layer 30 and the distance from the upper surface center point of the Ag fired layer 30 to the edge of the Ag fired layer 30. The electrical resistance between the point on the circuit layer 12 that is separated from the end of the Ag fired layer 30 by H by H is measured.

  In the present embodiment, since the circuit layer 12 is made of aluminum having a purity of 99.99%, the aluminum oxide film 12A naturally generated in the atmosphere is formed on the surface of the circuit layer 12 (upper surface in FIG. 3). However, the aluminum oxide film 12A is removed at the portion where the Ag fired layer 30 is formed, and the Ag fired layer 30 is formed directly on the circuit layer 12. That is, the aluminum constituting the circuit layer 12 and the lead-free glass layer 31 are directly joined.

  Here, the thickness to of the aluminum oxide film 12A that naturally occurs on the circuit layer 12 is set to 4 nm ≦ to ≦ 6 nm. In the present embodiment, the lead-free glass layer 31 has a thickness tg of 0.01 μm ≦ tg ≦ 5 μm, the Ag layer 32 has a thickness ta of 1 μm ≦ ta ≦ 100 μm, and the total thickness tg + ta of the Ag fired layer 30 is 1. .01 μm ≦ tg + ta ≦ 105 μm.

Next, the Ag paste constituting the Ag fired layer 30 will be described.
This Ag paste contains an Ag powder, a lead-free glass powder containing ZnO, a resin, a solvent, and a dispersant, and the content of the powder component composed of the Ag powder and the lead-free glass powder is It is set as 60 mass% or more and 90 mass% or less of the whole Ag paste, and the remainder is made into resin, a solvent, and a dispersing agent. In the present embodiment, the content of the powder component composed of Ag powder and lead-free glass powder is set to 85% by mass of the entire Ag paste.
The viscosity of this Ag paste is adjusted to 10 Pa · s or more and 500 Pa · s or less, more preferably 50 Pa · s or more and 300 Pa · s or less.

The Ag powder has a particle size of 0.05 μm or more and 1.0 μm or less. In this embodiment, an Ag powder having an average particle size of 0.8 μm was used.
The lead-free glass powder contains Bi ₂ O ₃ , ZnO, and B ₂ O ₃ as main components, and has a glass transition temperature of 300 ° C. or higher and 450 ° C. or lower, a softening temperature of 600 ° C. or lower, and a crystallization temperature. 450 ° C. or higher. The weight ratio A / G between the weight A of the Ag powder and the weight G of the lead-free glass powder is adjusted within a range of 80/20 to 99/1. In this embodiment, A / G = 80 / It was set to 5.

A solvent having a boiling point of 200 ° C. or higher is suitable. In this embodiment, diethylene glycol dibutyl ether is used.
The resin is used to adjust the viscosity of the Ag paste, and a resin that decomposes at 500 ° C. or higher is suitable. In this embodiment, ethyl cellulose is used.
In this embodiment, a dicarboxylic acid-based dispersant is added. In addition, you may comprise Ag paste, without adding a dispersing agent.

Here, the lead-free glass powder used in the present embodiment will be described in detail. The glass composition of the lead-free glass powder in this embodiment is
Bi ₂ O ₃ : 68 mass% or more and 93 mass% or less,
ZnO: 1% by mass or more and 20% by mass or less,
B ₂ O ₃ : 1% by mass or more and 11% by mass or less,
SiO ₂ : 5% by mass or less,
Al ₂ O ₃ : 5% by mass or less,
Fe ₂ O ₃ : 5% by mass or less,
CuO: 5 mass% or less,
CeO ₂ : 5% by mass or less,
ZrO ₂ : 5% by mass or less,
Alkali metal oxide: 2% by mass or less,
Alkaline earth metal oxide: 7% by mass or less,
It is said that.

That is, Bi ₂ O ₃ , ZnO, B ₂ O ₃ is an essential component, and this includes SiO ₂ , Al ₂ O ₃ , Fe ₂ O ₃ , CuO, CeO ₂ , ZrO ₂ , Li ₂ O, Na ₂ O, Alkali metal oxides such as K ₂ O and alkaline earth metal oxides such as MgO, CaO, BaO and SrO are appropriately added as necessary. Here, the reason why these oxides are within the above-mentioned range is shown below.

(Bi ₂ O ₃ )
Bi ₂ O ₃ is a main raw material of the lead-free glass powder, and is an essential component for lowering the melting point. When the content of Bi ₂ O ₃ is less than 68 wt%, the softening temperature becomes high. On the other hand, when the content of Bi ₂ O ₃ exceeds 93% by mass, the crystallization temperature is lowered. For this reason, the content of _Bi 2 _{O 3} and 68 wt% or more 93 wt% or less.

(ZnO)
ZnO has the effect of suppressing crystallization of lead-free glass containing Bi ₂ O ₃ . When the content of ZnO is less than 1% by mass, the effect of suppressing crystallization cannot be sufficiently achieved. On the other hand, when the content of ZnO exceeds 20% by mass, the softening temperature becomes high. For this reason, content of ZnO was made into 1 mass% or more and 20 mass% or less.

(B ₂ O ₃ )
B ₂ O ₃ is an essential component for vitrification. If the content of B ₂ O ₃ is less than 1% by mass, vitrification becomes insufficient and crystallization occurs easily. On the other hand, when the content of B ₂ O ₃ exceeds 11% by mass, the softening temperature is increased. For this _reason, the content of B 2 _{O 3} and 1 wt% to 11 wt% or less.

(SiO ₂ )
SiO ₂ has an effect of suppressing chemical durability and crystallization of glass. When the content of SiO ₂ exceeds 5% by mass, the glass transition temperature becomes high and firing at a low temperature cannot be performed. Therefore, the content of SiO ₂ was 5 mass% or less.

(Al ₂ O ₃ , ZrO ₂ )
Al ₂ O ₃ and ZrO ₂ have the effect of suppressing the chemical durability and crystallization of glass. When the content of Al ₂ O ₃ or the content of ZrO ₂ exceeds 5% by mass, the glass transition temperature becomes high and vitrification becomes difficult. Therefore, content of _Al 2 _{O 3,} and ZrO ₂ content of 5 wt% or less, respectively.

(Fe ₂ O ₃ , CuO, CeO ₂ )
Fe ₂ O ₃ , CuO, and CeO ₂ have an effect of suppressing crystallization of lead-free glass containing Bi ₂ O ₃ . When the content of Fe ₂ O _3, the content of CuO, and the content of CeO ₂ exceed 5% by mass, the glass transition temperature becomes high and vitrification becomes difficult. For this reason, the content of Fe ₂ O _3, the content of CuO, and the content of CeO ₂ were each 5% by mass or less.

(Alkali metal oxide)
The alkali metal oxide is contained as a raw material-derived component. When the total content of the alkali metal oxides exceeds 2% by mass, crystallization becomes easy and vitrification becomes difficult. For this reason, the sum total of content of the alkali metal oxide was made into 2 mass% or less.

(Alkaline earth metal oxide)
The alkaline earth metal oxide is contained in order to finely adjust the physical properties of the glass. When the total content of the alkaline earth metal oxides exceeds 7% by mass, the glass transition temperature becomes high and vitrification becomes difficult. For this reason, the total content of alkaline earth metal oxides was set to 7% by mass or less.

Such a lead-free glass powder containing ZnO is produced as follows. The above-mentioned various oxides, carbonates or ammonium salts are used as raw materials. This raw material is charged into a platinum crucible, an alumina crucible, a quartz crucible or the like and melted in a melting furnace. Although there is no restriction | limiting in particular in melting conditions, It is preferable to set it as the range of 900 degreeC or more and 1300 degrees C or less and 30 minutes or more and 120 minutes or less so that all the raw materials may be mixed uniformly by a liquid phase.
The obtained melt is dropped on carbon, steel, copper plate, twin rolls, water, etc., and rapidly cooled to produce a uniform glass lump.
The glass lump is pulverized with a ball mill, a jet mill or the like, and coarse particles are classified to obtain a lead-free glass powder. Here, in this embodiment, the center particle diameter d50 of the lead-free glass powder is set in the range of 0.1 μm or more and 5.0 μm or less.

Next, the manufacturing method of Ag paste is demonstrated with reference to the flowchart shown in FIG. First, the above-described Ag powder and lead-free glass powder are mixed to produce a mixed powder (mixed powder forming step S1). Moreover, a solvent and resin are mixed and an organic mixture is produced | generated (organic substance mixing process S2).
Then, the mixed powder, the organic mixture, and the dispersant are premixed by a mixer (preliminary mixing step S3). Next, the preliminary mixture is mixed while being kneaded using a roll mill (kneading step S4). The obtained kneading is filtered with a paste filter (filtration step S5). In this way, the aforementioned Ag paste is produced.

Next, the manufacturing method of the power module 1 which is this embodiment is demonstrated with reference to the flowchart shown in FIG.
First, an aluminum plate to be the circuit layer 12 and an aluminum plate to be the metal layer 13 are prepared, and these aluminum plates are laminated on one surface and the other surface of the ceramic substrate 11 through brazing materials, respectively, and pressed. -The said aluminum plate and the ceramic substrate 11 are joined by cooling after a heating (circuit layer joining process S11). The brazing temperature is set to 640 ° C to 650 ° C.

  Next, the cooler 40 (top plate portion 41) is joined to the other surface side of the metal layer 13 via the buffer layer 15 via the brazing material (cooler joining step S12). Note that the brazing temperature of the cooler 40 is set to 590 ° C to 610 ° C.

  And the above-mentioned Ag paste is apply | coated to the surface of the circuit layer 12 (Ag paste application | coating process S13). In addition, when apply | coating Ag paste, various means, such as a screen printing method, an offset printing method, and a photosensitive process, are employable. In this embodiment, the Ag paste is formed in a pattern by a screen printing method.

In a state where the Ag paste is applied to the surface of the circuit layer 12, the Ag paste is charged into the heating furnace and the Ag paste is fired (firing step S <b> 14). The firing temperature at this time is set to 350 ° C. to 645 ° C.
By this firing step S <b> 14, the Ag fired layer 30 including the lead-free glass layer 31 and the Ag layer 32 is formed. At this time, the lead-free glass layer 31 melts and removes the aluminum oxide film 12 </ b> A naturally generated on the surface of the circuit layer 12, and the lead-free glass layer 31 is formed directly on the circuit layer 12. In addition, fine conductive particles 33 having a particle size of about several nanometers are dispersed inside the lead-free glass layer 31. The conductive particles 33 are crystalline particles containing at least one of Ag and Al, and are presumed to have precipitated in the lead-free glass layer 31 during firing.

  Thus, the power module substrate 10 having the Ag fired layer 30 formed on the surface of the circuit layer 12 and the power module substrate with a cooler are produced.

Then, the semiconductor chip 3 is placed on the surface of the Ag fired layer 30 via a solder material, and soldered in a reduction furnace (solder joining step S15). At this time, part or all of the Ag layer 32 of the Ag fired layer 30 is melted in the solder layer 2 formed of the solder material.
Thereby, the power module 1 in which the semiconductor chip 3 is bonded onto the circuit layer 12 through the solder layer 2 is produced.

  In the power module substrate 10 and the power module 1 according to the present embodiment configured as described above, the circuit layer 12 is configured by brazing an aluminum plate having a purity of 99.99% or more to the ceramic substrate 11. Since the Ag fired layer 30 made of a fired body of Ag paste containing lead-free glass is formed on the surface, the solder layer 2 can be formed on the Ag fired layer 30 to join the semiconductor chip 3. That is, it is not necessary to form a Ni plating film as in the prior art, and the semiconductor chip 3 can be bonded onto the circuit layer 12 relatively easily and reliably. Therefore, the trouble which arises when forming a Ni plating film is lost, and the power module 1 can be produced satisfactorily.

Here, the Ag fired layer 30 includes a lead-free glass layer 31 and an Ag layer 32 formed on the lead-free glass layer 31, as shown in FIG. The naturally generated aluminum oxide film 12A can be melted and removed in the lead-free glass layer 31, and the lead-free glass layer 31 (Ag fired layer 30) can be formed directly on the surface of the circuit layer 12.
Since the fine conductive particles 33 having a particle size of about several nanometers are dispersed inside the lead-free glass layer 31, the lead-free glass layer 31 can also have conductivity. Specifically, in this embodiment, the electrical resistance value P in the thickness direction of the Ag fired layer 30 including the lead-free glass layer 31 is set to 0.5Ω or less.
Therefore, electricity can be reliably conducted between the semiconductor chip 3 and the circuit layer 12 via the Ag fired layer 30 and the solder layer 2, and the highly reliable power module 1 can be configured.

  Further, the Ag paste constituting the Ag fired layer 30 contains Ag powder and lead-free glass powder containing ZnO, and the softening temperature of the lead-free glass powder is set to 600 ° C. or less. It is possible to fire the Ag paste at a low temperature. Specifically, the firing temperature can be set to 350 ° C. or higher and 645 ° C. or lower. Therefore, it is possible to prevent problems such as deterioration of the circuit layer 12 and reduction in bonding strength between the circuit layer 12 and the ceramic substrate 11 due to firing of the Ag paste, and the high-quality power module substrate 10 and power module can be prevented. 1 can be produced.

  Furthermore, since the viscosity of the Ag paste is adjusted to 10 Pa · s or more and 500 Pa · s or less, more preferably 50 Pa · s or more and 300 Pa · s or less, in the Ag paste application step S13 for applying the Ag paste to the surface of the circuit layer 12 Thus, it is possible to apply a screen printing method or the like, and the Ag fired layer 30 can be selectively formed only on the portion where the semiconductor chip 3 is disposed. Therefore, the amount of Ag paste used can be reduced, and the manufacturing costs of the power module substrate 10 and the power module 1 can be greatly reduced.

In this embodiment, since the ceramic substrate 11 made of AlN (aluminum nitride) having excellent insulation and strength is used as the insulating layer, the reliability of the power module substrate 10 can be improved. Further, the circuit layer 12 can be easily formed by brazing an aluminum plate on the ceramic substrate 11.
Furthermore, in the present embodiment, the cooler 40 is disposed on the other side (lower side in FIG. 1) of the ceramic substrate 11 with the metal layer 13 and the buffer layer 15 interposed therebetween. It can prevent that the power module 1 becomes high temperature.

As mentioned above, although embodiment of this invention was described, this invention is not limited to this, It can change suitably in the range which does not deviate from the technical idea of the invention.
For example, the metal plate constituting the circuit layer and the metal layer has been described as a rolled plate of pure aluminum having a purity of 99.99%, but is not limited to this, and aluminum having a purity of 99% (2N aluminum) It may be sufficient if it is comprised with aluminum or aluminum alloy.

Moreover, about the raw material of Ag paste, and the compounding quantity, it is not limited to what was described in embodiment, You may use another lead-free glass powder, resin, a solvent, and a dispersing agent. The softening temperature should just be below melting | fusing point of aluminum, More preferably, it is 600 degrees C or less.
As the resin, an acrylic resin, an alkyd resin, or the like may be used. Further, α-terpineol, butyl carbitol acetate or the like may be used as the solvent.

Further, the thickness ratio between the lead-free glass layer and the Ag layer in the formed Ag fired layer is not limited to that illustrated in FIG.
Further, it is described that uses a ceramic substrate made of AlN as an insulating layer, is not limited thereto, it may be used a ceramic substrate made of Si ₃ N ₄ or Al ₂ O _3, or the like, insulating The insulating layer may be made of resin.

  Moreover, while brazing the aluminum board used as a circuit layer to a ceramic substrate, and brazing a cooler, it demonstrated as what forms an Ag baking layer on a circuit layer, but it is not limited to this, The Ag fired layer may be formed before brazing the aluminum plate to the ceramic substrate or before brazing the cooler.

Moreover, although demonstrated as what provided the buffer layer which consists of aluminum, the aluminum alloy, or the composite material containing aluminum (for example, AlSiC etc.) between the top plate part of the cooler and the metal layer, this buffer layer is not provided. Also good.
Furthermore, although demonstrated as what comprised the top-plate part of the cooler with aluminum, you may be comprised with the composite material etc. which contain aluminum alloy or aluminum. Furthermore, although it demonstrated as a cooler having the radiation fin and the flow path of a cooling medium, there is no limitation in particular in the structure of a cooler.

Below, the result of the confirmation experiment performed in order to confirm the effect of this invention is demonstrated.
As Example 1 of the present invention, the power module substrate described in the above embodiment was prepared. That is, Invention Example 1 was obtained by forming an Ag fired layer on a circuit layer made of an aluminum plate having a purity of 99.99% or more. As lead-free glass powder of this time, the _Bi 2 _{O 3} 90.6 wt%, the ZnO 2.6 _wt%, the B 2 _{O 3} 6.8 wt%, with lead-free glass powder containing. Further, ethyl cellulose was used as the resin, and diethylene glycol dibutyl ether was used as the solvent. Furthermore, a dicarboxylic acid-based dispersant was added.
As a conventional example, the power module substrate described in the above-described embodiment was prepared by forming a 5 μm thick Ni plating film on the surface of the circuit layer instead of the Ag fired layer.

With respect to these inventive example 1 and the conventional example, the solder wettability was evaluated. The evaluation of the solder wettability was performed by a spread test defined in JIS Z 3197. The spreading rate S is determined by measuring the height h of a solder layer formed by placing a ball-shaped solder material having a diameter d on a sample (Ag fired layer or Ni plating film) and heating it to a predetermined temperature. , (D−h) / d × 100. The larger the spreading ratio S, the better the wettability with the solder material, and the stronger the semiconductor chip can be bonded via the solder layer.
Here, in this example, the height of the solder layer after being held at 350 ° C. for 0.1 hour was measured using a Pb-10 wt% Sn material as the solder material. The results are shown in Table 1.

In the conventional example in which the Ni plating film is formed, the spreading rate S is 67 to 70%. On the other hand, in the present invention example 1 in which the Ag sintered layer is formed, the spreading rate S is 70 to 72%, and it is confirmed that the solder wettability is equal to or higher than that of the conventional example. The
Therefore, also in the present invention example 1 in which the Ag fired layer is provided on the circuit layer, it is possible to configure the power module by joining the semiconductor chip via the solder layer as in the conventional example in which the Ni plating film is formed. It was confirmed that.

Next, the result of measuring the electrical resistance value P in the thickness direction of the Ag fired layer in the power module substrate described in the above embodiment will be described.
While changing the Ag layer thickness ta and the lead-free glass layer thickness tg of the Ag fired layer, the weight ratio A / G between the weight A of the Ag powder and the weight G of the lead-free glass powder was changed, and Examples 2-6 of the present invention Was produced. As lead-free glass powder, the _Bi 2 _{O 3} 90.6 wt%, the ZnO 2.6 _wt%, the B 2 _{O 3} 6.8 wt%, with lead-free glass powder containing. Further, ethyl cellulose was used as the resin, and diethylene glycol dibutyl ether was used as the solvent. Furthermore, a dicarboxylic acid-based dispersant was added. Further, the Ag fired layer was formed into a square shape having a side length of 15 mm as viewed from above.
For the samples of Invention Examples 2 to 6 obtained in this way, using the tester (manufactured by KEITHLEY: 2010 MULTITIMER) according to the method described in FIG. 4 and FIG. The resistance value P was measured. The measurement results are shown in Table 2.

  In any of Invention Examples 2 to 6, it was confirmed that the electrical resistance value P in the thickness direction of the Ag fired layer was 0.5Ω or less. Moreover, it was confirmed that the electrical resistance value P in the thickness direction of the Ag fired layer is reduced by forming the lead-free glass powder in a small proportion and forming the lead-free glass layer with a small thickness tg.

Next, the composition of the lead-free glass powder was changed to adjust the softening temperature, crystallization temperature, and glass transition temperature of the lead-free glass powder. Using these lead-free glass powders, an Ag paste was constructed.
The weight ratio A / G between the weight A of the Ag powder and the weight G of the lead-free glass powder was 97.5 / 2.5. Moreover, Ag powder, lead-free glass powder, resin (ethyl cellulose), and a solvent (diethylene glycol dibutyl ether) were mixed. In this example, the addition amount of the powder component, the resin and the solvent is adjusted so that the content of the powder component composed of Ag powder and lead-free glass powder is 85 mass%, the resin is 3 mass%, and the solvent is 12 mass%. did.

Here, the softening temperature, glass transition temperature, and crystallization temperature of the above lead-free glass powder were measured. The measurement results are shown in Tables 3 and 4.
The softening temperature was the temperature of the yield point (deflection point) by performing thermomechanical analysis (TMA) using Thermoflex manufactured by Rigaku Corporation.
The glass transition temperature and the crystallization temperature were measured by performing differential thermal analysis (TG-DTA) using TG-TDA2000S manufactured by BRUKER AXS. The lead-free glass powder is heated at 10 ° C./min, the temperature of the endothermic peak appearing in the calorie change curve is defined as the glass transition temperature, and the temperature of the exothermic peak generated in the temperature region higher than the glass transition temperature is defined as the crystallization temperature. .

  Then, using this Ag paste, the Ag fired layer 30 of the power module substrate 10 disclosed in the above embodiment was formed. In the Ag paste application step S13 in which the Ag paste is applied to the surface of the circuit layer 12, the application thickness of the Ag paste is set to 10 μm. In the firing step S14, the firing temperature was 575 ° C. and the firing time was 10 minutes. Thereby, the Ag fired layer 30 in which the thickness ta of the Ag layer 32 was about 8 μm and the thickness tg of the lead-free glass layer 31 was about 1 μm was formed.

Then, the semiconductor chip 3 was placed on the surface of the Ag fired layer 30 via a solder material (Sn—Ag—Cu lead-free solder), and soldered in a reduction furnace.
In this embodiment, the ceramic substrate 11 is 30 mm × 20 mm, the circuit layer 12 is 13 mm × 10 mm, and the semiconductor chip 3 is 12.5 mm × 9.5 mm.

The joining reliability of the power module 1 obtained in this way was evaluated. As an evaluation of the bonding reliability, the bonding rates after 2000 times of the thermal cycle (−45 ° C. to 125 ° C.) were compared. The results are shown in Tables 3 and 4.
In addition, the joining rate was computed with the following formula | equation. Here, the initial bonding area is an area to be bonded before bonding.
Bonding rate = (initial bonding area-peeling area) / initial bonding area When the bonding rate is greater than 85% and 100% or less, “excellent”
"Good" with a joining rate greater than 75% and less than 85%
Bonding rate is greater than 50% and 75% or less
A bonding rate of 50% or less was regarded as “impossible”.

About Examples 1-8, the joining rate after repeating a thermal cycle 2000 times is favorable, and it is confirmed that sufficient joining strength is obtained.
In Samples A to E, although the practicability is satisfactory in the joining rate after 2000 cycles of the cooling and heating cycle, the joining rate is lower than in Examples 1 to 8. In particular, in the samples D and E in which ZnO is not contained in the lead-free glass powder, the crystallization temperatures are lowered to 330 ° C. and 403 ° C., and the joining rate is greatly reduced.

1 Power Module 2 Solder Layer 3 Semiconductor Chip (Semiconductor Element)
DESCRIPTION OF SYMBOLS 10 Power module substrate 11 Ceramic substrate 12 Circuit layer 12A Aluminum oxide film 30 Ag baking layer 31 Lead-free glass layer 32 Ag layer 33 Conductive particle 40 Cooler

Claims (9)

A power module substrate in which a circuit layer made of aluminum or an aluminum alloy is disposed on one surface of an insulating layer, and a semiconductor element is disposed on the circuit layer via a solder layer,
A power module substrate, wherein an Ag fired layer made of a fired body of an Ag paste containing ZnO-containing lead-free glass is formed on a surface of the circuit layer on which the solder layer is formed .   2. The power module according to claim 1, wherein the Ag fired layer includes a lead-free glass layer formed on the circuit layer and an Ag layer formed on the lead-free glass layer. Substrate.   3. The power module substrate according to claim 1, wherein the Ag fired layer has an electric resistance value P in the thickness direction set to 0.5Ω or less. 4. The glass composition of the lead-free glass is
Bi ₂ O ₃ : 68 mass% or more and 93 mass% or less,
ZnO: 1% by mass or more and 20% by mass or less,
B ₂ O ₃ : 1% by mass or more and 11% by mass or less,
SiO ₂ : 5% by mass or less,
Al ₂ O ₃ : 5% by mass or less,
Fe ₂ O ₃ : 5% by mass or less,
CuO: 5 mass% or less,
CeO ₂ : 5% by mass or less,
ZrO ₂ : 5% by mass or less,
Alkali metal oxide: 2% by mass or less,
Alkaline earth metal oxide: 7% by mass or less,
The power module substrate according to any one of claims 1 to 3, wherein the power module substrate is configured as described above. 5. The power module substrate according to claim 1, wherein the insulating layer is an insulating substrate selected from AlN, Si ₃ N _4, or Al ₂ O ₃ .   A power module substrate according to any one of claims 1 to 5, and a cooler disposed on the other surface side of the insulating layer of the power module substrate. A power module board with a cooler.   A power module substrate according to any one of claims 1 to 5, and a semiconductor element disposed on one surface side of the circuit layer of the power module substrate, the semiconductor element Is a power module characterized by being joined via a solder layer. A method for producing a power module substrate in which a circuit layer made of aluminum or an aluminum alloy is disposed on one surface of an insulating layer, and a semiconductor element is disposed on the circuit layer via a solder layer,
An application step of applying an Ag paste containing lead-free glass to a surface of the circuit layer on which the solder layer is formed; a baking step of baking the Ag paste by heat treatment in a state where the Ag paste is applied; With
A method for producing a power module substrate, comprising forming an Ag fired layer made of a fired body of an Ag paste containing lead-free glass on a surface of the circuit layer on which the solder layer is formed .   The method for producing a power module substrate according to claim 8, wherein a firing temperature in the firing step is 350 ° C. or more and 645 ° C. or less.

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