JP2014220541A - Method for manufacturing substrate for power module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a substrate for a power module capable of joining a ceramic substrate and a metal layer in a short time.SOLUTION: A laminate 30 formed by laminating a ceramic substrate 2 and metal layers 6, 7 by interposing a bonding material between them, and a cushion sheet 40 having a carbon layer 42 formed on both surfaces of a graphite layer 41 are laminated with each other and heated in a state where an increase in the height of these laminates is restricted, thereby manufacturing a substrate for a power module by joining the ceramic substrate 2 and the metal layers 6, 7.

Description

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

As a conventional power module, a metal layer serving as a circuit layer is laminated on one surface of a ceramic substrate, and an electronic component such as a semiconductor chip is soldered on the circuit layer, and on the other surface of the ceramic substrate. A structure in which a metal layer to be a heat dissipation layer is formed and a heat sink is joined to the heat dissipation layer is known.
Patent Document 1 proposes a power module substrate with a heat sink in which an aluminum heat sink is directly bonded to a ceramic substrate without providing a metal layer serving as a heat dissipation layer.

When a metal layer to be a circuit layer or a heat dissipation layer is bonded to a ceramic substrate in a laminated state, as shown in Patent Document 2, a laminate in which a metal layer is laminated on a ceramic substrate via a brazing material and a cushion sheet There is known a method of brazing a ceramic substrate and a metal layer by alternately superimposing them in the thickness direction and heating them while applying pressure by applying a biasing force such as a spring.
The cushion sheet is used to apply pressure evenly to the joint surface between the ceramic substrate and the metal layer, and a dense carbon layer is formed on both surfaces of the graphite cushion layer.
In this power module substrate manufacturing apparatus, guide posts are vertically fixed at the four corners of the base plate, and a pressure plate (pressure plate) parallel to the base plate is supported by the guide posts so as to be movable up and down. A plurality of springs are provided between the pressing plate and the fixed plate fixed to the upper end portion of the guide post to pressurize the stacked laminate and the cushion sheet.

JP 2008-205372 A JP 2008-192823 A

  However, in the apparatus for manufacturing a power module substrate described in Patent Document 1, a plurality of springs and fixing plates are used in addition to the base plate and the pressing plate in order to pressurize the laminated body and the cushion sheet that are arranged in an overlapping manner. Since these are large enough to cover the entire surface of the power module substrate, the heat capacity of the entire apparatus is increased. Therefore, there is a problem that it takes time until all the laminated bodies including this manufacturing apparatus are uniformly heated.

  This invention is made | formed in view of such a situation, Comprising: The manufacturing method of the board | substrate for power modules which can join a ceramic substrate and a metal layer in a short time is provided.

  A manufacturing apparatus suitable for use in the method for manufacturing a power module substrate according to the present invention is a method of bonding a power module substrate by pressing a laminated body in which a metal layer is laminated on a ceramic substrate through a bonding material at a high temperature. An apparatus for manufacturing, comprising: a cushion sheet disposed on both surfaces of the laminate; and a fixing means for restraining an increase in the laminate height of the laminate and the cushion sheet accompanying thermal expansion during heating. In addition, the cushion sheet is formed by forming carbon layers on both sides of the graphite layer.

A graphite layer with a large coefficient of thermal expansion is provided inside the cushion sheet, so if you hold the cushion sheet laminated on both sides of the laminate and heat it while it is heated, the graphite layer will try to thermally expand. On the other hand, since the stacking height of the laminate and the cushion sheet is restrained by the fixing means, the laminate is pressed in the stacking direction by the thermal expansion force. Therefore, the laminate can be heated in a pressurized state and can be firmly bonded. In addition, since the graphite layer is relatively soft and has cushioning properties, the laminate can be uniformly pressed following local thickness unevenness and surface irregularities of the ceramic substrate.
Thus, since it is the structure which restrains the thermal expansion of a laminated body and a cushion sheet with a fixing means, without providing pressurizing means, such as a spring, it is set as a simple structure using a bolt, a nut, etc., for example. Can do. For this reason, the heat capacity of the entire apparatus can be reduced, the heating time required for bonding can be shortened, and the ceramic substrate and the metal layer can be bonded in a short time.
In addition, the surface which contacts a laminated body is made into the comparatively hard surface by the carbon layer, and can detach | leave from a laminated body easily.

  Further, in the manufacturing apparatus, the fixing means includes a base plate on which the laminated body and the cushion sheet are placed, a plurality of guide posts attached to the upper surface of the base plate along the vertical direction, and the base plate It is comprised by the press plate arrange | positioned on the said laminated body and the said cushion sheet | seat stacked on top, and the stopper attached to the upper end part of the said guide post, and restrict | regulating the upward movement of the said press plate. Good.

  Furthermore, in the manufacturing apparatus, a male screw portion is formed at the upper end portion of the guide post, and the stopper may be constituted by a nut that is screwed into the male screw portion of the guide post.

  And the manufacturing method of the board | substrate for power modules of this invention laminates | stacks the laminated body which laminated | stacked the ceramic substrate and the metal layer through the bonding material, and the cushion sheet in which the carbon layer was formed on both surfaces of the graphite layer. The power module substrate is manufactured by bonding the ceramic substrate and the metal layer by heating in a state where the increase in the stacking height is constrained.

  According to the present invention, the heat capacity of the entire manufacturing apparatus can be reduced, and the laminate of the ceramic substrate and the metal layer can be uniformly pressed and heated quickly. Can be joined in a short time.

It is a longitudinal cross-sectional view which shows the power module using the board | substrate for power modules. It is a side view which shows the manufacturing apparatus of the board | substrate for power modules of this invention. It is sectional drawing which shows a cushion sheet.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 shows a power module 1 to which a power module substrate 3 manufactured according to the present invention is applied. The power module 1 in FIG. 1 is bonded to a power module substrate 3 having a ceramic substrate 2, an electronic component 4 such as a semiconductor chip mounted on the surface of the power module substrate 3, and the back surface of the power module substrate 3. The heat sink 5 is made up of.

  In the power module substrate 3, metal layers 6 and 7 are laminated on both surfaces of the ceramic substrate 2, one of the metal layers 6 becomes a circuit layer, and the electronic component 4 is bonded to the surface thereof. The other metal layer 7 is a heat radiating layer, and the heat sink 5 is attached to the surface thereof.

The ceramic substrate 2 is made of, for example, nitride ceramics such as AlN (aluminum nitride), Si ₃ N ₄ (silicon nitride), or oxide ceramics such as Al ₂ O ₃ (alumina), and the thickness thereof is For example, it is set to 0.635 mm.
The metal layers 6 and 7 are each made of aluminum having a purity of 99.90% by mass or more. According to JIS standards, 1N90 (purity 99.90% by mass or more: so-called 3N aluminum) or 1N99 (purity 99.99% by mass or more). : So-called 4N aluminum) can be used.
The power module substrate 3 is formed so as to be thicker than the metal layer 6 serving as a circuit layer because the metal layer 7 serving as a heat dissipation layer has a buffer function.

In the power module substrate 3 of the present embodiment, for example, the thickness of the metal layer 6 that is a circuit layer is 0.6 mm, and the thickness of the metal layer 7 that is a heat dissipation layer is 1.5 mm.
These metal layers 6 and 7 are formed into a desired external shape by etching after bonding a material punched into a desired external shape by pressing to the ceramic substrate 2 or joining a flat plate to the ceramic substrate 2. Either method can be employed.

  The ceramic substrate 2 and the metal layers 6 and 7 serving as the circuit layer and the heat dissipation layer are brazed with a brazing material such as Al—Si, Al—Ge, Al—Cu, Al—Mg, or Al—Mn. Has been.

  For joining the metal layer 6 and the electronic component 4, a solder material such as Sn—Ag—Cu, Zn—Al, or Pb—Sn is used. Reference numeral 8 in the figure indicates the solder joint layer. The electronic component 4 and the terminal portion of the metal layer 6 are connected by a bonding wire (not shown) made of aluminum or the like.

On the other hand, the shape and the like of the heat sink 5 are not particularly limited, but are formed by extrusion molding of an aluminum alloy, and a large number of flow paths 16 are formed along the length direction for circulating cooling water.
As a joining method between the heat sink 5 and the metal layer 7 serving as a heat dissipation layer, an Al—Si based, Al—Ge based, Al—Cu based, Al—Mg based, or Al—Mn based alloy brazing material is used. Brazing method, Nocolok brazing method using flux for Al-Si based brazing material, Ni plating on metal layer and heat sink, Sn-Ag-Cu based, Zn-Al or Pb-Sn based solder A method of soldering with a material is used, or it is mechanically fixed with screws in a state of being in close contact with silicon grease. FIG. 1 shows a brazed example.

An embodiment according to the present invention will be described for a manufacturing apparatus 100 for a power module substrate 3 used in the power module 1.
As shown in FIG. 2, the power module substrate manufacturing apparatus 100 of this embodiment is formed by laminating a ceramic substrate 2 and metal layers 6 and 7 via a brazing material foil (not shown) as a bonding material. A cushion sheet 40 disposed on both surfaces of the laminate 30, a fixing means 110 for holding the laminate 30 and the cushion sheet 40 in a stacked state, and a heating furnace (not shown). It is supposed to be configured.

The fixing means 110 includes a base plate 111, guide posts 112 attached to the four corners of the upper surface of the base plate 111 along the vertical direction, and a pressing plate 113 supported on the upper ends of the guide posts 112 so as to be movable up and down. A nut 114 is provided that is a stopper that restricts the upward movement of the pressing plate 113 by screwing into a male screw portion 112 a provided at the upper end portion of the guide post 112.
The pressing plate 113 is disposed in parallel with the base plate 111. The laminate 30 and the cushion sheet 40 are alternately laminated between the base plate 111 and the pressing plate 113, and the thickness direction (stacking direction) is held at a predetermined interval depending on the position of the nut 114. .

Specifically, after the laminated body 30 of the ceramic substrate 2 and the metal layers 6 and 7 is stacked on the base plate 111 via the cushion sheet 40, the pressing plate 113 is disposed thereon, and the pressing plate The nut 114 is screwed into the male threaded portion 112a of the guide post 112 protruding above the 113, and the nut 114 is slightly moved from the position where the lower surface of the nut 114 is in contact with the upper surface of the pressing plate 113 or from the contact position. The state is tightened.
The guide post 112 is formed of a composite material including carbon fiber having a very small thermal expansion coefficient.

As shown in FIG. 3, the cushion sheet 40 is formed in a state where carbon layers 42 are laminated on both surfaces of a graphite layer 41 by brazing.
The graphite layer 41 is formed by attaching scaly graphite thin films like a plurality of mica, and is formed by rolling natural graphite into a sheet shape after acid treatment. In this case, the thermal expansion coefficient of the graphite layer 41 is about 5 × 10 ⁻⁶ / ° C. in the direction parallel to the surface, but is approximately 2 × 10 ⁻⁴ / in the direction orthogonal to the surface (stacking direction). It is a big one with ℃.
The carbon layer 42 is formed of a thin carbon plate and is fired at a high temperature of about 3000 ° C.

That is, the cushion sheet 40 has a configuration in which the fired carbon layer 42 is disposed on both sides of the unfired graphite layer 41, and the internal graphite layer 41 has a bulk density of 0.5 Mg / m ³ or more. Although it is soft at ³ Mg / m ³ or less, the carbon layer 42 on the surface is formed on a relatively hard and smooth plane having a bulk density of 1.6 Mg / m ³ or more and 1.9 Mg / m ³ or less.
In addition, the thickness of the graphite layer 41 shall be 0.5 mm or more and 5.0 mm or less, and the thickness of the carbon layer 42 of both surfaces shall be 0.5 mm or more and 5.0 mm or less.

  For the brazing of the graphite layer 41 and the carbon layer 42, a brazing material having a melting point higher than that of the brazing material foil of the laminated body 30 that does not melt when the laminated body 30 is brazed is used. Examples of such a brazing material include a Cu—Ni—Mn brazing material having a melting point of 1273 to 1373K, and a Cu—Ni—M (Si, Cr, Mg, Co) —Mn brazing material. M (Si, Cr, Mg, Co) means containing at least one of Si, Cr, Mg, and Co.

  The cushion sheet 40 and the laminated body 30 thus formed are alternately laminated, and the fixing means 110 that holds them in the thickness direction are placed in a heating furnace and heated at a predetermined brazing temperature, whereby each laminated layer is obtained. The power module substrate 3 can be manufactured by brazing the ceramic substrate 2 of the body 30 and the metal layers 6 and 7.

At this time, since the graphite layer 41 having a large thermal expansion coefficient is provided inside the cushion sheet 40, the graphite layer 41 is heated when the cushion sheet 40 is heated while being laminated on both surfaces of the laminate 30. However, since the stacking height of the laminate 30 and the cushion sheet 40 is restrained by the nut 114, the laminate 30 is pressed in the stacking direction by the thermal expansion force. It becomes. Therefore, the laminated body 30 can be heated in a pressurized state and can be firmly bonded.
In addition, since the graphite layer 41 is relatively soft and has cushioning properties, the laminated body can be uniformly pressed following local thickness unevenness and surface irregularities of the ceramic substrate 2.

Thus, since it can be set as the simple structure of restraining thermal expansion of the laminated body 30 and the cushion sheet 440 with the nut 114, without providing pressurizing means, such as a spring, the thermal capacity of the whole apparatus is made small. The heating time required for joining can be shortened, and the ceramic substrate 2 and the metal layers 6 and 7 can be joined in a short time.
In addition, the surface of the cushion sheet 40 that contacts the laminated body 30 is a relatively hard surface by the carbon layer 42, and can be easily detached from the laminated body 30.

In order to confirm the effect of the present invention, a power module substrate was manufactured using the configuration of the above-described embodiment. Further, as a comparative example, a power module substrate was manufactured using a manufacturing apparatus provided with a spring biasing structure as proposed in Patent Document 1, and evaluated.
In the manufacturing apparatus of the comparative example, in addition to the base plate, the guide post, and the pressing plate, as a biasing structure by the spring, a spring and a fixing plate that supports the back portion of the spring are provided. A nut 114 is provided instead of the biasing structure by the spring and the fixing plate.

The laminated body is formed of a 28 mm square aluminum metal layer (circuit layer thickness 0. 0 mm) on both sides of a square ceramic substrate made of aluminum nitride having a plate thickness of 0.635 mm and a side of 30 mm with an Al-Si alloy brazing foil. 6 mm, the thickness of the heat radiation layer 1.5 mm). Further, as the cushion sheet, a 35 mm square sheet obtained by brazing a 1 mm thick carbon layer on both sides of a 1 mm thick graphite layer was used. And the cushion sheet | seat was laminated | stacked on both surfaces of each laminated body, it arrange | positioned between the base board and the press board, and the heat processing was performed, and the board | substrate for 15 units of power modules was manufactured, respectively.
In addition, in the manufacturing apparatus of the Example, it manufactured by tightening a nut to such an extent that a laminated body and a cushion sheet were not pressurized at normal temperature. In the manufacturing apparatus of the comparative example, the spring has to act an urging force by the pressing force in the stacking direction by the spring, was used as a ^{0.5 × 10 5 Pa~5 × 10 5} Pa.

  As a result, in the manufacturing apparatus of the comparative example, the power module substrate was manufactured with the manufacturing apparatus of the example, where heating time of 5 to 6 hours was required until the bonding force of the power module substrate that can be used properly was obtained. In this case, it was found that the time can be shortened to about 4 hours.

In addition, this invention is not limited to the said embodiment, A various change can be added in the range which does not deviate from the meaning of this invention.
For example, in the above embodiment, the cushion sheet is configured by brazing the carbon layer of the plate material to the graphite layer, but any cushion sheet can be used as long as it can be configured without restricting the thermal expansion of the graphite layer. The surface of the layer may be coated with a carbon layer, or may be formed by other methods.
In addition, the cushion sheet is not only a three-layer structure in which carbon layers are provided on both sides of the graphite layer, as in the above embodiment, but a plurality of graphite layers and carbon layers are alternately laminated, for example, You may comprise a five-layer structure. Further, when a plurality of cushion sheets are stacked, all the cushion sheets do not have to have the same configuration, and a cushion sheet having a thick graphite layer, a cushion sheet having a thin graphite layer, or the like may be mixed.
In addition, as the fixing means, a nut is fastened to the male thread portion of the guide post, but a clamp that can be fixed at an arbitrary position of the guide post may be used instead of the nut.
Further, in the above embodiment, aluminum having a purity of 99.90% or more is used for the metal layer, but aluminum or aluminum alloy having a purity of 99% or more may be used. Moreover, although the same material was used for the metal layer used as the circuit layer and the heat dissipation layer, both metal layers are not limited to this, and the circuit layer and the heat dissipation layer may be made of different materials. For example, aluminum can be used for the heat dissipation layer and copper can be used for the circuit layer.
Furthermore, in the above embodiment, a power module substrate is configured by laminating a metal layer serving as a circuit layer and a heat dissipation layer on both sides of the ceramic substrate, and a heat sink is attached to the heat dissipation layer. The power module substrate may be configured by bonding a heat sink directly to the back surface of the ceramic substrate by brazing or the like without providing it.

  Further, the ceramic substrate and the metal layer may be bonded by diffusion bonding (Transient Liquid Phase Diffusion Bonding). The bonding method will be briefly described. After forming a fixed layer containing Ag on the metal layer by sputtering, the fixed layer is laminated in contact with the ceramic substrate, and the laminate is pressurized and heated. As a result, Ag diffuses into the metal layer and lowers the melting point in the vicinity of the fixed layer of the metal layer, thereby forming a molten metal layer at the interface between the ceramic substrate and the metal layer. Further, as diffusion proceeds, the Ag concentration of the molten metal layer decreases and the melting point increases, so that it solidifies, and the ceramic substrate and the metal layer are joined.

DESCRIPTION OF SYMBOLS 1 Power module 2 Ceramic substrate 3 Power module substrate 4 Electronic component 5 Heat sink 6,7 Metal layer (aluminum metal layer)
DESCRIPTION OF SYMBOLS 8 Solder joint layer 16 Flow path 30 Laminate body 40 Cushion sheet 41 Graphite layer 42 Carbon layer 100 Power module board manufacturing apparatus 110 Fixing means 111 Base plate 112 Guide post 112a Male thread portion 113 Press plate 114 Nut (stopper)

Claims (1)

  A laminate in which a ceramic substrate and a metal layer are laminated with a bonding material interposed therebetween, and a cushion sheet in which a carbon layer is formed on both sides of a graphite layer are laminated, and heating is performed while restraining the increase in the lamination height. By doing this, the said ceramic substrate and the said metal layer are joined, and the board | substrate for power modules is manufactured, The manufacturing method of the board | substrate for power modules characterized by the above-mentioned.

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