JP2012009787A - Substrate for power module and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a substrate for a power module that can prevent warpage caused by temperature variation under heating or cooling and enhance reliability to joint and a method of manufacturing thereof .SOLUTION: In a substrate for a power module in which metal layers 6, 7 having different thicknesses are laminated on both surfaces of a ceramic substrate 2, the average grain size of crystal particles is smaller in the thinner metal layer 6 than that in the thicker metal layer 7, whereby occurrence of warpage caused by a heating treatment such as brazing or the like or a subsequent temperature environment can be prevented and thus the reliability to joint can be enhanced.

Description

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

  As a conventional power module, an aluminum metal layer 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, while the other surface of the ceramic substrate is There is known a structure in which a metal layer of aluminum serving as a heat dissipation layer is formed and a heat sink is joined to the metal layer.

  As a method for forming an aluminum metal layer serving as a circuit layer or a heat dissipation layer on such a ceramic substrate, for example, in Patent Document 1, an Al—Si based or Al—Ge based brazing material is interposed in a ceramic substrate. Then, the aluminum metal layer is overlaid, and the laminated body is pressurized and heated to melt the brazing material and to join the ceramic substrate and the aluminum metal layer.

In this case, the circuit layer and the heat dissipation layer are generally formed of the same plate material. However, in recent years, the heat dissipation layer itself has a buffer function for relaxing thermal expansion and contraction between the heat dissipation layer and the heat sink. Therefore, it has been studied to form the heat dissipation layer thick. As a result, there is a difference in the thickness between the circuit layer and the heat dissipation layer. Therefore, when the heat treatment for brazing is performed, the entire thin circuit layer side is warped, and the heat sink becomes The problem of obstructing the attachment of the device has arisen.
In addition to the heat applied during manufacturing such as brazing, the same warp occurs in the environment of use as long as it is used for a long time at a relatively high temperature of, for example, 50 ° C.

  As a joining method that does not depend on brazing, there is a technique described in Patent Document 2. In this joining method, an aluminum metal layer is formed on both surfaces of a ceramic substrate by placing the ceramic substrate in a mold and injecting a molten aluminum alloy into the mold.

JP 2008-311296 A JP 2007-36263 A

However, even in the circuit layer and the heat dissipation layer formed in this way, since the aluminum is melted and solidified, the crystal grain size becomes as large as several hundred μm, for example, and warping has occurred.
This invention is made | formed in view of such a situation, Comprising: The curvature which generate | occur | produces at the time of a heating or cooling is eliminated, and the board | substrate for power modules which can improve the reliability of joining, and its manufacturing method are provided.

  Since the heat dissipation layer itself is provided with a buffer function, the heat dissipation layer is formed thicker than the circuit layer. Therefore, if both metal layers have the same composition and the same properties, as described above, the thin circuit layer side Warping occurs. Here, when the circuit layer and the heat dissipation layer are formed of metal layers having different crystal grain sizes and the same thickness, the inventors of the present application may warp the metal layer having a larger crystal grain size. I found. And when laminating metal layers with different thicknesses on both sides of the ceramic substrate, by reducing the crystal grain size of the thin metal layer than the thick metal layer, the warp caused by the difference in thickness and the crystal It has been found that the warpage caused by the difference in particle size can be offset and the overall warpage can be eliminated.

The power module substrate of the present invention is a power module substrate in which metal layers having different thicknesses are laminated on both surfaces of a ceramic substrate, and the average grain size of the crystal grains constituting both metal layers is a thick metal. The metal layer thinner than the layer is formed smaller.
In the power module substrate of the present invention, the ratio of the thin metal layer to the thick metal layer is 0.2 or more and 0.9 or less, and the ratio of the average grain size of the corresponding crystal grains is 0.01 or more and 1 It may be the following.

  In the power module substrate of the present invention, more preferably, the ratio of the thickness of the thin metal layer to the thick metal layer is 0.2 or more and 0.8 or less, and the ratio of the average grain size of the corresponding crystal grains is It is good that it is 0.1 or more and 0.8 or less.

  In this way, the circuit layer and the metal layer serving as the heat dissipation layer are configured with crystal grain sizes that match the respective thicknesses, thereby preventing warping caused by heat treatment such as brazing or the subsequent temperature environment. Thus, the bonding reliability of the power module substrate can be improved.

  The power module of the present invention is characterized in that an electronic component is joined to one of the metal layers of the power module substrate by soldering, and a heat sink is joined to the other metal layer.

  The method for manufacturing a power module substrate of the present invention is a method for manufacturing a power module substrate in which metal layers having different thicknesses are laminated on both surfaces of a ceramic substrate, and is an average of crystal grains constituting both metal layers. The particle size is made smaller in the metal layer thinner than the thick metal layer, and both the metal layers are heated and bonded in a state where the both metal layers are disposed on both sides of the ceramic substrate via the brazing material. Features.

  According to the present invention, by using a metal layer having a different crystal grain size in accordance with the thickness of the metal layer having a different thickness for the power module substrate, warping due to heat treatment such as brazing or the subsequent temperature environment. It is possible to improve the bonding reliability.

It is a longitudinal section showing the whole power module composition of an embodiment of the present invention. It is a figure explaining the curvature resulting from the thickness and crystal grain diameter of a metal layer. It is a graph which shows the relationship between the crystal grain size ratio and curvature amount of a power module substrate.

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

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 soldered to the surface. 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.
Both the metal layers 6 and 7 are made of aluminum having a purity of 99.9 wt% or higher. According to JIS standards, 1N90 (purity 99.9 wt% or higher: so-called 3N aluminum) or 1N99 (purity 99.99 wt% or higher: 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 this case, the thickness of the metal layers 6 and 7 serving as the circuit layer and the heat dissipation layer is such that the ratio of the thickness of the metal layer 6 to the metal layer 7 (the thickness of the metal layer 6 / the thickness of the metal layer 7) is 0. The average grain size of the crystal grains forming these metal layers 6 and 7 is the ratio (average grain diameter of metal layer 6 / average grain diameter of metal layer 7). Is 0.01 or more and 1 or less. More preferably, the ratio of the thickness of the metal layer 6 to the metal layer 7 is 0.2 or more and 0.8 or less, and the ratio of the average grain size of the corresponding crystal grains is 0.1 or more and 0.8 or less. Good.
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 1.6 mm. The thickness ratio is 0.375. In the case of the exemplified thickness, for example, it is preferable that the average particle diameter of the metal layer 6 is 0.5 mm and the average particle diameter of the metal layer 7 is 1.0 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 power module substrate 3 of the present embodiment is an example in which the thickness of the metal layer 7 serving as a heat dissipation layer is thicker than the thickness of the metal layer 6 serving as a circuit layer. The thickness of the metal layer 7 to be used may be smaller than the thickness of the metal layer 6 to be the circuit layer. In that case, the ratio of the thickness of the metal layer 7 to the metal layer 6 (the thickness of the metal layer 7 / the thickness of the metal layer 6) is 0.2 or more and 0.9 or less, and the ratio of the corresponding average particle diameter ( The average particle size of the metal layer 7 / the average particle size of the metal layer 6) is 0.01 or more and 1 or less, and more preferably, the ratio of the thickness of the metal layer 7 to the metal layer 6 is 0.2 or more and 0.8. The ratio of the average grain size of the corresponding crystal grains is preferably 0.1 or more and 0.8 or less.
Hereinafter, unless otherwise specified, it is assumed that the thickness of the metal layer 7 is greater than the thickness of the metal layer 6.

The ceramic substrate 2 and the metal layers 6 and 7 serving as a circuit layer and a heat dissipation layer are laminated by brazing. As the brazing material, Al—Si, Al—Ge, Al—Cu, Al—Mg, Al—Mn, or the like is used. The brazing material is made of, for example, a brazing material foil. The brazing material foil is interposed between the ceramic substrate 2 and the two metal layers 6 and 7, and these are pressed in the thickness direction in a vacuum 600 The ceramic substrate and the metal layers 6 and 7 are joined by heating at ˜650 ° C. for about 1 hour.
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, as a joining method between the metal layer 7 serving as a heat dissipation layer and the heat sink 5, a soldering method using a soldering material such as a nocolok brazing method, a Sn—Ag—Cu type, a Zn—Al, or a Pb—Sn type may be used. Used, or mechanically fixed with screws while being in close contact with silicon grease. FIG. 1 shows a brazed example.

  The shape of the heat sink 5 is not particularly limited. The heat sink 5 is formed by extrusion molding of an aluminum alloy, and the cylinder 15 joined to the power module substrate 3 and the inside of the cylinder 15 are divided into a plurality of flow paths 16. The vertical wall 17 is integrally formed. The top plate portion 15 a of the cylindrical body 15 has a rectangular planar shape larger than the metal layer 7 of the power module substrate 3, and the vertical walls 17 are mutually spaced at equal intervals in the width direction of the cylindrical body 15. They are arranged in parallel and are provided along the length direction of the cylindrical body 15.

  In the power module 1, the metal layers 6, 7 serving as circuit layers or heat dissipation layers constituting the power module substrate 3 have different thicknesses, and the metal layers 6, 7 have crystal grain sizes matching the respective thicknesses. By comprising, the generation | occurrence | production of the curvature accompanying the temperature change by heat processing, such as brazing, can be prevented.

In the power module substrate 3 composed of the metal layers 6 and 7 having different thicknesses, if the crystal grain sizes of the metal layers 6 and 7 serving as the circuit layer and the heat dissipation layer are configured to be the same, FIG. As shown in the power module substrate 3a shown in FIG. 2, the thickness of the circuit layers and the metal layers 6a and 7a serving as the heat dissipation layers is different, so that the metal can be used by heat treatment such as brazing or use in a high temperature environment. Warpage is generated with the layer 6a side convex. Further, as in the power module substrate 3b shown in FIG. 2B, when the metal layers 6 and 7 serving as the circuit layer and the heat dissipation layer are formed of the metal layers 6a and 7b having different crystal grain sizes and the same thickness. Causes a warp in which the metal layer 7b side having a large crystal grain size protrudes.
Therefore, in the power module substrate 3 of the present embodiment, as in the power module substrate 3c shown in FIG. 2C, the grain size of the thin metal layer 6a is smaller than that of the thick metal layer 7c. By reducing the diameter, the warpage caused by the difference in thickness and the warpage caused by the difference in crystal grain size are offset, and the problem of warpage is solved.

  FIG. 3 shows the relationship between the crystal grain size ratio of the metal layer serving as the circuit layer and the heat dissipation layer constituting the power module substrate and the amount of warpage of the power module substrate.

A 30 mm square metal layer having an aluminum purity of 99.99 wt% was used, the thickness of the metal layer serving as the circuit layer was 0.6 mm, and the thickness of the metal layer serving as the heat dissipation layer was 1.6 mm. The ratio of the thicknesses of these metal layers (circuit layer thickness / heat dissipation layer thickness) is 0.375. The ceramic substrate was made of AlN and had a thickness of 0.635 mm. The metal layer and the ceramic substrate were joined by heating to 630 ° C. using an Al—Si brazing material having a thickness of 10 μm to 15 μm. The overall warpage of the power module substrate thus bonded was measured, and the amount of warpage per 28 mm length was shown in the graph.
Table 1 below shows data of a particle size ratio of 0.2 (Example 1) and a particle size ratio of 0.67 (Example 2) on the graph of FIG.
In Examples 3 to 5 in Table 2 below, each metal layer when the ratio of the average particle diameter of both metal layers (the average particle diameter of the circuit layer / the average particle diameter of the heat dissipation layer) is 0.1. The warpage amount of the power module substrate with respect to the thickness ratio (thickness ratio) is shown.

The crystal grain size was measured by macro-etching the metal layer with Barrett's solution (mixed solution of hydrochloric acid, nitric acid and hydrofluoric acid) and evaluating the area ratio using a polarizing microscope. As a method for obtaining the crystal grain size from the area ratio, an intercept method was used in which the crystal grain size was estimated from the number of particles cut by diameter on the material surface or the cut surface.
Moreover, the used metal layer heat-processed on the arbitrary processing conditions for 1 to 24 hours in the range of 200-400 degreeC with the incinerator, and produced the thing from which a crystal grain diameter ratio differs.

As can be seen from FIG. 3, when the crystal grain size of both metal plates is the same, the warpage amount was about 125 μm, whereas the grain size ratio was 0.1. Is suppressed to about 65 μm. Thus, the warp accompanying a temperature change can be suppressed by using a metal layer having a different crystal grain size depending on the thickness of the metal layer having a different thickness for the power module substrate.
As is clear from Table 2, when the difference in crystal grain size between the metal layers is such that the particle size ratio between the two metal layers is 0.1, the thickness of the corresponding metal layer is also large. In the case (Example 3 in Table 2), the amount of warpage is minimized. Thus, the warp accompanying a temperature change can be suppressed by making the particle size ratio and the thickness ratio of the two metal layers correspond to each other and achieving a balance.
In addition, although the ratio of the crystal grain size of both the metal layers 6 and 7 of the above-described Examples 1 to 5 was described in the state before the brazing joint, as shown in Table 1 even in the state after the brazing joint Each crystal grain size becomes large, but the ratio of crystal grain size hardly changes.

As mentioned above, although embodiment of this invention was explained in full detail with reference to drawings, the concrete structure is not restricted to this embodiment, The design change etc. of the range which does not deviate from the summary of this invention are included.
In the above embodiment, the metal layer is brazed to the ceramic substrate, but the bonded surfaces of the ceramic substrate and the metal layer are activated by irradiation with an ion beam in vacuum, and these are bonded. Direct bonding at room temperature is also possible.

DESCRIPTION OF SYMBOLS 1 Power module 2 Ceramic substrate 3, 3a, 3b, 3c Power module substrate 4 Electronic component 5 Heat sink 6, 6a, 7, 7a, 7b, 7c Metal layer 8 Solder joint layer 15 Cylindrical body 15a Top plate part 16 Flow path 17 Vertical wall

Claims (5)

  A power module substrate in which metal layers with different thicknesses are laminated on both sides of a ceramic substrate, and the average particle size of crystal grains constituting both metal layers is thinner than the thick metal layer. A power module substrate characterized by being formed small.   The ratio of the thickness of the thin metal layer to the thick metal layer is 0.2 or more and 0.9 or less, and the ratio of the average grain size of the corresponding crystal grains is 0.01 or more and 1 or less. The power module substrate according to claim 1.   The ratio of the thickness of the thin metal layer to the thick metal layer is 0.2 or more and 0.8 or less, and the ratio of the average grain size of the corresponding crystal grains is 0.1 or more and 0.8 or less. The power module substrate according to claim 2.   An electronic component is joined to one of the metal layers of the power module substrate according to any one of claims 1 to 3 by soldering, and a heat sink is joined to the other metal layer. Power module. A method for manufacturing a power module substrate in which metal layers having different thicknesses are laminated on both sides of a ceramic substrate, wherein the average grain size of crystal grains constituting both metal layers is smaller than that of a thick metal layer. A method for manufacturing a substrate for a power module, characterized in that the metal layer is formed in a small size, and both the metal layers are heated and bonded in a state of being disposed on both surfaces of the ceramic substrate via a brazing material.

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