JP5678684B2 - Power module substrate manufacturing method - Google Patents

Description

  The present invention relates to a method for manufacturing a power module substrate in which metal layers having different thicknesses are laminated on both surfaces of a ceramic substrate.

  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, and the other surface of the ceramic substrate. 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.

  Further, as a method for forming an aluminum metal layer to be a circuit layer or a heat dissipation layer on such a ceramic substrate, an aluminum metal layer is formed by interposing an Al—Si or Al—Ge brazing material in the ceramic substrate. It is known that a ceramic substrate and an aluminum metal layer are joined by melting the brazing material by melting and brazing the laminated body and heating the laminated body.

  In this case, the circuit layer and the heat radiating layer are generally formed of the same plate material, but the heat radiating layer itself has a buffer function for relaxing thermal expansion and contraction between the heat radiating layer and the heat sink. It is considered to form the layer thick. In this case, due to the difference in thickness between the circuit layer and the heat radiating layer, there is a problem that the entire substrate is warped through the heat treatment for brazing, and the subsequent attachment to the heat sink is hindered.

In Patent Document 1, when metal layers having different thicknesses are laminated on both surfaces of a ceramic substrate, warpage occurs in which the metal layer side having a large volume is concave (the metal layer side having a small volume is convex). It is disclosed.
Patent Document 2 discloses that a thick metal layer is made of a material having a smaller deformation resistance than a thin metal layer as a measure for preventing warpage. For example, a thick metal layer is made of aluminum having a purity of 99.99 to 99.9999% or more (so-called 4N aluminum to 6N aluminum), and a thin metal layer is less than purity of 99 to 99.99% or more. Warpage is reduced by constituting the aluminum (so-called 2N aluminum to 4N aluminum).

JP 2002-252433 A JP 2010-93225 A

  However, when the purity of the thin metal layer is lowered, there is a possibility that the solder, which is a bonding layer such as a chip to be mounted, is deteriorated by a thermal cycle, and there is a problem in reliability.

  The present invention has been made in view of such circumstances, and when metal layers having different thicknesses are laminated on both surfaces of a ceramic substrate, even if the same material is used for both metal layers, bonding is possible. Provided is a method for manufacturing a power module substrate, which can reduce warping that sometimes occurs and increase the reliability of bonding.

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, wherein both metal layers are disposed on both surfaces of the ceramic substrate. These are heated and bonded while being pressurized, then cooled to room temperature in a pressurized state, and further cooled in a state where the pressure is released to cause plastic deformation in the metal layer.

Since metal layers with different thicknesses have a large difference in thermal expansion and contraction, when they are bonded to a ceramic substrate, the thin metal layer side is projected due to residual stress generated in the cooling process after heat bonding. Warping occurs.
In this cooling process, if the cooling is further advanced to a low temperature of room temperature or lower, the deformation due to the heat shrinkage further proceeds. However, since both metal layers and the ceramic substrate are bonded and the deformation resistance of the ceramic substrate is large, if the warpage proceeds to a certain extent, it is restrained by the ceramic substrate and cannot be further deformed as warpage. When this is further cooled, plastic deformation occurs in the metal layer. This plastic deformation is the deformation in the direction opposite to the warp.If it is cooled to the plastic deformation region and returned to room temperature, the warpage is offset by the amount of plastic deformation, which is more than the warp that occurred at room temperature before cooling. , Get smaller.
Thus, the warp of the entire power module substrate can be reduced by the warp due to thermal expansion and contraction and the reverse deformation due to plastic deformation.

In the method for manufacturing a power module substrate of the present invention, the metal layer is made of aluminum having a purity of 99.90% by mass or more before being bonded to the ceramic substrate, and the cooling is less than −45 ° C. It is good to do until.
Furthermore, in the method for manufacturing a power module substrate of the present invention, the cooling may be performed from the thick metal layer side.

  In the power module intermediate of the present invention, the heat sink top plate member constituting the heat sink is temporarily fixed to one of the metal layers of the power module substrate manufactured by the above manufacturing method. It is characterized by.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the board | substrate for power modules which can reduce the curvature which generate | occur | produces by joining and can improve the reliability of joining can be provided. Moreover, since it is not necessary to change the material of both metal layers laminated | stacked on a ceramic substrate, problems, such as a solder deterioration by a thermal cycle, can be eliminated.

It is a longitudinal section showing the whole power module composition of an embodiment of the present invention. It is a graph which shows the relationship between temperature and the amount of curvature.

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.
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. In addition, Cu can also be used for the metal layers 6 and 7 besides aluminum.
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 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.
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.
Moreover, the thickness of both the metal layers 6 and 7 is not limited to the above-mentioned thickness, For example, the thing thinner than 0.6 mm and the thing thicker than 1.5 mm are used.

  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, an alloy such as Al—Si, Al—Ge, Al—Cu, Al—Mg, or Al—Mn is used.

  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, an alloy such as an Al—Si, Al—Ge, Al—Cu, Al—Mg, or Al—Mn alloy is used. Brazing method with a material, Nocolok brazing method using flux on Al-Si brazing material, Ni plating on metal layer and heat sink, Sn-Ag-Cu system, Zn-Al or Pb-Sn system, etc. The soldering method 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.

  The shape of the heat sink 5 is not particularly limited, but is formed by extrusion molding of an aluminum alloy, and a top plate member 15a joined to the power module substrate 3 and a bottom plate member 15b joined to the top plate member 15a Thus, the inside of the heat sink 5 is divided into a plurality of flow paths 16 via vertical walls 17. The top plate member 15a has a rectangular planar shape larger than the metal layer 7 of the power module substrate 3, and the vertical walls 17 are arranged in parallel to each other at equal intervals in the width direction of the heat sink 5. It is provided along the length direction of the heat sink 5.

The power module 1 of the present embodiment configured as described above is manufactured as follows.
A metal layer 6 serving as a circuit layer is laminated on one surface of the ceramic substrate 2 made of AlN via a brazing material foil, and a metal layer 7 serving as a heat dissipation layer is also disposed on the other surface of the ceramic substrate 2 via the brazing material foil. Laminate. This laminate and a sheet made of carbon and graphite thin films having cushioning properties and heat resistance are alternately stacked in the stacking direction and placed between pressing means, and these are stacked in the thickness direction (stacking direction). In a vacuum furnace under pressure. And it brazes by heating in this pressurization state.

When the thicknesses of the metal layers 6 and 7 serving as the circuit layer and the heat dissipation layer are different as in the power module substrate 3 shown in FIG. 1, there is a large difference in thermal expansion and contraction due to heating during brazing. Due to the residual stress generated during the cooling process, when the pressure state is released by returning to normal temperature, a warp is formed in which the thin metal layer 6 side is convex.
Therefore, in this cooling process, the cooling (amount of warpage) due to thermal contraction is further increased by further cooling to a low temperature below room temperature. Since both the metal layers 6 and 7 are bonded to the ceramic substrate 2 and the deformation resistance of the ceramic substrate 2 is large, if the warpage proceeds to a certain extent, it is constrained by the ceramic substrate 2 and cannot be further deformed as warpage. . When this is further cooled, it appears that there is no change in appearance, but plastic deformation occurs in the metal layer.
This plastic deformation is a deformation in the opposite direction to the warp. When cooled to the plastic deformation region and returned to room temperature, the warpage is offset by the amount of plastic deformation, and the warpage that occurred at room temperature before cooling However, the warpage at normal temperature after cooling is smaller.
Thus, the warpage of the power module substrate 3 as a whole can be reduced by warpage due to thermal expansion and contraction and reverse deformation due to plastic deformation.

FIG. 2 shows the relationship between the cooling temperature and the amount of warpage of the power module substrate.
A 28 mm square metal layer having an aluminum purity of 99.99 mass% is used for both the circuit layer and the heat dissipation layer, the thickness of the metal layer 6 serving as the circuit layer is 0.6 mm, and the thickness of the metal layer 7 serving as the heat dissipation layer is 1. It was 5 mm. The ceramic substrate 2 was made of AlN and had a thickness of 0.635 mm. The metal layers 6 and 7 and the ceramic substrate 2 used were a power module substrate 3 bonded using an Al—Si brazing material having a thickness of 10 μm to 15 μm.

  In order to fabricate the power module substrate thus configured, first, the back surface of the metal layer 6 and the front surface of the ceramic substrate 2 and the front surface of the metal layer 7 and the back surface of the ceramic substrate 2 are respectively sandwiched by brazing materials. The ceramic substrate 2 and the metal layers 6 and 7 are bonded to each other by heating at 600 to 650 ° C. for about 1 hour while pressing the laminated ceramic substrate 2 and the metal plates 6 and 7 in the thickness direction. .

  Thereafter, the ceramic substrate 2 and the metal layers 6 and 7 were cooled to room temperature while being pressurized, and the pressure was released. In this state, a warp having a convex on the thin metal layer 6 side was recognized. And it cooled to -70 degreeC further in the state which released the pressurization. At this time, the whole temperature was held for 7 minutes so as to be constant, and again returned from −70 ° C. to room temperature. The amount of warpage from the start of cooling to -70 ° C. for 7 minutes and then returning to room temperature was measured. The amount of warpage is shown in a graph by placing the power module substrate 3 on a surface plate in a cooler and measuring the warpage of the power module substrate 3 being cooled from above with a laser displacement meter. .

As is apparent from FIG. 2, the warpage amount was about 150 μm at 0 ° C. (point a) before cooling to −70 ° C., but the deformation due to thermal contraction was increased as the cooling proceeded to a low temperature ( The amount of warpage increased to about 300 μm when cooled to −45 ° C. or lower. And the range which seemed that there was no change appeared in the apparent curvature between this -45 and -70 ° C (b point to c point). After that, the amount of warpage decreased when the temperature was returned to room temperature, and the amount of warpage was about 70 μm at 0 ° C. (point d), which was reduced by about 80 μm from before cooling (point a).
Further, when the amount of warpage was about 180 μm, the amount of warpage at room temperature was about 100 μm when it was similarly cooled to −70 ° C. In this case, the amount of warpage was reduced as compared with that before cooling.
The ceramic substrate 2 and the metal layers 6 and 7 are cooled to −70 ° C. in a state where pressure is released. However, since the metal layer may be cooled to a temperature region where plastic deformation occurs, at least to less than −45 ° C. It only has to be cooled. The degree of cooling below −45 ° C. may be set according to the degree of warpage caused by the area, thickness, material, etc. of the power module substrate.
For example, it is more preferable to cool to −50 ° C.

Further, in order to reduce the warpage amount of the power module substrate, the entire power module substrate may be cooled as in the above-described embodiment, but in order to efficiently cool the thick metal layer 7 side, A cooling gas may be injected from the metal layer 7 side.
Specifically, the nozzle of the cooling spray containing the cooling gas (for example, HFC-134a) was sprayed from the position 5 cm away from the back side of the metal layer 7 for 20 seconds. The power module substrate 3 was cooled to −55 ° C. by the cooling gas, and the amount of warpage that was 180 μm before cooling was reduced to 120 to 130 μm. By blowing the cooling gas from the back side of the metal layer 7 of the power module substrate 3 (the concave side where the warp has occurred), the warp is efficiently warped in a short time compared to the case where the entire power module substrate 3 is cooled. The amount can be reduced.

As described above, according to the present embodiment, it is possible to reduce the warpage caused by the bonding between the ceramic substrate and the metal layer and to increase the reliability of the bonding. In addition, it is done by releasing the pressure from the pressurization and heating state at the time of joining and cooling, so there is no need to change the material of both metal layers, eliminating the problem that the solder deteriorates due to the heat cycle can do.
Further, since the warpage of the power module substrate is reduced, the subsequent mounting to the heat sink can be performed well, and the reliability can be improved even when the power module is formed.

In addition, in order to perform the brazing of the heat sink and the brazing of the power module substrate to the heat sink in a lump, only the top plate member of the heat sink is temporarily fixed to the power module substrate first to In this case, one metal layer of the power module substrate and the top plate member for the heat sink are bonded to each other with a solvent (such as octanediol) or an adhesive via a brazing material. It can be temporarily fixed by ultrasonic welding or the like. This temporary fixing does not have to have a strong adhesive force, but if it is adhered to the extent that it does not accidentally shift or peel before the power module substrate and the heat sink are brazed. Good. And when it is set as a power module, the board | substrate for power modules and a top plate member can be joined with a brazing material using the heat | fever when joining the top plate member and bottom plate member of a heat sink, and heating The power module can be efficiently manufactured by reducing the number of times.
Further, in the power module substrate configured as described above, since the amount of warpage is reduced, a gap between the power module substrate and the top plate member is provided small, such as an intervening solvent. The amount of the adhesive can be reduced and temporarily fixed.

  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.

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

Claims (4)

A method for manufacturing a power module substrate in which metal layers having different thicknesses are laminated on both surfaces of a ceramic substrate, wherein both metal layers are disposed on both surfaces of the ceramic substrate and heated and bonded together while being pressed. A method for producing a substrate for a power module , which is cooled to room temperature in a pressurized state and further cooled in a state where the pressure is released to cause plastic deformation of the metal layer.   2. The power module according to claim 1, wherein the metal layer is made of aluminum having a purity of 99.90 mass% or more before being bonded to the ceramic substrate, and the cooling is performed to less than −45 ° C. 3. Manufacturing method for industrial use. The method of manufacturing a power module substrate according to claim 1 or 2, wherein the cooling is performed from a thick metal layer side. A heat sink top plate member constituting a heat sink is temporarily fixed to one of the metal layers of the power module substrate manufactured by the manufacturing method according to claim 1. An intermediate for power modules.

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