JP5786569B2 - 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 dissipation layer were generally formed of the same plate material, but in order to have a buffer function for relaxing the thermal expansion and contraction between the heat dissipation layer and the heat sink, the heat dissipation layer itself, It has been studied to form the heat dissipation layer thick.

Patent Document 1 proposes a power module substrate in which metal layers having different thicknesses are laminated on both surfaces of a ceramic substrate, and the metal layer side having a large volume is concave (the metal layer side having a small volume is convex. It is disclosed that warping occurs.
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

As described above, if both metal layers have the same composition and the same properties, the entire power module substrate warps through the heat treatment for brazing due to the difference in thickness between the circuit layer and the heat dissipation layer. As a result, there is a problem that it is difficult to join the heat sink thereafter.
Further, the technique described in Patent Document 2 has a problem that the materials of the two metal layers must be changed, and cannot be applied when using the same material.

  The present invention has been made in view of such circumstances, and in the case where metal layers having different thicknesses are laminated on both surfaces of a ceramic substrate, warpage occurring at the time of bonding can be reduced, and bonding reliability can be reduced. It aims at providing the manufacturing method of the board | substrate for power modules which can improve property.

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. After these are heated and joined, the joined body is cooled in a state of being pressed in the thickness direction between parallel plates to cause plastic deformation in the metal layer, and then returned to room temperature and then pressed. It is characterized by solving .

The difference in thermal expansion and contraction is large in metal layers with different thicknesses, so when they are bonded to both sides of a ceramic substrate, the stress that causes warping with the thin metal layer side protruding in the cooling process after heat bonding Will occur. In this invention, after joining both metal layers and a ceramic substrate, generation | occurrence | production of curvature is suppressed by cooling in the state pressurized in the thickness direction. When the metal layer and the ceramic substrate are joined and cooled down to a low temperature below room temperature, the stress due to thermal shrinkage of the thick metal layer increases, but the metal layer and the ceramic substrate are pressed and restrained. In this state, the warp cannot be deformed. 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, by cooling both metal layers and the ceramic substrate in a state of being pressurized in the thickness direction, both metal layers can be caused to undergo plastic deformation opposite to the warp due to thermal expansion and contraction, and the power module. Warpage of the entire substrate can be reduced.

In the method for manufacturing a power module substrate according to the present invention, the ratio of the thickness of the thin metal layer to the thick metal layer laminated on both surfaces of the ceramic substrate is 0.2 or more and 0.9 or less. It is good to cool in the range of −5 ° C. or lower and −70 ° C. or higher in a state of being pressurized at 9.8 × 10 ⁴ Pa or more and 343 × 10 ⁴ Pa or less in the direction.
Thus, by setting the conditions of pressurization and cooling in accordance with the ratio of the thicknesses of both metal layers, the occurrence of warpage that occurs in the cooling process is prevented, and the bonding reliability of the power module substrate is improved. be able to.

  ADVANTAGE OF THE INVENTION According to this invention, when the metal layer of different thickness is laminated | stacked on both surfaces of a ceramic substrate, the curvature which generate | occur | produces by joining can be reduced and manufacture of the board | substrate for power modules which can improve the reliability of joining. A method can be provided. Moreover, since it can apply even if both metal layers are the same material, it can apply to manufacture of the board | substrate for various power modules.

It is a longitudinal cross-sectional view which shows the whole structure of the power module using the board | substrate for power modules of embodiment of this invention. It is a front view which shows the example of the pressurization apparatus used with the manufacturing method of this invention.

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 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 back surface of the power module substrate 3. The heat sink 5 is joined.

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), oxide ceramics such as Al ₂ O ₃ (alumina), SiC (silicon carbide), or the like. It is formed of carbide-based ceramics and has a thickness of 0.635 mm, for example.
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 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. .2 or more and 0.9 or less.
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.
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 1.6 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.

Further, the heat sink 5 has an appropriate shape, such as a flat plate, one in which a large number of pin-shaped fins are integrally formed by hot forging or the like, and one in which strip-like fins are formed in parallel by extrusion. Can be adopted.
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.

The power module 1 of the present embodiment configured as described above is manufactured as follows.
First, a metal layer 6 serving as a circuit layer is laminated on one surface of the ceramic substrate 2 via a brazing material foil, and a metal layer 7 serving as a heat dissipation layer is also laminated on the other surface of the ceramic substrate 2 via a brazing material foil. To do. And this laminated body S is installed in the pressurization apparatus 110 shown in FIG.
The pressure device 110 includes a base plate 111, guide posts 112 vertically attached to the four corners of the upper surface of the base plate 111, a fixed plate 113 fixed to the upper ends of the guide posts 112, and the base plates 111. A pressing plate 114 supported by a guide post 112 so as to freely move up and down between the fixing plate 113 and a spring provided between the fixing plate 113 and the pressing plate 114 to urge the pressing plate 114 downward. And urging means 115.
The fixed plate 113 and the pressing plate 114 are arranged in parallel to the base plate 111, and the above-described laminate S is arranged between the base plate 111 and the pressing plate 114. Carbon sheets 116 are disposed on both sides of the laminate S in order to make the pressure uniform.
In a state where the laminate S is pressurized by the pressurizing device 110, the pressurizing device 110 is installed in a heating furnace (not shown) and brazed by heating to a brazing temperature of, for example, 630 ° C. in a vacuum atmosphere. Thus, the power module substrate 3 is manufactured.

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. In the cooling process, when the cooling is further advanced to a low temperature of room temperature or lower, the deformation (warping amount) due to thermal shrinkage further increases.
In the present embodiment, the power module substrate on which the warp with the metal layer 6 side convex is applied with a load so as to correct the warp and cooled in a state of being pressurized in the thickness direction. . At that time, it is cooled within a range of −5 ° C. or lower and −70 ° C. or higher in a state of being pressurized at 9.8 × 10 ⁴ Pa (1 kg / cm ² ) or more and 343 × 10 ⁴ Pa (35 kg / cm ² ) or less. Is preferred. When cooling the power module substrate in a pressurized state, if the entire pressure device shown in FIG. 2 is put into the cooler, the power module substrate normally reaches the ambient temperature in about 5 minutes. In order to cool the power module substrate to its cooling temperature, it may be left for about 5 minutes after being put into the cooler. More preferably, it is good to cool at -10 degreeC--20 degreeC for 10 minutes.

In this case, since both the metal layers 6 and 7 and the ceramic substrate 2 are pressed in the thickness direction and are restrained so as not to warp, there appears to be no change during cooling. The metal layers 6 and 7 are plastically deformed by being pressed against the stress. The plastic deformation of both the metal layers 6 and 7 is a deformation in the opposite direction to the warp. When cooled to the plastic deformation region and returned to room temperature, the warp is offset by the amount of plastic deformation, and at normal temperature before cooling. The warp at normal temperature after cooling is smaller than the warp that has occurred. Therefore, the stress at normal temperature after cooling is smaller than the stress at normal temperature before cooling.
Thus, by cooling both the metal layers 6 and 7 and the ceramic substrate 2 in a state where they are pressed in the thickness direction, both the metal layers 6 and 7 undergo plastic deformation opposite to the warp due to thermal expansion and contraction. The overall warpage of the power module substrate 3 can be reduced.

Next, examples and comparative examples performed for confirming the effects of the present invention will be described.
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 6 mm. The ratio of the thicknesses of these metal layers (circuit layer thickness / heat dissipation layer thickness) is 0.375. 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 were bonded using an Al—Si brazing material having a thickness of 10 μm to 15 μm.
In manufacturing 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, 7 laminated with each other are heated in a vacuum atmosphere at 600 ° C. to 650 ° C. for about 1 hour while being pressed in the thickness direction. 6 and 7 were brazed and joined, and power module substrates of Samples 1 to 15 were manufactured.

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 the whole curvature about the board | substrate for power modules after these joining was measured. Table 1 shows the flatness per 28 mm length with these measurement results as the amount of warpage after joining.
Next, each sample 1-15 was subjected to three types of processing for improving the flatness, and the amount of warpage of each sample 1-15 after the improvement of the flatness was measured. Samples 1 to 5 were flattened by a method in which pressure was applied in the thickness direction at 53.9 × 10 ⁴ Pa (5.5 kg / cm ² ) after bonding, the pressure state was maintained for 20 minutes, and then the pressure was released. Improved. Samples 6 to 10 were cooled to -70 ° C with the pressure released after bonding, held for 20 minutes so that the entire temperature was constant, and then returned from -70 ° C to room temperature. . Moreover, about the samples 11-15, after joining, it pressurizes to thickness direction by 53.9 * 10 < ⁴ > Pa (5.5 kg / cm < ² >), it cools to -70 degreeC in the pressurization state, cooling and pressurization After holding for 20 minutes, the temperature was returned from −70 ° C. to room temperature, and the pressure was released. Table 1 shows the measurement results of the warpage amount after improving the flatness of these samples 1 to 15.
The flatness was measured by placing a power module substrate on a surface plate and using a laser displacement meter from above. The flatness change rate Z is calculated by Z = (X−Y) / X × 100 (%), where X is the flatness after bonding, and Y is the flatness after the flatness is improved. Value.

As can be seen from Table 1, with respect to Samples 11 to 15, by applying a predetermined load and cooling in the pressurized state, the amount of warpage was greatly reduced as compared to before flatness improvement. As a method for improving flatness, the amount of warpage was reduced when only pressing (samples 1 to 5) or only cooling (samples 6 to 10) was performed, but when pressing and cooling were performed simultaneously (sample) Compared with 11-15), the effect was small.
In the above description, the ceramic substrate and both metal layers are cooled to -70 ° C., but may be cooled to a temperature range where plastic deformation occurs in the metal layer. The degree of cooling 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 preferable to cool to at least −10 ° C. to −20 ° C. and hold this state for 10 minutes.

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 pressurization is once released after the ceramic substrate and the two metal layers are joined, and the pressurization is performed again in the cooling process. Then, cooling may be performed.

The ceramic substrate and the metal layer can be joined by soldering in addition to brazing. Moreover, you may join by the transient liquid phase joining method called TLP joining method (Transient Liquid Phase Bonding). In this transient liquid phase bonding method, a copper layer deposited on the surface of the metal layer is interposed at the interface between the metal layer and the ceramic substrate. By heating, copper diffuses into the aluminum of the metal layer, the copper concentration in the vicinity of the copper layer of the metal layer increases and the melting point decreases, and a metal liquid phase forms at the bonding interface in the eutectic region of aluminum and copper Is done. If the temperature is kept constant in a state in which this metal liquid phase is formed, the metal liquid phase reacts with the ceramic substrate, and copper further diffuses into the aluminum, so that the copper concentration in the metal liquid phase Gradually decreases, the melting point rises, and solidification proceeds with the temperature kept constant. As a result, a strong bond between the metal layer and the ceramic substrate can be obtained.
Moreover, the method of joining a ceramic substrate and a metal layer using an active metal brazing material can also be employ | adopted. For example, an active metal brazing material (Ag-27.4% by mass Cu-2.0% by mass Ti) containing Ti, which is an active metal, is used in a state where a laminate of a metal layer and a ceramic substrate is pressurized. By heating with Ti, the active metal Ti is preferentially diffused into the ceramic substrate, and the metal layer and the ceramic substrate can be joined via the Ag-Cu alloy.

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 110 Pressurizing device 111 Base plate 112 Guide post 113 Fixing plate 114 Press plate 115 Energizing means 116 Carbon sheet

Claims (2)

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 these are heated and bonded, and then bonded The power module substrate is characterized in that the body is cooled in a state of being pressed in the thickness direction between parallel plates to cause plastic deformation in the metal layer, and then the pressure is released after returning to room temperature . Production method. In the manufacturing method of the board | substrate for power modules of Claim 1, The ratio of the thickness of the thin metal layer with respect to the thick metal layer laminated | stacked on both surfaces of the said ceramic substrate is comprised by 0.2 or more and 0.9 or less, A method for producing a substrate for a power module, characterized by cooling in a range of −5 ° C. or lower and −70 ° C. or higher in a state of being pressurized at 9.8 × 10 ⁴ Pa or more and 343 × 10 ⁴ Pa or less in the thickness direction. .

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