JP2011210746A - Substrate for power module, and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a substrate for power module in which a metal layer and a heat sink are bonded without adversely affecting a junction between a ceramic substrate and the metal layer to improve reliability, and to provide a method of manufacturing the same.SOLUTION: In the substrate for power module 3 in which metal layers 6, 7 are laminated on both sides of the ceramic substrate 2 respectively, and the heat sink 5 is bonded to one metal layer 7, the metal layers 6, 7 contain crystal grains having an average particle diameter equal to or smaller than 0.5 μm, and the heat sink 5 and the metal layer 7 are directly bonded together by putting etched surfaces of both the heat sink 5 and the metal layer 7 together in a vacuum atmosphere after performing a surface cleaning etching on the surfaces of the metal layer 7 and the heat sink 5.

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 to be 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 an aluminum metal layer 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.

On the other hand, as a joining method between the metal layer and the heat sink, there are a method by screwing described in Patent Document 2, a method by brazing described in Patent Document 3, and the like.
Among them, in the method of joining by screwing, it is difficult to closely adhere the joining surface between the metal layer and the heat sink. For this reason, it is fastened with heat conductive grease interposed, but there is a limit to rapid heat transfer.
In addition, the method of bonding by brazing allows the metal layer and the heat sink to be brought into close contact with each other so that heat can be transferred quickly. There is a concern about adverse effects on the joint between the substrate and the metal layer.

  In addition, as a joining method between the metal layer and the heat sink, the application of the Nocolok brazing method is being studied as a flux brazing method that does not require expensive equipment and can be stably brazed relatively easily. In the joining method, at the time of brazing, there is a risk that the flux may erode between the ceramic substrate and the metal layer, causing cracks in the joint.

JP 2008-311296 A JP 2006-203108 A JP 2008-277654 A

  The present invention has been made in view of such circumstances, and can improve reliability by bonding the metal layer and the heat sink without adversely affecting the bonded portion between the ceramic substrate and the metal layer. A power module substrate and a manufacturing method thereof are provided.

The power module substrate of the present invention is a power module substrate in which a metal layer is laminated on both surfaces of a ceramic substrate, and a heat sink is bonded to one of the metal layers. The average particle size is 0.5 μm or less and is directly bonded to the surface of the heat sink.
The manufacturing method is a method for manufacturing a power module substrate in which metal layers are laminated on both surfaces of a ceramic substrate and a heat sink is bonded to one of the metal layers, and the average grain size of the crystal grains is 0. The surface of the metal layer having a thickness of 5 μm or less and the heat sink are subjected to surface purification etching in a vacuum atmosphere, and then the etched surfaces of the heat sink and the metal layer are overlapped and bonded in the vacuum atmosphere. And

  In the present invention, the surfaces of the heat sink and the metal layer are irradiated with an ion beam or plasma to perform surface purification etching, thereby activating the surfaces of the heat sink and the metal layer and directly bonding in the activated state. Since it can be joined and is not accompanied by heat treatment, it can be joined with a fine structure. Direct bonding means bonding without interstitial elements between the atoms without any reaction layer at the interface between the heat sink and the metal layer, and the bonding state is confirmed by TEM cross-sectional observation. can do.

In the power module substrate of the present invention, the metal layer is made of aluminum having a purity of 99.99 wt% or more.
By using this high-purity aluminum, the stress caused by the thermal expansion / contraction difference with the heat sink can be relaxed. In this case, if the power module substrate to which high-purity aluminum is bonded is used in a severe thermal cycle environment, wrinkles occur on the solder bonding surface of the aluminum metal layer, and there is a concern about the adverse effect on the bondability with electronic components. Since the crystal grains are fine, generation of wrinkles on the surface can be suppressed even if the crystal grains are used in a severe thermal cycle environment thereafter, and the reliability of bonding with the electronic component can be improved.

  The power module of the present invention is characterized in that an electronic component is joined by soldering to a metal layer opposite to the metal layer to which the heat sink of the power module substrate is joined.

  According to the present invention, since the metal layer having an average grain size of 0.5 μm or less and the surface of the heat sink are directly bonded in an activated state, a thermal effect is exerted on the bonded portion between the ceramic substrate and the metal layer. Since the metal layer is bonded with a fine structure, there is no generation of wrinkles on the solder joint surface with the electronic component even if the metal layer is used in a thermal cycle environment thereafter, and the bonding reliability can be improved. it can.

It is a longitudinal section showing the whole power module composition of an embodiment of the present invention. It is sectional drawing which shows the schematic structural example of the joining 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 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 and the like, and an electronic component 4 such as a semiconductor chip mounted on the surface of the power module substrate 3. Yes.

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 dissipation layer, and a heat sink 5 is attached to the surface thereof.
The ceramic substrate 2 is formed of nitride ceramics such as AlN (aluminum nitride) and Si ₃ N ₄ (silicon nitride), or oxide ceramics such as Al ₂ O ₃ (alumina).
For the metal layers 6 and 7, aluminum having a purity of 99.99 wt% or more is used, and 1N99 can be used in the JIS standard. The metal layers 6 and 7 have a fine crystal structure with an average grain size of 0.5 μm or less. The average crystal grain size is preferably 0.05 to 0.2 μm.

In this case, the individual dimensions of the thickness of each layer are not limited, but the thickness of the ceramic substrate 2 is, for example, 635 μm, and the thickness of the metal layer 6 serving as a circuit layer is 600 μm, which becomes a heat dissipation layer. The thickness of the metal layer 7 is 1600 μm.
Further, these metal layers 6 and 7 are bonded to the ceramic substrate 2 by stamping to a desired outer shape by pressing, or after joining a flat plate to the ceramic substrate 2 and then etching to a desired outer shape. Either method can be employed.

  In addition, dissimilar elements other than the constituent elements of the ceramic substrate 2 and the two metal layers 6 and 7 are present at the bonding interface between the ceramic substrate 2 and the metal layers 6 and 7 serving as the circuit layer and the heat dissipation layer. Instead, the ceramic substrate 2 and the metal layers 6 and 7 are directly bonded.

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 has a cylindrical body 15 joined to the metal layer 7 and a vertical wall that divides the interior of the cylindrical body 15 into a plurality of flow paths 16. 17 is formed integrally. As the aluminum alloy, an aluminum alloy having an aluminum concentration of 90% to 99%, for example, A6063 according to JIS standard is used. The top plate portion 15a of the cylindrical body 15 has a quadrangular planar shape larger than that of the metal layer 7, and the vertical walls 17 are arranged in parallel with each other at equal intervals in the width direction of the cylindrical body 15. It is provided along the length direction of the body 15.
The heat sink 5 and the metal layer 7 serving as a heat dissipation layer are also directly bonded in the same manner as the bonded portion between the ceramic substrate 2 and the metal layers 6 and 7 on the front and back sides. Specific methods of these direct bonding will be described later.

  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.

In order to manufacture the power module substrate 3 configured as described above, the ceramic substrate 2 and the metal layers 6 and 7 are respectively formed, and their bonding surfaces are activated and bonded as follows.
First, the ceramic substrate 2 and the one metal layer 6 are disposed in the vacuum chamber 22 of the bonding apparatus 21 shown in FIG. The joining apparatus 21 includes a horizontal stage 23 and an upper ram 24 that is driven in a direction of separating and approaching the stage 23 above the stage 23 in the vacuum chamber 22. An upper surface of the upper ram 23 and a lower end surface of the upper ram 23 are arranged to face each other, and an ion beam source 25 is provided to irradiate the ion beams toward these opposed surfaces.

Then, for example, the ceramic substrate 2 is placed on the stage 23, the metal layer 6 is held on the lower end surface of the upper ram 24, and the upper surface of the ceramic substrate 2 and the lower surface of the metal layer 6 are irradiated with an ion beam. Thus, these surfaces are purified and etched. For example, in the case of an ion beam, an inert gas such as argon is used, and the surface of the ceramic substrate 2 and the metal layer 6 is irradiated with an argon atom beam having an applied voltage of about 1.5 keV for about 10 minutes. An unstable and active surface is exposed on the substrate 2 and the metal layer 6.
Then, after the activation process, the upper ram 24 is lowered in the vacuum chamber 22 and the activated surfaces of the ceramic substrate 2 and the metal layer 6 are overlapped to join them.

  Next, the metal layer 6 is held so that the opposite surface of the ceramic substrate 2 bonded to one side faces the upper side of the stage 23, and the other metal layer 7 is held on the lower end surface of the upper ram 24. The surface of the ceramic substrate 2 and the metal layer 7 facing each other is irradiated with an ion beam to perform surface purification etching, and then the upper ram 24 is lowered to join the metal layer 7 to the ceramic substrate 2.

  Next, the heat sink 5 is bonded to the metal layer 7 serving as a heat dissipation layer. In this case as well, the metal layer 7 serving as a heat dissipation layer bonded to the ceramic substrate 2 and the heat sink 5 are opposed to each other, and these are held on the upper surface of the stage 23 and the lower end surface of the upper ram 24. After the surface is etched by irradiating the opposite surface with an ion beam, the upper ram 24 is lowered and the etched surfaces of the metal layer 7 and the heat sink 5 are overlapped and joined.

In this way, the power module substrate 3 in which the metal layers 6 and 7 are bonded to both surfaces of the ceramic substrate 2 and the heat sink 5 is bonded to the metal layer 7 serving as a heat dissipation layer has the ceramic substrate 2 and the metal layers 6 and 7. In addition, the interface is directly bonded without interposing a different element, and is firmly bonded, and the metal layer 7 and the heat sink 5 are also directly bonded without a different element interposed and are firmly bonded.
In this case, the bonding process of three times of bonding of one metal layer 6 to the ceramic substrate 2, bonding of the other metal layer 7, and bonding of the metal layer 7 and the heat sink 5 is performed. Since there is no heating even during the bonding, the subsequent bonding process does not adversely affect the bonded portion that was the previous process, and the metal layers 6 and 7 are not heated in this series of bonding processes. The grains are joined with a fine structure having an average particle diameter of 0.5 μm or less.

Thereafter, the electronic component 4 is soldered on the metal layer 6 that becomes the circuit layer. This soldering operation is performed in a reducing gas atmosphere in which nitrogen and hydrogen are mixed. Thereafter, wire bonding is performed between the electronic component 4 and the metal layer 6 in the atmosphere.
The power module 1 is completed through this series of steps.

  In the power module 1 manufactured in this way, the metal layers 6 and 7 have a fine structure, so that wrinkles are prevented from occurring on the surfaces of the metal layers 6 and 7. And since it is a soft pure aluminum metal, the stress based on a thermal expansion-contraction difference with the heat sink 5 can be relieved, and the reliability of a junction part can be maintained for a long period of time.

In order to confirm the effect of the present invention, a heat sink made of an aluminum alloy of A6063 was directly bonded to a metal layer having an aluminum purity of 99.99 wt% bonded to a ceramic substrate of aluminum nitride. The surfaces of the heat sink and the metal layer were irradiated with an argon atom beam having an applied voltage of 1.5 keV for 10 minutes to join them. As a comparative example, a heat sink made of an aluminum alloy of A6063 was joined to the metal layer by heating to 600 ° C. by a nocollock brazing method.
In either case, the ceramic substrate and the metal layer were directly bonded by irradiation with an argon atom beam with an applied voltage of 1.5 keV for 10 minutes.

Of these metal layers, the metal layer serving as the circuit layer was 600 μm, and the metal layer bonded to the heat sink as the heat dissipation layer was 1600 μm thick. In either case, the metal layer had an average crystal grain size of 0.5 μm or less. The crystal grain size was evaluated by macro-etching the surface of the metal layer with Barrett's solution (mixed solution of hydrochloric acid, nitric acid and hydrofluoric acid), and the area ratio was evaluated using a polarizing microscope. Specifically, it was determined by a method (intercept method) for estimating the particle size from the length of the line segment of each particle cut by a straight line on the material surface and the number of particles cut by a line segment of a certain length.
For the power module substrate thus manufactured, a semiconductor chip as an electronic component was joined to a metal layer with a thickness of 600 μm serving as a circuit layer by soldering, and then a thermal cycle test was performed. As the thermal cycle test, the temperature increase and cooling were repeated 3000 cycles in the temperature range of −40 ° C. to 125 ° C.

After the test, as a result of cross-sectional observation of the joint between the electronic component and the metal layer and the joint between the ceramic substrate and the metal layer, no abnormality was found in the example, but the comparative example was Flaws were observed on a part of the surface of the metal layer in the solder joint with the component, and the generation of wrinkles was remarkable particularly when the ceramic substrate and the metal layer were brazed.
Moreover, when the average crystal grain size of each metal layer was measured, the sample in the example did not change below 0.5 μm, but the sample in the comparative example was as large as 3.0 μm.

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 present invention, the ceramic substrate and the metal plate are preferably joined directly as described above, but this does not prevent ordinary brazing between them.

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 15 Cylindrical body 16 Flow path 17 Vertical wall 21 Joining device 22 Vacuum chamber 23 Stage 24 Upper ram 25 Ion beam source

Claims (4)

  A power module substrate in which a metal layer is laminated on both surfaces of a ceramic substrate and a heat sink is bonded to one of the metal layers, and the metal layer has an average grain size of 0.5 μm or less. A power module substrate, which is directly bonded to a surface of the heat sink.   The power module substrate according to claim 1, wherein the metal layer is made of aluminum having a purity of 99.99 wt% or more.   The electronic component is joined by soldering to the metal layer opposite to the metal layer to which the heat sink is joined of the power module substrate according to claim 1 or 2, and the heat sink is joined to the other metal layer. Power module characterized by   A method of manufacturing a power module substrate in which a metal layer is laminated on both surfaces of a ceramic substrate and a heat sink is bonded to one of the metal layers, and the metal layer has an average grain size of 0.5 μm or less And the surface of the heat sink is subjected to surface purification etching in a vacuum atmosphere, and then the etched surfaces of the heat sink and the metal layer are overlapped and bonded in the vacuum atmosphere. Production method.

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