JPH11171672A - Composite and heat sink using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an aluminum-silicon carbide composite with high thermal conductivity and low thermal expansion coefficient suitable as a heat sink for ceramic circuit boards. SOLUTION: This composite is obtained by impregnating a silicon carbide porous body with an aluminum-predominant metal. This composite is such one that there are cracks along the surface of the silicon carbide grains, each of their length being pref. 1-10 μm, the thermal conductivity at 25 deg.C is >=160 W/(m.K), and the thermal expansion coefficient at 25-250 deg.C is <=7.5×10<-6> /K.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat sink provided on the back surface of a ceramic circuit board on which semiconductor components are mounted and used for efficiently and quickly releasing heat generated from the semiconductor components.

[0002]

2. Description of the Related Art In recent years, with the increase in the area and the degree of integration of semiconductor devices in the power field, it has become a problem how to quickly release generated heat. In order to solve this problem, a substrate on which a semiconductor element is mounted is replaced with a conventionally used alumina substrate, and has a high thermal conductivity, excellent electrical insulation, and a thermal expansion coefficient close to that of silicon. Aluminum nitride substrates and silicon nitride substrates have been used because their mechanical properties are higher than those of alumina substrates.

[0003] A module using the ceramic substrate described above.
The circuit is usually a circuit formed of a metal having excellent conductivity such as copper or aluminum, a ceramic substrate, a pattern made of a metal for joining the ceramic substrate and a heat sink and securing heat radiation characteristics, It is composed of a solder layer for joining the metal to a heat sink, and a heat sink. In addition, a copper plate is frequently used as a heat sink material from the viewpoint of thermal conductivity and cost.

[0004]

However, the copper plate used for the heat sink has a coefficient of thermal expansion of 17 × 10 ^-6 / K.
On the other hand, the coefficient of thermal expansion of the ceramic substrate is 4
-5 × 10 ^-6 / K, and there is a large difference in the coefficient of thermal expansion between them. When soldering the ceramic substrate on which the circuit is formed to a heat sink, a large thermal stress is generated. There has been a problem that the substrate is cracked or cracked.

In order to solve this problem, a heat sink made of a composite mainly composed of silicon carbide and aluminum, which is made by impregnating a porous body having a skeleton structure of silicon carbide with an aluminum alloy or the like recently. Is being used. This heat sink has a coefficient of thermal expansion of about 7.5 × 10 ⁻⁶ / K, and has succeeded in greatly reducing the coefficient of thermal expansion as compared with a conventionally used copper heat sink. However, its thermal conductivity is about 160 W / (m · K),
Compared with the thermal conductivity of a copper plate, it is small, and there is a demand for both a lower thermal expansion coefficient and a higher thermal conductivity.

In the aluminum alloy-silicon carbide composite, a method of increasing the content of silicon carbide is generally employed to reduce the coefficient of thermal expansion.
It is not easy to make the volume% or more.

Further, the heat sink is used under a temperature condition of about 100 ° C. in actual use. However, since silicon carbide has a property of decreasing the thermal conductivity when the temperature is increased, the silicon carbide-aluminum composite includes There is also a problem that increasing the silicon content lowers the thermal conductivity under actual use conditions. For this reason, it is more difficult to achieve both a low coefficient of thermal expansion and a high coefficient of thermal conductivity.

[0008]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have conducted various experimental studies on a silicon carbide-aluminum composite and found that a composite produced under specific conditions contained aluminum in the composite. Cracks are generated along the surface of the silicon carbide particles, and the silicon carbide-aluminum composite (hereinafter, referred to as a composite) has been found to have a high thermal conductivity and a low thermal expansion coefficient. This has led to the invention.

That is, the present invention provides a composite comprising a porous silicon carbide body impregnated with a metal mainly composed of aluminum, wherein the metal has cracks along the surface of the silicon carbide particles. And preferably, the crack has a length of 1 to 10 μm.

According to the present invention, the thermal conductivity at 25.degree.
The composite according to claim 1, wherein the composite has a thermal expansion coefficient of 0 W / (m · K) or more and a thermal expansion coefficient at 25 to 250 ° C. of 7.5 × 10 ⁻⁶ / K or less.

Further, the present invention is a heat sink for a ceramic circuit board using the above-mentioned composite, and preferably, the heat sink, wherein the ceramic substrate is any of alumina, aluminum nitride or silicon nitride. is there.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.

A feature of the present invention is that when cracks are present in aluminum along the surface of silicon carbide particles forming a skeleton structure in a silicon carbide-aluminum composite,
The point is that it has been found that the coefficient of thermal expansion can be reduced without lowering the thermal conductivity of the obtained composite. Although the reason for this is not yet clear, the inventors believe that in the early stage of thermal expansion from room temperature, the expansion acts to fill in the cracks, thereby suppressing substantial expansion.

The crack in the present invention is generated along the surface of silicon carbide particles in a metal containing aluminum as a main component, and has a magnification of about 1000 times using a scanning electron microscope (SEM). And can be easily observed. FIG. 1 shows the structure of the composite obtained by the present invention by SEM.
Is a schematic view of a photographic image obtained when photographed in step (a), showing that cracks are present in a metal containing aluminum as a main component along the surface of silicon carbide particles. Further, the crack length referred to in the present invention refers to the longest distance of the tip of the crack. In the crack illustrated in FIG. 2, it is represented by the distance connecting the crack tip a and the crack tip c.

As described above, the composite of the present invention has cracks in a metal containing aluminum as a main component along the surface of silicon carbide particles. Has a die casting method
508350) and molten metal forging (Materia,
Vol. 36, No. 1, 1997, pp. 40-46)
Pressure casting method, spontaneous infiltration method (JP-A-2-197368)
In this method, the cooling rate at the time of impregnation of a metal containing aluminum as a main component can be increased.

Although the mechanism of crack generation is not clear,
The present inventors have found that, when the silicon carbide particles become large, the relaxation of thermal stress generated at the boundary between the silicon carbide particles and the metal containing aluminum as a main component during the cooling process during the production of the composite does not progress, and We believe that cracks will occur.

Hereinafter, the complex of the present invention will be described in more detail while exemplifying a method for obtaining the complex of the present invention.

First, the silicon carbide porous material used in the present invention can be obtained by a conventionally known method.
It can be obtained by using one or two or more kinds of silicon carbide powder as a raw material, adding silica or alumina as a binder to the raw material, molding, and firing at a temperature of about 800 ° C. to develop strength. it can.

In the present invention, the average particle diameter of the silicon carbide particles constituting the silicon carbide porous body is preferably from 10 to 200 μm. The average particle diameter is obtained by polishing an arbitrary cross section of the silicon carbide porous body, observing the cross section with a SEM, and obtaining the average particle diameter by an intercept method. That is, an arbitrary straight line is drawn on the SEM photograph, the length of about 100 or more silicon carbide particles crossed by the straight line is measured, and the length is multiplied by 3/2 to obtain an average value.

When the average particle size of the silicon carbide particles is less than 10 μm, the resulting composite tends to be less prone to cracks along the surface of the silicon carbide particles. This is because it is difficult to obtain a porous body, and it is difficult to increase the thermal conductivity of the obtained composite.
On the other hand, if it exceeds 200 μm, it is difficult to obtain a silicon carbide-based porous body having a high silicon carbide filling rate, and the strength of the obtained silicon carbide-based porous body is low. It is. For the above reasons, 20
A range of １００100 μm is more preferably selected.

In the production of a silicon carbide porous body (also referred to as a preform), the particle size of silicon carbide may change when mixed with a silica sol or the like as a binder. The silicon carbide raw material powder may be selected so as to have an average particle diameter in the above range on the formed preform. Generally, although the particle size of the raw silicon carbide powder slightly varies depending on the measurement method, Raw silicon carbide powder having a particle size substantially equal to the above range may be used. From the viewpoint of improving the thermal conductivity of the composite, it is preferable that the silicon carbide-based porous body has a silicon carbide filling rate of 50% or more.

The silicon carbide porous body is impregnated with a metal mainly composed of aluminum to obtain a composite. What is important in the present invention is the cooling rate after impregnation. Since cracks are caused by thermal stress caused by a difference in thermal expansion between silicon carbide particles and aluminum, the cooling rate is preferably high, and according to the study of the present inventors, 100 ° C. /
This is because, when a cooling rate of at least one minute is employed, desired cracks are easily generated and the number thereof can be increased.

When the length of the cracks in the metal containing aluminum as the main component along the surface of the silicon carbide particles is 1 to 10 μm, the number of the cracks having the length in the above range is further reduced. 10-30 per 10 pieces
The effect of the present invention is remarkable in a book, which is preferable. When the crack length is less than 1 μm, the coefficient of thermal expansion of the composite can be slightly reduced, but the effect is small. On the other hand, 10
If it exceeds μm, the cracks hinder heat conduction greatly, indicating that the thermal conductivity tends to decrease, and further, the strength of the composite itself starts to decrease. Further, the number of cracks having a crack length in the above range is 3 per silicon carbide particle.
Exceeding the number tends to lower the thermal conductivity of the composite.

The composite of the present invention has a thermal conductivity at 25 ° C. of 160 W / (m · K) or more, and moreover 25 to 250
It has the characteristic that the coefficient of thermal expansion at 7.5 ° C. is 7.5 × 10 ⁻⁶ / K or less. As described above, the composite of the present invention has a low coefficient of thermal expansion and a high thermal conductivity, and thus is suitable for use as a heat sink for various circuit boards on which semiconductors are mounted.

A heat sink using the composite of the present invention is:
Thermal expansion coefficient is 7.5 × 10 ⁻⁶ / K or less;
Since the coefficient of thermal expansion is close to that of a ceramic substrate made of aluminum nitride, silicon nitride, or the like, there is an advantage that cracks and cracks of the ceramic substrate, which have conventionally been problems, can be prevented. Further, the heat sink of the present invention has a capacity of 160 W /
Since it has a thermal conductivity of (m · K) or more, there is an advantage that heat can be quickly released.

Hereinafter, the present invention will be described in more detail with reference to Examples.

[0027]

EXAMPLES Example 1 100 parts by weight of silicon carbide powder having an average particle diameter of 30 μm was mixed with 1 part of silica sol having a solid content of 20%.
0 parts by weight were added and mixed well. The mixture is 4
Filled in a mold of 0mm x 30mm, 300kgf / c
Pressure molding was performed at m ² to obtain a molded body of 40 mm × 30 mm × 7 mm. The relative density of this molded product (preform) was measured to be 62% by volume.

After sintering the molded body in air at 800 ° C., the molded body is placed in a mold preheated to 650 ° C., and silicon is reduced to 6%.
Pour molten aluminum containing 1 ton / cm ²
And then cooled. Cooling at this time is 100
C./min. The surface of the obtained molded body was ground to obtain a composite.

From the above composite, a diameter of 3 m was obtained by grinding.
A test piece having a length of 10 mm, a diameter of 11 mm, and a thickness of 3 mm was obtained, and the coefficient of thermal expansion at 25 to 250 ° C. was measured with each test piece using a thermal dilatometer (TMA300, manufactured by Seiko Instruments Inc.).
The thermal conductivity at 25 ° C. was measured by a laser flash method (manufactured by Rigaku Corporation; LF / TCM-8510B). Further, the fracture surface of the composite was observed by SEM, and the presence or absence of cracks in the metal containing aluminum as a main component was determined by an SEM photograph.
The average length was measured for μm. Table 1 shows the results.

[0030]

[Table 1]

Example 2 A composite was prepared and evaluated in the same manner as in Example 1 except that silicon carbide powder having an average particle diameter of 10 μm was used. Table 1 shows the results.

Example 3 A composite was prepared and evaluated in the same manner as in Example 1 except that silicon carbide powder having an average particle diameter of 200 μm was used. Table 1 shows the results.

Example 4 A composite was prepared and evaluated in the same manner as in Example 1, except that silicon carbide powder having an average particle diameter of 50 μm was used and the cooling rate was set to 130 ° C. Table 1 shows the results.

Example 5 A composite was prepared and evaluated in the same manner as in Example 4 except that silicon carbide powder having an average particle diameter of 20 μm was used. Table 1 shows the results.

Example 6 A composite was prepared and evaluated in the same manner as in Example 1 except that silicon carbide powder having an average particle size of 100 μm was used. Table 1 shows the results.

Comparative Example 1 A heat sink was prepared and evaluated in the same manner as in Example 1 except that the cooling rate was 10 ° C./min. Table 1 shows the results.

Example 7 Using the composite of Example 1, 3
A plate of 0 mm × 65 mm × 3 mm was formed and used as a heat sink. Next, a predetermined circuit is formed on the front surface, and the heat sink is soldered to the heat dissipating copper plate of the aluminum nitride circuit board (size: 25 mm × 60 mm) having a heat dissipating copper plate having a thickness of 0.4 mm bonded on the back surface. By using and joining, the heat sink was integrated with the circuit board. Next, the circuit board integrated with the heat sink is-
No abnormalities were observed after one thousand cycles of a thermal shock test in which one cycle of temperature rise, hold, and drop between 40 ° C and + 125 ° C was a temperature cycle of 40 minutes.

Comparative Example 2 Using the composite of Comparative Example 1,
A circuit board integrated with a heat sink was produced and evaluated by the same operation as the comparative example, and cracks were found between the circuits of the aluminum nitride substrate.

[0039]

Since the heat sink of the present invention has a small coefficient of thermal expansion and high thermal conductivity, it is suitable as a heat sink for a ceramic circuit board on which semiconductor components are mounted.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a structure of a composite of the present invention.

FIG. 2 is an explanatory diagram of the length of a crack present in the composite of the present invention.

──────────────────────────────────────────────────の Continued on front page (51) Int.Cl. ⁶ Identification code FI H01L 23/373 H01L 23/36 M

Claims (5)

[Claims] 1. A composite comprising a porous silicon carbide body impregnated with a metal mainly composed of aluminum, wherein the metal has cracks along the surface of the silicon carbide particles. 2. The composite according to claim 1, wherein the crack has a length of 1 to 10 μm. 3. The thermal conductivity at 25 ° C. is 160 W /
3. The composite according to claim 1, wherein the composite has a thermal expansion coefficient of not less than (m · K) and a thermal expansion coefficient at 25 to 250 ° C. of 7.5 × 10 ⁻⁶ / K or less. 4. 4. A heat sink for a ceramic circuit board, comprising the composite according to claim 1, 2 or 3. 5. The heat sink according to claim 4, wherein the ceramic substrate is one of alumina, aluminum nitride, and silicon nitride.