JPH10335767A - Circuit substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a circuit substrate having high reliability which does not need an excessive thickness of the entire circuit substrate than required and does not cause cracks in an insulation base if the thickness of a circuit conductor comprising a copper plate is increased corresponding to a large power. SOLUTION: In a circuit substrate in which a circuit conductor 2 comprising a copper plate is brazed on an insulation base 1 composed of ceramics, the insulation base 1 and the circuit conductor 2 are brazed interposing a sub- conductor 4 composed of a metal having a yield force 25 MPa or less at 20 deg.C and volumetric resistivity 1.7 μΩxcm or less therebetween. Since a thermal stress between the insulation base 1 and the circuit conductor 2 is efficiently absorbed and mitigated by the sub-conductor 4, no cracks occur in the insulation base at the time of brazing and using, and as a current efficiently flows even in the sub-conductor 4, the thickness of the entire circuit substrate can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit board for mounting a high power semiconductor element used in a power module of an electric vehicle or the like, and more particularly, to a thick insulating board made of a copper plate on an insulating board made of ceramics. The present invention relates to a circuit board formed by brazing circuit conductors.

[0002]

2. Description of the Related Art In a device such as an electric vehicle which uses a large amount of power as a driving source, it is necessary to handle a large amount of power in a power module for controlling the driving power.

A circuit board used in such a power module has a metallized metal layer such as tungsten and molybdenum on a surface of an insulating base made of ceramics such as an aluminum oxide sintered body.
A circuit conductor made of a copper plate is joined to this metallized metal layer via a brazing material such as silver brazing.

In such a circuit board, ceramics such as an aluminum oxide sintered body constituting an insulating base are excellent in electrical insulation and heat resistance, and a copper plate constituting a circuit conductor is inexpensive and has a volume resistivity of 1.7 μΩ. -Low in cm (20 ° C) and excellent in conductivity, and furthermore, brazing material such as silver brazing is suitable for power modules because it has excellent bonding reliability at high temperatures.

[0005]

However, in this conventional circuit board, it is necessary to increase the thickness of the circuit conductor made of a copper plate as the power to be handled becomes larger. 1m thick
m or more, when a circuit conductor made of a copper plate is brazed to a metallized metal layer adhered to an insulating base made of ceramics, a large thermal stress occurs between the insulating base and the circuit conductor due to a difference in thermal expansion coefficient between the two. When this occurs, this thermal stress is greatly applied to the insulating substrate, and as a result, a defect is induced in that the insulating substrate is cracked.

Further, as the thickness of the circuit conductor formed of a copper plate is increased, the thickness of the entire circuit board including the circuit conductor is increased, and it becomes impossible to meet the demand for a thin module. There is also a problem that the thickness of the entire circuit board including the above must be reduced as much as possible.

SUMMARY OF THE INVENTION The present invention has been devised to solve the above-mentioned drawbacks, and has as its object to reduce the thickness of the circuit conductor made of a copper plate without unnecessarily increasing the thickness of the entire circuit board and corresponding to high power. It is an object of the present invention to provide a highly reliable circuit board that does not cause cracks in an insulating base even if the thickness is large.

[0008]

A circuit board according to the present invention is a circuit board obtained by brazing a circuit conductor made of a copper plate to an insulating base made of ceramic, wherein the insulating base and the circuit conductor are maintained at 20 ° C. It is characterized in that it is brazed with a sub-conductor made of a metal having a yield stress of 25 MPa or less and a volume resistivity of 1.7 μΩ · cm or less.

Further, in the circuit board according to the present invention, the thickness of the circuit conductor is 1 mm or more and the thickness of the sub-conductor is 0.2 to 1 mm.

According to the circuit board of the present invention, the insulating base made of ceramics and the circuit conductor made of copper plate have a temperature of 20 ° C.
Is less than 25MPa and the volume resistivity is
Since it is brazed with a sub-conductor made of a metal of 1.7μΩcm or less, when brazing a circuit conductor made of a copper plate to an insulating base made of ceramics or when using a circuit board in a power module, etc. Even if a thermal stress is generated between the insulating base and the circuit conductor due to a difference in thermal expansion coefficient between the two, the thermal stress is well absorbed by the sub-conductor, and as a result, a large thermal stress is applied to the insulating base. No voltage is applied and no cracks occur.

In addition, when a large current is applied to the circuit conductor according to a large power, the current flows to the sub-conductor as well as the circuit conductor. Therefore, the thickness of the circuit conductor made of the weight plate is reduced. Can be.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, a circuit board according to the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a sectional view showing an embodiment of a circuit board according to the present invention, wherein 1 is an insulating base, and 2 is a circuit conductor.

The insulating substrate 1 is made of a ceramic such as an aluminum oxide sintered body, an aluminum nitride sintered body, a silicon carbide sintered body, a silicon nitride sintered body. A ceramic powder consisting of aluminum oxide powder, silicon oxide powder, calcium oxide powder, magnesium oxide powder, etc., is mixed with an appropriate organic binder and solvent to form a slurry, which is formed using a well-known doctor blade method. Then, a ceramic green sheet is obtained by forming the sheet into a sheet shape. Thereafter, the ceramic green sheet is subjected to a suitable punching process or cutting process to obtain a predetermined shape, followed by firing at a temperature of about 1600 ° C.

On the surface of the insulating substrate 1, a metallized metal layer 3 made of a sintered body of a refractory metal such as tungsten or molybdenum is formed in a predetermined pattern. Between the insulating substrate 1
A circuit conductor 2 made of a copper plate is brazed via a brazing material 5 such as a silver braze with a subconductor 4 made of a metal having a yield stress at 20 ° C. of 25 MPa or less and a volume resistivity of 1.7 μΩ · cm or less interposed therebetween. ing.

The metallized metal layer 3 applied to the surface of the insulating base 1 acts as a base metal for brazing the circuit conductor 2 to the insulating base 1. A metal paste obtained by adding and mixing a binder and a solvent is printed and applied in a predetermined pattern on a ceramic green sheet serving as the insulating substrate 1 by a conventionally known screen printing method.
It is adhered to the surface of the insulating base 1 by firing together with the ceramic green sheet to be formed.

If the metallized metal layer 3 is plated on the surface thereof with a metal having excellent wettability with a brazing material such as nickel to a thickness of about 0.1 to 10 μm by plating, the metallized metal layer 3 and the circuit The conductor 2 can be easily and firmly brazed through the brazing material 5 such as silver brazing with the sub-conductor 4 made of a silver plate or a gold plate interposed therebetween. Accordingly, the metallized metal layer 3 is preferably plated with a metal having excellent wettability with a brazing material such as nickel on the surface thereof to a thickness of about 0.1 to 10 μm by a plating method.

The circuit conductor 2 brazed to the metallized metal layer 3 with the sub-conductor 4 interposed between the metallized metal layer 3 and the insulating base 1 is made of a copper plate having a thickness of about 1 to 2 mm and acts as a main conductor through which a large current flows. For example, electrodes of a semiconductor element are connected to the circuit conductor 3 via a large number of large-diameter bonding wires having a diameter of about 0.5 mm.

The circuit conductor 2 has a volume resistivity at 20 ° C.
Since it is made of a highly conductive copper plate having a thickness of 1.7 μΩ · cm and has a thickness of 1 mm or more, a large current can be applied to the circuit conductor 2 to cope with a large power.
If the width and width are about 25 mm, a large current of about 200 amperes can flow.

In order to braze the circuit conductor 2 made of a copper plate on the metallized metal layer 3 with the sub-conductor 4 interposed therebetween, the sub-conductor 4 and the circuit conductor 2 made of a copper plate are placed on the metallized metal layer 2. A method is adopted in which, for example, a foil brazing material made of, for example, a silver-copper eutectic alloy is interposed therebetween and the brazing material interposed therebetween is heated and melted for brazing.

The sub-conductor 4 sandwiched between the metallized metal layer 3 and the circuit conductor 2 made of a copper plate is made of a metal having a yield stress at 20 ° C. of 25 MPa or less and a volume resistivity of 1.7 μΩ · cm or less; When the circuit conductor 2 made of a copper plate is brazed to the metallized metal layer 3 or when the circuit board is used in a power module or the like, due to a difference in thermal expansion coefficient between the insulating base 1 and the circuit conductor 2. In addition to acting to absorb and relax the generated thermal stress, it functions as a part of a conductive path when a large current flows together with the circuit conductor 2 as the main conductor.

Since the sub-conductor 4 has a small yield stress of 25 MPa or less and is easily plastically deformed, even if a thermal stress is generated between the insulating base 1 and the circuit conductor 2, the thermal stress is reduced by the sub-conductor 4. Is well absorbed by plastic deformation.
As a result, no large thermal stress is applied to the insulating base 1, and therefore, no crack is generated in the insulating base 1 even if the circuit conductor 2 made of a copper plate is made thicker than 1 mm.

In this case, when the yield stress at 20 ° C. exceeds 25 MPa, the sub-conductor 4 is used for brazing the circuit conductor 2 made of a copper plate to the insulating substrate 1 with the sub-conductor 4 interposed therebetween, or It tends to be difficult to sufficiently absorb and mitigate the thermal stress generated when used in power modules and the like. Therefore, the sub-conductor 4 is specified as a metal having a yield stress at 20 ° C. of 25 MPa or less.

Further, the sub-conductor 4 has a volume resistivity of 1.
Since it is as small as 7 μΩ · cm or less, a large current can be satisfactorily passed together with the circuit conductor 2 made of a copper plate as a main conductor, and the thickness of the circuit conductor 2 is made thinner by that amount, so that the thickness of the entire circuit board becomes unnecessary. Thickness can be prevented. As a result, as compared with the case where the circuit board is constituted only by the circuit conductor 2 made of a copper plate, a highly reliable circuit board which does not generate cracks in the insulating base 1 and can cope with higher power. Can be.

In this case, the sub-conductor 4 has a volume resistivity of
If it exceeds 1.7 μΩ · cm, it becomes difficult to flow a current through the sub-conductor 4 as well as the circuit conductor 2, and the thickness of the circuit conductor 2 cannot be reduced accordingly, and the thickness of the entire circuit board is 4 results in an unnecessarily thick one. Therefore, the sub-conductor 4 is specified as a metal having a volume resistivity of 1.7 μΩ · cm or less.

When the thickness of the sub-conductor 4 is less than 0.2 mm, the difference in thermal expansion coefficient between the two when the circuit conductor made of a copper plate is brazed to the insulating base 1 or when the circuit board is used in a power module or the like. Therefore, it becomes difficult to sufficiently absorb and alleviate the thermal stress generated due to this, and cracks tend to occur in the insulating base 1. If the thickness exceeds 1 mm, the sub-conductor 4 is a soft and easily deformable metal. Therefore, when an external force is applied to the circuit conductor 2 when a semiconductor element is connected to the circuit conductor 2, the circuit conductor 2 becomes When the circuit conductor 2 is deformed, it tends to be difficult to firmly connect the semiconductor element and the circuit conductor 2 when the circuit conductor 2 is deformed. Therefore, the sub-conductor 4
It is preferable that the thickness is set in the range of 0.2 to 1 mm.

If the thickness of the sub-conductor 4 is at least one third of the thickness of the circuit conductor 2 made of a copper plate, the joining reliability between the insulating base 1 and the circuit conductor 2 made of a copper plate is extremely high. Becomes Further, when the thickness exceeds one half, there is no remarkable difference in the improvement of the reliability. Therefore, the thickness of the circuit conductor 2 made of a copper plate is one third to two thirds.
It is preferable to set the thickness in a range of 1 /.

Such a subconductor 4 may be made of a metal such as silver (having a yield stress of about 23 MPa) or gold (having a yield stress of about 15 MPa), for example. It may be used as a plating layer.

In the case where a silver plate or a gold plate is used, it is preferable to use pure silver or pure gold because these materials tend to be hardened and hardly plastically deformed when they contain impurities.

[0030]

[Example] Purity of 90mm width x 170mm length x 2mm thickness
27 mm width on the upper surface of the insulating substrate 1 made of 96% alumina substrate
A metallized metal layer 3 made of tungsten having a length of 82 mm and a thickness of 15 μm is applied, and the metallized metal layer 3 is plated with nickel having a thickness of 3 μm, and the metallized metal layer 3 has a width of 25 mm ×
A silver plate as the sub-conductor 4 and a copper plate as the circuit conductor 2 having a length of 80 mm and the thickness shown in Table 1 are interposed between a silver plate having a thickness of 50 μm and
A sample was prepared by brazing a brazing material 5 made of copper eutectic brazing. Then, after brazing, and at every 100 cycles up to 500 cycles of a temperature cycle test at −40 to + 125 ° C., the presence or absence of cracks generated on the alumina substrate was checked. Table 1 shows the results.

[0031]

[Table 1]

As can be seen from Table 1, cracks did not occur after brazing in any case where the sub-conductor 4 using a silver plate was sandwiched between the insulating base 1 and the circuit conductor 2 formed of a copper plate. In the case of a sample having a silver plate thickness of 0.2 mm or more,
No cracks were observed at least up to 300 cycles in the temperature cycle test, indicating high bonding reliability. On the other hand, in the conventional circuit board (sample No. 1), cracks occurred after brazing.

Thus, according to the circuit board of the present invention,
When the circuit conductor made of copper plate is brazed to the insulating substrate made of ceramics or when the circuit substrate is used in power modules, etc., no cracks occur in the insulating substrate, confirming that the circuit substrate is highly reliable. did it.

Also, in a test using a gold plate instead of a silver plate, the circuit board of the present invention provided good results similar to those shown in Table 1.

[0035]

According to the circuit board of the present invention, the insulating base made of ceramic and the circuit conductor made of copper are interposed between the insulating base and the circuit conductor made of copper.
Since the yield stress at ℃ is 25MPa or less and the volume resistivity is 1.7μΩcm or less, the sub-conductor made of metal is brazed. When used in a module or the like, the thermal stress generated due to the difference in the thermal expansion coefficient between the two is well absorbed and relaxed by plastic deformation of the sub-conductor, so that large thermal stress is applied to the insulating base. Therefore, no crack is generated in the insulating base even if the thickness of the circuit conductor made of copper is increased.

Further, when a large current flows through the circuit conductor, the current also flows well through the sub-conductor, so that the thickness of the circuit conductor formed of the copper plate can be reduced, and the thickness of the entire circuit board can be reduced. A large current can be passed without making the thickness unnecessarily thick.

As described above, according to the present invention, even if the thickness of the circuit conductor formed of a copper plate is increased without unnecessarily increasing the thickness of the entire circuit board, and even if the thickness of the circuit conductor made of a copper plate is increased in response to high power, cracks are formed on the insulating base. A highly reliable circuit board free from occurrence can be provided.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of an embodiment of a circuit board of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Insulating base made of ceramics 2 ... Circuit conductor made of copper plate 4 ... Subconductor 5 ... Brazing material

Claims (2)

[Claims] 1. A circuit board formed by brazing a circuit conductor made of a copper plate to an insulating base made of ceramics,
The insulating base and the circuit conductor have a yield stress at 20 ° C. of 25 MPa or less and a volume resistivity of 1.7 μΩ.
-A circuit board characterized by being brazed with a sub-conductor made of a metal having a size of not more than cm. 2. The circuit board according to claim 1, wherein the thickness of the circuit conductor is 1 mm or more, and the thickness of the sub-conductor is 0.2 to 1 mm.