JP2000277662A - Ceramic circuit board - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable a ceramic circuit board to be improved in the withstand voltage characteristic and protected against cracking which occurs when the board is mounted and put in operation, by a method wherein an alumina board whose alumina purity is above a specified value is used as a ceramic board, and voids of the alumina board are set smaller than a prescribed value. SOLUTION: A ceramic circuit board 1 is composed of a ceramic board 2 and a metal circuit board 3 of prescribed shape bonded to the board 2. An alumina board whose alumina purity is above 99.5% and void content is below 5 vol.% and which has few defects is used as the ceramic board 2. By this setup, the ceramic board 2 is enhanced enough in denseness even if a small amount of sintering auxiliary is used and becomes higher in thermal conductivity than a conventional alumina board. The alumina board itself is lessened in thickness so as to be reduced in heat resistance. A circuit board more excellent in heat dissipating properties can be formed by synergism of these effects.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ceramic circuit board, and more particularly, to a ceramic circuit board having improved withstand voltage characteristics, and capable of effectively preventing the occurrence of cracks during mounting and use and improving reliability. About.

[0002]

2. Description of the Related Art In recent years, as a circuit substrate such as a substrate for a power transistor module or a substrate for a switching power supply module, a copper plate, an aluminum plate,
Ceramic circuit boards in which metal plates such as various clad plates are joined are widely used. In addition, as the ceramic substrate, alumina (Al) which is inexpensive and has high versatility is used.
_{A 2} O ₃ ) substrate, an aluminum nitride (AlN) substrate, a silicon nitride (Si ₃ N ₄ ) substrate, and the like, which have electrical insulating properties and excellent thermal conductivity, are generally used.

A ceramic circuit board 1 having a circuit formed of a copper plate or the like as described above, for example, as shown in FIGS. 1 to 3, has a copper plate as a metal circuit board 3 joined to one surface of a ceramic substrate 2. Is formed by bonding a copper plate as the back metal plate 4 to the other surface.

As a method for integrally forming various metal plates or metal layers on the surface of the ceramic substrate 2, the following direct bonding method, refractory metal metallizing method, active metal method, and the like are used. In the direct bonding method, for example, a copper circuit board or the like punched into a predetermined shape is contacted and placed on the ceramic substrate 2 and heated, and Cu—Cu ₂ O, Cu—
A eutectic liquid phase such as O is generated, the wettability with the ceramic substrate is increased by the liquid phase, and then the liquid phase is cooled and solidified to directly join the ceramic substrate and the copper circuit board, etc. The so-called copper direct bonding method (DBC method: D
direct bonding Copper method). The refractory metal metallization method is a method in which a refractory metal such as Mo or W is baked on the surface of a ceramic substrate to form a metal circuit layer integrally. The active metal method is a method of integrally joining a metal plate on the ceramic substrate 2 via an Ag-Cu brazing material layer containing an active metal such as a Group 4A element such as Ti, Zr, and Hf. . According to this active metal method, the brazing material layer can increase the bonding strength with the copper circuit board by the Cu and Ag components, while the Ti,
The bonding strength between the brazing material layer and the ceramic substrate is enhanced by the Zr and Hf components.

Further, as a specific method of forming a circuit,
There are known methods of using a copper plate that has been patterned by press working or etching in advance, and performing patterning by a technique such as etching after bonding. The ceramic circuit boards obtained by the direct bonding method or the active metal brazing method each have a high bonding strength between the ceramic substrate and the metal circuit board, and have a simple structure, so that they can be miniaturized and mounted. The advantages are that the manufacturing process can be shortened and the like, and it is possible to cope with a large current type or highly integrated semiconductor chip.

In recent years, the output of a semiconductor device using a ceramic circuit board and the integration of a semiconductor element have been rapidly advanced, and the thermal stress and thermal load repeatedly acting on the ceramic circuit board have also tended to increase. Ceramic circuit boards are also required to have sufficient bonding strength and durability against the above thermal stress and thermal cycle.

In order to cope with the increase in the heat load and to further improve the durability of the circuit board, the thickness of the ceramic substrate constituting the circuit board is reduced to about 0.25 to 0.38 mm to reduce the thermal resistance. Attempts have been made to reduce or improve the flexibility to prevent the occurrence of tears. On the other hand, attempts have been made to integrally bond a metal circuit board (circuit layer) to an alumina substrate having a relatively high purity of about 96% as a ceramic substrate by the direct bonding method or the active metal method to form a ceramic circuit substrate. I have.

[0008]

However, in the conventional ceramic circuit board, a high bonding strength has been obtained by improving the type of the ceramic substrate and the bonding method of the metal plate. There is a problem that the cycle performance and the bending strength are not sufficiently obtained, and a pressure leakage or the like is easily generated, and the reliability and the product yield of the semiconductor device using the ceramic circuit board are lowered.

That is, the thermal cycle load is greatly increased in accordance with the high integration and high output of the semiconductor elements mounted on the ceramic circuit board, and the board is cracked due to thermal stress and the function of the circuit board is lost. There was a problem that was done. In addition, since the ceramic substrate itself has low bending strength and a small amount of bending, if the ceramic circuit board is screwed and fixed to the mounting board at the time of assembly, the ceramic substrate may be broken by the slight tightening force of the screw. There is also a problem that the product yield of the semiconductor device using the circuit board is reduced. Further, the strength of the ceramic substrate is low and the strength is insufficient due to the thinness, and there is a problem that the power transistor module incorporating the circuit board is easily broken by a tightening force when the power transistor module is mounted on the radiation fin. In addition, cracks often occur due to thermal stress generated during use, and the reliability of the semiconductor device is disadvantageously reduced.

In particular, when the thickness of the ceramic substrate is reduced to 0.25 to 0.38 mm as described above, minute pinholes or voids (voids) formed on the surface of the ceramic substrate are used to withstand the entire circuit substrate. There is also a problem that the dielectric breakdown characteristics are greatly reduced, and a leak withstand voltage or the like is likely to occur.

The present invention has been made to solve the above problems, and has high bending strength (flexural strength) in addition to high withstand voltage characteristics and excellent heat cycle characteristics.
It is an object of the present invention to provide a highly reliable ceramic circuit board which does not cause cracking or dielectric breakdown even when a large bending load is applied.

[0012]

Means for Solving the Problems To achieve the above object, the present inventors have developed various structures for improving the withstand voltage characteristics of ceramic circuit boards in particular and for preventing cracks occurring during mounting and use. investigated. As a result, when a high-purity alumina substrate having a purity of 99.5% by weight or more is used as a ceramic substrate, and the voids (pores) generated inside the alumina substrate are reduced to a predetermined value or less, compared with the conventional alumina substrate. The Vickers hardness, thermal conductivity, flexural strength, and toughness are improved, and a circuit board such as a copper plate, aluminum plate, or clad plate is integrally joined to this alumina substrate by a direct joining method or an active metal method. It has been found that when used as a substrate, the insulation resistance, bending strength, and deflection of the entire ceramic circuit board can be increased, and a ceramic circuit board with less cracking can be obtained.

The present invention has been completed based on the above findings.
It is. That is, the ceramic circuit board according to the present invention
For the board, a metal circuit board of a predetermined shape is connected to a ceramic substrate.
In the combined ceramic circuit board,
Substrate is alumina (Al₂O ₃) Purity is 99.5% or more
And an alumina substrate having a void fraction of 5% by volume or less.
It is characterized by the following.

The void ratio of the alumina substrate is 3% by volume.
The following is more preferable. Further, the withstand voltage of the alumina substrate is 25 KV / mm or more, and the toughness value is 3.24.
MPa · m ^1/2 or more, thermal conductivity 28 W / m · K or more, Vickers hardness 1500 or more, flexural strength 400
It is preferably at least MPa.

Further, the present invention is characterized in that the metal circuit board is joined to the ceramic substrate by a direct joining method. Further, the metal circuit board is a copper circuit board, and this copper circuit board is
You may comprise so that it may be joined to a ceramics substrate by a -O eutectic compound. Further, when the metal circuit board is made of Ti, Z
It may be configured to be bonded to a ceramic substrate via an active metal layer containing at least one selected from r and Hf.

In the ceramic circuit board according to the present invention,
The ceramic substrate used for
Purity is 99.5% by weight or more, void ratio (porosity)
Is 5% by volume or less of high purity and few defects in alumina (Al
₂O₃) A substrate is used. The above alumina substrate, for example,
For example, it is prepared by the following production method. Sand
In other words, it is composed of α-alumina crystals and has a particle size
Finer than ever, with a purity of 99.5-99.99%
As a sintering aid for high-purity alumina powder ₂, M
Less than 0.5% by weight of a metal oxide such as gO or CaO is preferable.
Or 0.3% by weight or less, and
The raw material mixture to which the binder is added
And the obtained sheet-like molded body is heated to about 600 ° C.
After completely degreased at a temperature of 1200 to 1700 ° C.
It is manufactured by sintering for as long as 48 hours. For example, this
When performing degreasing and sintering simultaneously by appropriate temperature adjustment
A law may be applied.

As described above, the high-purity alumina substrate obtained by using the high-purity alumina raw material powder and sintering for a long period of time is sufficiently densified even if the amount of the sintering aid is small, and the void Rate (porosity) is 5 vol. % Or less, and even when the thickness is reduced to about 0.25 to 0.38 mm, the withstand voltage is 25 KV / mm or more, and excellent insulation resistance is obtained. Further, the alumina substrate has a toughness value of 3.42 MPa · m ^1/2 or more, a thermal conductivity of 28 W / m · K or more, and an average value of 31 W / m · K.
K or more, Vickers hardness is 1500 or more, bending strength is 4
It is not less than 00 MPa, the average value is not less than 500 MPa, and it has excellent properties in terms of thermal conductivity (heat dissipation) and mechanical strength as compared with a conventional 96% purity alumina substrate. In particular, the effect of increasing the thermal conductivity compared to the conventional alumina substrate and the effect of reducing the thermal resistance by reducing the thickness of the alumina substrate itself are synergistic, resulting in a circuit board with better heat dissipation. Can be formed.

The metal constituting the metal circuit board may be a eutectic compound with a substrate component, such as a simple substance of copper, aluminum, iron, nickel, chromium, silver, molybdenum, and cobalt, or an alloy thereof, or a clad material thereof. Generate
The metal is not particularly limited as long as it is a metal to which the direct bonding method or the active metal method can be applied.
Aluminum or its alloy or clad material is preferred.

The thickness of the metal circuit board is determined in consideration of the current carrying capacity, etc., while the thickness of the alumina substrate as the ceramic substrate is in the range of 0.25 to 1.2 mm, while the thickness of the metal circuit board is If the thickness is set in the range of 0.1 to 0.5 mm and the two are combined, it is less susceptible to deformation or the like due to a difference in thermal expansion. In particular, the thickness of the alumina
By reducing the thickness to about 0.38 mm, the thermal resistance is reduced, and the heat dissipation of the circuit board can be synergistically improved.

When a copper circuit board is used as a metal circuit board and bonded by a direct bonding method, oxygen is added in an amount of 100 to 100.
By using a copper circuit board made of tough pitch electrolytic copper containing 0 ppm and further forming a copper oxide layer of a predetermined thickness on the copper circuit board surface in advance as described later,
It is preferable because the amount of generated Cu-O eutectic can be increased and the bonding strength between the substrate and the copper circuit board can be further improved.

The oxide layer such as the copper oxide layer is formed by, for example, performing a surface oxidation treatment in which a metal circuit board is heated in the atmosphere at a temperature of 150 to 360 ° C. for 20 to 120 seconds. Here, the thickness of the copper oxide layer is 1
If it is less than μm, the amount of Cu—O eutectic generated is small, so that there are many unjoined portions between the substrate and the copper circuit board, and the effect of improving the joining strength is small. On the other hand, when the thickness of the copper oxide layer is 1
Even if the thickness is excessively larger than 0 μm, the effect of improving the bonding strength is small and the conductive properties of the copper circuit board are rather hindered. Therefore, the thickness of the copper oxide layer formed on the surface of the copper circuit board is preferably in the range of 1 to 10 μm. And for the same reason, the range of 2 to 5 μm is more desirable.

In the ceramic circuit board according to the present invention, the active metal layer formed when the metal circuit board is bonded to the alumina substrate by the active metal method includes Ti, Zr, and Hf.
A bonding composition paste comprising at least one active metal selected from the group consisting of an Ag-Cu-based brazing material having an appropriate composition ratio, and prepared by dispersing the brazing material composition in an organic solvent Is formed on the surface of a ceramic substrate (alumina substrate) by screen printing or the like.

Specific examples of the bonding composition paste include the following. That is, a composition consisting of 15 to 35% by weight of Cu, 1 to 10% of at least one active metal selected from Ti, Zr, and Hf and a balance substantially composed of Ag is dispersed in an organic solvent. It is good to use the bonding composition paste prepared in this way.

The active metal is a component for improving the wettability and reactivity of the brazing material with respect to the ceramic substrate. An appropriate amount of the active metal is 1 to 10% by weight based on the whole bonding composition.

According to the ceramic circuit board having the above structure, a high-purity alumina substrate having an alumina purity of 99.5% or more and a void fraction of 5% by volume or less is used. Since it is dense and has high strength as compared with the alumina substrate, excellent resistance to stress distortion can be obtained, and the occurrence of cracks can be effectively suppressed.

In addition, since the alumina substrate is dense and has few surface defects due to voids, even when the substrate thickness is reduced, a decrease in withstand voltage characteristics is small and dielectric breakdown (breakdown voltage leakage) occurs. Also less.

Further, as compared with the conventional alumina substrate,
Since the thermal conductivity also increases, the transient thermal resistance of the module formed of the circuit board can be reduced, and excellent heat dissipation can be exhibited. Further, the strength is higher than that of the conventional alumina substrate, and the point that the substrate can be used thinner and the point that the thermal conductivity is improved synergistically make it possible to further enhance the heat dissipation of the circuit board.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described more specifically with reference to the accompanying drawings based on the following examples.

Examples 1 and ₂ 0.2% by weight of SiO ₂ as a sintering aid was added to high-purity alumina powder having an average particle size of 1.5 μm and having a purity of 99.9%. Example 1 and 0.5% by weight (Example 2) were added, and an organic binder was further added to prepare raw material mixtures. Each raw material mixture is sheet-formed by a doctor blade method to prepare a plate-like molded body, and this molded body is 800 ° C. in a vacuum of 10 ⁻⁴ Torr.
For 8 hours to completely degrease. This degreased body is heated at a temperature of 1
By sintering at 600 ° C. for 20 hours, alumina substrates 2 for Examples 1 to 2 each having a length of 29 mm × a width of 69 mm × a thickness of 0.30 mm were prepared as shown in FIGS.

Comparative Examples 1 to 3 On the other hand, the alumina substrates for Comparative Examples 1 to 3 had the purity (91 to 96%) shown in Table 1 and were 29 mm long × 63 mm wide.
X An alumina substrate 2 having a thickness of 0.30 mm was prepared.

The Al ₂ O ₃ purity, void ratio, dielectric strength, bending strength, toughness, thermal conductivity, and Vickers hardness of each alumina substrate prepared or prepared as described above were measured and shown in Table 1. The results shown were obtained.

On the other hand, as shown in FIGS.
A copper circuit board as a metal circuit board 3 made of tough pitch electrolytic copper having a thickness of 0.2 mm is disposed in contact with the front side of each alumina substrate 2, while a back metal plate 4 made of 0.20 mm thick tough pitch electrolytic copper is provided on the back side. The back copper plate was placed in contact to form a laminate.

Next, the interior of each laminate was inserted into a heating furnace whose temperature was set at 1075 ° C. and the laminate was heated for 1 minute.
Bonding the metal circuit board 3 or the back copper plate 4 directly to both sides of the
Examples 1 and 2 and Comparative Example 1 joined by the BC method)
Each of the ceramic circuit boards 1 according to Nos. 1 to 3 was prepared.

Each of the ceramic circuit boards 1 thus prepared is, as schematically shown in FIGS.
Has a structure in which a metal plate path plate 3 having a predetermined circuit pattern shape is integrally joined to the front surface side, and a single plate-shaped back copper plate 4 is integrally joined to the back surface side.

With respect to each of the ceramic circuit boards 1 according to Examples 1 and 2 and Comparative Example 1 prepared as described above, both ends of the circuit pattern surface on the front side are supported with a support span of 50 mm, while the back side on the back side is supported. A load was applied to one point at the center of the copper plate 4 to measure the three-point bending strength, and the maximum deflection of the alumina substrate 2 with respect to a plane including both edges was measured. The flexural strength value of each ceramic circuit board 1 indicates a load value when the alumina substrate is broken as a stress value for the alumina substrate alone. Also, the maximum deflection is
It was measured as the amount of deflection at the time when the alumina substrate was broken. Table 2 shows the measurement results.

[0036]

[Table 1]

[0037]

[Table 2]

As is clear from the results shown in Tables 1 and 2, the ceramic circuit boards 1 according to Examples 1 and 2
According to the above, a high-purity alumina substrate 2 having an alumina purity of 99.5% or more and a void fraction of 2% by volume or less is used. It has been found that, as compared with the prior art, it is dense and has high bending strength, so that excellent resistance to stress strain can be obtained and generation of cracks can be effectively suppressed. In particular, as shown in Table 2, according to the circuit boards according to Examples 1 and 2, even when evaluated with the DBC circuit board, the bending strength and the breaking strength were lower than those of the conventional circuit board of Comparative Example 1. It has been found that both the maximum deflection and the maximum deflection are greatly increased, so that excellent resistance to cracking is exhibited.

Further, since the alumina substrate 2 for each embodiment is dense and has few surface defects due to voids, even when the substrate thickness is reduced to 0.30 mm, the withstand voltage is 25 KV / mm or more. It was confirmed that the risk of dielectric breakdown (breakdown voltage leakage) was significantly reduced.

Furthermore, the thermal conductivity of the alumina substrate of each embodiment is also as high as 31 W / m · K as compared with the conventional alumina substrate (thermal conductivity 17 to 24 W / m · K) shown in each comparative example. Therefore, the transient thermal resistance of the module formed by the circuit board can be reduced, and excellent heat dissipation can be exhibited. Further, as compared with the conventional alumina substrate shown in each comparative example, in each of the examples, the flexural strength is increased, the point that the thinner substrate can be used, and the point that the thermal conductivity is improved are synergistic. It has been found that it is possible to further enhance the heat dissipation of the slab.

In the above Examples and Comparative Examples, a circuit board in which a copper circuit board or the like is integrally bonded to an alumina substrate using a copper direct bonding method (DBC method) is described. Use the active metal method to obtain Ti, Zr,
In the case where a copper circuit board or the like is integrally joined to an alumina substrate via an Ag-Cu brazing material layer containing an active metal such as Hf, substantially the same effects as those in Examples 1 and 2 were obtained.

[0042]

As described above, according to the ceramic circuit board of the present invention, a high-purity alumina substrate having an alumina purity of 99.5% or more and a void fraction of 5% by volume or less is used. Since it is dense and has high strength as compared with a conventional 96% pure alumina substrate, excellent resistance to stress strain can be obtained and cracks can be effectively suppressed.

In addition, since the alumina substrate is dense and has few surface defects due to voids, even when the substrate thickness is reduced, there is little decrease in withstand voltage characteristics and dielectric breakdown (breakdown voltage leakage) occurs. Also less.

Further, as compared with the conventional alumina substrate,
Since the thermal conductivity also increases, the transient thermal resistance of the module formed of the circuit board can be reduced, and excellent heat dissipation can be exhibited. Further, the strength is higher than that of the conventional alumina substrate, and the point that the substrate can be used thinner and the point that the thermal conductivity is improved synergistically make it possible to further enhance the heat dissipation of the circuit board.

[Brief description of the drawings]

FIG. 1 is a plan view showing a configuration on a pattern surface side of a ceramic circuit board.

FIG. 2 is a cross-sectional view of the ceramic circuit board shown in FIG.

FIG. 3 is a rear view showing the configuration on the back side of the ceramic circuit board shown in FIG. 1;

[Explanation of symbols]

1 ceramic circuit board (alumina circuit board) 2 ceramic substrate _(Al 2 _{O 3} substrate) 3 metal circuit plate (copper circuit board) 4 back metal plate (copper plate)

Claims (10)

[Claims] 1. A ceramic circuit board in which a metal circuit board having a predetermined shape is joined to a ceramic substrate, wherein the ceramic substrate has an alumina (Al ₂ O ₃ ) purity of 99.
A ceramic circuit board, which is an alumina substrate having a void ratio of 5% or more and a void fraction of 5% by volume or less. 2. The ceramic circuit board according to claim 1, wherein the void ratio of the alumina substrate is 3% by volume or less. 3. The dielectric strength of an alumina substrate is 25 KV / m.
2. The ceramic circuit board according to claim 1, wherein the length is at least m. 4. An alumina substrate having a toughness of 3.2 MPa ·
2. The ceramic circuit board according to claim 1, wherein the value is m ^1/2 or more. 5. The thermal conductivity of an alumina substrate is 28 W / m ·
2. The ceramic circuit board according to claim 1, wherein K is not less than K. 6. The alumina substrate has a Vickers hardness of 150.
2. The ceramic circuit board according to claim 1, wherein the value is 0 or more. 7. The alumina substrate has a bending strength of 400 MPa.
2. The ceramic circuit board according to claim 1, wherein: 8. The ceramic circuit board according to claim 1, wherein the metal circuit board is joined to the ceramic substrate by a direct joining method. 9. The ceramic circuit board according to claim 1, wherein the metal circuit board is a copper circuit board, and the copper circuit board is joined to the ceramic substrate by a Cu—O eutectic compound. 10. The ceramic circuit according to claim 1, wherein the metal circuit board is joined to the ceramic substrate via an active metal layer containing at least one selected from Ti, Zr, and Hf. substrate.