JPH0568877B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a method for manufacturing a highly thermally conductive circuit board using a substrate substantially made of aluminum nitride ceramic (hereinafter referred to as an AlN substrate).

[Technical Background of the Invention and Problems Therein] Various materials have been conventionally used for circuit boards, such as ceramic substrates such as Al ₂ O ₃ and resin substrates. Among them, Al ₂ O ₃ ceramic substrates have excellent mechanical strength and electrical insulation.
Furthermore, since it is easy to form into a green sheet, high-density wiring such as multilayer wiring is possible, and it is widely used. On the other hand, with the recent progress in miniaturization of electronic devices, the mounting density of electrical elements (ICs, etc.) on circuit boards has become higher. Furthermore, when considering the mounting of power semiconductors and the like, the amount of heat generated on the circuit board tends to increase, and effective heat dissipation is required.

However, the thermal conductivity of the Al ₂ O ₃ ceramic substrate is as low as about 20 W/m·K, and when a large amount of heat is generated, it is not expected that heat will be dissipated from the substrate side. Therefore,
Considering heat dissipation from the board side during high-density mounting and modules with power semiconductors, etc., a circuit board that has the characteristics required for a circuit board such as mechanical strength and electrical insulation, and has good thermal conductivity. development is required.

With the recent progress in fine ceramics technology, ceramic materials with excellent mechanical strength, such as SiC and AlN, have been developed. These materials also have excellent thermal conductivity, and their application as structural materials is being studied. There is also a movement to use SiC as a circuit board by taking advantage of its good thermal conductivity, but due to its high dielectric constant and low dielectric strength, it cannot be used for mounting elements to which high frequencies and high voltages are applied. There is a problem when considering this.

AlN substrates have good electrical insulation and thermal conductivity, making them promising for application to circuit boards. However, AlN has poor wettability with metals and is difficult to bond with conductor layers, as is used, for example, in crucibles for melting metal aluminum. Therefore, there is no circuit board in which conductor paths are directly formed on an AlN substrate, and at best, a power semiconductor such as a thyristor is fixed with an organic adhesive and used as a heat sink.

JP-A-52-37914, JP-A-50-132022, etc. disclose a technology for directly bonding a copper plate to ceramic, and it is conceivable to use this technology to form a conductor layer on an AlN substrate. However, there are limits to the formation of fine patterns, and multilayer wiring, which is essential for high-density wiring, is also difficult.

[Object of the invention] The present invention has been made in consideration of the above points, and
An object of the present invention is to provide a method for manufacturing a highly thermally conductive circuit board that has excellent electrical insulation, mechanical strength, and thermal conductivity.

[Summary of the Invention] The present invention is based on applying an AlN substrate on which an oxide layer is formed to a circuit board.

As a result of research into the application of AlN substrates to circuit boards, the present inventors discovered that forming an oxide layer on the surface of an AlN substrate makes it an extremely excellent circuit board. That is, by forming an oxide layer, not only the conductor layer can be bonded, but also the glass layer can be bonded. In particular, good adhesion of the glass layer has various advantages.

AlN ceramics are known to have poor wettability with metals, etc., but they also have poor wettability with glass. However, by forming an oxide layer, the wettability with respect to metal is improved, and a conductor layer can be formed. Furthermore, if a glass layer is directly formed on an AlN substrate that does not have an oxide layer, bubbles will occur in the glass layer, making it impossible to obtain a strong bond. This is thought to be because the AlN substrate generates gases such as ammonia gas when exposed to high temperatures in the atmosphere. In contrast, it has an oxide layer
Such bubbles do not occur in the glass layer formed on the AlN substrate, and a good bond to the substrate can be obtained.

Obtaining such good bonding of the glass layers is very important for circuit boards. For example, when performing sealing, it is necessary to bond the cap and the base, and at this time sealing with glass can be performed. Glass bonding can also be used when considering adhesion between the lead frame and the base. Further, when performing printed multilayer wiring, a dielectric layer is required for interlayer insulation, and a glass layer can be used as the dielectric layer.

Furthermore, this is particularly advantageous when a resistor is formed on the substrate. Since resistor paste generally has conductor powder dispersed in glass, it is necessary to bond the glass well. Furthermore, in the case of a resistor, reproducibility of its resistance value is important from the point of view of circuit design and the like. If an AlN substrate without an oxide layer is used, as mentioned above, wrinkles will occur in the resistor, which is a glass layer, resulting in very large variations in resistance value, but according to the present invention, wrinkles will occur. Since a good bond can be obtained without any problems, the reproducibility of the resistance value is very good and the variation is small.

Now, regarding this oxide layer, when considering the bonding strength between the glass layer and the AlN substrate, if the oxide layer is too thin, its effect will not be exhibited, so it is preferably about 0.5 μm or more. Also, if it is too thick, the oxide layer and AlN
The thickness is preferably about 100 μm or less because the oxidized layer may peel off due to the difference in thermal expansion coefficient with the substrate. This oxide layer is made of alumina, boehmite, etc., but since it is inferior in thermal conductivity compared to AlN, it is better to be as thin as possible within the range where the same level of bonding strength can be obtained, and preferably about 2 to 20 μm.

The method for forming an oxide layer on an AlN substrate is as follows:
For example, high temperature treatment in the atmosphere can be mentioned.
The thickness of the oxide layer formed changes depending on the heat treatment temperature and treatment time. As the processing temperature of this high-temperature processing increases, the processing time becomes shorter,
It is preferable to carry out the treatment at 1000 to 1300°C for about 3 to 0.5 hours. In addition, heat treatment in an oxidizing atmosphere such as water vapor (relatively low temperature compared to air is required;
~140℃ is sufficient. It is preferable to carry out under high pressure such as 2 to 3 atmospheres. ), immersion in an acidic solution, and other methods.

The AlN substrate used in the present invention includes Y and Y in the AlN raw material.
A method of adding additives such as rare earth elements and alkaline earth elements (approximately 0.01 to 15 wt% in terms of metal elements) and performing pressureless sintering and hot pressing, and using an AlN raw material containing approximately 7 wt% or less of oxygen. It is manufactured by atmospheric pressure sintering with the addition of additives, hot pressing, or hot pressing using only the AlN raw material without substantially adding any additives. The present invention can be applied to any AlN substrate manufactured by any method, regardless of the presence or absence of additives.

Thermal conductivity of AlN substrate is 40W/m・K or more,
For example, 100W/m・K and alumina ceramic
It has a mechanical strength of 40 to 50 Kg/mm ² (alumina 25 Kg/mm ² ) and an electrical dielectric strength of 140 to 170 kV/cm (alumina 100 kV/cm), which is required for circuit boards. Its properties are better than those of alumina. By taking advantage of these good properties of AlN ceramics, it is possible to obtain circuit boards with high thermal conductivity. In the present invention, since the surface of the AlN substrate is oxidized, it cannot be strictly called AlN, but since the oxide layer is extremely thin, the various properties of the high thermal conductivity circuit board obtained by the present invention depend on the oxidation layer. It is almost the same as the AlN substrate without it.

As the glass layer of the present invention, those commonly used for alumina substrates can be used. for example,
Glasses such as PbO-SiO ₂ -B ₂ -O ₃ and BaO-SiO ₂ -B ₂ O ₃ can be used. Considering the effects of changes in the oxidation state of the AlN substrate, the softening point is
It is preferable to bond glass having a temperature of 350 to 950°C at a temperature of about 400 to 1000°C. In addition, in the present invention, the elements constituting the glass layer may be diffused into the oxide layer to some extent, and in the highly thermally conductive circuit board obtained by the present invention, the oxide layer may contain a small amount of such diffusing components. There is no problem if you do.

In addition, by using an AlN substrate with an oxide layer in this way, there is no formation of wrinkles in the thick film pattern formed on the substrate, so when forming a thick film resistor pattern, the resistance value can be formed with good reproducibility. It has the advantage of being able to The thick film resistor paste may be one commonly used for alumina, such as RuO ₂ paste,
LaF ₆ paste etc. can be used. Also,
Usual Ag, Au, Cu, Ni, as conductor paste
Al paste etc. can be used.

[Effects of the Invention] As explained above, according to the present invention, a glass layer can be firmly bonded onto an AlN substrate,
It is possible to obtain a highly thermally conductive circuit board that takes advantage of the electrical insulation and mechanical strength of AlN ceramics.

This circuit board generates a relatively large amount of heat and is suitable for high-density mounting and for mounting power semiconductors.

[Embodiments of the invention] Examples of the present invention will be described below.

Example 1 An AlN substrate containing 3wt% Y ₂ O ₃ was exposed to 1250 nm in the atmosphere.
Oxidation treatment was performed at ℃ for 1 hour. A 6μm Al ₂ O ₃ layer was formed on the surface of the obtained AlN substrate.Next, glass paste (Nippon Electric Glass LS0120M) was applied to the predetermined areas and the binder was removed.
Calcination was performed at 350°C for 5 minutes. Then, place a 42 alloy lead frame and heat at 420℃ for 10min.
The main firing was carried out under the following conditions. Next, after mounting and bonding the IC, an alumina cap was bonded and sealed using the glass by firing at 420° C. for 10 minutes.

It was confirmed that no bubbles were observed in the glass layer and that good bonding was achieved. The bonding strength of both the lead frame and the cap was 500 g or more under normal load before breaking, which is a value that does not pose any practical problems. Thermal resistance was 15% lower than that of a similar CERDIP 28 pin package in alumina. Therefore, in the case of alumina, the input that could only be 1W is 1.2W.
Now you can enter up to

Example 2 An AlN substrate was immersed in a 10% phosphoric acid solution and subjected to oxidation treatment. An oxide film of 3 μm was formed on the surface of the substrate. On this AlN substrate, Au paste (dupont991) was printed in a 325 mesh pattern,
After being left at room temperature for 10 minutes, it was dried at 120°C for 10 minutes, and then fired at 850°C for 10 minutes. Glass paste (Dupont 9950) was printed and formed as a dielectric layer so as to have through holes for conduction between the upper and lower conductor layers, and fired under the same process as the above conditions. This glass layer formed good adhesion with the AlN substrate. This process was repeated three times to achieve three-layer wiring. There was no shortening between the layers, the bonding strength was sufficient, and the reliability was high.

Example 3 An AlN substrate containing 6wt% Y ₂ O ₃ was heated at 121°C for 2
It was placed in water vapor at atmospheric pressure and left for 168 hours for oxidation treatment. A boehmite film (like Al ₂ O ₃ ) with a thickness of 3 to 5 μm was formed on the surface of the substrate. then
After printing Ag-Pd paste (ESL9601) in a predetermined pattern and drying it at 125℃ for 10 minutes,
A conductor path was formed by firing at 930°C for 10 minutes. Next, resistive paste (dupont16 series 100kΩ/□, 1kΩ/□,
10kΩ/□, 1000kΩ/□) was printed using a 250 mesh screen. When fired at 850°C for 10 min. in air, good adhesion was obtained without forming any bubbles in the resistor, and all resistance values were ±15%.
It was delivered within.

As a comparative example, when a resistor was similarly formed on an AlN substrate without oxidation treatment, it showed a very large variation of about ±30%. This tendency was the same for hot-pressed AlN substrates that did not contain termination aids.

As explained in the examples above, by forming an oxide layer on the AlN substrate regardless of whether it contains additives or not, it becomes possible to bond the conductor layer and the glass layer. A highly thermally conductive circuit board can be obtained by fully utilizing high thermal conductivity, high pressure resistance, and mechanical strength.

Claims (1)

[Claims] 1. High thermal conductivity characterized by forming an oxide layer on the surface of an aluminum nitride ceramic substrate in advance, and then forming a glass layer by firing a glass paste on the aluminum nitride ceramic substrate with the oxide layer formed on the surface. Method of manufacturing circuit boards.