JP7655046B2 - Manufacturing method for insulating circuit board - Google Patents

Description

The present invention relates to a method for manufacturing an insulating circuit board used in power modules that control high currents and high voltages.

Conventional insulating circuit boards used in power modules and the like include a ceramic substrate made of, for example, AlN (aluminum nitride), _Al2O3 ( _alumina ), _Si3N4 (silicon _nitride ), etc., a circuit layer formed on one side of the ceramic substrate, and a heat dissipation layer formed on the other side of the ceramic substrate. In addition, brazing filler metal is generally used to bond the ceramic substrate to a metal plate that serves as the circuit layer or the heat dissipation layer. When the metal plate is a copper or copper alloy plate, a brazing filler metal containing an active metal is used.

Patent Document 1 discloses a power module substrate in which a copper plate made of copper or a copper alloy is laminated and bonded to the surface of a ceramic substrate, in which a nitride layer is formed on the surface of the ceramic substrate between the copper plate and the ceramic substrate, and the Ag-Cu eutectic structure layer has a thickness of 15 μm or less. In this case, the ceramic substrate and the copper plate are bonded by inserting a layer of Ag and a nitride-forming element therebetween and heating them in a vacuum heating furnace while being pressurized in the lamination direction (pressure 1 to 35 kgf/cm ² ). It is described that the pressure in the vacuum heating furnace is set in the range of 10 ^-6 Pa to 10 ^-3 Pa, and the heating temperature is set in the range of 790°C to 850°C.

JP 2013-179263 A

As disclosed in Patent Document 1, conventionally, the degree of vacuum reached in the heating furnace during bonding is confirmed using the vacuum level attached to the heating furnace. This bonding method does not impair the bondability initially, but when subjected to a thermal cycle load in which the cooling and heating environments are repeatedly switched between, a problem occurs in which the reliability of the bond is reduced.

The present invention was made in consideration of these circumstances, and aims to reexamine the bonding conditions and achieve good bonding reliability even in an environment where cooling and heating are repeated.

The method for producing an insulated circuit board of the present invention is a method for producing an insulated circuit board having a copper layer on a surface of a ceramic substrate by laminating a ceramic substrate and a copper plate made of copper or a copper alloy with a brazing material interposed therebetween, and heating the ceramic substrate and the copper plate in a heating furnace while applying pressure in the lamination direction, and the degree of vacuum in the heating furnace in a temperature range of 600°C or higher and 770°C or lower is set to 1.0 x ^10-2 Pa or lower.

Since the bonding is between a ceramic substrate and a copper plate, the bonding temperature during bonding is 790°C to 850°C, as described in Patent Document 1, and the degree of vacuum at that time is set in the range of ^10-6 Pa to ^10-3 Pa. However, the degree of vacuum may decrease and the desired degree of vacuum may not be obtained before the maximum bonding temperature and maximum degree of vacuum are reached. It is assumed that the decrease in the degree of vacuum (vacuum deterioration) is the cause of the deterioration of bonding reliability in an environment where cooling and heating are repeated. In other words, gas is generated in the heating furnace in a specific temperature range before the maximum bonding temperature is reached, and vacuum deterioration occurs due to the influence of the gas. The inventor of the present invention made the above-mentioned invention with the recognition that the bonding reliability can be improved by suppressing this vacuum deterioration.

That is, the present invention controls the maximum bonding temperature and maximum degree of vacuum while suppressing gas generation, and controls the degree of vacuum during the temperature rise process, specifically, the degree of vacuum in the temperature range of 600° C. to 770° C. to be 1.0×10 ⁻² Pa or less. Even if a small amount of gas is generated in the heating furnace in this temperature range, by ensuring a degree of vacuum of 1.0×10 ⁻² Pa or less, bonding reliability can be improved.

In this method for manufacturing an insulating circuit board, the ratio of the ion intensity of _H2 , CO, and _CO2 to the total ion intensity of mass numbers of 2 or more and 200 or less measured by a quadrupole mass spectrometer is preferably 20.3% or less for _H2 , 50.9% or less for CO2, and 4.0% or less for _CO2 .

If these gases are generated in amounts greater than a certain level, they will cause vacuum deterioration. In addition, _H2 and CO reduce the oxide film on the copper surface, and the _H2O generated by the reduction will cause problems such as oxidizing the components of the joining material. They will also penetrate into the molten joining material and cause voids. Therefore, by controlling the concentrations of these gases to a certain level or less, the effects of the gases can be suppressed and the joining reliability can be improved.

According to the present invention, by controlling the degree of vacuum in a specified temperature range during bonding, good bonding reliability can be obtained even in an environment where cooling and heating are repeated.

1 is a cross-sectional view showing an example of an insulating circuit board manufactured by the manufacturing method of the present invention. 2 is a cross-sectional view showing a layer structure of a laminate before bonding of the insulating circuit board of FIG. 1. FIG. 3 is a front view showing a pressing jig for pressing the laminate of FIG. 2 . FIG. 4 is a cross-sectional view showing a backing plate used in the pressing jig. FIG. 1 is a schematic diagram of a test apparatus. 1 is a graph showing the relationship between temperature, ion intensity, and vacuum degree during a bonding process.

Hereinafter, an embodiment of the method for producing an insulating circuit board of the present invention will be described.
The insulating circuit board manufactured by the manufacturing method of this embodiment is, for example, a power module substrate used in a power supply circuit. As shown in Figure 1, this power module substrate 10 has a ceramic substrate 11, a circuit layer 12 formed on one surface of the ceramic substrate 11, and a heat dissipation layer 13 formed on the other surface of the ceramic substrate 11.

The ceramic substrate 11 prevents electrical connection between the circuit layer 12 and the heat dissipation layer 13, and may be made of aluminum nitride (AlN), _silicon nitride ( _Si3N4 ), _aluminum oxide ( _Al2O3 ), etc., of which silicon nitride is preferred due to its high strength. The thickness of the ceramic substrate 11 is set within the range of 0.2 mm to 1.5 mm.

The circuit layer 12 is made of copper or a copper alloy, which has excellent electrical properties. The heat dissipation layer 13 is also made of copper or a copper alloy. The circuit layer 12 and heat dissipation layer 13 are formed, for example, by brazing a copper plate made of oxygen-free copper with a purity of 99.96% by mass or more to the ceramic substrate 11 with, for example, an active metal brazing material. The thickness of the circuit layer 12 and heat dissipation layer 13 is set within the range of 0.1 mm to 5 mm.

As shown in FIG. 2, the power module substrate 10 thus constructed is formed by laminating copper plates 12' and 13' on both sides of a ceramic substrate 11 with brazing material 21 between them. A brazing material made of Ag-Ti or Ag-Cu-Ti is used as the brazing material 21. This brazing material 21 may be supplied in the form of a foil, or may be formed by applying a brazing material paste to the ceramic substrate 11 by screen printing or the like.

The laminate S of the ceramic substrate 11 and the copper plates 12', 13' laminated with the brazing material 21 interposed therebetween is placed in a heating furnace under pressure, heated to the joining temperature in a vacuum atmosphere, and then cooled to form a circuit layer 12 on one side of the ceramic substrate 11 and a heat dissipation layer 13 on the other side.
In this case, the pressing tool shown in FIG. 3, for example, is used.
This pressure application tool 110 comprises a base plate 111, a number of guide posts 112 attached vertically to the four corners of the upper surface of the base plate 111, a fixed plate 113 fixed to the upper ends of the guide posts 112, a pressure plate 114 supported by the guide posts 112 so as to be freely movable up and down between the base plate 111 and the fixed plate 113, and a biasing member 115 such as a spring provided between the fixed plate 113 and the pressure plate 114 to bias the pressure plate 114 downward.

The base plate 111, the fixed plate 113, the pressure plate 114, the guide post 112, etc. are made of a carbon composite material that is a compound of carbon fiber. The fixed plate 113 and the pressure plate 114 are disposed parallel to the base plate 111, and the laminate S is disposed between the base plate 111 and the pressure plate 114.
In this case, a contact plate 30 is disposed on the side of the base plate 111 and the pressing plate 114 that contacts the laminate S to ensure uniform pressure.

As shown in FIG. 4, the backing plate 30 has a structure in which a graphite sheet 32 is sandwiched between two carbon sheets 31 .
The carbon sheet 31 is formed into a flat plate shape from a heat-resistant hard carbon material and sintered at a high temperature of about 3000° C. This carbon sheet 31 is configured to have a relatively hard, smooth flat surface with a bulk density of 1.6 Mg/m ³ or more and 1.9 Mg/m ³ or less (1600 kg/m ³ or more and 1900 kg/m ³ or less). For example, G-347 (thermal conductivity: 116 W/mK, elastic modulus: 10.8 GPa) manufactured by Asahi Graphite Co., Ltd. can be used.

On the other hand, the graphite sheet 32 is made of a soft graphite material having cushioning properties, and is formed by laminating a plurality of thin flake graphite films like mica, and is obtained by treating natural graphite with an acid, forming it into a sheet, and rolling it. The graphite sheet 32 is soft, with a bulk density of 0.5 Mg/ ^m3 or more and 1.3 Mg/ ^m3 or less. For example, T-5 (thermal conductivity: 75.4 W/mK, elastic modulus: 11.4 GPa) manufactured by Asahi Graphite Co., Ltd., graphite sheet PF (compressibility 47%, recovery rate 14%) manufactured by Toyo Tanso Co., Ltd., or the like can be used.
When the contact plates 30 are disposed on both end surfaces of the laminate S, the carbon sheets 31 come into contact with both end surfaces of the laminate S.

Then, with the backing plates 30 placed on both end surfaces of the laminate S, a pressure tool 110 is used to apply pressure in the lamination direction, and the entire laminate S including the pressure tool 110 is heated in a vacuum atmosphere.
The bonding conditions are, for example, pressing the laminate S with a pressure of 0.1 MPa to 3.5 MPa, heating in a vacuum atmosphere of 10 ⁻⁶ Pa to 10 ⁻³ Pa at a maximum bonding temperature of, for example, 790° C. to 850° C. for 1 to 60 minutes.

During this heat treatment, solvent is removed from the brazing material 21 as the temperature inside the heating furnace is increased, but it is important to maintain the degree of vacuum inside the heating furnace at 1.0×10 ⁻² Pa or less in the temperature range of 600° C. to 770° C.
During the time until the maximum bonding temperature and maximum degree of vacuum are reached, gases such as _H2 , CO, and _CO2 are generated in the heating furnace from the internal structures, such as carbon in the pressurizing jig 110. The generation of these gases causes vacuum deterioration and may prevent the desired degree of vacuum from being obtained, so the effect of the generated gases is reduced by setting the degree of vacuum in the heating furnace to 1.0 x ^10-2 Pa or less in the temperature range of 600°C to 770°C.

For example, a copper oxide film is generally formed on the surface of the copper plates 12', 13', but when _H2 or CO is present, the oxide film is reduced, which generates _H2O , which makes the silver, titanium, etc. contained in the copper and brazing material 21 easily oxidized. Titanium is particularly susceptible to oxidation, and when it is oxidized by _H2O generated by the reduction of the copper oxide film, the oxidized titanium is adsorbed to the carbon caused by the dispersant around the silver in the brazing material 21. Therefore, the amount of titanium that contributes to the bonding of _the ceramic substrate ( _Si3N4 ) 11 is reduced, and the generation of titanium nitride is inhibited. In addition, if gases such as _H2 , CO, and _CO2 are generated in large amounts, bubbles may accumulate in the brazing material layer, and even if it becomes molten, they may not be able to escape and may remain as voids. The combined action of these factors deteriorates the bonding reliability.

In this embodiment, in order to reliably control the degree of vacuum in the heating furnace to 1.0×10 ⁻² Pa or less in the temperature range of 600° C. to 770° C., for example, the carbon used in the pressurizing jig 110 is heated to a high temperature before use to release gas from the carbon in advance. Specifically, the pressurizing jig 110 is heated to, for example, 770° C. (this is called baking). This causes gas components to be released from the pressurizing jig 110. In this case, all the members of the pressurizing jig 110 are baked. Furthermore, when the pressurizing jig 110 is placed in a heating furnace and baked, the structures in the heating furnace (for example, carbon insulation) are also baked and gas is released.

In actual bonding, the laminate S is pressurized and heated using the pressure jig 110 and heating furnace that have been subjected to the pre-baking.
As a result, since the gas has been released in advance, the influence of the gas during heating is reduced, and the degree of vacuum can be maintained at 1.0×10 ⁻² Pa or less in the temperature range of 600° C. to 770° C. Even if a small amount of gas is generated, the influence can be reduced and the degree of vacuum can be maintained at 1.0×10 ⁻² Pa or less. It is even better to maintain this degree of vacuum in the temperature range of 550° C. to 700° C.

In this manner, while controlling the degree of vacuum in the temperature range of 600°C or more and 770°C or less, the temperature is raised to a bonding temperature of 790°C or more and 850°C or less, and the degree of vacuum at the maximum temperature is set to ^10-6 Pa or more and ^10-3 Pa or less. After maintaining this bonding temperature and degree of vacuum for 1 to 60 minutes, the substrate is cooled, whereby copper plates 12', 13' are bonded to ceramic substrate 12, and insulating circuit substrate 10 is formed in which copper layers (circuit layer 12, heat dissipation layer 13) are formed on both sides of ceramic substrate 11.
This insulating circuit board 10 has a reduced effect of gas during the bonding process, and therefore has high bonding reliability and can maintain stable bonding not only at the initial stage but also over the long term.
The pressing jig 110 can be used multiple times, but it is advisable to carry out a baking step before each use.

The present invention is not limited to the configuration of the above embodiment, and various changes can be made to the details of the configuration without departing from the spirit of the present invention.
In addition to the above, it is effective to limit the use of carbon parts in the heating furnace by, for example, changing the heat insulating material in the heating furnace to a metal one, thereby reducing the number of carbon parts in the heating furnace.
Further, in the embodiment, the power module substrate 10 has been described as an insulating circuit substrate, but the present invention is also applicable to insulating circuit substrates other than power module substrates.

A copper plate (37 mm x 37 mm x 0.3 mm) made of oxygen-free copper (OFC) with a purity of 99.99% by mass or more was laminated on _one side of a ceramic substrate ( ₄₀ mm x 40 mm x 0.32 mmt) made of Si ^{3 N 4 via a brazing paste made of Ag-8.8% by mass Ti, and the laminate was pressed with a pressure of 1.5 MPa using the pressing tool described in the embodiment, heated to a bonding temperature of about 825 ° C. for 30 minutes in a vacuum atmosphere of 10 -5} Pa or less, and then cooled to produce an insulating circuit board. At this time, two types of insulating materials were prepared for the heating furnace, one made of metal and one made of carbon, and the carbon insulating materials were used, one that had been pre-baked and one that had not been pre-baked. Two types of pressing tools were also used, one that had been pre-baked and one that had not been pre-baked.

Further, the maximum degree of vacuum in the temperature range of 600° C. to 770° C., the maximum bonding temperature, and the degree of vacuum at that temperature were each measured.
In addition, the H ₂ concentration, CO concentration, and CO ₂ concentration in the heating furnace were measured in the temperature range of 600 ° C. or more and 770 ° C. or less. In the measurement, as shown in FIG. 5, a quadrupole mass spectrometer 53 (Qulee with YTP model BGM-202) manufactured by ULVAC, Inc. was connected to the port 52 of the heating furnace 51, and the gas components in the heating furnace 51 were analyzed while the port 52 was evacuated with a turbo molecular pump 54. The degree of vacuum was measured with a Penning gauge 55 (CT-3DA manufactured by Daia Vacuum Co., Ltd.) connected to the port 52. In this case, an oil diffusion pump 57 is provided in the vacuum piping 56 of the heating furnace 51, and the port 52 for gas analysis is installed on the opposite side of the oil diffusion pump 57.

The ion intensity was measured every 10 seconds by the quadrupole mass spectrometer 53, and the ion intensity ratio of the gas components in the heating furnace 51 was calculated. The ratio of each ion intensity of _H2 , CO, and CO2 to the total ion intensity of the mass numbers of 2 to 200 measured by the quadrupole mass spectrometer ₅₃ (each ion intensity/total ion intensity of the mass numbers of 2 to 200 x 100) was calculated. The maximum values in the temperature range of 600°C to 770°C are shown in Table 1.

The bonded insulating circuit board was then subjected to a thermal cycle test at -40°C to 150°C for 2000 cycles. After the test, the bonding surface between the ceramic substrate and the copper layer of the insulating circuit board was subjected to ultrasonic testing. The ultrasonic testing image was then binarized to determine the bonded area excluding the peeled portion. The bonding rate was calculated by dividing the bonded area by the area of the interface to be bonded.
The results are shown in Table 1.

As shown in Table 1, in Examples 1 and 2 in which the degree of vacuum was controlled to 1.0× ^10-2 Pa or less in the temperature range of 600°C to 770°C during the heating furnace temperature rise process, there was little decrease in the bonding rate not only immediately after bonding but also after a thermal cycle load, and excellent bonding reliability was maintained. In this case, it is found that it is effective to change the insulating material in the heating furnace to metal or to bake the carbon insulating material in the air in addition to baking the pressure jig.

6 is a graph showing the transition of the temperature and degree of vacuum in the heating furnace and the ion intensity of each gas component in Example 1, where the left vertical axis is the ion intensity (A), the right vertical axis is the degree of vacuum (Pa), and the horizontal axis is the temperature (° C.) Also shown is the transition of the pressure value of the Penning gauge.
As can be seen from FIG. 6, the ionic strength of each gas component increases up to 770° C., and the gas is released into the heating furnace 51, but the degree of vacuum at that time is 1.0×10 ⁻² Pa or less.

In contrast, vacuum degradation was large in the temperature range of 600°C to 770°C, and Comparative Examples 1 and 2, in which the degree of vacuum exceeded 1.0 x ^10-2 Pa, showed excellent bonding reliability at the initial stage of bonding, but the bonding rate decreased after thermal cycle loading, resulting in poor bonding reliability. In this case, even if the carbon insulation material in the heating furnace is pre-baked, if the pressure jig is used without pre-baking, vacuum degradation will occur and bonding reliability after thermal cycles will deteriorate.

10 Power module substrate (insulated circuit substrate)
11 Ceramic substrate 12 Circuit layer (copper layer)
13 Heat dissipation layer (copper layer)
12', 13' Copper plates 21, 22 Brazing material layer 30 Backing plate 31 Carbon sheet 32 Graphite sheet 51 Heating furnace 52 Port 53 Quadrupole mass spectrometer 54 Turbo molecular pump 54
55 Penning gauge 56 Vacuum piping 57 Oil diffusion pump 110 Pressurizing jig 111 Base plate 112 Guide post 113 Fixing plate 114 Pressing plate 115 Pressing member

Claims (3)

A method for producing an insulated circuit board having a copper layer on a surface of the ceramic board, comprising stacking a ceramic board and a copper plate made of copper or a copper alloy with a brazing material interposed therebetween, and heating the ceramic board and the copper plate in a heating furnace while applying pressure in the stacking direction with a pressure jig to bond the two together, the method comprising the steps of: setting the degree of vacuum in the heating furnace to 1.0 x ^10-2 Pa or less in a temperature range of 600°C to 770°C , and then heating to a maximum bonding temperature of 790°C to 850°C in a vacuum atmosphere of 10-6 ^Pa to 10-3 ^Pa . 2. The method for producing an insulating circuit board according to claim 1, characterized in that in the temperature range, the ratios of the ion intensities of _H2 , CO, and _CO2 to the total ion intensities having mass numbers of 2 or more and 200 or less, as measured by a quadrupole mass spectrometer, are 20.3% or less for _H2 , 50.9% or less for CO2, and 4.0% or less for _CO2 . 2. The method for producing an insulating circuit board according to claim 1, wherein the pressing tool contains carbon that has been heated in advance to release gas.