JP7513984B2 - METHOD FOR RELAXING RESIDUAL STRESS IN CERAMIC-METAL BONDED BODY, ... AND METHOD FOR MANUFACTURING CERAMIC-METAL BONDED BODY - Google Patents

Description

The present invention relates to a bonded body of a ceramic member and a metal member, and more specifically, to a method for alleviating residual stress occurring in the ceramic in a ceramic-metal bonded body, the ceramic-metal bonded body, and a method for manufacturing the ceramic-metal bonded body.

For example, ceramic-metal joints, in which a metal member is joined to a ceramic member with high thermal conductivity and high electrical insulation, such as aluminum nitride (AlN), are widely used as mounting substrates for electronic components in power modules.

Ceramic-metal joints are formed, for example, by the active metal method, in which a metal plate is attached to a ceramic substrate via a brazing material, or by the molten metal joining method, in which molten metal is brought into contact with the surface of the ceramic substrate and cooled to solidify, thereby directly joining the metal plate to the ceramic substrate. If the ceramic-metal joint is a circuit substrate, the metal plate is further etched to form a circuit. Since the ceramic and metal members have different thermal expansion coefficients, the difference in thermal expansion during the heating process during joining and the cooling process after joining causes tensile residual stress in the ceramic member.

For example, in the case of a circuit board for mounting power semiconductors using a conventional AlN (aluminum nitride)-Cu (copper) joint, cracks may occur in the AlN near the edge of the Cu joint metal part after about 50 thermal cycle tests (TCT).

For example, Patent Document 1 discloses a method for preventing cracks from occurring in such ceramic members, which includes a step of forming a metal layer on the surface of a ceramic insulating substrate, a step of forming a protective layer to cover the portion of the metal layer where the metal circuit is to be formed, and a step of selectively removing the metal layer and the ceramic insulating substrate that are not protected by the protective layer to form a step, thereby forming the metal circuit forming surface, which is the bonding interface between the ceramic insulating substrate and the metal circuit, to be higher than the exposed ceramic surface where the metal circuit is not formed, and forming the end face of the metal circuit, the end face of the metal circuit forming surface, and the exposed ceramic surface to be continuously and smoothly connected, thereby improving the bonding reliability between the insulating substrate and the metal circuit, and providing a ceramic circuit substrate that can ensure excellent heat cycle resistance.

Patent Document 2 also discloses a metal-ceramic bonded substrate that has excellent bond strength between the ceramic substrate and the metal plate and excellent heat cycle resistance, by spraying a slurry containing abrasive grains in a liquid onto the surface of the ceramic substrate so that the residual stress of the ceramic substrate is -50 MPa or less and the arithmetic mean roughness Ra of the bonded surface of the ceramic substrate with the metal plate is 0.15 to 0.30 μm, and then bonding the metal plate to the ceramic substrate obtained by this treatment.

Patent Document 3 discloses a circuit board in which a conductive metal plate is attached to a ceramic substrate using an adhesive made of composite powder mechanically interlocked and bonded by a mechanical alloy method, and then the surface of the metal plate is subjected to a peening treatment, thereby applying compressive stress to the metal plate, alleviating residual stress and improving thermal shock resistance, thereby preventing peeling at the joint and cracking of the substrate.

Patent Document 4 also discloses a technique for improving heat cycle resistance by forming a shot peening layer on the surface of the metal part of a metal-ceramic composite substrate produced using a molten metal bonding method.

JP 2001-274545 A JP 2014-101248 A Japanese Patent Application Laid-Open No. 63-254031 Japanese Patent Application Laid-Open No. 10-87385

However, in the above-mentioned Patent Document 1, sandblasting or the like is performed as an etching process for selectively removing the metal layer and the ceramic insulating substrate surface that are not protected by the protective layer to form steps, and this may cause damage to the surface of the ceramic substrate exposed between the circuits, etc., reducing the strength and heat cycle resistance of the circuit substrate, or may cause variations in the strength and heat cycle resistance of the circuit substrate due to difficulty in uniform processing. In addition, it is difficult to form a continuous, smooth connection between the end face of the metal circuit, the end face of the metal circuit forming surface, and the exposed ceramic surface all around the circumference of the metal circuit.

In Patent Document 2, a ceramic-metal bonded body is manufactured after wet blasting or the like is performed on the ceramic members in advance, and it is not possible to improve the heat cycle resistance of an already formed bonded substrate. Furthermore, it is necessary to uniformly blast the entire surface of the ceramic substrate, which may make it difficult to manufacture a metal-ceramic bonded substrate with little variation. Furthermore, in Patent Documents 3 and 4, there is a risk that the metal members forming the circuits, etc. may be worn or damaged by peening.

In order to solve the above problems, the present invention aims to provide a method for relaxing residual stress that reduces the tensile residual stress perpendicular to the bonding interface between a ceramic member and a metal member, reduces the occurrence of cracks or breakage in the ceramic member during the thermal cycles associated with use as a circuit board or the like, reduces damage to the metal member surface and the ceramic member surface, and provides a ceramic-metal bonded body and a method for manufacturing the ceramic-metal bonded body.

The inventors conducted various studies on the mechanism by which cracks occur in ceramic members when a circuit board is subjected to a thermal cycle. As a result, when the stress distribution near the joint edge of the circuit board before cracks or fractures occur in the ceramic member was measured using X-rays, it was found that a tensile stress of 400 MPa or more remained in the direction perpendicular to the joint boundary with the metal on the ceramic surface within 0.1 mm from the joint edge (joint boundary) between the AlN and the metal. It is believed that this residual stress exceeded the material strength of the AlN during TCT, causing the cracks to occur.

In order to solve the above problems, the present invention provides a method for relaxing residual stress in a ceramic-metal joined body, characterized in that in a joined body in which a metal member is joined to a surface of a ceramic member, the ceramic member and the metal member are plate materials, the ceramic member is larger than the metal member, a surface of the ceramic member is exposed in the vicinity of a joint boundary between the ceramic member and a lower end of a side surface of the metal member, and a compressive stress is applied to the surface of the ceramic member in the vicinity of the joint boundary between the ceramic member and the lower end of a side surface of the metal member by honing, peening, or blasting treatment, thereby reducing tensile residual stress in a direction perpendicular to the joint boundary on the surface of the ceramic member in the vicinity of the joint boundary.

It is preferable to apply the compressive stress after forming masking portions on the surface of the metal member and the surface of the ceramic member except for the vicinity of the joint boundary.

The ceramic member may be composed mainly of any one of _{Al2O3 , AlN} , and _Si3N4 , and the metal member may be composed mainly of Cu, Al, or an alloy containing either Cu or Al, and the ceramic member and the metal member may be joined by a brazing method or a molten metal joining method.

The present invention also provides a ceramic-metal bonded body in which a metal member is bonded to a surface of a ceramic member, characterized in that the ceramic member and the metal member are plate materials, the ceramic member is larger than the metal member, the surface of the ceramic member is exposed in the vicinity of a bond boundary with a lower end of a side surface of the metal member, and the tensile residual stress in a direction perpendicular to the bond boundary in the surface of the ceramic member in the vicinity of the bond boundary is 250 MPa or less.

The stress gradient in the direction perpendicular to the bond boundary on the surface of the ceramic member near the bond boundary may be 200 MPa/0.1 mm or less. The ceramic-metal bonded body may also be a circuit board.

The present invention also provides a method for producing a ceramic-metal bonded body, characterized in that the method includes a step of relaxing the residual stress in the ceramic-metal bonded body.

According to the present invention, compressive stress is applied to the surface of the ceramic member near the joint boundary between the ceramic member and the lower end of the side of the metal member by honing, peening, or blasting, thereby reducing the residual tensile stress perpendicular to the joint boundary between the ceramic member and the metal member, preventing cracks in the ceramic member, and improving heat cycle resistance.

1A to 1C show an example of a ceramic-metal bonded body to which the present invention is applied, where (a) is a top view, (b) is a vertical cross-sectional view, and (c) is a bottom view. FIG. 2 is an enlarged view of part A in FIG. FIG. 2 is an enlarged view of part B in FIG. 3 is a graph showing a residual stress distribution in a direction perpendicular to a bonded boundary of ceramic members in part C of FIG. 2 . 3 is a schematic cross-sectional view showing an example of a masking portion in part C of FIG. 2 when a compressive stress is applied. FIG.

The present invention is a method for relaxing residual stress in a ceramic-metal bonded body, characterized in that in a bonded body in which a metal member is bonded to the surface of a ceramic member, compressive stress is applied to the surface of the ceramic member near the bonded boundary between the ceramic member and the lower end of the side of the metal member by honing, peening, or blasting, thereby reducing the tensile residual stress in the direction perpendicular to the bonded boundary on the surface of the ceramic member near the bonded boundary.

The following describes an embodiment of the present invention with reference to the drawings. In this specification and the drawings, elements having substantially the same functional configuration are designated by the same reference numerals to avoid redundant description.

FIG. 1 is a diagram showing an example of a ceramic-metal bonded body 1. As the ceramic member 2, for example, ceramics containing AlN (aluminum nitride), Al ₂ O ₃ (alumina), Si ₃ N ₄ (silicon nitride) or the like as a main component (for example, containing 80 mass % or more of AlN, Al ₂ O ₃ , or Si ₃ N ₄ ) are used. In particular, it is preferable to apply the present invention to ceramics containing AlN or Al ₂ O ₃ as a main component, which have lower strength and toughness than silicon nitride. As the metal member 3, for example, Cu, Al, or an alloy containing either Cu or Al as a main component, is used. The ceramic member 2 and the metal member 3 are bonded by a brazing method (active metal method) in which the metal member 3 is bonded to the ceramic member 2 using a brazing material containing an active metal, or a molten metal bonding method in which the metal member 3 is bonded to the ceramic substrate 2 by solidifying a molten metal.

Figure 1 shows a circuit board as one embodiment of a ceramic-metal bonded body 1. In bonding using the active metal method, a ceramic member 2 made of a ceramic substrate (plate material) and a metal member 3 made of a metal plate are bonded via a brazing material containing an active metal such as titanium, and after bonding, the metal member 3 and the brazing material layer (not shown) are etched to form a circuit pattern, etc., to produce a circuit board for a power module, etc.

In addition, in the case of bonding using the molten metal bonding method, a ceramic substrate (ceramic member 2) is placed in a mold, and then molten aluminum or aluminum alloy is poured into the mold so that it comes into contact with the ceramic substrate. The molten metal is then cooled and solidified to form a metal plate (metal member 3) made of aluminum or aluminum alloy that is directly bonded to the ceramic substrate, producing a circuit board (ceramic-metal bonded body 1) with a shape such as that shown in Figure 1.

When the ceramic-metal bonded body 1 is a circuit board, the ceramic member 2 preferably has a thickness of 0.2 to 2.0 mm, more preferably 0.3 to 1.0 mm, and the metal member 3 preferably has a thickness of 0.2 to 5.0 mm. When the metal member 3 is a circuit pattern, the metal member 3 preferably has a thickness of 0.2 to 2.0 mm, more preferably about 0.25 to 1.0 mm. The (insulating) distance between the circuit patterns is preferably 1.0 mm or more, more preferably 1.2 mm or more, and even more preferably 1.5 mm or more.

Figure 2 is an enlarged plan view of part A in Figure 1(a). Figure 3 is an enlarged longitudinal cross-sectional view of part B in Figure 1(b), showing the vicinity of the joint boundary 11, which is the joint boundary between the surface of the ceramic member 2 and the lower end of the side of the metal member 3. In the example of Figure 3, the angle between the side of the metal member 3 and the surface of the ceramic member 2 is 90°. Note that in the present invention, the angle between the side of the metal member 3 and the surface of the ceramic member 2 is not limited to 90°, and can be applied to ceramic-metal joints in which the angle is, for example, 15° to 120°, preferably 45° to 90°.

In such a ceramic-metal joint 1, residual stress occurs on the surface of the ceramic member 2 at the joint with the metal member 3. FIG. 4 shows the distribution of residual stress on the surface of the ceramic member 2 in the direction perpendicular to the joint boundary 11 between the surface of the ceramic member 2 and the side of the metal member 3 in part C of FIG. 2 (the direction of the arrow in part C of FIG. 2), where the "+" on the vertical axis indicates tensile stress and the "-" indicates compressive stress, the horizontal axis indicates part C of FIG. 2, and the left and right dashed lines indicate the position of the lower end of the side of the metal member 3 (joint boundary 11). As shown in FIG. 4, the residual stress occurring on the surface of the ceramic member 2 is the largest tensile stress at the joint boundary 11 between the ceramic member 2 and the side of the metal member 3, and large tensile stress is also occurring near the joint boundary 11. In addition, near the center of the metal member 3 (vertical axis in FIG. 4) away from the joint boundary 11, the tensile stress is smaller than that at the joint boundary 11 and its vicinity, and there is also an area where the stress is compressive. The present invention aims to relieve the tensile stress that occurs in the ceramic member 2, particularly in the vicinity of the bond boundary 11, by performing honing, peening, or blasting on the surface of the ceramic member near the bond boundary 11.

The honing, peening, or blasting process is preferably performed in a direction approximately perpendicular to the bonding interface between the ceramic member 2 and the metal member 3, in the vicinity of the bonding boundary 11 within as narrow an area as possible from the bonding boundary 11, so as not to apply compressive stress to positions away from the bonding boundary 11 between the ceramic member 2 and the metal member 3, and so as not to damage the circuit portion of the metal member 3.

Furthermore, in order to apply compressive stress concentrated in the vicinity of the joint boundary 11 between the ceramic member 2 and the metal member 3, it is preferable to apply compressive stress after forming a masking portion 5 on the surface of the metal member 3 and the surface of the ceramic member 2 excluding the vicinity of the joint boundary 11, as shown in FIG. 5. FIG. 5 is a schematic cross-sectional view showing an example of a state in which the masking portion 5 is formed in part C of FIG. 2. Examples of materials that can be used for the masking portion 5 include masking tape, dry film, and resist ink. To form the masking portion 5 in a narrow area such as the surface of the ceramic substrate 2, it is preferable to apply the resist ink using an inkjet printer or dispenser.

In this specification, the vicinity of the bond boundary 11 refers to the portion where the tensile stress described above is large, and refers to the surface area of the ceramic member 2 within a range (distance) of approximately 0.3 mm in the vertical direction from the bond boundary 11.

The peening process may be, for example, shot peening, in which a nozzle with a tip diameter of 3 mm is used to spray steel ball particles with a diameter of approximately 50 μm at 100 m/sec. The shot material may also include soft plastic balls such as nylon balls.

The blasting treatment may be, for example, shot blasting in which a nozzle with a tip diameter of 4 mm is used to project Al ₂ O ₃ having a grain size of JIS #100 to 150 at 0.3 to 0.5 MPa, preferably 0.35 to 0.45 MPa, at 0.75 Nm ³ /min.

By carrying out the above-mentioned treatment, the residual stress (tensile stress) near the bond boundary 11 between the ceramic member 2 made of AlN and the side surface of the metal member 3 can be reduced to 250 MPa or less, and the stress gradient near the bond boundary 11 can be reduced to 200 MPa/0.1 mm or less, resulting in a ceramic-metal bonded body 1 with significantly improved heat cycle resistance. In the method for relaxing residual stress in a ceramic-metal bonded body according to the present invention, the residual stress (tensile stress) near the bond boundary 11 can be reduced to 200 MPa or less, or even 100 MPa or less.

Residual stress can be measured by, for example, the sin ² ψ method using a CrKα X-ray beam with an irradiation diameter of 0.1 mm or a CuKα X-ray beam with an irradiation diameter of 0.2 mm, or the 2D method or cosα method using an X-ray two-dimensional detector. Note that the X-ray beam can also be MnKα, CoKα, or VKα in addition to CrKα or CuKα.

The above describes preferred embodiments of the present invention, but the present invention is not limited to these examples. It is clear that a person skilled in the art can come up with various modified or revised examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

Comparative Example
A metal member made of a Cu plate having a length of 32 mm, a width of 32 mm and a thickness of 0.25 mm was bonded to both sides of a flat ceramic member (AlN substrate manufactured by Tokuyama Corporation) made of highly thermally conductive AlN having a length of 34 mm, a width of 34 mm and a thickness of 0.635 mm by using an active metal bonding method (AMC) with a Ti-containing Ag-Cu-based brazing material to create an AlN-Cu bonded body (bonded substrate).

Furthermore, a circuit-shaped etching resist (ink) was formed by screen printing on the surface of the Cu plate formed (bonded) on one of the main surfaces (front surfaces) of the AlN substrate of the AlN-Cu bonded body, and after curing with ultraviolet light, unnecessary parts of the Cu plate and brazing material were removed by etching with a chemical solution, and then the etching resist was removed to form a circuit pattern consisting of multiple islands (Cu circuit plates), producing an AlN-Cu bonded circuit substrate, which is a ceramic-metal bonded body. The angle between the surface of the AlN substrate and the side surface of the Cu circuit plate was 90°. The distance between the islands (circuits) was 1.2 mm.

In the center of the length of the Cu circuit board of this circuit board, a flux of X-rays was irradiated onto the surface of the AlN substrate at a position at a predetermined distance in the vertical direction from the joint boundary (circuit pattern end) between the surface of the AlN substrate and the side of the Cu circuit board, and the residual stress of the ceramic substrate surface was measured by the 2D method of X-ray stress measurement. The measurement conditions were tube voltage 38 kV, tube current 16 mA, and a flux of CrKα rays with a diameter of 0.2 mm was used to measure the (202) diffraction ring of AlN appearing near the diffraction angle 2θ ₀ = 150.3 deg, and the stress was calculated with Young's modulus (longitudinal elastic modulus) E = 315 GPa and Poisson's ratio ν = 0.24. As a result, a residual tensile stress of 350 MPa was generated in the direction perpendicular to the bond boundary in a φ0.2 mm region on the surface of the AlN substrate at a location 0.1 mm away from the bond boundary in the vertical direction, and the extrapolated value of the bond end (bond boundary) was 450 MPa. The absolute value of the stress gradient in the direction perpendicular to the bond boundary on the ceramic substrate surface near the bond boundary (between 0.1 and 0.2 mm in the vertical direction from the bond boundary) was about 300 MPa/0.1 mm. When this bonded body was subjected to a thermal cycle test (TCT) from 125°C to -40°C, a crack occurred at one end of the bond after 250 cycles.

<Example>
An AlN-Cu bonded circuit board was produced under the same manufacturing conditions as those of the comparative example. A masking tape (N-300 Elep Masking for Printed Circuit Boards, manufactured by Nitto Denko Corporation) was applied to the entire surface of the Cu circuit board of this circuit board. Next, the masking tape was applied to the ceramic substrate surface except for an area within 0.3 mm in the vertical direction from the lower end (bonding boundary) of the side of the Cu circuit board, and a masking portion was formed on the ceramic substrate surface except for the vicinity of the bonding boundary. In this masked AlN-Cu bonded circuit board, wet honing of alumina media was performed along the bonding boundary between the surface of the AlN substrate and the side of the Cu circuit board. As the conditions of the wet honing, alumina powder with a particle size of JIS #120 (sieve opening 125 μm) was used, the nozzle diameter was 4 mm, the injection pressure was 0.4 MPa, and 0.75 Nm ³ /min. In this way, the compressive stress by wet honing was applied to the surface of the ceramic substrate near the bonding boundary, and then the masking portion (masking tape) was removed.

The Al-Cu bonded circuit board obtained in this way was used to measure the residual stress on the surface of the AlN substrate in a φ0.2 mm region 0.1 mm away from the bond boundary in the vertical direction, using the same method as in the comparative example, and the stress gradient was calculated. As a result, the residual stress was a tensile stress of 200 MPa, and the stress gradient had dropped to approximately 120 MPa/0.1 mm. Furthermore, no cracks were observed up to 500 TCT cycles.

As described above, by honing the surface of the ceramic substrate near the bonding interface between the ceramic and metal sides, it was possible to achieve a residual stress perpendicular to the interface of 250 MPa or less and an interface stress gradient of 200 MPa/0.1 mm or less, improving TCT resistance by more than two times. In addition, by using masking, honing was not performed on unnecessary parts, suppressing damage to the circuit board and ceramic substrate surface and enabling processing with little variation.

The present invention can be applied to a bonded body in which a metal member is bonded to the surface of a ceramic member.

1 Ceramic-metal bonded body 2 Ceramic member 3 Metal member 5 Masking portion 11 Bonding interface

Claims (7)

In a joined body in which a metal member is joined to a surface of a ceramic member,
the ceramic member and the metal member are plate materials,
The ceramic member is larger than the metal member, and a surface of the ceramic member is exposed in the vicinity of a joint boundary between the ceramic member and a lower end of a side surface of the metal member,

A method for relaxing residual stress in a ceramic-metal bonded body, comprising: applying a compressive stress to a surface of the ceramic member in the vicinity of a bond boundary between the ceramic member and a lower end of a side surface of the metal member by honing, peening, or blasting, thereby reducing a tensile residual stress in a direction perpendicular to the bond boundary on the surface of the ceramic member in the vicinity of the bond boundary. The method for relaxing residual stress in a ceramic-metal joint described in claim 1, characterized in that the compressive stress is applied after forming masking portions on the surface of the metal member and the surface of the ceramic member excluding the vicinity of the joint boundary. 3. The method for relaxing residual stress in a ceramic-metal joint according to claim 1, wherein the ceramic member is composed mainly of any one of Al ₂ O _{3 ,} AlN, and Si ₃ N ₄ , the metal member is composed mainly of Cu, Al, or an alloy composed mainly of either Cu or Al, and the ceramic member and the metal member are joined by a brazing method or a molten metal joining method. In a joined body in which a metal member is joined to a surface of a ceramic member,
the ceramic member and the metal member are plate materials,
The ceramic member is larger than the metal member, and a surface of the ceramic member is exposed in the vicinity of a joint boundary between the ceramic member and a lower end of a side surface of the metal member,
A ceramic-metal bonded body, characterized in that a tensile residual stress in a direction perpendicular to the bonded boundary on a surface of the ceramic member in the vicinity of the bonded boundary is 250 MPa or less. 5. The ceramic-metal joint according to claim 4 , wherein a stress gradient in a direction perpendicular to the bonded boundary on a surface of the ceramic member in the vicinity of the bonded boundary is 200 MPa/0.1 mm or less. 6. The ceramic-metal bonded body according to claim 4, wherein the ceramic-metal bonded body is a circuit board. A method for producing a ceramic-metal bonded body, comprising a step of relaxing residual stress in the ceramic-metal bonded body by the method according to any one of claims 1 to 3 .

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