Insulated circuit board

Abstract

Provided is an insulated circuit board in which a metal layer formed from pure aluminum is bonded to at least one surface of a ceramic substrate. In this circuit board: the pure aluminum is aluminum having a purity of 99.9%mass or more; a slip plane of metal crystal in the metal layer and the bonding interface with the ceramic substrate form an angle of 40° or less; and a second metal layer formed from any of aluminum, an aluminum alloy, copper, or a copper alloy may be bonded on the metal layer.

Description

Insulated Circuit Board

[0001] The present invention relates to an insulating circuit board for mounting various semiconductor elements, which is used in automobiles, railway vehicles, elevators, industrial equipment, etc. This application claims priority based on Japanese Patent Application No. 2023-164127, filed on September 27, 2023, the contents of which are incorporated herein by reference.

[0002] Known insulated circuit boards are those in which metal layers are bonded to both sides of an insulating substrate made of a ceramic substrate such as aluminum nitride or silicon nitride, with one metal layer being used as a circuit layer and the other as a heat dissipation layer. Since the metal layer in this case is required to have high properties such as stress buffering effect, thermal conductivity, and electrical conductivity, a high-purity metal, such as 4N-Al, with a purity of 99.9% by mass (so-called 3N) or more is used.

[0003] For example, Patent Document 1 discloses a power module substrate having a ceramic substrate, a circuit layer formed on one surface of the ceramic substrate, and a metal layer (heat dissipation layer) formed on the other surface of the ceramic substrate, and describes that the circuit layer and the heat dissipation layer are made of a high-purity aluminum material, for example, a rolled plate of aluminum having a purity of 99.99% by mass or more (so-called 4N-Al), and that impurities are further removed.

[0004] Japanese Patent Application Laid-Open No. 2015-185707

[0005] With regard to this 4N-Al, recrystallized grains with various orientations are generated depending on the rolling conditions, the load and temperature during bonding. The thickness of the metal layer made of this 4N-Al is in the range of 0.1 to 3.0 mm, and to add the strength required for a device mounting portion, an aluminum alloy material (such as A6063 as specified by JIS (Japanese Industrial Standards)) is sometimes bonded to the metal layer under pressure by vacuum brazing.

[0006] Conventionally, the crystal orientation of the 4N-Al layer near the bonding interface (up to 100 μm) between the ceramic substrate and the 4N-Al layer, as measured by electron backscatter diffraction (EBSD) for both the edge and center cross sections of the circuit pattern, is predominantly (001). This can be achieved by applying rolling conditions developed for capacitors, and this (001) orientation is said to have high resistance to shear strain. However, once a crack occurs at the aluminum edge near the interface, a similar response to stress also occurs in the center near the interface, causing delamination to progress from the edge toward the center, which has been an issue.

[0007] The present invention has been made in consideration of the above circumstances, and aims to suppress the progression of peeling from the edge of the metal layer toward the center even in an environment where cooling and heating are repeated, thereby maintaining high reliability as an insulated circuit board.

[0008] The insulating circuit board of the present invention comprises a ceramic substrate and a metal layer made of pure aluminum having a purity of 99.9% by mass or more bonded to at least one surface of the ceramic substrate, wherein the angle between the slip plane of metal crystals in the metal layer and the bonding interface with the ceramic substrate is 40° or less.

[0009] The slip plane of a metal crystal refers to a crystal plane where slip deformation occurs due to shear strain, and in the case of aluminum, which has a face-centered cubic lattice structure, this is the {111} plane. Because the angle between this slip plane and the bonding interface with the ceramic substrate is small, at 40° or less, the stress that the metal layer experiences due to repeated heating and cooling is greatly alleviated by the slip, thereby increasing the time until fracture and making it less likely to fracture. Therefore, this insulated circuit board can prevent the progression of peeling due to cracks in the metal layer in an environment where repeated heating and cooling occurs, and maintain high reliability.

[0010] In the insulating circuit board of the present invention, a second metal layer made of aluminum, an aluminum alloy, copper, or a copper alloy may be bonded onto the metal layer.

[0011] Even when two bonding processes are performed by separately bonding the metal layer and the second metal layer, the stress relaxation effect of the lower metal layer can prevent the progression of peeling at the interface between the ceramic substrate and the metal layer.

[0012] In the insulating circuit board of the present invention, the pure aluminum may have a purity of 99.99% by mass or more.

[0013] In the insulating circuit board of the present invention, the pure aluminum preferably has a Cu content of 30 ppm or less, an Fe content of 20 ppm or less, an Mn content of 6 ppm or less, and a Cr content of 6 ppm or less, where ppm is a mass percentage (mass ppm).

[0014] By keeping the content of these elements within a specified range, recrystallization due to heating during bonding can be controlled, and the variation in grain size can be reduced. This reduces the unevenness of stress generated in the metal layer, even in an environment where repeated cooling and heating occur, and prevents the metal layer from peeling from the ceramic. Combined with the excellent properties of high-purity aluminum, such as the stress buffering effect, thermal conductivity, and electrical conductivity, this allows the insulating circuit board to maintain high reliability.

[0015] Therefore, by performing elemental analysis on the metal plate that will become the metal layer, it is possible to predict the joint reliability after joining, and this can also be used as a guideline for selecting materials.

[0016] In the insulating circuit board of the present invention, the angle between the slip plane of the metal crystal in the metal layer and the bonding interface with the ceramic substrate may be the smallest angle among the angles between the {111} plane of the metal layer expressed in Miller indices and the bonding interface.

[0017] In the insulating circuit board of the present invention, the ceramic substrate can be made of nitride ceramics or oxide ceramics.

[0018] In the insulating circuit board of the present invention, a heat dissipation layer made of pure aluminum with a purity of 99.9% by mass or more may be bonded to a surface of the ceramic substrate opposite to the bonding surface with the metal layer, and the angle between the sliding plane of the metal crystals in the heat dissipation layer and the bonding interface with the ceramic substrate may be 40° or less.

[0019] In the insulating circuit board of the present invention, a second heat dissipation layer made of aluminum, an aluminum alloy, copper, or a copper alloy may be bonded onto the heat dissipation layer.

[0020] In the insulating circuit board of the present invention, a heat sink-attached insulating circuit board can be formed by joining a heat sink onto the heat dissipation layer, and the heat sink may be made of aluminum, an aluminum alloy, copper, or a copper alloy.

[0021] In the insulating circuit board of the present invention, the pure aluminum in the heat dissipation layer may have a purity of 99.99% by mass or more.

[0022] In the insulating circuit board of the present invention, the pure aluminum of the heat dissipation layer preferably has a Cu content of 30 ppm or less, an Fe content of 20 ppm or less, an Mn content of 6 ppm or less, and a Cr content of 6 ppm or less.

[0023] In the insulating circuit board of the present invention, the angle between the slip plane of the metal crystal in the heat dissipation layer and the bonding interface with the ceramic substrate may be the smallest angle among the angles between the {111} plane, expressed in Miller indices for the metal layer, and the bonding interface.

[0024] According to the present invention, the progress of peeling of the metal layer can be suppressed even in an environment where cooling and heating are repeated, and high reliability as an insulating circuit board can be maintained.

[0025] 1 is a longitudinal sectional view showing an insulating circuit board with a heat sink according to one embodiment of the present invention; FIG. 2 is a sectional view showing the arrangement of components during a first bonding step in manufacturing the insulating circuit board with a heat sink shown in FIG. 1; FIG. 3 is a sectional view showing a first bonded body obtained by the first bonding step; FIG. 4 is a sectional view showing the arrangement of components during a second bonding step; FIG. 5 is an IPF map of a cross section near the bonding interface between a first circuit layer and a ceramic substrate, where (a) shows the case where a conventional pure aluminum plate is used and (b) shows the case where the pure aluminum plate of this embodiment is used; FIG. 6 is a schematic diagram illustrating the slip plane of the metal crystals of the first circuit layer bonded to the ceramic substrate; FIG. 7 is a sectional view showing an insulating circuit board according to another embodiment of the present invention; FIG. 8 is a Weibull plot of the micro Vickers indentation diameter ratio, Vickers indentation diameter ratio, and total elongation ratio at break in the TD and RD directions for each aluminum plate used as a material; and FIG. 9 is a diagram showing the relationship between a site selected for calculating the area ratio of each crystal orientation in the IPF map and the position on the IPF scale. It is a diagram showing the change in crystal orientation before and after a gas-phase thermal reliability test, where (a) shows the case where a conventional pure aluminum plate is used and (b) shows the case where the pure aluminum plate of this embodiment is used.

[0026] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0027] 1, an insulated circuit board 10 according to one embodiment of the present invention includes 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. A heat sink 20 is bonded to the heat dissipation layer 13, and the insulated circuit board 10 and the heat sink 20 are integrated together. This is called an insulated circuit board 100 with a heat sink.

[0028] The ceramic substrate 11 is made of, for example, aluminum nitride (AlN) or silicon nitride (Si ３ N ４ ) or alumina (Al ２ O ３ The thickness of the ceramic substrate is not limited, but is set within the range of 0.2 mm to 1.5 mm, for example.

[0029] The circuit layer 12 has a two-layer structure, including a first circuit layer 14 bonded to the ceramic substrate 11 and a second circuit layer 15 bonded to the upper surface of the first circuit layer 14. The first circuit layer 14 can be made of pure aluminum with a purity of 99.9% by mass or higher (known as 3N aluminum), and more preferably, pure aluminum with a purity of 99.95% by mass or higher. For example, high-purity aluminum equivalent to 1N99 as defined by the Japanese Industrial Standards (JIS) can be used. The JIS defines the standard chemical composition of wrought aluminum as having an aluminum purity of 99.99% by mass or higher (known as 4N aluminum), Si of 60 ppm or less, Fe of 40 ppm or less, and Cu of 80 ppm or less. For example, 1N99 aluminum corresponds to 16 ppm Fe, 24 ppm Si, and 61 ppm Cu.

[0030] In contrast, in the present invention, the aluminum purity is set to 99.99 mass% or more, the Cu content is set to 30 ppm or less, the Fe content is set to 20 ppm or less, the Mn content is set to 6 ppm or less, and the Cr content is set to 6 ppm or less. By casting with a reduced Cu content and applying the rolling process described below, recrystallization during heating in the joining process described below can be controlled, and variation in crystal grain size can be suppressed.

[0031] The purity of aluminum can be determined by the method described in 5.1 of JIS H4170-1991.

[0032] On the other hand, the second circuit layer 15 is made of an aluminum alloy in this embodiment, for example, JIS A6063 alloy, but is not limited to this and may be made of other metals, such as copper or a copper alloy.

[0033] Although there are no limitations on the thickness of the first circuit layer 14 and the second circuit layer 15, the thickness of the first circuit layer 14 is set to 0.1 mm to 3.0 mm, and the thickness of the second circuit layer 15 is set to 0.2 mm to 3.0 mm, and both have the same shape (rectangular flat plate). In the example shown in Fig. 1, the first circuit layer 14 and the second circuit layer 15 are formed on the ceramic substrate 11 so as to be separated into a plurality of islands indicated by symbols A and B.

[0034] The heat dissipation layer 13 is made of pure aluminum with a purity of 99.9% by mass or more, more preferably 99.95% by mass or more, similar to the first circuit layer, and has an aluminum purity of 99.99% by mass or more, a Cu content of 30 ppm or less, an Fe content of 20 ppm or less, a Mn content of 6 ppm or less, and a Cr content of 6 ppm or less. The thickness of this heat dissipation layer 13 is not particularly limited, but is set to 0.1 mm to 3.0 mm, and is formed in a rectangular plate shape.

[0035] The heat sink 20 has pin-shaped fins 24 formed by forging or the like. The material can be copper, aluminum, a copper alloy, or an aluminum alloy, such as the aluminum alloy A6063. The central portion 21 of the heat sink 20 is thinner than the thick peripheral wall portion 22, and a mounting recess 23 for housing at least a portion of the insulating circuit board 10 is formed on the central portion 21 and surrounded by the peripheral wall portion 22. A number of pin-shaped fins 24 are erected parallel to one another on the surface of the central portion 21 opposite the mounting recess 23.

[0036] The heat dissipation layer 13 of the insulating circuit board 10 is bonded onto the central portion 21 of the heat sink 20 , and most of the insulating circuit board 10 is housed in the housing recess 23 .

[0037] Furthermore, the shape of the heat sink 20 is not limited to the above, and it may be a heat sink in which the pin-shaped fins 24 are formed on a single plate without a peripheral wall portion 22, or it may be a flat heat sink without the pin-shaped fins 24. The shape of the fins is not limited to pin shapes, and they may be plate-shaped or prismatic. Furthermore, the method of forming these fins is not limited to forging, and joining may also be used.

[0038] The first circuit layer 14 and the second circuit layer 15, and the heat dissipation layer 13 and the central portion 21 of the heat sink 20 are joined using a double-sided brazing clad brazing sheet 40 described below, with a core material 41 of the double-sided brazing clad brazing sheet 40 remaining between them. When copper is used for the second circuit layer 15, a double-sided brazing clad brazing sheet is not necessarily required, and by adjusting the joining conditions described below, joining using a single-sided brazing clad brazing sheet or joining by so-called solid-phase diffusion can be adopted.

[0039] Next, a method for manufacturing the insulating circuit board 100 with a heat sink will be described.

[0040] [Method for manufacturing pure aluminum plate] A pure aluminum plate having an aluminum purity of 99.99 mass% or more, a Cu content of 30 ppm or less, an Fe content of 20 ppm or less, a Mn content of 6 ppm or less, and a Cr content of 6 ppm or less (the remainder being unavoidable impurities) is manufactured as the aluminum plate for forming the first circuit layer 14 and the heat dissipation layer 13.

[0041] Conventional pure aluminum sheets are subjected to hot rolling, cold rolling, heat treatment, and weak rolling (cold) in this order to make the cube orientation (100) plane predominant after joining, whereas the pure aluminum sheet of the present embodiment is produced by hot rolling followed by cold rolling without heat treatment and weak rolling.

[0042] Specifically, the steel sheet is hot-rolled at a temperature of 300°C to 550°C to a reduction of 85% or more, cooled, and then cold-rolled at a reduction of 80% or more. The reduction is the percentage (t1-t2) / t1, where t1 is the thickness before rolling and t2 is the thickness after rolling.

[0043] In the present embodiment, the metal layer refers to the first circuit layer 14 and the heat dissipation layer 13. Hereinafter, in this specification, the first circuit layer 14 and the heat dissipation layer 13 may also be referred to as the metal layer. In addition, in the present embodiment, the second metal layer refers to the second circuit layer 15 and the heat sink 20.

[0044] [Method for manufacturing an insulated circuit board with a heat sink] When an insulated circuit board with a heat sink 100 is manufactured using a pure aluminum plate, the manufacturing process is carried out through a first bonding step in which a first circuit layer 14 is formed on one surface of a ceramic substrate 11 and a heat dissipation layer 13 is formed on the other surface, and a second bonding step in which a second circuit layer 15 is formed on the first circuit layer 14 and a heat dissipation layer 13 is bonded to a heat sink 20.

[0045] The steps will be explained below in order.

[0046] (First Bonding Step) As shown in Fig. 2, the first circuit layer metal plate 140, ceramic substrate 11, and heat dissipation layer metal plate 130 are laminated with brazing foil 30 (e.g., 7 μm to 20 μm thick) of Al-Si, Al-Ge, Al-Cu, Al-Mg, Al-Mn, or Al-Si-Mg interposed between them, and the laminate is heated while being pressed in the lamination direction, and then cooled. As a result, as shown in Fig. 3, the first circuit layer metal plate 140 is bonded to one surface of the ceramic substrate 11 to form the first circuit layer 14, and the heat dissipation layer metal plate 130 is bonded to the other surface to form the heat dissipation layer 13. This is called the first bonded body 110.

[0047] In this embodiment, the thickness of the first circuit layer 14 and the heat dissipation layer 13 is set to 0.9 mm.

[0048] The bonding conditions at this time are not necessarily limited, but it is preferable to hold the bonding in a vacuum atmosphere at a pressure of 0.3 MPa to 1.5 MPa in the stacking direction and at a heating temperature of 630°C to 655°C for 20 minutes to 120 minutes.

[0049] In this embodiment, the brazing filler metal foil 30 is used, but the present invention is not limited to this, and a brazing filler metal paste may be used. In this case, the brazing filler metal paste may be applied to the ceramic substrate 11, or may be applied to the first circuit layer metal plate 140 and the heat dissipation layer metal plate 130.

[0050] In addition, the first circuit layer 14 and the heat dissipation layer 13 in the first bonded body 11 have an average crystal grain size of 1 mm or more and 2 mm or less, measured in a plan view, and the standard deviation of the crystal grain size is less than 1 mm. As described above, the contents of additive elements such as Cu in the first circuit layer 14 and the heat dissipation layer 13 are kept low, and the rolling method described above is applied, so that crystal growth during recrystallization due to heating during bonding becomes uniform within the plane, and the standard deviation is kept small.

[0051] (Second Joining Step) In the second joining step, the above-described double-sided brazing clad brazing sheet is used.

[0052] As shown in FIG. 4 , the heat dissipation layer 13 of the first joining body 110 is placed on the central portion 21 of the heat sink 20 via a double-sided brazing sheet 40, and these are laminated together. At the same time, the metal plate 150 for the second circuit layer is laminated on the first circuit layer 14 of the first joining body 110 via a double-sided brazing sheet 40, and these are joined together by heating in a nitrogen atmosphere, an argon atmosphere, or a vacuum while being pressurized in the lamination direction.

[0053] The double-sided brazing clad brazing sheet 40 is, for example, a clad material having an A3000 series aluminum alloy (such as A3003) as a core material 41 and an Al-Si based brazing material as a brazing material 42 on both sides thereof, and is formed by rolling the brazing material 42 overlaid on both sides of the core material 41. The thickness of the core material 41 is preferably 0.05 mm to 0.6 mm, and the thickness of the brazing material 42 is preferably 0.03 mm to 0.1 mm.

[0054] The joining conditions include a pressure of 0.001 MPa to 0.5 MPa, and a heating temperature of 580° C. to 630° C., both inclusive, held for 1 minute to 60 minutes.

[0055] In the heatsink-equipped insulating circuit board 100 configured as described above, various semiconductor elements 50, such as an IGBT (Insulated Gate Bipolar Transistor), a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), or an FWD (Free Wheeling Diode), are bonded to the circuit layer 12 (second circuit layer 15) depending on the required function. The solder material used to bond these semiconductor elements 50 is, for example, an Sn—Sb-based, Sn—Ag-based, Sn—Cu-based, Sn—In-based, or Sn—Ag—Cu-based solder material (so-called lead-free solder material). Furthermore, when an aluminum alloy is used for the second circuit layer 15, a thin film of Ni or the like must be formed on the surface of the second circuit layer 15 by plating or the like to enhance the wettability of the solder material.

[0056] In the insulating circuit board 100 with a heat sink manufactured as described above, heating and cooling are repeated after two bonding processes, which causes recrystallization of the pure aluminum that constitutes the first circuit layer 14 and the heat dissipation layer 13, and as the crystal grains grow, the distribution of crystal grain size and crystal orientation also becomes new.

[0057] Furthermore, when a cross section of the first circuit layer 14 near the bonding interface with the ceramic substrate 11 was measured using a combination of a scanning electron microscope (SEM) and electron backscatter diffraction (EBSD), the crystal orientation was observed as shown in Fig. 5. Fig. 5 is an inverse pole figure orientation map (IPF map) of the first circuit layer 14 near the bonding interface with the ceramic substrate 11. Fig. 5(a) shows a case where a conventional pure aluminum plate was used as the first circuit layer 14, and Fig. 5(b) shows a case where the pure aluminum plate of this embodiment was used. Although the scales are different, (1) in the upper row shows the state after the first bonding step, and (2) in the lower row shows the state after the second bonding step.

[0058] As shown by the arrows in Figure 5, in the case of a conventional aluminum plate, the area occupancy rate of the (001) orientation is large, whereas in the case of this embodiment, there are few (001) orientations, and many orientations appear that are 5° to 50° off from the (001) orientation and 10° to 40° off from the (101) orientation, between the (001) orientation and the (111) orientation according to the inverse pole figure. In the lower part (2) of Figure 5, the black part B is the ceramic substrate, and the layer that appears on top of it is the brazing material trace layer K.

[0059] Regardless of the material (a) or (b), similar IPF maps were obtained after the first bonding process (1) and after the second bonding process (2), demonstrating that there was little change in crystal orientation due to the second bonding process. Furthermore, after the first bonding process (1), the entire thickness of the first circuit layer 14 is shown, and from Figure 5 (1), it can be seen that the crystal orientation is the same from the surface of the first circuit layer 14 to the vicinity of the bond with the ceramic substrate 11. Therefore, there is no difference in crystal orientation whether measuring the cross section after the first bonding process or the second bonding process. Furthermore, it can be seen that, for thicknesses of approximately 0.1 mm to 3.0 mm, even if the surface of the first circuit layer 14 is measured, it can be applied to the measurement results for the entire first circuit layer 14.

[0060] In environments where repeated heating and cooling occur, the difference in the linear expansion coefficient between ceramics and metals causes stress at the bonding interface, which is known to cause defects such as fracture of the metal layer. The direction of the stress is parallel to the interface near the bonding interface. This stress is relieved by plastic deformation such as slip deformation and twinning in the metal layer and grain refinement, ultimately leading to fracture of the metal structure.

[0061] In the insulating circuit board 100 with a heat sink of this embodiment, even in an environment where heating and cooling are repeated, fractures (cracks) are unlikely to occur in the first circuit layer 14 and the heat dissipation layer 13, and the board has high reliability.

[0062] This is thought to be because, as mentioned above, the crystal orientations of the first circuit layer 14 and heat dissipation layer 13 bonded to the ceramic substrate 11 are often between the (001) and (111) orientations, which makes it easier for stress relaxation due to slippage to occur in the metal layer, increasing the time until fracture.

[0063] The mechanism will be explained below.

[0064] 6A and 6B are schematic diagrams showing the slip planes of metal crystals in a metal layer 14 bonded to a ceramic substrate 11. In Fig. 6A, the slip plane (111) is formed at an incline with respect to the bonding interface, while in Fig. 6B it is formed at a right angle. In other words, the angle between the slip plane (111) and the bonding interface is smaller in Fig. 6A than in Fig. 6B.

[0065] In order for slippage to occur easily, it is preferable that the slip plane and the bonded interface are in a positional relationship that is close to parallel; in other words, it is preferable that the angle between the bonded interface and the slip plane is small. In other words, slippage occurs more easily in the state shown in Figure 6(a) than in (b), which increases the time until final fracture and increases reliability. In other words, it can be said that slippage occurs more easily and is less likely to lead to fracture of the metal layer when the bonded interface and the slip plane are close to parallel.

[0066] Furthermore, the crystal structure of aluminum, which is the metal layer (first circuit layer 14 and heat dissipation layer 13) used in the insulating circuit board of this embodiment, is a face-centered cubic lattice, and its slip plane, expressed in Miller indices, is the {111} plane. Due to the symmetry of this face-centered cubic lattice, there are eight planes equivalent to the (111) plane, including the (111) plane.

[0067] In reality, the aluminum layer recrystallizes due to the heat treatment and other processes included in the manufacturing method described above, forming a structure in which the crystal cubic lattice is rotated three-dimensionally. To discuss the plane orientation of the crystal grains after this rotation using data obtained from EBSD, it is easy to analyze using Euler angles. The method for calculating the angle between the slip plane and the bonding interface in the structure after this rotation (recrystallization) is as follows:

[0068] As mentioned above, to analyze the crystal plane orientation after rotation using Euler angles, we assume that the plane equivalent to the bonding interface is the (001) plane. Furthermore, the mathematical description of the slip plane after rotation (i.e., after recrystallization) is the plane obtained by applying a rotation operator to the {111} plane before rotation. The angle between planes is generally calculated by calculating the angle between the normal vectors of each plane. Therefore, we simply calculate the angle between the normal vector of the (001) plane before rotation and the normal vector of the {111} plane after applying the rotation operator. However, as mentioned above, the {111} plane contains seven planes equivalent to the (111) plane, and the same phenomenon occurs on equivalent planes. Therefore, we calculate the angle between the slip plane and the bonding interface for all eight equivalent planes contained in the {111} plane using the above method, and use the smallest one for discussion.

[0069] Based on the above, a general formula is constructed to calculate the angle between the slip plane of the metal layer bonded to the ceramic through the bonding process and the bonding interface.

[0070] First, the normal vector T of the (001) plane before rotation normal and the normal vector S of the {111} plane normal is expressed as a matrix in Eq. 1:

[0071]

[0072] where (pqr)=(1 1 1),(1 1 -1),(1 -1 1),(-1 1 1),(1 -1 -1),(-1 1 -1),(-1 -1 1),(-1 -1 -1).

[0073] By the way, the three-dimensional rotation of the unit lattice can be expressed by Euler angles (Φ, θ, Ψ), so the rotation operator R Φ,θ,Ψ is expressed as the following equation 2 (Eq.2).

[0074]

[0075] The Euler angles that represent the orientation of this unit cell are obtained by SEM-EBSD measurement.

[0076] This rotation operator R Φ,θ,Ψ S normal The normal vector S' of the sliding surface after rotation (i.e., in the product state) obtained by applying normal is expressed as the following equation 3 (Eq. 3).

[0077]

[0078] This rotation causes the normal vector S' of the sliding surface to normal Since it has an apparent size, S(~)' in the following equation (Eq.4) can be normalized. normal get.

[0079]

[0080] Therefore, the angle ζ between the slip plane of the metal layer joined to the ceramic through the joining process and the joining interface is selected as the smallest angle among the {111} planes, and is obtained by the following Eq. 5 (Eq. 5).

[0081]

[0082] The pure aluminum plate of this embodiment was bonded to a ceramic substrate, and SEM-EBSD measurements of the central portion of the pattern in a plan view revealed that Φ, θ, and Ψ were within the ranges of 50° to 350°, 70° to 120°, and 10° to 220°, respectively. On the other hand, similar measurements using a conventional pure aluminum plate revealed that Φ, θ, and Ψ were within the ranges of 70° to 90°, 80° to 90°, and −10° to 10°, respectively.

[0083] The angle between the slip plane {111} calculated from this Euler angle and the bonding interface between the ceramic substrate and the metal layer was 20° to 40° in this embodiment, and 45° to 60° in the conventional example in which a cube orientation was formed after bonding.

[0084] Similar results were obtained using the EBSD measurement results of the cross section.

[0085] This result indicates that the {111} plane, which is the slip plane, is closer to a plane parallel to the bonding interface with the ceramic substrate in the aluminum of this embodiment than in the conventional example. In other words, in this embodiment, the stress applied to the aluminum metal layer during the thermal cycle test is largely relieved by slip, which is less likely to lead to horizontal fracture, i.e., cracks, in cross section than in the conventional example, and is thought to have led to improved thermal reliability.

[0086] In the insulated circuit board 100 with a heat sink of this embodiment, the angle between the sliding surfaces of the first circuit layer 14 and the heat dissipation layer 13 and the bonding interface between these aluminum layers and the ceramic substrate is small, at 40° or less. This prevents the progression of peeling of the metal layers (the first circuit layer 14 and the heat dissipation layer 13) even in an environment where cooling and heating are repeated, and allows the insulated circuit board to maintain high reliability.

[0087] In this embodiment, the circuit layer 12 has a two-layer structure and includes a second circuit layer 15, and a heat sink 20 is similarly bonded to the heat dissipation layer 13. Because the second circuit layer and the heat sink are bonded simultaneously, this embodiment is manufactured through a total of two bonding steps, but as described above, it has been confirmed that there is no change in the crystal orientation of the metal layer even after the second bonding step, and even when two bonding steps are performed, the stress relaxation effect of the lower metal layer can prevent the progression of peeling.

[0088] In the first bonded body 110 shown in FIG. 3 , which does not have the second circuit layer 15 and the heat sink 20, the stress load on the first circuit layer 14 and the heat dissipation layer 13 is smaller than that in the insulating circuit board 100 with a heat sink, and therefore it is clear that the effect of preventing the progression of peeling is greater.

[0089] Furthermore, both the first circuit layer 14 and the heat dissipation layer 13 contain 30 ppm or less of Cu, 20 ppm or less of Fe, 6 ppm or less of Mn, and 6 ppm or less of Cr, and the standard deviation of the crystal grain size measured in a planar view is less than 1 mm. Therefore, the first circuit layer 14 and the heat dissipation layer 13 are excellent in properties such as stress buffering effect, thermal conductivity, and electrical conductivity, and together with these effects, the insulating circuit board can maintain high reliability.

[0090] As described above, the first circuit layer 14 and the heat dissipation layer 13 are obtained by cold rolling after hot rolling, and pure aluminum plates are used that are manufactured without undergoing heat treatment or weak rolling after cold rolling, as is the case with ordinary pure aluminum plates.

[0091] If a conventional pure aluminum sheet is subjected to heat treatment or weak rolling after cold rolling, cube orientation is likely to appear after the joining process. In this case, the ordinary pure aluminum sheet has high shear strain resistance and high resistance to deformation parallel to the joining interface, which makes it difficult to slide, resulting in a poor effect of suppressing crack generation as in this embodiment.

[0092] While the above-described embodiment illustrates an insulated circuit board 100 with a heat sink 20 having fins 24, an insulated circuit board 60 as shown in FIG. 7 may also be used. This insulated circuit board 60 has a two-layer structure in which a circuit layer 12 having a two-layer structure (first circuit layer 14 and second circuit layer 15) is bonded to one surface of a ceramic substrate 11, and a plate-shaped second heat dissipation layer 61 is bonded to the other surface of the ceramic substrate 11 on top of a heat dissipation layer 13, forming a two-layer structure. The heat dissipation layer 13 and the second heat dissipation layer 61 are bonded together using a brazing sheet 40 with double-sided brazing clad, as in the case of bonding the heat dissipation layer 13 to the heat sink 20 in the above-described embodiment. Therefore, a core material 41 remains between the heat dissipation layer 13 and the second heat dissipation layer 61. In this case, the heat dissipation layer 13 is the metal layer of the present invention, and the second heat dissipation layer 61 can also be the second metal layer of the present invention.

[0093] Furthermore, in this embodiment, the first circuit layer 14 and the heat dissipation layer 13 are made of aluminum, but because copper also has a face-centered cubic lattice crystal structure, they can be similarly applied to circuit layers and heat dissipation layers made of copper, and peeling can be suppressed by setting the angle between the slip plane {111} plane and the bonding interface with the ceramic substrate to 40° or less.

[0094] Two types of 4N aluminum plates (0.9 mm thick) with a purity of 99.99% by mass or more were prepared using different manufacturing methods. The aluminum plates of the examples were obtained by hot rolling at a rolling reduction of 87% at a temperature of 450°C, cooling, and then cold rolling at a rolling reduction of 85%. On the other hand, the aluminum plates of the comparative examples were obtained by similarly hot rolling and cold rolling, followed by heat treatment at a temperature of 250°C for 60 minutes, and then cold rolling at a rolling reduction of 15%.

[0095] Each aluminum plate was subjected to elemental analysis by Glow Discharge Mass Spectrometry (GD-MS), and the results are shown in Table 1. The total content of the following elements was 81 ppm in the example and 120 ppm in the comparative example, and the purity of aluminum in each case was 99.99%.

[0096]

[0097] Using a micro-Vickers tester and a Vickers tester, a diamond pyramidal indenter was pressed against these aluminum plates with a predetermined force, and the width dimensions of the indentation in the rolling direction (RD) of the aluminum plate and the direction perpendicular to the rolling direction (TD) of the aluminum plate were measured, and the ratio of these (TD / RD) was calculated. The pressing force of the indenter was 2 kgf for the micro-Vickers test and 3 kgf for the Vickers test. The micro-Vickers value is strongly influenced by the surface layer of the aluminum plate, while the Vickers value is more strongly influenced by the interior than the surface layer. For both the micro-Vickers indentation and the Vickers indentation, the number of indentations was 10.

[0098] Furthermore, tensile tests were carried out on each aluminum plate in the rolling direction (RD) and the direction perpendicular to the rolling direction (TD) in accordance with JIS Z 2241. From the obtained stress-strain curves, the total elongation at break was measured and the ratio (TD / RD) was calculated. The number of samples for the total elongation at break was 5.

[0099] The measurement results of the micro Vickers indentation, Vickers indentation, and total elongation at break were compared using a Weibull plot. The results are shown in Figure 8. In Figure 8, (a) shows the ratio (TD / RD) of the Vickers indentation, (b) shows the micro Vickers indentation, and (c) shows the ratio (TD / RD) of the total elongation at break. In all cases, the filled squares represent Examples, and the open squares represent Comparative Examples.

[0100] To visually distinguish the differences between the data of the comparative examples and the examples, a Weibull plot was performed with the logarithm of the TD / RD ratio of the Vickers indentation, micro Vickers indentation, and total elongation at break plotted on the horizontal axis. The slope corresponds to the variability of the data, and all have approximately the same slope.

[0101] For the Vickers indentation, micro-Vickers indentation, and total elongation at break, the TD / RD ratio for all of the Examples is approximately 1.0, i.e., isotropic, whereas the Comparative Examples show anisotropy, with TD > RD for the micro-Vickers indentation and total elongation at break.

[0102] Furthermore, the Vickers indentation of the comparative example behaved differently from the others, with RD > TD. This is presumably because recovery by intermediate heat treatment and subsequent weak rolling resulted in a limited area where RD > TD being present in a region deeper than the region evaluated by micro-Vickers. Since this region was limited, the Vickers test sampled singular points in the thickness direction of the test piece, resulting in behavior different from that of the micro-Vickers indentation and total elongation at fracture.

[0103] Although the mechanism is not clear, the comparative example, which is mostly cubic oriented after bonding, exhibits anisotropy, whereas the example is isotropic. By rolling so that the example exhibits such characteristics, after bonding, more crystal grains are obtained that have slip planes that are relatively close to parallel to the bonding surface, compared to cubic oriented grains.

[0104] Next, a silicon nitride (Si) substrate with a thickness of 0.32 mm was used as a ceramic substrate. ３ N ４A ) plate was prepared, and pure aluminum plates of the examples and comparative examples were bonded to both sides of the ceramic substrate using an Al-Si brazing filler metal to produce a first bonded body having a first circuit layer on one side of the ceramic substrate and a heat dissipation layer on the other side. Next, a metal plate for a second circuit layer made of an aluminum alloy such as JIS A6063 alloy and a heat sink were bonded via a double-sided brazing clad brazing sheet to produce a sample insulated circuit board with a heat sink. The double-sided brazing clad brazing sheet used had an Al-Si brazing filler metal formed on both sides of a core material made of JIS A3003 alloy. The bonding conditions were as described above.

[0105] Furthermore, by adjusting the above-mentioned bonding conditions, it is possible to use copper, copper alloy, aluminum, or aluminum alloy for the second circuit layer and the heat sink.

[0106] This sample was subjected to a gas-phase thermal cycle reliability test.

[0107] In this gas-phase thermal cycle reliability test, the sample was exposed to each of the following environments in the order of low temperature: -40°C for 30 minutes, room temperature: 25°C for 10 minutes, and high temperature: 150°C for 30 minutes. This sequence was repeated for 1000 cycles and 2000 cycles, and the peeling rate was measured from ultrasonic flaw detection images. The peeling rate was defined as the ratio of the area where peeling occurred (the area of ​​the white part in the ultrasonic flaw detection image) to the area to be joined of the metal layer before joining.

[0108] The results are shown in Table 2. The average values ​​obtained by measuring two samples for each of the Examples and Comparative Examples are shown.

[0109]

[0110] The average value of the ratio of the peeled area to the total area measured by ultrasonic flaw detection after 2000 cycles was 3.29% for the Example and 5.75% for the Comparative Example, which shows that the Example had a smaller peeled area and a smaller degree of peeling progression. Furthermore, although the difference between the Example and the Comparative Example was small after 1000 cycles, the difference became larger after 2000 cycles, indicating that the Example had high long-term reliability.

[0111] Next, crystal orientation analysis was performed on the cross section of the metal layer of the example by SEM-EBSD.

[0112] A cross section along the thickness direction of the metal layer was cut out by cutting and mechanical polishing, and a smooth cross-sectional sample was prepared by ion milling. The crystal orientation was measured using a field emission scanning electron microscope (FE-SEM) equipped with an EBSD detector (SU-70 manufactured by Hitachi High-Tech Corporation) at an acceleration voltage of 15 kV, a measurement range of 1.2 mm × 7.2 mm, and a measurement step of 15 μm, and the crystal orientation distribution was determined.

[0113] Prior to conducting cross-sectional crystal orientation analysis, we decided to search for representative orientations across the entire substrate based on the IPF map obtained from SEM-EBSD measurements of the first bonded body in a planar view. Using the color display shown on the IPF scale, we grouped crystal grains into groups with small RGB differences, measured their areas, and calculated the area ratio of each crystal orientation.

[0114] As shown in Figure 9(b), in the aluminum plate of the example, the grains between the (001) and (111) orientations had the largest area, accounting for 45.7% in terms of area ratio. On the other hand, in the comparative example shown in Figure 9(a), the crystal grains near the (001) orientation were the most numerous, accounting for 83.1%. Therefore, it can be said that the crystal grains with the above orientations are representative in each example.

[0115] Figure 10 shows IPF maps of the cross-sectional samples of the heat sink-equipped insulating circuit board for the comparative example (a) and the example (b), where (3) in the figure shows the initial state of bonding, and (4) shows the state after 2000 cycles in the gas-phase thermal cycle reliability test. Also, E indicates the vicinity of the edge of the metal layer, and C indicates the center of the surface of the metal layer. The area shown in the lower part of Figure 5 (2) is the center of the surface of the metal layer, and is the same as C in (3) of Figure 10.

[0116] 10, as in Fig. 9, crystal grains having an orientation between the (001) and (111) orientations were confirmed on the IPF scale in the Example, while crystal grains having an orientation close to the (001) orientation were confirmed in the Comparative Example. Therefore, since the measurement results on the surface of the first bonded body can be used as a substitute, the Euler angles were obtained for representative points in each crystal grain based on the results in Fig. 9, and the angle ζ between the bonded interface and the slip plane was calculated. The results were 20.03° for the Example and 53.58° for the Comparative Example, revealing that the angle between the bonded interface and the slip plane was smaller in the Example.

[0117] 10, a so-called brazing material trace layer is generated at the bonding interface due to penetration of the brazing material components during bonding. However, this layer is refined by stress generated at the bonding interface during thermal reliability testing. Furthermore, this stress cannot be fully absorbed by the brazing material trace layer, and it spreads to the upper structure, refining the crystal grains. As a result, when the substrate after thermal reliability testing is observed cross-sectionally, not only the brazing material trace layer but also the upper structure are refined, making it indistinguishable.

[0118] When comparing the Example and Comparative Example, the (001) orientation was dominant at the edge of the substrate in both cases immediately after bonding, and in both cases the aluminum material fractured after 2000 cycles.

[0119] However, as can be seen by comparing columns (4)(E) of (a) and (b), the refinement of crystal grains is more advanced in the comparative example than in the example, and the aluminum material of the example shows refinement and changes in crystal grain orientation only near the crack. In other words, it can be said that the refinement of crystal grains as a stress relaxation process is suppressed near the crack in the example.

[0120] On the other hand, when comparing the central portion of the substrate shown in columns (4)(C) of (a) and (b), the crystal grains begin to become finer in both the example and the comparative example.

[0121] While no significant change in the crystal plane orientation was observed in the Example, the Comparative Example experienced a change in crystal plane orientation from (001) to (101) after thermal reliability testing. However, because this change was a rotation around an axis perpendicular to the bonding interface (i.e., a rotation parallel to the bonding interface), the change in the angle between the slip plane and the bonding interface due to thermal reliability testing was small. Therefore, this comparison can be treated in the same way as the Comparative Example, which has a (001) orientation, and the Example, which has an orientation between (001) and (111). Therefore, even after thermal reliability testing, the angle between the bonding interface and the slip plane remained smaller in the center of the Example substrate, and stress relaxation was enhanced by slip, which is thought to have allowed the Example to maintain high reliability. As mentioned above, the first bonded body 110 shown in FIG. 3 also achieved high reliability equivalent to or greater than that of the heatsink-equipped insulating circuit board.

[0122] It can be used as an insulating circuit board that maintains high reliability by suppressing the progression of peeling of the metal layer even in environments where it is subjected to repeated cooling and heating.

[0123] REFERENCE SIGNS LIST 10 Insulated circuit board 11 Ceramic substrate 12 Circuit layer 13 Heat dissipation layer (metal layer) 14 First circuit layer (metal layer) 15 Second circuit layer (second metal layer) 20 Heat sink (second metal layer) 21 Central portion 22 Peripheral wall portion 23 Accommodating recess 24 Pin-shaped fin 30 Brazing foil 40 Double-sided brazing clad brazing sheet 41 Core material 42 Brazing material 50 Semiconductor element 60 Insulated circuit board 61 Second heat dissipation layer (second metal layer) 100 Insulated circuit board with heat sink 110 First bonding body 130 Metal plate for heat dissipation layer 140 Metal plate for first circuit layer 150 Metal plate for second circuit layer

Claims

1. An insulating circuit board comprising: a ceramic substrate; and a metal layer made of pure aluminum having a purity of 99.9% by mass or more bonded to at least one surface of the ceramic substrate, wherein the angle between the sliding plane of metal crystals in the metal layer and the bonding interface with the ceramic substrate is 40° or less.

2. The insulating circuit board according to claim 1, characterized in that a second metal layer made of aluminum, an aluminum alloy, copper or a copper alloy is bonded onto the metal layer.

3. The insulating circuit board according to claim 1, characterized in that the pure aluminum has a purity of 99.99% by mass or more.

4. The insulating circuit board according to claim 1, characterized in that the pure aluminum has a Cu content of 30 ppm or less, an Fe content of 20 ppm or less, an Mn content of 6 ppm or less, and a Cr content of 6 ppm or less.

5. The insulating circuit board according to claim 1, characterized in that the angle between the slip plane of the metal crystal in the metal layer and the bonding interface with the ceramic substrate is the smallest angle among the angles between the {111} plane, expressed in Miller indices for the metal layer, and the bonding interface.

6. The insulating circuit board according to claim 1, wherein the ceramic substrate is made of a nitride ceramic or an oxide ceramic.

7. The insulated circuit board according to claim 1, characterized in that a heat dissipation layer made of pure aluminum having a purity of 99.9% by mass or more is bonded to the surface of the ceramic substrate opposite to the bonding surface with the metal layer, and the angle between the sliding plane of the metal crystals in the heat dissipation layer and the bonding interface with the ceramic substrate is 40° or less.

8. The insulating circuit board according to claim 7, characterized in that a second heat dissipation layer made of any one of aluminum, an aluminum alloy, copper, and a copper alloy is bonded onto the heat dissipation layer.

9. An insulated circuit board with a heat sink, in which a heat sink is bonded onto the heat dissipation layer of the insulated circuit board according to claim 7, characterized in that the heat sink is made of any one of aluminum, an aluminum alloy, copper, and a copper alloy.

10. The insulating circuit board according to claim 7, characterized in that the pure aluminum of the heat dissipation layer has a purity of 99.99% by mass or more.

11. The insulating circuit board according to claim 7, characterized in that the pure aluminum of the heat dissipation layer has a Cu content of 30 ppm or less, an Fe content of 20 ppm or less, an Mn content of 6 ppm or less, and a Cr content of 6 ppm or less.

12. The insulating circuit board according to claim 7, characterized in that the angle between the slip plane of the metal crystal in the heat dissipation layer and the bonding interface with the ceramic substrate is the smallest angle among the angles between the {111} plane, expressed in Miller indices for the metal layer, and the bonding interface.

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