WO2024176598A1

WO2024176598A1 - Circuit board and power module

Info

Publication number: WO2024176598A1
Application number: PCT/JP2023/045774
Authority: WO
Inventors: 優太津川; 清治矢野; 良太青野
Original assignee: デンカ株式会社
Priority date: 2023-02-24
Filing date: 2023-12-20
Publication date: 2024-08-29

Abstract

This circuit board comprises a silicon nitride plate, a copper plate, and a bonding layer that bonds the silicon nitride plate and the copper plate. The bonding layer includes, from the silicon nitride plate side, a first bonding layer containing titanium nitride, a second bonding layer having a higher elemental silicon content than the first bonding layer, and a third bonding layer having a higher elemental silver content than the first bonding layer and the second bonding layer, and the second bonding layer is interposed between the first bonding layer and the third bonding layer so as to partition the first bonding layer from the third bonding layer.

Description

Circuit boards and power modules

This disclosure relates to a circuit board and a power module.

The circuit boards mounted on electronic devices are manufactured by joining ceramic and metal plates with a brazing material. Furthermore, the multi-layer structure allows the circuit board to be endowed with a variety of properties. For example, Patent Document 1 discloses a circuit board that can suppress the deterioration of electrical properties due to heat generation and cooling by making the ceramic base material and metal base material into multiple layers. Furthermore, Patent Document 2 discloses a circuit board that has high durability against heat cycles by laminating a metal layer and a sprayed layer on a ceramic plate.

JP 2019-067805 A JP 2021-080537 A

It has been found that when a copper plate is bonded to a silicon nitride plate in an assembly board or individual board, the bonding strength of the bonding layer between the silicon nitride plate and the copper plate is insufficient. Therefore, the present disclosure provides a circuit board with high bonding strength. The present disclosure also provides a power module that includes such a circuit board.

One aspect of the present disclosure provides the following circuit board:

[1] A circuit board comprising a silicon nitride plate, a copper plate, and a bonding layer bonding the silicon nitride plate and the copper plate, the bonding layer having, from the silicon nitride plate side, a first bonding layer containing titanium nitride, a second bonding layer having a higher silicon content than the first bonding layer, and a third bonding layer having a higher silver content than the first bonding layer and the second bonding layer, the second bonding layer being interposed between the first bonding layer and the third bonding layer so as to isolate the first bonding layer from the third bonding layer.

The bonding layer of the circuit board has a second bonding layer interposed between the first bonding layer and the third bonding layer so as to isolate the first bonding layer and the third bonding layer. By providing such a bonding layer on the circuit board, the bonding strength between the silicon nitride plate and the copper plate in the circuit board can be improved.

The circuit board of [1] above may be any one of [2] to [6] below.

[2] The circuit board according to [1], wherein titanium elements are not present at the interface between the copper plate and the bonding layer.
[3] The circuit board according to [1] or [2], wherein when the carbon content in the first bonding layer is C1 atomic % and the carbon content in the third bonding layer is C3 atomic %, the average value of C1/C3 is 0.5 or less.
[4] The circuit board according to any one of [1] to [3], wherein the difference between the maximum and minimum thicknesses of the first bonding layer is 70 nm or less.
[5] The circuit board according to any one of [1] to [4], wherein the second bonding layer has a thickness of 50 nm or more.
[6] The circuit board according to any one of [1] to [5], wherein t1/t2 is 5 or less, when the average thickness of the first bonding layer is t1 and the average thickness of the second bonding layer is t2.

In the circuit board of [2] above, no titanium element is present at the interface between the copper plate and the bonding layer. As no titanium element is present at the interface between the copper plate and the bonding layer, the first bonding layer and the second bonding layer are sufficiently formed, and the bonding strength between the silicon nitride plate and the copper plate can be sufficiently improved. In the circuit board of [3] above, carbon derived from the binder in the bonding layer can be sufficiently removed, and the first bonding layer, the second bonding layer, and the third bonding layer are sufficiently formed, and the bonding strength between the silicon nitride plate and the copper plate can be sufficiently increased.

In the circuit board of [4] above, the difference between the maximum and minimum thicknesses of the first bonding layer is 70 nm or less. With the first bonding layer having such a thickness, the first bonding layer is sufficiently formed without variation over a wide range, and the bonding strength between the silicon nitride plate and the copper plate can be sufficiently high. In the circuit board of [5] above, the second bonding layer has a thickness of 50 nm or more. In such a circuit board, the second bonding layer is formed with a sufficient thickness, and the bonding strength between the silicon nitride plate and the copper plate can be sufficiently high.

In the circuit board of [6] above, when the average thickness of the first bonding layer is t1 and the average thickness of the second bonding layer is t2, t1/t2 is 5 or less. By having the average thickness of the first bonding layer and the average thickness of the second bonding layer in this range, the first bonding layer and the second bonding layer are well balanced, and the bonding strength between the silicon nitride plate and the copper plate can be sufficiently increased.

One aspect of the present disclosure provides the following power module:

[7] A power module comprising a circuit board described in any one of [1] to [6] above and a semiconductor element electrically connected to the copper plate of the circuit board.

The power module has high bonding strength between the silicon nitride plate and the copper plate of the circuit board. Therefore, the power module has excellent durability.

The present disclosure can provide a circuit board with high bonding strength. The present disclosure can also provide a power module that includes such a circuit board.

FIG. 5A to 5C are cross-sectional views of an example of a laminate for explaining a process in a method for manufacturing a circuit board. FIG. FIG. 2 is a plan view of the structure with a portion of the pressing jig cut away. FIG. 2 is a front view of the structure with a portion of the pressing jig cut away. FIG. 2 is a side view of the structure with a portion of the pressing jig cut away. 7 is a cross-sectional view of the structure shown in FIG. 4 taken along line VII-VII. 13 is a cross-sectional view of the structure showing a state in which the pressure plate is raised and the spring washer is restored to its original state. FIG. FIG. 9 is an enlarged cross-sectional view showing a portion of the composite body and the spacer of FIG. 8 . 1A is an STEM image of a cross section of a circuit board in Example 1. FIG. 1B is an STEM image of a cross section of a circuit board in Comparative Example 1. 1A is a diagram showing the distribution of titanium elements by EDX analysis of a cross section of the circuit board in Example 1. FIG. 1B is a diagram showing the distribution of silicon elements by EDX analysis of a cross section of the circuit board in Example 1. 1A is a diagram showing the distribution of silver and copper elements by EDX analysis of a cross section of the circuit board in Example 1; FIG. 1B is a diagram showing the distribution of silver and copper elements by EDX analysis of a cross section of the circuit board in Example 1; 1A is a diagram showing the distribution of titanium elements by EDX analysis of a cross section of the circuit board in Comparative Example 1. FIG. 1B is a diagram showing the distribution of silicon elements by EDX analysis of a cross section of the circuit board in Comparative Example 1. 1A is a diagram showing the distribution of silver and copper elements by EDX analysis of a cross section of the circuit board in Comparative Example 1; 1 is an EDX line profile in Example 1. 1A is a photograph showing the state when the copper plate is peeled off from the circuit board in Example 1. FIG. 1B is a photograph showing the state when the copper plate is peeled off from the circuit board in Comparative Example 1. 1A is an SEM image of the peeled surface after the copper plate is peeled off from the circuit board in Example 1. FIG. 1B is an SEM image of the peeled surface after the copper plate is peeled off from the circuit board in Comparative Example 1. 1A is a diagram showing the distribution of silicon elements by EDX analysis of the peeled surface after peeling the copper plate from the circuit board in Example 1. FIG. 1B is a diagram showing the distribution of silicon elements by EDX analysis of the peeled surface after peeling the copper plate from the circuit board in Comparative Example 1.

Below, an embodiment of the present disclosure will be described. However, the following embodiment is an example for explaining the present disclosure, and is not intended to limit the present disclosure to the following content. The upper or lower limit of the numerical range specified in this disclosure may be replaced with any value shown in the examples. Furthermore, the upper and lower limit values described individually may be combined in any combination. Unless otherwise specified, the materials or components exemplified in this disclosure may be used alone or in combination of two or more types. In the description, the same elements or elements having the same functions are given the same reference numerals, and duplicate descriptions are omitted. Furthermore, the positional relationships such as up, down, left, and right used in the description shall be based on the positional relationships shown in the drawings, unless otherwise specified.

Figure 1 is a schematic cross-sectional view of a circuit board according to one embodiment. The circuit board 100 has a pair of copper plates 50 on both sides of a silicon nitride plate 10, and a bonding layer 20 that bonds each main surface of the silicon nitride plate 10 to the main surface of the copper plate 50. The bonding layer 20 has, from the silicon nitride plate 10 side, a first bonding layer 21, a second bonding layer 22, and a third bonding layer 23, in this order.

The shape of the circuit board 100 is not particularly limited, and it may be an aggregate board, or an individual board obtained by dividing an aggregate board. The thickness of the circuit board may be 0.9 to 2.7 mm, or 1.3 to 2.2 mm.

The silicon nitride plate 10 may be made of a silicon nitride sintered body obtained by sintering silicon nitride powder. There are no particular limitations on the shape of the silicon nitride plate 10 as long as it is plate-like. The thickness of the silicon nitride plate 10 may be, for example, 0.2 to 2.0 mm, or 0.32 to 1.1 mm.

The copper plate 50 is not particularly limited in shape as long as it can be bonded to the main surface of the silicon nitride plate 10. The thickness of the copper plate 50 may be, for example, 0.1 to 1.2 mm, or may be 0.2 to 1.0 mm. The copper plate 50 may have a plating film on its surface.

The bonding layer 20 has, from the silicon nitride plate 10 side, a first bonding layer 21 containing titanium nitride, a second bonding layer 22 having a higher concentration of silicon element than the first bonding layer 21, and a third bonding layer 23 having a higher concentration of silver element than the first bonding layer 21 and the second bonding layer 22. The second bonding layer 22 is interposed between the first bonding layer 21 and the third bonding layer 23 so as to isolate the first bonding layer 21 and the third bonding layer 23. A circuit board 100 having such a bonding layer 20 can increase the bonding strength between the silicon nitride plate 10 and the copper plate 50.

The bonding layer 20 is a layer having a first bonding layer 21, a second bonding layer 22, and a third bonding layer 23. The bonding layer 20 contains titanium, silver, and silicon elements. The bonding layer 20 may further contain one or two metals selected from the group consisting of copper, tin, and an active metal. The two or more metals may be an alloy. The active metal may contain one or two or more metals selected from the group consisting of titanium, hafnium, zirconium, and niobium.

The silver contained in the bonding layer 20 may be contained in the bonding layer 20 as an alloy, such as an Ag-Cu eutectic alloy. The silver content in the bonding layer 20 may be 60-95 mass% in Ag equivalent. The silver and copper contents in the bonding layer 20 may be 75-100 mass%, 85-99 mass%, or 90-98 mass% in Ag and Cu equivalent, respectively. This makes it possible to sufficiently reduce the residual stress in the bonding layer 20 while improving the density of the bonding layer 20.

The content of the active metal in the bonding layer 20 may be 0.5 to 5 parts by mass per 100 parts by mass of Ag and Cu combined. By making the content of the active metal 0.5 parts by mass or more, it is possible to improve the bond between the silicon nitride plate 10 and the bonding layer 20. On the other hand, by making the content of the active metal 5 parts by mass or less, it is possible to suppress the formation of a brittle alloy layer at the bonding interface.

The metal contained in the bonding layer 20 may be contained as a nitride, oxide, carbide or hydride. As an example, the bonding layer 20 may contain titanium nitride and/or titanium hydride (TiH ₂ ). This allows the silicon nitride plate 10 and the bonding layer 20 to react sufficiently to form the first bonding layer 21, the second bonding layer 22, and the third bonding layer 23. This allows the bonding strength between the silicon nitride plate 10 and the copper plate 50 to be sufficiently high. In the bonding layer 20, the content of TiH ₂ relative to a total of 100 parts by mass of Ag and Cu may be, for example, 1 to 8 parts by mass.

The elements contained in the bonding layer 20 can be identified and quantified by the following procedure. First, a cross section of the circuit board is observed, for example, with a scanning transmission electron microscope (STEM). In the image of the cross section, analysis is performed with an energy dispersive X-ray analyzer (EDX) along a line perpendicular to the main surface of the silicon nitride plate 10, from the silicon nitride plate 10 toward the copper plate 50. This allows the distribution of each element to be detected as a line profile and quantified. Here, the cross section in this disclosure refers to a cross section perpendicular to the main surface of the silicon nitride plate 10 on which the bonding layer 20 is provided, and passing through the bonding layer 20.

The first bonding layer 21 is a layer containing titanium nitride. The titanium nitride content may be 50 to 90 atomic %, 55 to 85 atomic %, or 60 to 80 atomic %. The titanium element and the nitrogen element may be detected in the first bonding layer 21 in greater amounts than the second bonding layer 22 and the third bonding layer 23. That is, the first bonding layer 21 may have a higher titanium nitride content than the second bonding layer 22 and the third bonding layer 23.

The titanium nitride phase contained in the first bonding layer 21 may be formed in a layer along the contact surface between the silicon nitride plate 10 and the first bonding layer 21. By forming the titanium nitride phase in a continuous layer along the contact surface between the silicon nitride plate 10 and the first bonding layer 21, the bonding strength between the silicon nitride plate 10 and the copper plate 50 can be further increased. The fact that the titanium nitride phase is formed in a layer along the contact surface between the silicon nitride plate 10 and the first bonding layer 21 can be confirmed by analyzing a cross section, for example, with an electron beam microanalyzer (EPMA).

The thickness of the first bonding layer 21 can be determined as the length of the line at which titanium and nitrogen elements are detected with a content of 10 atomic % or more. The titanium and nitrogen elements contained in the first bonding layer 21 may be 10 to 45 atomic %, or 20 to 40 atomic %.

The thickness of the first bonding layer 21 may be 100 to 300 nm, or 150 to 250 nm. The difference between the maximum and minimum thicknesses of the first bonding layer 21 may be 70 nm or less, 50 nm or less, or 45 nm or less. When the difference between the maximum and minimum thicknesses of the first bonding layer 21 is within this range, the first bonding layer 21 contains a sufficient amount of titanium nitride, and the bonding strength between the silicon nitride plate 10 and the copper plate 50 can be improved. The average thickness t1 of the first bonding layer 21 can be calculated as the average thickness value measured at five or more lines. The average thickness t1 of the first bonding layer 21 may be 175 to 235 nm, 180 to 230 nm, or 190 to 220 nm.

The second bonding layer 22 is a layer with a higher silicon element content than the first bonding layer 21. The silicon element originates from the silicon nitride plate 10, and when the silicon nitride plate 10 and the copper plate 50 are heat-bonded via the brazing material layer, it is believed that the silicon nitride reacts with the titanium of the brazing material layer due to heating, forming the second bonding layer 22 as a layer containing titanium silicide.

The silicon element content of the second bonding layer 22 may be 10-40 atomic %, or 20-30 atomic %. The second bonding layer 22 with a silicon element content in this range is formed by a sufficient reaction between the silicon nitride plate 10 and the titanium in the brazing material layer. This allows the bonding strength between the silicon nitride plate 10 and the copper plate 50 to be sufficiently improved.

The second bonding layer 22 is formed so as to isolate the first bonding layer 21 from the third bonding layer 23. That is, the second bonding layer 22 is formed over the entire area between the first bonding layer 21 and the third bonding layer 23. Such a second bonding layer 22 is formed by the sufficient reaction between the silicon nitride and the titanium in the brazing material layer when the silicon nitride plate 10 and the copper plate 50 are heat-bonded via the brazing material layer. Therefore, the bonding strength between the silicon nitride plate 10 and the copper plate 50 of the circuit board 100 can be improved.

The thickness of the second bonding layer 22 can be determined as the length of the line at which the silicon element content is 10 atomic % or more. The thickness of the second bonding layer 22 may be 50 nm or more, 50 to 120 nm, 60 to 110 nm, or 80 to 100 nm. By having the thickness of the second bonding layer 22 in such a range, the reaction between the silicon nitride and the titanium in the bonding layer can proceed sufficiently, improving the bonding strength between the silicon nitride plate and the copper plate.

The average thickness t2 of the second bonding layer 22 can be determined by calculating the average thickness measured at five or more lines. The average thickness t2 of the second bonding layer 22 may be 60 to 100 nm, or 70 to 90 nm. A second bonding layer 22 with an average thickness t2 in this range is formed by the reaction between the silicon nitride plate 10 and the brazing material layer progressing sufficiently. Such a second bonding layer 22 can sufficiently increase the bonding strength between the silicon nitride plate 10 and the copper plate 50.

The ratio (t1/t2) of the average thickness t1 of the first bonding layer 21 to the average thickness t2 of the second bonding layer 22 may be 5 or less, 4 or less, or 3 or less. By having (t1/t2) within this range, the first bonding layer 21 and the second bonding layer 22 are formed in a balanced manner, and the bonding strength between the silicon nitride plate 10 and the copper plate 50 can be sufficiently high. The lower limit of the above ratio may be 1.0 or 1.5.

The third bonding layer 23 is a layer having a higher silver element content than the first bonding layer 21 and the second bonding layer 22. The silver element content in the third bonding layer 23 may be 20 to 90 atomic %, or may be 30 to 80 atomic %. The thickness of the third bonding layer 23 may be 1.0 to 2.0 μm, or may be 1.2 to 1.7 μm.

Titanium elements do not have to be present at the interface between the third bonding layer 23 and the copper plate 50. In the circuit board 100, the reaction between the silicon nitride plate 10 and the titanium in the brazing material layer has progressed sufficiently, so that the titanium elements tend to remain in the first bonding layer 21 and the second bonding layer 22 near the silicon nitride plate 10 side. The absence of titanium elements at the interface between the third bonding layer 23 and the copper plate 50 results in a circuit board 100 in which the reaction between the silicon nitride plate 10 and the titanium in the brazing material layer has progressed sufficiently. As a result, the bonding strength between the silicon nitride plate 10 and the copper plate 50 can be made sufficiently high.

The bonding layer 20 may contain carbon derived from the organic binder. The carbon may be contained in the third bonding layer 23. The average value of the ratio (C1/C3) of the carbon content (C1) contained in the first bonding layer 21 to the carbon content (C3) contained in the third bonding layer 23 may be 0.5 or less, 0.45 or less, or 0.4 or less. When the average value of (C1/C3) is in this range, the thickness of the first bonding layer 21 becomes sufficiently large. As a result, the bonding strength between the silicon nitride plate 10 and the copper plate 50 can be sufficiently high. The lower limit of the above ratio may be 0.2 or 0.3. The average value of (C1/C3) is obtained as follows. (C3) and (C1) are measured on five or more lines, and (C1/C3) is calculated from each measurement value. The above average value is obtained from five or more (C1/C3) values. In addition, each of the five or more (C1/C3) values may be 0.5 or less, 0.45 or less, or 0.4 or less.

The bond strength between the silicon nitride plate 10 and the copper plate 50 can be determined in accordance with JIS K 6854-1:1999 "Adhesives - Peel Adhesion Strength Test Method" by peeling the copper plate 50 vertically upward at 90° from the circuit board 100 and measuring the strength at the time of peeling. The bond strength may be 500 to 800 N/cm, or 600 to 700 N/cm. A circuit board 100 having such bond strength reduces variation in bond strength on the circuit board 100, and the quality of the circuit board 100 can be stabilized.

An example of a manufacturing method for a circuit board 100 is described below with reference to Figures 2 to 9. The manufacturing method in this example includes a step (a) of producing a plurality of composites 40 each including a silicon nitride plate 10, a copper plate 50, and a brazing material layer 30 that bonds them together, a step (b) of placing a spacer 70 between a pair of composites 40 to produce a laminate 80, a step (c) of heating the laminate 80 to bond the silicon nitride plate 10 and the copper plate 50 with the brazing material layer 30, and a step (d) of producing a circuit board from the bonded assembly.

A partition line may be formed on the main surface of the silicon nitride plate 10 used in step (a). For example, a laser beam may be irradiated onto the main surface of the silicon nitride plate 10 to provide a scribe line as the partition line. Examples of the laser beam to be irradiated include a carbon dioxide laser and a YAG laser. By intermittently irradiating the laser beam from such a laser source, a scribe line that serves as the partition line is formed. Such a partition line can be used as a cutting line when dividing the bonded body.

The brazing material applied to the main surface of the silicon nitride plate 10 contains, for example, silver, copper, tin, active metals, and metal compounds containing these as constituent elements, an organic solvent, and an organic binder. The viscosity of the brazing material may be, for example, 5 to 20 Pa·s. The organic solvent content in the brazing material may be, for example, 5 to 25 mass %, and the organic binder content may be, for example, 2 to 15 mass %.

The brazing filler metal may contain silver in the form of a metal element or a metal compound (alloy), and may contain, in addition to silver, one or more metals selected from the group consisting of copper, tin, and active metals. The two or more metals may be in the form of an alloy. The active metal may contain one or more metals selected from the group consisting of titanium, hafnium, zirconium, and niobium. The silver content in the brazing filler metal may be 45 to 95 mass% or 50 to 95 mass% in terms of Ag. The total silver and copper content in the brazing filler metal may be 65 to 100 mass%, 70 to 99 mass%, or 90 to 98 mass% in terms of Ag and Cu, respectively.

The content of the active metal in the brazing filler metal may be 0.5 to 8 parts by mass per 100 parts by mass of the total of Ag and Cu. By making the content of the active metal 0.5 parts by mass or more, it is possible to improve the bond between the silicon nitride plate and the brazing filler metal. On the other hand, by making the content of the active metal 8 parts by mass or less, it is possible to suppress the formation of a brittle alloy layer at the bonded interface.

The above metals contained in the brazing material may be contained as nitrides, oxides, carbides, or hydrides. As an example, the brazing material may contain titanium nitride and/or titanium hydride (TiH ₂ ). This allows the bonding strength between the silicon nitride plate 10 and the copper plate 50 to be sufficiently high. The content of TiH ₂ relative to a total of 100 parts by mass of Ag and Cu may be, for example, 1 to 8 parts by mass.

The tin content in the brazing material may be 0.5 to 6 parts by mass per 100 parts by mass of the total of Ag and Cu. By making the tin content 0.5 parts by mass or more, it is possible to improve the bond between the silicon nitride plate 10 and the bonding layer 20. On the other hand, by making the tin content 5 parts by mass or less, it is possible to suppress the formation of a brittle alloy layer at the bonding interface.

A brazing material is applied to both main surfaces of the silicon nitride plate 10 by a roll coater method, a screen printing method, a transfer method, or the like, and then dried to provide one or more brazing material layers 30. The brazing material layers 30 may be provided at positions where the copper plates 50 are to be joined to the silicon nitride plate 10. Once the brazing material layers 30 are formed, the silicon nitride plates 10 and the copper plates 50 are stacked with the brazing material layer 30 sandwiched between them to produce multiple composites 40.

In step (b), as shown in FIG. 2, a pair of composites 40 are laminated so as to sandwich a spacer 70, to produce a laminate 80. A commercially available carbon plate may be used as the spacer 70. The density of the carbon plate may be 1.6 to 1.9 g/cm ^3. The thickness of the carbon plate may be 0.5 to 1.0 mm. The surface roughness Ra may be 5 to 10 μm. The spacers 70 and composites 40 used in the laminate 80 may be repeatedly laminated in any number. The number of composites 40 and spacers 70 to be laminated is not particularly limited, and may be adjusted depending on the size of the heating furnace to be used. By laminating in this manner, a plurality of composites 40 can be heated at once, and a bonded body can be efficiently produced.

The laminate 80 may be heated in step (c) while being pressurized in the stacking direction. An example of a method for pressurizing the laminate 80 is described below with reference to Figures 3, 4, 5, 6, 7, and 8. The structure 90 in Figure 3 includes a pressurizing jig 1 and a laminate 80 sandwiched between a base plate 2 and a cover plate 3 of the pressurizing jig 1. The laminate 80 is formed by alternately stacking a plurality of spacers 70 and a plurality of composites 40. The laminate 80 is rectangular in plan view, but may have other shapes. A spacer 70 is disposed between the laminate 80 and the pressurizing jig 1. The spacer 70 also has the function of preventing bonding between the composites 40 and bonding between the composites 40 and the pressurizing jig 1.

As shown in FIG. 3, the pressing jig 1 comprises a plate-shaped base plate 2 (foundation portion) on which the laminate 80 is placed, and a cover plate 3 (cover portion) arranged opposite the base plate 2. The cover plate 3 and the base plate 2 sandwich the laminate 80. The pressing jig 1 comprises a plurality of spring washers 4 arranged on the cover plate 3, and a pressing plate 5 (pressing portion) arranged on the spring washers 4. The pressing jig 1 has a pressing structure 6 that compresses the spring washers 4 with the pressing plate 5 to pressurize the laminate 80.

The base plate 2 is, for example, rectangular, and has an area that extends beyond the laminate 80 in a plan view, i.e., a protruding area 2a that does not overlap the laminate 80. In the protruding area 2a of the base plate 2, multiple (for example, two) pillars 8 (pillar portions) are erected along the short sides of the base plate 2. The pillars 8 are also arranged opposite each other on both sides of the longitudinal direction (left-right direction) of the base plate 2, and the laminate 80 is arranged between a pair of pillars 8 facing each other on the left and right. An engagement hole 8a is formed in the upper part of the pillar 8, through which a locking claw 9 is inserted.

The cover plate 3 has a rectangular shape corresponding to the shape of the laminate 80, and has one surface 3a (hereinafter referred to as the "lower surface") and the other surface 3b (hereinafter referred to as the "upper surface") as shown in Figs. 5 and 7. The lower surface 3a is flat and abuts against the spacer 70 arranged closer to the cover plate 3. As shown in Fig. 3, the upper surface 3b has a plurality of counterbores 3c formed therein, and the spring washer 4 is housed in each counterbores 3c. The depth of the counterbores 3c may be a dimension such that the entire spring washer 4 is housed therein when the helical spring washer 4 is compressed and flattened. The counterbores 3c are donut-shaped (annular) in plan view, and the annular spring washer 4 is housed therein, preventing the spring washer 4 from shifting out of position. The counterbores 3c function as positioning portions for the spring washer 4.

As shown in FIG. 5, the pressure plate 5 has a surplus area 5a that faces the protruding area 2a of the base plate 2 in a plan view. As shown in FIG. 3 and FIG. 4, a post through hole 5b through which the post 8 passes is formed in the surplus area 5a. A storage groove 5c is provided on the upper surface of the surplus area 5a so as to pass through the post through hole 5b. The locking claw 9 fits into the storage groove 5c.

The pressure plate 5 is placed so as to overlap the cover plate 3, and is further pressed down so as to abut against the spring washer 4. When the pressure plate 5 is pressed down, a support 8 is passed through the support through hole 5b provided in the excess area 5a of the pressure plate 5. The upper part of the support 8 protrudes above the support through hole 5b. An engagement hole 8a is provided in the upper part of the support 8, and a locking claw 9 (locking portion) is inserted into the engagement hole 8a. The locking claw 9 passes through the engagement hole 8a from the lateral direction and locks the pressure plate 5 by abutting against the surface (upper surface) of the pressure plate 5 opposite the surface (lower surface) that abuts against the spring washer 4. A storage groove 5c corresponding to the outer shape of the locking claw 9 is formed in the upper surface of the pressure plate 5, and the locking claw 9 slides along the storage groove 5c.

When the locking claw 9 locks the pressure plate 5, the pressure plate 5 is restricted from moving upward. By restricting the movement of the pressure plate 5, the spring washer 4 is prevented from returning to its original position, and the spring washer 4 is held in a compressed state. The pressure structure 6 in this embodiment is configured to include at least the pressure plate 5, the support 8, the engagement hole 8a, and the locking claw 9.

The spring washer 4 is formed of, for example, an alloy such as Inconel 750 or Inconel 718, silicon nitride, or a carbon composite. The spring washer 4 is in an uncompressed state when it is not in contact with the cover plate 3 (see FIG. 8). The spring washer 4 is an example of a compression spring that is wound in a spiral shape without overlapping and has both ends 4a and 4b spaced apart. More specifically, the spring washer 4 has a C-shape, and both

ends

4a and 4b are spaced apart from each other and are offset in the axial direction (for example, vertical direction) assuming a spiral shape. Here, within the countersink 3c of the cover plate 3, one end that is in contact with the bottom of the countersink 3c is the lower end 4a, and the other end on the opposite side is the upper end 4b. When the spring washer 4 is compressed by the pressure structure 6 through the cover plate 3, the lower end 4a and the upper end 4b approach each other, the spiral shape of the spring washer 4 is eliminated, and the spring washer 4 becomes substantially flat (see Figure 7).

The multiple spring washers 4 are arranged so as to fit inside the outer edge 3x of the cover plate 3 without protruding beyond the outer edge 3x of the cover plate 3. The multiple spring washers 4 are also arranged along the outer edge 3x of the cover plate 3. For example, the outer edge 3x of the cover plate 3 according to this embodiment is rectangular. Three spring washers 4 are arranged in a row along each short side of the outer edge 3x, and five spring washers 4 are arranged in a row along each long side. By arranging the multiple spring washers 4 along the outer edge 3x of the cover plate 3, it becomes easier to apply uniform pressure to the laminate 80 via the cover plate 3.

The multiple spring washers 4 are arranged at equal intervals. Specifically, a total of 15 spring washers 4 are arranged in three rows and five columns (multiple rows and multiple columns), and all of the spring washers 4 are arranged at equal intervals. By arranging the spring washers 4 at equal intervals, it becomes easier for the cover plate 3 to apply uniform pressure to the laminate 80. Note that only the multiple spring washers 4 arranged along the outer edge 3x of the cover plate 3 may be arranged at equal intervals. Furthermore, when the center of the cover plate 3 is defined in a plan view, the multiple spring washers 4 may be arranged so as to be point symmetrical with respect to this center.

A separation region 4x is formed between the lower end 4a and upper end 4b of the spring washer 4 (between both ends). The multiple spring washers 4 are arranged so that the separation region 4x faces the outer edge 3x of the cover plate 3. In other words, if the spring washer 4 is assumed to be ring-shaped in a plan view, the spring washer 4 is arranged so that the separation region 4x is closer to the outer edge 3x of the cover plate 3 than the center point of the ring. To further add, if a line is assumed to connect the separation regions 4x of the multiple spring washers 4, this line is arranged to run along the outer edge 3x of the cover plate 3.

When the spring washer 4 is compressed, the repulsive force (pressure) is greatest at the lower end 4a and upper end 4b, which face each other across the separation area 4x. By arranging this separation area 4x to face the outer edge 3x of the cover plate 3, it becomes easier to reliably hold down the portion along the outer edge 3x of the cover plate 3, and it becomes easier to apply uniform pressure to the laminate 80 via the cover plate 3.

9 shows an enlarged cross section of the laminated structure of the composite 40 and the spacer 70 in FIG. 8 along the lamination direction. In FIG. 9, the spacer 70, the laminate 80, and the spacer 70 are laminated in this order from the base plate 2 toward the cover plate 3. The laminate 80 is formed by laminating a pair of composites 40 so as to sandwich the spacer 70. The composite 40 is formed by laminating a pair of copper plates 50 so as to sandwich the silicon nitride plate 10 with the brazing material layer 30 between them. The laminate 80 may be formed by repeatedly laminating any number of composites 40 and spacers 70. This laminated structure makes it possible to suppress the variation in reaction during heating in step (c) and obtain a joint having high joint strength. The number of spacers 70 and composites 40 constituting the laminate 80 is not particularly limited.

In step (c), the laminate 80 is heated in a heating furnace while still under pressure to cause a reaction between the silicon nitride plate 10 and the copper plate 50 and the brazing material layer 30, thereby obtaining a bonded body in which the silicon nitride plate 10 and the copper plate 50 are bonded. By heating, the brazing material layer 30 becomes the bonding layer 20 described above. Therefore, the bonding layer 20 may contain a reaction product between the silicon nitride plate 10 and the copper plate 50 and the brazing material layer 30.

Step (c) may include a step of degreasing the carbon in the brazing material layer 30. The heating temperature in the degreasing step may be 370 to 420°C, 380 to 415°C, or 390 to 410°C. The heating time at the above heating temperature may be 3.5 hours or more, or 3.75 hours or more. In order to shorten the time required for the entire manufacturing process, the heating time may be 5 hours or less, or 4.5 hours or less. The degreasing step can remove excess carbon derived from the organic binder contained in the brazing material layer 30 by thermal decomposition, so that carbon and titanium do not react when heated at 700 to 900°C, and the generation of titanium nitride and titanium silicide can be sufficiently promoted.

The atmosphere in the heating furnace may be an inert gas such as nitrogen, and may be performed under reduced pressure below atmospheric pressure, or may be performed under vacuum. The heating furnace may be a continuous type that continuously produces a plurality of composites 40, or may be a batch type that produces one or a plurality of composites 40. The heating temperature may be 700 to 900°C or 750 to 850°C from the viewpoint of promoting the reaction. The heating time may be 3 hours or less, or 2 hours or less. The pressure in the furnace during heating may be any of normal pressure, reduced pressure, and vacuum. From the viewpoint of promoting the reaction, the pressure in the furnace may be 10 ^-2 to 10 ^-7 Pa.

The heating rate in the heating furnace in step (c) may be 70 to 120°C/min, or may be 80 to 110°C/min. By keeping the heating rate in this range, the time required for the entire manufacturing process can be sufficiently shortened. In addition, the temperature reduction rate after heating may also be in the same range as the above heating rate.

After step (c), an annealing step may be performed. The annealing temperature in the annealing step may be 400 to 650°C, or 500 to 600°C. The annealing time at the annealing temperature may be 1.5 to 3.0 hours, or 2.0 to 2.5 hours. After cooling, the bonded body may be removed from the pressure jig. By performing the annealing step, it is possible to reduce distortion of the laminate 80 due to heating, and to produce a circuit board with sufficiently excellent heat cycle characteristics.

In step (d), resist printing and etching are performed on the resulting bonded body to obtain the circuit board 100. Since the carbon in the brazing material layer 30 has been sufficiently removed from the circuit board 100, the bonding layer 20 has a first bonding layer 21, a second bonding layer 22, and a third bonding layer 23. Therefore, the bonding strength between the silicon nitride plate 10 and the copper plate 50 in the circuit board 100 can be made sufficiently high.

The circuit board 100 may be used to manufacture a power module. The power module can be manufactured by mounting a semiconductor element electrically connected to the copper plate of the circuit board using solder and wire bonding, etc., and housing the circuit board and the semiconductor element in the housing space of the housing, and then sealing with resin.

Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments. For example, the structures and shapes of the metal plates and joints joined to each of the pair of main surfaces of the silicon nitride plate may be different from each other. Also, the present disclosure may be a circuit board in which a joining layer and a copper plate are provided on only one main surface of the silicon nitride plate.

The contents of this disclosure will be explained in more detail with reference to examples and comparative examples, but this disclosure is not limited to the following examples.

Example 1
[Preparation of circuit board]
A silicon nitride plate having a thickness of 0.32 mm, a copper plate (size: length×width×thickness=170 mm×120 mm×0.8 mm), and a brazing material were prepared. The copper plate was obtained by punching.

The brazing material contained Ag, Cu, TiH ₂ , and Sn. The mass ratio of Ag to Cu was 9: 1. The brazing material contained 5 parts by mass of Sn and 3.5 parts by mass of TiH ₂ with respect to 100 parts by mass of Ag and Cu in total.

The brazing filler metal was applied to the main surface of the silicon nitride plate by screen printing (mesh number: 150) and dried to form a brazing filler metal layer. The application area of the brazing filler metal layer was the same as the area of the main surface of the copper plate to be joined to the silicon nitride plate. The thickness of the applied brazing filler metal layer was 0.03 mm.

After forming the brazing material layer, the copper plate was placed on top of the silicon nitride plate so that the brazing material layer was in contact with the main surface of the copper plate, creating a composite. In this way, 20 composites of silicon nitride plate, brazing material layer, and copper plate were created.

A pair of carbon plates (thickness: 0.5 mm, density: 1.8 g/cm ³ ) were placed between adjacent composites to produce a laminate as shown in Fig. 2. As shown in Fig. 9, spacers were placed on both main surfaces of the laminate and fixed in place by a pressing jig.

While being fixed by the pressure tool, the laminate was heated to 400°C in a vacuum ( ^10-3 Pa) at a heating rate of 100°C/min, and degreasing was performed at 400°C for 3.5 hours. Thereafter, the laminate was heated from 400°C to 750°C at a heating rate of 100°C/min in an atmosphere of ^10-3 Pa, and heated at 750°C for 3 hours. Thereafter, the laminate was cooled to 600°C at a cooling rate of 100°C/min, and an annealing step was performed in which the laminate was held at 600°C for 2 hours. After cooling, the bonded body was removed from the pressure tool. In this way, a bonded body was produced in which a silicon nitride plate and a copper plate were bonded via a bonding layer. A circuit board was produced using the bonded body produced.

(Comparative Example 1)
A circuit board was produced in the same manner as in Example 1, except that degreasing was carried out at 350° C. for 3 hours.

[Circuit board evaluation]
<Elemental mapping of the layer state of a cross section of a circuit board>
The cross section of the obtained circuit board was observed at a magnification of 25,000 times using a scanning transmission electron microscope (STEM, Hitachi High-Tech Corporation, product name: HD-2700). The observed cross section was subjected to elemental mapping of Ti, Si, Ag, and Cu at an acceleration voltage of 200 kV using an energy dispersive X-ray analyzer (EDX, Oxford, product name: XMAXN 100TLE). The STEM image of Example 1 is shown in FIG. 10(A), and the results of elemental mapping are shown in FIG. 11(A), FIG. 11(B), FIG. 12(A), and FIG. 12(B). The STEM image of Comparative Example 1 is shown in FIG. 10(B), and the results of elemental mapping are shown in FIG. 13(A), FIG. 13(B), FIG. 14(A), and FIG. 14(B). FIG. 11(A) and FIG. 13(A) show the distribution of titanium element, and FIG. 11(B) and FIG. 13(B) show the distribution of silicon element. Moreover, FIG. 12(A) and FIG. 14(A) show the distribution of silver element, and FIG. 12(B) and FIG. 14(B) show the distribution of copper element.

The results of the STEM images and element mapping showed that in Example 1, the second bonding layer 22 was interposed between the first bonding layer 21 and the third bonding layer 23 so as to isolate them (see Figures 11(A), 11(B), and 12(A)). On the other hand, in Comparative Example 1, the second bonding layer 22A was confirmed, but unlike the second bonding layer 22, the second bonding layer 22A was a discontinuous layer, and there were portions where the first bonding layer 21 and the third bonding layer 23 were in direct contact (see Figures 13(A), 13(B), and 14(A)).

In Comparative Example 1, as shown in Figures 13(A), 14(A), and 14(B), titanium elements were scattered at the interface between the copper plate 50 and the third bonding layer 23. On the other hand, in Example 1, as shown in Figures 11(A), 12(A), and 12(B), titanium elements were not present at the interface between the copper plate 50 and the third bonding layer 23.

<Measurement of Thickness of First Bonding Layer 21 and Second Bonding Layer 22>
In each of the above STEM images, the line profile of each element was measured by EDX analysis along the line from the silicon nitride plate to the copper plate. The result of the EDX analysis in Example 1 is shown in FIG. 15. In FIG. 15, the length of the line where titanium element and nitrogen element are detected at 10 atomic % or more was defined as the thickness of the first bonding layer 21. In addition, the length of the line where titanium element and silicon element are detected at 10 atomic % or more was defined as the thickness of the second bonding layer 22. The line profile was performed at five arbitrary points in each STEM image, and the thicknesses of the first bonding layer 21 and the second bonding layer 22 were measured in the same manner. In Comparative Example 1, five arbitrary points were selected from the portion where the second bonding layer 22A was formed and measured. The average value t1 of the thickness of the first bonding layer 21 and the average value t2 of the thickness of the second bonding layer at the five measured points were calculated, and t1/t2 was calculated. The measurement results are shown in Table 1. Table 1 also shows the maximum value, minimum value, and difference between the measured thicknesses of the bonding layers.

Example 1 showed a thicker second bonding layer than Comparative Example 1. This indicates that the reaction between the silicon nitride plate and the brazing material layer caused by heating has progressed sufficiently.

<Ratio of C (carbon) contained in first bonding layer 21 and third bonding layer 23>
In the above line profile, the portion where silver element is detected at 40 atomic % or more was determined as the third bonding layer 23. The atomic % of C (carbon) (C1) of the first bonding layer 21 was measured at a position 100 nm away from the position where titanium element was first detected at 10 atomic % or more along the line toward the copper plate 50 side, measured from the silicon nitride plate 10 side. In addition, the atomic % of C (carbon) (C3) of the third bonding layer 23 was measured at a position 1000 nm away from the position where it was first detected along the line toward the copper plate 50 side, measured from the position where it was first detected. Furthermore, the ratio of C (carbon) (C1/C3) was calculated from each measured value. The measurement of the atomic % of carbon was performed for each line by arbitrarily selecting five different lines. The results are shown in Table 2. Table 2 also shows the measured atomic % of carbon and the average value, maximum value, minimum value, and difference between the maximum value and the minimum value of the calculated (C1)/(C3).

It was confirmed that in Example 1, the amount of carbon detected in the first bonding layer relative to the third bonding layer was less than in Comparative Example 1. Therefore, it was shown that in Example 1, the carbon derived from the binder in the first bonding layer was reduced by degreasing for a long time at high temperature. Therefore, it is considered that the reaction between carbon and titanium was suppressed, and the reaction between nitrogen derived from silicon nitride and titanium progressed sufficiently.

<Bonding strength and bonding state of copper plates>
In Example 1 and Comparative Example 1, the copper plate bonded to the silicon nitride plate was pulled vertically upward using a tensile tester, and the bonding strength was measured at the time when the copper plate peeled off from the silicon nitride plate as shown in Figures 16(A) and 16(B). The stage of the tensile tester used was FGS-100VC manufactured by Nidec-Shimpo Corporation. The force gauge used was FGP-20 manufactured by Nidec-Shimpo Corporation. The peeling speed was 50 mm/min. The results were 650 N/cm in Example 1 and 450 N/cm in Comparative Example 1. Furthermore, SEM images of the peeled surface and elemental analysis by EDX of the fractured cross section were performed. The elemental analysis conditions were the same as those described above.

　FIG. 16(A) shows a photograph of the appearance of the copper plate of Example 1 when peeled off, FIG. 17(A) shows the results of SEM image observation of the peeled surface of Example 1, and FIG. 18(A) shows the results of elemental analysis by EDX of Example 1. Similarly, FIG. 16(B) shows a photograph of the appearance of the copper plate of Comparative Example 1 when peeled off, FIG. 17(B) shows the results of SEM image observation of the peeled surface of Comparative Example 1, and FIG. 18(B) shows the results of elemental analysis by EDX of Comparative Example 1. The black parts 101 in FIG. 17(A) and FIG. 17(B) represent parts containing silicon elements, and the gray parts 105 represent parts containing silver elements. Meanwhile, each dot of the dotted pattern parts 102 in FIG. 18(A) and FIG. 18(B) represents parts containing silicon elements, and the black parts 106 represent parts containing silver elements.

From Figures 17(A), 17(B), 18(A), and 18(B), in Example 1, a large amount of silicon element was attached to the peeled surface, causing fracture near the silicon nitride plate 10. In contrast, in Comparative Example 1, the amount of silicon element attached to the peeled surface was less than in Example 1, and it was confirmed that interfacial peeling occurred between the third bonding layer 23 and the copper plate 50. In Example 1, more silicon element was attached to the peeled surface than in Comparative Example 1, and the bonding strength was also higher as described above. From these results, it was found that in the circuit board of Example 1, the first bonding layer and second bonding layer were sufficiently formed, and therefore the silicon nitride plate 10 and bonding layer 20 were bonded more firmly than in Comparative Example 1.

The present disclosure provides a circuit board with high bonding strength. It also provides a power module using such a circuit board.

100...circuit board, 1...pressure jig, 2...base plate, 3...cover plate, 3x...outer edge, 4...spring washer, 4a...lower end, 4b...upper end, 4x...separation area, 5...pressure plate, 5b...post through hole, 6...pressure structure, 8...post portion (post) 9...locking claw (locking portion), 8a...engagement hole, 10...silicon nitride plate, 20...bonding layer, 21...first bonding layer, 22...second bonding layer, 23...third bonding layer, 50...copper plate, 30...brazing material layer, 40...composite, 70...spacer, 80...laminated body, 101, 106...black portion, 105...gray portion, 102...dot pattern portion.

Claims

A circuit board comprising a silicon nitride plate, a copper plate, and a bonding layer that bonds the silicon nitride plate and the copper plate,
The bonding layer includes, from the silicon nitride plate side, a first bonding layer containing titanium nitride, a second bonding layer having a higher silicon element content than the first bonding layer, and a third bonding layer having a higher silver element content than the first bonding layer and the second bonding layer,
A circuit board, wherein the second bonding layer is interposed between the first bonding layer and the third bonding layer so as to isolate the first bonding layer from the third bonding layer.
The circuit board according to claim 1, wherein no titanium element is present at the interface between the copper plate and the bonding layer.
The circuit board according to claim 1, in which the average value of C1/C3 is 0.5 or less when the carbon content in the first bonding layer is C1 atomic % and the carbon content in the third bonding layer is C3 atomic %.
The circuit board according to claim 1, wherein the difference between the maximum and minimum thicknesses of the first bonding layer is 70 nm or less.
The circuit board according to claim 1, wherein the second bonding layer has a thickness of 50 nm or more.
The circuit board according to claim 1, in which t1/t2 is 5 or less, where t1 is the average thickness of the first bonding layer and t2 is the average thickness of the second bonding layer.
A power module comprising: the circuit board according to any one of claims 1 to 6; and a semiconductor element electrically connected to the copper plate of the circuit board.