Silicon nitride circuit board and method for manufacturing the same

Abstract

The present invention provides a silicon nitride circuit board and a method for manufacturing the same that can improve the bonding strength between the silicon nitride substrate and the metal plate. [Solution] The silicon nitride circuit board 1 comprises a silicon nitride substrate 10 and linear first to third metal plates 11 to 13 joined to the silicon nitride substrate 10 via a brazing material 101. The thickness T1 of the first to third metal plates 11 to 13 is 0.5 mm or more. The bonding strength of the silicon nitride substrate 10 to the first to third metal plates 11 to 13 is higher than the breaking strength of the first to third metal plates 11 to 13. The manufacturing method of the silicon nitride circuit board 1 includes a molding step of forming a sheet molded body which is a precursor of the silicon nitride substrate 10, a heat treatment step of heat treating the sheet molded body to obtain the silicon nitride substrate 10, and a metal plate forming step of forming the first to third metal plates 11 to 13 on the silicon nitride substrate 10, wherein silicon powder containing 100 ppm to 1900 ppm of Fe is used as a raw material in the sheet molded body molding step.

Description

Technical Field

[0001] The present invention relates to a silicon nitride circuit board and a method for manufacturing the silicon nitride circuit board.

Background Art

[0002] Conventionally, a silicon nitride circuit board is known which includes a silicon nitride substrate and a metal plate bonded to the silicon nitride substrate via a brazing material layer, and in which a circuit pattern is formed by the metal plate. Such a silicon nitride circuit board has higher heat resistance than, for example, a FR-4 substrate generally used as an electric circuit board, and thus a power semiconductor such as a power transistor is mounted thereon, for example. In such a silicon nitride circuit board, in order to obtain higher heat resistance, it is necessary to increase the bonding strength between the silicon nitride substrate and the metal plate. This is because if the bonding strength between the silicon nitride substrate and the metal plate is low, the metal plate will peel off from the silicon nitride substrate due to the difference in linear expansion coefficient. For example, Patent Document 1 describes that setting the surface roughness of the silicon nitride substrate to 0.5 μm or more increases the bonding strength between the silicon nitride substrate and the metal plate.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In recent years, along with the higher performance of industrial equipment and transportation equipment, the output of power semiconductors mounted on silicon nitride circuit boards has been increasing. When the output of a power semiconductor increases, the amount of heat generated by the power semiconductor increases. For this reason, higher heat resistance than ever before has been required for silicon nitride circuit boards. Accordingly, an object of the present invention is to provide a silicon nitride circuit board capable of improving the bonding strength between a silicon nitride substrate and a metal plate, and a method for manufacturing the same.

Means for Solving the Problems

[0005] To solve the above problems, the present invention provides a silicon nitride circuit board comprising a silicon nitride substrate and a metal plate joined to the silicon nitride substrate via a brazing material, wherein the thickness of the metal plate is 0.5 mm or more, and the bonding strength of the silicon nitride substrate to the metal plate is higher than the fracture strength of the metal plate.

[0006] Furthermore, in order to solve the above problems, the present invention provides a method for manufacturing a silicon nitride circuit board, comprising: a molding step of molding a sheet molded body which is a precursor of the silicon nitride substrate; a heat treatment step of heat-treating the sheet molded body to obtain a silicon nitride substrate; and a metal plate forming step of forming the patterned metal plate on the silicon nitride substrate, wherein the molding step comprises the steps of preparing a slurry by adding raw material powder containing silicon powder, a sintering aid and a binder to a dispersion medium; coating the slurry onto a substrate to form a sheet; and drying the sheet-shaped slurry, wherein the silicon powder contains 100 ppm to 1900 ppm of Fe. [Effects of the Invention]

[0007] According to the silicon nitride circuit board and its manufacturing method according to the present invention, it is possible to improve the bonding strength between the silicon nitride substrate and the metal plate. [Brief explanation of the drawing]

[0008] [Figure 1] (a) is a plan view showing a silicon nitride circuit board according to an embodiment of the present invention. (b) is a cross-sectional view taken along line AA in Figure 1(a). [Figure 2] (a) is a plan view showing the silicon nitride circuit board and power transistor. (b) is a side view of the silicon nitride circuit board and power transistor. [Figure 3] This flowchart shows an example of a manufacturing method for silicon nitride circuit boards. [Figure 4](a) is an explanatory diagram showing an example of the configuration of a peel strength testing machine in a silicon nitride circuit board and its surrounding area during peel strength measurement. (b) shows the state in which the first metal plate has fractured during the peel strength test. [Modes for carrying out the invention]

[0009] [Embodiment] Figure 1(a) is a plan view showing a silicon nitride circuit board 1 according to an embodiment of the present invention. Figure 1(b) is a cross-sectional view taken along line AA in Figure 1(a). Figure 2(a) is a plan view showing the silicon nitride circuit board 1 and a power transistor 2 mounted on the silicon nitride circuit board 1. Figure 2(b) is a side view of the silicon nitride circuit board 1 and the power transistor 2.

[0010] The silicon nitride circuit board 1 comprises a silicon nitride substrate 10, first to fourth metal plates 11 to 14 joined to one main surface 10a of the silicon nitride substrate 10 via a brazing material 101, and a fifth metal plate 15 joined to the other main surface 10b of the silicon nitride substrate 10 via a brazing material 102. The first to fourth metal plates 11 to 14 are formed by etching a portion of a single plate material joined to one main surface 10a of the silicon nitride substrate 10 by the brazing material 101, and have the same thickness. The thickness of the fifth metal plate 15 is, for example, thicker than the first to fourth metal plates 11 to 14, but may be the same thickness as the first to fourth metal plates 11 to 14.

[0011] The brazing materials 101 and 102 are, for example, active metal brazing materials of the silver (Ag)-copper (Cu)-titanium (Ti) system. The first to fifth metal plates 11 to 15 are made of a highly conductive metal, such as copper or a copper alloy. However, they are not limited to this, and the first to fourth metal plates 11 to 14 may be made of aluminum or an aluminum alloy.

[0012] The silicon nitride substrate 10 is rectangular in shape. The power transistor 2 is mounted on one end of the silicon nitride substrate 10 in the longitudinal direction. The power transistor 2 has a body 20 sealed with a encapsulant, a collector terminal 21, a base terminal 22, and an emitter terminal 23 extending from the body 20, and a metal heat spreader 24 integrated with the body 20. The heat spreader 24 is joined to the fourth metal plate 14, for example, by soldering or by a conductive adhesive such as silver paste. A heat sink 3 is attached to the side of the fifth metal plate 15 opposite to the silicon nitride substrate 10. In Figure 2(b), the heat sink 3 is shown by a dashed line. The heat generated in the body 20 of the power transistor 2 is dissipated from the heat spreader 24 through the fourth metal plate 14, the silicon nitride substrate 10, and the fifth metal plate 15, and then from the heat sink 3.

[0013] The first to third metal plates 11 to 13 extend parallel to each other in a predetermined direction. In this embodiment, this predetermined direction is the longitudinal direction of the silicon nitride substrate 10. That is, the silicon nitride substrate 10 is rectangular in shape, with the extension direction of the first to third metal plates 11 to 13 being the longitudinal direction. The first to third metal plates 11 to 13 include a linear portion that extends along the longitudinal direction of the silicon nitride substrate 10. In this embodiment, the entirety of the first to third metal plates 11 to 13 is linear. The collector terminal 21, base terminal 22, and emitter terminal 23 of the power transistor 2 are connected to one end of each of the first to third metal plates 11 to 13, for example, by soldering.

[0014] In Figure 1(a), W1 indicates the width of the silicon nitride substrate 10 in the short direction, and W2 indicates the width of the first metal plate 11 in the same direction. W2 is 20% or more of W1. In Figure 1(b), T1, T2, and T3 indicate the thickness of the first metal plate 11, the thickness of the silicon nitride substrate 10, and the thickness of the brazing material 101, respectively. Also in Figure 1(b), T4 and T5 indicate the thickness of the fifth metal plate 15 and the thickness of the brazing material 102, respectively. The thickness T1 of the first metal plate 11 is 0.5 mm or more and 2.0 mm or less. The thickness T2 of the silicon nitride substrate 10 is thinner than the thickness T1 of the first metal plate 11, for example, 0.2 mm or more and 0.5 mm or less. The thickness T4 of the fifth metal plate 15 is, for example, greater than or equal to the thickness T1 of the first metal plate 11. The thicknesses T3 and T5 of the brazing materials 101 and 102 are, for example, between 0.01 mm and 0.10 mm.

[0015] (Manufacturing method for silicon nitride circuit boards) Here, the method for manufacturing the silicon nitride circuit board 1 will be described. The method for manufacturing the silicon nitride circuit board 1 includes a molding step of forming a sheet molded body which is a precursor of the silicon nitride substrate 10, a heat treatment step of heat-treating the sheet molded body to obtain the silicon nitride substrate 10, and a metal plate forming step of forming first to fifth metal plates 11 to 15 on one main surface 10a and the other main surface 10b of the silicon nitride substrate 10.

[0016] Figure 3 is a flowchart illustrating the manufacturing method of the silicon nitride circuit board 1 in more detail. Steps S1 to S3 in this flowchart represent the molding process, step S4 represents the heat treatment process, and steps S5 to S7 represent the metal plate formation process.

[0017] In step S1, a raw material powder including silicon (Si) powder, sintering aid powder, and binder powder is added to a dispersion medium, and the mixture is pulverized and mixed to prepare a slurry. As the sintering aid, for example, a magnesium compound, or a mixture of a rare earth element oxide and a magnesium compound is used. The binder is not particularly limited as long as it is thermally decomposed at a temperature of 600°C or lower and does not leave a large amount of ash, carbon content or the like, and for example, acrylic resins such as polyacrylic acid and polyacrylate, cellulose resins such as methyl cellulose and ethyl cellulose, polyvinyl alcohol, polyvinyl butyral and the like can be used.

[0018] In step S2, the slurry prepared in step S1 is applied onto a base material and formed into a sheet shape. The base material is not particularly limited as long as it has a heat resistance temperature sufficiently higher than the vaporization temperature of the dispersion medium contained in the slurry, and an appropriate type such as a resin film, a resin plate, a metal plate, a ceramic plate, or a glass plate can be used.

[0019] In step S3, the slurry formed into a sheet shape is dried to obtain a sheet molded body that is a precursor of the silicon nitride substrate 10. When the slurry molded on the base material is put into a heating and drying furnace or the like and dried to remove the dispersion medium, a sheet molded body in which the powdery Si, the sintering aid, and the binder are bound is obtained.

[0020] In step S4, the sheet molded body obtained in step S3 is subjected to heat treatment. More specifically, a degreasing treatment for removing the binder contained in the sheet molded body, a nitriding treatment for bonding nitrogen (N) to silicon contained in the sheet molded body after the degreasing treatment to nitride the sheet molded body, and a treatment for sintering the nitrided sheet molded body to obtain the silicon nitride substrate 10 are performed. Each of these treatments may be performed sequentially in separate furnaces, or may be performed continuously in the same furnace.

[0021] In step S5, paste-shaped brazing materials 101 and 102 are applied to one main surface 10a and the other main surface 10b of the silicon nitride substrate 10 by, for example, a screen printing method, metal plate materials are stacked on the applied paste-shaped brazing materials 101 and 102, and the assembly is heated to about 800°C under a loaded state to perform brazing. In step S6, a resist is applied to a part of the plate material brazed to the one main surface 10a side of the silicon nitride substrate 10 and the entire plate material brazed to the other main surface 10b side of the silicon nitride substrate 10 to perform masking and patterning. In step S7, the unmasked portion of the plate material is etched and removed with a solvent. The resist has dissolution resistance to this solvent, and prevents the portion of the plate material coated with the resist from being dissolved by the solvent. Thereby, the silicon nitride circuit board 1 including the silicon nitride substrate 10 and the first to fifth metal plates 11 to 15 is obtained.

[0022] In the present embodiment, the silicon powder used in the process of step S1 contains 100 ppm or more and 1900 ppm or less of iron (Fe). Thereby, the bonding strength between the silicon nitride substrate 10 and the first to fifth metal plates 11 to 15 is enhanced. Fe is contained in the silicon powder in the form of iron-based oxides (Fe₂O₃, Fe₃O₄, FeO). Although the principle by which the bonding strength is increased when the silicon powder contains a predetermined amount of iron has not yet been completely elucidated, it is considered that the iron-based oxides function as a nitriding catalyst to promote the growth of crystal grains of β-phase silicon nitride (β-Si₃N₄), which is related to the reduction of the grain boundary phase. The β-phase silicon nitride has a columnar or acicular crystal structure, and has higher strength, toughness and higher heat resistance than α-phase silicon nitride.

[0023] Furthermore, it is believed that the TiN formed by the bonding of Ti in the brazing materials 101, 102 and N in silicon nitride at the interface between the silicon nitride substrate 10 and the silicon nitride substrate 10 contributes significantly to the bonding strength between the silicon nitride substrate 10 and the silicon nitride substrate 10. In this embodiment, it is presumed that the silicon powder, which is the raw material for the silicon nitride substrate 10, contains 100 ppm to 1900 ppm of iron (Fe), which increases the proportion of silicon nitride particles on the surface of the silicon nitride substrate 10, and thus increases the amount of TiN at the interface between the brazing materials 101, 102 and the silicon nitride substrate 10, thereby improving the bonding strength. Next, the method and results of the peel strength test to measure the bonding strength in the silicon nitride circuit board 1 will be described.

[0024] (Peel strength test) Figure 4(a) is an explanatory diagram showing an example of the configuration of a silicon nitride circuit board 1 and a peel strength tester 4 in its surrounding area during peel strength measurement. In the silicon nitride circuit board 1, a portion of the longitudinal direction of the first metal plate 11 is peeled from the silicon nitride substrate 10, and the bending portion 110 is bent in a direction perpendicular to the silicon nitride substrate 10. The method for peeling a portion of the longitudinal direction of the first metal plate 11 from the silicon nitride substrate 10 is not particularly limited, but for example, a release agent may be used, or the first metal plate 11 may be made easier to peel by applying a paste-like brazing material 101 in step S5 above and then removing some of the brazing material 101.

[0025] The peel strength tester 4 has a first gripping part 41 that grips the peeled portion 111, which is the part of the first metal plate 11 that has been peeled from the silicon nitride substrate 10, and a second gripping part 42 and a third gripping part 43 that grip one end and the other end of the silicon nitride substrate 10 in the longitudinal direction, respectively. The distance D between the portion of the silicon nitride substrate 10 gripped by the second gripping part 42 and the portion gripped by the third gripping part 43 is, for example, three times the width W1 of the silicon nitride substrate 10 in the short direction. The bent portion 110 is located in the center between the portion gripped by the second gripping part 42 and the portion gripped by the third gripping part 43.

[0026] The peel portion 111 extends perpendicular to the silicon nitride substrate 10, and its tip is gripped by the first gripping portion 41. When performing a peel strength test, a force is applied to pull the peel portion 111, causing the first gripping portion 41 and the silicon nitride substrate 10, which is gripped by the second gripping portion 42 and the third gripping portion 43, to move relative to each other in a direction perpendicular to the silicon nitride substrate 10. A load sensor (not shown) measures the force required to peel the first metal plate 11 from the silicon nitride substrate 10. If the first metal plate 11 does not peel from the silicon nitride substrate 10 even when the pulling force on the peel portion 111 is increased, the peel strength test is stopped when the peel portion 111 breaks. Figure 4(b) shows the state in which the first metal plate 11 has broken at the break point 112.

[0027] Next, Table 1 shows the results of the peel strength test when the Fe content of the silicon powder used in step S1 was varied between 100 ppm and 1900 ppm. The silicon nitride circuit board 1 subjected to this peel strength test had a first metal plate 11 made of copper (Cu), a thickness T1 of the first metal plate 11 of 0.5 mm, a width W2 of the first metal plate 11 of 2.5 mm, a thickness T2 of the silicon nitride substrate 10 of 0.32 mm, and a width W1 in the short direction of the silicon nitride substrate 10 of 13.0 mm. [Table 1]

[0028] As shown in Table 1, regardless of whether the Fe content was between 100 ppm and 1900 ppm, the first metal plate 11 fractured before it peeled off the silicon nitride substrate 10. Furthermore, when peel strength tests were performed on multiple samples of silicon powder with an Fe content of 10 ppm used in step S1, the first metal plate 11 peeled off the silicon nitride substrate 10 without fracture, and its peel strength was 70-80 kN / m.

[0029] In other words, in the silicon nitride circuit board 1 of this embodiment, where the Fe content of the silicon powder, which is the raw material for the silicon nitride substrate 10, is 100 ppm or more and 1900 ppm or less, the bonding strength between the first metal plate 11 and the silicon nitride substrate 10 is higher than the breaking strength of the first metal plate 11. Furthermore, in the silicon nitride circuit board 1 of this embodiment, when both ends in the longitudinal direction of the silicon nitride substrate 10 are grasped and the peeled portion 111, which is a part of the first metal plate 11 that has been peeled away from the silicon nitride substrate 10, is pulled in a direction perpendicular to the silicon nitride substrate 10, the first metal plate 11 breaks. Similar results are obtained when peel strength tests are performed on the second metal plate 12 and the third metal plate 13.

[0030] Thus, according to this embodiment, it is possible to improve the bonding strength between the silicon nitride substrate 10 and the first to fifth metal plates 11 to 15. In other words, in the silicon nitride circuit board 1 of this embodiment, the first to fifth metal plates 11 to 15 are less likely to peel off from the silicon nitride substrate 10 even at high temperatures, making it possible to mount power semiconductors that generate a lot of heat at high output.

[0031] (Summary of the embodiments) Next, the technical concept understood from the embodiments described above will be described using the reference numerals and other symbols from the embodiments. However, the reference numerals in the following description are not limited to the components in the claims that are specifically shown in the embodiments.

[0032] [1] A silicon nitride circuit board (1) comprising a silicon nitride substrate (10) and metal plates (11-13) joined to the silicon nitride substrate (10) via a brazing material (101), wherein the thickness (T1) of the metal plates (11-13) is 0.5 mm or more, and the bonding strength of the silicon nitride substrate (10) to the metal plates (11-13) is higher than the breaking strength of the metal plates (11-13).

[0033] [2] The silicon nitride circuit board (1) according to [1] above, wherein the metal plates (11-13) include straight portions extending linearly in a predetermined direction, and when both ends of the silicon nitride substrate (10) are gripped in the predetermined direction and a portion of the straight portions is peeled off from the silicon nitride substrate (10), the peeled portion (peeled portion 111) is pulled in a direction perpendicular to the silicon nitride substrate (10), the metal plates (11-13) break.

[0034] [3] The silicon nitride circuit board (1) according to [2] above, wherein the silicon nitride substrate (10) is rectangular in shape with the predetermined direction being the longitudinal direction, and the width (W2) of the metal plates (11-13) in the direction perpendicular to the predetermined direction is 20% or more of the width (W1) of the silicon nitride substrate (10) in the short direction.

[0035] [4] A method for manufacturing a silicon nitride circuit board (1) as described in any of [1] to [3] above, comprising: a molding step of molding a sheet molded body which is a precursor of the silicon nitride substrate (10); a heat treatment step of heat-treating the sheet molded body to obtain the silicon nitride substrate (10); and a metal plate forming step of forming the metal plates (11 to 14) patterned on the silicon nitride substrate (10), wherein the molding step comprises the steps of preparing a slurry by adding raw material powder containing silicon powder, a sintering aid and a binder to a dispersion medium; coating the slurry onto a substrate to form a sheet; and drying the slurry formed into a sheet, wherein the silicon powder contains 100 ppm to 1900 ppm of Fe,

[0036] Although embodiments of the present invention have been described above, these embodiments do not limit the invention as defined in the claims. Furthermore, it should be noted that not all combinations of features described in the embodiments are necessarily essential for solving the problem of the invention. [Explanation of symbols]

[0037] 1...Silicon circuit board 10...Silicon nitride board 101, 102... Brazing material; 11-15... Metal plates 1 to 5 111... Detachment

Claims

1. The device comprises a silicon nitride substrate and a metal plate joined to the silicon nitride substrate via a brazing material. The thickness of the aforementioned metal plate is 0.5 mm or more. The silicon nitride substrate has a bonding strength with the metal plate that is higher than the fracture strength of the metal plate. Silicon nitride circuit board.

2. The metal plate includes a straight portion extending linearly in a predetermined direction, When both ends of the silicon nitride substrate are grasped in the predetermined direction, and a portion of the straight section is peeled from the silicon nitride substrate, and the peeled portion is pulled in a direction perpendicular to the silicon nitride substrate, the metal plate breaks. The silicon nitride circuit board according to claim 1.

3. The silicon nitride substrate is rectangular in shape, with the predetermined direction being the longitudinal direction. The width of the metal plate in the direction perpendicular to the predetermined direction is 20% or more of the width of the silicon nitride substrate in the short-side direction. The silicon nitride circuit board according to claim 2.

4. A method for manufacturing a silicon nitride circuit board according to any one of claims 1 to 3, The process includes: a molding step for forming a sheet molded body which is a precursor of the silicon nitride substrate; a heat treatment step for heat-treating the sheet molded body to obtain the silicon nitride substrate; and a metal plate forming step for forming the patterned metal plate on the silicon nitride substrate. The molding process includes the steps of: preparing a slurry by adding raw material powder containing silicon powder, a sintering aid, and a binder to a dispersion medium; coating the slurry onto a substrate to form a sheet; and drying the sheet-shaped slurry. The silicon powder contains 100 ppm to 1900 ppm of Fe. A method for manufacturing silicon nitride circuit boards.

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