Manufacturing method and straightening method for an insulated circuit board with a heatsink, and manufacturing method for a power module.

Abstract

To provide a manufacturing method of an insulation circuit board having an excellent flatness of a component surface.SOLUTION: A manufacturing method of an insulation circuit board comprises a bonding step and a heating step. The bonding step includes the steps of: bonding a first circuit layer 121 formed by pure aluminum to one surface 11a of a ceramic substrate 11; bonding a second circuit layer 122 formed by an aluminum alloy to a surface 121a of the first circuit layer 121 on the side opposite to the ceramic substrate 11 side; bonding a metal layer 13 formed by pure aluminum to the other surface 11b of the ceramic substrate 11; and bonding a heat sink 20 formed by an aluminum alloy to a surface 13a of the metal layer 13 on the side opposite to the ceramic substrate 11 side to manufacture a bonding body 52. The heating step includes a step of heating the bonding body 52 in a non-weight to 200°C or more and 300°C or less.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a method for manufacturing an insulated circuit board with a heat sink by correcting the insulated circuit board when the flatness of the mounting surface on which a semiconductor element is mounted changes due to the external environment during the manufacturing process, a method for correcting the insulated circuit board with a heat sink when the flatness of the mounting surface changes due to the external environment after manufacturing the insulated circuit board with a heat sink, and a method for manufacturing a power module using the corrected insulated circuit board with a heat sink.

Background Art

[0002] The insulated circuit board shown in Patent Document 1 includes, for example, a ceramic substrate made of AlN (aluminum nitride), Si3N4 (silicon nitride), or the like, and a circuit layer formed by joining a metal plate having excellent conductivity and heat dissipation to one surface of the ceramic substrate, and a heat dissipation layer having excellent heat dissipation on the other surface. Such an insulated circuit board is manufactured by joining a ceramic substrate and a plurality of metal plates. Before mounting a semiconductor element, the transportation and storage of the insulated substrate may be performed by cooling the insulated substrate to a temperature below freezing point.

[0003] In the circuit layer constituting the insulated circuit board, the insulated circuit board may be deformed by being cooled to a temperature below freezing point, and the flatness of the mounting surface may deteriorate compared to before cooling. If such deformation of the insulated circuit board remains at the time of mounting the semiconductor element, there is a risk that the semiconductor element and the solder material may be displaced from the desired position and mounted, which is not preferable. Conventionally, as a method for correcting an insulated circuit board having warpage or the like, regardless of whether the insulated substrate is cooled, a load has been applied to the insulated circuit board.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] However, in straightening methods that apply load to the insulated circuit board, stress can concentrate on the board, potentially causing cracks and other damage, which could degrade product quality. Therefore, it is desirable to improve the flatness of the insulated circuit board without applying load before product shipment or before semiconductor element mounting.

[0006] Therefore, the present invention was created to solve these problems, and aims to provide a method for manufacturing an insulated circuit board with good flatness of the mounting surface, a method for straightening an insulated circuit board, and a method for manufacturing a power module. [Means for solving the problem]

[0007] The present invention provides a method for manufacturing an insulated circuit board with a heat sink, comprising: a bonding step of manufacturing a bonded body by bonding a first circuit layer made of pure aluminum to one side of a ceramic substrate, bonding a second circuit layer made of an aluminum alloy to the side of the first circuit layer opposite to the ceramic substrate, bonding a metal layer made of pure aluminum to the other side of the ceramic substrate, and bonding a heat sink made of an aluminum alloy to the side of the metal layer opposite to the ceramic substrate; and a heating step of heating the bonded body to 200°C or more and 300°C or less without load.

[0008] By heating the joint at 200°C to 300°C without load, the second circuit layer softens and absorbs the internal stress, i.e., the stress accumulated in the first circuit layer. As a result, the surface of the second circuit layer, which forms the outermost layer, is formed to a flatness suitable for mounting electronic components such as semiconductor chips. For example, even if the second metal layer warps significantly due to exposure to low temperatures during transportation or storage, the flatness can be restored to its value before cooling.

[0009] The manufacturing method of the power module of the present invention includes a mounting step of mounting electronic components on the surface of the second circuit layer after the heating step.

[0010] Furthermore, the present invention relates to a method for straightening an insulated circuit board with a heat sink, wherein a first circuit layer made of pure aluminum is bonded to one side of a ceramic substrate, a second circuit layer made of an aluminum alloy is bonded to the side of the first circuit layer opposite to the ceramic substrate side, a metal layer made of pure aluminum is bonded to the other side of the ceramic substrate, and a heat sink made of an aluminum alloy is bonded to the side of the metal layer opposite to the ceramic substrate side, the method comprising a heating step of heating the insulated circuit board with the heat sink to 200°C or more and 300°C or less without load. [Effects of the Invention]

[0011] According to the present invention, the joint can be heated to 200°C to 300°C without load, allowing the stress accumulated in the first circuit layer to be absorbed by the second circuit layer, and the flatness of the mounting surface formed by the second circuit layer can be made suitable for mounting. [Brief explanation of the drawing]

[0012] [Figure 1] This figure shows an insulating circuit board with a heat sink according to an embodiment of the present invention. [Figure 2] This diagram shows the manufacturing process of an insulated circuit board with a heatsink, as shown in Figure 1. [Figure 3] (a) to (c) are diagrams illustrating the manufacturing process shown in Figure 2. [Figure 4] This figure shows a power module using an insulated circuit board with a heatsink, as shown in Figure 1. [Figure 5] This graph shows the change in the flatness of the experimental sample. [Figure 6] This graph shows the relationship between the storage temperature of the experimental sample and the change in flatness. [Modes for carrying out the invention]

[0013] Embodiments of the present invention will be described below with reference to the drawings. [Outline configuration of an insulated circuit board with a heatsink] As shown in Figure 1, the insulated circuit board 1 with a heat sink comprises an insulated circuit board 10 and a fin-integrated heat sink 20 on which a plurality of fins are erected.

[0014] [Configuration of an insulated circuit board] The insulating circuit board 10 comprises a ceramic substrate 11, a circuit layer 12 laminated on one surface 11a of the ceramic substrate 11, and a metal layer 13 laminated on the other surface 11b of the ceramic substrate 11. The ceramic substrate 11 is an insulating material that prevents electrical connection between the circuit layer 12 and the metal layer 13, and is formed from, for example, aluminum nitride (AlN), silicon nitride (Si3N4), etc., with a plate thickness of 0.2 mm to 1.2 mm.

[0015] The circuit layer 12 comprises a first circuit layer 121 bonded to one surface 11a of the ceramic substrate 11, and a second circuit layer 122 bonded to the surface of the first circuit layer 121 opposite to the ceramic substrate 11. The first circuit layer 121 uses pure aluminum with a purity of 99% by mass or higher. According to JIS standards, pure aluminum in the 1000 series, especially 1N90 (purity of 99.9% by mass or higher: so-called 3N aluminum) or 1N99 (purity of 99.99% by mass or higher: so-called 4N aluminum), can be used. The metal layer 13 is made of pure aluminum or an aluminum alloy with a purity of 99% by mass or higher. According to JIS standards, aluminum in the 1000 series, especially 1N99 (purity of 99.99% by mass or higher: so-called 4N aluminum), can be used.

[0016] In this embodiment, the first circuit layer 121 and the metal layer 13 are aluminum plates made from rolled pure aluminum (so-called 4N aluminum) with a purity of 99.99% by mass or higher, and their thickness is set to 0.4 mm to 1.6 mm, with the thickness dimensions of the first circuit layer 121 and the metal layer 13 being the same. On the one hand, the second circuit layer 122 is formed of an aluminum plate made of an aluminum alloy such as A6063, and its thickness is set to 0.5 mm to 1.5 mm.

[0017] [Configuration of Heat Sink] The heat sink 20 joined to this insulating circuit board 10 is formed of a plate material made of an aluminum alloy, and is preferably composed of the same aluminum alloy such as A6063 as the second circuit layer 122. On one surface 20a of this heat sink 20, a metal layer 13 is joined, and on the other surface 20b, a plurality of pin-shaped fins 25 are erected. The tip positions of the pin-shaped fins 25 are aligned on the horizontal plane and are formed to have substantially the same erected height from the surface of the surface 20b. Note that the shape of the fins erected on the heat sink 20 is not particularly limited, and in addition to the pin-shaped fins 25 as in this embodiment, diamond-shaped fins, strip-shaped fins, etc. can also be formed. The metal layer 13 of the insulating circuit board 10 is laminated on the surface 20a on the opposite side of the pin-shaped fins 25 of this heat sink 20 via an Al-Si-Mg-based brazing material, and these are pressurized and heated in the lamination direction to join the heat sink 20 to the insulating circuit board 10.

[0018] Next, a method for manufacturing the insulating circuit board 1 with a heat sink according to this embodiment will be described. As shown in FIG. 2, the method for manufacturing the insulating circuit board 1 with a heat sink includes a first joining step of joining the first circuit layer 121 and the metal layer 13 to the ceramic substrate 11 to manufacture a first joined body 51, a second joining step of joining the second circuit layer 122 and the heat sink 20 to the first joined body 51 to manufacture a second joined body 52, and a heating step of heating the second joined body. Hereinafter, these steps will be described in this order.

[0019] (First Joining Step) As shown in Figure 3(a), a first laminate is constructed by laminating a first circuit layer 121, a ceramic substrate 11, and a metal layer 13 via Al-Si, Al-Ge, Al-Cu, Al-Mg, Al-Mn, or Al-Si-Mg brazing foils 41 and 42, respectively. Furthermore, the first laminate is heated under pressure in the lamination direction and then cooled to produce a first bonded body 51. In the first bonded body 51, the first circuit layer 121 is bonded to one surface 11a of the ceramic substrate 11, and the metal layer 13 is bonded to the other surface 11b. The brazing foils 41 and 42 melt upon heating and further diffuse into the circuit layer 12 and metal layer 13, firmly bonding them to the ceramic substrate 11. The bonding conditions at this time are not necessarily limited, but it is preferable to bond in a vacuum atmosphere with a load in the stacking direction of 0.1 MPa to 3.4 MPa, and hold at a heating temperature of 610°C to 655°C for 1 minute to 60 minutes.

[0020] (Second joining process) As shown in Figure 3(b), an Al-Si-Mg brazing foil 43 is interposed between the first circuit layer 121 and the second circuit layer 122 of the first joint 51, and an Al-Si-Mg brazing foil 44 is interposed between the metal layer 13 of the first joint 51 and the heat sink 20 to form a second laminate. Furthermore, by heating this second laminate under pressure in the lamination direction, the second circuit layer 122 is bonded to the surface 121a of the first circuit layer 121 of the first joint 51 that is opposite to the ceramic substrate 11 side, and the heat sink 20 is bonded to the surface 13a of the metal layer 13 of the first joint 51 that is opposite to the ceramic substrate 11 side, thereby producing the second joint 52 shown in Figure 3(c). The load in the lamination direction is preferably 0.1 MPa to 3.4 MPa, and it is preferable to maintain the temperature at 580°C to 615°C for 3 minutes to 60 minutes. In the second bonded body 52, the ceramic substrate 11, the circuit layer 12, and the metal layer 13 are bonded together to form an insulating circuit board 10, and the heat sink 20 is further bonded to the insulating circuit board 10 and positioned opposite to the circuit layer 12.

[0021] In this way, the manufactured second bond 52 can be used as an insulated circuit board 1 with a heat sink.

[0022] (Transportation / storage process) The second assembly 52 is transported and stored as needed, and during this time, it may be exposed to low temperatures due to the influence of ambient temperature, etc. While these low temperatures are not limited to any specific temperature, if 25°C is the temperature of the environment in which the second assembly 52 is placed, which serves as the reference temperature for measuring changes in flatness (hereinafter referred to as the reference temperature), then any temperature lower than this reference temperature is considered a low temperature, for example, below freezing point between -30°C and 0°C. Thus, the second joint 52, when exposed to low temperatures, shows deformation of the first circuit layer 121 of the circuit layer 12 compared to before exposure to low temperatures. Furthermore, the surface 122a of the second circuit layer 122, opposite to the first circuit layer 121, also exhibits warping and other deformations, resulting in a deterioration of flatness. Here, flatness is defined as "the magnitude of deviation of a planar feature from a geometrically correct plane" as specified in JIS B0621. In this invention, exposure to low temperatures is referred to as the transportation and storage process.

[0023] (Heating process) In this invention, a heating process is performed on the second joint 52 that has been exposed to low temperatures. In the heating process, the second joint 52 is placed in a heating furnace and heated, for example, at 200°C to 300°C for 10 to 30 minutes without applying any load to the second joint 52. This heating process softens the second circuit layer 122, which is located outside the first circuit layer 121, thereby absorbing the stress generated in the first circuit layer 121 inside the second joint 52 with the second circuit layer 122 on the outside. After the warping of the first circuit layer 121 and the second circuit layer 122 is reduced, the second joint 52 is allowed to dissipate heat at a reference temperature (e.g., 25°C) and solidify again, so that the flatness of the surface 122a of the second circuit layer 122 returns to or is approximately the same as the flatness before the transport and storage process (i.e., before exposure to low temperatures). In this way, by performing the heating process, the surface 122a of the second bonded body 52 is formed to a flatness suitable for mounting electronic components 30 such as semiconductor chips, thereby completing the insulated circuit board 1 with a heat sink.

[0024] [Power Module Configuration] Then, as shown in Figure 4, electronic components 30 such as semiconductor chips are mounted on the surface 122a of the heat sink-equipped insulating circuit board 1, thereby enabling the manufacture of the power module 100. The electronic components 30 constituting the power module 100 are joined to the Ni plating (not shown) formed on the surface of the circuit layer 12 using solder materials such as Sn-Ag-Cu, Zn-Al, Sn-Ag, Sn-Cu, Sn-Sb, or Pb-Sn. Reference numeral 31 in Figure 4 indicates the solder joint layer. Furthermore, the electronic components 30 and the terminals of the circuit layer 12 are connected, for example, by bonding wires (not shown) made of aluminum, or by soldering the electronic components 30.

[0025] According to this embodiment, even if the flatness of the surface 122a (mounting surface) of the second circuit layer 122 deteriorates due to transportation and storage processes during manufacturing (i.e., exposure to low temperatures), the flatness of the second circuit layer 122 can be improved by performing a heating process, and an insulated circuit board 1 with a heat sink can be manufactured. Then, using the insulated circuit board 1 with a heat sink manufactured in this way, electronic components 30 such as semiconductor chips can be mounted at predetermined positions on the surface 122a of the second circuit layer 122, which is formed with warping and other defects suppressed, to manufacture a power module 100.

[0026] The present invention can be implemented in ways not limited to the above description and illustrated examples. Dimensions and ratios are also not limited to those shown in the illustrations. The improvement in flatness can also be achieved when the environment around the second joint 52 is cooled to a temperature lower than the reference temperature using mechanical equipment such as a freezer.

[0027] The flatness of an insulated circuit board with a heat sink can be improved not only during the manufacturing process, but also after the manufacturing process, for example, before the purchaser of the insulated circuit board with a heat sink mounts electronic components onto it. In this case, the heating process can be performed on the insulated circuit board with a heat sink as a method for correcting the insulated circuit board with a heat sink.

[0028] In the above embodiment, the method for manufacturing an insulated circuit board with a heat sink involved two separate bonding steps: first, manufacturing the first bonded body 51, and then bonding the second circuit layer 122 and the heat sink 20 to the first bonded body 51 to manufacture the second bonded body 52 (corresponding to the bonded body of the present invention). However, a laminate can also be constructed by interposing brazing foil between the ceramic substrate 11, the first circuit layer 121, the second circuit layer 122, the metal layer 13, and the heat sink, and then heating and cooling this laminate under pressure in the lamination direction to manufacture the bonded body in a single bonding step. Even if there is warping or other distortion on the surface of the second circuit layer of this bonded body, the heating step can be used to form the second circuit layer with a flatness suitable for mounting electronic components. [Examples]

[0029] A. Experiment (1-1) Experimental Sample An insulating circuit board with a heat sink was prepared as an experimental sample. This circuit board consisted of a 0.32 mm thick ceramic substrate 11 made of silicon nitride (Si3N4), with a circuit layer 12 on one side and a metal layer 13 on the other side, and a heat sink 20 bonded to the side of the metal layer 13 opposite to the ceramic substrate 11. The change in the flatness (JIS B0621) of the experimental sample was observed by changing the temperature. In the circuit layer 12, the first circuit layer 121 is made of 4N aluminum with a thickness of 0.9 mm, and the second circuit layer 122 is made of 6063 series aluminum alloy with a thickness of 0.8 mm and is formed to have the same outer shape as the first circuit layer 121, with its peripheral edge joined to match the peripheral edge of the first circuit layer 121. The metal layer 13 is made of 4N aluminum with a thickness of 0.9 mm, and the heat sink 20 is made of 6063 series aluminum alloy and is larger than the metal layer 13.

[0030] (1-2) Observation Method As the first step, the flatness f1 (JIS B0621) of the experimental sample was measured at a reference temperature (25°C). The target surface was surface 122a of the second circuit layer 122 of the experimental sample, located opposite the first circuit layer 121. A height and flatness measuring instrument (HM-1000: laser type) manufactured by Keyence Corporation was used for this measurement. The reference temperature was the temperature of the laboratory where the experiment was conducted. In the second step, the experimental sample was subjected to temperature changes through cooling and heating. First, the experimental sample was exposed to a low temperature of -10°C for 2 hours, and then allowed to cool at a reference temperature. Next, the experimental samples were heated and maintained at a constant temperature (heating process). Multiple experimental samples were prepared, and each sample was heated for 15 minutes at a different temperature (hereinafter referred to as the test temperature). The test temperatures were 150°C for the first sample, 200°C for the second sample, 250°C for the third sample, 300°C for the fourth sample, and 350°C for the fifth sample. After 15 minutes of heating, the samples were allowed to cool in a laboratory at a reference temperature. As a third step, the flatness f2 of each experimental sample that underwent the second step was measured in the laboratory at the reference temperature, and the change Δ1 mm [= (flatness f2 after cooling and heating) - (flatness f1 before cooling and heating)] from the flatness f1 measured in the first step was calculated. The change Δ1 mm of flatness for each experimental sample is shown in Figure 5. Furthermore, separate from the observations at the aforementioned test temperatures of 150°C to 350°C, another experiment was conducted at naturally occurring test temperatures of -10°C to 60°C. First, six experimental samples were prepared and the flatness f4 was measured at each of their reference temperatures. Next, each experimental sample was exposed to the test temperature for 2 hours. Each experimental sample had a different test temperature set, which was one of -10°C, 0°C, 10°C, 20°C, 40°C, or 60°C. After maintaining the exposure to the test temperature for 2 hours, each experimental sample was allowed to cool or dissipate heat in a laboratory at the reference temperature (25°C), and the flatness f5 was measured at the reference temperature (25°C). The change in flatness from f4, Δ2mm [= flatness f5 - flatness f4], was calculated. The change in flatness Δ2mm for each experimental sample is shown in Figure 6.

[0031] (1-3) Experimental results The experimental samples (first to fifth samples) that underwent the heating process were checked to see if they fell within the specified range of flatness. In this experiment, flatness was evaluated as "good" if the flatness of the second circuit layer 122 was less than 200 μm, and as "poor" if the flatness of the second circuit layer 122 was 200 μm or more. The second, third, and fourth samples were kept in a heated state between 200°C and 300°C after cooling, resulting in a change in flatness Δ1 of less than 0.05 mm. This confirmed that the flatness of the second circuit layer 122 remained below 200 μm even after heating, and that the heated experimental samples exhibited the same flatness as those before the cooling treatment. On the other hand, in the fifth sample, which was kept in a heated state at 350°C after cooling, the change in flatness Δ1 was greater than 0.6 mm. In other words, the surface 122a heated at a temperature exceeding 300°C deviated significantly from its position before cooling to -10°C. As a result, the flatness of the second circuit layer 122 after heating did not fall within the predetermined range (less than 200 μm), and the flatness could not be sufficiently improved. In the first sample, which was kept at a heated state of 150°C after cooling, the change in flatness Δ1 was 60 μm, but the flatness of the second circuit layer 122 after heating was 200 μm or more, indicating that the flatness could not be sufficiently improved. Furthermore, as shown in Figure 6, in samples that have not undergone a heating process, the change in Δ2 increases as the storage temperature decreases, indicating a deterioration in flatness.

[0032] B. Simulation (2-1) Simulation details Using an insulated circuit board with a heatsink, which has a configuration similar to experimental sample A, as an experimental model, the change in flatness with temperature was simulated. Simulation tool: "ABAQUS" by Intermesh Japan Co., Ltd. The simulations were as follows: the first simulation involved heating from 25°C to 250°C without any cooling; the second simulation involved cooling from 25°C to 0°C before heating to 250°C; and the third simulation involved cooling from 25°C to -20°C before heating to 250°C. In each simulation, after heating to 250°C, the temperature was allowed to dissipate back to the starting temperature of 25°C. Table 1 shows the flatness at each step of each simulation and the change Δ3 (flatness at step 9 - flatness at step 1).

[0033] [Table 1]

[0034] (2-2) Simulation results In the experimental model created by simulation, it was confirmed that the change in flatness Δ3 (flatness at step 9 - flatness at step 1) between the flatness before and after the temperature change was equivalent to the experimental result shown in Figure 5 (change Δ1 between 200°C and 300°C). Note that the negative value of the change Δ3 is due to calculation errors in the simulation. Since the temperature at which the flatness returns to the amount of warping before the temperature change is determined by the material constituting the insulating circuit board with a heat sink, such as a ceramic substrate, the simulation can also be used as a tool to design a board that can be corrected before mounting electronic components. [Explanation of symbols]

[0035] 1. Insulated circuit board with heatsink 10 Insulated circuit board 11. Ceramic substrate 11a,11b,20a,20b,121a,122a,13a side 12 circuit layers 121 First circuit layer 122 Second circuit layer 13 Metal layer 20 Heatsinks 25 pin-shaped fins 30 Electronic Components 51 First zygote 52 Second zygote 100 Power Modules

Claims

1. A method for manufacturing an insulated circuit board with a heat sink on which electronic components are mounted, A bonding process for manufacturing a bonded body, comprising: bonding a first circuit layer made of pure aluminum to one side of a ceramic substrate; bonding a second circuit layer made of an aluminum alloy to the side of the first circuit layer opposite to the ceramic substrate; bonding a metal layer made of pure aluminum to the other side of the ceramic substrate; and bonding a heat sink made of an aluminum alloy to the side of the metal layer opposite to the ceramic substrate; A heating step is performed to heat the joint to 200°C or more and 300°C or less without load before the aforementioned electronic components are mounted. A method for manufacturing an insulated circuit board with a heat sink, characterized by having the following features.

2. A method for manufacturing a power module, characterized by comprising a mounting step of mounting an electronic component on the side of the second circuit layer of an insulating circuit board with a heat sink, manufactured by the manufacturing method described in claim 1, that is opposite to the side of the first circuit layer.

3. The method for manufacturing a power module according to claim 2, wherein a transport and storage step is provided between the bonding step and the heating step, and in the transport and storage step, the bonded body is exposed to a sub-zero temperature.

4. A method for straightening an insulated circuit board with a heat sink, wherein a first circuit layer made of pure aluminum is bonded to one side of a ceramic substrate, a second circuit layer made of an aluminum alloy is bonded to the side of the first circuit layer opposite to the ceramic substrate side, a metal layer made of pure aluminum is bonded to the other side of the ceramic substrate, a heat sink made of an aluminum alloy is bonded to the side of the metal layer opposite to the ceramic substrate side, and an electronic component is mounted on the side of the second circuit layer opposite to the first circuit layer side. Before the aforementioned electronic components are installed, A method for straightening an insulated circuit board with a heat sink, characterized by comprising a heating step of heating the insulated circuit board with a heat sink to 200°C or more and 300°C or less without load.

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