Semiconductor module and manufacturing method

Abstract

To realize semiconductor modules with high heat dissipation efficiency. [Solution] A semiconductor module comprising a semiconductor device, a sealing resin, and a graphite film, The sealing resin is located on the upper surface of the semiconductor device. A semiconductor module wherein the graphite film has a thickness of 0.4 μm or more and 40 μm or less, the thermal conductivity in the film plane direction is 1500 W / m·K or more, the thermal conductivity in the film thickness direction is 4 W / m·K or more, and the graphite film is in surface contact with the encapsulating resin or the surface of the encapsulating resin.

Description

[Technical Field]

[0001] The present invention relates to a semiconductor module having high heat dissipation efficiency and a method for manufacturing the same, and is particularly preferred as a semiconductor module equipped with semiconductor devices such as power semiconductors that generate a large amount of heat during operation. [Background technology]

[0002] In semiconductor module technology, there is a need for simple heat dissipation methods for semiconductor devices. In semiconductor modules with multiple semiconductor devices mounted, such as intelligent power modules (IPMs), there are concerns that semiconductor devices that generate a large amount of heat may adversely affect other semiconductor devices, and minimizing this impact is a major challenge. In particular, when semiconductor modules contain semiconductor devices that generate a large amount of heat, such as power semiconductors and central processing units (CPUs), how to dissipate that heat is a crucial issue. An example of a semiconductor module structure is shown in Figure 1. In Figure 1, 1, 1A, 1B, and 1C are various semiconductor devices, 2 is encapsulation resin (molding resin, filler resin), 3 is a wiring board such as an interposer, 4 is a module board, and 5 is solder. The encapsulation resin is in direct contact with the semiconductor devices, encapsulating and protecting them. Typical encapsulation resins include epoxy resin, urethane resin, silicone resin, and silicone gel.

[0003] In semiconductor modules, the electrodes of semiconductor devices are typically joined to aluminum or copper electrodes formed on a wiring substrate via solder. In such a structure, the heat generated from various semiconductor devices is mainly transferred to a thermally conductive and heat-resistant module substrate, such as ceramic, via the wiring substrate and dissipated into the atmosphere. Furthermore, in the case of semiconductor modules that generate even greater heat, a heat sink is often provided in contact with the module substrate for cooling. In this case, the generated heat is transferred from the module substrate to the heat sink via a thermal interface material (TIM). For convenience, in this invention, the side of the semiconductor module with the module substrate and the heat sink connected to it is referred to as the bottom surface, and the opposite side with the sealing resin is referred to as the top surface.

[0004] When heat dissipation from the bottom surface alone is insufficient, it is necessary to improve heat dissipation from the top surface of the semiconductor module. One method used to increase heat dissipation from the top surface of a semiconductor module is to install a heat sink on the top surface of the module. However, such a configuration is complex and expensive and is only used in cases where the heat generation is particularly large. Therefore, there is a strong demand for a simple method to improve the heat dissipation efficiency from the top surface of the module. Of course, in order to obtain a greater heat dissipation effect, in addition to improving the heat dissipation efficiency from the top surface using a simple method, a heat sink may also be installed on the top surface, but in any case, there is a strong demand for a simple method to improve the heat dissipation efficiency from the top surface of the module.

[0005] It is known that graphite thermal diffusion films are used as a heat management tool for small electronic devices such as mobile phones and personal computers. In this method, a graphite film oriented in the direction of the film surface, with an electrically insulating polymer film formed on its surface, is inserted into the electronic device. By promoting thermal diffusion in the direction of the film surface, heat is dissipated from a wide area of ​​the small electronic device, thereby increasing the heat dissipation efficiency. For this purpose, graphite films with a thickness of about 25 μm to 100 μm are used. This is because, considering the size and heat generation of small electronic devices, a film thickness of 25 μm or more is necessary to obtain a large thermal diffusion effect. Furthermore, in this application, it is important to increase the heat dissipation efficiency by thermal diffusion in the direction of the film surface, and the thermal conductivity (thermal conductivity) in the direction of film thickness does not need to be particularly considered, so graphite films are used with an electrically insulating polymer film formed on the surface. On the other hand, since semiconductor modules have a small area, in order to improve the heat dissipation of individual semiconductor modules by installing graphite films inside the semiconductor module, new technological development is required regarding the thickness, physical properties, arrangement, and installation method of the graphite film. However, such specific technological development has not been carried out.

[0006] On the other hand, for power semiconductors, which are semiconductor devices that generate a very large amount of heat, there are several proposals to use graphite for heat dissipation. Patent Document 1 describes a power semiconductor whose lead frame, which comprises an insulating substrate mounted via a metal layer, is made of a composite material including graphite. Patent Document 2 describes a power module comprising an insulating substrate on which power semiconductors are mounted, a cooler having a mounting surface on which the insulating substrate is placed, a side surface perpendicular to the mounting surface, and a facing surface opposite to the mounting surface, and a graphite cover that covers at least a portion of each of the mounting surface, the side surface, and the facing surface. Patent Document 3 describes a power module comprising a plurality of power semiconductor devices arranged in the X-axis direction, and a flat plate-shaped heat diffusion member including a pyrolysis graphite plate and a metal layer covering it, wherein the equivalent thermal conductivity of the heat diffusion member in the Y-axis direction is made higher than the equivalent thermal conductivity in the X-axis direction, and the heat of each power semiconductor device is diffused in the Y-axis direction to suppress thermal interference between semiconductor devices. Patent Document 4 describes a power semiconductor module in which a surface conductor made of plate-shaped graphite is provided on the main surface and back surface of an insulating plate, and the growth axes of the graphite used as the surface conductor and the graphite used as the back surface conductor are arranged to be perpendicular to each other. Patent Document 5 describes a method for dissipating heat in a power semiconductor module using an interlayer thermal bonding material made of a graphite film with an electrical conductivity of 14,000 S / cm or more in the film plane direction. However, both of these patents were inventions aimed at improving the heat dissipation efficiency from the bottom surface in a power semiconductor module.

[0007] In other words, the necessary thickness, physical properties, arrangement, and installation method of the graphite film required to improve heat dissipation efficiency from the top surface of a small semiconductor module by installing the graphite film inside the semiconductor module and utilizing the thermal diffusion function of the graphite film had never been specifically considered. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2018-163932 [Patent Document 2] Japanese Patent Publication No. 2018-166184 [Patent Document 3] Japanese Patent Publication No. 2021-72326 [Patent Document 4] Patent No. 6299407 [Patent Document 5] Patent No. 6552935 [Overview of the project] [Problems that the invention aims to solve]

[0009] The objective of this invention is to realize a simple heat dissipation and cooling method from the top surface of a semiconductor module, by providing a graphite film with the necessary thermal properties and thickness in the necessary location using the necessary installation method. The method of this invention can significantly improve the heat dissipation efficiency from the top surface of a semiconductor module, thereby improving the overall cooling capacity of the module. Furthermore, by using the graphite film of this invention as a heat dissipation path (heat diffusion layer) in a semiconductor module containing multiple semiconductor devices, the adverse effects of devices that generate a large amount of heat on other semiconductor devices can be reduced. This invention is particularly effective in semiconductor modules containing devices that generate a large amount of heat, such as power semiconductors and CPUs. [Means for solving the problem]

[0010] The inventors of this invention conducted research to improve the heat dissipation and cooling efficiency from the top surface of a semiconductor module using a simple method. The material used was a graphite film having high thermal conductivity in the direction of the film surface. To improve heat dissipation efficiency, a comprehensive examination of (1) the required thermal conductivity of the graphite film, (2) the required thickness of the graphite film, and (3) the optimal location and method of installation of the graphite film was necessary. As a result, the inventors found the conditions that the graphite film should possess, the optimal module structure, and a method for manufacturing such a structure, leading to the present invention. A typical semiconductor module structure of the present invention is shown in Figure 2. Figure 2(a) shows a structure in which a graphite film having the necessary values ​​regarding thermal conductivity, thickness, etc., is installed in a sealing resin. Figure 2(b) shows a structure in which the graphite film is installed so as to be in direct contact with the surface of the sealing resin. By using such a structure, the heat dissipation efficiency from the top surface of the semiconductor module can be significantly improved.

[0011] The present invention (1) A semiconductor module comprising a semiconductor device, a sealing resin, and a graphite film, wherein the sealing resin is on the upper surface of the semiconductor device, the thickness of the graphite film is 0.4 μm or more and 40 μm or less, the thermal conductivity in the film plane direction is 1500 W / m·K or more, the thermal conductivity in the film thickness direction is 4 W / m·K or more, and the graphite film is in surface contact with the sealing resin or the surface of the sealing resin. (2) The graphite film has an electrical conductivity of 16,000 S / cm or more in the direction of the film surface, and a density of 1.8 g / cm³. 3 The above is the semiconductor module described in (1). (3) A semiconductor module comprising the heat dissipation method described in (1) or (2), wherein through holes are formed in a part of the graphite film. (4) The semiconductor module described in (1) or (2) has a plurality of semiconductor devices, (5) The semiconductor module described in (1) or (2), wherein the semiconductor device is a power semiconductor and a heat sink is provided on one side. (6). The semiconductor module according to (1) or (2), wherein the semiconductor module has a substrate and a heat sink is provided on the substrate. (7). The semiconductor module according to (1) or (2), wherein the semiconductor module has a plurality of semiconductor devices, and power semiconductors and semiconductor devices other than power semiconductors are mixed. (8). A semiconductor module having a semiconductor device, a sealing resin, and a graphite film as a heat dissipation path, wherein the sealing resin is on the upper surface of the semiconductor device, and the graphite film is a graphite film having a thickness of 0.4 μm or more and 40 μm or less, a thermal conductivity in the film surface direction of 1500 W / m·K or more, and a thermal conductivity in the film thickness direction of 4 W / m·K or more, and is a graphite film having a thickness of 40 μm or more produced by laminating and pressing a plurality of sheets, and the graphite film is in surface contact with the inside of the sealing resin or the surface of the sealing resin. (9). The semiconductor module according to (8), wherein there are a plurality of the graphite films, and the plurality of graphite films do not exist on a semiconductor device with a small calorific value. (10). The semiconductor module according to (1) or (8), wherein the sealing resin is a silicone gel. (11). The semiconductor module according to (1) or (8), wherein the sealing resin is an epoxy resin. (12). A method for manufacturing the semiconductor module according to (1) or (8), wherein the graphite film is produced by graphitizing an aromatic heat-resistant polymer. (13). A method for manufacturing the semiconductor module according to (10), characterized in that after applying the silicone gel on the surface of the graphite film, it is adhered to the sealing resin of the semiconductor module. (14). A method for manufacturing the semiconductor module according to (11), characterized in that after applying an uncured epoxy resin on the surface of the graphite film, it is adhered to the sealing resin of the semiconductor module. relates to.

Effect of the Invention

[0012] This invention enables simple heat dissipation and cooling from the top surface of a semiconductor module, improving the overall cooling capacity of the semiconductor module. Furthermore, in semiconductor modules containing multiple semiconductor devices, it can mitigate the adverse effects of high-heat-generating devices on other semiconductor devices. These effects are particularly pronounced in semiconductor modules containing devices that generate significant heat, such as power semiconductors and CPUs. [Brief explanation of the drawing]

[0013] [Figure 1] Schematic diagram of a semiconductor module structure [Figure 2] Schematic diagram of the semiconductor module of the present invention [Figure 3] Structural examples of semiconductor modules using graphite films with a perforated structure. [Figure 4] Example of a semiconductor module structure in which heat dissipation paths of graphite films are selectively placed on multiple semiconductor devices. [Figure 5] Schematic diagram of the evaluation module [Modes for carrying out the invention]

[0014] (Explanation of the principle of the invention) First, we will explain why forming a graphite film with a specific thickness and physical properties in surface contact with the encapsulating resin or on the surface of the encapsulating resin improves the heat dissipation efficiency of a semiconductor module. Surface contact in this invention will be discussed later. In a typical semiconductor module, most of the heat generated by the semiconductor flows downwards and is dissipated to the module substrate or heat sink. In such a structure, the heat that reaches the upper surface of the semiconductor module exists as a heat spot on the encapsulating resin and contributes almost nothing to the heat dissipation of the module. Furthermore, the heat spot can also be a factor in the degradation of the encapsulating resin. In contrast, in the structure of this invention, the heat that reaches the upper surface of the module is diffused by the installed graphite film, and the area that can contribute to heat dissipation is increased, thereby improving the heat dissipation efficiency of the module. It also helps to mitigate heat spots. Therefore, the graphite film used for the purpose of heat diffusion needs to have a high heat diffusion effect (heat conduction) in the direction of the film surface.

[0015] On the other hand, since heat diffusion is limited to the surface area of ​​the module, improving heat dissipation efficiency requires a suitable range of graphite film thickness and suitable physical properties. This is because, while a thicker graphite film increases the heat diffusion effect in the film plane direction, it also has the effect of blocking heat in the film thickness direction (i.e., increasing thermal resistance). When a graphite film is added, thermal resistance (bulk thermal resistance) is generated in the film thickness direction of the graphite film, as well as a new interfacial thermal resistance at the interface between the upper and lower surfaces of the graphite film. This blocks the flow of heat in the thickness direction of the graphite film. The thermal conductivity of the film surface (basal plane) of an ideal crystalline graphite is 1900-2000 W / m·K, while the thermal conductivity in the film thickness direction (C-axis direction) is about 5-7 W / m·K. Therefore, to reduce the bulk thermal resistance in the film thickness direction of the graphite film, it is best to achieve a high thermal conductivity in the film thickness direction close to these theoretical values ​​and to make the graphite film thinner. On the other hand, the interfacial thermal resistance between the graphite film and the sealing resin is large because it is essentially a contact between two solids. This is because the actual contact area at the interface between solids is very small, resulting in what is essentially point contact, and there is an air layer with low thermal conductivity at the interface. Therefore, in order to reduce interfacial thermal resistance, it is important to make the contact between surfaces into surface contact. In this invention, surface contact refers to a state in which there is little air layer at the interface between surfaces, and 20% or more of the entire contact surface is in direct contact without gaps or an air layer in between. Since the graphite film has very high thermal conductivity in the direction of the film surface, if about 20% of the entire contact surface is in direct contact, heat can rapidly diffuse to the remaining 80% through the graphite film, so even a 20% direct contact area is effective. Of course, higher percentages such as 40%, 60%, or 80% are preferable, and 100% is ideal.

[0016] The present invention relates to a semiconductor module comprising a semiconductor device, a sealing resin, and a graphite film, wherein the sealing resin is located on the upper surface of the semiconductor device, the thickness of the graphite film is 0.4 μm or more and 40 μm or less, the thermal conductivity in the film surface direction is 1500 W / m·K or more, the thermal conductivity in the film thickness direction is 4 W / m·K or more, and the graphite film is in surface contact with the sealing resin or the surface of the sealing resin.

[0017] The present invention also relates to a semiconductor module having a semiconductor device, a sealing resin, and a graphite film as a heat dissipation path, wherein the sealing resin is on the upper surface of the semiconductor device, and the graphite film is a graphite film with a thickness of 40 μm or more, produced by laminating and pressing together multiple graphite films having a thickness of 0.4 μm or more and 40 μm or less, a thermal conductivity of 1500 W / m·K or more in the film surface direction, and a thermal conductivity of 4 W / m·K or more in the film thickness direction, and the graphite film is in surface contact with the sealing resin or the surface of the sealing resin.

[0018] (Conditions that graphite films must meet) The conditions that a graphite film should possess to improve the heat dissipation efficiency of a semiconductor module were determined through verification experiments. As a result, when the entire top surface of a semiconductor module is covered with a graphite film, the conditions are: thickness of 20 μm or less and 0.4 μm or more; thermal conductivity in the film plane direction of 1500 W / m·K or more; and thermal conductivity in the film thickness direction of 4 W / m·K or more. The thickness of the graphite film is preferably 15 μm or less and 0.7 μm or more, and more preferably 10 μm or less and 1 μm or more. A higher thermal conductivity in the film plane direction is desirable, and there is no upper limit from the viewpoint of heat dissipation efficiency, but it is difficult to exceed the thermal conductivity of graphite with an ideal crystal structure of 2000 W / m·K. Therefore, the practical upper limit is 2000 W / m·K.

[0019] Furthermore, when the graphite film is used partially, such as only on the upper portion of some semiconductor devices, a graphite film with a thickness of 40 μm or less and 0.4 μm or more, a thermal conductivity of 1500 W / m·K or more in the film plane direction, and a thermal conductivity of 4 W / m·K or more in the film thickness direction can be used. The reason why the film thickness of the graphite film is permissible up to 40 μm when used partially is that the thermal blocking effect in the film thickness direction is reduced because the film only covers a part of the semiconductor module. When used partially, the thickness of the graphite film is preferably 25 μm or less and 1 μm or more, and more preferably 15 μm or less and 2 μm or more. The upper limit of the thermal conductivity in the film plane direction is substantially 2000 W / m·K, as described above.

[0020] Measuring the thermal conductivity of thin graphite films, especially those with a thickness of 10 μm or less, is relatively time-consuming. Therefore, electrical conductivity in the plane direction is used as a simple method for estimating the thermal conductivity of a graphite film in the plane direction. It is known that thermal conductivity and electrical conductivity in the plane direction are almost proportional, and an electrical conductivity of 16,000 S / cm in the plane direction of a graphite film is approximately equivalent to a thermal conductivity of 1,500 W / m·K in the plane direction.

[0021] Furthermore, it is known that thermal conductivity and electrical conductivity in the film plane direction are almost proportional, and thermal conductivity in the film thickness direction can also be estimated from electrical conductivity in the film thickness direction. If the electrical conductivity in the film thickness direction is 5 S / cm, it can be considered that the thermal conductivity in the film thickness direction is approximately 4 W / m·K. Also, if the electrical conductivity in the film plane direction is approximately 16000 S / cm and the thermal conductivity is approximately 1500 W / m·K, then the electrical conductivity in the film thickness direction is approximately 5 S / cm and the thermal conductivity is approximately 4 W / m·K.

[0022] When air layers are formed between graphite layers, thermal and electrical conduction is hindered in both the film plane direction and the film thickness direction, resulting in reduced thermal and electrical conductivity. Therefore, the graphite film used in this invention needs to have few air layers between the graphite layers, which corresponds to a density of 1.8 g / cm³ for the graphite film of this invention. 3 This means that the above is necessary.

[0023] The area of ​​the graphite film is preferably larger than that of the semiconductor device, and is preferably 5 to 100 times the area of ​​the semiconductor device, as this allows for a large area where heat can be rapidly diffused, and is also preferable from the viewpoint of reducing the area of ​​the graphite film in order to suppress the cost of the graphite film, with 10 to 50 times being more preferable. If the semiconductor module has multiple semiconductor devices, it is preferable that the area of ​​the graphite film is 5 to 50 times the total area of ​​those multiple semiconductor devices.

[0024] The area of ​​the graphite film is preferably 10-90% of the area of ​​the semiconductor module, as this allows for a large area of ​​rapid heat dissipation and reduces the cost of the graphite film. A more preferable area is 20-80%. Note that the area of ​​the semiconductor device and the area of ​​the semiconductor module mentioned above both refer to the area as viewed from the top (upper surface) of the module.

[0025] The average distance between the graphite film and the semiconductor device is preferably 0.5 to 5 mm, from the viewpoint of bringing the graphite film as close as possible to the semiconductor device in order to effectively dissipate heat while reliably preventing the graphite film from directly contacting the semiconductor device in order to prevent damage to the graphite film, and more preferably 1.0 to 2.5 mm.

[0026] The average distance between the graphite film and the encapsulating resin surface is preferably 0 to 5 mm, and more preferably 0.1 to 2.5 mm. This is because, while it is desirable to prevent the graphite film from being completely exposed to the encapsulating resin surface to prevent damage to the graphite film, when the graphite film is simply installed (bonded) to the encapsulating resin surface for cost reduction and workability reasons, the graphite film inevitably ends up being almost entirely on the encapsulating resin surface. This is to strike an appropriate balance between these factors.

[0027] By placing a metal wire between the graphite film and the semiconductor device, or in other words, by connecting the semiconductor device and the graphite film with a metal wire, heat from the semiconductor device can be transferred to the graphite film through the metal wire, thereby improving heat dissipation efficiency. However, since placing a metal wire between the graphite film and the semiconductor device is a difficult task, it is preferable not to place a metal wire between the graphite film and the semiconductor device.

[0028] Another method involves bonding a graphite film to the semiconductor device with an adhesive, and then bonding the other end of the graphite film to the lead frame with an adhesive. This method dissipates heat from the semiconductor device by transferring it from the adhesive layer to the graphite film, then back to the adhesive layer, and finally to the lead frame. However, this is a time-consuming and difficult process, so it is preferable not to include such an adhesive layer.

[0029] (Fabrication of graphite film) The graphite film used in this invention can be produced by heat treatment of a heat-resistant aromatic polymer film, and aromatic polyimide films can be preferably used for the purposes of this invention. Among them, polyimide commonly called Kapton type and abbreviated as PMDA-ODA type (pyromellitic dianhydride-4,4'-oxydianiline type) can be effectively used. In this invention as well, PMDA-ODA type polyimide was used. The polyimide film was thermally decomposed in an inert gas such as nitrogen and further carbonized at 1000°C. The carbonized film was further graphitized in argon gas at a maximum temperature of 2300~3100°C to obtain graphite films with various physical properties such as thermal conductivity depending on the maximum processing temperature for graphitization. The obtained graphite film has high heat resistance up to 2000°C in a vacuum or in an inert gas, and also has heat resistance of 500~600°C in air. For example, the temperature of the encapsulating resin in power semiconductor modules can rise to 175-200°C, but graphite films have sufficient heat resistance to such conditions. Furthermore, since graphite is an extremely chemically stable substance, decomposition products resulting from chemical degradation will not adversely affect the device or module.

[0030] As the film thickness of the raw polyimide film increases, irregular structures are likely to be introduced into the graphite layers of the obtained graphite film. In such cases, air layers are often inserted between the layers. Since the influence of the air layer on the thermal conductivity is greater in the film thickness direction than in the film surface direction, when an air layer is formed in the graphite film, particularly the thermal conductivity and electrical conductivity in the film thickness direction become small, making it unsuitable as the graphite film of the present invention. Whether there is an air layer between the layers can be estimated by measuring the density of the graphite film, and the density is one of the criteria for determining whether the graphite film is suitable as the graphite thin film of the present invention. The density can be determined from the volume and weight obtained from the film area and thickness. The density of the graphite film of the present invention is 1.8 g / cm 3 or more, and it is preferable that it is 1.9 g / cm 3 or more, and most preferably it is 2.0 g / cm 3 or more. Further, as a method for simply estimating the density, the change in film thickness when the graphite film is pressurized may be measured. For example, when the change in film thickness is within 20% under a pressure of 5 Kgf / cm 2 , it can be estimated that the density of the graphite film is 1.8 g / cm 3 or more. The thermal conductivities in the thickness direction of the graphite films with film thicknesses of 75.0 μm and 50.0 μm used in Experiments 1 and 2 described later were less than 4 W / m·K, and the densities of both were less than 1.8 g / cm 3 . That is, with the production method by heat treatment of a heat-resistant aromatic polymer film, it is difficult to produce a graphite film having the physical properties required by the present invention and a thickness exceeding 40 μm.

[0031] In the case of PMDA-ODA type polyimide, the thickness of the obtained graphite film is approximately half of the thickness of the raw polyimide film. Therefore, the thickness of the graphite film can be controlled by the thickness of the raw polyimide film. To produce a graphite film with a thickness of 40 μm or less, generally a polyimide film with a thickness of 80 μm or less may be used. When the thickness of the graphite is 40 μm or less, the graphite film has a certain flexibility. Further, from the cross-sectional SEM observation of the graphite film, it is observed that the graphite film with a thickness of 40 μm or less hardly contains an air layer between the graphite layers, and the density is 1.8 g / cm 3The above is the case. In the graphite film of the present invention with a thickness of 40 μm or less, the thermal conductivity can be controlled by the heating temperature during graphitization. For example, in order to make the thermal conductivity in the direction of the film surface 1500 W / m·K or higher, the graphitization temperature should be 2700°C or higher.

[0032] (Creation of a surface-contact interface) As mentioned above, in order to reduce interfacial thermal resistance, it is important to ensure surface contact so that a larger area of ​​contact is made at the interface, thereby eliminating the presence of an air layer. In this invention, it is necessary to make the graphite film surface contact with the encapsulating resin of the semiconductor module or with the surface of the encapsulating resin, so the method for manufacturing it is described below.

[0033] Examples of encapsulating resins of the present invention include epoxy resins, urethane resins, silicone resins, and silicone gels, with epoxy resins and silicone gels being preferred.

[0034] When solid surfaces come into contact, the net contact area is generally considered to be about 1-0.01% of the surface area due to surface irregularities and undulations. In other words, in actual contact between solid surfaces, contact occurs only at limited points, such as between some of the convex parts of the surfaces. That is, it is point contact. Because a layer of air with low thermal conductivity (thermal conductivity at 20°C: 0.026 W / m·K) exists in most of the contact interface (other than the net contact points), the interfacial thermal resistance of the contact interface becomes large. When a graphite film is formed in a encapsulating resin, such interfacial thermal resistance exists on both the upper and lower surfaces of the graphite film, and it is extremely important to reduce the interfacial thermal resistance between the graphite film and the encapsulating resin. In other words, in the structure of the semiconductor module of the present invention (Figure 2), reducing this interfacial thermal resistance is essential to overcome the heat blocking effect and increase the heat dissipation efficiency by heat diffusion.

[0035] Achieving surface contact is extremely difficult when the contacting object is solid, but it can be formed relatively easily when the contacting object is a liquid encapsulating resin or when the encapsulating resin is flexible. Typical resins used for encapsulating semiconductor modules are epoxy resins and flexible silicone gels. For example, if the encapsulating resin is flexible like silicone gel, the graphite film can be coated and impregnated with silicone gel or silicone oil, and then brought into contact with the silicone gel. On the other hand, if the encapsulating resin is solid like epoxy resin, surface contact can be achieved, for example, by bonding the graphite film to an uncured, flexible epoxy resin and then curing it. However, since semiconductor modules are generally manufactured using the transfer molding method, it may not be easy to place the graphite layer in an uncured epoxy resin as a practical process. In such cases, a method can be used in which a graphite film coated with uncured epoxy resin is bonded to the encapsulating resin.

[0036] Furthermore, it is preferable to form a silicone gel coating or an uncured epoxy resin coating on the graphite film under reduced pressure. This is to reduce the possibility of an air layer existing between the graphite surface and the silicone gel or uncured epoxy resin. In addition, since the basal surface of the graphite film tends to repel resin, it is preferable to chemically treat, corona treat, or plasma etch the graphite surface to improve wettability and achieve strong adhesion. By taking such measures, the contact between the encapsulating resin and the graphite film can be made into surface contact, and the interfacial thermal resistance can be reduced. The effects of the present invention cannot be obtained if the graphite film is simply placed on the encapsulating resin of the semiconductor module so that the solids are in contact with each other without forming such surface contact.

[0037] (Application to semiconductor modules) Examples of semiconductor devices of the present invention include power semiconductors, CPUs, image processing units (GPUs), and memory. The present invention is particularly preferred for semiconductor devices that generate a lot of heat, such as power semiconductors, CPUs, and GPUs.

[0038] The interlayer bonding force of graphite crystals is relatively weak, and delamination is possible, especially when air layers are present between layers. In semiconductor modules with a graphite film in the encapsulating resin, there is a possibility of a decrease in the strength of the encapsulating resin due to delamination, so graphite films with many air layers are relatively undesirable. The graphite film of the present invention with a thickness of 40 μm or less is dense and relatively resistant to delamination, and can be preferably used. However, if there is still concern about a decrease in the mechanical strength of the encapsulating resin due to the presence of the graphite film, it is preferable to provide a structure in which holes are drilled through a part of the graphite film. The encapsulating resin on the upper and lower surfaces of the graphite film is integrated through the holes, preventing a decrease in mechanical strength due to delamination. An example of the structure of a semiconductor module using a graphite film with such a perforated structure is shown in Figure 3. The lower part of Figure 3 is a view of the graphite film with through holes from the upper side. As shown in Figure 3, the shape, location, and number of through holes can be freely selected. From the viewpoint of improving the heat diffusion effect of the graphite film, it is preferable that the area of ​​the holes be as small as possible, and that the total area of ​​the holes be 10% or less of the area of ​​the graphite film without holes. Furthermore, in order to achieve effective heat dissipation, it is desirable that there are no large holes or numerous holes directly above semiconductor devices that generate a lot of heat. In addition, from the viewpoint of preventing delamination, it is preferable to cover the edges of the graphite film and the cross-sections of through holes, which are prone to delamination, with sealing resin.

[0039] The heat dissipation method of the present invention is extremely effective for semiconductor modules containing multiple semiconductor devices with different heat generation capacities. In such cases, heat diffused from a semiconductor device with a large heat generation through the graphite film may adversely affect adjacent semiconductor devices. This effect can be avoided to some extent by selecting the optimal distance between the semiconductor device and the graphite film (the thickness of the sealing resin between the semiconductor device and the graphite film). It is preferable that the distance between a semiconductor device with a large heat generation and the graphite film be small, and that the distance between a semiconductor device with a small heat generation and the graphite film be large. By using a graphite film of the optimal thickness, the adverse effect of heat from a semiconductor device with a large heat generation on adjacent semiconductor devices can be reduced. It is also possible to reduce the thermal effect by providing through-holes in the graphite film on semiconductor devices with a small heat generation.

[0040] The heat dissipation method of the present invention is also effective as a method for selectively cooling multiple semiconductor devices, and is particularly effective for the effective heat dissipation of semiconductor modules that include devices that generate a large amount of heat, such as power semiconductors and CPUs. In such cases, for example, as shown in Figure 4, a graphite film is provided as a heat dissipation path on the upper surface of semiconductor devices that generate a large amount of heat (referred to here as 1 and 1B), while conversely, a graphite film is not provided on the upper surface of semiconductor devices that generate little heat or semiconductor devices that are sensitive to heat (referred to here as 1A and 1C). In this way, the heat from semiconductor devices that generate a large amount of heat can be effectively diffused while minimizing the adverse effects of heat on other (especially adjacent) semiconductor devices. The thickness, width, and spacing of the graphite film used in the installation of such selective heat dissipation paths should be determined based on the amount of heat generated, arrangement, area, and heat resistance of the semiconductor devices.

[0041] For the purpose of forming a partial heat dissipation path as shown in Figure 4, a relatively thick graphite film (thickness 20-40 μm) is preferred within the scope of the present invention. Furthermore, multiple graphite films can be stacked to create a laminated graphite film with a total thickness of more than 40 μm. Density 1.8 g / cm³ 3In order to achieve a graphite film with a thickness of 40 μm or more, for example, multiple graphite films within the scope of the present invention can be laminated and pressed together by pressure pressing to produce a laminated graphite film with a thickness of 40 μm or more, and this can be used to form a heat dissipation passage. When used in a very small area in a partial manner, there is no need to worry about the blocking effect in the film thickness direction, so from the viewpoint of heat dissipation performance, there is no particular limit to the total thickness of the laminated graphite films, and the thicker the film, the better the heat diffusion performance in the direction of the film surface, which is preferable. On the other hand, laminating many graphite films is time-consuming, and there are also limitations on the thickness of the semiconductor module, so the total thickness of the laminated graphite films is preferably 1 mm or less, and 0.5 mm, 0.2 mm, etc. are also acceptable.

[0042] In summary, the present invention is a method for dissipating heat from a semiconductor module, characterized by improving the heat dissipation efficiency to the upper surface of the semiconductor module and enhancing its cooling capacity by providing a heat dissipation passage (thermal diffusion layer) made of a graphite film having specific physical properties. By optimizing the shape and installation position of this heat dissipation passage, it is possible to selectively and efficiently cool multiple semiconductor devices and reduce the thermal adverse effects on other semiconductor devices. [Examples]

[0043] (Verification of the conditions that must be met) To verify the requirements that the graphite film used for the purposes of this invention must meet, an evaluation module simulating a semiconductor module was created. Figure 5 shows a schematic diagram of the evaluation module used. The size of the evaluation module is as follows: A thermocouple for temperature measurement was attached to a ceramic heater (size: 1 cmφ, manufactured by Jiangsu Fist Special Ceramic Co., Ltd., model number MCH012) simulating a semiconductor device, with a base area of ​​3 × 3 cm². 2It was set in the center of a copper case. The inside of the case was sealed with silicone gel (manufactured by Shin-Etsu Chemical Co., Ltd., model number KE-1066), so that the thickness of the silicone gel on the top surface of the heater was 2 mm. The reason for using silicone gel as the sealing resin is that, along with epoxy resin, silicone gel is widely used as a sealing resin in semiconductor modules such as power semiconductor modules, and because it is a flexible gel, it can reduce the effect of interfacial thermal resistance, making it suitable for estimating the effect of the graphite film itself.

[0044] (Experiments No. 1-17) A constant power was supplied to the ceramic heater of the evaluation module, and the temperature of the ceramic heater (temperature of the thermocouple) was adjusted to 120°C. Next, graphite films of various thicknesses (area 3 × 3 cm²) were used. 2 Silicone gel (several hundred μm thick) was applied to both sides of the evaluation module, and it was placed over the evaluation module so that it covered the entire top surface of the silicone gel, preventing air from entering. In the evaluation module covered with the graphite film, the same power as above was applied to the ceramic heater, and the temperature of the ceramic heater (temperature of the thermocouple) was measured. Table 1 shows the thickness of the graphite film used, the thermal conductivity in the planar direction, the ceramic heater temperature, and the difference (ΔT) from the ceramic heater temperature without the graphite film (120°C).

[0045] The thickness of the graphite film was measured using a contact-type thickness gauge, the CT2501, manufactured by Heidenhain Co., Ltd.

[0046] The thermal conductivity of the graphite film in the plane direction was calculated by multiplying the thermal diffusivity, density, and specific heat of the graphite film in the plane direction. The thermal diffusivity of the graphite film was measured at 23°C under vacuum and at 10Hz using a thermal diffusivity measuring device (LaserPit, manufactured by ULVAC, Inc.) using the optical AC method. The density of the graphite film was measured using a dry automatic densimeter, Accupic II 1340 (manufactured by Shimadzu Corporation). The specific heat was determined by differential scanning calorimetry (DSC) using a DSC Q1000, manufactured by TA Instruments Corporation.

[0047] For example, in Experiment No. 8, a significant decrease in ceramic heater temperature of 36°C was observed by using a graphite film with a thickness of 6.7 μm (thermal conductivity in the planar direction of 1700 W / m·K). On the other hand, with a graphite film thickness of 75.0 μm (thermal conductivity in the planar direction of 1300 W / m·K) (Experiment No. 1), the heater temperature rose to 141°C, an increase of 21°C. Comparative experiments of three types of graphite films (thickness 14.9 μm) (Experiments No. 5-7) show the effect of thermal conductivity in the planar direction on the heater temperature decrease, indicating that a thermal conductivity value of 1500 W / m·K or higher is necessary to achieve a temperature decrease of 5°C or more. Similar experiments were also conducted with a graphite film thickness of 0.4 μm (Experiments No. 13-15), and it was found that even with this thickness, a thermal conductivity value of 1500 W / m·K or higher is necessary to achieve a temperature decrease of 5°C or more. Furthermore, with graphite films less than 0.2 μm thick, even with a thermal conductivity of 1800 W / m·K in the planar direction, only a temperature reduction of 2°C could be achieved, and no effect was observed with films having a thermal conductivity of 1410 W / m·K in the planar direction. (Experiment No. 16, 17)

[0048] [Table 1] Based on the above results, a graphite film exhibiting a significant effect was defined as one in which a decrease in heater temperature of 5°C or more was observed, and the preferred range of graphite thickness and the required thermal conductivity value in the film plane direction were determined. Specifically, the appropriate thickness conditions for the present invention are 20 μm or less and 0.4 μm or more. A thickness of 15 μm or less and 0.6 μm or more is more preferable. The required thermal conductivity value in the graphite film plane direction is 1500 W / m·K or more, and a value of 1700 W / m·K or more is more preferable. Within this range of thickness and thermal conductivity, an effective improvement in heat dissipation efficiency can be achieved through the thermal diffusion effect of the graphite film.

[0049] (Experiments No. 18-23) The actual size of a semiconductor module is 1 cm² in area. 2 From about 100cm 2 There are even some that are as large as this. As shown in the experiment above, when the graphite film of the present invention is used to cover the entire top surface of the module, the magnitude of its heater temperature reduction effect is also influenced by factors such as the area of ​​the semiconductor module and the thickness of the silicone gel. The results in Table 1 show that the module area was 3 × 3 cm². 2 This is the result in this case, and since the thermal diffusion effect becomes greater in larger area modules, it is considered preferable to use thicker graphite films as well. Therefore, for a module area of ​​10 × 10 cm², 2 An additional experiment was conducted in this case. Base area: 3 x 3 cm 2 Instead of the copper case, a base area of ​​10 x 10 cm 2The experiment was conducted in the same manner as Experiment No. 1, except that a copper case was used. The size of the ceramic heater was the same as in the experiment in Table 1, with a diameter of 1 cm. The results are shown in Table 2. Experiments No. 18, 19, and 20 used the same graphite film as Experiments No. 1, 2, and 3, but the decrease in heater temperature was approximately 10°C greater. The experimental results (Experiments No. 21, 22, and 23) of three types of graphite films with different physical properties and a film thickness of 19.8 μm clearly show that these graphite films are also suitable for the purpose of the present invention. From these results and the previous results, it was determined that the upper limit of the thickness suitable for the graphite film of the present invention when used to cover the entire upper surface of the module is 25.0 μm or less. In reality, the optimal thickness and thermal properties of the graphite film are thought to be influenced by factors other than the module area, such as the thickness of the silicone gel. However, factors such as the optimal thickness of the sealing resin can be adjusted from the obtained results to produce the best effect.

[0050] [Table 2]

[0051] (Experiments No. 24-25) The graphite film of the present invention does not necessarily need to be used to cover the entire top surface of the semiconductor module. As mentioned above, through holes may be formed in the graphite film, or it may be formed only on specific semiconductor devices as a heat dissipation path. In such structures, the heat blocking effect of the graphite film is presumed to be mitigated, thus widening the usable thickness range of the graphite film. Using a graphite film with a thickness of 40 μm and a thermal conductivity of 1500 W / m·K in the film plane direction, the following two comparative experiments were conducted: (Experiment No. 24) was the case where the graphite film covered the entire surface, and (Experiment No. 25) was the case where the area of ​​the graphite film was 1 / 2 of the area on the semiconductor module, 1.5 × 3 cm². 2 The graphite film is 3 x 3 cm 2It is located in the center of the evaluation device. Experiment No. 24 was conducted in the same manner as Experiment No. 1, except that a graphite film with a thickness of 40 μm and a thermal conductivity of 1500 W / m·K in the film plane direction was used instead of a graphite film with a thickness of 75.0 μm and a thermal conductivity of 1300 W / m·K in the film plane direction. The measurement results showed that the heater temperature in Experiment No. 24 was 128°C, and the heater temperature in Experiment No. 25 was 115°C. From these results, it was found that when the area of ​​the graphite film is 1 / 2 the area on the semiconductor module, a graphite film with a thickness of 40 μm and a thermal conductivity of 1500 W / m·K can be preferably used. The area of ​​the heat dissipation path of the graphite film can be freely selected, such as making it very small, so from this perspective there is no upper limit to the film thickness. However, as mentioned above, for film thicknesses exceeding 40 μm, the thermal conductivity in the film plane direction is 1500 W / m·K or higher, the thermal conductivity in the film thickness direction is 4 W / m·K or higher, and the density is 1.8 g / cm³. 3 It is difficult to fabricate graphite films exceeding the above thickness. In other words, it is difficult to fabricate a graphite film suitable for exhibiting excellent heat dissipation effects at a thickness exceeding 40 μm. Therefore, when used alone, the upper limit of the thickness of the graphite film of the present invention is set to 40 μm. [Industrial applicability]

[0052] This invention can be used as a cooling method for semiconductor modules, and is particularly effective for cooling modules that include semiconductor devices that generate heat, such as power semiconductors. [Explanation of Symbols]

[0053] Figure 1.1, 1A, 1B, 1C: Semiconductor devices, 2: Encapsulation resin (molding resin, filler resin), 3: Wiring board, 4: Module board, 5: Solder. Figure 2(a). 1, 1A, 1B, 1C: Semiconductor devices, 2: Encapsulation resin (molding resin, filler resin), 3: Wiring board, 4: Module board, 5: Solder, 6: Graphite film. Figure 2(b). 1, 1A, 1B, 1C: Semiconductor devices, 2: Encapsulation resin (molding resin, filler resin), 3: Wiring board, 4: Module board, 5: Solder, 6: Graphite film. Figure 3.1, 1A, 1B, 1C: Semiconductor device, 2: Encapsulation resin (molding resin, filler resin), 3: Wiring board, 4: Module board, 5: Solder, 6: Graphite film. Figure 4.1, 1A, 1B, 1C: Semiconductor device, 2: Encapsulation resin (molding resin, filler resin), 3: Wiring board, 4: Module board, 5: Solder, 6: Graphite film. Figure 5.1D: Ceramic heater, 1E: Wiring for ceramic heater, 2A: Silicone gel, 2B: Silicone gel surface, 7: Thermocouple, 7A: Wiring for thermocouple, 8: Copper case, 8A: Hole in copper case for wiring.

Claims

1. A semiconductor module comprising a semiconductor device, a sealing resin, and a graphite film, The sealing resin is located on the upper surface of the semiconductor device. A semiconductor module wherein the graphite film has a thickness of 0.4 μm or more and 40 μm or less, the thermal conductivity in the film plane direction is 1500 W / m·K or more, the thermal conductivity in the film thickness direction is 4 W / m·K or more, and the graphite film is in surface contact with the encapsulating resin or the surface of the encapsulating resin.

2. The graphite film has an electrical conductivity of 16,000 S / cm or higher in the direction of the film surface, and a density of 1.8 g / cm³. ３ The semiconductor module according to claim 1.

3. A semiconductor module comprising the heat dissipation method according to claim 1 or 2, wherein through holes are formed in a part of the graphite film.

4. The semiconductor module according to claim 1 or 2, wherein the semiconductor module has a plurality of semiconductor devices.

5. The semiconductor module according to claim 1 or 2, wherein the semiconductor device is a power semiconductor and a heat sink is provided on one side.

6. The semiconductor module according to claim 1 or 2, wherein the semiconductor module has a substrate and a heat sink is provided on the substrate.

7. The semiconductor module according to claim 1 or 2, wherein the semiconductor module has a plurality of semiconductor devices, and the power semiconductor and semiconductor devices other than power semiconductors are mixed together.

8. A semiconductor module comprising a semiconductor device, a sealing resin, and a graphite film as a heat dissipation path, The sealing resin is located on the upper surface of the semiconductor device. The aforementioned graphite film is a graphite film with a thickness of 40 μm or more, which is produced by laminating and pressing together multiple graphite films having a thickness of 0.4 μm or more and 40 μm or less, a thermal conductivity of 1500 W / m·K or more in the film surface direction, and a thermal conductivity of 4 W / m·K or more in the film thickness direction. A semiconductor module in which the graphite film is in surface contact with the encapsulating resin or the surface of the encapsulating resin.

9. The semiconductor module according to claim 8, wherein there are multiple graphite films, and the multiple graphite films are not located on a semiconductor device with low heat generation.

10. The semiconductor module according to claim 1 or 8, wherein the sealing resin is a silicone gel.

11. The semiconductor module according to claim 1 or 8, wherein the sealing resin is an epoxy resin.

12. A method for manufacturing a semiconductor module according to claim 1 or 8, wherein the graphite film is produced by graphitizing an aromatic heat-resistant polymer.

13. A method for manufacturing a semiconductor module according to claim 10, characterized in that the silicone gel is applied to the surface of the graphite film and then bonded to the encapsulating resin of the semiconductor module.

14. A method for manufacturing a semiconductor module according to claim 11, characterized in that an uncured epoxy resin is applied to the surface of the graphite film, and then bonded to the encapsulating resin of the semiconductor module.

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