JP6847686B2 - Interlayer heat bonding method, cooling system - Google Patents

Description

The present invention relates to an interlayer heat bonding method for rapidly transferring the heat of a heat generating source to a cooling / heat dissipation unit, and particularly to an interlayer heat bonding method and a cooling system between two members having different linear thermal expansion coefficients.

In recent years, due to the increase in heat generation due to the speeding up of microprocessors and the high performance of LED (Light Emitting Diode) chips, electronic devices such as mobile phones, personal computers, PDAs (Personal Digital Assistants), game machines, and LED lighting have become available. Dealing with high heat has become a major issue to be solved. There are methods that use heat conduction, heat radiation, and heat convection for heat dissipation and cooling. Heat sinks and air-cooled fans are used as cooling methods that use heat convection, and ceramic plates are used as heat radiation. There are various types of heat conductive (diffusion) sheets, heat conductive resins, etc. that utilize conduction. In order to effectively dissipate and cool the heat of the heat generation source, it is necessary to combine these heat dissipation and cooling methods and efficiently transfer the heat of the heat generation part to the heat dissipation and cooling part such as the circuit board, cooling fins, and heat sink. For that purpose, it is important to reduce the thermal resistance between the heat generating part and the heat radiating / cooling part.

Even if the heat generating member and the heat radiating / cooling member are simply joined to each other, the bonding between the layers becomes point contact due to the unevenness of the surface of the member, and as a result, an air layer with low thermal conductivity (thermal conductivity: 0.02 W) / MK) is present, which causes a large thermal resistance. An interlayer thermal bonding member (Thermal Interface Material, hereinafter abbreviated as TIM) is used to reduce the thermal resistance between such interlayers, and is used by sandwiching it between metals or between a member such as a metal and a ceramic. In this case, it is important that the thermal conductivity of the TIM itself is high and that the thermal resistance at the interface between the member and the TIM is small. In the present invention, the two members that are thermally joined are simply referred to as "members".

In order to reduce the thermal resistance at the interface, it is necessary to increase the contact area of the interface, and for this purpose, a mixture of a flexible polymer material and a highly thermally conductive inorganic filler has been used. (Polymer / inorganic composite type TIM; hereinafter abbreviated as composite type TIM). The interface is in surface contact with the composite TIM, and the air layer is removed from the layers, so that the thermal resistance between the layers can be reduced. However, such a composite type TIM has a problem that if the amount of the inorganic filler added is increased in order to realize high thermal conductivity, the flexibility is impaired and the thermal resistance at the interface is increased. Therefore, the current situation is that the thermal conductivity of the composite TIM itself is only about 1 to 2 W / mK for the normal product and about 5 W / mK for the high thermal conductive product.

The thermal resistance value, which is a practical characteristic of the TIM, is the sum of the thermal resistance of the TIM itself and the thermal resistance at the interface, and the thermal resistance value of the composite TIM is ^{about 0.4 to 4.0 K · cm 2} / W. Is. However, this thermal resistance value changes depending on the magnitude of the pressure applied to the joint surface, and it is necessary to indicate the magnitude of the pressure in order to display the thermal resistance value. Generally, a force of 0.1 MPa to 0.5 MPa is applied to TIM, but the magnitude of the thermal resistance value changes about 2 to 4 times depending on the magnitude of such pressure. Of course, the smaller this pressure dependence is, the more preferable it is. This is because it is not preferable that the pressure on the thermal junction changes due to external factors such as temperature change and vibration, and the thermal resistance value changes accordingly.

Further, the value of the thermal resistance value is also affected by the unevenness of the surface of the member that is actually held narrow. Therefore, the value of thermal resistance (0.4 to 4.0 ° C. cm ² / W) in a general composite TIM is a characteristic when the member is held narrowly with a member considered to be almost a mirror surface, and has irregularities. When used between practical members, the value is larger than that value. In the actual composite type TIM, the thickness of the TIM is increased in consideration of the unevenness of the member to be narrowed, and the thickness of the general composite type TIM is about 0.5 to 5 mm. This is because the TIM needs to enter the uneven portion of the member by pressurization and remove the air layer. Therefore, when a composite TIM is used between practical members, its thermal resistance value is often several times (about 1.5 to 5 times) the value when it is sandwiched between mirror surface members (catalog value). ..

In the composite TIM, polymers such as acrylic resin, epoxy resin, and silicone resin are used as the flexible polymer material, but as long as these matrix polymers are used, the environment at a severe high temperature exceeding 150 ° C. I can't stand it. For example, it is known that when a composite type TIM is subjected to a continuous heat resistance test in an environment of 150 ° C., its thermal resistance value increases significantly in many cases. In other words, it is a new product that can withstand the increase in heat generation due to the recent increase in the speed of microprocessors and the performance of LED chips, or the use in harsh thermal environments of 150 ° C or higher, such as around automobile engines, and has low thermal resistance. TIM is strongly requested.

Further, in the case of heat-bonding between members having a large temperature change, countermeasures against expansion and contraction due to heat of the members become a big issue. Generally, the length and volume of a substance increase as the temperature rises, and the rate of increase is expressed as the rate of increase in 1 ° C. with the coefficient of thermal expansion, which is in units of ^10-6 / K. For example, copper: 16.6, nickel: 13.4, iron 11.8 to 12.1, stainless steel (SUS410): 10.4, quartz: 10.3, titanium: 8.5, zirconia: 8 8.8, Alumina: 5.4, Cemented Carbide: 5.0, Chromium: 4.9, Titanium 4.5, CBN: 0.2 to 2.9, Invar: 0.2 to 2.0, etc. is there. From the point of view of TIM, the coefficient of thermal expansion of such a member does not matter when the temperature of the member is relatively low. However, in a harsh thermal environment such as 150 ° C or higher, or in an environment where there is a large temperature difference or temperature change, the contact state changes due to expansion and contraction due to the difference in the coefficient of thermal expansion between two different members, resulting in heat. The problem of changing resistance characteristics emerges. Generally, it is considered that the presence of a flexible substance of the composite TIM is indispensable in order to cope with such a difference in the coefficient of thermal expansion between the members, but as already mentioned, the flexibility is increased. There is a contradictory problem that the polymer to be applied is vulnerable to heat.

As described above, when the temperature of the member is 150 ° C. or higher and the temperature cycle is repeatedly applied, the difference in the coefficient of thermal expansion between the members becomes a big problem. The effect is particularly large when the difference in the coefficient of thermal expansion between two different members is 3 × 10 ^{-6 K or more.} That is, there is currently a TIM capable of stably heat-bonding members such that the temperature of at least one member is 150 ° C. or higher and the difference in thermal expansion coefficient between the members is 3 × 10 ^{-6 / K or higher.} Instead, such a TIM is strongly requested.

Graphite has excellent heat resistance, chemical resistance, high thermal conductivity, and high electrical conductivity, so it can be used as a structural material, reinforcing material, sliding material, conductive material, heat dissipation sheet, etc. in a wide range of fields such as energy, space, and medicine. It is used and occupies an important position as an industrial material. Among such graphite, a graphite film obtained by a method of directly heat-treating a special polymer and carbonizing / graphitizing it (polymer firing method) is used as a TIM, and a thin graphite film TIM (thickness: 10 nm) is used. It is disclosed that excellent thermal characteristics can be obtained by setting the temperature to ~ 15 μm, the thermal conductivity in the film surface direction to 500 W / mk or more, and the anisotropy of the thermal conductivity in the film surface direction and the thickness direction to 100 or more). ing. (Patent Document 1)

Japanese Unexamined Patent Publication No. 2014-133669

The present invention further improves the invention described in Patent Document 1, and further provides an interlayer thermal bonding method capable of realizing excellent thermal bonding even between members having different coefficients of thermal expansion between the members, and layers thereof. It is an object of the present invention to provide a cooling system using a thermal bonding method.

In order to improve heat resistance, the present inventors have considered a TIM having a structure that does not use a flexible polymer material that has been used in conventional composite TIMs, and have created a graphite film that has extremely excellent heat resistance and is well oriented in the film surface direction. We considered using it as a TIM. Graphite has lubricity derived from its structure, and such lubricity is caused by slippage between graphite layers. That is, in order to exhibit lubricity, it is considered effective to use high-quality, highly oriented graphite with a well-developed graphite layer structure, and it is plane-oriented (the ab plane is oriented in the film surface direction). It was considered that by sandwiching a high-quality graphite film (with such high orientation) between the members, it is possible to absorb the strain caused by the temperature difference between the members and the difference in the linear thermal expansion coefficient. Such lubrication characteristics of graphite can be evaluated by the coefficient of linear thermal expansion of graphite. The linear thermal expansion coefficient in the ab plane direction of the high quality graphite crystal is 1 × 10 ^-6 / K or less, while the linear thermal expansion coefficient in the c-axis direction is 32 × 10 ^-6 / K or less.

It is naturally considered that the thicker the graphite film, the greater the effect of absorbing the strain caused by the difference in the temperature of the members and the difference in the coefficient of linear thermal expansion between the members. However, on the other hand, increasing the thickness of the TIM is not preferable because it increases the bulk thermal resistance. Therefore, it is necessary to develop a graphite film that has a strain absorbing effect even if it is thin. However, on the other hand, it is considered that the thermal resistance at the interface may increase because the thin graphite film cannot cope with the unevenness of the member as described above. Therefore, it is expected that the interfacial thermal resistance value cannot be reduced by simply using a thin graphite film as the TIM. Therefore, we aimed to develop a method that has an excellent strain reduction effect even if it is thin and can further reduce the interfacial thermal resistance. If such contradictory conditions can be overcome, a completely new TIM can be developed.

The present inventors have studied various ways to reduce the interfacial resistance by using a high-quality graphite film, which is considered to have a large strain reducing effect, as a TIM. As a result, the effects of the thickness of the graphite film, the thermal conductivity and its anisotropy of the film, the wrinkles of the film, etc. were examined in detail, and by applying the optimum wrinkles to a specific graphite film, at least one of the members was 150. The present invention has been completed by discovering that excellent thermal bonding is possible even between members having different thermal expansion coefficients at ° C or higher.

That is, the interlayer thermal bonding method of the present invention is an interlayer thermal bonding method in which an interlayer thermal bonding member is sandwiched between two members having different coefficients of thermal expansion, and the interlayer thermal bonding material has a thickness of 200 nm to 80 μm. This is an interlayer thermal bonding method in which the density of the graphite film is 1.40 g / cm3 to 2.26 g / cm3, and the thermal conductivity of the graphite film in the plane direction is 500 W / mK to 2000 W / mK. ..

This is because (1) the bulk thermal resistance can be reduced by using a thinner graphite film as compared with the conventional composite TIM, and (2) a graphite film having excellent thermal conductivity characteristics in the ab plane direction is used. By doing so, the multi-point bonding effect (described later) can be realized, and by providing the optimum wrinkles on such a graphite film, the interfacial resistance can be made extremely small, and (3) expansion due to the difference in the coefficient of thermal expansion between the members. -Shrinkage can be absorbed by the lubricity of graphite and the anisotropy of the coefficient of thermal expansion (anisotropy in the а-b plane direction and the c-axis direction). The dependence can be reduced, and (5) such thermal bonding characteristics can be stably realized even if the temperature of at least one member is 150 ° C. or higher and the difference in the coefficient of thermal expansion between the members is large, which is severe. It is based on the discovery of a new fact that it is stable even in continuous heat resistance evaluation tests and thermal cycle tests.

Further, it is preferable that the wrinkles (Ra) on the graphite surface are in the range of 0.1 μm to 10 μm. The specific value of the thermal resistance value that can be expressed in the present invention is 3.0 ° C. cm ² / W or less when pressurized at 0.2 MPa (when a load is applied). This thermal resistance value is equivalent to the characteristics of the composite TIM, but considering that the TIM of the present invention has extremely excellent durability and heat resistance, it is an extremely characteristic and beneficial interlayer thermal bonding method. It can be said that.

Further, the average linear thermal expansion coefficient (between 0 ° C. and 200 ° C.) of the graphite film of the present invention in the film surface direction ^{is preferably ± 5 × 10 −6} / K or less. The coefficient of linear thermal expansion in the film thickness direction ^{is preferably 1 × 10 -5} / K or more, but since the graphite film of the present invention is extremely thin, its value does not significantly affect the thermal bonding characteristics. By using such graphite, stable thermal bonding is performed even between two members whose average linear thermal expansion coefficient difference in the range of 0 ° C to 200 ° C is 3 × 10 ^{-6 / K or more.} You can do it. Needless to say, stable thermal bonding can be realized between members whose average coefficient of linear thermal expansion is 3 × 10 ^{-6 / K or less.}

In order to realize excellent thermal resistance characteristics, not only the thickness and thermal conductivity characteristics of the TIM graphite film but also the size and uniformity of wrinkles on the surface of graphite are important. As a result of various studies, the present inventors have found that the graphite film TIM preferably has a surface roughness (Ra) of graphite in the range of 0.1 to 10 μm before being narrowed. By using such a graphite film, not only the characteristic that the thermal resistance value at the time of pressurizing 0.2 MPa is 3.0 ° C. cm ² / W or less can be realized, but also the pressure dependence thereof can be reduced. .. The value of the pressure dependence is that the difference in thermal resistance between when pressurized at 0.1 MPa and when pressurized at 0.5 MPa is within 3 times. The fact that the pressure dependence of the thermal resistance value is extremely small and that it exhibits low thermal resistance characteristics even with a small pressurization means that strong mechanical tightening is not required, and even if the mechanical tightening is loosened, even if the mechanical tightening is loosened. It is a practically extremely useful property that its thermal resistance is hardly affected.

By such a TIM of the present invention, the temperature of at least one member is 150 ° C. or higher, and the difference in the coefficient of thermal expansion between the members is 3 × 10 ^-6 K or higher, which is stable between the two members. Interlayer thermal bonding is possible, because graphite is an extremely stable material with excellent durability, graphite has lubricity, the coefficient of thermal expansion of the а-b surface of graphite is small, and thermal expansion in the c-axis direction is possible. This is due to the fact that the coefficient is large and that the wrinkles formed on the graphite surface can alleviate the strain caused by the difference in the coefficient of thermal expansion between the members. As a result, the graphite TIM of the present invention can be preferably used in a harsh thermal environment. The TIM of the present invention can increase the thermal resistance value within 20% in a continuous heat resistance evaluation test at 150 ° C. for 240 hours and a temperature cycle test (96 hours) between 200 ° C. and room temperature. The room temperature means 20 ° C.

Further, this heat bonding method can be applied even when the member has irregularities. The unevenness of the member that can be dealt with in the present invention is in the range of 6.0 μm or less in Ra display, and the unevenness of such a member corresponds to the surface unevenness of many realistic members, so that the present invention is useful. It becomes an invention ,.

The method for producing the graphite TIM of the present invention is not particularly limited, but it is preferably produced by the steps of carbonizing and graphitizing the polymer film.

The method for imparting optimum wrinkles (roughness) in the present invention is not particularly limited, but the polymer film, the carbonized film, or the graphite film is formed at a plurality of points in at least one treatment step of carbonization and graphitization. It is preferable to heat-treat while holding and pressurizing. Further, in at least one of the carbonization and graphitization treatment steps, one surface of at least one of the polymer film, the carbonized film, and the graphite film and the surface roughness Ra is 0.1 μm or more and 10 μm. It is preferable that the following spacers are laminated and heat-treated while pressurizing. The spacer at this time is not particularly limited as long as it has the necessary unevenness, durability, and heat resistance, but it is an example of a preferable spacer that the spacer is felt made of a carbon material such as carbon fiber or graphite fiber. is there.

The type of the polymer raw material is not particularly limited, but a condensed aromatic polymer is preferable. Further, the condensed aromatic polymer is an aromatic polyimide film having a thickness in the range of 300 nm to 125 μm, and it is preferable to heat-treat the polyimide film at a temperature of 2400 ° C. or higher.

That is, the present invention that has solved the above problems is as follows.
(1) An interlayer thermal bonding method in which an interlayer thermal bonding member is sandwiched between two members having different thermal expansion coefficients. The interlayer thermal bonding member contains a graphite film having a thickness of 200 nm to 80 μm, and is a graphite film. the density was ^{1.40g / cm 3 ~2.26g / cm 3} , the thermal conductivity in the plane direction of the graphite film Ri 500W / mK~2000W / mK der, also,
The difference in the average coefficient of linear thermal expansion between 0 ° C. and 200 ° C. of the two members is 3 × 10 ^-6 / K or more, and further.
0 ° C. to 200 DEG average linear thermal expansion coefficient in a plane direction at ° C. is ^{-5 × 10 -6 ~ + 5 ×} 10 -6 / K der Ru interlayer heat bonding method of the graphite film.
( 2 ) The interlayer thermal bonding method according to (1), wherein the arithmetic average roughness Ra of the surface of the graphite film before being sandwiched between the two members is 0.1 μm to 10 μm.
( 3 ) The interlayer thermal bonding method according to (1) or (2) , wherein the thermal resistance value when a load of 0.2 MPa is applied is 3.0 ° C. cm ² / W or less.
(4) R _0.1P / R _0.5 is the ratio of the thermal resistance value when a load is applied to 0.1 MPa (R _0.1P) and thermal resistance when applying a load of 0.5 MPa (R _0.5P) The interlayer thermal bonding method according to any one of (1) to (3), wherein _{P is 1.0 to 3.0.}
( 5 _{) The thermal resistance value (R 240H} ) after the continuous heat resistance evaluation test at 150 ° C. for 240 hours is 1.0 to 1.2 times the _{thermal resistance value (R 0H} ) before the continuous heat resistance evaluation test. The interlayer thermal bonding method according to any one of (1) to (3).
( 6 _{) The thermal resistance value (R 96H} ) after the temperature cycle evaluation test between 200 ° C. and room temperature every two hours for 96 hours continuously is 1 of the thermal resistance value (R _{0H) before the temperature cycle evaluation test.} The interlayer thermal bonding method according to any one of (1) to (3), which is 0 to 1.2 times.
( 7 ) A cooling system using the interlayer thermal bonding method according to any one of (1) to ( 6).

According to the present invention, it is possible to realize a thermal bonding and cooling system that can be used in an environment where the temperature of at least one member is 150 ° C. or higher, which was completely impossible with the conventional composite TIM. The thermal bonding method / cooling system of the present invention has a different coefficient of linear thermal expansion and can be preferably used for thermal bonding between practical parts having irregularities, such as a continuous heat resistance evaluation test at 150 ° C. and 240 hours and 200 ° C. In the temperature cycle evaluation test (96 hours) between −room temperature, the increase in thermal resistance value can be 20% or less.

The cross-sectional schematic diagram which shows the state of the wrinkle of the graphite film of this invention. (A) is a cross-sectional image of a graphite film having large wrinkles, (b) is an image of a cross-sectional view of a graphite film having wrinkles of an appropriate size, and (c) is an image of a cross-sectional view of a graphite film having almost no wrinkles. An actual example showing the state of wrinkles in the graphite film of the present invention. (A) Graphite film with large wrinkles, (b) Graphite film with appropriate size wrinkles, (c) Graphite film with almost no wrinkles. Cross-sectional SEM photograph of the graphite film (thickness 2.6 μm) of the present invention. An example of a graphite film with moderate and uniform wrinkles. The schematic diagram which shows an example of the manufacturing method of the graphite TIM of this invention which presses by sandwiching the film to be processed with a flat substrate. Image of the measurement method and measurement position (thick black lines at 5 points) of the surface roughness (arithmetic mean roughness Ra) of the graphite film. However, 5α is the midpoint of each side of the graphite film of about 50 mm square, 5β is the midpoint of the five line segments 5a, and 5γ is the center of gravity of the graphite film. The surface roughness (arithmetic mean roughness Ra) is measured in the direction of the line segment 5a at five points. A method for measuring thermal resistance between members with different coefficients of thermal expansion. 6a is the first member, 6b is the graphite TIM (interlayer heat bonding member), 6c is the second member, 6d is the weight, and 6e is the heating source. Also, ■ (black square) is the temperature measurement location. A method for evaluating the heat resistant properties of graphite TIM. 7a is the first member, 7b is the graphite TIM (interlayer thermal bonding member), 7c is the second member, 7d is the silicon grease, and 7e is the weight. The evaluation is performed by changing the materials of the first member and the second member. (A) is a method for measuring thermal resistance, and (B) is a method for heat resistance test and temperature cycle evaluation test.

Hereinafter, embodiments of the present invention will be described in detail. All academic and patent documents described in this specification are incorporated herein by reference. Further, unless otherwise specified in the present specification, "A to B" representing a numerical range means "A or more (including A and larger than A) and B or less (including B and smaller than B)".

(A) Graphite film In the present invention, the thickness of the graphite film is in the range of 200 nm or more and 80 μm or less. The thickness is more preferably 300 nm or more, and most preferably 400 nm or more. Further, the graphite film of the present invention is more preferably 70 μm or less, further preferably 60 μm or less, and particularly preferably 50 μm or less. If it exceeds 80 μm, there is a problem that the bulk thermal resistance increases. On the other hand, if the thickness is less than 200 nm, there is a problem that it becomes extremely difficult to handle as a self-supporting film. If the thickness is in the range of 200 nm to 80 μm ^{, excellent thermal resistance characteristics of 3.0 ° C. cm 2} / W or less can be realized. This characteristic is equal to or higher than that of the conventional composite TIM, and further has overwhelmingly excellent heat resistance and durability.

The thermal resistance characteristics of the TIM (interlayer thermal bonding member) obtained in this patent will be compared and discussed with the thermal resistance characteristics of the graphite TIM obtained in the prior patent of the present inventors (Patent Document 1). According to the results of the prior patent, in a graphite film having a thickness in the range of 18 nm to 13 μm, a characteristic of 0.98 to 0.33 ° C. cm ² / W was obtained as a characteristic when ^{pressurized at 1.0 kgf / cm 2.} ing. (Note: In this patent, the magnitude of the load to be pressurized is described in MPa. The load of 1.0 Kgf / cm ² is approximately equal to 0.1 MPa.) However, these characteristics are only the thermal resistance measuring device. It is a characteristic between the electrode surfaces that have been mirror-polished by. Therefore, such a characteristic value is not realized when it is applied between practical members having irregularities, and its thermal resistance characteristic changes when it is actually applied between members having irregularities. The thermal resistance value at this time becomes larger as it is used between members having large irregularities, and is often worse than the characteristics of the conventional composite TIM. On the other hand, the thermal resistance value achieved by the TIM of the present invention is 3.0 ° C. cm ² / W or less, and this characteristic is an excellent value equal to or higher than the characteristic of the composite TIM.

The graphite film of the present invention has a density in ^{the range of 1.40 to 2.26 g / cm 3} , and has a thermal conductivity in the film surface direction of 500 W / mK or more. Due to such characteristics, the bulk thermal resistance can be reduced. It is more preferably a density in the range of 1.50~2.26g / cm ^3, it is most preferred in the range of 1.60~2.26g / cm ^3. The density of 2.26 g / cm ³ is the density of graphite that does not contain an air layer at all, and whether or not the graphite film contains an air layer can be confirmed by measuring the density of the graphite film. Since the thermal conductivity of the air layer is extremely low, it is desirable that the air layer does not exist inside the graphite film, and the density condition is a guide for knowing the existence of the air layer.

The thermal conductivity in the film surface direction is 500 W / mK or more and 2000 W / mK or less. It is more preferably 600 W / mK or more, further preferably 800 W / mK or more, and most preferably 1000 W / mK or more. It is preferable that the thermal conductivity in the film surface direction is large, but in the case of graphat, it is known that the maximum value in the а−b surface direction is 2000 W / mK. As described above, the reason why it is important that the thermal conductivity in the film surface direction is large in the present invention can be explained as follows. That is, in the graphite film TIM, the bonding with the member is basically a point bonding, but the heat flowing in from a certain bonding point has extremely high thermal conductivity in the graphite film surface direction, so that the heat is quickly formed in the graphite film TIM. It spreads and can eventually flow from many junctions to the opposite member. That is, the high thermal conductivity in the film plane direction has the same effect as contacting substantially more joint points than the actual joint points. Since such an effect increases as the thermal conductivity in the film surface direction increases, it is considered that the thermal conductivity in the film surface direction is important for realizing low thermal resistance characteristics. It is a fact clarified for the first time by the present invention that such thermal conductivity in the film surface direction is important for improving the TIM characteristics.

Further, the average linear thermal expansion coefficient of the graphite TIM of the present invention in the range of 0 ° C. to 200 ° C. in the film surface direction is preferably within ^{± 5 × 10-6 / K.} It is more preferably within ± 4 × ^{10-6 / K, further preferably within ± 3 × 10-6} / K, and most preferably within ^{± 2 × 10-6 / K.} By using graphite having such an average coefficient of thermal expansion, the difference between the average coefficient of linear thermal expansion between 0 ° C. and 200 ° C. is stable between two members such that ^{the difference is 3 × 10 -6 / K or more.} Thermal bonding is possible.

Further, in the graphite TIM of the present invention, it is preferable that the surface arithmetic mean roughness (Ra) of graphite before being narrowed is in the range of 0.1 μm to 10 μm. The value of the arithmetic roughness Ra of this surface is preferable in order to reduce the interfacial thermal resistance and to realize the thermal resistance characteristic of ^{3.0 ° C. cm 2 / W or less.} The Ra value in the range of 0.1 μm to 10 μm is a numerical value defined in the case of 200 nm to 80 μm, which is the range of the thickness of the graphite film of the present invention. It is preferable to consider the arithmetic mean roughness Ra of the surface in consideration of the thickness. As a result of examination from such a viewpoint, it was found that the more preferable range of Ra is the range of 0.1 μm to 4 μm in the case of the thinnest 200 nm and the range of 0.4 μm to 10 μm in the case of the thickest 80 μm. It was. As described above, the interlayer thermal bonding member of the present invention has a graphite film having a thickness of 200 nm to 80 μm that can be practically handled and manufactured, and has a large thermal conductivity in the film surface direction and an optimum surface roughness. By having, the interfacial thermal resistance can be reduced and ^{excellent thermal resistance characteristics of 3.0 ° C. cm 2} / W or less can be realized. Further, by having an optimum surface roughness and an appropriate linear thermal expansion coefficient, a stable interlayer thermal bonding method between members having different expansion coefficients can be realized.

(B) Method for Producing Polymer Film The method for producing a graphite film is not particularly limited, but it can be obtained, for example, by a method of heat-treating a polymer film. The polymer film is preferably an aromatic polymer. Among them, polyamide, polyimide, polyquinoxaline, polyoxadiazole, polybenzimidazole, polybenzoxazole, polybenzthiazole, polyquinazolinedione, polybenzoxazineone, polyquinazoline, benzimidazolone benzophenanthroline ladder polymer, and derivatives thereof. It is preferable that it is at least one kind.

Among the above polymer films, an aromatic polyimide film is preferable because it can be converted to graphite. Further, among aromatic polyimide films, a film having a controlled molecular structure and its higher-order structure and having excellent orientation is preferable because it is easier to convert to high-quality graphite. The aromatic polyimide film can be produced by a known method. For example, it can be produced by casting an organic solvent solution of polyamic acid, which is a polyimide precursor, on a support such as an endless belt or a stainless steel drum, and drying and imidizing the solution. The method for imidization is not particularly limited, and a thermal cure method in which the precursor polyamic acid is converted to an imide by heating, a dehydrating agent typified by an acid anhydride such as acetic anhydride, picolin, quinoline, isoquinoline, etc. Examples include a chemical cure method for imid conversion using a tertiary amine such as pyridine as an imidization accelerator. In addition, a polyimide thin film can be produced by a vapor deposition polymerization method.

Specific methods for producing a film by the chemical cure method include the following methods. First, a dehydrating agent having a stoichiometry or higher and an imidization accelerator having a catalytic amount are added to the polyamic acid solution, and the solution is cast or applied onto an organic film such as a support plate or PET, or a support such as a drum or an endless belt. A film having a self-supporting property is obtained by forming a film and evaporating an organic solvent. Next, this is further heated and dried while being imidized to obtain a polyimide film made of a polyimide polymer. The temperature at the time of heating is preferably a temperature in the range of 150 ° C. to 550 ° C. The rate of temperature rise during heating is not particularly limited, but it is preferable to gradually or intermittently heat the temperature so that the maximum temperature reaches the above temperature. Further, it is preferable to include a step of fixing or stretching the film in order to prevent shrinkage in the process of manufacturing the polyimide. By such a treatment, the orientation can be increased.

The thickness of the graphite film finally obtained varies depending on the type of the starting polymer film, but in the case of aromatic polyimide, it is often about 60 to 40% of the thickness of the original polymer film. .. Therefore, in order to finally obtain a graphite film having a thickness of 200 nm to 80 μm, the thickness of the starting polymer film is preferably in the range of about 300 nm to 125 μm.

(C) Method for Producing Graphite Film The method for producing the graphite film TIM of the present invention is not particularly limited, but it is preferably produced by carbonization or graphitization of the polymer film. Here, a method for carbonizing and graphitizing a polymer film will be described. Such carbonization and graphitization steps may be continuously carried out in the same furnace, or may be carried out in separate furnaces. The method of carbonization is not particularly limited, and for example, the polymer film as a starting material is preheated in an inert gas or in a vacuum to carry out carbonization. As the inert gas, nitrogen, argon or a mixed gas of argon and nitrogen is preferably used. Preheating is usually performed at a temperature of about 600 to 1000 ° C. At the stage of preheating, it is effective to apply tension in the plane direction to the extent that the film does not break so that the orientation of the starting polymer film is not lost.

The method of graphitization is not particularly limited, and for example, the film carbonized by the above method is set in a high-temperature furnace to perform graphitization. Graphitization is carried out in an inert gas, but argon is the most suitable inert gas, and a small amount of helium may be added to argon. The higher the treatment temperature, the higher the quality of graphite that can be converted, preferably 2400 ° C. or higher, more preferably 2600 ° C. or higher, and most preferably 2800 ° C. or higher.

(D) Optimal wrinkle shape and evaluation When a graphite film is prepared from a polymer film such as polyimide, the carbonized film has a dimension in the film surface direction of about 75 to 85% of the original polymer film at the time of carbonization. Often contracts. In addition, when the shrinkage and expansion of the film during carbonization and graphitization are left to nature, the finally obtained graphite film in the film surface direction is 85 to 95, which is the size of the original polymer film. It is often about%. Wrinkles occur in the graphite film obtained by expansion and contraction in the steps of carbonization and graphitization. FIG. 1 shows a schematic cross-sectional view showing the state of wrinkles generated in the graphite film. When it is produced by the above general method, it is impossible to control the size of the wrinkles that occur, and when large wrinkles as shown in (a) occur or in (c). There are cases where there are almost no wrinkles as shown. Further, in some cases, large wrinkles may be partially generated, resulting in non-uniform wrinkles such that there are partially wrinkle-free parts. As a result of the examination, such wrinkles of the graphite film affect the characteristics of TIM, and it is preferable to control the size and wrinkles of the wrinkles in order to realize excellent thermal bonding characteristics. In order to produce a graphite film with controlled size wrinkles, it is necessary to add new ideas to the manufacturing method.

FIG. 2 is an image diagram of wrinkles in a graphite film. (A) is an example of a graphite film having uniform and large wrinkles, (b) is an example of a graphite film having uniform and optimally sized wrinkles for realizing low thermal bonding, and (c) is an example of a graphite film having almost no wrinkles. It is a graphite film. (A), (b), and (c) are examples of actual surface photographs of the graphite film in the image diagram of the schematic cross section shown in FIG. 1, and correspond to (a), (b), and (c) of FIG. 1, respectively. ..

FIG. 3 is a cross-sectional SEM photograph of the graphite film (film thickness 2.6 μm) of the present invention having the optimum wrinkles shown in FIG. 2 (c).

As described above, in the graphite film TIM of the present invention, it is desirable to control the graphite film TIM within an appropriate range as shown in FIG. 2 (c) or FIG. 3, but a method for realizing such wrinkles has not been known conventionally. Therefore, in order to give appropriate wrinkles, it was necessary to develop a new wrinkle giving method and to specify wrinkles with appropriate numerical values.

The size of wrinkles can be expressed by the arithmetic mean surface roughness Ra, such as a stylus type surface roughness meter, an optical method such as a laser microscope, STM (Scanning Tunneling Microscope), AFM (Atomic Force Microscope), etc. It can be determined by the method. As the provisions relating to these, for example, JIS B0601-2001 can be applied or applied mutatis mutandis. In the present invention, the size of wrinkles is defined by this Ra display.

Furthermore, the present inventors have found that not only the size of wrinkles but also the uniform wrinkles are preferable for the stable realization of low thermal resistance characteristics. In principle, the uniformity of wrinkles is the ratio of the value (Ra) of each location when the arithmetic average roughness Ra of multiple locations is measured to the average value (Rave) of Ra obtained from the measurement results at all multiple locations. (Ra / Rave) may be taken, but it is impossible to measure Ra at all points in a certain area, and it is difficult to quantitatively describe wrinkles. Therefore, we quantified the uniformity of wrinkles by the method shown in FIG. First, a graphite film as a sample was cut into a square of about 50 mm, and the surface roughness Ra was measured in the direction of the five line segments shown by the black line in FIG. Here, 5α is the midpoint of each side of the graphite film of about 50 mm square, 5β is the midpoint of five black line segments, and 5γ is the center of gravity of the graphite film of about 50 mm square. As a result, when measured by such a method, it is preferable that the ratio of (Ra / Rave) is in the range of 0.2 to 5.0 in order to realize the excellent thermal resistance characteristics of the present invention. Do you get it. However, the value of (Ra / Rave), which is an index for evaluating the uniformity of wrinkles specified in the present invention, is only the value when measured by the method shown in FIG. 5, and was measured by a method other than this method. Even if the value of (Ra / Rave) deviates from the range of 0.2 to 5.0, it does not mean that it deviates from the range of the present invention.

(E) Optimal wrinkle forming method The method for forming wrinkles having an appropriate size of Ra in the present invention is not particularly limited, but the conventional method for producing a graphite film from a polymer film produces appropriate wrinkles. It is difficult to form. As described above, when carbonization is performed using an aromatic polyimide as the polymer film, the carbonized film shrinks to about 75 to 85% of the original polymer film in the direction of the film surface during carbonization. Often. In addition, when the shrinkage and expansion of the film during carbonization and graphitization are left to nature, the finally obtained graphite film in the film surface direction is 85 to 95, which is the size of the original polymer film. It is often about%. Due to such natural contraction and expansion, the size of wrinkles in the graphite film cannot be optimally controlled.

In at least one step of the carbonization step or the graphite step, the present inventors laminate a spacer having an unevenness of an appropriate size with at least one side of a sample such as a polymer film, a carbonized film, or a graphite film. The present invention has been completed by finding that an appropriate wrinkle can be formed by sandwiching this with a smooth jig and pressurizing it from both sides with an appropriate pressure and treating it at a carbonization temperature and a graphitization temperature. FIG. 4 shows a schematic diagram for forming wrinkles in the graphite film by pressing. 4a is a graphite film, 4b is a spacer, and 4c is a press jig.

In the present invention, shrinkage and expansion are controlled by a press treatment using a spacer to form appropriate wrinkles, and specifically, it is suitable for both sides of a polymer film, a carbonized film, or a graphite film. Spacers having irregularities of various sizes are laminated, sandwiched between smooth press plates, pressed from both sides at an appropriate pressure, and treated at a carbonization temperature and a graphitization temperature. This allows the formation of appropriate wrinkles. The carbonization is a treatment performed on the polymer film, and the graphitization is a treatment performed on the carbonized film. Moreover, you may perform regraphitization treatment if necessary. Regraphitization is a process performed on a graphite film. When the polymer film is carbonized and then graphitized, the press treatment may be performed by either carbonization or graphitization, or both treatments may be performed.

The material of the press plate and the spacer used for pressing the film to be treated is not particularly limited as long as it has durability against a high treatment temperature, but a carbon material or a graphite-based material is generally preferable. For example, a substrate made of isotropic graphite CIP (Cold Isotropic Press) or glassy carbon can be used.

The spacer used in the present invention has irregularities of an appropriate size, and the surface roughness (Ra) of the spacer is preferably 10 μm or less, preferably 8 μm or less, and more preferably 5 μm or less. The surface roughness of the spacer is preferably 0.1 μm or more, more preferably 0.2 μm or more, and more preferably 0.4 μm or more.

It is difficult to unambiguously determine the press conditions because various factors such as the shape of the spacer, the type of object to be treated (polymer film, carbonized film, graphite film), and thickness are intricately entwined. It may be set within the range of. That is, since the polymer film shrinks in the carbonization process and stretches in the graphitization process, if the pressurizing force is too strong, the film will be finely cracked in the shrinkage process. In addition, non-uniform wrinkles occur in the process of graphitization. On the other hand, if the pressurizing pressure is too small, the unevenness of the spacer cannot be reflected as the unevenness of the graphite film. Therefore, the method of pressurization is not a uniform pressure for carbonization and graphitization, but it is preferable to change the size in consideration of expansion and contraction of each.

Regardless of whether it is continuously pressurized or intermittently pressurized. For example, the press pressure can be appropriately set from the range of ^{1 gf / cm 2} or more and 2000 gf / cm ^{2 or less.} Pressing pressure, 2 gf / cm ² or more, 5 gf / cm ² or more, it is preferable to 10 gf / cm ² or more. The 1000 gf / cm ² or less, 600 gf / cm ² or less, it is preferable to 400 gf / cm ² or less.

The press time is appropriately set in the range of a short time to a long time according to various conditions, and a plurality of presses may be performed. However, in the graphitization press, it is desirable to continue the press until the maximum temperature is reached, rather than ending the press at an early stage. During the graphitization treatment, the dimension of the graphite film in the film surface direction grows until the maximum temperature is reached. Therefore, if the pressing is finished at an early stage, uneven wrinkles may be formed due to the subsequent stretching. Further, in the press at the time of graphitization, it is desirable to start the press after the growth of the graphite film starts. If the pressing is started from the stage before the start of elongation of the graphite film, the elongation of the graphite film is greatly suppressed, and there is a risk of causing non-uniform wrinkles. As for the timing of pressing at the time of graphitization, it is desirable to start the pressing at a stage of, for example, 2200 ° C. or higher, preferably 2400 ° C. or higher, more preferably 2600 ° C. or higher, and continue the pressing to the maximum temperature reached. Further, in the present invention, the pressing may be repeated for a short time and pressed even near the maximum temperature. Details such as press pressure, time, and timing may be optimized as appropriate.

The pressing means may be a pressing means (press mechanism) capable of mechanical pressure control, such as one using the weight of the press plate itself, or one in which a graphite or carbon weight stone is placed on the press plate. It may be a non-mechanical means such as. The non-mechanical means is suitable for applying a constant weak load from beginning to end during carbonization and graphitization, and can form wrinkles of an appropriate height on the graphite film, which is preferable.

The spacer is not particularly limited as long as it has durability against a processing temperature and a press pressure, and includes, for example, a film or a woven fabric made of powdery particles or a fibrous substance. As the powdery particles, glassy carbon particles, graphite particles, graphite scales, carbon fibers as fibrous substances, carbon-based / graphite-based particles of graphite fibers, or fibers are preferably used.

The roughness (unevenness) of the surface of the spacer molded into a film or woven fabric has a great influence on the unevenness formation of the graphite film to be produced, but the roughness of the spacer surface and the surface roughness of the graphite film are not necessarily large. It does not match. It is considered that this is because the unevenness of the spacer surface serves to support the graphite film at points in some cases.

Further, the surface of the press jig may be provided with irregularities, and the irregularities may be used as a spacer. In this case, it is preferable that the surface of the press jig has a certain shape and surface roughness by polishing with sandpaper, sandblast, an abrasive, or the like. For such a purpose, a press plate made of CIP material or glassy carbon whose surface is uniformly roughened to a certain degree may be used as the press jig. Further, it is also preferable to use a press plate made of CIP material or glassy carbon in which the carbon fibers are pressed at a high temperature and the carbon fibers are fixed to the surface.

The various methods for providing the optimum roughness of the graphite film TIM as described above may be appropriately selected according to the manufacturing process, and are not limited to the contents of the examples shown below. Further, the method for producing a graphite TIM film of the present invention is excellent in productivity because a large number of sheets can be stacked and fired at one time. Further, it can be applied even when the thickness of the film to be treated is extremely thin as in the range of the present invention and is physically easily torn. According to the method for producing a graphite TIM film of the present invention as described above, wrinkles having an appropriate size can be formed and the arithmetic mean roughness (Ra) can be controlled within an appropriate range.

(F) Interlayer Thermal Bonding Method The interlayer thermal bonding method using the TIM of the present invention includes a step of sandwiching the TIM of the present invention between the members to be thermally bonded. By sandwiching the TIM of the present invention between layers, interlayer thermal bonding is performed to transfer heat from a heat generation source or a member thermally bonded to the heat generation source to a second member having a temperature lower than that. Can be done. The graphite film is sandwiched between a member close to the heat source and a member far from the heat source, and the graphite film and each member are in direct contact with each other.

In order to realize heat-resistant interlayer thermal bonding, it is preferable to heat-bond the layers only with a graphite film. As a method of realizing thermal bonding between layers without using an adhesive layer, it may be fixed by simply mechanical pressure. Mechanically tightening with screws, screws, springs, etc. is effective and preferable for direct thermal bonding. However, considering that low thermal resistance can be realized under low pressure, which is a feature of the present invention, and that the pressure dependence of thermal resistance is small, it is not always necessary to tighten strongly, and even if the tightening pressure changes. Since the influence is small, it is possible to realize a practically extremely effective interlayer thermal bonding. By such a method, thermal bonding between members having at least one temperature of 150 ° C. or higher becomes possible.

The thermal bonding formed in this way is effective when the bonding members have different coefficients of thermal expansion and the difference is large or the temperature difference between the members is large. The average coefficient of linear thermal expansion (between 0 ° C and 200 ° C) of graphite in the film surface direction of the present invention is ± 5 × ^10-6 / K or less, and the graphite film has lubricity based on its layer structure. Therefore, stable thermal bonding is possible even when the difference in the coefficient of linear thermal expansion between the two members is 3 × 10 ^{-6 / K or more.}

The thermal junction formed in this way is also effective when the temperature change of the junction portion changes significantly. For example, it is effective for thermal bonding of a portion where the temperature is 150 ° C. or higher when used but is room temperature when not in use. Thermal bonding by such a method enables thermal bonding in which the thermal bonding characteristics do not change by 20% or more in, for example, a continuous heat resistance evaluation test at 150 ° C. for 240 hours. Further, for example, even in a continuous temperature cycle evaluation test between 200 ° C. and room temperature (every 2 hours, 96 hours), thermal bonding is possible so that the thermal bonding characteristics do not change by 20% or more. A heat bonding method having such excellent heat resistance has not been known so far.

Therefore, the thermal bonding method of the present invention is an excellent method especially when used in a high temperature environment or an environment with a large temperature difference. In particular, it is an extremely effective joining method when the temperature of at least one of the heat joining members is 150 ° C. or higher, and it is effective in a harsh environment such as an LED, a power semiconductor, or around an automobile engine. Can be done.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited by the following Examples, and is carried out with appropriate modifications to the extent that it can be adapted to the gist of the above and the following. It is also possible, and they are within the technical scope of the invention.
<Measurement method of physical properties>
First, the method for measuring the physical properties in the following examples is shown below.
(1) Thickness of Graphite Film 50 × 50 mm The thickness of ^{the graphite film cut out into 2} was measured at arbitrary 5 points with a contact type thickness gauge, and the average value was taken as the thickness of the graphite film.

(2) Density of graphite film The density of graphite film was measured using a dry automatic densitometer Accupic II 1340 (manufactured by Shimadzu Corporation). The densities of five graphite films cut into 50 × 50 mm ² were measured one by one, and the average value was taken as the density.

(3) the thermal diffusivity of the thermal conductivity of the graphite film of the graphite film, using a thermal diffusivity measuring apparatus due to the periodic heating method (ULVAC-RIKO Co. "LaserPit" device), 25 ° C., under vacuum (10 ^{- (Approximately 2} Pa), the measurement was performed using a frequency of 10 Hz. This is a method in which a thermocouple is attached at a point separated from the point of laser heating by a certain distance and the temperature change is measured. The thermal conductivity (W / mK) was ^{calculated by multiplying the thermal diffusivity (m 2} / s), the density (kg / m ³ ) and the specific heat (798 kJ / (kg · K)).

(4) Arithmetic mean roughness Ra of graphite film
The surface roughness (arithmetic mean roughness) Ra of the graphite film was measured in a room temperature atmosphere using a surface roughness measuring machine Surfcom DX (manufactured by Tokyo Precision Co., Ltd.) based on JIS B 0601. A 5 cm square graphite film was used for the measurement, and the surface roughness Ra was measured at the five points shown by the line segment 5a in FIG. To determine the reference length, cut out the part of the reference length L from the chart drawn at a feed rate of 0.05 mm / sec according to JIS B 0633, and set the center line of the cut out part as the X-axis and the vertical direction as the Y-axis. When expressed by the roughness curve Y = f (X), the arithmetic mean roughness Ra is the value obtained by the following equation (1) expressed in μm. The value of Ra was obtained at each of the five points of the graphite film (the central portion 5β of the five line segments 5a), and the average value was further obtained, which was used as the arithmetic mean roughness Ra of the graphite film.

(5) Calculation of Ra / Rave ratio of graphite film Obtained from the measurement results of all the values (Ra) of each point when the arithmetic mean roughness Ra of 5 places is measured for the selected graphite film. The ratio of Ra to the average value (Rave), (Ra / Rave), was determined, and the value was defined as the intramembrane variation.

(6) Measurement of Thermal Resistance Characteristics The thermal resistance of the graphite TIM of the present invention was measured using a precision thermal resistance measuring device manufactured by Hitachi Technology and Service. This measuring device is a device capable of precise thermal resistance measurement, and the error is ± 0.002 ° C. cm ² / W. Sample dimensions are 10 x 10 mm ² . The load is in the range of 10 to 50 N (corresponding to 0.1 MPa to 0.5 MPa), and the measurement temperature is 60 ° C. Specifically, first, the wattage (W) to be added was adjusted so that the interface temperature became 60 ° C., and the measurement was performed 10 times after the temperature change became constant, and the average value was used as the measured value. The above is a standard method for measuring a thermal resistance value, but the material of the measuring rod of the thermal resistance measuring device is copper, and the joint surface is mirror-finished. Therefore, the material and the state of surface irregularities are different from those of practical members. Therefore, we first made alumina and copper rods as typical members and measured the thermal resistance value. Moreover, the surface roughness was measured, and if necessary, the surface was polished to change the surface roughness. The measuring method is the same as the above standard thermal resistance measuring method.

(7) Continuous heat resistance evaluation test, temperature cycle evaluation test The continuous heat resistance evaluation test and temperature cycle evaluation test were carried out by the method shown in FIG. The condition of the continuous heat resistance evaluation test is a continuous test in air at 150 ° C. for 240 hours. The test procedure is as follows. First, silicon grease 7d is applied to the lower surface of 7a and the upper surface of 7c in FIG. 7A, and the thermal resistance value of the unit (7a-7b-7c) is measured by applying a pressure of 0.2 MPa. Next, the silicon grease is removed, a weight (stainless steel block corresponding to a load of 0.2 MPa) is placed on this unit (B), and the entire unit is put into a furnace at 150 ° C. and heated for 240 hours. After heating, the weight was removed, silicon grease was applied again to the lower surface of (a) and the upper surface of (c), and the thermal resistance value was measured. The thermal resistance values before and after heating were compared and the rate of change was calculated. In the temperature cycle evaluation test, a heat cycle between 200 ° C. and room temperature is performed every 2 hours. First, the thermal resistance value is measured by the above method, and then the unit is put into a furnace whose temperature is set to 200 ° C. Then, take it out to room temperature after 2 hours. After 2 hours at room temperature, the operation of putting the mixture into the furnace at 200 ° C. was repeated. Such an operation was continuously performed for 96 hours, the thermal resistance values before and after the test were compared, and the rate of change was calculated.

(8) Preparation of Samples Used in Examples and Comparative Examples The following describes a standard method for preparing graphite films having different thicknesses and wrinkles used in Examples and Comparative Examples. Here, those with the same thickness and firing conditions are displayed with the same alphabet (A, B ..., etc.), and samples with different wrinkle conditions are accompanied by A-1, A-2, etc. It is described by numbers.

Acetic anhydride 20 g in 100 g of a 18 wt% DMF solution of polyamic acid prepared from pyromellitic dianhydride, 4,4'-diaminodiphenyl ether, and p-phenylenediamine (4/3/1 in molar ratio). A curing agent consisting of 10 g of isoquinolin was mixed, stirred, defoamed by centrifugation, and then cast and coated on an aluminum foil. The process from stirring to defoaming was performed while cooling to 0 ° C. The laminate of the aluminum foil and the polyamic acid solution was heated at 120 ° C. for 150 seconds to obtain a self-supporting gel film. This gel film was peeled off from the aluminum foil and fixed to the frame. This gel film was heated at 300 ° C., 400 ° C., and 500 ° C. for 30 seconds each to have an average linear thermal expansion coefficient of 1.6 × ^10-5 cm / cm / ° C. and a birefringence of 0.14 at 100 to 200 ° C. Polyimide films with different thicknesses were manufactured.

The obtained polyimide film was heated to 1000 ° C. in nitrogen gas at a rate of 10 ° C./min using an electric furnace, and kept at 1000 ° C. for 1 hour for pretreatment. Next, the obtained carbonized film was sandwiched between spacers made of graphite fiber felt having different surface roughness Ra values, placed between graphite blocks whose surface was polished, and set in a graphite heater furnace. The temperature was raised to 2900 ° C. at a heating rate of 20 ° C./min, held at the maximum temperature for 10 minutes, and then lowered at a rate of 40 ° C./min. The graphitization treatment was performed in an argon atmosphere. At this time, a load was applied to the sample so as to be 50 gf / cm ^2. The carbonized film was sandwiched between spacers made of graphite fiber felt having different surface roughness Ra values, placed between the surface-polished graphite blocks, and set in a graphite heater furnace to prepare a graphite film.

The thickness of the produced graphite film, the thermal conductivity in the film surface direction, the average linear thermal expansion coefficient (between 0 ° C. and 200 ° C.) in the film surface direction, the density, and Ra are as follows.
(A-1) Thickness: 100 μm, Thermal conductivity: 800 W / mK, Coefficient of linear thermal expansion: 2.6 × ^10-6 / K, Density: 1.4 g / cm ³ , Ra: 5 μm
(B-1) Thickness: 80 μm, Thermal conductivity: 1000 W / mK, Coefficient of linear thermal expansion: 1.2 × ^10-6 / K, Density: 1.8 g / cm ³ , Ra: 0.4 μm
(B-2) Thickness: 80 μm, Thermal conductivity: 1000 W / mK, Coefficient of linear thermal expansion: 1.2 × ^10-6 / K, Density: 1.8 g / cm ³ , Ra: 2 μm
(B-3) Thickness: 80 μm, Thermal conductivity: 1000 W / mK, Coefficient of linear thermal expansion: 1.6 × ^10-6 / K, Density: 1.8 g / cm ³ , Ra: 6 μm
(C-1) Thickness: 48 μm, Thermal conductivity: 1300 W / mK, Coefficient of linear thermal expansion: 1.0 × ^10-6 / K, Density: 2.0 g / cm ³ , Ra: 6 μm
(D-1) Thickness: 20 μm, Thermal conductivity: 1580 W / mK, Coefficient of linear thermal expansion: 1.2 × ^10-6 / K, Density: 2.1 g / cm ³ , Ra: 2 μm
(E-1) Thickness: 8.1 μm, Thermal conductivity: 1780 W / mK, Coefficient of linear thermal expansion: 1.2 × ^10-6 / K, Density: 2.1 g / cm ³ , Ra: 2.4 μm
(F-1) Thickness: 2.2 μm, Thermal conductivity: 1800 W / mK, Coefficient of linear thermal expansion: 1.0 × ^10-6 / K, Density: 2.2 g / cm ³ , Ra: 5.5 μm
(G-1) Thickness: 1.0 μm, Thermal conductivity: 1780 W / mK, Coefficient of linear thermal expansion: 0.9 × ^10-6 / K, Density: 2.1 g / cm ³ , Ra: 3.3 μm
(H-1) Thickness: 0.4 μm, Thermal conductivity: 1800 W / mK, Linear thermal expansion coefficient: 0.9 × ^10-6 / K, Density: 1.9 g / cm ³ , Ra: 0.8 μm
(I-1) Thickness: 0.2 μm, Thermal conductivity: 1710 W / mK, Coefficient of linear thermal expansion: 3.2 × ^10-6 / K, Density: 1.9 g / cm ³ , Ra: 4 μm
(I-2) Thickness: 0.2 μm, Thermal conductivity: 1710 W / mK, Coefficient of linear thermal expansion: 1.4 × ^10-6 / K, Density: 1.9 g / cm ³ , Ra: 0.03 μm
(I-3) Thickness: 0.2 μm, Thermal conductivity: 1710 W / mK, Coefficient of linear thermal expansion: 4.2 × ^10-6 / K, Density: 1.9 g / cm ³ , Ra: 12 μm
(J-1) Thickness: 0.1 μm, Thermal conductivity: 1580 W / mK, Coefficient of linear thermal expansion: 5.2 × ^10-6 / K, Density: 1.9 g / cm ³ , Ra: 5 μm

[Examples 1 to 10]
The 10 types of samples (B) to (I) above were prepared by the method shown in FIG. 6 for copper (coefficient of thermal expansion: 16.6 × ^10-6 / K, surface roughness: Ra = 0.4 μm) and. The thermal resistance value was measured by sandwiching it between members of alumina (coefficient of thermal expansion: 5.4 × ^{10-6 / K, surface roughness: Ra = 2.3 μm).}

From this result, the interlayer thermal bonding member sandwiched between the two members is a graphite film having a thickness of 200 nm to 80 μm and a density of 1.40 to 2.26 g / cm ^{3 in} the film surface direction. When the thermal conductivity is 500 W / mK or more, excellent thermal resistance characteristics between two members having different coefficients of thermal expansion (copper and alumina in this case) (at the time of pressurization of 0.2 MPa at 20 ° C.) It was found that a thermal resistance value of 3.0 ° C. cm ² / W or less and a pressure dependence ( _{ratio of R 0.1P} / R _0.5P ) of 3.0 or less can be realized. In order to realize such thermal bonding characteristics, the linear thermal expansion coefficient of the graphite film before ^{being narrowed is ± 5 × 10-6} / K or less, and its surface roughness (Ra) is 0.1 μm to 10 μm. It is preferably in the range.

[Comparative Examples 1 to 4]
The four types of samples (A-1), (I-2), (I-3), and (J-1) were held between the same members as in Examples 1 to 8 and their thermal resistance values were measured. The results of are shown in Table 2. The thickness of (A-1) and (J-1) of these samples is outside the scope of the present invention. The thicknesses of (I-2) and (I-3) are such that the range of wrinkles within the scope of the present invention is outside the scope of the present invention.

In such a graphite film, the thermal resistance value at 20 ° C. at the time of pressurization of 0.2 MPa ^{could not be set to 3.0 ° C. cm 2} / W or less. Moreover, in the sample of A-1, the pressure dependence could not be set to 3 or less.

[Examples 11 to 20]
Using the graphite film C-1 by the method shown in FIG. 7, a heat resistance evaluation test of the thermal bonding characteristics between two members having different coefficients of thermal expansion was performed. In the experiment, the interlayer thermal bonding material 7b was sandwiched between the first member 7a and the second member-7c, and silicon grease was applied to the lower surface of the first member 7a and the upper surface of the second member 7b to form a unit ( 7a-7b-7c) The overall thermal resistance value was measured using a precision thermal resistance measuring device manufactured by Hitachi Technology and Service (load: 0.2 MPa). Next, the silicon grease was removed, a weight (stainless steel block corresponding to a load of 0.2 MPa) was placed on the unit (7a-7b-7c), and the whole was heated at 150 ° C. for 240 hours. After heating, the weight was removed, silicon grease was applied to the lower surface of 7a and the upper surface of 7c again, and the heat resistance value was measured. The thermal resistance values before and after heating were compared and the rate of change was calculated. The experimental results are shown in Table 3. As a result, it was found that good thermal bonding is possible even when the difference in the coefficient of thermal expansion of the two members is 3 × 10 ^{-6 K or more.} Needless to say, thermal bonding between members having a difference in coefficient of thermal expansion of 3 × ^10-6 K or less can be carried out without any problem.

Using the unit prepared by the above method, a temperature cycle evaluation test (every 2 hours) was carried out continuously for 96 hours between room temperature and −200 ° C., and the results are shown in Table 3. As a result of such a heat resistance evaluation test, if the graphite TIM of the present invention is used, its thermal bonding characteristics can be suppressed to 20% or less (1.2 times or less) in such a durability test. It was confirmed by the above-mentioned heat resistance evaluation test at 150 ° C. and 240 hours and the temperature cycle evaluation test between room temperature and 200 ° C. that the electrical conductivity and thermal conductivity of the graphite film (C-1) did not change at all. Therefore, it can be concluded that the above-mentioned change in thermal resistance characteristics as TIM is a change in interfacial thermal resistance. Further, it was found that the thermal resistance value after the heat resistance evaluation test can be returned to the value before the measurement by changing the pressing force at the time of measurement, and the interfacial resistance value can be regenerated. This is a great practical advantage of the present invention.

The above-mentioned solid interlayer heat bonding method can be widely used in the heat bonding portion which is the key to various cooling systems. For example, it is used in a cooling system such as a device for cooling a CPU that becomes hot, a vehicle body cooling system including an engine cooling system, an intake air cooling system, an air fin cooler cooling system, a gas cooling system, and a kiln cooling system.

4a Graphite film 4b Spacer 4c Press jig 5a Line segment 5α Midpoint of each side 5β Midpoint of line segment 5γ Center of gravity 6a First member 6b Interlayer heat bonding member 6c Second member 6d Weight 6e Heat source 7a First Member 7b Interlayer heat bonding member 7c Second member 7d Silicon grease 7e Weight

Claims (7)

This is an interlayer thermal bonding method in which an interlayer thermal bonding member is sandwiched between two members having different coefficients of thermal expansion.
The interlayer thermal bonding member contains a graphite film having a thickness of 200 nm to 80 μm, and contains a graphite film.
Density of the graphite film was ^{1.40g / cm 3 ~2.26g / cm 3} ,
Thermal conductivity in the plane direction of the graphite film is Ri 500W / mK~2000W / mK der, also,
The difference in the average coefficient of linear thermal expansion between 0 ° C. and 200 ° C. of the two members is 3 × 10 ^-6 / K or more, and further.
0 ° C. to 200 DEG average linear thermal expansion coefficient in a plane direction at ° C. is ^{-5 × 10 -6 ~ + 5 ×} 10 -6 / K der Ru interlayer heat bonding method of the graphite film. The interlayer thermal bonding method according to claim 1, wherein the arithmetic average roughness Ra of the surface of the graphite film before being sandwiched between the two members is 0.1 μm to 10 μm. The interlayer thermal bonding method according to claim 1 or 2 , wherein the thermal resistance value when a load of 0.2 MPa is applied is 3.0 ° C. cm ² / W or less. Thermal resistance when subjected to a load of 0.1 MPa (R _0.1P) as the ratio of the thermal resistance (R _0.5P) when a load is applied to 0.5MPa R _0.1P / R _0.5P 1 The interlayer thermal bonding method according to any one of claims 1 to 3, which is 0 to 3.0. _{Claim that the} thermal resistance value (R 240H) after the continuous heat resistance evaluation test at 150 ° C. for 240 hours is 1.0 to 1.2 times the _{thermal resistance value (R 0H} ) before the continuous heat resistance evaluation test. The interlayer thermal bonding method according to any one of 1 to 3. _{The thermal resistance value (R 96H} ) after the temperature cycle evaluation test between 200 ° C. and room temperature every two hours for 96 hours continuously is 1.0 to 1.0 of the thermal resistance value (R _{0H) before the temperature cycle evaluation test.} The interlayer thermal bonding method according to any one of claims 1 to 3, which is 1.2 times. A cooling system using the interlayer thermal bonding method according to any one of claims 1 to 6.

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