Method of manufacturing an electronic device

A method of manufacturing an electronic device includes: placing a resin film on a component; and while heating the resin film to be softened, pressing end portions of a plurality of carbon nanotubes against the softened resin film to bring the end portions into contact with the component, and causing the softened resin film to climb up side surfaces of the carbon nanotubes.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-164678, filed on Aug. 25, 2016, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a heat dissipating sheet, an electronic device, and a method of manufacturing the electronic device.

BACKGROUND

In a server and a personal computer, a heat spreader is attached to an electronic component such as a central processing unit (CPU) to dissipate heat generated in the electronic component to the outside.

When the thermal resistance between the heat spreader and the electronic component is high, heat in the electronic component cannot be transferred to the heat spreader quickly. For this reason, a heat dissipating sheet with excellent thermal conductivity is interposed between the electronic component and the heat spreader in some cases.

There are various types of heat dissipating sheets. An indium sheet is an example of the heat dissipating sheets. However, since the expensive indium is employed in the indium sheet, the cost of the heat dissipating sheets cannot be lowered. Moreover, indium has a thermal conductivity of 80 W/m·K, with which it is difficult to efficiently dissipate heat from an electronic component.

A thermal conductive polymer can be used as the heat dissipating sheet, but the thermal conductivity of a thermally conductive polymer is not sufficiently high.

To deal with these problems, a heat dissipating sheet using carbon nanotubes is under consideration as an alternative of the indium sheet and the thermally conductive polymer.

Carbon nanotubes have a thermal conductivity of about 1,500 W/m·K to 3,000 W/m·K, which is much higher than that of indium (50 W/m·K). Thus, carbon nanotubes are attractive for use in the heat dissipating sheet.

Although there are various proposed techniques for application of carbon nanotubes to a heat dissipating sheet, each of them have room for improvement.

For example, it is proposed to disperse carbon nanotubes into a resin to form the heat dissipating sheet. In this method, since the carbon nanotubes are oriented in various directions, it is difficult to transfer heat along the carbon nanotubes from the upper surface to the lower surface of the sheet.

Alternatively, it is also proposed to grow carbon nanotubes along the normal direction of the substrate, and then completely fill the spaces between the carbon nanotubes with resin. Since all the carbon nanotubes are oriented in substantially the same direction in this method, it is supposedly easy to transfer heat from the upper surface to the lower surface of the sheet along the carbon nanotubes.

In this method, however, resin is interposed between end portions of the carbon nanotubes and the electronic component, which increases the thermal resistance between the electronic component and the carbon nanotubes.

Note that techniques related to the present application are disclosed in the following literature:

Japanese Laid-open Patent Application No. 2013-115094; and

SUMMARY

According to one aspect discussed herein, there is provided a method of manufacturing an electronic device, the method including placing a resin film on a component; and while heating the resin film to be softened, pressing end portions of a plurality of carbon nanotubes against the softened resin film to bring the end portions into contact with the component, and causing the softened resin film to climb up side surfaces of the carbon nanotubes.

DESCRIPTION OF EMBODIMENTS

Prior to describing the present embodiment, matters studied by the inventor of the present application are described.

FIGS. 1A to 1Hare sectional views of the studied electronic device in the course of manufacturing thereof.

This electronic device employs a heat dissipating sheet formed by carbon nanotubes impregnated with resin as follows.

First, as illustrated inFIG. 1A, a plurality of carbon nanotubes2are grown on a silicon substrate1by the hot-filament chemical vapor deposition (CVD) method, thereby obtaining a heat dissipating sheet3provided with a plurality of carbon nanotubes2.

Next, as illustrated inFIG. 1B, a rubber sheet4stretched from its natural length is prepared. Examples of the material for the rubber sheet4include silicone rubber, natural rubber, and synthetic rubber.

Then, the rubber sheet4is pressed against the heat dissipating sheet3, so that the rubber sheet4is fixed to the rubber sheet4by its adhesive force.

Next, as illustrated inFIG. 1C, while keeping the rubber sheet4to be stretched, the heat dissipating sheet3is peeled off from the substrate1.

Thereafter, as illustrated inFIG. 1D, the stretched rubber sheet4is returned to its natural length. Thus, the interval of the carbon nanotubes2is narrowed by the contraction of the rubber sheet4, so that the surface density of the carbon nanotubes2in the heat dissipating sheet3increases, which in turn increases the thermal conductivity of the heat dissipating sheet3.

Next, as illustrated inFIG. 1E, a heat spreader6made of metal excellent in thermal conductivity such as copper is prepared, and resin7is fixed to a surface of the heat spreader6.

In this example, a thermoplastic resin is used as the resin7, and its thickness T is set to substantially the same length L of the carbon nanotubes2.

Then, while heating the resin7to be softened by an unillustrated hot press machine, the heat dissipating sheet3is compressively attached to the resin7. Thereafter, the resin7is cooled naturally to be solidified.

Subsequently, as illustrated inFIG. 1F, the rubber sheet4is peeled off from the heat dissipating sheet3.

Next, as illustrated inFIG. 1G, while pressing a pressing plate8of a hot press machine against the heat dissipating sheet3, the resin7is heated to be softened by a heater provided in the pressing plate8, thereby embedding the carbon nanotubes2into the softened resin7.

At this time, since the thickness T of the resin7is set to the substantially same length L of the carbon nanotubes2in this example as described above, the carbon nanotubes2can be embedded into the resin7over their entire length.

However, once the pressing plate8comes into contact with the resin7, the resin7has nowhere to escape except in the substrate's lateral direction. Thus, there is no sufficient space for the resin7to escape, so that the resin7remains under end portions2aof the carbon nanotubes2and is interposed between the end portions2aand the heat spreader6.

Thereafter, as illustrated inFIG. 1H, an electronic component8such as a CPU is placed on the resin7. Then, while heating the resin7to be softened by an unillustrated hot press machine, the electronic component8is compressively attached to the heat dissipating sheet3.

By these steps, the basic structure of the electronic device used for the study is completed.

According to this method of manufacturing the electronic device, the heat dissipating sheet3can be fixed to both the heat spreader6and the electronic component8by the adhesive force of the resin7.

Moreover, since the thickness of the resin7is set to the substantially the same length of the carbon nanotubes2, the resin7can prevent the carbon nanotubes2from being crushed by the force generated in the step ofFIG. 1Hwhen the electronic component8is compressively attached to the heat dissipating sheet3.

However, when the resin7is made thick as the same length of the carbon nanotubes2, the resin7cannot escape except in the substrate's lateral direction when the carbon nanotubes2are embedded into the resin7in the step ofFIG. 1G.

As a result, the resin7remains between the end portions2aof the carbon nanotubes2and the heat spreader6, and hence the end portion2acannot be brought into contact with the heat spreader6. In such a state, heat in the electronic component8cannot be transferred directly to the heat spreader6via the carbon nanotubes2. Thus, the thermal resistance between the heat spreader6and the electronic component8increases, which cannot exploit the beneficial high conductivity of the carbon nanotubes2.

In addition, according to the investigation conducted by the inventor of the present application, it was revealed that this method has following problem.

FIGS. 2A and 2Bare schematic plan views illustrating the problem.

FIG. 2Ais a diagram drawn based on a photograph of the heat dissipating sheet3taken after the surface density of the carbon nanotubes2is increased by the step ofFIG. 1D. Note that the heat dissipating sheet3is not impregnated with the resin7in this state.

As illustrated inFIG. 2A, the heat dissipating sheet3appears normal in this state.

On the other hand,FIG. 2Bis a diagram drawn based on a photograph of the heat dissipating sheet3taken after the surface density of the carbon nanotubes2is increased by the step ofFIG. 1Dand then the heat dissipating sheet3is impregnated with the resin7by the step ofFIG. 1G.

As illustrated inFIG. 2B, it was found that the heat dissipating sheet3cracks when the heat dissipating sheet3was impregnated with the resin7after increasing the surface density of the carbon nanotubes2.

This is conceivably because when the resin7is impregnated into spaces between the carbon nanotubes2whose surface density is increased, the resin7exerts force against the carbon nanotubes2to widen the intervals between the carbon nanotubes2.

When the heat dissipating sheet3is cracked in this manner, the thermal conductivity of the heat dissipating sheet3significantly decreases, and the heat dissipating sheet3cannot be put into practical use.

In the followings, embodiment capable of decreasing thermal resistance is described.

Embodiment

FIGS. 3A to 3Lare sectional views of an electronic device in the course of manufacturing thereof according to the present embodiment.

First, as illustrated inFIG. 3A, a silicon substrate is prepared as a substrate21. Then, the surface of the substrate21is thermally oxidized to form a silicon oxide film to a thickness of about 300 nm as an underlying film22.

The material for the substrate21is not limited to silicon. A substrate made of any one of aluminum oxides, magnesium oxides, glass, and stainless steel may be used as the substrate21. Moreover, instead of the rigid substrate21, a metallic foil such as a stainless steel foil and an aluminum foil may be used.

Next, as illustrated inFIG. 3B, an aluminum film is formed on the underlying film22by sputtering method to a thickness of about 10 nm, and the aluminum film is used as an underlying metal film23.

Other than aluminum, the material for the underlying metal film23includes molybdenum, titanium, hafnium, zirconium, niobium, vanadium, tantalum, tungsten, copper, gold, platinum, palladium, titanium silicide, aluminum oxides, titanium oxides, and titanium nitride. Moreover, an alloy film containing any of these materials may be formed as the underlying metal film23.

Next, an iron film is formed on the underlying metal film23by sputtering method to a thickness of about 2.5 nm as a catalytic metal film24.

The material for the catalytic metal film24is not limited to iron. The catalytic metal film24may be famed of any one of iron, cobalt, nickel, gold, silver, platinum, and alloys of these materials.

Furthermore, instead of the catalytic metal film24, metallic fine particles of the same material as the catalytic metal film24may be attached onto the underlying metal film23. In this case, the metallic fine particles are sieved by a differential electrostatic classifier or the like, so that only ones with a predetermined diameter are supplied to the underlying metal film23.

Subsequently, as illustrated inFIG. 3C, by using the catalytic action of the catalytic metal film24, a plurality of carbon nanotubes26of about 100 μm to 500 μm long are grown by hot-filament CVD method. These carbon nanotubes26are grown straight along a normal direction n of the substrate21by the action of the underlying film22.

Conditions for growing the carbon nanotubes26are not particularly limited. In this example, a mixed gas of acetylene gas and argon gas is used as a source gas, and the total gas pressure of the source gas in an unillustrated growth chamber is set to about 1 kPa. Note that the ratio of the acetylene gas and the argon gas is about 1:9 for example. The temperature of the hot filament is about 1,000° C., and the substrate temperature is about 620° C. to 660° C.

Under such growth conditions, the carbon nanotubes26grow at a speed of about 4 μm/min.

Note that the underlying metal film23and the catalytic metal film24are condensed into metal particles25when the source gas is introduced into the growth chamber, and the carbon nanotubes26grow only on these metal particles25.

Under the growth conditions in this example, the surface density of the carbon nanotubes26is approximately 1×1011nanotubes/cm2, with the diameter of each carbon nanotube26being 4 nm to 8 nm, approximately 6 nm in average.

In each carbon nanotube26, about three to six graphene sheets are stacked from the center axis of the carbon nanotube26to its outside, and the average number of the graphene sheets becomes about four. Although such a carbon nanotube famed by stacking a plurality of graphene sheets is called a multilayered carbon nanotube, a single-layer nanotube may be formed in the alternative.

Method for growing the carbon nanotubes26is not limited to the hot-filament CVD, but may be thermal CVD method and remote plasma CVD method. Moreover, instead of acetylene, the source for carbon may be either hydrocarbon such as methane and ethylene, or alcohol such as ethanol and methanol.

The carbon nanotubes are not limited to those illustrated in this example, the height of the carbon nanotubes, surface density of the carbon nanotubes26, the diameter of the carbon nanotubes26, and the graphene sheet of the carbon nanotubes26may be approximately 50 μm to 5000 μm, approximately 1×1010nanotubes/cm2to 1×1013nanotubes/cm2, approximately 2 nm to 50 nm, and about one to one hundred fifty, respectively.

Next, as illustrated inFIG. 3D, a rubber sheet30stretched from its natural length is placed on one end portions26aof the carbon nanotubes26.

Then, a surface30aof the rubber sheet30is pressed against the one end portions26aof the carbon nanotubes26with a roller31, so that the end portions26aare fixed to the surface30aby the adhesive force of the rubber sheet30.

The material for the rubber sheet30is not particularly limited, and may be any rubber sufficiently adhesive to allow the carbon nanotubes26to be fixed thereto in this step. Examples of such a material include silicone rubber, natural rubber, and synthetic rubber.

FIG. 4is a plan view of the rubber sheet30stretched in this step.

As illustrated inFIG. 4, the rim of the rubber sheet30is gripped by a plurality of clips32, and is stretched radially by these clips32.

Subsequently, as illustrated inFIG. 3E, the carbon nanotubes26are peeled off from the substrate21, and the carbon nanotubes26are transferred to the rubber sheet30.

Then, as illustrated inFIG. 3F, the stretched rubber sheet30is relaxed by its elastic force. Thus, the intervals between the carbon nanotubes26are narrowed by the contraction of the rubber sheet30, thereby increasing the surface density of the carbon nanotubes26.

The contraction amount of the rubber sheet30is not particularly limited. In this example, the rubber sheet30is contracted in a manner that the area of the rubber sheet30shrinks to about one third of that before this step, thereby increasing the surface density of the carbon nanotubes26as three times higher than that before this step.

Next, as illustrated inFIG. 3G, a heat spreader is prepared as a first component33, and a first resin film34is placed on a surface of the first component33. The material and the thickness of the first resin film34are not particularly limited. In this example, an epoxy-based thermosetting resin film with the thickness of about 5 μm to 10 μm is used as the first resin film34.

The melting point of the first resin film34is about 170° C., and the thermosetting temperature thereof is about 160° C. to 180° C.

Then, while heating the first resin film34to its melting point (170° C.) by an unillustrated hot press machine, the first resin film34is pressed onto the first component33, thereby attaching the first resin film34to the first component33.

Thereafter, the rubber sheet30described above is placed over the first resin film34, so that the carbon nanotubes26transferred to the rubber sheet30face the first resin film34.

Subsequently, as illustrated inFIG. 3H, while heating the first resin film34by an unillustrated hot press machine, the other end portions26bof the carbon nanotubes26are pressed against the first resin film34, thereby fixing the end portions26bto the first resin film34.

The heating temperature in this step is not particularly limited. In this example, the first resin film34is heated to its melting point of about 170° C. Thus, the first resin film34can be softened sufficiently, making it easy to fix the carbon nanotubes26to the first resin film34.

Thereafter, the first resin film34is naturally cooled to room temperature to be solidified.

Next, as illustrated inFIG. 3I, the rubber sheet30is peeled off from the carbon nanotubes26.

Subsequently, as illustrated inFIG. 3J, a second resin film35is placed on the one end portions26aof the carbon nanotubes26.

The material and thickness of the second resin film35are not particularly limited. In this example, like the first resin film34, an epoxy-based thermosetting resin film with a thickness of about 5 μm to 10 μm is used as the second resin film35.

The melting point and the thermosetting temperature of the second resin film35are the same as those of the first resin film34. That is, the melting point of the second resin film35is about 170° C., and its thermosetting temperature is about 160° C. to 180° C.

Then, while heating the second resin film35to its melting point (170° C.) by an unillustrated hot press machine, the second resin film35is pressed against the one end portions26aof the carbon nanotube26, thereby attaching the second resin film35to the one end portions26a.

First, a wiring substrate41on which a second component43is mounted is prepared, and is positioned so that the second component43faces the first component33.

The second component43is a semiconductor element, such as a CPU, that generates heat during its operation, and is mounted on the wiring substrate41by solder bumps42.

Then, after the first component33and the second component43are aligned, the second resin film35is brought into close contact with the second component43.

Next, as illustrated inFIG. 3L, the wiring substrate41is placed between a first pressing plate51and a second pressing plate52of a hot press machine.

Then, the first resin film34and the second resin film35are heated to their melting point (170° C.) and softened by an illustrated heater embedded in the pressing plates51and52. Simultaneously, the wiring substrate41and the first component33are pressed with the pressing plates51and52, so that the end portions26a,26bof the carbon nanotubes26are pressed against the resin films35and34, respectively.

By performing this step, as illustrated in a dotted-line circle A, the one end portions26aof the carbon nanotubes26penetrate the second resin film35in its thickness direction D1and come into contact with a surface of the second component43. Then, by the capillary force acting from the carbon nanotubes26to the second resin film35, the softened second resin film35climbs up the side surfaces26sof the carbon nanotubes26, thereby forming climb portions35xof the second resin film35.

Similarly, as illustrated in a dotted-line circle B, the other end portions26bof the carbon nanotubes26penetrate the first resin film34in its thickness direction D2and come into contact with a surface of the first component33. Then, the softened first resin film34climbs up the side surfaces26sof the carbon nanotubes26, climb portions34xis formed in the first resin film34.

Then, by maintaining this state for approximately 30 minutes, the softened resin films34and35are thermally cured.

By the steps so far, the basic structure of a heat dissipating sheet55including the carbon nanotubes26and the opposing resin films34and35is completed. The heat dissipating sheet55is fixed to the first component33and the second component43by the adhesive forces of the resin film34and the resin film35, respectively.

Note that the end portions of the first component33, which is a heat spreader, are bonded to the wiring substrate41with an adhesive56in this step.

By these steps, an electronic device57according to the present embodiment is completed.

In the electronic device57, the heat generated in the second component43is transferred to the first component33, which is the heat spreader, via the heat dissipating sheet55, so that the heat dissipation of the second component43is promoted

Especially, the carbon nanotubes26in the heat dissipating sheet55are excellent in their thermal conductivity. Therefore, heat generated in the second component43is quickly transferred through the heat dissipating sheet55, so that the second component43is cooled efficiently.

Furthermore, an intermediate portion26xof the carbon nanotube26, which is a portion of the carbon nanotube26excluding the end portions26aand26b, is uncovered with the resin films34and35and exposed from these resin films34and35. Therefore, the flexibility of the carbon nanotubes26is not reduced due to the resin films34and35. As a result, even when the distance between the first component33and the second component43changes due to heat generated in the second component43, the carbon nanotubes26can flexibly deform to accommodate to that change, allowing the components33and43to be kept connected via the heat dissipating sheet55.

Moreover, the thickness of the second resin film35is sufficiently thinner than the length of the carbon nanotubes26in the present embodiment.

Therefore, the second resin film35softened in the step ofFIG. 3Lescapes along the side surface26sof the carbon nanotube26, so that the second resin film35is unlikely to remain between the one end portion26aof the carbon nanotube26and the second component43.

As a result, the one end portion26aof the carbon nanotube26comes into contact with the second component43, whereby it is prevented that the thermal resistance between the end portion26aand the second component43increases due to the second resin film35.

For the same reason, the other end portions26bof the carbon nanotube26comes into contact with the first component33, whereby it is prevented that the thermal resistance between the end portion26band the first component33increases due to the first resin film34.

The inventor of the present application investigated how much the thermal resistance of the heat dissipating sheet55decreases by excluding the resin films34and35from the end portions26aand26b.

FIG. 5is a sectional view illustrating a method of this investigation.

In this investigation, while making the second component43generate heat, the temperature difference ΔT between the first component33and the second component43was measured.

The higher the thermal conductivity of the heat dissipating sheet55, the smaller the temperature difference ΔT. Therefore, the temperature difference ΔT serves as an index to estimate the thermal conductivity of the heat dissipating sheet55.

Moreover, in this investigation, a plurality of samples with different thicknesses of the resin films34and35were prepared, and the temperature difference ΔT was measured for each sample.

FIG. 6depicts the results of the investigation.

The horizontal axis inFIG. 6represents the thickness of the resin films34and35before the resin films34and35are softened by the step ofFIG. 3L. Note that the first resin film34and the second resin film35have the same thickness in the same sample.

The vertical axis inFIG. 6represents the temperature difference ΔT described above.

As illustrated inFIG. 6, the temperature difference ΔT increases as the thickness of the resin films34and35increases.

This is because when the second resin film35becomes thicker for example, the second resin film35softened in the step ofFIG. 3Lhas less space for escape and is more likely to remain between the one end portions26aof the carbon nanotubes26and the second electronic component43.

Therefore, in order to decrease the thermal difference between the components33and43by lowering the temperature difference ΔT, it is preferable that the resin films34and35are thin as possible.

However, when the resin films34and35are too thin, adhesiveness between the resin films and the components33and43is deteriorated, so that the heat dissipating sheet55may peel off from the components33and43.

Such a problem becomes prominent when the thickness of the resin films34and35before softened in the step ofFIG. 3Lis thinner than 2 μm. Therefore, in order to improve the reliability of the electronic device57by preventing the heat dissipating sheet55from peeling off from the components33and43, it is preferable that the thickness of the resin films34and35before softened in the step ofFIG. 3Lis equal to or thicker than 2 μm.

Moreover, the inventor of the present application prepared samples of the heat dissipating sheet55whose resin films34and35is different in thickness for each sample, and observed the appearance of each sample using a scanning electron microscope (SEM). The observation results are described below.

FIG. 7Ais a diagram drawn based on an SEM image of a heat dissipating sheet55in which the thicknesses of the resin films34and35are 5 μm. Note that the thicknesses of the resin films34and35in this investigation indicate the thicknesses of the resin films34and35before softened in the step ofFIG. 3L. This is also the case for the followingFIGS. 8A to 9B.

As illustrated inFIG. 7A, in this case, although the one end portion26aof the carbon nanotube26is covered with the second resin film35, the portion of the carbon nanotube26locating higher than the one end portion26ais uncovered with the second resin film35and exposed from the second resin film35.

FIG. 7Bis a schematic sectional view of the heat dissipating sheet55in this case.

As illustrated inFIG. 7B, the one end portion26aof the carbon nanotube26is in contact with the second component43, and the other end portion26bis in contact with the first component33.

FIG. 8Ais a diagram drawn based on an SEM image of a heat dissipating sheet55in which the thicknesses of the resin films34and35are 10 μm.

As illustrated inFIG. 8A, in this case as well, the one end portion26aof the carbon nanotube26is covered with the second resin film35, and the portion of the carbon nanotube26locating higher than the one end portion26ais uncovered with the second resin film35and exposed from the second resin film35.

FIG. 8Bis a schematic sectional view of the heat dissipating sheet55in this case.

As illustrated inFIG. 8B, the end portions26band26aof the carbon nanotube26are in contact with the component33and43respectively.

On the other hand,FIG. 9Ais a diagram drawn based on an SEM image of a heat dissipating sheet55in which the thicknesses of the resin films34and35are 20 μm. Then,FIG. 9Bis a schematic sectional view of the heat dissipating sheet55in this case.

As illustrated inFIG. 9A, in this case, the portion of the carbon nanotube26locating higher than the one end portion26ais covered with the second resin film35.

As a result, the elasticity of the carbon nanotube26is deteriorated by the second resin film35, which makes it difficult for the carbon nanotube26to accommodate to the uneven surfaces of the components33and43, which may causes the heat dissipating sheet55to peel off from the components33and43.

Moreover, as illustrated inFIG. 9B, the second resin film35exists thickly on the surface of the second component43in this case. Therefore, the second resin film remains between the end portion26aof the carbon nanotube26and the second component43, which increases the thermal resistance between the heat dissipating sheet55and the second component43.

According to these investigation results, it is preferable to set the thicknesses of the resin films34and35to equal to or thinner than 10 μm, for the purpose of maintaining the elasticity of the carbon nanotubes26, as well as excluding the resin films34and35from the portion between the end portions26aand26band components33and43.

In the meantime, the surface density of the carbon nanotubes26is increased in the step ofFIG. 3Fin the present embodiment.

According to the investigation conducted by the inventor of the present application also revealed that, by increasing the surface density of the carbon nanotubes26in this manner, the thermal conductivity of the heat dissipating sheet55increases approximately 60% than in the case where the surface density of the carbon nanotubes26is not increased.

Moreover, the investigation conducted by the inventor of the present application also revealed that, by increasing the surface density of the carbon nanotubes26, the mechanical strength of the heat dissipating sheet55improved.

FIG. 10Ais a schematic sectional view of a heat dissipating sheet55that is not subjected to the step ofFIG. 3F, and theFIG. 10Bis an SEM image of this heat dissipating sheet55.

As illustrated inFIGS. 10A and 10B, since the density of the carbon nanotubes26is low in this case, the carbon nanotubes26buckle under the pressure from the pressing plates51and52in the step ofFIG. 3L. As a result, the softened second resin film35cannot climb up the side surface of the carbon nanotube26, so that the second resin film35remains between the end portion26aof the carbon nanotube26and the second component43.

On the other hand,FIG. 11Ais a schematic sectional view of a heat dissipating sheet55that is subjected to the step ofFIG. 3Faccording to the present embodiment, and theFIG. 11Bis an SEM image of this heat dissipating sheet55.

As illustrated inFIGS. 11A and 11B, since the density of the carbon nanotubes26is increased in this case, mechanical strength of the heat dissipating sheet55is increased to the extent that the carbon nanotubes26does not buckle under the pressure from the pressing plates51and52in the step ofFIG. 3L.

Moreover, it was also revealed that by preventing the carbon nanotubes26from buckling, the softened second resin film35can easily climb up the side surface of the carbon nanotube26, so that the second resin film35is unlikely to remain between the end portions26aand the second component43.

Although, the present embodiment is described in detail, the present embodiment is not limited to the above.

For example, although the heat spreader is used as the first component33and CPU is used as the second component43, any components located in a heat dissipation path may be used as the component33and34, and these component may be connected each other via the heat dissipating sheet55.