Patent Publication Number: US-10770370-B2

Title: Electronic device and heat dissipating sheet

Description:
CROSS-REFERENCE TO RELATED APPLICATION(s) 
     This application is a divisional of application Ser. No. 15/660,124, filed Jul. 26, 2017, which 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. 2005-150362; 
     Japanese Laid-open Patent Application No. 2006-147801; 
     Japanese Laid-open Patent Application No. 2006-303240; 
     Japanese Laid-open Patent Application No. 2010-118609; 
     Japanese Laid-open Patent Application No. 2010-267706; 
     Japanese Laid-open Patent Application No. 2012-199335; 
     Japanese Laid-open Patent Application No. 2010-199367; 
     Japanese Laid-open Patent Application No. 2013-115094; and 
     Japanese Laid-open Patent Application No. 2014-60252. 
     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. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1H  are sectional views of an electronic device in the course of manufacturing thereof, the device being used for a study; 
         FIG. 2A  is a diagram drawn based on a photograph of a heat dissipating sheet taken after the surface density of carbon nanotubes is increased; 
         FIG. 2B  is a diagram drawn based on a photograph of the heat dissipating sheet taken after the surface density of the carbon nanotubes is increased and then the heat dissipating sheet is impregnated with resin; 
         FIGS. 3A to 3L  are sectional views of an electronic device in the course of manufacturing thereof according to the present embodiment; 
         FIG. 4  is a plan view of a rubber sheet stretched in the present embodiment; 
         FIG. 5  is a sectional view illustrating how the thermal resistance of a heat dissipating sheet according to the present embodiment was investigated; 
         FIG. 6  is a diagram depicting results of an investigation on the relation between the thickness of resin films and the temperature difference in the heat dissipating sheet in the present embodiment; 
         FIG. 7A  is a diagram drawn based on a scanning electron microscopic (SEM) image of a heat dissipating sheet according to the present embodiment having resin films whose thicknesses are 5 μm; 
         FIG. 7B  is a schematic sectional view of the heat dissipating sheet according to the present embodiment having resin films whose thicknesses are 5 μm; 
         FIG. 8A  is a diagram drawn based on an SEM image of a heat dissipating sheet according to the present embodiment having resin films whose thicknesses are 10 μm; 
         FIG. 8B  is a schematic sectional view of the heat dissipating sheet according to the present embodiment having resin films whose thicknesses are 10 μm; 
         FIG. 9A  is a diagram drawn based on an SEM image of a heat dissipating sheet according to the present embodiment having resin films whose thicknesses are 20 μm; 
         FIG. 9B  is a schematic sectional view of the heat dissipating sheet according to the present embodiment having resin films whose thicknesses are 20 μm; 
         FIG. 10A  is a schematic sectional view of a heat dissipating sheet that is not subjected to the step of  FIG. 3F ; 
         FIG. 10B  is an SEM image of the heat dissipating sheet that is not subjected to the step of  FIG. 3F ; 
         FIG. 11A  is a schematic sectional view of a heat dissipating sheet that is subjected to the step of  FIG. 3F ; and 
         FIG. 11B  is an SEM image of the heat dissipating sheet that is subjected to the step of  FIG. 3F . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Prior to describing the present embodiment, matters studied by the inventor of the present application are described. 
       FIGS. 1A to 1H  are 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 in  FIG. 1A , a plurality of carbon nanotubes  2  are grown on a silicon substrate  1  by the hot-filament chemical vapor deposition (CVD) method, thereby obtaining a heat dissipating sheet  3  provided with a plurality of carbon nanotubes  2 . 
     Next, as illustrated in  FIG. 1B , a rubber sheet  4  stretched from its natural length is prepared. Examples of the material for the rubber sheet  4  include silicone rubber, natural rubber, and synthetic rubber. 
     Then, the rubber sheet  4  is pressed against the heat dissipating sheet  3 , so that the rubber sheet  4  is fixed to the rubber sheet  4  by its adhesive force. 
     Next, as illustrated in  FIG. 1C , while keeping the rubber sheet  4  to be stretched, the heat dissipating sheet  3  is peeled off from the substrate  1 . 
     Thereafter, as illustrated in  FIG. 1D , the stretched rubber sheet  4  is returned to its natural length. Thus, the interval of the carbon nanotubes  2  is narrowed by the contraction of the rubber sheet  4 , so that the surface density of the carbon nanotubes  2  in the heat dissipating sheet  3  increases, which in turn increases the thermal conductivity of the heat dissipating sheet  3 . 
     Next, as illustrated in  FIG. 1E , a heat spreader  6  made of metal excellent in thermal conductivity such as copper is prepared, and resin  7  is fixed to a surface of the heat spreader  6 . 
     In this example, a thermoplastic resin is used as the resin  7 , and its thickness T is set to substantially the same length L of the carbon nanotubes  2 . 
     Then, while heating the resin  7  to be softened by an unillustrated hot press machine, the heat dissipating sheet  3  is compressively attached to the resin  7 . Thereafter, the resin  7  is cooled naturally to be solidified. 
     Subsequently, as illustrated in  FIG. 1F , the rubber sheet  4  is peeled off from the heat dissipating sheet  3 . 
     Next, as illustrated in  FIG. 1G , while pressing a pressing plate  8  of a hot press machine against the heat dissipating sheet  3 , the resin  7  is heated to be softened by a heater provided in the pressing plate  8 , thereby embedding the carbon nanotubes  2  into the softened resin  7 . 
     At this time, since the thickness T of the resin  7  is set to the substantially same length L of the carbon nanotubes  2  in this example as described above, the carbon nanotubes  2  can be embedded into the resin  7  over their entire length. 
     However, once the pressing plate  8  comes into contact with the resin  7 , the resin  7  has nowhere to escape except in the substrate&#39;s lateral direction. Thus, there is no sufficient space for the resin  7  to escape, so that the resin  7  remains under end portions  2   a  of the carbon nanotubes  2  and is interposed between the end portions  2   a  and the heat spreader  6 . 
     Thereafter, as illustrated in  FIG. 1H , an electronic component  8  such as a CPU is placed on the resin  7 . Then, while heating the resin  7  to be softened by an unillustrated hot press machine, the electronic component  8  is compressively attached to the heat dissipating sheet  3 . 
     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 sheet  3  can be fixed to both the heat spreader  6  and the electronic component  8  by the adhesive force of the resin  7 . 
     Moreover, since the thickness of the resin  7  is set to the substantially the same length of the carbon nanotubes  2 , the resin  7  can prevent the carbon nanotubes  2  from being crushed by the force generated in the step of  FIG. 1H  when the electronic component  8  is compressively attached to the heat dissipating sheet  3 . 
     However, when the resin  7  is made thick as the same length of the carbon nanotubes  2 , the resin  7  cannot escape except in the substrate&#39;s lateral direction when the carbon nanotubes  2  are embedded into the resin  7  in the step of  FIG. 1G . 
     As a result, the resin  7  remains between the end portions  2   a  of the carbon nanotubes  2  and the heat spreader  6 , and hence the end portion  2   a  cannot be brought into contact with the heat spreader  6 . In such a state, heat in the electronic component  8  cannot be transferred directly to the heat spreader  6  via the carbon nanotubes  2 . Thus, the thermal resistance between the heat spreader  6  and the electronic component  8  increases, which cannot exploit the beneficial high conductivity of the carbon nanotubes  2 . 
     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 2B  are schematic plan views illustrating the problem. 
       FIG. 2A  is a diagram drawn based on a photograph of the heat dissipating sheet  3  taken after the surface density of the carbon nanotubes  2  is increased by the step of  FIG. 1D . Note that the heat dissipating sheet  3  is not impregnated with the resin  7  in this state. 
     As illustrated in  FIG. 2A , the heat dissipating sheet  3  appears normal in this state. 
     On the other hand,  FIG. 2B  is a diagram drawn based on a photograph of the heat dissipating sheet  3  taken after the surface density of the carbon nanotubes  2  is increased by the step of  FIG. 1D  and then the heat dissipating sheet  3  is impregnated with the resin  7  by the step of  FIG. 1G . 
     As illustrated in  FIG. 2B , it was found that the heat dissipating sheet  3  cracks when the heat dissipating sheet  3  was impregnated with the resin  7  after increasing the surface density of the carbon nanotubes  2 . 
     This is conceivably because when the resin  7  is impregnated into spaces between the carbon nanotubes  2  whose surface density is increased, the resin  7  exerts force against the carbon nanotubes  2  to widen the intervals between the carbon nanotubes  2 . 
     When the heat dissipating sheet  3  is cracked in this manner, the thermal conductivity of the heat dissipating sheet  3  significantly decreases, and the heat dissipating sheet  3  cannot be put into practical use. 
     In the followings, embodiment capable of decreasing thermal resistance is described. 
     Embodiment 
       FIGS. 3A to 3L  are sectional views of an electronic device in the course of manufacturing thereof according to the present embodiment. 
     First, as illustrated in  FIG. 3A , a silicon substrate is prepared as a substrate  21 . Then, the surface of the substrate  21  is thermally oxidized to form a silicon oxide film to a thickness of about 300 nm as an underlying film  22 . 
     The material for the substrate  21  is not limited to silicon. A substrate made of any one of aluminum oxides, magnesium oxides, glass, and stainless steel may be used as the substrate  21 . Moreover, instead of the rigid substrate  21 , a metallic foil such as a stainless steel foil and an aluminum foil may be used. 
     Next, as illustrated in  FIG. 3B , an aluminum film is formed on the underlying film  22  by sputtering method to a thickness of about 10 nm, and the aluminum film is used as an underlying metal film  23 . 
     Other than aluminum, the material for the underlying metal film  23  includes 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 film  23 . 
     Next, an iron film is formed on the underlying metal film  23  by sputtering method to a thickness of about 2.5 nm as a catalytic metal film  24 , 
     The material for the catalytic metal film  24  is not limited to iron. The catalytic metal film  24  may be famed of any one of iron, cobalt, nickel, gold, silver, platinum, and alloys of these materials. 
     Furthermore, instead of the catalytic metal film  24 , metallic fine particles of the same material as the catalytic metal film  24  may be attached onto the underlying metal film  23 . 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 film  23 . 
     Subsequently, as illustrated in  FIG. 3C , by using the catalytic action of the catalytic metal film  24 , a plurality of carbon nanotubes  26  of about 100 μm to 500 μm long are grown by hot-filament CVD method. These carbon nanotubes  26  are grown straight along a normal direction n of the substrate  21  by the action of the underlying film  22 . 
     Conditions for growing the carbon nanotubes  26  are 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 nanotubes  26  grow at a speed of about 4 μm/min. 
     Note that the underlying metal film  23  and the catalytic metal film  24  are condensed into metal particles  25  when the source gas is introduced into the growth chamber, and the carbon nanotubes  26  grow only on these metal particles  25 . 
     Under the growth conditions in this example, the surface density of the carbon nanotubes  26  is approximately 1×10 11  nanotubes/cm 2 , with the diameter of each carbon nanotube  26  being 4 nm to 8 nm, approximately 6 nm in average. 
     In each carbon nanotube  26 , about three to six graphene sheets are stacked from the center axis of the carbon nanotube  26  to 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 nanotubes  26  is 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 nanotubes  26 , the diameter of the carbon nanotubes  26 , and the graphene sheet of the carbon nanotubes  26  may be approximately 50 μm to 5000 μm, approximately 1×10 10  nanotubes/cm 2  to 1×10 13  nanotubes/cm 2 , approximately 2 nm to 50 nm, and about one to one hundred fifty, respectively. 
     Next, as illustrated in  FIG. 3D , a rubber sheet  30  stretched from its natural length is placed on one end portions  26   a  of the carbon nanotubes  26 . 
     Then, a surface  30   a  of the rubber sheet  30  is pressed against the one end portions  26   a  of the carbon nanotubes  26  with a roller  31 , so that the end portions  26   a  are fixed to the surface  30   a  by the adhesive force of the rubber sheet  30 . 
     The material for the rubber sheet  30  is not particularly limited, and may be any rubber sufficiently adhesive to allow the carbon nanotubes  26  to be fixed thereto in this step. Examples of such a material include silicone rubber, natural rubber, and synthetic rubber. 
       FIG. 4  is a plan view of the rubber sheet  30  stretched in this step. 
     As illustrated in  FIG. 4 , the rim of the rubber sheet  30  is gripped by a plurality of clips  32 , and is stretched radially by these clips  32 . 
     Subsequently, as illustrated in  FIG. 3E , the carbon nanotubes  26  are peeled off from the substrate  21 , and the carbon nanotubes  26  are transferred to the rubber sheet  30 . 
     Then, as illustrated in  FIG. 3F , the stretched rubber sheet  30  is relaxed by its elastic force. Thus, the intervals between the carbon nanotubes  26  are narrowed by the contraction of the rubber sheet  30 , thereby increasing the surface density of the carbon nanotubes  26 . 
     The contraction amount of the rubber sheet  30  is not particularly limited. In this example, the rubber sheet  30  is contracted in a manner that the area of the rubber sheet  30  shrinks to about one third of that before this step, thereby increasing the surface density of the carbon nanotubes  26  as three times higher than that before this step. 
     Next, as illustrated in  FIG. 3G , a heat spreader is prepared as a first component  33 , and a first resin film  34  is placed on a surface of the first component  33 . The material and the thickness of the first resin film  34  are 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 film  34 . 
     The melting point of the first resin film  34  is about 170° C., and the thermosetting temperature thereof is about 160° C. to 180° C. 
     Then, while heating the first resin film  34  to its melting point (170° C.) by an unillustrated hot press machine, the first resin film  34  is pressed onto the first component  33 , thereby attaching the first resin film  34  to the first component  33 . 
     Thereafter, the rubber sheet  30  described above is placed over the first resin film  34 , so that the carbon nanotubes  26  transferred to the rubber sheet  30  face the first resin film  34 . 
     Subsequently, as illustrated in  FIG. 3H , while heating the first resin film  34  by an unillustrated hot press machine, the other end portions  26   b  of the carbon nanotubes  26  are pressed against the first resin film  34 , thereby fixing the end portions  26   b  to the first resin film  34 . 
     The heating temperature in this step is not particularly limited. In this example, the first resin film  34  is heated to its melting point of about 170° C. Thus, the first resin film  34  can be softened sufficiently, making it easy to fix the carbon nanotubes  26  to the first resin film  34 . 
     Thereafter, the first resin film  34  is naturally cooled to room temperature to be solidified. 
     Next, as illustrated in  FIG. 3I , the rubber sheet  30  is peeled off from the carbon nanotubes  26 . 
     Subsequently, as illustrated in  FIG. 3J , a second resin film  35  is placed on the one end portions  26   a  of the carbon nanotubes  26 . 
     The material and thickness of the second resin film  35  are not particularly limited. In this example, like the first resin film  34 , an epoxy-based thermosetting resin film with a thickness of about 5 μm to 10 μm is used as the second resin film  35 . 
     The melting point and the thermosetting temperature of the second resin film  35  are the same as those of the first resin film  34 . That is, the melting point of the second resin film  35  is about 170° C., and its thermosetting temperature is about 160° C. to 180° C. 
     Then, while heating the second resin film  35  to its melting point (170° C.) by an unillustrated hot press machine, the second resin film  35  is pressed against the one end portions  26   a  of the carbon nanotube  26 , thereby attaching the second resin film  35  to the one end portions  26   a.    
     Next, the step illustrated in  FIG. 3K  is described. 
     First, a wiring substrate  41  on which a second component  43  is mounted is prepared, and is positioned so that the second component  43  faces the first component  33 . 
     The second component  43  is a semiconductor element, such as a CPU, that generates heat during its operation, and is mounted on the wiring substrate  41  by solder bumps  42 . 
     Then, after the first component  33  and the second component  43  are aligned, the second resin film  35  is brought into close contact with the second component  43 . 
     Next, as illustrated in  FIG. 3L , the wiring substrate  41  is placed between a first pressing plate  51  and a second pressing plate  52  of a hot press machine. 
     Then, the first resin film  34  and the second resin film  35  are heated to their melting point (170° C.) and softened by an illustrated heater embedded in the pressing plates  51  and  52 . Simultaneously, the wiring substrate  41  and the first component  33  are pressed with the pressing plates  51  and  52 , so that the end portions  26   a,    26   b  of the carbon nanotubes  26  are pressed against the resin films  35  and  34 , respectively. 
     By performing this step, as illustrated in a dotted-line circle A, the one end portions  26   a  of the carbon nanotubes  26  penetrate the second resin film  35  in its thickness direction D 1  and come into contact with a surface of the second component  43 . Then, by the capillary force acting from the carbon nanotubes  26  to the second resin film  35 , the softened second resin film  35  climbs up the side surfaces  26   s  of the carbon nanotubes  26 , thereby forming climb portions  35   x  of the second resin film  35 . 
     Similarly, as illustrated in a dotted-line circle B, the other end portions  26   b  of the carbon nanotubes  26  penetrate the first resin film  34  in its thickness direction D 2  and come into contact with a surface of the first component  33 . Then, the softened first resin film  34  climbs up the side surfaces  26   s  of the carbon nanotubes  26 , climb portions  34   x  is foamed in the first resin film  34 . 
     Then, by maintaining this state for approximately 30 minutes, the softened resin films  34  and  35  are thermally cured. 
     By the steps so far, the basic structure of a heat dissipating sheet  55  including the carbon nanotubes  26  and the opposing resin films  34  and  35  is completed. The heat dissipating sheet  55  is fixed to the first component  33  and the second component  43  by the adhesive forces of the resin film  34  and the resin film  35 , respectively. 
     Note that the end portions of the first component  33 , which is a heat spreader, are bonded to the wiring substrate  41  with an adhesive  56  in this step. 
     By these steps, an electronic device  57  according to the present embodiment is completed. 
     In the electronic device  57 , the heat generated in the second component  43  is transferred to the first component  33 , which is the heat spreader, via the heat dissipating sheet  55 , so that the heat dissipation of the second component  43  is promoted 
     Especially, the carbon nanotubes  26  in the heat dissipating sheet  55  are excellent in their thermal conductivity. Therefore, heat generated in the second component  43  is quickly transferred through the heat dissipating sheet  55 , so that the second component  43  is cooled efficiently. 
     Furthermore, an intermediate portion  26   x  of the carbon nanotube  26 , which is a portion of the carbon nanotube  26  excluding the end portions  26   a  and  26   b,  is uncovered with the resin films  34  and  35  and exposed from these resin films  34  and  35 . Therefore, the flexibility of the carbon nanotubes  26  is not reduced due to the resin films  34  and  35 . As a result, even when the distance between the first component  33  and the second component  43  changes due to heat generated in the second component  43 , the carbon nanotubes  26  can flexibly deform to accommodate to that change, allowing the components  33  and  43  to be kept connected via the heat dissipating sheet  55 . 
     Moreover, the thickness of the second resin film  35  is sufficiently thinner than the length of the carbon nanotubes  26  in the present embodiment. 
     Therefore, the second resin film  35  softened in the step of  FIG. 3L  escapes along the side surface  26   s  of the carbon nanotube  26 , so that the second resin film  35  is unlikely to remain between the one end portion  26   a  of the carbon nanotube  26  and the second component  43 . 
     As a result, the one end portion  26   a  of the carbon nanotube  26  comes into contact with the second component  43 , whereby it is prevented that the thermal resistance between the end portion  26   a  and the second component  43  increases due to the second resin film  35 . 
     For the same reason, the other end portions  26   b  of the carbon nanotube  26  comes into contact with the first component  33 , whereby it is prevented that the thermal resistance between the end portion  26   b  and the first component  33  increases due to the first resin film  34 . 
     The inventor of the present application investigated how much the thermal resistance of the heat dissipating sheet  55  decreases by excluding the resin films  34  and  35  from the end portions  26   a  and  26   b.    
       FIG. 5  is a sectional view illustrating a method of this investigation. 
     In this investigation, while making the second component  43  generate heat, the temperature difference ΔT between the first component  33  and the second component  43  was measured. 
     The higher the thermal conductivity of the heat dissipating sheet  55 , the smaller the temperature difference ΔT. Therefore, the temperature difference ΔT serves as an index to estimate the thermal conductivity of the heat dissipating sheet  55 . 
     Moreover, in this investigation, a plurality of samples with different thicknesses of the resin films  34  and  35  were prepared, and the temperature difference ΔT was measured for each sample. 
       FIG. 6  depicts the results of the investigation. 
     The horizontal axis in  FIG. 6  represents the thickness of the resin films  34  and  35  before the resin films  34  and  35  are softened by the step of  FIG. 3L . Note that the first resin film  34  and the second resin film  35  have the same thickness in the same sample. 
     The vertical axis in  FIG. 6  represents the temperature difference ΔT described above. 
     As illustrated in  FIG. 6 , the temperature difference ΔT increases as the thickness of the resin films  34  and  35  increases. 
     This is because when the second resin film  35  becomes thicker for example, the second resin film  35  softened in the step of  FIG. 3L  has less space for escape and is more likely to remain between the one end portions  26   a  of the carbon nanotubes  26  and the second electronic component  43 . 
     Therefore, in order to decrease the thermal difference between the components  33  and  43  by lowering the temperature difference ΔT, it is preferable that the resin films  34  and  35  are thin as possible. 
     However, when the resin films  34  and  35  are too thin, adhesiveness between the resin films and the components  33  and  43  is deteriorated, so that the heat dissipating sheet  55  may peel off from the components  33  and  43 . 
     Such a problem becomes prominent when the thickness of the resin films  34  and  35  before softened in the step of  FIG. 3L  is thinner than 2 μm. Therefore, in order to improve the reliability of the electronic device  57  by preventing the heat dissipating sheet  55  from peeling off from the components  33  and  43 , it is preferable that the thickness of the resin films  34  and  35  before softened in the step of  FIG. 3L  is equal to or thicker than 2 μm. 
     Moreover, the inventor of the present application prepared samples of the heat dissipating sheet  55  whose resin films  34  and  35  is 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. 7A  is a diagram drawn based on an SEM image of a heat dissipating sheet  55  in which the thicknesses of the resin films  34  and  35  are 5 μm. Note that the thicknesses of the resin films  34  and  35  in this investigation indicate the thicknesses of the resin films  34  and  35  before softened in the step of  FIG. 3L . This is also the case for the following  FIGS. 8A to 9B . 
     As illustrated in  FIG. 7A , in this case, although the one end portion  26   a  of the carbon nanotube  26  is covered with the second resin film  35 , the portion of the carbon nanotube  26  locating higher than the one end portion  26   a  is uncovered with the second resin film  35  and exposed from the second resin film  35 . 
       FIG. 7B  is a schematic sectional view of the heat dissipating sheet  55  in this case. 
     As illustrated in  FIG. 7B , the one end portion  26   a  of the carbon nanotube  26  is in contact with the second component  43 , and the other end portion  26   b  is in contact with the first component  33 . 
       FIG. 8A  is a diagram drawn based on an SEM image of a heat dissipating sheet  55  in which the thicknesses of the resin films  34  and  35  are 10 μm. 
     As illustrated in  FIG. 8A , in this case as well, the one end portion  26   a  of the carbon nanotube  26  is covered with the second resin film  35 , and the portion of the carbon nanotube  26  locating higher than the one end portion  26   a  is uncovered with the second resin film  35  and exposed from the second resin film  35 . 
       FIG. 8B  is a schematic sectional view of the heat dissipating sheet  55  in this case. 
     As illustrated in  FIG. 8B , the end portions  26   b  and  26   a  of the carbon nanotube  26  are in contact with the component  33  and  43  respectively. 
     On the other hand,  FIG. 9A  is a diagram drawn based on an SEM image of a heat dissipating sheet  55  in which the thicknesses of the resin films  34  and  35  are 20 μm. Then,  FIG. 9B  is a schematic sectional view of the heat dissipating sheet  55  in this case. 
     As illustrated in  FIG. 9A , in this case, the portion of the carbon nanotube  26  locating higher than the one end portion  26   a  is covered with the second resin film  35 . 
     As a result, the elasticity of the carbon nanotube  26  is deteriorated by the second resin film  35 , which makes it difficult for the carbon nanotube  26  to accommodate to the uneven surfaces of the components  33  and  43 , which may causes the heat dissipating sheet  55  to peel off from the components  33  and  43 . 
     Moreover, as illustrated in  FIG. 9B , the second resin film  35  exists thickly on the surface of the second component  43  in this case. Therefore, the second resin film  35  remains between the end portion  26   a  of the carbon nanotube  26  and the second component  43 , which increases the thermal resistance between the heat dissipating sheet  55  and the second component  43 . 
     According to these investigation results, it is preferable to set the thicknesses of the resin films  34  and  35  to equal to or thinner than 10 μm, for the purpose of maintaining the elasticity of the carbon nanotubes  26 , as well as excluding the resin films  34  and  35  from the portion between the end portions  26   a  and  26   b  and components  33  and  43 . 
     In the meantime, the surface density of the carbon nanotubes  26  is increased in the step of  FIG. 3F  in 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 nanotubes  26  in this manner, the thermal conductivity of the heat dissipating sheet  55  increases approximately 60% than in the case where the surface density of the carbon nanotubes  26  is not increased. 
     Moreover, the investigation conducted by the inventor of the present application also revealed that, by increasing the surface density of the carbon nanotubes  26 , the mechanical strength of the heat dissipating sheet  55  improved. 
       FIGS. 10A to 11B  are diagrams illustrating the investigation results. 
       FIG. 10A  is a schematic sectional view of a heat dissipating sheet  55  that is not subjected to the step of  FIG. 3F , and the  FIG. 10B  is an SEM image of this heat dissipating sheet  55 . 
     As illustrated in  FIGS. 10A and 10B , since the density of the carbon nanotubes  26  is low in this case, the carbon nanotubes  26  buckle under the pressure from the pressing plates  51  and  52  in the step of  FIG. 3L . As a result, the softened second resin film  35  cannot climb up the side surface of the carbon nanotube  26 , so that the second resin film  35  remains between the end portion  26   a  of the carbon nanotube  26  and the second component  43 . 
     On the other hand,  FIG. 11A  is a schematic sectional view of a heat dissipating sheet  55  that is subjected to the step of  FIG. 3F  according to the present embodiment, and the  FIG. 11B  is an SEM image of this heat dissipating sheet  55 . 
     As illustrated in  FIGS. 11A and 11B , since the density of the carbon nanotubes  26  is increased in this case, mechanical strength of the heat dissipating sheet  55  is increased to the extent that the carbon nanotubes  26  does not buckle under the pressure from the pressing plates  51  and  52  in the step of  FIG. 3L . 
     Moreover, it was also revealed that by preventing the carbon nanotubes  26  from buckling, the softened second resin film  35  can easily climb up the side surface of the carbon nanotube  26 , so that the second resin film  35  is unlikely to remain between the end portions  26   a  and the second component  43 . 
     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 component  33  and CPU is used as the second component  43 , any components located in a heat dissipation path may be used as the component  33  and  34 , and these component may be connected each other via the heat dissipating sheet  55 . 
     All examples and conditional language recited herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.