Patent Publication Number: US-7903422-B2

Title: Mobile terminal device and method for radiating heat therefrom

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a divisional application of U.S. Ser. No. 11/940,758 filed Nov. 15, 2007 and issued as U.S. Pat. No. 7,616,446 which is a divisional of U.S. Ser. No. 11/304,179 filed on Dec. 15, 2005 and issued as U.S. Pat. No. 7,330,354 on Feb. 12, 2008, the contents of which is fully incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates to a mobile terminal device and a method for radiating heat therefrom, more particularly, to a structure of a circuit board in a mobile terminal device, on which heat-generating components are mounted, and to a method for radiating heat using the circuit board. 
     2. Description of the Related Art 
     Mobile terminal devices as typified by mobile phones are provided with various functions, such as a video phone function, a photographing function and a TV broadcast receiving function, in addition to a voice communications function and a mail sending/receiving function. For the purpose of realizing these functions, efforts have been made to improve the processing capacities of mobile terminal devices. As mobile terminal devices are made high in function and performance, the power consumed in the electronic components mounted inside the mobile terminal devices increases. As a result, the temperature rises in the mobile terminal devices have lead to problems. 
     For the purpose of preventing the temperature rises described above, countermeasures are taken in electronic apparatuses, such as personal computers. For example, heat-radiating heatsinks made of metal, fans, etc., are installed on the surfaces of electronic components that generate much heat so that heat can be radiated from the inside of the housing to the outside. In addition, Japanese Published Unexamined Patent Application No. Hei 11-95871 discloses a heat-radiating structure of an electronic apparatus, wherein heat conduction films are provided on ribs that are used to raise the rigidity of the housing of a mobile personal computer, and heat radiation plates bonded to heat generating components are made so as to contact with the heat conduction films of the ribs so that the heat generated from the heat generating components is dispersed efficiently. 
     However, since mobile terminal devices of recent years have been requested to be reduced further in size and thickness, it is difficult to provide such heat-radiating structures as used in personal computers because of space limitations. Hence, in a mobile terminal device, it is necessary to transfer the heat generated in electronic components to the housing and to radiate the heat from the housing to the outside. However, the surface area of the housing of the mobile terminal device is small and does not have sufficient heat-radiating efficiency. Hence, if the amount of heat generated in the electronic components is large, the temperature of the housing eventually rises. 
     In addition, mobile terminal devices are also required to be reduced in thickness, the clearance between the electronic components and the housing cannot be increased, and the heat generated in the electronic components is transferred immediately to the surface of the housing. As a result, a problem of local temperature rising at the surface of the housing result. 
     In particular, if the temperature of the operation member, such as the keys that the user operates, and the temperature of the housing in the vicinity thereof are raised, discomfort is given to the user during operation. For this reason, it is desired to propose a mobile terminal device capable of effectively radiating the heat generated in electronic components. 
     SUMMARY OF THE INVENTION 
     The present invention is intended to provide a mobile terminal device capable of suppressing local temperature rising due to the heat generated in electronic components disposed inside and to provide a method for radiating the heat from the device. 
     A mobile terminal device according to the present invention comprises: 
     a circuit board, 
     heat-generating components mounted on the circuit board, and 
     at least one first heat conduction layer provided inside the circuit board, in which a member having a heat conductivity higher than that of the main constituent material of the circuit board is arranged in the direction of the face of the circuit board, wherein 
     the heat generated in the heat-generating components is dispersed to the whole face of the circuit board via the heat conduction layer. 
     The present invention can have a configuration wherein the heat-generating components are disposed on one face of the circuit board. 
     In addition, the present invention can have a configuration further comprising a operation member on the face other than the face of the circuit board on which the heat-generating components are mounted, wherein at least one heat conduction suppressing layer having a heat conductivity lower than that of the main constituent material of the circuit board is provided inside the circuit board and between the first heat conduction layer and the operation member. 
     Furthermore, the present invention can have a configuration wherein a connection member formed of a heat conduction member having a heat conductivity higher than that of the main constituent material of the circuit board and making contact with both of the first heat conduction layer and the heat-generating components is provided between the first heat conduction layer and the heat-generating components, and the first heat conduction layer is thermally connected to the heat-generating components by the connection member. 
     Furthermore, the present invention can have a configuration wherein the heat-generating component is provided with a heat-radiating terminal that makes contact with the first heat conduction layer in a state that the heat-generating component is mounted, and the heat conduction layer is thermally connected to the heat-generating component by the heat-radiating terminal. The heat-radiating terminal is a terminal not contributing to the electrical connection of the heat-generating component and is preferably connected to the housing of the heat-generating component, or a circuit board or a component inside the heat-generating component. The heat-radiating terminal can be formed of a solder ball constituting BGA (Ball Grid Array). 
     Furthermore, the present invention can have a configuration wherein the first heat conduction layer is formed on the whole face or part of the circuit board excluding the areas on which connection wires for connecting the front and back faces of the circuit board are formed as viewed in the normal direction of the circuit board. 
     Furthermore, the present invention can also have a configuration wherein non-heat-resistant components are mounted on the circuit board, and the first heat conduction layer is formed on the whole face or part of the circuit board excluding the areas on which the non-heat-resistant components are mounted as viewed in the normal direction of the circuit board. 
     Furthermore, the present invention can also have a configuration wherein the circuit board is fixed by protruding portions provided beforehand on the housing of the mobile terminal device, and the members of the circuit board between the first heat conduction layer and the protruding portions are removed in at least part of the areas corresponding to the protruding portions, and the heat conduction layer is thermally connected to at least part of the protruding portions. 
     Furthermore, the present invention can also have a configuration wherein another circuit board is fixed to the circuit board, at least one second heat conduction layer in which a member having a heat conductivity higher than that of the main constituent material of the other circuit board is arranged in the direction of the face of the other circuit board is provided inside or on the surface of the other circuit board, the first heat conduction layer of the circuit board is thermally connected to the second heat conduction layer of the other circuit board by the connection member, and the heat generated in the heat-generating components mounted on the circuit board is transferred to the other circuit board via the connection member. 
     Furthermore, the present invention can also have a configuration wherein the circuit board held in a first housing, an other circuit board held in a second housing and a connection circuit board for connecting the circuit board to the other circuit board are provided, at least one second heat conduction layer in which a member having a heat conductivity higher than that of the main constituent material of the other circuit board is provided in the direction of the face of the other circuit board is arranged inside or on the surface of the other circuit board, at least one third heat conduction layer in which a member having a heat conductivity higher than that of the main constituent material of the connection circuit board is arranged in the direction of the face of the connection circuit board is provided inside or on the surface of the connection circuit board, the first heat conduction layer of the circuit board is thermally connected to the third heat conduction layer of the connection circuit board by the connection member, and the third heat conduction layer of the connection circuit board is thermally connected to the second heat conduction layer of the other circuit board by the connection member, and the heat generated in the heat-generating components mounted on the circuit board is transferred to the other circuit board via the connection member and the connection circuit board. 
     Furthermore, in the present invention, the mobile terminal device is preferably a mobile phone, a mobile computer or a mobile game machine. 
     Furthermore, a heat radiating method according to the present invention is a heat radiating method for a mobile terminal device equipped with a circuit board on which at least one heat-generating component is mounted, wherein at least one first heat conduction layer in which a member having a heat conductivity higher than that of the main constituent material of the circuit board is arranged in the direction of the face of the circuit board is provided inside the circuit board, and the heat generated in the heat-generating component is dispersed in the direction of the face of the circuit board by the heat conduction layer. 
     The present invention further comprising the operation member on the face other than the face of the circuit board on which the heat-generating components are mounted can have a configuration wherein at least one heat conduction suppressing layer having a heat conductivity lower than that of the main constituent material of the circuit board is arranged inside the circuit board and between the first heat conduction layer and the operation member, and the transfer of the heat generated in the heat-generating components to the operation member is suppressed by the heat conduction suppressing layer. 
     Furthermore, the present invention can also have a configuration wherein the first heat conduction layer is thermally connected to the heat-generating components by the connection member formed of a member having a heat conductivity higher than that of the main constituent material of the circuit board, and the heat generated in the heat-generating components is transferred to the first heat conduction layer via the connection member. 
     Furthermore, the present invention can also have a configuration wherein protruding portions for fixing the circuit board are provided on the housing of the mobile terminal device, the members of the circuit board between the first heat conduction layer and the protruding portions are removed in at least part of the areas corresponding to the protruding portions so that the first heat conduction layer is thermally connected to the protruding portions, and the heat generated in the heat-generating components is transferred to the housing via the heat conduction layer and the protruding portions. 
     Furthermore, the present invention can also have a configuration wherein at least one second heat conduction layer in which a member having a heat conductivity higher than that of the main constituent material of the other circuit board is provided in the direction of the face of the other circuit board is arranged inside or on the surface of the other circuit board fixed to the circuit board, the first heat conduction layer of the circuit board is thermally connected to the second heat conduction layer of the other circuit board by the connection member, and the heat generated in the heat-generating components mounted on the circuit board is transferred to the other circuit board via the connection member. 
     Furthermore, the present invention comprising the circuit board held in the first housing, the other circuit board held in the second housing and the connection circuit board for connecting the circuit board to the other circuit board can also have a configuration wherein at least one second heat conduction layer in which a member having a heat conductivity higher than that of the main constituent material of the other circuit board is arranged in the direction of the face of the other circuit board is provided inside or on the surface of the other circuit board, at least one third heat conduction layer in which a member having a heat conductivity higher than that of the main constituent material of the connection circuit board is arranged in the direction of the face of the connection circuit board is provided inside or on the surface of the connection circuit board, the first heat conduction layer of the circuit board is thermally connected to the third heat conduction layer of the connection circuit board by the connection member, and the third heat conduction layer of the connection circuit board is thermally connected to the second heat conduction layer of the other circuit board by the connection member, and the heat generated in the heat-generating components mounted on the circuit board is transferred to the other circuit board via the connection member and the connection circuit board. 
     With the configuration of the present invention described above, the heat generated in the electronic components is dispersed in the direction of the face of the circuit board by at least one heat conduction layer arranged in the circuit board on which the electronic components are mounted, transferred from the whole face of the circuit board to the operation member, such as keys, and the whole of the housing, further transferred to the other circuit board connected to the circuit board and the components mounted on the other circuit board, and still further transferred to the other housing accommodating the other circuit board, and then radiated to the outside. Hence, the temperatures of the operation member and the housing do not rise locally, and heat radiation can be carried out efficiently. With this simple heat-radiating structure, discomfort during operation due to local temperature rises can be relieved without significantly increasing the cost or the like and without increasing the thickness of the mobile terminal device. 
     The mobile terminal device and the heat-radiating method according to the present invention have effects described below. 
     A first effect of the present invention is that the local temperature rise at the operation member, such as keys, and the housing due to the heat generated in electronic components can be suppressed and that the temperature of the surface of the mobile terminal device can be made uniform. The reason is that the heat generated in the electronic components is dispersed in the direction of the face of the circuit board by at least one first heat conduction layer arranged in the circuit board on which the electronic components are mounted, transferred from the whole face of the circuit board to the operation member and the whole of the housing, further transferred to the other circuit board connected to the circuit board and the components mounted on the other circuit board, and still further transferred to the other housing accommodating the other circuit board, and then radiated to the outside. 
     In addition, a second effect of the present invention is that the heat generated in the electronic components can be radiated effectively without significantly increasing the cost or the like and without increasing the thickness of the mobile terminal device. The reason is that the heat generated in the electronic components can be dispersed by a simple structure wherein heat conduction members are arranged in layers inside the circuit board and then radiated by the operation member and the whole of the housing, without installing heatsinks made of metal, fans or the like that are large in size. 
     Furthermore, high-performance electronic components can be used by adopting this kind of heat-radiating structure. Furthermore, the rigidity of the circuit board can be raised by providing the heat transfer layer inside the circuit board. As a result, the deformation of the circuit board due to external forces encountered during dropping, key operation and the like can be suppressed, and the connection reliability of the electronic components can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing the configuration of a mobile terminal device; 
         FIG. 2  is a block diagram showing the basic functions of the mobile terminal device; 
         FIG. 3  is a schematic sectional view showing a structure of the key operation section of the mobile terminal device according to the first embodiment of the present invention; 
         FIG. 4  is a schematic view showing how heat is transferred in the key operation section of the mobile terminal device according to the first embodiment of the present invention; 
         FIG. 5  is a sectional view showing another structure of the key operation section of the mobile terminal device according to the first embodiment of the present invention; 
         FIG. 6  is a sectional view showing still another structure of the key operation section of the mobile terminal device according to the first embodiment of the present invention; 
         FIGS. 7A ,  7 B and  7 C are schematic perspective views showing the structures of electronic components to be mounted on the circuit board of the mobile terminal device according to the first embodiment of the present invention; 
         FIG. 8  is a sectional view showing a structure of the key operation section of the mobile terminal device according to the first embodiment of the present invention; 
         FIG. 9  is a sectional view showing a structure of the key operation section of the mobile terminal device according to the first embodiment of the present invention; 
         FIG. 10  is a plan view showing a variation of the shape of a heat conduction layer according to the first embodiment of the present invention; 
         FIG. 11  is a plan view showing variation of the shape of the heat conduction layer according to the first embodiment of the present invention; 
         FIG. 12  is a plan view showing variation of the shape of the heat conduction layer according to the first embodiment of the present invention; 
         FIG. 13  is a schematic view showing how heat is transferred in the key operation section of a conventional mobile terminal device; 
         FIG. 14  is a schematic sectional view showing a structure of the circuit board of a mobile terminal device according to a second embodiment of the present invention; and 
         FIG. 15  is a schematic sectional view showing another structure of the circuit board of the mobile terminal device according to the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As described with respect to the background art, as mobile terminal devices are made multifunctional and high in performance, the amount of heat generated in electronic components increases. However, since mobile terminal devices are also requested to be reduced in size, weight and thickness, the heatsinks made of metal and the fans being used for personal computers and other electronic apparatuses cannot be used for the mobile terminal devices. As a result, the temperature of the operation member, such as the keys, and the temperature of the housing are raised locally by the heat generated in the electronic components, thereby causing a problem of giving discomfort to the user during key operation. 
     For the purpose of solving this problem, a structure wherein a heat conduction member is arranged inside the housing (for example, Japanese Published Unexamined Patent Application No. 2002-323936), a structure wherein a metal chassis is provided inside the housing (for example, Japanese Published Unexamined Patent Application No. Hei 2-262724), a structure wherein the housing itself is formed of a member having a high heat conductivity, such as a magnesium alloy (for example, Japanese Published Unexamined Patent Application No. 2000-253115), etc., have been proposed to raise the heat-radiating effect of the housing. 
     However, since the mobile terminal devices are required to be reduced in thickness, an operation member, such as numeric keys and direction keys, are directly installed, via a key sheet, on a circuit board on which electronic components are mounted. As a result, the heat generated in the electronic components is transferred directly to the operation member. Hence, even if the heat conductivity of the housing is improved, the temperature rise at the operation member cannot be suppressed, and the problem of giving discomfort to the user during key operation cannot be solved. 
     In the present invention, instead of a structure wherein a heat conduction member is arranged inside the housing and the housing itself is formed of a member having a high heat conductivity, a heat conduction member is arranged inside a circuit board itself on which heat-generating components are mounted, and the heat generated in the electronic components is dispersed immediately to the whole of the circuit board. With this kind of structure, the local temperature rise at the operation member and the housing in the vicinity thereof is suppressed even in a structure wherein the operation member is directly installed on the circuit board via a key sheet, whereby the problem of giving discomfort to the user during key operation is solved. 
     Next, embodiments according to the present invention will be described in detail referring to the accompanying drawings.  FIG. 1  is a view showing the basic configuration of a mobile terminal device.  FIG. 2  is a block diagram showing the functions of this mobile terminal device. In addition,  FIG. 3  is a schematic view showing a sectional structure of the key operation section thereof, and  FIG. 4  is a schematic view showing how heat is transferred. Furthermore,  FIG. 5 ,  FIG. 6 ,  FIG. 8  and  FIG. 9  are views showing variations of the sectional structure of the key operation section,  FIGS. 7A ,  7 B and  7 C are schematic perspective views showing the structures of electronic components to be mounted on a circuit board according to this embodiment, and  FIG. 10  to  FIG. 12  are views showing variations of the shape of a heat conduction layer. Moreover,  FIG. 13  is a schematic view showing how heat is transferred in a conventional mobile terminal device. Although a mobile phone is taken as an example of mobile terminal devices and is described below, such mobile terminal devices should only be equipped with at least an operation member and a display means, and the configuration thereof can be applied similarly to any mobile devices, for example, mobile computers such as PDAs, slim digital cameras and mobile game machines. 
     As shown in  FIG. 1  and  FIG. 2 , a mobile terminal device  1  (mobile phone) generally comprises a display section  3 , formed of a liquid crystal display device, an EL light-emitting device or the like, for displaying standby screens, various function setting screens, web screens, etc.; a key operation section  2  having multiple keys, such as numeric keys, direction keys and special function keys; a wireless communications section  8  and an antenna  9  for carrying out telephone communications, E-mail sending/receiving, web screen receiving, etc.; a speaker  4  and a microphone  5  for inputting/outputting sound and for outputting ring tone; an internal storage device  6  for storing various setting values; and a control section  7  for controlling these. Furthermore, various electronic components for realizing various functions of the mobile terminal device  1  are disposed inside the key operation section  2 . 
     The sectional structure of the key operation section  2  of the mobile terminal device  1  is as shown in  FIG. 3 , for example. The key operation section  2  comprises an upper housing  11 ; a lower housing  12  fitted to the upper housing  11 ; a circuit board  13  arranged inside the upper housing  11  and the lower housing  12  and held by the protruding portions  11   a  and  12   a  of the respective housings; a key sheet  15  provided on a face of the circuit board  13  on the side of the key operation face of the key operation section  2 ; keys  16 , such as numeric keys, direction keys and special function keys, arranged on the key sheet  15 ; and at least one electronic component  14  arranged on the other face of the circuit board  13  on the opposite side of the key operation face of the key operation section  2 . Inside the circuit board  13 , at least one heat conduction layer  17  having the shape of a layer is provided wherein a member (hereinafter referred to as a heat conduction member) having a heat conductivity at least higher than that of the main constituent material (a material constituting the substrate excluding wiring materials and the like) of the circuit board  13  is arranged in the direction of the face of the circuit board  13 . 
     The kind and thickness of this heat conduction member are not particularly limited, but the heat conduction layer may be formed of a metal material, such as copper or aluminum, or may be formed of a material having high electrical insulation, such as carbon. Furthermore, the heat conduction member may be formed of thin films of these materials, and may also be formed of thin sheets wherein these materials are dispersed in a resin or the like. The method for producing the circuit board  13  is not particularly limited, but a method for producing general multi-layer circuit boards may also be used. In addition, for example, the circuit board  13  can also be produced by using a method wherein two substrates on which wiring patterns are formed beforehand are made, the heat conduction layer  17  is formed on the surface of one of the substrates, and the other substrate is bonded to the heat conduction layer  17  using an adhesive or the like. Although the heat conduction layer  17  is formed at a nearly central position in the thickness direction of the circuit board  13  in the figure, the heat conduction layer  17  should only be formed inside the circuit board  13 . In other words, the heat conduction layer  17  may be formed at a position close to the face on which the electronic components  14  are mounted or may also be formed at a position close to the other face that is close to the key operation face of the key operation section  2 . 
     In addition, although two electronic components  14  are mounted on the circuit board  13  in the figure, the number, size, disposition and the like of the electronic components  14  are not limited. Among the electronic components  14 , at least one electronic component  14  generating heat (a heat-generating component whose temperature is raised to a temperature at least higher than the ambient temperature when driven, for example, a power amplifier, a charging IC or the like) should only be included. Furthermore, the configuration of each electronic component  14  is not limited. In other words, the electronic component  14  may be a component for realizing its individual function, such as a transistor or a transformer, or may be an assembly comprising multiple components for realizing their individual functions, such as an amplifier or a voltage converter. Furthermore, although the electronic components  14  are mounted on only one face of the circuit board  13  in the figure, in the case that there is space on the other face of the circuit board  13  (on the side of the key operation face), the electronic components  14  may also be mounted on the side of key operation face. 
     Next, how the heat generated in the electronic components  14  mounted on the circuit board  13  is transferred will be described referring to the figures. 
     First, in the case of a conventional mobile terminal device wherein no heat conduction layer is formed inside the circuit board  13 , when heat is generated in either one of the electronic components  14  mounted on one face of the circuit board  13  as shown in  FIG. 13  (a heat-generating portion  19  is indicated in black in the figure), the generated heat is transferred from the electronic component  14  to the circuit board  13  since the generated heat is hard to be transferred to an air layer  18  but apt to be transferred in the direction of an object making contact with the electronic component  14 . Hence, the generated heat is transferred from the electronic component  14  to the circuit board  13 , thereby being dispersed isotropically inside the circuit board  13 . Since the thickness of the circuit board  13  is sufficiently smaller than the dimensions in the direction of the face, the heat dispersed into the circuit board  13  is promptly transferred to the opposite face of the circuit board  13 , and further transferred to the respective keys  16  via the key sheet  15  installed on the opposite face of the circuit board  13 . For this reason, only the keys  16  disposed close to the electronic component  14  generating heat and the upper housing  11  in the vicinity thereof tend to be heated to high temperatures. 
     The keys  16  are formed separate from the housing (the upper housing  11  in this case) so that they can be pressed. In addition, the key sheet  15  is disposed separate from the housing so as not to be pressed by anything other than the keys  16 . Hence, even if a heat conduction member or a metal chassis is provided on the inside face of the housing or even if the housing itself is formed of a heat conduction member, as in the conventional examples, the heat directly transferred to the keys  16  cannot be radiated effectively. Therefore, it is impossible to suppress the local temperature rise at the keys  16  and the housing. 
     Contrary to this, in the case of the mobile terminal device  1  according to this embodiment, as shown in  FIG. 4 , although the heat generated in the electronic components  14  mounted on one face of the circuit board  13  is similarly transferred to the circuit board  13 , since the heat conduction layer  17  being excellent in heat conductivity is arranged in the shape of a layer in the direction of the face of the circuit board  13  inside the circuit board  13 , the heat transferred to the circuit board  13  is promptly dispersed in the direction of the face of the circuit board  13  via the heat conduction layer  17 . Furthermore, the heat dispersed to the whole of the heat conduction layer  17  is transferred to the whole face on the opposite side of the circuit board  13 , and then transferred to all the keys  16  via the key sheet  15 . Moreover, the heat is also transferred to the upper housing  11  and the lower housing  12  via the protruding portions  11   a  and  12   a  that are used to hold and fix the circuit board  13 . For this reason, even if the electronic components  14  generate heat, it is possible to prevent the problem of raising the temperatures of only the keys  16  and the upper housing  11  in the vicinity of the electronic components  14 . 
     Although only the heat conduction layer  17  is provided inside the circuit board  13  in  FIG. 3  and  FIG. 4 , it is also possible to provide a structure wherein the heat conducted to the heat conduction layer  17  is hard to be transferred to the key sheet  15  and the keys  16 . For example, as shown in  FIG. 5 , a heat conduction suppressing layer  22  having a given thickness is provided closer to the side of the key operation face than the heat conduction layer  17  inside the circuit board  13 , and the heat conducted to the heat conduction layer  17  is radiated to sections other than the key sheet  15  and the keys  16  (for example, the upper housing  11  and the lower housing  12  via the protruding portions  11   a  and  12   a ) by the heat conduction suppressing layer  22 . With this configuration, the temperature rise at the keys  16  can be suppressed. The heat conduction suppressing layer  22  may be formed of, for example, a member at least having a heat conductivity smaller than that of the main constituent material of the circuit board  13 , or areas having a small heat conductivity may be formed in the circuit board  13  by providing minute pores in the circuit board  13  so as to function as the heat conduction suppressing layer  22 . 
     Furthermore, although only the heat conduction layer  17  having the shape of a layer is provided inside the circuit board  13  in  FIG. 3  and  FIG. 4 , it is also possible to provide a structure wherein the heat generated in the electronic components  14  can be promptly transferred to the heat conduction layer  17 . For example, as shown in  FIG. 6 , at least one connection portion  17   a  connected to the heat conduction layer  17  is formed at the portion of the circuit board  13  on which the electronic component  14  generating heat is mounted, and the electronic component  14  is mounted so as to make contact with the connection portion  17   a  (for example, the connection portion  17   a  is formed so as to protrude from the circuit board  13 , and the protruding portion is crushed when the electronic component  14  is mounted). With this configuration, the heat conduction layer  17  is thermally connected to the bottom faces of the electronic component  14 , and the heat generated in the electronic component  14  can be transferred promptly to the heat conduction layer  17 . The method for forming the connection portion  17   a  is not particularly limited. However, for example, it may be possible that at least one hole reaching the heat conduction layer  17  from the component mounting face is formed at the portion of the circuit board  13  on which the electronic component  14  is mounted and that a heat conduction member (the member may be the same member as that of the heat conduction layer  17  or may be a member different therefrom) is embedded inside the hole. Furthermore, it may also be possible that at least one through hole passing through the circuit board  13  from the side of the key operation face is formed and that the above-mentioned heat conduction member is embedded inside the through hole. 
     In addition, in the case that the connection portion  17   a  described above is formed, it may also be possible that a member having a high heat conductivity, such as a metal plate  23 , is provided beforehand on the component mounting face of the circuit board on which the electronic component  14  is mounted as shown in  FIG. 7A  and that this metal plate  23  is made so as to contact with the connection portion  17   a . Furthermore, in the case that the electronic component  14  is connected to the circuit board  13  by BGA (Ball Grid Array), it may also be possible that at least one connection solder ball  24   a  not contributing to the electrical connection of the electronic component  14  is provided among the solder balls constituting the BGA  24  as shown in  FIG. 7B  (the position of the connection solder ball  24   a  is not limited to that shown in the figure), that a hole reaching the heat conduction layer  17  is formed at the position opposed to the connection solder ball  24   a , and that, when the electronic component  14  is mounted, the connection solder ball  24   a  is melted and embedded inside the hole so as to make contact with the heat conduction layer  17 . Furthermore, in the case that the electronic component  14  is connected to the circuit board  13  by pins, it may also be possible that at least one connection pin  25   a  not contributing to the electrical connections of the electronic component  14  is provided among the pins  25  as shown in  FIG. 7C  (the position of the connection pin  25   a  is not limited to that shown in the figure), that a hole reaching the heat conduction layer  17  is formed at the position opposed to the connection pin  25   a , and that, when the electronic component  14  is mounted, the connection pin  25   a  is made so as to contact with the heat conduction layer  17 . 
     The connection solder ball  24   a  and the connection pin  25   a  described above may be connected to the housing of the electronic component  14  or a member inside the electronic component  14  (for example, an individual component, an internal circuit board or the like constituting the electronic component  14 ). The generated heat can be transferred promptly to the outside without being stored inside the electronic component  14  by directly thermally contacting the connection solder ball  24   a  or the connection pin  25   a  with the heating element inside the electronic component  14 . Hence, the temperature rise at the electronic component  14  itself can be suppressed, and the performance of the electronic component  14  can be improved. 
     Furthermore, although one heat conduction layer  17  is provided inside the circuit board  13  in  FIG. 3  to  FIG. 6 , the heat conduction layer  17  according to the present invention should only be formed so that the heat conduction can be facilitated in the direction of the face of the circuit board  13 . For example, as shown in  FIG. 8 , multiple heat conduction layers  17  (two layers in the figure) may also be provided inside the circuit board  13 . Moreover, it may also be possible to have a configuration wherein the multiple heat conduction layers  17  are thermally connected mutually by at least one connection portion  17   a  in a way similar to that described above. 
     In addition, although the structure wherein the circuit board  13  is fixed by the protruding portion  11   a  of the upper housing  11  and the protruding portion  12   a  of the lower housing  12  is provided in  FIG. 3  to  FIG. 6  and  FIG. 8 , it is also possible to provide a structure wherein the heat transferred to the heat conduction layer  17  can be transferred promptly to the upper housing  11  and the lower housing  12 . For example, as shown in  FIG. 9 , it is also possible to have a structure wherein the portions of the heat conduction layer  17 , corresponding to the protruding portions  11   a  and  12   a , are exposed partly or wholly so that the heat conduction layer  17  is thermally connected to at least part of the protruding portions  11   a  and  12   a  and so that the heat transferred to the heat conduction layer  17  is transferred promptly to the upper housing  11  and the lower housing  12 . 
     Furthermore, in the structures shown in  FIG. 3  to  FIG. 6  and  FIG. 8  and  FIG. 9 , the heat conduction layer  17  may be formed on the whole face of the circuit board  13 . However, the heat conduction layer  17  should only be structured so as to expand in the direction of the face of the circuit board  13 . Hence, the heat conduction layer  17  is not necessarily required to be formed on the whole face of the circuit board  13 , as long as the heat conduction layer  17  can promptly transfer the heat generated in the electronic components  14  in the direction of the face of the circuit board  13 . For example, as shown in  FIG. 10 , the heat conduction layer  17  may have a grid shape or a slit shape (the grids or slits may have regular shapes or irregular shapes being different in width or pitch). 
     Furthermore, in the case that the heat conduction layer  17  is formed of a material having high electrical insulation, such as carbon, the heat conduction layer  17  is not short-circuited to the connection wires for connecting the front and back faces of the circuit board  13 . However, in the case that the heat conduction layer  17  is formed of a metal material, such as copper or aluminum, the heat conduction layer  17  is in danger of being short-circuited to the connection wires. In such a case, it is possible to form the heat conduction layer  17  on the whole face or part of the circuit board  13  excluding the areas for the connection wires  20 ,  21  as viewed in the normal direction of the circuit board  13  as shown in  FIG. 11 . 
     Furthermore, in the case that the heat conduction layer  17  is provided on the whole face of the circuit board  13 , the heat generated in some of the electronic components  14  is also transferred to the other electronic components  14 , thereby raising the temperatures of the other electronic components  14 . The electronic components  14  are classified into two types: those hardly affected by temperature (heat-resistant components  14   a  that operate normally or have unvarying characteristics even when the temperature rises) and those easily affected by temperature (non-heat-resistant components  14   b  that do not operate normally or have characteristics deteriorated when the temperature rises). In the case that these two types of electronic components  14  are mixed together, it is undesirable that the temperatures of the non-heat-resistant components  14   b  are raised by the heat generated by some of the electronic components  14 . To solve this problem, it may be possible that, as shown in  FIG. 12 , the heat conduction layer  17  is formed on the whole face or part of the circuit board  13  excluding the areas on which non-heat-resistant components  14   b  are mounted, as viewed in the normal direction of the circuit board  13 , thereby to prevent the temperatures in the vicinities of the non-heat-resistant components  14   b  from rising. 
     The heat generated in the electronic components  14  can thus be transferred promptly in the direction of the face of the circuit board  13  by arranging at least one heat conduction layer  17  having the shape of a layer inside the circuit board  13  as described above. With this configuration, it is possible to prevent the problem of the local temperature rise at the keys  16  and the housing in the vicinities of the electronic components  14  generating heat, and the heat can be radiated efficiently to the outside. Furthermore, since the local temperature rise can be suppressed, the degree of freedom in the selection of the electronic components  14  increases, and it is possible to use electronic components  14  having high performance. Still further, the rigidity of the circuit board  13  can be raised by forming the circuit board  13  to have a multi-layer structure, whereby the reliability of the mobile terminal device  1  can be enhanced. 
     Next, a mobile terminal device and a heat radiating method according to a second embodiment of the present invention will be described referring to  FIG. 14  and  FIG. 15 .  FIG. 14  and  FIG. 15  are sectional views schematically showing the structures of circuit boards inside the mobile terminal device according to the second embodiment. In this embodiment, although a mobile phone is also taken as an example of mobile terminal devices and described below, such mobile terminal devices should only be equipped with at least an operation member and a display means. 
     The first embodiment described above is configured so that the heat generated in the electronic components  14  is transferred to the whole of the housing (the upper housing  11  and the lower housing  12 ) for holding the circuit board  13  on which the electronic components  14  are mounted, the key sheet  15  and the keys  16 , whereby the heat is radiated. However, for the purpose of further raising heat radiation efficiency, it is also possible to have a configuration wherein the heat generated in the electronic components  14  is transferred to the other components constituting the mobile terminal device  1 . 
     For example, the mobile terminal devices  1  of recent years are equipped with a slot for accommodating a recording medium, such as a memory card. Even if the temperature of this slot rises to some extent, no problem occurs in operation. Hence, for example, as shown in  FIG. 14 , in a structure wherein a second circuit board  13   a  equipped with a slot  26  or the like is fixed to the circuit board  13  according to the first embodiment, at least one heat conduction layer  17  (the layer may be the same as or different from the heat conduction layer  17  of the circuit board  13 ) is also provided inside or on the surface of the second circuit board  13   a , and the heat conduction layer  17  of the circuit board  13  is thermally connected to the heat conduction layer  17  of the second circuit board  13   a  by a thermal connection member (a connection portion  17   a  also serving as a support rod for fixing the second circuit board  13   a , a metal wire or the like). 
     With this structure, the heat generated in the electronic components  14  is transferred from the heat conduction layer  17  inside the circuit board  13  to the heat conduction layer  17  of the second circuit board  13   a  via the connection portion  17   a , the temperature of the slot  26  is raised, and the heat thus is radiated, whereby heat-radiating paths can be increased. As a result, it is possible to further suppress heat transfer to components that may cause problems when the temperature rises, such as the keys  16 . 
     In addition, the mobile terminal devices  1  of recent years mostly have a foldable structure to realize downsizing. In the case of a mobile phone, as shown in  FIG. 1 , it is composed of two sections: a section wherein the key operation section  2  is disposed, and a section wherein the display section  3  is disposed. In this case, the temperature rise at the components that the user touches directly, such as the keys  16 , cause problems. However, the sections that the user does not touch directly, such as the display section  3  and the housing for holding the display section  3 , do not cause problems even if temperature rises to some extent. Hence, for example, as shown in  FIG. 15 , in a structure wherein the circuit board  13  according to the first embodiment is connected to a circuit board  13   b  on the display section side via a connection member, such as a flexible circuit board  13   c , at least one heat conduction layer  17  (the layer may be the same as or different from the heat conduction layer  17  of the circuit board  13 ) is also provided inside the circuit board  13   b  on the display section side, and the circuit board  13  is thermally connected to the circuit board  13   b  on the display section side by a thermal connection member (the heat conduction layer  17 , a metal wire or the like provided inside or on the surface of the flexible circuit board  13   c ). 
     With this structure, the heat generated in the electronic components  14  mounted on the circuit board  13  is transferred from the heat conduction layer  17  inside the circuit board  13  to the heat conduction layer  17  of the circuit board  13   b  on the display section side via the flexible circuit board  13   c . Hence, the temperatures of the housing for fixing the circuit board  13   b  on the display section side and other sections are raised, and the heat is radiated, whereby heat-radiating paths can be increased. As a result, it is possible to further suppress heat transfer to components that may cause problems when the temperature rises, such as the keys  16 . 
     Although the heat conduction layer  17  is provided inside the circuit board  13  according to each of the embodiments described above, the circuit board  13  itself may be formed of a material having an excellent heat conductivity in which a heat conduction material is dispersed. Furthermore, although the mobile terminal device according to each of the embodiments described above is configured so that the keys  16  are disposed on one face of the circuit board  13  via the key sheet  15  and that the electronic components  14  are mounted on the other face, the device may also be configured so that these are disposed on the same face of the circuit board  13 .