JPH09114552A - Electronic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To cool a heat generating element to a prescribed temperature without preventing arranging constitution by providing a through-hole for a first wiring substrate and thermally connecting the heat generating element to be a cooling object and a heat radiating panel through this through-hole. SOLUTION: The heat radiating panel 11 of nearly the same size as the bottom surface of a housing is laid inside of the housing 7 and CPU 2 being the heat generating element is loaded on a second wiring substrate 3. Then an extended metallic plate 13 is fixed to the CPU 2 with a frame 14 through a flexible heat conductive member 12. Then the second wiring substrate 3 is electrically connected with a first wring substrate 1 through a connector 15. The second wired substrate 3 is fixed by sticking to the heat generating panel 11 or thermally connecting by inserting the flexible heat conductive member 12 between a heat generating block 16 obtained by providing a through-hole to the first wiring substrate 1 and the extended heat generating panel 13. Thereby CPU 2 being the heat generating element is cooled to be the prescribed temperature without preventing arranging constitution.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cooling structure for a small electronic device, and more particularly to a cooling device for cooling a heating element to keep it at a predetermined temperature.

[0002]

2. Description of the Related Art A conventional electronic device is disclosed in Japanese Patent Laid-Open No. 4-4879.
No. 7, JP-A-55-125699, an example in which a metal plate is shaped like a leaf spring, or an elastic resin is interposed between a heat-generating member and a heat-dissipating member for cooling. It is shown. Also, JP-A-63-25090
No. 0, JP-A-3-255697, and Japanese Utility Model Publication No. 5-
As described in Japanese Patent No. 29153, an independent metal plate or a metal plate that forms a part of a housing is interposed between a heat generating member and a metal housing wall to dissipate heat generated by the heat generating member. It is transported by heat conduction to the wall of the metal casing, which radiates heat.

[0003]

In the above-mentioned conventional example, Japanese Patent Laid-Open No.
In the examples described in Japanese Patent Application Laid-Open No. 3-250900, Japanese Patent Application Laid-Open No. H3-255697, and Japanese Utility Model Application Laid-Open No. 5-29153, the metal housing wall does not always have a short conduction distance depending on the arrangement of components. . Therefore, the arrangement of parts or the structure of the case is limited, for example, by disposing the heat generating member near the case. On the other hand, in electronic devices that require high performance, the heating elements mounted on the wiring board are uniquely arranged on the wiring board due to the speedup of electronic circuits and the layout of other components. Will end up. Also, regarding the heat dissipation member, there is a restriction on the installation place of the heat dissipation member in order to obtain high heat dissipation performance. In the above conventional example, the heat generating element and the heat radiating member can be realized only on the same side and close to the wiring board. Therefore, the realization of the optimal arrangement configuration is greatly restricted, which hinders downsizing and high performance of electronic devices.

An object of the present invention is to cool the heating element to a predetermined temperature without disturbing the arrangement of the heating element on the wiring board and the position where the heat dissipation member is installed. To do.

[0005]

The object is to provide a keyboard mounted in a housing and an electronic device having a wiring board inside the housing, wherein the wiring board is mounted with an element to be cooled. And a first wiring board on which elements other than the element to be cooled are mounted,
A heat dissipation plate is provided on the bottom of the housing, the keyboard, the second wiring board, the first wiring board, and the heat dissipation plate are arranged in this order, and a through hole is provided in the first wiring board. This is achieved by including means for thermally connecting the element to be cooled and the heat dissipation plate via the.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIG. Personal computer according to the present embodiment
An electronic device such as a computer has a first wiring board 1 on which a plurality of heating elements including a heat generating element are mounted and a second wiring board 3 on which a main heating element 2 is mounted. A palm rest 8 and a keyboard 9 are mounted on a housing 7 in which 6 and the like are housed, and a retractable display 10 is further provided. The main heating element 2 is, for example, C of an electronic computer.
It is a PU (hereinafter referred to as CPU2). The second wiring board 3 can be removed from the wiring board 1, and can be easily replaced or upgraded.
Inside the housing 7, a heat dissipation plate 11 having a size substantially the same as the bottom surface of the housing is laid. In the CPU 2 mounted on the second wiring board 3, an enlarged metal plate 13 is attached together with a frame 14 via a flexible heat conduction member 12 described in detail later. The second wiring board 3 is electrically connected to the wiring board 1 via the connector 15. Then, the second wiring board 3
The attachment is performed by thermally connecting the heat dissipation block 16 bonded to or integrally formed with the heat dissipation plate 11 and the enlarged heat dissipation plate 13 via the flexible heat conduction member 12. The heat dissipation block 16 is provided so as to penetrate through the hole provided in the wiring board 1 as illustrated.

Details regarding cooling of the CPU 2 shown in FIG. 1 will be described with reference to FIG. CPU2, for example,
In the tape-shaped carrier package, the back surface of the element is adhered to the second wiring board 3 and the front surface is pressed against the flexible heat conducting member 12. In addition, this soft heat conducting agent 12 is
2 and the expanded metal plate 13 suspended by the electrically insulating frame 14 fixed to the second wiring board 3, and the flexible heat conducting member 12 and the expanded metal plate 13 are pressure-welded to each other. ing. The enlarged metal plate 13 is a CPU that is a heating element.
2 diffuses the generated heat and also provides mechanical protection for the device. Further, the thermal conductivity of the expanded metal plate 13 is higher than that of the flexible thermal conductive member 12. The heat sink 11 is made of Al, M
A metal plate having a high thermal conductivity such as g and Cu is laid on the bottom of the housing 7 with substantially the same area as the bottom surface of the housing. By laying it on the bottom of the housing, a wide heat dissipation area can be secured without spatial restrictions on the mounted parts and the like, and by contacting the housing 7, heat can be efficiently dissipated to the outside air. Further, on the bottom surface of the housing, the heat dissipation block 16 can be placed at any position.
The CPU 2 on the wiring board 3 can be installed.
There is no restriction on the mounting position of. In order to enhance the heat diffusion effect of the heat radiating plate 11, a heat pipe or the like may be crawled and connected, or the heat radiating plate and the heat pipe may be integrally formed.

The second wiring board 3 is electrically connected to the wiring board 1 through the connectors 15a and 15b, and at the same time, the expanded metal plate 13 and the heat dissipation block 16 provided through the wiring board 1. Are thermally connected via the flexible heat conducting member 17. The flexible heat-conducting member 17 absorbs manufacturing tolerances such as variations in the height of connector attachment and variations in the inclination of the enlarged metal plate, secures a contact area for connection with a small thermal resistance, and protects against impacts applied from the outside of the housing 7. It also functions as a cushioning material. Further, the surfaces of the flexible heat conducting members 12 and 17 have adhesiveness, and the heat radiating plate 11 and the second wiring board 3 are
Since they are pressed against each other, a pressing force is generated by the reaction force of the pressure contact even if they are not bonded by using an adhesive or the like, so that the personal computer does not wobble even when carried.

On the other hand, the heat path from the back surface of the wiring board is CP
Depending on the amount of heat generated by U2, a narrow gap in the air layer may be used (heat conduction of air), and if the amount of generated heat is small, no heat path may be provided. The flexible heat conducting member 12,
Reference numeral 17 (similarly to the flexible heat conduction member 20 described later) is, for example, a mixture of Si rubber with a filler such as aluminum oxide, or a gel-like high heat conduction Si resin.

According to the present embodiment, the main heat generating element and the heat radiating plate are separated from each other by the wiring board or the like because the heat radiating plate is installed at a position where high heat radiating performance can be ensured due to the structure of the electronic circuit. Even in the positional relationship, the heat generated in the heating element can be diffused and the heat can be conducted to the heat radiating plate by the heat radiating block penetrating the wiring board, so that the heat can be efficiently radiated outside the housing. According to one embodiment of the present invention, in the positional relationship between the heating element and the heat dissipation plate, when the wiring board is installed between them (the positional relationship between the heating element and the heat dissipation member is different from that of the wiring board). Side), a heat dissipation plate of almost the same size as the bottom surface of the housing is laid on the bottom surface of the housing, and the heat dissipation block is provided at an arbitrary position of the heat dissipation plate so as to penetrate the wiring board. Since it is thermally connected to the heat sink at the shortest distance, it can be cooled efficiently with a small thermal resistance. In addition, depending on the surface on which the heating element is mounted or the structure of the package, the heat can be conducted to the radiator plate through the wiring board after the heat is diffused by the expansion radiator plate on the back side of the substrate on which the heating element is mounted. Therefore, there is an effect that cooling can be performed efficiently.

FIG. 3 shows another embodiment. The difference from the embodiment shown in FIG. 2 is that a heat conducting member is also provided between the rear surface of the second wiring board 3 and the keyboard 9.
A metal plate 19 is attached to the back surface of the second wiring board 3 via an insulating sheet 18, and is connected to the back surface of the keyboard 9 via a flexible heat conducting member 20. As a result, a heat conduction path from the rear surface of the heating element to the keyboard side is formed through the wiring board. The rear surface of the keyboard to which the flexible heat conducting member 20 is connected is a metal plate forming the lower surface of the keyboard or a metal plate further provided below the keyboard. As a result, in addition to the heat radiated from the heat radiating plate at the bottom of the housing, the heat conducted to the keyboard side is diffused in the keyboard direction and can be radiated from the keyboard surface to the outside air. A cooling effect is obtained. It is more effective when the amount of heat generated by the CPU 2 is large.

FIG. 4 shows another embodiment. In this embodiment, the second wiring board 3 on which the CPU 2 is mounted is electrically connected to the wiring board 1 by the connector 15 shown in FIG.
However, the surface of the CPU 2 is directly connected to the heat dissipation block 16 provided on the heat dissipation plate 11 laid on the bottom of the housing 7 via the flexible heat conduction member 12. different. The size of the cross section of the heat dissipation block 16 is larger than the contact area with the flexible heat conduction member 12, and diffuses the heat of the heat generating element 2 in the cross section direction and the heat dissipation plate 11
Conducts heat to. In the present embodiment, as compared with the embodiment shown in FIG. 2, the number of flexible heat conducting members used is small, so that the heat generating element and the heat radiating plate at the bottom of the housing can be connected with a small heat resistance.

FIG. 5 shows another embodiment. This embodiment has an electronic circuit configuration similar to that of FIGS.
2 are mounted on different surfaces with respect to the heat dissipation plate 11 laid on the bottom of the housing 7 on the second wiring board 3. The back surface of the second wiring board 3 is provided with an enlarged metal plate 19 via an insulating sheet 18.
Is attached and connected to the heat dissipation block 16 provided on the heat dissipation plate 11 laid on the bottom of the housing 7 via the flexible heat conduction member 20. In the present embodiment, the heat generated by the CPU 2 is conducted through the second wiring board 3, and is first diffused in the plane direction by the expanded metal plate 19. Then, after the heat is conducted to the heat radiation block 16 through the flexible heat conduction member 20, it is sufficiently diffused by the heat radiation plate 11 and radiated to the outside air. Since the flexible heat-conducting member 20 has a smaller thermal conductivity than metal, the contact area with the flexible heat-conducting member is increased and the thermal resistance is reduced by first diffusing heat with the expanded metal plate 19. Since the wiring board 3 has a plurality of copper wiring layers formed therein, the thermal conductivity thereof is generally higher than that of the flexible heat conducting member. Therefore, even if heat is conducted through the wiring board, the total thermal resistance is small. Furthermore, since the thermal conductivity in the plane direction is increased by the copper wiring layer, an in-plane heat diffusion effect can be expected.

FIG. 6 shows another embodiment. This embodiment has a structure similar to that of FIG. 2, but has enlarged metal plates 13 and 1 provided on the front surface of the CPU 2 and the back surface of the second wiring board 3.
9 is different in that heat is conducted to the heat radiating plate 11 laid on the bottom of the housing 7 via the heat radiating block 16. The surface of CPU2 is
It is thermally connected to the first expanded metal plate 13 via the flexible heat-conducting member 12, while the back surface of the wiring board 3 has the insulating sheet 1
It is thermally connected to the second expanded metal plate 19 via 8. The first expanded metal plate 13 and the second expanded metal plate 19 are screwed through the second wiring board 3 by a metal rod or screw 21 having a high thermal conductivity such as aluminum or copper. The first expanded metal plate 13 and the heat dissipation block 16 are connected via a flexible heat conduction member 17. In the heat path on the back surface of the wiring board 3, after the heat is diffused by the second expanded metal plate 19, the heat is conducted to the first expanded metal plate 13 by the metal rod 21. Therefore, in the first expanded metal plate 13, the heating element 2
The heat generated in 2) is conducted to the heat dissipation block 16 from both sides thereof via the flexible heat conduction member 17, and high heat dissipation performance is obtained. Note that the structure on the back surface of the wiring board 3 can be realized with a small thermal resistance, as described in FIG.

Similar to FIG. 6, in FIG. 7, heat conduction paths are formed on both sides of the heat generating element 2, but metal rods are not provided, and the first enlarged metal plate 13 is attached to the wiring board 3 by the frame 14. . The material of the frame 14 is preferably metal, and is attached to the wiring board with an electrically insulating double-sided tape or the like. In this embodiment, the first expanded metal plate 13 and the second expanded metal plate 13
The thermal resistance between the expanded metal plate 19 and the expanded metal plate 19 is increased by increasing the cross-sectional area of the heat conduction path and the contact area with the wiring board by using the frame 14. It is also possible to connect with a small thermal resistance also by heat conduction via 3. Similar to the embodiment shown in FIG.
Also in the present embodiment, in the first enlarged metal plate 13,
The heat generated in the heat generating element 2 is conducted to the heat dissipation block 16 from both sides thereof via the flexible heat conduction member 17, so that high heat dissipation performance is obtained. In addition, in FIG. 6 and FIG.
The CPU 2 can have the same configuration regardless of which surface of the wiring board 3 is mounted, and is not restricted by the mounting arrangement of the electronic circuit.

The embodiment shown in FIGS. 8 and 9 has a wiring board structure similar to that of the above-mentioned embodiment, but is an embodiment in which a relatively large through hole of the heat dissipation block is not provided on the wiring board. Is. The embodiment shown in FIG. 8 has a CPU 2
Is mounted on the surface of the second wiring board 3 that does not face the wiring board 1, and the embodiment shown in FIG. 9 is the case where the second wiring board 3 is mounted on the surface that faces the wiring board 1. In FIG. 8, the expanded metal plate 13 is provided with the heating element 2 through the flexible heat conducting member 12.
A plurality of metal rods 22 (high thermal conductivity material such as aluminum and copper) provided on the surface and at the end of the enlarged metal plate 13
It is fixed with screws. The metal rod 22 is provided on the heat dissipation plate 11 that penetrates the wiring board 1 and is laid on the bottom surface of the housing 7. On the other hand, in the embodiment of FIG. 9, the second expanded metal plate 19 is provided on the back surface of the second wiring board 3 via the insulating sheet 18, and the expanded metal plate 13 is connected via the flexible heat conduction member 20. . Other parts are the same as those in the embodiment shown in FIG. The structure shown in the embodiments of FIGS. 1 to 7 in which a relatively large through hole is provided in the wiring board may be difficult in designing the wiring pattern of the board. 8 and 9
In the embodiment, the through-hole diameter provided in the wiring board is reduced by dispersing the plurality of heat conduction paths from the expanded metal plate 13 to the heat dissipation plate 11 laid on the bottom surface of the housing, and the heat resistance up to the heat dissipation plate is increased. The wiring pattern design can be facilitated without doing so.

Although the above-described embodiment shows the case where the second wiring board on which the CPU 2 is mounted is constructed separately and independently, it can be applied to the case where the CPU is mounted on a single wiring board. In particular, the embodiments shown in FIGS. 8 and 9 can be applied to the case where a CPU is mounted on a single wiring board having the same structure. Further, as an example, FIG. 10 shows an example in which the structure of the embodiment of FIG. 6 is applied to a case where a CPU is mounted on a single wiring board. From the enlarged metal plates 13 and 19 provided on the front surface of the CPU 2 and the back surface of the wiring board 1, the housing 7
Heat is conducted to the heat dissipation plate 11 laid on the bottom. The surface of the CPU 2 is provided with the first enlarged metal plate 13 via the flexible heat conduction element 12.
On the other hand, a second enlarged metal plate 19 is provided on the back surface of the wiring board 1 with the insulating sheet 18 interposed therebetween. The first expanded metal plate 13 and the second expanded metal plate 19 are made of a metal rod or screw 21 having a high thermal conductivity such as aluminum or copper and the wiring board 1
Is screwed through. The first expanded metal plate 13 and the heat dissipation plate 11 laid on the bottom of the housing 7 are the flexible heat conduction members 17
Connected via The heat path on the back surface of the wiring board 1 is provided with the metal rods 2 after the heat is diffused by the second expanded metal plate 19.
At 1, the heat is conducted to the first expanded metal plate 13. Therefore, in the first expanded metal plate 13, the heat generated in the heating element 2 is conducted to the heat radiating plate 11 laid on the bottom of the housing 7 from both sides thereof via the flexible heat conducting member 17, which is high. Heat dissipation performance can be obtained. FIG. 11 shows another embodiment. This embodiment has a structure similar to that of FIG. 9, but is a case where the second wiring board 3 is installed relatively at the end portion of the wiring board 1. In the present embodiment, the second expanded metal plate 19 is provided on the back surface of the second wiring board 3 via the insulating sheet 18, and the first expanded metal plate 13 is connected via the flexible heat conduction member 20. The enlarged metal plate 13 is fixed to the heat dissipation block 16 by screws or the like at the end of the enlarged metal plate 13.
The heat dissipation block 16 is provided on the heat dissipation plate 11 laid on the bottom surface of the housing 7 at the side of the wiring board 1. In FIG. 11, the structure in which the positional relationship of the CPU 2 is similar to that of FIG. 9 is shown, but it is also applicable to the examples shown in FIGS. 1 to 8. According to the present embodiment, there is no need to provide through holes in the wiring board, and the wiring pattern design of the wiring board 1
The CPU 2 can be cooled without any restrictions.

The above embodiment is an example mainly applied to a portable electronic device such as a personal computer or a word processor, and therefore the keyboard surface temperature during operation,
Alternatively, it is desirable that the surface temperature of the casing when the casing is moved after the power is turned off is kept at a temperature that does not cause discomfort. Empirically, in consideration of human body temperature, the temperature difference between the outside air temperature and the temperature of the part where the hand is touched may be about 10 ° C. or less. In the embodiments of FIGS. 1 to 11 described above, a heat radiating plate is laid on the bottom of the housing, and the CPU and the heat radiating plate are connected with a small thermal resistance, so that the heat generated by the heating element is transferred from the bottom surface side of the housing. The rate of heat dissipation is increased. As a result, the temperature rise on the keyboard surface can be suppressed. On the other hand, pay attention to the surface temperature of the housing. FIG. 12 shows the elapsed time, the case surface temperature, and the sensible temperature of the finger when the case surface is touched with a finger after the power is turned off when the case surface temperature is 55 ° C. (when the outside air temperature is 35 ° C.). It is a relationship. As shown in FIG. 12, if the housing surface temperature is 55 ° C. (the temperature difference from the outside air temperature is 20 ° C.), the sensible temperature of the finger does not exceed 45 ° C. (the temperature difference from the outside air temperature is 10 ° C.). . Therefore, in order to suppress the heat radiation from the keyboard surface, the ratio of the heat radiation from the bottom surface of the housing is controlled so that the temperature difference from the outside air temperature is 10 ° C or less on the keyboard surface and 20 ° C or less on the housing bottom surface. Hold down. As a result, the cooling can be performed without making the operator feel uncomfortable. The control of the ratio of the heat radiation amount can be controlled mainly by the cross-sectional area of the heat radiation block, the cross-sectional area and thickness of the flexible heat conducting member, and the thickness and size of the enlarged heat radiation plate shown in FIGS. 1 to 11.

[0019]

According to the present invention, even if the positional relationship between the heating element and the heat dissipation member is different from the wiring board,
It is possible to cool the heating element to a predetermined temperature without disturbing the optimum arrangement configuration with respect to the position of the heating element on the wiring board and the position where the heat dissipation member is installed.

[Brief description of the drawings]

FIG. 1 is a perspective sectional view of an embodiment of the present invention.

FIG. 2 is a detailed cross-sectional view of the embodiment shown in FIG.

FIG. 3 is a sectional view of another embodiment of the present invention.

FIG. 4 is a sectional view of another embodiment of the present invention.

FIG. 5 is a sectional view of another embodiment of the present invention.

FIG. 6 is a sectional view of another embodiment of the present invention.

FIG. 7 is a cross-sectional view of another embodiment of the present invention.

FIG. 8 is a sectional view of another embodiment of the present invention.

FIG. 9 is a sectional view of another embodiment of the present invention.

FIG. 10 is a sectional view of another embodiment of the present invention.

FIG. 11 is a cross-sectional view of another embodiment of the present invention.

FIG. 12 is a diagram for explaining the effect of the present invention.

[Explanation of symbols]

1 ... Wiring board, 2 ... CPU, 3 ... Second wiring board, 7 ...
Housing, 11 ... Radiating plate, 12 ... Flexible heat conducting member, 17 ... Flexible heat conducting member, 20 ... Flexible heat conducting member, 13 ... Expanded metal plate, 19 ... Expanded metal plate, 16 ... Radiating block, 21 ... Metal rod , 22 ... Metal rod

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takeshi Nakagawa 810 Shimoimaizumi, Ebina-shi, Kanagawa Hitachi Ltd. Office Systems Division (72) Inventor Susumu Iwai 810 Shimoimaizumi, Ebina, Kanagawa Stock Company Hitachi Factory Office Systems Division (72) Inventor Hitoshi Matsushima 502 Jintamachi, Tsuchiura City, Ibaraki Prefecture

Claims (9)

[Claims] 1. An electronic device comprising a keyboard mounted in a housing and a wiring board inside the housing, wherein the wiring board includes a second wiring board on which an element to be cooled is mounted, and the cooling board. 1st with elements other than the target element
And a heat dissipation plate is provided on the bottom of the housing, and the keyboard, the second wiring board, the first wiring board, and the heat dissipation board are arranged in this order, and An electronic device comprising a through hole, and means for thermally connecting the element to be cooled and the heat dissipation plate through the through hole. 2. The thermal connection means according to claim 1, wherein the thermally connecting means is thermally provided through a flexible heat conducting member and a heat radiating block provided between the element to be cooled and the heat radiating plate. An electronic device that is connected to. 3. The thermal connection means according to claim 1, wherein the thermally connecting means is provided between the element to be cooled and the heat radiating plate and has flexibility, a metal plate, and a heat conducting member. And an electronic device that is thermally connected via a heat dissipation block. 4. The heat connecting means according to claim 1, wherein a metal plate is attached to the back surface of the second wiring board via an insulating sheet, and the metal plate and the heat radiation block are made of flexible heat. An electronic device that is thermally connected via a conductive member. 5. The first metal plate according to claim 1, wherein the means for thermally connecting is a first metal plate thermally connected to the element to be cooled through a heat conducting member having flexibility. A second metal plate thermally connected to the back surface of the second wiring board via an insulating sheet, and the first metal plate and the heat dissipation block via a flexible heat conducting member. An electronic device that is thermally connected to the heat sink. 6. The electronic device according to claim 5, wherein the first metal plate and the second metal plate are thermally connected. 7. An electronic device comprising a keyboard mounted in a housing and a wiring board having an element to be cooled mounted in the housing, wherein the wiring board has an element to be cooled mounted thereon. And a first wiring board on which elements other than the element to be cooled are mounted,
A heat dissipation plate provided on the bottom of the housing, one or a plurality of rod-shaped metal members penetrating the first wiring board provided on the heat dissipation plate, and the heat dissipation plate of the rod-shaped metal member opposite to the heat dissipation plate. An electronic device comprising: a metal plate provided at an end of the metal plate; and means for thermally connecting the metal plate and the element to be cooled. 8. An electronic device comprising a keyboard mounted in a housing and a wiring board having an element to be cooled mounted in the housing, wherein a heat radiating plate provided at the bottom of the housing and the cooling board. A first metal plate thermally connected to a target element through a flexible heat conduction member, and a first metal plate thermally connected to a rear surface of a main heating element mounting portion of the wiring board via an insulating sheet. A second metal plate, one or more rod-shaped metal members that penetrate the wiring board and fix the first metal plate and the second metal plate, the first metal plate and the An electronic device comprising means for thermally connecting to a heat sink. 9. The electronic device according to claim 1, 7 or 8, wherein the temperature difference between the keyboard surface temperature and the casing surface temperature and the outside air temperature is 10 ° C. or less and 20 ° C. or less, respectively.