Key sheet, press switch and electronic device provided with the press switch

It is an object of the present invention to provide a key sheet and the like which can suppress local elevation of temperature, and effectively diffuse heat loss from electronic circuits. The key sheet includes: a viscoelastic sheet 16b having a viscoelastic property, and having a first surface and a second surface; a button section 16a located on the side of the first surface of the viscoelastic sheet 16b; a thermally-conductive sheet 14 located along the first surface or the second surface of the viscoelastic sheet 16b, the thermally-conductive sheet 14 having a thermal conductivity equal to a specific value; and a contact section 16d projected from the second surface of the viscoelastic sheet 16b, the contact section occupies a position corresponding to the button section 16a.

TECHNICAL FIELD

The present invention relates to a key sheet, a press switch and an electronic device provided with the key sheet, and more particularly to a key sheet improved in heat radiation performance as an element useful for a portable electronic device, a press switch, and an electronic device provided with the key sheet.

BACKGROUND OF THE INVENTION

In recent years, an electronic device or more specifically a portable electronic device has been needed to be improved in size, thickness and function. Therefore, it is necessary to enhance the density of electronic components to be mounted on a printed-circuit board of the portable electronic device, and to improve the portable electronic device in heat radiation performance.

As shown inFIGS. 13 to 15, the portable electronic device of this type is exemplified by a mobile phone. As shown inFIG. 13, the electronic device100includes a lower housing103, an upper housing105, and a hinge unit104for connecting the lower housing17with the upper housing105to allow opening and closing movements of the lower housing17and the upper housing105. The lower housing103has an operation input unit102and a sound input unit103accommodated therein, while the upper housing104has a screen106and a sound output unit107accommodated therein.

The lower housing101has a front member101aand a rear member101b. As shown inFIG. 14, a printed-circuit board121for communications and input/output control, and a key sheet122having an elastic sheet section122aretain key tops122b,122c, and122d, and a flexible electrically-insulating sheet123. When the key tops122b,122c, and122dare operated, the press switches corresponding to the key tops122b,122c, and122dselectively assume open and closed state (seeFIG. 15).

More specifically, the flexible electrically-insulating sheet123has an area which corresponds to a key contact section120, and has bores125as shown inFIG. 15. The flexible electrically-insulating sheet123includes a thermally-conductive sheet123aconstituted by a sheet made of electrically-nonconducting and thermally-conductive material, an electrically-conductive film123blocated on the opposite side of the thermally-conductive sheet123afrom the printed-circuit board121, a resin sheet123clocated on the opposite side of the electrically-conductive film123bfrom the thermally-conductive sheet123a, and dome-shaped sections124constituted as switch elements corresponding to contact points120on the printed-circuit board121, made of metal, and received in the bores125(see patent document 1). The above-mentioned press switch can diffuse heat loss from the electronic circuit129to avoid the temperature elevation of a surface for key operations on the side to be operated.

Patent document 1: Japanese Patent Laid-Open Publication 2006-310035

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

In the conventional press switch, the thermally-conductive sheet123ais in contact with the printed-circuit board121, and covered with the electrically-conductive film123band the key sheet122. As a result, the conventional electronic device is increasingly reduced in heat radiation performance, and makes it difficult to diffuse heat loss from the electronic circuit129. Further, the electrically-insulating sheet is generally low in heat conductivity in comparison with the thermally-conductive sheet. As a result, the conventional electronic device is further reduced in heat radiation performance by reason that the electrically-insulating sheet is used in the conventional electronic device.

It is therefore an object of the present invention to provide a key sheet, a press switch and an electronic device, each of which can prevent the housing the button section from being excessively heated by the electronic circuit by controlling and suppressing local elevation of temperature resulting from heat loss from the electronic circuit, and enhance heat radiation performance to effectively diffuse heat loss from the electronic circuit.

Means for Solving the Problems

The key sheet according to the present invention comprises: a viscoelastic sheet having a viscoelastic property, and having a first surface and a second surface; a button section located on the side of the first surface of the viscoelastic sheet; a thermally-conductive sheet located along the first surface or the second surface of the viscoelastic sheet, the thermally-conductive sheet having a thermal conductivity equal to a specific value; and a contact section projected from the second surface of the viscoelastic sheet, the contact section occupies a position corresponding to the button section.

The key sheet thus constructed is increased in heat radiation performance by reason that the key sheet is in the vicinity of the electronic components mounted on the printed-circuit board, and the thermally-conductive included in the key sheet is close to external air.

In the key sheet according to the present invention, the button section may include a first button section, a second button section, and a third button section which is not on a straight line passing through the first and second button sections, the thermally-conductive sheet is in an area identified by the first to third button sections.

The key sheet thus constructed can suppress local elevation of temperature to even the temperature distribution by reason that the thermally-conductive sheet is in an area identified by the first to third button sections.

In the key sheet according to the present invention, the thermally-conductive sheet may have a first surface and a second surface, the thermally-conductive sheet is located under the condition that the second surface of the thermally-conductive sheet is in contact with the first surface of the viscoelastic sheet.

Under the condition that the second surface of the thermally-conductive sheet is in contact with the first surface of the viscoelastic sheet, the key sheet according to the present invention may further comprise an electrically-insulating cover layer having an insulating property. The electrically-insulating cover layer is in contact with the viscoelastic sheet, and covers a peripheral section of the thermally-conductive sheet.

Under the condition that the second surface of the thermally-conductive sheet is in contact with the first surface of the viscoelastic sheet, in the key sheet according to the present invention, the thermally-conductive sheet may have an opening, the contact section occupies a position of the opening, and is in contact with the first surface of the viscoelastic sheet.

The button section can be illuminated by the LED mounted on the printed-circuit board.

Under the condition that the second surface of the thermally-conductive sheet is in contact with the first surface of the viscoelastic sheet, in the key sheet according to the present invention, the thermally-conductive sheet may have a character-shaped opening.

The button section can be illuminated through the character-shaped opening.

In the key sheet according to the present invention, the thermally-conductive sheet having a first surface and a second surface, the thermally-conductive sheet may be located under the condition that the first surface of the thermally-conductive sheet is in contact with the second surface of the viscoelastic sheet.

In the key sheet according to the present invention, the thermally-conductive sheet has an opening, the contact section may occupy a position of the opening, and may be in contact with the second surface of the viscoelastic sheet.

The key sheet according to the present invention may further comprise an electrically-insulating cover layer having an insulating property, the electrically-insulating cover layer is in contact with the viscoelastic sheet, and covers a peripheral section of the thermally-conductive sheet. The electrically-insulating cover layer may have a white or glossy-colored upper section. The electrically-insulating cover layer may be white or glossy.

In the key sheet according to the present invention, the electrically-insulating cover layer may have a visible light reflective property.

The key sheet thus constructed can guide visible light to a specific section in the housing to illuminate the button sections through the electrically-insulating cover layer without irregular color. In this case, the electrically-insulating cover layer may have a white or glossy-colored upper section. The electrically-insulating cover layer may be white or glossy.

In the key sheet according to the present invention, the thermally-conductive sheet may be constituted by a sheet made of graphite.

The press switch comprises: a printed-circuit board provided with an electronic circuit, the printed-circuit board having a first surface and a second surface; a switch section located on the first surface of the printed-circuit board, the switch section having a push point to change the connection state of the electronic circuit, and a key sheet defined in claim1, and located in relation to the push point.

The press switch thus constructed can suppress local elevation of temperature to even the temperature distribution by reason that the thermally-conductive sheet is in an area identified by the first to third button sections.

In the press switch according to the present invention, the thermally-conductive sheet may be constituted by a sheet made of electrically-conductive material, and the printed-circuit board may have an electrically-conductive layer electrically connected to the thermally-conductive sheet.

The electronic device according to the present invention comprises the above press switch.

The electronic device thus constructed can suppress local elevation of temperature to even the temperature distribution by reason that the thermally-conductive sheet is in an area identified by the first to third button sections by reason that the heat radiation performance is increased by reason that the key sheet is in the vicinity of the electronic components mounted on the printed-circuit board, and the thermally-conductive included in the key sheet is close to external air.

The term “thermally-conductive sheet” is intended to indicate a sheet larger in thermal conductivity than the electrically-insulating cover layer and other members.

The following description is directed to specific values of thermal conductivity of the thermal conductive sheet. For example, the thermally-conductive sheet may be made of graphite, and may be set to 700 (W/(m·K)) in thermal conductivity on the surface of the thermally-conductive sheet (in X-Y direction) under the condition that the thermally-conductive sheet made of graphite is 100 μm in thickness (in Z-direction). The thermally-conductive sheet may be set to 850 (W/(m·K)) in thermal conductivity in any direction on the surface (in X-Y direction) under the condition that the thermally-conductive sheet made of graphite is 70 μm in thickness (in Z-direction). The thermally-conductive sheet made of graphite may be set to 1600 (W/(m·K)) in thermal conductivity in any direction on the surface (in X-Y direction) under the condition that the thermally-conductive sheet made of graphite is 25 μm in thickness (in Z-direction).

The thermally-conductive sheet may be made of aluminum, and may be set to 237 (W/(m·K)) in thermal conductivity on the surface of the thermally-conductive sheet (in X-Y direction). The thermally-conductive sheet may be made of copper, and may be set to 398 (W/(m·K)) in thermal conductivity on the surface of the thermally-conductive sheet (in X-Y direction).

Advantageous Effect of the Invention

The present invention is to provide a key sheet, a press switch, and an electronic device improved in heat radiation performance can effectively diffuse heat loss from an electronic circuit and the like.

EXPLANATION OF THE REFERENCE NUMERALS

PREFERRED EMBODIMENTS OF THE INVENTION

First Embodiment

FIG. 1is a perspective view showing an electronic device according to the first embodiment of the present invention. As shown inFIG. 1, the electronic device1includes a lower housing17, an upper housing105, and a hinge unit104for connecting the lower housing17with the upper housing105to allow the upper housing105to be pivotally movable with respect to the lower housing17. An operating section102and a sound input section103are in the lower housing17, while a sound output section107and a screen106are in the upper housing105. The lower housing17has a housing member17aon the rear side to be operated and a housing member17bon the rear side. As shown inFIG. 2, a printed-circuit board11for communications and input/output control, and a key sheet16for press switches are further in the lower housing17.

FIG. 3(a) is a perspective view showing a key sheet for press switches of the electronic device according to the first embodiment, whileFIG. 3(b) is an exploded perspective view showing a key sheet for press switches of the electronic device according to the first embodiment. As shown inFIGS. 3(a) and3(b), the key sheet16includes button sections16aconstituted by a plurality of button sections16a-1,16a-2,16a-3, . . . , a viscoelastic sheet16b, and a thermally-conductive sheet14.FIGS. 4(a) and4(b) are cross-sectional views showing a press switch according to the first embodiment of the present invention.

As shown inFIG. 5, a plurality of press switches10, each of which is shown inFIG. 4, are accommodated in the lower housing17of the electronic device1improved in size and thickness. Additionally, the electronic device1may be constituted by a mobile phone, a personal digital assistant (PDA), or an electronic device improved in size and thickness.

As shown inFIG. 4, the printed-circuit board11is covered on one surface with an electrically-insulating sheet13. In the press switch10according to the first embodiment, the first and second contact sections11aand11bon the printed-circuit board11are located on the inside of a flexible click section13aof the electrically-insulating sheet13, and can be electrically connected to each other. As shown inFIG. 5, the press switches10, the printed-circuit board11, and the key sheet16are in the lower housing17.

As shown inFIG. 5, the button sections16ato be selectively pushed are operatively arranged on the flexible viscoelastic sheet16bof the key sheet16. The viscoelastic sheet16bof the key sheet16has projections16cextending from the lower surface of the viscoelastic sheet16bto the electrically-insulating sheet13, and portions16dto be respectively engaged with the click sections13aof the electrically-insulating sheet13.

As shown inFIG. 4, the key sheet16includes a thermally-conductive sheet14located on the upper surface of the viscoelastic sheet16b, and an insulating layer15located on the upper surface of the thermally-conductive sheet14. For example, as shown inFIG. 3(a), the thermally-conductive sheet14has a portion in an area30surrounded by button sections16a-1,16a-2, and16a-3which did not located in the same straight line. Here, the thermally-conductive sheet14is larger in thermal conductivity than the printed-circuit board11, the electrically-insulating sheet13, the viscoelastic sheet16b, and the front member17aof the lower housing17. The thermally-conductive sheet14is made from graphite sheet, metal sheet, or the like which exceeds other materials in thermal conductivity. The viscoelastic sheet16bis constituted by a sheet made of silicon rubber or the like.

The electrically-insulating sheet13is constituted by a sheet made of electrically-insulating resin such as for example polyethylene terephthalate (PET), and an adhesive layer or an electrically-insulating adhesive layer (not shown). Further, the electrically-insulating cover layer15is also constituted by a sheet made of electrically-insulating resin such as for example polyethylene terephthalate (PET).

When the click section13a(pressure point) is pressed, the press switch10of the electronic circuit assumes a conduction state by reason that the first contact section11ais electrically connected to the second contact section11b. The click section13aof the electrically-insulating sheet13on the printed circuit board11is constituted as a circular portion projected on the operation side of the electronic device1, and distant from the printed circuit board11.

Additionally, the click section13amay not assume a convex shape when the click section13ais not in the pushed state (a state in which the electronic device is not operated through the press switch). The click section13aand the center section12cmay assume a predetermined position when the click section13ais in a released state.

Further, the electrically-insulating sheet13may have a restorative force necessary to assume an original position. On the other hand, a member constituted as a contact section or a member to be electrically connected to the member has a restorative force necessary to assume an original position. Therefore, the click section13ahas flexibility to allow the center section12cof the third contact section12to be changed in response to a force from the button section16a.

More specifically, as shown inFIG. 4, two or more second contact sections11bformed on the printed-circuit board11are in spaced relationship with each other, the first contact section11ais between the second contact sections11b. As another example, two or more second contact sections11bmay be formed on a circumferential line of a circle under the condition that the first contact section11amay be formed at a center of the circle.

As shown inFIG. 4, the third contact section12constituted by, for example, a metal diaphragm (dish-shaped electrically-conductive plate spring having the shape of a circular arc in cross section) is electrically connected to the second contact sections11b, and adhered to the inner surface of the click section13aof the electrically-insulating sheet13.

The third contact section12allows the center section12cto function as a movable contact point. When the button section16ais pushed down by the user, the third contact section12sags downwards in the center in response to a force from the button section16aof the key sheet16through the click section13aof the electrically-insulating sheet13to assume a state in which the first contact section11ais electrically connected to the second contact section11bthrough the third contact section12.

When, on the other hand, the force for pushing the button section16aof the key sheet16is released, the third contact section12assumes a state in which the first contact section11ais not electrically connected to the second contact section11b, and restores to its original state.

The electrically-insulating sheet13is adhered to and retained by the printed-circuit board11as an insulation protection layer, while the third contact section12is adhered to the electrically-insulating sheet13.

The electrically-insulating cover layer15is adhered to the thermally-conductive sheet14as an insulation protection layer. The thermally-conductive sheet14is located throughout all parts of the body. The upper surface and the peripheral portion of the thermally-conductive sheet14are covered with the electrically-insulating sheet15. It is preferable that the peripheral portion of the thermally-conductive sheet14be electrically insulated by the electrically-insulating sheet15.

In this embodiment, the third contact section12is retained and adhered by an adhesive layer (not shown) to the click section13aof the electrically-insulating sheet13under the condition that the lower portions of the third contact section12are positioned and electrically connected to the second contact sections11bon the printed-circuit board11.

The key sheet16is located along one side of the electrically-insulating sheet13, while the printed-circuit board11is located along the other side of the electrically-insulating sheet13. The button sections16aoperatively arranged on the key sheet16are exposed as keys through openings of the housing member17aon the side to be operated. On the other hand, the contact sections16dfrom the lower side of the key sheets16are respectively in contact with the click section13aof the electrically-insulating sheet13.

As shown inFIG. 5, light-emitting members such as for example LEDs (light emitting diodes)18are mounted on the upper side of the printed-circuit board11, in other words, a surface facing the key sheet16, and used to illuminate, from the inside of the lower housing17, the button sections16asuch as for example keys and the like to be used to input numbers and characters. On the other hand, heat generating components19such as power amplifiers and other electronic components are mounted on the lower side of the printed-circuit board11, and generate heat loss in the lower housing17.

In the electronic device1, a plurality of electronic components (not shown) are mounted on the printed-circuit board11as a control circuit for communications and input/output control and accommodated in the lower housing17. When the electronic device1is running, heat generating components19such as power amplifiers and the like mounted on the printed-circuit board11generates heat loss in the lower housing17. As a result, the heat generating components19and its surrounding components are heated, and produce an increase in temperature the inside of the lower housing17.

In this state, heat loss from the printed-circuit board11is diffused through the thermally-conductive sheet14formed along the viscoelastic sheet16bof the key sheet16(in a spreading direction). As a result, the printed-circuit board11can suppress local elevation of temperature of the button sections16aand the housing member17ain the vicinity of the heat generating components19of the electronic device1.

In the conventional press switch disclosed in the patent document 1, a thermally-conductive sheet in contact with the printed-circuit board is covered with an electrically-conductive film and a key sheet, and away from external air. As a result, it is difficult to effectively diffuse heat loss from the heat-generating electronic component19. The heat radiation performance is decreased. On the other hand, the heat radiation performance of the electronic device1is increased by reason that the thermally-conductive sheet14included in the key sheet16is close to external air in the press switch10according to the first embodiment.

The thermally-conductive sheet14is improved in radiation effect under the condition that, for example, the thermally-conductive sheet14is made of graphite, and 700 or more (W/(m·K)) in thermal conductivity in the direction of the thermally-conductive sheet14. As a result, thermally-conductive sheet14can be reduced in thickness to 100 μm or less. Therefore, the key sheet16is reduced in thickness. The electronic device1can be further reduced in thickness by comprising a press switch10reduced in thickness.

FIG. 6is a diagram showing a result obtained from a computer simulation on the temperature distribution of the housing member17aon the basis of position, heat loss, and the like of the heat-generating electronic component19of the electronic device according to the first embodiment of the present invention.FIG. 6(a) is a diagram showing the temperature distribution of the housing member as a result obtained from a computer simulation.FIG. 6(b) is a graph showing the temperature distribution of the cross section taken along the line X-X shown inFIG. 6(a).

The computer simulation has been executed under the condition that the housing member17aon the side to be operated is 0.9 millimeters in thickness and 0.3 (W/(m·K)) in thermal conductivity, the printed-circuit board11is 0.5 millimeters in thickness and 35 (W/(m·K)) in thermal conductivity, the viscoelastic sheet16bis 0.5 millimeters in thickness (the height of the viscoelastic sheet16babove the lower end of the projection16cis 0.1 millimeters) and 0.2 (W/(m·K)) in thermal conductivity, the heat-generating electronic component19is 1.0 millimeters in thickness and 1 (W/(m·K)) in thermal conductivity, the thermally-conductive sheet14is constituted by a sheet made of graphite, the electrically-insulating sheet13is 0.1 millimeters in thickness and 700 (W/(m·K)) in thermal conductivity (in a direction along its surface), and the printed-circuit board11has a section corresponding to the button sections16a, the section is covered with the electrically-insulating sheet13.

From this computer simulation, it will be understood that, in the electronic device1according to the first embodiment, the temperature of the housing member17ais equalized within the section covered with the electrically-insulating sheet13, and kept below the designated level. Further, fromFIG. 6(b), it will be understood that the peripheral portion of housing (both ends in horizontal) is hardly influenced by heat loss from the electronic circuit, and the local elevation of temperature of the section to be operated is kept within a few degrees.

On the other hand,FIG. 7is a view showing a result obtained from a contrastive computer simulation on the temperature distribution of the operational surface of the housing member of the electronic device under the condition that the thermally-conductive sheet14is limited in size by the button section16a.FIG. 7(a) is a view schematically showing the temperature distribution zoned by isothermal lines over the operational surface of the housing member of the electronic device.FIG. 7(b) is a graph showing a temperature distribution of a cross section taken along the X-X line shown inFIG. 7(a).

In this case, the temperature distribution of the electronic device shown inFIG. 7(a) is influenced by heat loss from the heat generating electronic component, and not even. On the other hand, the temperature distribution of the electronic device shown inFIG. 7(b) is even without being influenced by heat loss from the heat generating electronic component.

From a result obtained from a computer simulation on the first embodiment shown inFIG. 6and a result obtained from a contrastive computer simulation shown inFIG. 7, it will be understood that the electronic device1according to the first embodiment of the present invention effectively prevent local elevation of temperature resulting from heat loss from the heat-generating electronic component19and its vicinity by enhancing an even distribution effect of heat loss along the surface of the key sheet16.

The button sections16aof the key sheet16can be illuminated with light from light emitting diodes18mounted on the printed-circuit board11through openings of the thermally-conductive sheet14. The button section16aof the key sheet16can be illuminated with light from the light emitting diode18mounted on the printed-circuit board11through character-shaped openings of the thermally-conductive sheet14.

In this embodiment, the electrically-insulating cover layer15or the viscoelastic sheet16bhas a notched section corresponding to a contact section (not shown), the thermally-conductive sheet14is exposed and electrically connected to the grounded pattern of the printed-circuit board11through conductive layer and metal spring. Therefore, the thermally-conductive sheet14electrically connected to the grounded pattern of the printed-circuit board11can prevent the electronic device1from functioning improperly by preventing static electrical charge from flowing into each contact section.

The electronic device can be improved without being increased in the number of assembling process by reason that the key sheet16includes a thermally-conductive sheet14provided along the viscoelastic sheet16b, the thermally-conductive sheet14is stacked when the key sheet16is mounted on the electronic device.

Even if the thermally-conductive sheet made of graphite is reduced in thickness, the thermally-conductive sheet reduced in thickness can be enhanced in thermal conductivity on the basis of conventionally-known technique for enhancing the thermal conductivity of the thermally-conductive sheet made of graphite and reduced in thickness.

FIG. 8is a diagram showing a table of specific values in thermal conductivity of the thermally-conductive sheet. As shown inFIG. 8, the thermally-conductive sheet may be made of graphite, and set to 700 (W/(m·K)) in thermal conductivity in a direction based on the surface of the thermally-conductive sheet (in X-Y direction) under the condition that the thermally-conductive sheet is 100 μm in thickness (in Z-direction). The thermally-conductive sheet may be 850 (W/(m·K)) in thermal conductivity in a direction based on the surface (in X-Y direction) under the condition that the thermally-conductive sheet made of graphite is 70 μm in thickness (in Z-direction). The thermally-conductive sheet made of graphite may be 1600 (W/(m·K)) in thermal conductivity in a direction based on the surface (in X-Y direction) under the condition that the thermally-conductive sheet made of graphite is 25 μm in thickness (in Z-direction).

As another example, the thermally-conductive sheet may be made of aluminum, and set to 237 (W/(m·K)) in thermal conductivity in a direction based on the surface of the thermally-conductive sheet (in X-Y direction). The thermally-conductive sheet may be made of copper, and set to 398 (W/(m·K)) in thermal conductivity in a direction based on the surface of the thermally-conductive sheet (in X-Y direction).

Second Embodiment

FIG. 9is a perspective view showing the outline of an electronic device according to the second embodiment of the present invention. As shown inFIG. 9, the electronic device according to the second embodiment is the same in appearance as the electronic device according to the first embodiment. The constitutional units of the electronic device according to the second embodiment substantially the same in construction as those of the electronic device according to the first embodiment will be simply described hereinafter and bear the same reference characters as those of the electronic device according to the first embodiment. On the other hand, the difference between the electronic devices according to the first and second embodiments will be described in detail hereinafter.

As shown inFIG. 10, a lower housing17is equipped with a printed-circuit board11for communication and input/output controls and a key sheet26for press switches are in.FIG. 11(a) is a perspective view showing a key sheet26for press switches of the electronic device according to the second embodiment of the present invention, whileFIG. 11(b) is an exploded perspective view showing a key sheet26for press switches of the electronic device according to the second embodiment of the present invention. The key sheet26includes a plurality of button sections26a-1,26a-2,26a-3, . . . , a viscoelastic sheet26b, and a thermally-conductive sheet24.FIGS. 12(a) and12(b) are cross-sectional views showing a press switch according to the second embodiment of the present invention.

As shown inFIG. 12(a), the printed-circuit board11has a surface covered with an electrically-insulating sheet13. In the press switch20according to the second embodiment, the first and second contact sections11aand11bformed on the printed-circuit board11are just below a flexible click section13aof the electrically-insulating sheet13. The press switch20assumes an operation state in which the first and second contact sections11aand11bare electrically connected with each other when the click section13ais pushed to the first contact section11a. The press switch20and the printed-circuit board11are in the lower housing17of the electronic device1. Further, the key sheet26is in the lower housing17.

More specifically, as shown inFIG. 12(a), the first contact section11ais located between the second contact sections11belectrically connected to the third contact section12formed on the click section13a.

The third contact section12sags downwards in the center in response to a force from the button section26aof the key sheet26through the click section13aof the electrically-insulating sheet13to assume a state in which the first contact section11ais electrically connected to the second contact section11bthrough the third contact section12. When, on the other hand, the force from the button section26aof the key sheet26through the click section13aof the electrically-insulating sheet13is released from the third contact section12, the third contact section12is away from the first contact section11ato assume a state in which the first contact section11ais not electrically connected to the second contact section11bthrough the third contact section12.

More specifically, as shown inFIG. 12(a), the third contact section12is adhered to and retained by the electrically-insulating sheet13, and adhered to the printed-circuit board11.

On the other hand, the key sheet26includes a thermally-conductive sheet24and an electrically-insulating cover layer25on the opposite side of the button sections26a. Here, the thermally-conductive sheet24is larger in thermal conductivity the electrically-insulating cover layer25of the key sheet26and the printed-circuit board11, and may be made of, for example, graphite or metal. The electrically-insulating cover layer25may be made of, for example, resin such as for example polyethylene terephthalate.

Here, each of the thermally-conductive sheet24and the electrically-insulating cover layer25has, for example, a circular-shaped opening based on the profile shape of the contact section26dof the key sheet26. As shown inFIG. 12(a), the thermally-conductive sheet24has an inner peripheral section overlapped with the electrically-insulating cover layer25.

As shown inFIG. 12(a), the key sheet26according to the second embodiment is constituted by an integrally formed three-layered sheet including an electrically-insulating cover layer25, a thermally-conductive sheet24, and a viscoelastic sheet26b.

As shown inFIG. 11(a), the thermally-conductive sheet24has a portion in an area30surrounded by button sections26a-1,26a-2, and26a-3which did not located in the same straight line. The remaining parts of the key sheet according to the second embodiment are the same in construction as those of the key sheet according to the first embodiment.

From the foregoing description, it will be understood that the key sheet according to the second embodiment of the present invention can effectively diffuse heat loss from the printed-circuit board11to prevent local elevation of temperature by reason that the heat loss from the printed-circuit board11is diffused through the thermally-conductive sheet24located along the viscoelastic sheet26b.

Further, the button sections26aof the key sheet26can be evenly illuminated with light from the light emitting diode mounted on the printed-circuit board11through an opening of the contact section of the thermally-conductive sheet24.

In the second embodiment, the button section26aof the key sheet26can be evenly illuminated with light from the light emitting diode mounted on the printed-circuit board11through an opening for the contact section26dby reason that part or all of the electrically-insulating cover layer25.

As shown in12(b), the electrically-insulating cover layer25may be constituted by a transparent sheet. The electrically-insulating sheet27may be constituted by a white or glossy sheet. Additionally, the electrically-insulating sheet27may be constituted by a white or glossy sheet made of resin such as for example polyethylene terephthalate, and may have openings based on the profile shape of the contact sections26dof the key sheet26.

In the key sheet thus constructed, the opening of the electrically-insulating sheet27is larger in size than that of the transparent electrically-insulating sheet, and smaller in size than or equal to that of the thermally-conductive sheet. As a result, the passage of light from the LED18mounted on the printed-circuit board11to the button section26aof the key sheet26can be increased in comparison with the construction shown inFIG. 12(a). Therefore, the illumination of the button section26acan further increased.

In the second embodiment, the thermally-conductive sheet24is electrically connected to the grounded pattern of the printed-circuit board11through conductive layer and metal spring. As a result, the thermally-conductive sheet14electrically connected to the grounded pattern of the printed-circuit board11can prevent the electronic device1from functioning improperly by preventing static electrical charge from flowing into each contact section.

From the foregoing description, it will be understood that the electronic device according to the second embodiment of the present invention can be improved without being increased in the number of assembling processes by reason that the key sheet26is constituted by a layered sheet including a thermally-conductive sheet24located along the viscoelastic sheet26b.

INDUSTRIAL APPLICABILITY

From the foregoing description, it will be understood that the key sheet according to the present invention can effectively diffuse heat loss from heat-generating electronic components to prevent local elevation of temperature by reason that the heat loss from heat-generating electronic components is diffused through the thermally-conductive sheet located along the viscoelastic sheet, and useful for a small and thin-model electronic device to be frequently carried and touched with one's hand.