Touch panel

FIELD OF THE INVENTION

The present invention relates to a touch panel to be placed on a screen of a liquid crystal display (LCD) or a cathode ray tube (CRT). The touch panel is used for inputting a position following an instruction displayed on the screen and viewed through the touch panel. The position is input through a user pressing the panel with a finger or pen.

BACKGROUND OF THE INVENTION

Recently, a compact and lightweight portable terminal has been popular in the market, and a touch panel, which is used as an input device of the terminal, is required to be compact and lightweight. Further, the touch panel is required to be free from inconvenience even if the terminal is carelessly dropped.

Resistive touch panels have been widely used among the touch panels because the resistive touch panel has rather high detecting accuracy and can be manufactured at an inexpensive cost, and numbers of the touch panels are formed by combining plastic materials.

The conventional touch panel discussed above is described hereinafter with a relevant drawing.

FIG. 4

is a sectional view of the conventional touch panel. The touch panel includes the following elements:

(a) An upper electrode sheet

1

including a flexible transparent film and a transparent electrode disposed on one of the faces of the sheet

1

, the electrode being made of indium tin oxide (ITO) and the like;

(b) A lower transparent plastic board

2

having a transparent electrode;

(c) An insulating spacer

3

A disposed on the periphery of both of the sheet

1

and board

2

for the transparent electrodes to face each other at a given distance; and

(d) A conical spacer

3

, made of soft insulating material, disposed like dots at a given pitch on the transparent electrode of the lower-transparent-plastic board

2

.

The conventional touch panel is usually placed on a screen of an LCD or a CRT, and an operator, who views an instruction displayed on the screen through the touch panel, presses the upper electrode sheet

1

at a given position from above with his/her finger or a pen. This action entails the upper electrode sheet

1

to bow downward partially, and the bowed portion of sheet

1

thus contacts with a corresponding portion of the transparent electrode disposed on the lower plastic board

2

, so that the pressed position can be detected.

The transparent film of the upper electrode sheet

1

is usually made of polyethylene terephthalate because it is inexpensive and flexible enough for the input operation, and further, the ITO, which is formed on one of the faces of the film, is stabilized easily. The lower board

2

is usually made of polycarbonate because this material is inexpensive, highly transparent and processed easily.

Another conventional touch panel is shown in FIG.

5

. This touch panel includes the following elements:

(a) An upper electrode sheet

1

including flexible transparent film and a transparent electrode disposed on one of the faces of sheet

1

, the electrode being made of indium tin oxide (ITO) and the like;

(b) A lower electrode sheet

4

having a transparent electrode;

(c) An insulating spacer

3

A disposed on the periphery of both sheets

1

and

4

for the transparent electrodes to face each other at a given distance;

(d) A transparent holder board

5

made of a rigid transparent plastic board and disposed under the lower face of the sheet

4

, where no electrode is formed;

(e) A transparent bonding layer

6

for bonding the board

5

to the sheet

4

; and

(f) A conical spacer

3

, made of soft insulating material, disposed like dots at a given pitch on the transparent electrode of the lower board

2

.

This touch panel operates similarly to the panel shown in

FIG. 4

previously described, i.e., the touch panel is placed on a screen, and an operator presses the upper electrode sheet

1

at a given position from above. This action entails the upper electrode sheet

1

to bow downward partially, and the bowed portion of sheet

1

thus contacts with a corresponding portion of the transparent electrode disposed on the lower plastic board

2

, so that the pressed position can be detected.

In this touch panel, the electrode sheets

1

and

4

are made of polyethylene terephthalate, and the holder board

5

is made of polycarbonate because of the same reason discussed above.

Both of the conventional touch panels discussed above employ polyethylene terephthalate combined with polycarbonate. These two materials are largely different in coefficients of linear expansion. Therefore, when the touch panel is exposed in a severe temperature or moisture environment, dimensions of members of the panel change differently from each other, which may cause waviness, i.e., distortion on the film, particularly in the case that the film is thin. This phenomenon lowers visibility of the touch panel and makes the distance between the upper and lower transparent electrodes narrower if the waviness is large, thus causing an unexpected short between both of the electrodes.

SUMMARY OF THE INVENTION

A reliable touch panel of the present invention features the following advantages. Even when being exposed in a severe temperature or moisture environment, the touch panel does not has waviness produced on an upper or lower electrode sheet. The panel thus hardly has the electrodes short-circuit and maintains excellent visibility.

The touch panel includes a transparent film, an upper electrode sheet having a first transparent electrode disposed on the transparent film, and a lower electrode board having a rigid transparent plastic board and a second transparent electrode. The second transparent electrode is disposed on the transparent plastic board and faces the first transparent electrode. The first transparent film and the transparent plastic board are made of respective materials containing an identical main component to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1

is a sectional view of a touch panel in accordance with a first exemplary embodiment of the present invention.

FIG. 2

is a sectional view of another touch panel having an upper electrode sheet different from that of the touch panel shown in

FIG. 1

in accordance with the first exemplary embodiment.

FIG. 3

is a sectional view of a touch panel in accordance with a second exemplary embodiment of the present invention.

FIG. 4

is a sectional view of a conventional touch panel.

FIG. 5

is a sectional view of another conventional touch panel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(Exemplary Embodiment 1)

FIG. 1

is a sectional view of a touch panel in accordance with a first exemplary embodiment of the present invention. An flexible upper electrode sheet

11

includes a transparent electrode

11

A which is made of indium tin oxide (ITO) and the like and is formed beneath the lower face of a rectangular transparent film. A lower electrode board

12

includes a transparent plastic board shaped substantially the same as the upper electrode sheet

11

and a transparent electrode

12

A formed on the plastic board. Between the upper electrode sheet

11

and lower electrode board

12

, an insulating spacer

13

having a given width is disposed at the periphery of the sheet

11

and board

12

. Thus, the electrodes

11

A and

12

A face each other at a given space.

In the drawings corresponding to this embodiment, respective thicknesses of the electrodes

11

A and

12

A are depicted exaggeratedly for an articulation purpose.

The transparent film, a base material of the upper electrode sheet

11

, is made of a polycarbonate film. The lower electrode board

12

is made of a material containing an identical main component and an identical structure to that of the upper electrode sheet

11

, and a different functional group than the sheet

11

. Both materials contain the identical main component to each other and the different functional groups bonded with the component for allowing the sheet

11

and board

12

to be processed most appropriately, respectively.

The transparent plastic board, a base material of the lower electrode board

12

, employs the same material as the upper electrode

11

, i.e., a polycarbonate board of thickness of 0.5-2.0 mm.

On the transparent electrode

12

A in the area surrounded by the spacer

13

, conical spacers

14

formed of flexible insulating material are disposed like dots at a given pitch.

The upper electrode sheet

11

may have a hard-coated layer on the face opposite to the face having the electrode

11

A. The hard-coated layer is made of thermosetting resin such as acrylic epoxy resin or urethane resin, or photo-curable resin of acrylate resin. The electrode sheet

11

is formed through forming the transparent electrodes

11

A sequentially on a raw rolled film and cutting the film at a given interval to be the sheet

11

.

The touch panel discussed above is placed on the screen of an LCD or a CRT. An operator, who views an instruction displayed on the screen through the touch panel, depresses and bows the upper electrode sheet

11

at a particular portion with his/her finger or a pen following the instruction. The electrodes

11

A and

12

A corresponding to bowed portion of the upper electrode sheet

11

and lower electrode board

12

thus contact with each other, thereby detecting the depressed position.

The depressed position is detected by the following way. While the transparent electrode

11

A has opposing edges to which a certain voltage is applied, a voltage at the position is detected, and a corresponding position between the opposing edges is thus detected. Then, the certain voltage is released from the edges. Then, while the transparent electrode

11

A has other opposing edges to which a certain voltage is applied, a voltage at the position is detected, and a corresponding position between the other opposing edges is thus detected similarly to the above.

Since being made of the respective materials containing the identical main component, the sheet

11

and board

12

of the touch panel in accordance with this first embodiment have respective dimensions that differ very little from each other even in a severe temperature or moisture environment. Therefore, the upper electrode sheet

11

hardly has waviness, the lower electrode board

12

can have reduced warp, and the touch panel thus maintains high visibility. The reduced waviness and warp eliminate an unexpected short-circuit between the electrodes

11

A and

12

A, thus allowing the touch panel to have stable quality. A portable electronic apparatus with this touch panel can thus be operated even in a critical environment.

The sheet

11

and board

12

discussed above are made of polycarbonate resin, however, they may be made of alicyclic olefin resin. This alicyclic olefin resin, since having a higher light-transmission and a higher heat-resistance than the polycarbonate resin, produces less dimensional differences of respective elements even in an ambient temperature or environment. As a result, this alicyclic olefin resin easily provides a touch panel having a high light-transmission and an excellent visibility. The alicyclic olefin resin, since featuring a low double-refraction, provides an inner touch panel having excellent visibility.

Versatile material such as polyether sulphone resin or polyarylate can be used as the base material of the upper electrode sheet

11

and the lower electrode board

12

. In this case, a high quality touch panel is also obtainable.

The sheet

11

and board

12

are not necessarily made of the same material, but can be made of different materials as far as the materials have only a small difference in linear-expansion coefficients. In this case, the same advantage as discussed previously can be expected.

A resistive touch panel is usually guaranteed to work properly within a temperature ranging from −20° C. to +70° C. The upper electrode sheet

11

having a thickness of approximate 170 &mgr;m and a length of one side not more than 80 mm is widely used. Based on these factors, upon being made of the materials having a difference in coefficients of linear expansion of not more than 1×10

−5

(1/° C.), the sheet

11

and board

12

have the dimensional difference between respective changes within the guaranteed temperature range regulated to not more than 40 &mgr;m, i.e., approximately one third of the thickness of the sheet

11

. As a result, waviness on the sheet

11

and warp of the board

12

are reduced, so that the panel maintains excellent visibility.

In the above discussion, the sheet

11

is made of a sheet of a transparent film. However, as shown in sectional view of

FIG. 2

, an upper electrode sheet

31

may be formed of two thin transparent films

33

A,

33

B bonded to each other with a transparent adhesive

32

and a transparent electrode

31

A disposed beneath the film

33

B at the side where no film

33

A is bonded.

In this case, the thin transparent films

33

A,

33

B and the lower electrode board

12

, upon being made of the same material or materials having differences in linear-expansion coefficients not more than 1×10

−5

(1/° C.), can also provide a quality touch panel having an excellent visibility similarly to the previously discussed panel. In this case, the upper electrode sheet

31

becomes more flexible, and thus, the place around the spacer

13

on the upper sheet

31

can be depressed more easily than other cases discussed above. The more flexible sheet

31

also contributes to the touch panel with light-touch operation and durability.

(Exemplary Embodiment 2)

Elements similar to those in the first embodiment are denoted by the same reference marks as those in the first embodiment, and the descriptions thereof are thus omitted.

FIG. 3

is a sectional view of a touch panel in accordance with the second embodiment of the present invention. An upper electrode sheet

11

includes a transparent film shaped approximately as a rectangle and a transparent electrode

11

A disposed beneath the lower face of the film. The lower electrode sheet

41

includes a transparent film shaped in approximately the same form and a transparent electrode

41

A disposed on the upper face of this film. The sheet

11

and sheet

41

are formed of the same transparent film such as polycarbonate film or alicyclic olefin film. A spacer

13

is formed at a given width around the sheet

11

and sheet

41

, and thus, the transparent electrodes

11

A and

41

A face each other at a given space.

In the drawing corresponding to the second embodiment, the thickness of electrodes

11

A,

41

A are exaggeratedly illustrated for an articulation purpose similar to the first embodiment.

On the transparent electrode

41

A of the area surrounded by the spacer

13

, conical spacers

14

formed of flexible insulating material are disposed like dots at given pitch.

Beneath the lower face of the sheet

41

, where electrode

41

A is not formed, a transparent holder board

43

is solidly bonded via a transparent adhesive layer

42

. The holder board

43

, which has a thickness of 0.5-2.0 mm, is formed of a transparent rigid plastic board made of a material containing an identical main component to that of the sheet

11

and sheet

41

.

The transparent holder board

43

, the upper electrode sheet

11

, and the lower electrode sheet

41

are made of respective materials containing an identical main component and an identical structure to each other, and different functional groups than each other. That is, the materials belong to an identical resin group. The transparent adhesive layer

42

preferably contains, as a main component, acrylate resin, polyester resin, or mixture of them, and has an elastic coefficient ranging 1×10

5

−1×10

7

dyn/cm

2

and a thickness of not less than 1 &mgr;m.

The touch panel in accordance with the second embodiment is placed on a screen on an LCD or a CRT similar to the first embodiment. An operator, who views an instruction displayed on the screen through the touch panel, depresses and bows the upper-electrode sheet

11

at a particular portion with his/her finger or a pen following the instruction. A specific portion corresponding to the bowed portion of the electrodes

11

A and

41

A of the upper electrode sheet

11

and the lower electrode sheet

41

contact with each other, thereby detecting the depressed position.

The position may be detected similar to embodiment 1.

The transparent adhesive layer

42

between the sheet

41

and board

43

functions as a shock absorber when the upper sheet

11

is depressed, and thereby, protects the sheets

11

,

41

and the electrodes

11

A,

41

A. As a result, the touch panel has an improved durability.

The touch panel in accordance with this second embodiment employs the same base material group in the upper and lower sheets

11

,

41

and the holder board

43

, namely, each element has the same linear-expansion coefficient. Therefore, even if the touch panel is exposed in a severe temperature or moisture environment, the touch panel has elements having little difference in dimensional changes, and has reduced stresses due to the changes between the elements. Waviness on the sheets

11

,

41

and warp of the holder board

43

are thus scarcely produced. As a result, a quality touch panel with excellent visibility and free from an unexpected short-circuit between the electrodes

11

A and

41

A is obtainable.

As discussed in the first embodiment, the upper sheet

11

, lower sheet

41

and holder board

43

are not necessarily made of the same material. They can be made of different materials as far as the materials have differences in the linear-expansion coefficient of not more than 1×10

−5

(1/° C.). Then, the touch panel produces the advantage similar to the panel discussed previously.

The upper electrode sheet

11

may have a hard-coated layer disposed on the face opposite to that having the electrode

11

A, as discussed in the first embodiment. Further, the sheet

11

can be formed of two transparent films bonded to each other.