Display unit, manufacturing method thereof and electronic device equipped with same

A display unit comprises a transparent touch switch and a liquid crystal display having a display surface. At least an input area of the transparent touch switch is laminated to the display surface of the liquid crystal display by clear adhesive. The clear adhesive has a permeability after hardening which is close to the permeability of the liquid crystal display and the transparent touch switch. Clear fillers are disposed in the clear adhesive to prevent warping of the transparent touch switch and the liquid crystal display.

BACKGROUND OF THE INVENTION
 The present invention relates to a display unit having a touch switch, a
 manufacturing method of the same, and an electronic device equipped with
 such a display unit.
 Heretofore, as shown in FIG. 7, as a method for disposing a transparent
 touch switch on a liquid crystal display, a transparent touch switch 4 is
 adhered and secured by a double coated adhesive 10 having a thickness of
 about 0.5 mm to 1 mm and provided at the periphery of a liquid crystal
 display which is concealed and held between at least two transparent
 substrates 1a and 1b, and optical films 2a and 2b are pasted on the
 surface of the transparent substrates.
 However, when the transparent touch switch is disposed on the liquid
 crystal display, the image quality thereof degrades because light coming
 from the outside is reflected on the surface of the transparent touch
 switch and on the surface between the transparent touch switch and the
 display panel. In particular, the screen is darkened and the image quality
 degrades remarkably when the touch switch is placed on a reflective type
 liquid crystal display.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to improve the image quality of a
 display unit equipped with a transparent touch switch.
 Another object of the present invention is to provide a method for
 manufacturing a display unit equipped with a transparent touch switch and
 having an improved image quality.
 Another object of the present invention is to provide an electronic device
 having a display unit equipped with a transparent touch switch and having
 an improved image quality.
 In order to solve the above-mentioned problem in the prior art, the
 foregoing and other objects of the present invention are carried out by a
 display unit comprising a transparent touch switch and a liquid crystal
 display having a display surface. The display surface of the liquid
 crystal display is adhered to the back of an input area of the transparent
 touch switch by clear adhesive. The transparent touch switch is preferably
 of an analog resistance film type, a digital resistance film type, an
 electrostatic capacity type or an ultrasound type. The liquid crystal
 display is formed and supported between two transparent films or
 transparent substrates. Because this arrangement allows the reflectance
 between the transparent touch switch and the liquid crystal display to be
 reduced as compared to the prior art display units, the image quality is
 improved.
 In another embodiment of the display unit of the present invention, clear
 fillers are mixed into the clear adhesive. By this construction, uneven
 display which is otherwise caused by the hardening and shrinkage of the
 clear adhesive when a touch switch having a large input area is adhered
 may be suppressed without damaging the image quality.
 In another aspect, a method is provided for manufacturing a display unit
 according to the present invention. A transparent touch switch is provided
 and a clear adhesive is applied to at least an input area on a rear
 surface of the transparent touch switch. The transparent touch switch is
 then turned over and aligned with a liquid crystal display such that the
 rear surface of the transparent touch switch having the clear adhesive
 confronts a display surface of the liquid crystal display. After the
 adhesive begins to form a drip, the transparent touch switch is slowly and
 continuously moved towards the liquid crystal display. The clear adhesive
 is allowed to flow between the transparent touch switch and the liquid
 crystal display until the clear adhesive expands to a predetermined area
 on the display surface of the liquid crystal display. Thereafter, the
 clear adhesive is cured and the transparent touch switch and liquid
 crystal display are laminated together. By this manufacturing method, the
 clear adhesive is allowed to be filled between the transparent touch
 switch and the liquid crystal display without the formation of bubbles
 therein.
 In another aspect of the present invention, there is provided an electronic
 device having a display unit in which a transparent touch switch is
 adhered surface-to-surface, at least at an input area thereof, with a
 display surface of a liquid crystal display by a clear adhesive, and an
 input pen having a mechanism for absorbing a force applied to the
 transparent touch switch. Because the input pen allows the force applied
 to the transparent touch switch to be suppressed below a predetermined
 value, no uneven display occurs on the display surface of the liquid
 crystal display.
 The display unit constructed as described above allows the image quality to
 be improved and may be manufactured readily without requiring an expensive
 manufacturing machine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Preferred embodiments of the present invention will be explained below with
 reference to the drawings.
 [First Embodiment]
 FIG. 1 is a schematic side view showing a first embodiment of the inventive
 display unit. In the display unit of the present embodiment, a liquid
 crystal display comprises a liquid crystal layer sandwiched between two
 transparent substrates (e.g., glass) 1a and 1b each having a thickness of,
 for example, 0.7 mm. A polarizing plate 2a and a polarizing plate 2b
 adhered to a reflecting plate (not shown) are attached to the transparent
 substrates 1a and 1b, respectively. A driver IC 3 for driving the liquid
 crystal display is mounted at a terminal section of the transparent
 substrates 1a. A touch switch 4 of an analog resistance film type using,
 for example, 0.7 mm thick glass is adhered on the display side of the
 liquid crystal display by a clear adhesive 5. The clear adhesive 5
 surrounds the periphery of the liquid crystal display and the entire input
 area of the touch switch 4. If clear adhesive having a refractive index of
 1.47 to 1.57 after hardening is used, the reflection between the touch
 switch 4 and the liquid crystal display is almost eliminated and the image
 quality of the display unit is improved because it is almost equal to the
 refractive index of the glass substrate (i.e., 1.52).
 It is also most preferable to use a clear adhesive whose refractive index
 after hardening is equal to the refractive index of glass of 1.52. That
 is, because the refractive index of the glass to which the touch switch is
 adhered and that of the clear adhesive after hardening are equal,
 reflection between the touch switch and the liquid crystal display is
 eliminated and the image quality is improved considerably.
 The touch switch used here is not confined only to that of the analog
 resistance film type, and other types of touch switches, such as a
 resistance film digital type or an ultrasound type, may be used.
 [Second Embodiment]
 FIG. 2 shows a second embodiment of the inventive display unit. The basic
 structure thereof is the same as that of the first embodiment. Because the
 clear adhesive 5 for adhering the touch switch 4 to the liquid crystal
 display has a hardening shrinkage factor of about 5%, there is a
 possibility that the touch switch and the liquid crystal display warp and
 cause an unevenness on the display screen when the adhesive is hardened.
 Therefore, in this embodiment clear fillers 6 are mixed in the clear
 adhesive 5 to lower the hardening shrinkage factor to prevent warping of
 the touch switch and the liquid crystal display.
 The clear fillers 6 of the present embodiment are preferably true spherical
 plastic particles whose diameter is 12 microns and whose refractive index
 is 1.47 to 1.57. That is, it is preferable to use clear fillers having a
 refractive index close to the refractive index of the clear adhesive after
 hardening.
 The percentage of the fillers mixed in the adhesive is set at 2 wt %. When
 they are mixed more than that, the adhesive layer becomes cloudy, thus
 degrading the image quality. However, it is possible to mix more when the
 refractive index of the fillers and the adhesive are almost same (e.g.
 1.52) because the cloudiness of the adhesive layer is reduced.
 [Third Embodiment]
 FIG. 3 is a flowchart and FIGS. 4(A)-4(C) are a sequence of steps showing a
 method for manufacturing a display unit according to the present invention
 having a transparent touch switch 4 and a liquid crystal display 11.
 First, the transparent touch switch 4 is flipped back and a blob of clear
 adhesive 5 is applied centrally on a predetermined area of a rear surface
 4a of the transparent touch switch (FIG. 4(A)). Preferably, the clear
 adhesive 5 is applied to at least an input area of the transparent touch
 switch. A degassed clear adhesive may be used, as necessary. The clear
 adhesive 5 can be, for example, an ultraviolet hardening clear adhesive of
 which about 0.7 g is applied in the case where the liquid crystal display
 has a length of about 6 inches.
 Next, as shown in FIG. 4(B), the transparent touch switch 4 is turned
 upside down and aligned in registry with a display surface 11a of the
 liquid crystal display 11 so that the rear surface 4a of the transparent
 touch switch 4 confronts the display surface 11a of the liquid crystal
 display. As the adhesive 5 begins to drip and forms a drop 5a, the
 transparent touch switch 4 is laminated on the liquid crystal display 11.
 During the lamination step, the transparent touch switch 4 is slowly and
 continuously moved toward the liquid crystal display 11 so that bubbles
 are not produced when a surface 5b of the drop 5a contacts the display
 surface 11a of the liquid crystal display (FIG. 4(C)).
 As shown in FIG. 5, for example, the surface 5b of the drop 5a which
 contacts the display surface 11a of the liquid crystal display becomes
 bumpy and uneven if the transparent touch switch 4 is moved too quickly
 toward the display surface 11a of the liquid crystal display 11. The bumps
 produce bubbles in the adhesive which prevent formation of a uniform
 adhesive layer and which ultimately lowers the strength and the image
 quality of the display unit. In order to avoid this problem, according to
 the method of the present invention, the transparent touch switch 4 is
 moved slowly and continuously towards the liquid crystal display 11 during
 the lamination step until the surface of the drop 5a contacts the display
 surface 11a of the liquid crystal display and the adhesive 5 expands and
 spreads over the display surface to an area preferably substantially equal
 to the area of the rear surface 4a of the transparent touch switch covered
 by the adhesive during the adhesive application step. The transparent
 touch switch 4 and the liquid crystal display 11 are then maintained in
 this position for a predetermined period of time to allow the adhesive 5
 to flow slowly between the transparent touch switch and the liquid crystal
 display, without the application of an external pressure, until the
 adhesive expands and spreads to a predetermined area on the display
 surface 11a of the liquid crystal display and forms a continuous adhesive
 layer (FIGS. 1, 2).
 The amount of time which the adhesive 5 is left to expand and spread
 between the transparent touch switch 4 and the liquid crystal display 11
 depends on the viscosity of the adhesive selected. Other factors include
 the weight of the transparent touch switch 4 and the quantity of the
 adhesive applied. For example, when the viscosity of the adhesive is
 preferably within a range of 1000 to 6000 cp., the transparent touch
 switch 4 and the liquid crystal display 11 are left for about 30 to 60
 minutes in order to allow the adhesive to expand between the transparent
 touch switch and the liquid crystal display by normal expansion of the
 adhesive without application of an external pressure.
 After confirming that the adhesive 5 has expanded to a predetermined area
 on the display surface 11a of the liquid crystal display 11, the adhesive
 is cured by, for example, irradiating ultraviolet rays uniformly from the
 top thereof to harden it. In the present embodiment, the thickness of the
 resulting adhesive layer is about 0.1 mm to 0.2 mm, which is thinner than
 the thickness of 0.4 mm to 0.8 mm. in the resulting adhesive layers of
 prior art display units. This allows for miniaturization of the display
 unit by reducing the overall thickness of the display unit.
 When the viscosity of the adhesive is high, e.g. 10000 cp., it will not
 expand and spread between the transparent touch switch and the liquid
 crystal display to form a desired adhesive layer. However, since the
 viscosity of the adhesive lowers by the application of heat, an adhesive
 having an initial viscosity of about 20000 cp. may expand to form a
 desired adhesive layer if heat is applied to the adhesive during the
 predetermined period of time which the adhesive is left to expand between
 the transparent touch switch 4 and the liquid crystal display 11.
 The adhesive 5 is not limited only to an ultraviolet hardening type
 adhesive. That is, the adhesive 5 may be a two-liquid type epoxy adhesive
 so long as it generates almost no gas. However, a thermal hardening type
 adhesive having a high viscosity which requires heat treatment cannot be
 used because hardening of the such adhesive advances when heat is applied
 while the adhesive is left to expand between the transparent touch switch
 4 and the liquid crystal display 11.
 The clear adhesive 5 may be applied to the side of the liquid crystal
 display 11. In this case, when ultraviolet rays are irradiated from the
 side of the transparent touch switch 4 to cure the adhesive, such as an
 ultraviolet hardening type clear adhesive, the lamination is carried out
 on a transparent stage, such as quartz glass, and the ultraviolet rays are
 irradiated through the quartz glass.
 Although in the method of the present embodiment only one blob of the
 adhesive 5 is applied to the rear surface of the transparent touch switch
 4, it is understood that two or more blobs may be applied at preselected
 areas of the rear surface. Alternatively, the blob or blobs of the clear
 adhesive may be applied on the display surface of the liquid crystal
 display, following which the transparent touch switch and the liquid
 crystal display are then laminated to form a clear adhesive layer
 therebetween.
 [Fourth Embodiment]
 FIG. 6 shows an embodiment of an electronic device having the inventive
 display unit equipped with the transparent touch switch according to the
 foregoing embodiments. In this embodiment, the electronic device comprises
 a tablet and input pen. Unevenness of display, which is otherwise caused
 on the liquid crystal screen when inputting information on the touch
 switch surface by a pen, is prevented according to the present invention
 by using a special pen 7. That is, when a force of about 200 g to 300 g or
 more is applied to the touch switch by the tip of the pen, there is a
 possibility that uneven display occurs on the screen of the liquid crystal
 display by the force which acts via the adhesive. The present invention
 provides a pen 7 having a mechanism for absorbing the force of 200 g or
 more which causes the uneven display, i.e., a mechanism by which no force
 of 200 g or more can be applied to the touch switch by the pen.
 As shown in FIG. 6, a pen 7 according to the present invention has a
 displaceable tip member 8 having a front end for contacting the liquid
 crystal screen of the display unit to input information and a rear end
 which abuts against one end of a spring 9. The spring 9 is selected to
 undergo contraction when, for example, a force of 200 g or more is axially
 applied thereto by the tip member 8. In this manner, because no force of
 200 g or more is applied to the liquid crystal display by the pen 7, it is
 possible to input information by the pen without damaging the image
 quality of the liquid crystal display. The structure of the pen 7 is not
 confined specifically to that described above.
 Furthermore, because the operating force of the touch switch of the analog
 resistance film type is 15 g to 100 g, an electronic device which does not
 suffer from operational difficulties of the touch switch and which does
 not damage the image quality of the liquid crystal display may be realized
 by providing a pen in which no force of 100 g or more can be applied to
 the touch switch by the pen according to the present invention, which can
 be achieved by providing a spring which contracts with a force of 100 g or
 more.
 Thus the present invention provides a display unit comprising a transparent
 touch switch and a liquid crystal display comprised of a liquid crystal
 layer disposed between a pair of transparent substrates. A display surface
 of the liquid crystal display is adhered to the transparent touch switch
 using a clear adhesive. By this construction, the reflectance between the
 transparent touch switch and the liquid crystal display is effectively
 reduced as compared with prior art display units, thereby improving the
 image quality of the display unit. Furthermore, by providing a clear
 adhesive having a refractive index which is equal to the refraction
 indices of the transparent touch switch and the transparent substrates
 supporting the liquid crystal layer, the reflection of light between the
 transparent touch switch and liquid crystal display is almost eliminated,
 thereby further improving the image quality of the display unit.
 Accordingly, the present invention allows the image quality of the display
 unit equipped with the transparent touch switch to be improved and allows
 for miniaturization of the display unit by reduction of its overall
 thickness. Furthermore, the strength of the display unit is enhanced by
 adhering the transparent touch switch and the display surface of the
 liquid crystal display in surface-to-surface relationship while preventing
 formation of bubbles in the resulting adhesive layer. Moreover, the
 display unit may be provided at low cost because no investment for an
 expensive facility is required for its manufacture.