Patent Publication Number: US-8111345-B2

Title: Display device comprising a first positioning wall having a height larger than the height of the bonding surface of the substrates and a second positioning wall having a height smaller than the height of the bonding surface

Description:
This application claims priority to Japanese Patent Application No. 2006-041312 filed Feb. 17, 2006 and Japanese Patent Application No. 2007-036416 filed Feb. 16, 2007. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a display device, and more particularly to a display device including a backlight unit having positioning ribs formed on the luminescent surface thereof and a panel unit mounted on the luminescent surface of the backlight unit wherein the position of the panel unit is defined by the positioning ribs. 
     2. Description of the Related Art 
     In general, a liquid-crystal display (LCD) device has a structure configured by sandwiching a liquid-crystal (LC) panel unit between a backlight unit and a shield front.  FIG. 17  is a perspective view showing the configuration of a conventional LCD device as observed from the front side.  FIG. 18  is an exploded perspective view of the LCD device of  FIG. 17 . The LCD device  100  includes a backlight unit  11  having a rectangular luminescent surface  11   a  on the front side, a LC panel unit  12  mounted on the luminescent surface  11   a  and having a light-switching function, and a shield front  13  defining a display surface  13   a  for the panel unit  12 . 
     The LC panel unit  12  includes a display panel  21 , a plurality of TCPs (Tape Carrier Packages)  22  coupled to the display panel  21 , and a circuit board  23  coupled mechanically and electrically to the TCPs  22 . ICs for driving the display panel  21  are mounted on the TCPs  22 , and the circuit board  23  outputs control signals controlling the drive of the display panel  21  by the TCPs  22 . 
     The display panel  21  includes a LC layer (not shown), front and rear transparent substrates  24 ,  25  bonded together at a bonding surface to sandwich therebetween the liquid crystal layer, and a pair of polarizing plates (not shown) disposed on the external surface of the transparent substrates  24 ,  25 . The display panel  21  has thereon a plurality of pixels defined in a two-dimensional array. The rear substrate  24  is configured as an active matrix substrate, mounting thereon switching elements such as TFTs (Thin Film Transistors) and the like disposed in a matrix for controlling the switching of the pixels. The rear substrate  24  is thus hereinafter referred to as TFT substrate  24 . The front substrate  25  mounts thereon color filters (CFs) having a color which corresponds to each pixel, and is hereinafter referred to as a counter substrate or CF substrate  25 . By driving the TFT elements via the circuit board  23  and the TCPs  22 , the display panel is controlled for each pixel. 
     The TCPs  22  and the circuit board  23  are disposed, for example, on the periphery of the display panel  21  along two adjacent sides of the rectangular display panel  21 . On these two sides, the edge of the TFT substrate  24  protrudes from the edge of the counter substrate  25  as observed normal to the display panel  21 , and the TCPs  22  are coupled to the peripheral area of the TFT substrate  24  exposed from the edge of the counter substrate  25 . 
     The backlight unit  11  is provided with, on the front side thereof, a frame-like backlight chassis  31  defining the luminescent surface  11   a  of the backlight unit  11 . On the backlight chassis  31 , a positioning rib set including a plurality of positioning ribs  42  is formed in the vicinity of four corners of the luminescent surface  11   a  for defining the position of the display panel  21 . A pair of positioning ribs  42  are formed on each corner of the display panel  21 , with the corner being interposed between the pair of positioning ribs  42 . Each rib has a shape of elongated cuboid and is disposed along an edge of the display panel  21 . Further, in consideration of the range of variation of the peripheral dimensions of the display panel  21  and the range of variation of the distance between the positioning ribs  42 , each rib is disposed so as to form a clearance of 0.2 to 0.3 mm between the edge of the display panel  21  and the positioning ribs  42 . 
       FIG. 19A  is a top plan view showing the LCD device of  FIG. 18  in which the LC panel unit  12  is mounted on the backlight unit  11 , and  FIG. 19B  is a sectional view showing a section taken along line b-b of  FIG. 19A . The backlight unit  11  is, for example, an edge-emission type backlight unit, and consecutively includes, on the luminescent surface  11   a  thereof, an optical sheet  33 , a optical guide plate  34  and a reflection sheet (not shown). The backlight unit  11  includes a lamp or light source (not shown) adjacent to both the side edges of the optical guide plate  34 . Around the luminescent surface  11   a , the backlight chassis  31  has a flat mounting surface  35  on which the TFT substrate  24  is mounted, and a recess  36  formed nearer to the luminescent surface  11   a  than the mounting surface  35  and being capable of accommodating a polarizing plate  26 . Numeral  28  indicates the edge of the display area of the display panel  21 . 
     In the assembly process of the LCD device  100 , the four corners of the display panel  21  are positioned so as to be located inside the positioning ribs  42  in the in-plane direction of the display panel  21 , and then, the LC panel unit  12  is mounted on the backlight unit  11 . In this process, the TFT substrate  24  is mounted on the mounting surface  35  with polarizing plate  26  being received in the recess  36  of the backlight chassis. Further, the shield front  13  is overlaid thereon so as to cover the backlight unit  11  and the LC panel unit  12 , and is pressed down to a predetermined position, thereby completing the assembly. The backlight unit  11  and the shield front  13  are fixed to each other by means of screwing, caulking, pawl fitting or the like. 
     It is known that the conventional LCD device  100  incurs the problem of poor image quality in which shadow of dust is projected onto the display surface  13   a  due to the dust adhered onto the luminescent surface  11   a  of the backlight unit. This poor image quality is attributable to the following causes. In the LCD device  100 , the TFT substrate  24  and the counter substrate  25  is configured by a transparent body such as made of glass, and the edge of the transparent body is not polished, and has minute irregularities such as shown in  FIG. 19B  by reference numeral  101 . Accordingly, if the LCD device  100  is subjected to vibration, the edge of the transparent substrate body, which is in contact with the positioning ribs  42 , suffers from abrasion with respect to the same to generate the dust. This dust passes through the gap between the bottom surface of the TFT substrate  24  and the mounting surface  35  of the backlight unit  11 , penetrates into the lower part of the display panel  21  and adheres onto the luminescent surface  11   a  of the backlight unit. 
     In order to solve the above-described problem, measures of forming an inclined surface on the side of each of the positioning ribs  42  opposing the display panel  21  is employed.  FIG. 20  is a sectional view showing the LCD device which has an inclined surface similar to the surface of equivalent to  FIG. 19B . In this LCD device  100 , the inclined surface  112  is formed on the side of the display panel  21  opposing the positioning ribs  42 . 
     In the LCD device  100 , the height of the boundary between a positioning wall  111  and the inclined surface  112  opposing the edge surface of the display panel  21  is set smaller than the height of the top surface of the TFT substrate  24 . As a result thereof, when the edge portion of the display panel  21  is brought into contact with the positioning ribs  42 , only a part of the edges of the TFT substrate  24  is in contact with the positioning ribs  42 . It is considered to be able to suppress the generation of dust due to friction between the display panel  21  and the positioning ribs  42  by reducing the contact area between the edge of the display panel  21  and the positioning ribs  42 . The LCD device in which the inclined surface is formed on the side of the positioning ribs is described, for example, in JP-2001-4983A. 
     Research was conducted by the inventor for investigating the incidence rate of the poor image quality after actually manufacturing the LCD device  110  of  FIG. 20 , and it was found in the research that the poor image quality due to the dust cannot be suppressed satisfactorily. The reason why the poor image quality due to the dust cannot be suppressed satisfactorily is conceived as follows. 
     The display panel  21  is formed by bonding the TFT substrate  24  and the counter substrate  25  together, with a LC layer sandwiched therebetween. Accordingly, depending on processing accuracy of the bonding, the position of the edge of the TFT substrate  24  and the position of the edge of the counter substrate  25  cannot be aligned perfectly, thereby leaving a certain degree of misalignment therebetween. Depending on the bonding surface between the TFT substrate  24  and the counter substrate  25 , the edge of the counter substrate  25  may protrude outwards from the edge of the TFT substrate  24 . 
     In the LCD device shown in  FIG. 20 , the TFT substrate  24  and the counter substrate  25  have a thickness of, for example, 0.7 mm, and in consideration of the range of variation in the dimension of the positioning ribs  42 , the boundary between the positioning wall  111  and the inclined surface  112  is set at a height of about 0.5 to 0.6 mm from the mounting surface  35  of the backlight unit so that a part thereof does not contact with the counter substrate  25 . The angle of the inclined surface  112  is set, for example, at about 10 to 30° so as to be able to mount the LC panel unit  12  with ease during the assembly of the LCD device  110 . 
     The misalignment between the edge of the TFT substrate  24  and the edge of the counter substrate  25  amounts to, for example, a maximum of about 0.4 mm. Accordingly, in case of largely protruded edge of the counter substrate  25 , even if the inclined angle is set at 30°, as shown by the numeral  113  of  FIG. 21 , the corner of the counter substrate  25  abuts the inclined surface  112  of the positioning ribs  42  before the TFT substrate  24  is brought into contact with the positioning ribs  42 . The dust causing the above-described poor image quality is generated by friction between the corner of the counter substrate  25  and the inclined surface  112  of the positioning ribs. 
     Here, countermeasures can be conceived that the angle of the inclined surface  112  is increased sufficiently for preventing the corner of the counter substrate  25  from contacting with the inclined surface  112  of the positioning ribs. However, according to the simulation of the present inventor, assuming that the misalignment between the edge of the TFT substrate  24  and the edge of the counter substrate  25  amounts to a maximum of 0.4 mm, even if the height of the boundary between the positioning wall  111  and the inclined surface  112  is reduced to a height of 0.5 mm from the mounting surface  35  of the backlight unit, contact of the corner of the counter substrate  25  with the inclined surface  112  of the positioning ribs cannot be prevented unless the inclined angle is set to larger than 68°, as shown in  FIG. 22A . 
     However, if the angle of the inclined surface  112  is excessively large, as shown in  FIG. 22B , the LC panel unit  12  is likely to be stopped after overriding the positioning ribs  42  during the assembly of the LCD device  120 . Accordingly, if the constituent members such as the shield front  13  are attached onto the LCD device  12  in this situation, there may occur a dimensional error, or the LC panel unit  12  may be damaged due to stresses. In particular, in a LCD device having a display panel  21  of large size, since the LC panel unit  12  is heavy, the LC panel unit  12  is more likely to be stopped on the ribs  42 . 
     SUMMARY OF THE INVENTION 
     In order to realize a LCD device having an excellent image quality, it is essential to prevent the LC panel unit  12  from overriding on the positioning ribs  42  and to suppress the generation of dust due to abrasion caused by friction between the LC panel unit  12  and the positioning ribs  42 . 
     In view of the above, it is an object of the present invention to provide a display device which is capable of preventing the panel unit from overriding the positioning ribs and suppressing the generation of dust due to friction between the panel unit and the positioning ribs. 
     The present invention provides, in a first aspect thereof, a display device including: a display panel including first and second substrates consecutively layered on a mounting surface and bonded together at a bonding surface, the display panel having a first panel portion in which an edge of the first substrate protrudes from an edge of the second substrate and a second panel portion in which an edge of the first substrate is substantially flush with an edge of the second substrate; and a positioning rib set protruding from the mounting surface to define a location for the display panel on the mounting surface, the positioning rib set including a first rib having a first positioning wall which opposes an edge of the first panel portion and has a height larger than a height of the bonding surface as observed from the mounting surface, and a second rib having a second positioning wall which opposes an edge of the second panel portion and has a height smaller than a height of the bonding surface as observed from the mounting surface. 
     The present invention provides, in a second aspect thereof, a display device including: a display panel including first and second substrates consecutively layered on a mounting surface and bonded together at a bonding surface, the display panel having a first panel portion in which an edge of the first substrate protrudes from an edge of the second substrate and a second panel portion in which an edge of the first substrate is substantially flush with an edge of the second substrate; and a positioning rib set protruding from the mounting surface to define a location for the display panel on the mounting surface, the positioning rib set including a first rib having a first positioning wall which opposes an edge of the first panel portion and has a height larger than a height of the bonding surface as observed from the mounting surface, and a second rib having a second positioning wall which opposes an edge of the second panel portion and has a height smaller than a height of the bonding surface as observed from the mounting surface and a retracted wall retracted from the second positioning wall with respect to the edge of the second panel portion. 
     The present invention provides, in a third aspect thereof, a display panel including a display panel mounted on a mounting surface; and a positioning rib protruding from the mounting surface to define a location for the display panel on the mounting surface, the positioning rib extending along at least a portion of an edge of the display panel and including a plurality of protrusions protruding toward the edge of the display unit. The protrusions may have a circular surface or an edged surface. 
     The present invention providers, in a fourth aspect thereof, a display device including: a display panel including first and second substrates consecutively layered on a mounting surface and bonded together at a bonding surface; and 
     a positioning rib protruding from the mounting surface to define a location for the display panel on the mounting surface, at least a portion of the positioning rib having a positioning wall which opposes an edge of the display panel, and a groove formed on the positioning wall in a height adjacent to the bonding surface. 
     In accordance with the display device of the first aspect, for example, of the present invention, the first positioning wall of the first rib having a lager height than the bonding surface suppresses the first panel portion of the display panel from overriding onto the first rib. The second positioning wall of the second rib having a smaller height than the bonding surface allows only a portion of the display panel to abut the second rib. Thus, the present invention reduces the contact area between the display panel and the rib assembly, thereby suppressing occurrence of dust caused by friction or abrasion of the display panel. 
     In accordance with the display device of the second to fourth aspects of the present invention, the positioning rib set formed in the display device provides an advantage similar to the advantage of the first aspect of the present invention. 
     In the present invention, the rib set may be formed integrally with the mounting surface, or may be formed separately from the mounting surface. The mounting surface may be configured by a backlight unit or other mounting unit. 
     The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view showing a LCD device according to a first embodiment of the present invention; 
         FIG. 2  is a top plan view showing a state in which a LC panel unit is mounted on the backlight unit in the LCD device of  FIG. 1 ; 
         FIGS. 3A and 3B  are sectional views taken along lines a-a and b-b, respectively, in  FIG. 2 ; 
         FIG. 4A  is an enlarged top plan view showing the portion encircled by the dotted line IV in  FIG. 2 ; 
         FIG. 4B  is a sectional view taken along line a-a of  FIG. 4A ; 
         FIG. 5  is an enlarged schematic perspective view showing the portion encircled by the dotted line V in  FIG. 1 ; 
         FIG. 6  is a perspective view showing the configuration of a LCD device according to a first modification of the first embodiment, similarly to  FIG. 5 ; 
         FIG. 7  is a perspective view showing, similarly to  FIG. 5 , the configuration of a LCD device according to a second modification of the first embodiment; 
         FIG. 8  is a perspective view showing, similarly to  FIG. 5 , the configuration of a LCD device according to a third modification of the first embodiment; 
         FIG. 9  is a perspective view showing, similarly to  FIG. 5 , the configuration of a LCD device according to a fourth modification of the first embodiment; 
         FIG. 10  is a top plan view showing, similarly to  FIG. 2 , the configuration of a LCD device according to a fifth modification of the first embodiment; 
         FIG. 11  is a top plan view showing the configuration of a LCD device according to a sixth modification of the first embodiment; 
         FIG. 12  is a top plan view showing, similarly to  FIG. 2 , the configuration of a LCD device according to a seventh modification of the first embodiment, similarly to  FIG. 2 ; 
         FIG. 13A  is a perspective view showing, similarly to  FIG. 5 , the configuration of a LCD device according to a second embodiment of the present invention; 
         FIG. 13B  is a sectional view taken along the line b-b of  FIG. 13A ; 
         FIG. 14  is a top plan view showing, similarly to  FIG. 4 , the configuration of a LCD device according to a fourth embodiment of the present invention; 
         FIG. 15  is a top plan view showing, similarly to  FIG. 14 , the configuration of a LCD device according to a first modification of the fourth embodiment; 
         FIG. 16  is a top plan view showing, similarly to  FIG. 14 , the configuration of a LCD device according to a second modification of the fourth embodiment; 
         FIG. 17  is a perspective view showing the configuration of a conventional LCD device; 
         FIG. 18  is an exploded perspective view showing the LCD device of  FIG. 17 ; 
         FIG. 19A  is a top plan view showing a condition in which a LC panel unit is mounted on the backlight unit in the LCD device of FIG.  18 ; 
         FIG. 19B  is a sectional view taken along the line b-b of  FIG. 19A ; 
         FIG. 20  is a sectional view showing the configuration of another conventional LCD device; 
         FIG. 21  is a sectional view showing the problems of the LCD device of  FIG. 20 ; 
         FIG. 22A  is a sectional view showing the configuration of a LCD device having an inclined surface of which the inclination angle is different from that of the inclined surface of the LCD device of  FIG. 20 ; and 
         FIG. 22B  is a sectional view showing the problems of the LCD device of  FIG. 22A . 
         FIG. 23A  is a perspective view of a LCD device according to a third embodiment of the present invention, and  FIG. 23B  is a sectional view taken along line b-b in  FIG. 23A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, the present invention will be described further in detail with reference to the accompanying drawings and based on embodiments of the present invention. Similar constituent elements are designated by similar reference numerals throughout the drawings.  FIG. 1  is an exploded perspective view of a LCD device according to a first embodiment of the present invention. The LCD device, generally designated at numeral  10 , has a configuration similar to that of the conventional LCD device  100  shown in  FIGS. 17 to 19  except for the shape of the positioning ribs formed on the backlight unit  11 . 
     In the display panel  21 , the TFT substrate  24  shown as the bottom side of the display panel unit  12  has an edge protruding from the edge of the counter substrate  25  shown as the top side of the display panel unit  12 , on two adjacent sides of the rectangular display panel  21  to which TCPs  22  are connected. The TFT substrate  24  has an edge aligned with the edge of the counter substrate on the remaining two adjacent sides of the display unit  12 . In other words, the edge of the display panel  21  includes a first edge portion  21   a  in which the edge of the TFT substrate  24  protrudes from the corresponding edge of the counter substrate  25 , and a second edge portion  21   b  in which the edge of the TFT substrate  24  is substantially flush with the corresponding edge of the counter substrate  25 . 
     In the present embodiment, a positioning rib set  32  for positioning the display panel  21  includes positioning ribs of first through third types, wherein the first type rib  32   a  has a first height, the second type rib  32   b  has a second height smaller than the first height, and the third type rib  32   c  is a combination of the first and second type ribs, i.e., has a portion having the first height and another portion having the second height. The first height is determined corresponding to the first edge portion  21   a , i.e., the edge of the first panel portion, and the second height is determined corresponding to the second edge portion  21   b , i.e., the edge of the second panel portion, as will be described hereinafter. 
       FIG. 2  shows detail of the LCD device  10  of  FIG. 1 , wherein the LCD panel unit  12  shown in  FIG. 1  is mounted on the backlight unit  11  within the space defined by the positioning ribs  32   a ,  32   b ,  32   c . The first type rib  32   a  is disposed along the side of the display panel  21  to which the TCPs  22  are connected and has the first height, the second type rib  32   b  is disposed along the side of the display panel  21  to which no TCPs  22  are connected and has the second height, and the third type rib  32   c  is disposed along the side of the display panel  21  to which no TCPs  22  are connected and includes the first portion C 1  having the first height and the second portion C 2  having the second height. The third type rib  32   c  is disposed at the boundary between the first edge portion  21   a  and the second edge portion  21   b , with the first portion C 1  corresponding to the first edge portion  21   a  and the second portion C 2  corresponding to the second edge portion  21   b.    
       FIGS. 3A and 3B  are sectional views taken along lines a-a and b-b, respectively, shown in  FIG. 2 . The first type rib  32   a , as shown in  FIG. 3A , has a positioning wall  37  having a height substantially equal to the height of the top surface (outer surface) of the counter substrate  25 , or equal to the thickness of the display panel unit  12 . The second type rib  32   b , as shown in  FIG. 3B , has a positioning wall  37  having a height lower than the height of the top surface (inner surface) of the TFT substrate  24 , or smaller than the thickness of the TFT substrate  24 . The TFT substrate  24  and the counter substrate  25  have a thickness of, for example, 0.7 mm, and the polarizing plates  26 ,  27  have a thickness of, for example, 0.2 mm. In  FIG. 3A , the first type rib  32   a  has a height H 1  of 1.6 mm, for example, and in  FIG. 3B , the second type rib  32   b  has a height H 2  of 0.6 mm, for example. 
       FIG. 4A  is an enlarged top plan view showing the portion encircled by the dotted line IV shown  FIG. 2 , and  FIG. 4B  is a sectional view taken along line a-a shown in  FIG. 4A . The section taken along line b-b shown in  FIG. 3A  is similar to the section shown in  FIG. 3B .  FIG. 5  is an enlarged schematic view showing the portion encircled by the dotted line V shown in  FIG. 1 . In the present embodiment, the positioning ribs  32   a ,  32   b ,  32   c  are formed integrally with the backlight chassis  31  by using a molding technique; however, the positioning ribs each may be formed by attaching a rib piece onto the backlight chassis  31  after forming the same. 
     In the LCD device  10 , as shown in  FIG. 3A  and  FIG. 4B , the first type rib  32   a  and the first portion C 1  of the third type rib  32   c  have a height substantially same as the height of the top surface of the counter substrate  25 . By providing a sufficiently large step difference needed for preventing the display panel  21  from overriding the first type rib  32   a , the override of the display panel  21  onto the first type rib  32   a  can be avoided. In  FIG. 2 , the override of the display panel  21  in the direction opposite to the X-axis direction or Y-axis direction can be avoided by the first type positioning ribs  32   a  disposed for the first edge portion  12   a , and the override in the X-axis direction or Y-axis direction can be avoided by the first portion C 1  of the third type positioning ribs  23   c  disposed for the first edge portion  12   a.    
     In the LCD device  10 , the first edge portion  21   a  of the panel unit  12  including only the TFT substrate  24  opposes or abuts the first type rib  32   a  and the first portion C 1  of the third type rib  32   c . Further, as shown in  FIG. 3B , since the second type rib  32   b  is set slightly lower than the top surface of the TFT substrate  24 , the second edge portion  21   b  of the panel unit  12  abuts the second type rib  32   b  and the second portion C 2  of the third type rib  232   c  only at the TFT substrate  24 . This reduces the contact area between the edge portion of the display panel  21  and the positioning ribs  32 . 
     According to the LCD device  10  of the present embodiment, since the first type rib  32   a  and the first portion of the third type rib  32   c  have a height substantially same as the height of the top surface of the counter substrate  25 , the override of the display panel  21  onto the first type rib  32   a  can be avoided. Further, since the second type rib  32   b  has a height slightly lower than the height of the top surface of the TFT substrate  24 , the contact area between the edge of the display panel  21  and the positioning ribs  32   b  can be reduced, thereby suppressing the generation of dust due to friction between the display panel  21  and the positioning ribs  32 . 
     Although the first type rib  32   a  has a height substantially equal to the height of the top surface of the counter substrate  25 , the first edge portion  21   a  of the panel unit  12  abuts the first type rib  32   a  only at the TFT substrate  24 . This reduces the contact area between the edge portion of the display panel  21  and the positioning ribs  32 , thereby suppressing the generation of dust due to friction between the display panel  21  and the positioning ribs  32 . That is, the override of the display panel  21  onto the positioning ribs  32  as well as the generation of dust due to friction between the edge portion of the display panel  21  and the positioning ribs  32  can be avoided. 
       FIG. 6  is a perspective view showing, similarly to  FIG. 5 , the configuration of a LCD device according to a first modification of the first embodiment. In the LCD device generally designated at numeral  51 , the first type rib  32   a  and the first portion C 1  of the third type rib  32   c  has a round edge opposing to the first edge portion  21   a  of the display panel  21 . The round edge of the first type rib  32   a  and the first portion C 1  of the third type rib  32   c  effectively suppresses the override of the display panel  21  onto the first type rib  32   a  or the third type rib  32   c . Further, the second type rib  32   b  as well as the second portion C 2  of the third type rib  32   c  has an inclined edge surface  62  opposing to the second edge portion  21   b  of the display panel  21  and extending from the positioning wall  37 . 
     In the LCD device  51  of the first modification, the round edge of the first type rib  32   a  as well as the inclined edge surface  62  of the second type rib and the second portion C 2  of the third type rib  32   c  allows the assembly process of the LCD device to mount the LC panel unit  12  on the backlight unit  11  with ease. 
     The third type rib  32   c  has the first portion C 1  integrally formed with the second portion C 2  in this modification; however, the first portion C 1  may be formed separately from the second portion C 2  in the third type rib  32   c .  FIG. 7  is a perspective view showing, similarly to  FIG. 5 , the configuration of a LCD device according to a second modification of the first embodiment. In the LCD device generally designated at numeral  52 , the first portion C 1  and the second portion C 2  of the third type rib  32   c  are formed separately from each other, differently from the LCD device  51  shown in  FIG. 6 . The LC panel unit  12  having a smaller size and thus a relatively light weight, in addition to the smaller contact area between the edge portion of the display panel  21  and the positioning ribs  32 , further suppresses the generation of dust. 
     Adjacent to the boundary between the first edge portion  21   a  and the second edge portion  21   b  of the panel unit  12 , it is possible to provide only the first portion C 1  or the first type rib  32   a  for the panel unit  12 .  FIG. 8  is a perspective view showing, similarly to  FIG. 5 , the configuration of a LCD device according to a third modification of the first embodiment. In the LCD device generally designated at numeral  53 , the second portion C 2  of the third type rib  32   c  in the LCD device  51  shown in  FIG. 6  is omitted, and thus only the first portion C 1  or the first type rib  32   a  is provided in the third modification. If the LC panel unit  12  has a small size and a relatively light weight, the contact area between the edge portion of the display panel  21  and the positioning ribs  32  is small and thus further suppresses the generation of dust. 
       FIG. 9  is a perspective view showing the configuration of a LCD device according to a fourth modification of the first embodiment, similarly to  FIG. 5 . In the LCD device generally designated at numeral  54 , a wall surface  64  parallel to the positioning wall  37  is formed to extend from the inclined edge surface  62  in the second portion C 2  of the third type rib  32   c , in addition to the configuration shown in  FIG. 6 . Further, a flat surface  65  is formed adjacent to the wall surface  64 . The flat surface  65  is configured as the top surface of the portion C 2 , and the top of the first portion C 1  and the top of the second portion C 2  are flush with each other in the third type rib  32   c.    
     In the LCD device  54  of the fourth modification, the wall surface  64  formed in the second portion C 2  allows the assembly process of the LCD device to mount the LC panel unit  12  onto the backlight unit  11  with ease. 
     In the first embodiment and the first to fourth modifications thereof, there are shown examples in which the positioning ribs  32  are disposed in the vicinity of the corners of the display panel  21 . However, the location of the positioning ribs  32  is not limited to those in he above configurations. 
       FIGS. 10 to 12  are top plan views showing, similarly to  FIG. 2 , the configuration of LCD devices according to fifth to seventh modifications, respectively, of the first embodiment. In the LCD device  55  of  FIG. 10 , a corner rib  32   d  is formed for each of the corners of the display panel  21 , wherein the corner rib  32   d  includes a first type rib  32   a  and a second type rib  32   b  integrally formed with each other to form a L-shaped corner, or includes only a first type rib  32   a  or a second type rib  32   b  having a L-shaped bent corresponding to the corner of the display panel  21 . The first type rib  32   a  is provided corresponding to the first edge portion  21   a , and the second type rib  32   b  is provided corresponding to the second edge portion  21   b  of the panel unit  12 . 
     In the LCD device  56  shown in  FIG. 11 , straight positioning ribs  32   e  extend  32   e  along the sides of the display panel  21  except for the part of the first edge portion  21   a  wherein the TCPs  22  and the circuit board  23  are mounted. Each straight positioning rib  32   e  includes a first type rib  32   a  and a second type rib  32   b  integrally formed with each other. 
     In the LCD device  57  shown in  FIG. 12 , a single elongate positioning rib  32   f  includes a second type rib  32   b  extending along the second edge portion  21   b  and a pair of first type ribs  32   a  integrally formed with the ends of the second type rib  32   b  and disposed for the first edge portion  21   a . Other first type positioning ribs  32   a  are disposed for the corner of the display panel  21  corresponding to the first edge portion  21   a.    
     In those LCD devices  55  to  57  shown in  FIGS. 10 to 12 , as in the case of the LCD device  10  of the first embodiment, the first type rib  32   a  in the corner rib  32   d , straight rib  32   e  and single elongate rib  32   f  has the first height equivalent to the height of the top surface of the counter substrate  25 , whereas the second type rib  32   b  has the second height slightly lower than the height of the top surface of the TFT substrate  24 . 
       FIG. 13A  is a perspective view showing, similarly to  FIG. 5 , the configuration of a LCD device according to a second embodiment of the present invention.  FIG. 13B  is a sectional view taken along line b-b shown in  FIG. 13A . In the LCD device generally designated at numeral  70 , the second type rib  32   b  has a positioning wall  71  opposing the edge surface of the counter substrate  25 , and a retracted wall  72  formed between the bottom of the second type rib  32   b  and the positioning wall  71 . The retracted wall  72  is formed at a position retracted from the positioning wall  71  with respect to the display panel  21 . 
     The distance between the surface of the positioning wall  71  and the surface of the retracted wall  72 , as observed normal to the display panel  21 , is set sufficiently larger than the range of misalignment estimated between the edge surface of the TFT substrate  24  and the edge surface of the counter substrate  25 . The second type positioning ribs  32   b  are formed only in a portion opposing the second edge portion  21   b  of the display panel  21 . The bottom surface of the positioning wall  71  is set higher than the boundary between the TFT substrate  24  and the counter substrate  25  and has a height of, for example, 0.9 mm, and the top surface of the second type rib  32   b  has a height of, for example, 1.6 mm. 
     The top inner portion of the positioning ribs  32  opposing the display panel  21  has a round edge. In the LCD device  70  of the present embodiment, since only part of the counter substrate  25  abuts the positioning ribs  32  at the second edge portion  21   b  of the display panel  21 , the contact area between the display panel  21  and the positioning ribs  32  is small to reduce the dust formed by abrasion, as in the case of the first embodiment. 
       FIG. 23A  shows in a perspective view, similarly to  FIG. 5 , a LCD device according to a third embodiment of the present invention, and  FIG. 23B  shows a sectional view taken along line b-b in  FIG. 23B . The LCD device, generally designated at numeral  74 , is similar to the LCD device  10  shown in  FIG. 5  except that the second type positioning ribs  32   b  in the present embodiment include a second positioning wall  71  opposing the second outer edge portion  21   b  of the display panel  21  and a groove  75  formed on the second positioning wall  71  at the height where the bonding surface of the display panel  21  and the second positioning wall  71  intersect. One of the second type positioning ribs  32   b  positioned adjacent to the first type positioning ribs  32   a  opposes only the second outer edge portion  21   b  of the display panel  21 . The bottom of the groove  75  configures a retracted wall of the second type positioning ribs  32   b  in the present embodiment. 
     The height of the center of the groove  75  with respect to the mounting surface  35  is substantially equal to the height of the bonding surface between the TFT substrate  24  and the counter substrate  25 , and may be 0.7 mm for example. The groove  75  may have a width of 0.4 mm and a depth of 0.3 to 0.5 mm, whereas the top surface of the second type positioning rib  32   b  is 1.6 mm high, for example. 
     In a modification from the LCD device  74  of the present embodiment, the edge of the counter substrate  25  protrudes from the edge of the TFT substrate  24  in the second outer edge portion  21   b  of the display panel  21 , and only a portion of the counter substrate  25  is in contact with the second positioning wall  71  at a location higher than the groove  75 . In another modification from the LCD device  74  of the present embodiment, the edge of the TFT substrate  24  protrudes from the edge of the counter substrate  25  in the second outer edge portion  21   b , and a portion of the TFT substrate  24  is in contact with the second positioning wall  71  at a location lower than the groove  75 . In both the modifications, the contact area between the second type positioning ribs  32   b  and the display panel  21  is reduced compared to the conventional display panel, to thereby reduce the amount of dust caused by friction, as in the case of the second embodiment. 
     In another modification of the LCD device  74 , the first type positioning rib  32   a  may have a groove similarly to the groove  75  formed on the second type positioning rib  32   b.    
     The contact area may be adjusted by adjusting the location and width of the groove  75 . In addition, in any of the cases where the edge of the TFT substrate protrudes and the edge of the counter substrate protrudes, the display panel  21  is positioned by the second positioning wall  71 . Thus, these modifications are especially suited for the case of positioning based on the outer profile of the display panel. Although the groove  75  shown in  FIG. 23B  has a rectangular cross section, the cross section of the groove may be any of V-character, U-character, semi-circular and other shapes. 
       FIG. 14  is a top plan view showing, similarly to  FIG. 4 , the configuration of a LCD device according to a fourth embodiment of the present invention. The LCD device, generally designated at numeral  80 , has a configuration similar to that of the conventional LCD device  100  shown in  FIGS. 17 to 19  except for the shape of the positioning ribs  32  formed on the backlight unit  11  in the present embodiment. In the LCD device  80 , the positioning ribs  32  have a cylindrical shape extending normal to the display panel  21 . 
     The positioning ribs  32  have a height substantially equal to the height of the top surface of the counter substrate  25 . The section taken along line a-a shown in  FIG. 14  is similar to that shown in  FIG. 3A , and the section taken along line e-e is similar to that shown in  FIG. 19B . 
     According to the LCD device  80  of the present embodiment, the positioning ribs  32  having a cylindrical shape extending normal to the display panel  21  reduces the contact area between the edge of the display panel  21  and the positioning ribs  32 , thereby suppressing the generation of dust due to abrasion or friction between the display panel  21  and the positioning ribs  32 . Further, the top surface of the positioning ribs  32  having a height substantially equal to the height of the top surface of the counter substrate  25  prevents the display panel  21  from overriding the first type rib  32   a  by providing a sufficient large step difference needed for preventing the display panel  21  from overriding the first type rib  32   a.    
       FIG. 15  is a top plan view showing, similarly to  FIG. 14 , the configuration of a LCD device according to a first modification of the fourth embodiment. In the LCD device generally designated at numeral  81 , each of the positioning ribs  32  has a plurality of protrusions  91  protruding toward the edge surface of the display panel  21 . The protrusions  91  have a semi-circular shape as observed normal to the display panel  21 , and are arranged at a regular pitch along the edge of the display panel  21 . 
     The positioning ribs  32  have a uniform shape along the direction normal to the display pane  121  and have a height substantially equal to the height of the top surface of the counter substrate  25 . The section taken along line a-a shown in  FIG. 15  is similar to that shown in  FIG. 3A , and the section taken along line e-e is similar to that shown in  FIG. 19B . In the LCD device  81  of the present modification, the reliability of the positioning ribs  32  is improved in comparison to the LCD device  80  of the fourth embodiment by supporting the edge surface of the display panel  21  by the positioning ribs  32  having a plurality of protrusions  91 , substantially without a significant increase of the contact area. 
       FIG. 16  is a top plan view showing, similarly to  FIG. 14 , the configuration of a LCD device according to a second modification of the fourth embodiment. In the LCD device generally designated at numeral  82 , the protrusions  91  provided in the LCD device  81  of  FIG. 15  are modified to have a V-character shape in  FIG. 16 . In the LCD device  82  of the present modification, the advantages similar to those in the LCD device  81  of the first modification can be obtained. 
     Further, in the LCD devices  80  to  82  shown in  FIGS. 14 to 16 , as in the case of the first embodiment, the positioning ribs  32  opposing the second edge portion  21   b  of the display panel  21  may have a positioning wall  37  having a height slightly lower than the height of the top surface of the TFT substrate  24 . In an alternative, the positioning wall  71  opposing the edge surface of the counter substrate  25  may be provided, as in the case of the second embodiment. Such a configuration may provide a smaller contact area between the display panel  21  and the positioning ribs  32 , thereby suppressing the generation of dust. 
     As described above, the present invention has been described based on the preferred embodiments thereof. However, the display device according to the present invention is not limited to the above-described embodiments, and the display devices to which various modifications and alterations have been applied in the configurations of the above-described embodiments will fall within the scope of the present invention. For example, the display device of the present invention is not limited to a LCD device, and may be applied to any of the display devices having the mounting surface and the positioning rib.