Patent Publication Number: US-2012033447-A1

Title: Backlight unit

Description:
TECHNICAL FIELD 
     The present invention relates to a backlight unit using light-emitting diodes (hereinafter, referred to as LEDs) as a backlight source, more specifically, it relates to a backlight unit that enables a LED-mounted board with LEDs as a backlight source fixed on a substrate to be exchangeable, and that enables positioning of the LED-mounted board to be done correctly so as to suppress unevenness of luminance and variation in luminance between before and after replacing. 
     BACKGROUND ART 
     A liquid crystal display device is used for displaying in many electronic devices due to its features such as being light, thin, and low power consumption comparing to a CRT (cathod-ray tube). The liquid crystal display device is to change a direction of liquid crystal molecules aligned in a certain direction by an electric field, and to display images by changing light transmittance of a liquid crystal layer. As the method of displaying by the liquid crystal display device, there are a reflective type and a semi-transmissive type. A reflective type of a liquid crystal display device uses low power consumption since it does not need a backlight, however, the display device becomes hard to see in a dark environment. For this reason, many of the liquid crystal display devices used are transmissive or semi-transmissive types using backlights. 
     As backlight source, there are a direct-under type arranged on a back surface of a liquid crystal display panel, and a side light type that arranges light source on a side surface of a light guide plate being arranged in a display area on a back surface of a liquid crystal display panel. Backlight source of the side light type reflects light from the light source to the direction of the liquid crystal panel being guided by the light guide plate. For this reason, the backlight source of the side light type has features in which a liquid crystal display device can be thinner than the direct-under type can make, and it can easily equalize the luminance. 
     Further, many of the backlight sources used are cold cathode discharge tubes and LEDs. Comparing to the cold cathode discharge tube, LEDs have advantages such as being small, low power consumption, a long operating life, free of mercury as cause of environmental contamination, low occurrence of high-frequency noise due to capability for DC lightning, easy to light at low temperature, and so on. For this reason, backlight sources composed of LEDs are often used in small and medium-sized mobile electronic devices, especially, in cellular phone units, portable navigation units, and so on. 
     On the other hand, backlight sources composed of LEDs have a feature that if a distance to a light receiving surface of a light guide plate is too close, luminance becomes uneven, and if the distance is too far, luminance becomes low. For this reason, as disclosed in the following Patent Literature 1, it is designed so that a predetermined distance can be kept between the LED and the light receiving surface of the light guide plate. In addition, the backlight source disclosed in the following Patent Literature 1 has a LED-mounted board with LEDs fixed on a substrate attached to a metallic frame by double-faced adhesive tape with fine thermal conductivity in order to improve the effectiveness of heat dissipation. 
     However, although backlight sources composed of LEDs has a long operating life, it has a demerit that resin covering light-emitting devices of LED changes its color across the ages. For this reason, as disclosed in the following Patent Literature 2, there is a structure wherein the a LED-mounted board with LEDs fixed on a substrate can be easily attached and detached. The backlight unit composed of LEDs disclosed by the following Patent Literature 2 adapts a structure wherein an attaching/detaching slot to attach or detach a LED-mounted board is mounted on a bottom surface of the backlight unit, and the attaching/detaching slot is sealed by a cover in a state when the LED-mounted board is attached. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: JP 2007-163620A 
         Patent Literature 2: JP 2008-218039A 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     A backlight unit without necessity of exchanging a LED-mounted board can perform positioning of the LED-mounted board at high accuracy since the LED-mounted board can be fixed by double-faced adhesive tape using a fixture or device for positioning, and the LED-mounted board rarely causes backlash due to vibration of vehicle movement or walking. On the contrary, a backlight unit capable of exchanging a LED-mounted board cannot easily attach or detach the LED-mounted board using the fixture or device for positioning of the LED-mounted board, since it is an operation for attaching/detaching the LED-mounted board through a narrow attaching/detaching slot. Moreover, it is difficult to adapt a structure wherein the LED-mounted board is fixed by double-faced adhesive tape when the LED-mounted board needs to be exchanged. 
     Further, although the structure is capable of exchanging the LED-mounted board as the liquid crystal display device disclosed by the above Patent Literature 2, if the LED substrate is fixed by a simple stopper, the LED-mounted board is not positioned at the predetermined position, therefore, there exists a problem that vibration causes the LED-mounted board to be backlashed to worsen the accuracy of the distance between the light receiving surface of the light guide plate and the LED-mounted board. In addition, the LED-mounted board may move along the direction parallel to the light receiving surface of the light guide plate. In particular, since LED is so called a dot light source, comparing to a linear light source like a cold cathode tube, if it moves even slightly along the direction parallel to the light receiving surface, it gives extremely enormous influence on luminance of light emitted from the light guide plate. 
     The present invention is made in order to solve such problems of conventional technologies, and it is aimed to provide a backlight unit that enables a LED-mounted board with LEDs as a backlight source fixed on a substrate to be exchangeable, and that enables positioning of the LED-mounted board to be done correctly so as to suppress unevenness of luminance and variation in luminance between before and after replacing. 
     Solution to Problem 
     In order to achieve the above-mentioned object, there is provided a backlight unit of the present invention including a LED-mounted board having light-emitting diodes (hereinafter, referred to as LEDs) as a backlight source, a light guide plate to which the LEDs are disposed to face, and which has a light receiving surface to which light from the LEDs incidents and an emission surface from which the incident light is output, and a frame which holds the LED-mounted board and the light guide plate, and which has an attaching/detaching slot of the LED-mounted board. The LED-mounted board may have an engaging section that engages between the frame and the LED-mounted board each other being formed in the backlight unit held removably in the frame. 
     The backlight unit of the present invention includes an engaging section that engages between a frame, having a LED-mounted board and a light guide plate, and the LED-mounted board. For this reason, according to the backlight unit of the present invention, although the LED-mounted board makes backlash due to vibration of vehicle movement or walking, displacement can be avoided since an engaging section formed between the frame and the LED-mounted board fixes positioning, and changes in distance between the light receiving surface of the light guide plate and the LED-mounted board can be avoided. 
     Further, according to the backlight unit of the present invention, the engaging section is preferably made of a hole formed on the LED-mounted board and a protrusion formed in the frame. 
     According to the backlight unit of the present invention, since the protrusion for positioning formed in the frame engages the hole formed on the LED-mounted board, it becomes possible to avoid displacement of the LED substrate under the condition of further intense vibration, and it becomes possible to further avoid changes in distance between the light receiving surface of the light guide plate and the LED-mounted board. Note that the hole formed on the LED-mounted board may be either a circle, a oval, or a rectangular. Basically, it would be enough if the protrusion for positioning formed in the frame engages tightly the hole formed on the LED-mounted board and it is hard to separate even under the condition of intense vibration. 
     According to the backlight unit of the present invention, the LED-mounted board is preferably composed of a flexible printed wiring board on which the LED is mounted, and a metallic reinforcing plate to which the flexible printed wiring board is implemented, and the hole is preferably formed on the reinforcing plate. 
     According to the backlight unit of the present invention, even if the LED-mounted board is likely to be deformed like a printed wiring board, the LED-mounted board is implemented on the metallic reinforcing plate while the protrusion for positioning formed in the frame engages the hole formed on the reinforcing plate, therefore, it is possible to conduct positioning of the LED-mounted board correctly. In addition, since the reinforcing plate is metallic and has a good thermal conductivity, the heat dissipation of LED becomes fine. Therefore, according to the backlight unit of the present invention, it is possible to increase the current to apply to LED, therefore, a brighter backlight unit can be obtained. 
     According to the backlight unit of the present invention, the frame forms a pressing section that elastically deforms extending from the frame, and the protrusion is formed so as to project from the pressing section. 
     The pressing section that elastically deforms in the backlight unit of the present invention can form a pressing section composed of a rib having elasticity only by cutting a part of a die for creating a frame if the frame is injection molded, and if the frame is being pressed, it is possible to form a pressing section composed of an arm having elasticity only by applying bending work. For this reason, according to the backlight unit of the present invention, it is possible to easily form a pressing section that elastically deforms in a frame. In addition, according to the backlight unit of the present invention, positioning of the LED-mounted board can be conducted by a simple configuration that implements the LED-mounted board in a frame and that causes the protrusion projecting from the pressing section to joint the hole of the LED-mounted board. 
     According to the backlight unit of the present invention, the protrusion may have a sloping side surface, and the pressing section may be to be elastically deformed by sliding the LED-mounted board on the sloping side surface of the protrusion when the LED-mounted board is attached or removed. 
     According to the backlight unit of the present invention, since the protrusion has a slope on which the LED-mounted board slides, to implement the LED-mounted board, the LED-mounted board is to be slide along the slope to automatically elastically deform the pressing section, therefore, the LED-mounted board can be implemented in a frame easily. Further, to remove the LED-mounted board, it is easy to remove the LED-mounted board easily by deforming the pressing section. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an exploded perspective view of a liquid crystal display device according to an embodiment; 
         FIG. 2  is a perspective diagram showing an external appearance of a backlight unit of a liquid crystal display device according to an embodiment; 
         FIG. 3  is a cross sectional diagram along the line III-III in  FIG. 2 , and a partially enlarged view thereof; 
         FIG. 4  is a cross sectional diagram along the line IV-IV in  FIG. 2 ; 
         FIG. 5A  is a cross sectional diagram of a LED-mounted board and a partially enlarged view thereof, and  FIG. 5B  is a cross sectional diagram of a LED-mounted board in a view from a direction different from  FIG. 5A  and a partially enlarged view thereof; 
         FIG. 6A  is a perspective diagram showing relationship between a light guide plate and LED light source, and  FIG. 6B  is a plan view showing an arrangement of a light control unit; 
         FIG. 7  is a perspective diagram showing a shape of a frame, and partially enlarged view thereof; 
         FIG. 8  is a perspective diagram showing a shape of a slide, and partially enlarged view thereof; 
         FIG. 9  is a perspective diagram showing attaching and detaching of a LED-mounted board; 
         FIG. 10  is a perspective diagram showing attaching and detaching of a slide, and partially enlarged view thereof; and 
         FIG. 11A  is a cross sectional diagram showing a first modified example, and  FIG. 11B  is a cross sectional diagram showing a second modified example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the embodiments and appended drawings. The following embodiments do not intend to limit the invention to what are described here, but the present invention can be adapted to various modifications within the scope of the appended claims. Note that, the drawings used in this specification for explanation are displayed in different scale size for each layer or member so that each layer or member can fit to a screen so as to be in a perceptible size, and they are not necessarily displayed in proportion as actual size. 
     A liquid crystal display device according to the embodiment of the present invention will be explained with reference to  FIG. 1  to  FIG. 10 . A backlight unit  10  that is used for the liquid crystal display device of the present embodiment is, as shown in  FIG. 10 , arranged on a back surface of a liquid crystal panel  50  which is transmissive or semi-transmissive. The liquid crystal panel  50  is sandwiched between the backlight unit  10  and an upper case  51 . On the backlight unit  10 , from the back surface of the liquid crystal panel  50 , that is, from the upper side of  FIG. 1 , a lower case  11 , a light guide plate  12 , an optical sheet  13  and a frame  14  are overlaid. In addition, on the backlight unit  10 , a LED-mounted board  15  is held by the frame  14  so as to face the side surface which is the light receiving surface of the light guide plate  12 , and is inserted as shown in  FIG. 2  so that a slide cover  16  can be slide from a cover insertion slot  14   f  formed on the side surface of the frame  14 . 
     The lower case  11  is, as shown in  FIG. 1 , formed by extruding a stainless plate. The figure does not shown, however, a white reflection sheet is stuck on inner sides of the lower case  11 . As shown in  FIG. 5A  and  FIG. 5B , the LED-mounted board  15  is formed by a flexible printed wiring board  17  fixed on a reinforcing plate  18  by double-faced adhesive tape  19 . The flexible printed wiring board  17  is mounted with a plurality of LEDs  20 , and composed of a main unit  17   a  adhered to the reinforcing plate  18 , and a connection unit  17   b  to be connected to an external power source (not shown). The reinforcing plate  18  is created by being pressed from a stainless-steel plate. Further, a hole  18  is placed between two adjacent LEDs  20 , and at both ends of the reinforcing plate  18 . For the double-faced adhesive tape  19 , the one with high thermal conductivity is selected so as to successfully conduct heat of the LED  20  to the reinforcing plate  18 . And here, size of the flexible printed wiring board  17  is set to be smaller than size of the reinforcing plate  18  so that an outer periphery of the reinforcing plate  18  locates outside of an outer periphery of the flexible printed wiring board  17 . 
     The light guide plate  12  is composed of a transparent acrylic resin (PMMA: polymethyl methacrylate resin) and is created by mold injection. The light guide plate  12  is formed in a rectangular shape of plate, and as shown in  FIG. 6A , a side surface where lights incident from LED  20  of the LED-mounted board  15  is extended to the side of LED  20 , and has an extension unit  12   a  that abuts on the main unit  17   a  of the flexible printed wiring board  17  located between LEDs  20  of the flexible printed wiring board  17 . Further, centering a position of the side surface of the flexible printed wiring board corresponding to each LED, a light control unit  12   b  is formed in a form of ripple. In the light control unit  12   b , as shown in the lower enlarged drawing in  FIG. 3 , a reflection surface whose angle is a predetermined angle θ, about 10 degree, for example, is created in a form of serrate. This light control unit  12   b  in ripple changes light path of the incident light from a plurality of LEDs  20  at right direction to the incident direction, equalizing luminance at right angle to the incident direction. 
     The optical sheet  13  is configured from, as shown in  FIG. 1 , a plurality of sheets, for example four sheets including, a first diffusion sheet that diffuses in order to equalize the luminance, a y-axis prism sheet that focuses light traveling in the direction of y-axis (vertical direction of a display) on the side of the liquid crystal panel  50 , a x-axis prism sheet that focuses light traveling in the direction of x-axis (horizontal direction of a display) on the side of the liquid crystal panel  50 , and a second diffusion sheet that diffuses in order to equalize the luminance. 
     A frame  14  is composed of synthetic resin, such as polycarbonate, and is created by mold injection. The frame  14  is in a form of frame in which a central part is widely open so that lights output from the light guide plate  12  can irradiate a display region of the liquid crystal panel  50  without being blocked. Further, according to the backlight unit  10  of the present embodiment, as shown in  FIG. 14 , the frame  14  joints the lower case  11  and stores the light guide plate  12  and the optical sheet  13 . The lower case  11  is slightly shorter than the frame  14  (refer to  FIG. 2 ) so that the LED-mounted board  15  can be attached and detached without removing the lower case  11  from the frame  14 , and an attaching/detaching slot  11   a  is formed between an edge of the lower case  11  and the frame  14 . 
     Further, an engaging section is formed so that the frame  14  and the LED-mounted board  15  are mutually engaged. Its specific example shows in the present embodiment, as shown in  FIG. 3  and  FIG. 7 , the frame  14  has a plurality of ribs  14   a  vertically arranged from a side of surface parallel to the optical sheet  13  of the frame  14  at a position corresponding to a hole  18   a  of the reinforcing plate  18  of the LED-mounted board  15 . Each rib  14   a  is formed to be narrow and capable of elastically deforming, like a cantilever. In addition, each of ribs  14   a  have a project  14   b  that projects from the rib  14   a . These ribs  14   b  joint a plurality of holes  18   a  formed on the reinforcing plate  18  respectively. The protrusion  14   b  is in a tapered chevron having a sloping surface  14   c . As shown in  FIG. 9 , in a state before the LED-mounted board  15  is implemented in the frame  14 , a gap L 1  between the protrusion  14   b  of the frame  14  and the extension unit  12   a  of the light guide plate  12  is smaller than a thickness L 2  of the flexible printed wiring board  17  added by the reinforcing plate  18 . 
     For this reason, when the LED-mounted board  15  is implemented between the frame  14  and the light guide plate  12  from an upper attaching/detaching slot  14   d , the outer periphery side of the reinforcing plate  18  slides down the sloping surface  14   c  of the protrusion  14   b  so that the rib  14   a  elastically deforms in the opposite direction of the light guide plate  12 , and the protrusion  14   b  joints the hole  18   a . This joint determines the positioning of the LED-mounted board in the frame  14 . The position of a LED-mounted board was easy to move in an ordinary backlight unit having a LED-mounted board capable of easily being attached and removed, however, the backlight unit  10  according to this embodiment enables the protrusion  14   b  joints the hole  18   a  of the reinforcing plate  18  so that the LED-mounted board  15  can be rigidly and exactly positioned between the frame  14  and the light guide plate  12 . In other words, by mutually engaging between the frame  14  and the LED-mounted board  15  in the manner above using the protrusion  14   b  of the frame  14  and the hole  18   a  of the reinforcing plate  18  in the LED-mounted board  15 , it can be possible to avoid a displacement of LED  20  in the direction (an inserting direction of the slide cover  16  in  FIG. 4  and  FIG. 8 ) parallel to the light receiving surface of the light guide plate  12 , and a displacement of LED  20  in the vertical direction (a thickness direction of the light guide plate  12  in the cross-sectional diagram in  FIG. 3 ) to the light receiving surface of the light guide plate  12 . Especially, unlike a linear light source such as a cold cathode tube, LED  20  is a so-called dot light source and the position of this LED  20  is determined with regard to the light guide plate  12  so that light emitted from the light guide plate  12  has the most high efficiency. Therefore, if LED  20  moves even slightly to the direction parallel to the light receiving surface of the light guide plate  12 , or to the vertical direction to the light receiving surface of the light guide plate  12 , emitted light from the light guide plate  12  may decrease, however, the present invention may prevent the displacement of LED  20 . Especially, in a plurality of LEDs  20  implemented on the LED-mounted board  15 , the hole  18   a  joints the protrusion  14   b  between the adjacent LEDs  20 . Therefore, even near the central part of the LED-mounted board  15  where the displacement of LED  20  in particular likely to occur, the displacement of LED  20  can be avoided. 
     In a state where the protrusion  14   b  joints the hole  18   a , as shown in  FIG. 3 , the rib  14   a  presses the side of the reinforcing plate  18  of the LED-mounted board  15  in the direction of the light guide plate  12  by an elastic deformation. Since the reinforcing plate  18  is rigid, the LED-mounted board  15  is pressed in the direction of the light guide plate  12  evenly, and a gap between LED  20  and the light guide plate  12  is successfully positioned at predetermined position (L 3  in  FIG. 6B ). Therefore, according to the backlight unit  10  of the present embodiment, even though the light guide plate  12  moves due to vibrations such as vehicle movements, walking, or the like, by the elastic deformation in synchronization with it, the LED-mounted board  15  moves in the direction of the light guide plate  12  by the elastic deformation of the rib  14   a  so as to maintain constant distance between the light guide plate  12  and the LED-mounted board  15 . 
     Further, in the backlight unit  10  of the present embodiment, the light guide plate  12  forms the extension unit  12   a  (refer to  FIG. 6  or  FIG. 7 ) that extends in the direction of the side of the LED-mounted board  15  so as to locate between LEDs  20  of the LED-mounted board  15 . When the LED-mounted board  15  is implemented between the frame  14  and the light guide plate  12  from the upper attaching/detaching slot  14   d , this extension unit  12   a  is to abut on the main unit  17   a  of the flexible printed wiring board  17  on the LED-mounted board  15 . Having such configuration, the main unit  17   a  of the flexible printed wiring board  17  on the LED-mounted board  15  is pressed so as to be sandwiched between the reinforcing plate  18  of the LED-mounted board  15  and the extension unit  12   a  of the light guide plate  12  so as to be able to maintain constant distance between the LED  20  of the LED-mounted board  15  and the light guide plate  12 . 
     Moreover, since the flexible printed wiring board  17  that fixes LED  20  is fixed on the surface of the stainless-steel reinforcing plate  18  with the double-faced adhesive tape  19  having a fine thermal conductivity, heat generated in LED  20  is to be transferred by conduction to the reinforcing plate  18  and to be cool down. For this reason, according to the backlight unit of the present embodiment, it is possible to apply larger electric current to LED  20  than the backlight unit using ordinary LEDs, so a brighter backlight unit can be obtained. 
     Note that when conducting desorption of the LED-mounted board  15  from the gap between the frame  14  and the light guide plate  12  via the attaching/detaching slot  11   a  of the lower cover, as shown in  FIG. 9 , the LED-mounted board  15  can be pulled out since the hole  18   a  (refer to  FIG. 5 ) of the reinforcing plate  18  slides down the sloping surface  14   c  of the protrusion  14   b  and the rib  14   a  is to be elastically deformed in an opposite direction of the light guide plate  12  to cancel the joint between the protrusion  14   b  and the hole  18   a.    
     Further, after implementing the LED-mounted board  15  from the attaching/detaching slot  14   b , as shown  FIG. 10 , the slide cover  16  is inserted from a slide cover insertion slot  14   f  (refer to  FIG. 1  and  FIG. 2 ) of the frame  14 , and the attaching/detaching slot  14   d  of the lower case  11  is to be sealed by sliding to implement the slide cover  16  into a chase  14   e  formed on a side wall of the LED-mounted board  15  by butting of dies at the time of production of the frame  14 , and a gap  11   b  between the lower case  11  and the light guide plate  12  which is arranged by perpendicular Z-bending of the lower case  11 . 
     At the same time, since one end of the slide cover  16  forms a ligula  16   a  that is bent into an L-shape, the ligula  16   a  can seal the slide cover insertion slot  14   f  (refer to  FIG. 1  and  FIG. 2 ) of the frame  14 . For this reason, as for the backlight unit  10  of the present embodiment, the attaching/detaching slot  14   d  of the LED-mounted board  15 , the slot which is formed between the lower case  11  and the frame  14 , and the slide cover insertion slot  14   f  formed on the side surface of the frame  14  are sealed at the same by the slide cover  16 , and it is possible to prevent an foreign object from entering from the attaching/detaching slot  14   d  and the slide cover insertion slot  14   f . Further, the ligula  16   a  of the slide cover  16  can be used as a stopper when the slide cover  16  is inserted. 
     Moreover, as shown in  FIG. 3  and  FIG. 5 , the connection unit  17   b  of the flexible printed wiring board  17  is bent into an S-shape, passes between the light guide plate  12  and the slide cover  16 , and between the slide cover  16  and the lower case  11 , so as to be exposed on the outside surface of the lower case  11 . Having such configuration, similar to the backlight unit disclosed in the Patent Literature 2 above, a wiring surface of the flexible printed wiring board  17  is not to touch the slide cover  16  so as to prevent disconnection of the flexible printed wiring board  17 . 
     Note that the upper case  51  is also a pressed processed product made of a stainless-steel plate. As shown in  FIG. 2 , the upper case  51  is in a shape of box, and its central part is widely open so that the display region of the liquid crystal panel  50  can be viewed. Further, the upper case  51  joints the frame  14 , and stores the liquid crystal panel  50  between the upper case  51  and frame  14 . According to the above-mentioned configuration, lights emitted from LED  20  of the LED-mounted board  15  is to be incident inside the light guide plate via the side surface of the light guide plate  12 , to be reflected and diffused at a reflected plate, to be further diffused and focused in the determined direction by the optical sheet  13 , and to be emitted on the back surface of the liquid crystal panel  50 . 
     Note that in the backlight unit  10  of the present embodiment described above, the rib  14   a  formed into the frame  14  is set as a cantilever, therefore it is easy to form this rib  14   a  since it needs only cutting a part of the die. However, the rib  14   a  may be the one wherein the hole  18   a  formed on the reinforcing plate  18  can joint the protrusion  14   b  formed on the rib  14   a , and is not limited to be in a shape of cantilever. For example, it may be in a shape of a vertically-coupled doubly-supported-beam as shown in  FIG. 11A  as a backlight unit  10 A of the first modification, or may be in a shape of a horizontally-coupled doubly-supported beam as shown in  FIG. 11B  as a backlight unit  10 B of the second modification. As far as it is in the shape of the doubly-supported beam, it can apply power, which is parallel to the light receiving surface of the light guide plate  12 , to the LED-mounted board  15 , the accuracy of positioning becomes more successfully, and it reduces backlash of the LED-mounted board  15 . Note that in  FIG. 11A  and  FIG. 11B , same reference signs are assigned for the same part of the structure as the backlight unit  10  of the present embodiment shown in  FIGS. 1 to 10 , and further detail explanations will be omitted. 
     Moreover, the above present embodiment has illustrated that the frame  14  and the LED-mounted board  15  engage mutually, using the protrusion  14   b  of the frame  14  and the hole  18   a  of the reinforcing plate  18  in the LED-mounted board  15 , however, its structure may be the one, for example, where a hole is formed on the side of the frame  14  and a protrusion is formed on the LED-mounted board  15 , the engaging between the frame and the LED-mounted board is not limited to the engaging section with a protrusion and a hole, but may be a structure where the both side has a hook so that these hooks are engaging each other. 
     REFERENCE SIGNS LIST 
     
         
           10 ,  10 A,  10 B backlight unit 
           11  lower case 
           11   a  opening 
           11   b  gap 
           12  light guide plate 
           12   a  extension unit (of the light guide plate) 
           12   b  optical control unit 
           13  optical sheet 
           14  frame 
           14   a  rib 
           14   b  protrusion 
           14   c  sloping surface 
           14   d  attaching/detaching slot 
           14   e  chase 
           14   f  cover insertion slot 
           15  LED-mounted board 
           16  slide cover 
           16   a  ligula 
           17  flexible printed wiring board 
           17   a  main unit (of the flexible printed wiring board) 
           17   b  connection unit (of the flexible printed wiring board) 
           18  reinforcing plate 
           18   a  hole (of the reinforcing plate) 
           19  double-faced adhesive tape 
           20  LED 
           50  liquid crystal panel 
           51  upper case