Patent Publication Number: US-2023161377-A1

Title: Foldable display device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of and priority to Korean Patent Application No. 10-2021-0161540 filed on Nov. 22, 2021, the entirety of which is incorporated herein by reference for all purposes. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a display device and particularly to, for example, without limitation, a foldable display device. 
     2. Discussion of the Related Art 
     As an information-oriented society progresses, a demand for a display device displaying an image has increased in various forms. In a display device field, a cathode ray tube (CRT) having a relatively large volume has been rapidly replaced by a flat panel display (FPD) device having a thin profile, a light weight and a low power consumption and applicable to a relatively large size. The FPD devices may include a liquid crystal display (LCD) device, a plasma display panel (PDP), an organic light emitting diode (OLED) display device, and a field emission display (FED) device. 
     Further, a flexible display device capable of being folded and unfolded freely by forming a transistor and a conductive line on a flexible substrate has been the subject of a next generation display device. 
     The flexible display device may be classified into a foldable display device, a bendable display device, and a rollable display device. 
     Further, the foldable display device may have an in-folding type where a display surface is hidden inside when folded and an out-folding type where a display surface is exposed outside when folded. In general, the foldable display device may have a unidirectional folding type where only one of the in-folding and the out-folding is steadily performed. 
     As a result, the foldable display device of a bidirectional folding type where the in-folding and the out-folding are selectively performed in the same folding area has been researched and developed. In the bidirectional type, since the in-folding and the out-folding opposite to each other are performed, a reproduction accuracy of a folding shape is reduced. 
     For example, an abnormal folding shape of the out-folding may be observed due to deformation of the in-folding having an omega shape, or an abnormal omega shape of the in-folding may be observed due to deformation of the out-folding. 
     The description provided in the discussion of the related art section should not be assumed to be prior art merely because it is mentioned in or associated with that section. The discussion of the related art section may include information that describes one or more aspects of the subject technology. 
     SUMMARY 
     The inventors of the present disclosure have recognized the problems and disadvantages of the related art, have performed extensive research and experiments, and developed a new invention. Accordingly, one or more embodiments of the present disclosure are directed to a foldable display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     Furthermore, one or more embodiments of the present disclosure provide a bidirectional foldable display device where a transfer of a deformation due to a folding stress to a display panel may be minimized, and a reproduction accuracy of a folding shape may be improved by forming a plurality of holes in a plate top of a back plate and forming at least one groove in a plate bottom of the back plate. 
     In addition, one or more embodiments of the present disclosure provide a foldable display device where a precise omega shape of an in-folding may be obtained and a stress of a folding shoulder area may be minimized by adjusting widths of a hole area, an in-folding area and an out-folding area of a back plate. 
     Additional features, advantages, and aspects of the present disclosure are set forth in part in the description that follows and in part will become apparent from the present disclosure or may be learned by practice of the inventive concepts provided herein. Other features, advantages, and aspects of the present disclosure may be realized and attained by the descriptions provided in the present disclosure, or derivable therefrom, and the claims hereof as well as the appended drawings. 
     To achieve these and other advantages of the present disclosure, as embodied and broadly described herein, in one or more aspects, a foldable display device may include: a display panel displaying an image; a cover window over the display panel; and a back plate under the display panel. The back plate may include a plate top having a plurality of holes, a plate bottom having at least one groove corresponding to the plurality of holes, and an adhesive layer attaching the plate top and the plate bottom. 
     In another aspect, a foldable display device may include: a display panel displaying an image; a plate top under the display panel, the plate top including a plurality of holes; and a plate bottom under the plate top, the plate bottom including at least one groove corresponding to the plurality of holes, wherein the plate top and the plate bottom may have a cross-section of an omega shape in an in-folding operation where a display surface of the display panel is hidden inside with reference to a folding axis, and wherein the plate top and the plate bottom may have a cross-section of a U shape in an out-folding operation where the display surface of the display panel is exposed outside with reference to the folding axis. 
     In another aspect, a foldable display device may include: a display panel configured to display an image; and a back plate under the display panel, the back plate including: a plate top having a plurality of holes disposed in a hole area; a plate bottom having a plurality of grooves disposed in a groove area; and an adhesive layer bonding the plate top and the plate bottom, wherein the foldable display device performs bidirectional folding with respect to a folding axis. 
     Other systems, methods, features and advantages will be, or will become, apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the present disclosure, and be protected by the following claims. Nothing in this section should be taken as a limitation on those claims. Further aspects and advantages are discussed below in conjunction with embodiments of the disclosure. 
     It is to be understood that both the foregoing description and the following description of the present disclosure are exemplary and explanatory, and are intended to provide further explanation of the disclosure as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the disclosure, are incorporated in and constitute a part of this disclosure, illustrate embodiments of the disclosure, and together with the description serve to explain principles of the disclosure. 
       In the drawings: 
         FIG.  1    is a plan view showing a foldable display device according to a first example embodiment of the present disclosure; 
         FIG.  2    is an example of a cross-sectional view taken along a line II-II of  FIG.  1   ; 
         FIGS.  3 A and  3 B  are examples of cross-sectional views showing an in-folding operation and an out-folding operation, respectively, of a foldable display device according to a first example embodiment of the present disclosure; 
         FIG.  4    is a plan view showing a plate top of a back plate of a foldable display device according to a first example embodiment of the present disclosure; 
         FIGS.  5  to  7    are examples of plan views showing a plate top of a back plate of a foldable display device according to second to fourth example embodiments, respectively, of the present disclosure; 
         FIG.  8    is a graph showing a strain of a cover window according to widths of a hole area of a back plate of a foldable display device according to a first example embodiment of the present disclosure; 
         FIG.  9    is a table showing rendering of an omega shape according to widths of a hole area of a back plate of a foldable display device according to a first example embodiment of the present disclosure; 
         FIG.  10    is a cross-sectional view showing a foldable display device according to a fifth example embodiment of the present disclosure; and 
         FIG.  11    is a cross-sectional view showing a foldable display device according to a sixth example embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference is now be made in detail to aspects of the present disclosure, examples of which may be illustrated in the accompanying drawings. In the following description, when a detailed description of well-known functions or configurations may unnecessarily obscure aspects of the present disclosure, the detailed description thereof may be omitted. The progression of processing steps and/or operations described is an example; however, the sequence of steps and/or operations is not limited to that set forth herein and may be changed, with the exception of steps and/or operations necessarily occurring in a particular order. Like reference numerals refer to like elements throughout unless stated otherwise. In one or more aspects, identical elements (or elements with identical names) in different drawings may have the same or substantially the same functions and properties unless stated otherwise. Names of the respective elements used in the following explanations are selected only for convenience and may be thus different from those used in actual products. 
     Advantages and features of the present disclosure, and implementation methods thereof, are clarified through following example embodiments described with reference to the accompanying drawings. The present disclosure may, however, be embodied in different forms and should not be construed as limited to the example embodiments set forth herein. Rather, these example embodiments are provided so that this disclosure is thorough and complete and fully conveys the scope of the present disclosure to those skilled in the art. Furthermore, the present disclosure is only defined by claims and their equivalents. 
     The shapes, sizes, areas, ratios, angles, numbers, and the like disclosed in the drawings for describing embodiments of the present disclosure are merely examples, and thus, the present disclosure is not limited to the illustrated details. 
     When the term “comprise,” “have,” “include,” “contain,” “constitute,” “make up of,” “formed of,” or the like is used, one or more other elements may be added unless a term such as “only” or the like is used. The terms of a singular form may include plural forms unless the context clearly indicates otherwise. The word “exemplary” is used to mean serving as an example or illustration. Any implementation described herein as an “example” is not necessarily to be construed as preferred or advantageous over other implementations. 
     In construing an element, the element is construed as including an error or tolerance range even where no explicit description of such an error or tolerance range is provided. 
     Where positional relationships are described, for example, where the positional relationship between two parts is described using “on,” “over,” “under,” “above,” “below,” “beneath,” “near,” “close to,” or “adjacent to,” “beside,” “next to,” or the like, one or more other parts may be located between the two parts unless a more limiting term, such as “immediate(ly),” “direct(ly),” or “close(ly),” is used. For example, when a structure is described as being positioned “on,” “over,” “under,” “above,” “below,” “beneath,” “near,” “close to,” or “adjacent to,” “beside,” or “next to” another structure, this description should be construed as including a case in which the structures contact each other as well as a case in which one or more additional structures are disposed or interposed therebetween. Furthermore, the terms “front,” “rear,” “back,” “left,” “right,” “top,” “bottom,” “downward,” “upward,” “upper,” “lower,” “up,” “down,” “column,” “row,” “vertical,” “horizontal,” and the like refer to an arbitrary frame of reference. 
     In describing a temporal relationship, when the temporal order is described as, for example, “after,” “subsequent,” “next,” “before,” “preceding,” “prior to,” or the like, a case that is not continuous may be included unless a more limiting term, such as “just,” “immediate(ly),” or “direct(ly),” is used. 
     It is understood that, although the term “first,” “second,” or the like may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be a second element, and, similarly, a second element could be a first element, without departing from the scope of the present disclosure. 
     In describing elements of the present disclosure, the terms “first,” “second,” “A,” “B,” “(a),” “(b),” or the like may be used. These terms are merely for differentiating one element from another element, and the essence, sequence, order, or number of a corresponding element should not be limited by these terms. 
     For the expression that an element or layer is “connected,” “coupled,” or “adhered” to another element or layer, the element or layer can not only be directly connected, coupled, or adhered to another element or layer, but also be indirectly connected, coupled, or adhered to another element or layer with one or more intervening elements or layers disposed or interposed between the elements or layers, unless otherwise specified. 
     For the expression that an element or layer “contacts,” “overlaps,” or the like with another element or layer, the element or layer can not only directly contact, overlap, or the like with another element or layer, but also indirectly contact, overlap, or the like with another element or layer with one or more intervening elements or layers disposed or interposed between the elements or layers, unless otherwise specified. 
     The term “at least one” should be understood as including any and all combinations of one or more of the associated listed items. For example, the meaning of “at least one of a first item, a second item, and a third item” denotes the combination of items proposed from two or more of the first item, the second item, and the third item as well as only one of the first item, the second item, or the third item. 
     The expression of a first element, a second elements “and/or” a third element should be understood as one of the first, second and third elements or as any or all combinations of the first, second and third elements. By way of example, A, B and/or C can refer to only A; only B; only C; any or some combination of A, B, and C; or all of A, B, and C. 
     In one or more aspects, the terms “between” and “among” may be used interchangeably simply for convenience. For example, an expression “between a plurality of elements” may be understood as between a plurality of elements or among a plurality of elements. For example, an expression “among a plurality of elements” may be understood as between a plurality of elements or among a plurality of elements. In one or more examples, the number of elements may be two. In one or more examples, the number of elements may be more than two. 
     Features of various embodiments of the present disclosure may be partially or wholly coupled to or combined with each other and may be variously inter-operated, linked or driven together. The embodiments of the present disclosure may be carried out independently from each other or may be carried out together in a co-dependent or related relationship. In one or more aspects, the components of each apparatus according to various embodiments of the present disclosure are operatively coupled and configured. 
     Unless otherwise defined, the terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It is further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is, for example, consistent with their meaning in the context of the relevant art and should not be interpreted in an idealized or overly formal sense unless expressly defined otherwise herein. 
     Reference is now made to the present disclosure, examples of which are described in detail with reference to the accompanying drawings. In adding reference numerals to elements of each of the drawings, although the same elements may be illustrated in other drawings, like reference numerals may refer to like elements unless stated otherwise. In addition, for convenience of description, a scale, size, and thickness of each of the elements illustrated in the accompanying drawings may differ from an actual scale, size, and thickness, and thus, embodiments of the present disclosure are not limited to a scale, size, and thickness illustrated in the drawings. 
       FIG.  1    is a plan view showing a foldable display device according to a first example embodiment of the present disclosure, and  FIG.  2    is an example of a cross-sectional view taken along a line II-II of  FIG.  1   . 
     In  FIGS.  1  and  2   , a foldable display device  110  according to a first example embodiment of the present disclosure includes a display panel  120 , a cover window  130  and a back plate  140 . 
     The foldable display device  110  may selectively perform an in-folding operation where a display surface of an upper surface of the display panel  120  is hidden inside with respect to a folding axis FX and an out-folding operation where the display surface of the display panel  120  is exposed outside with respect to the folding axis FX. 
     The foldable display device  110  may include an in-folding area IFA which is a bending portion of the in-folding operation, an out-folding area OFA which is a bending portion of the out-folding operation, and a non-folding area NFA which is a non-bending portion of the in-folding operation and the out-folding operation and is disposed outside the out-folding area OFA. 
     For example, a second width w 2  of the in-folding area IFA along a direction perpendicular to the folding axis FX may be smaller than a third width w 3  of the out-folding area OFA along the direction perpendicular to the folding axis FX. (w 2 &lt;w 3 ) 
     The display panel  120  may display an image using a plurality of pixels and may include a gate line, a data line, a thin film transistor, and a capacitor. 
     The display panel  120  may include a display area DA for displaying an image and a non-display area NDA surrounding the display area DA. 
     For example, the display panel  120  may include an organic light emitting diode panel or a liquid crystal panel. 
     When the display panel  120  is an organic light emitting diode panel, each pixel may include a switching thin film transistor, a driving thin film transistor, a storage capacitor and a light emitting diode. When the display panel  120  is a liquid crystal panel, each pixel may include a thin film transistor, a storage capacitor and a liquid crystal capacitor. 
     The cover window  130  may be disposed on the display panel  120  and protect the display panel  120  from an external impact, an external moisture and an external heat. The cover window  130  may include a glass or a plastic such as polymethylmethacrylate (PMMA), polyimide (PI) and/or polyethyleneterephthalate (PET). 
     Although not shown, a transparent adhesive layer and a polarizing layer may be disposed between the display panel  120  and the cover window  130 . 
     The back plate  140  may support the display panel  120  and protect the display panel  120  from an external impact, an external moisture and an external heat. The back plate  140  may include a plate top  142 , an adhesive layer  144  and a plate bottom  146  sequentially disposed under the display panel  120 . 
     The plate top  142  may increase flexibility of the entire back plate  140 . A first thickness t 1  of the plate top  142  may be smaller than a third thickness t 3  of the plate bottom  146 . 
     The plate top  142  may include a plurality of holes  142   a  symmetrically disposed with respect to the folding axis FX in a hole area HA. The hole area HA may produce the in-folding area IFA which has a rigidity relatively lower than the non-folding area NFA due to the plurality of holes  142   a  and is bent during the in-folding operation. 
     As a result, the in-folding area IFA may have an omega (Ω) shape during the in-folding operation by adjusting the first width w 1  of the hole area HA, and a stress in a folding shoulder area FSA (of  FIG.  3 A ) which is a portion of a maximum tensile force may be minimized. 
     A first width w 1  of the hole area HA along the direction perpendicular to the folding axis FX may be smaller than the second width w 2  of the in-folding area IFA along a direction perpendicular to the folding axis FX. (w 1 &lt;w 2 ) 
     For example, the first thickness t 1  of the plate top  142  may be within a range of about 10 μm to about 100 μm (preferably, a range of about 10 μm to about 30 μm). 
     When the first thickness t 1  of the plate top  142  is greater than about 100 μm, the folding stress may increase and the plurality of holes  142   a  may be visible or seen from a front of the display panel  120 . When the first thickness t 1  of the plate top  142  is smaller than about 10 μm, a constraint effect of deformation of the in-folding area IFA may be reduced. 
     For example, the plate top  142  may include a stainless steel, an amorphous metal, an amorphous silicon, an invar, a laminate thereof, an alloy thereof and/or a plating thereof. 
     The stainless steel may be formed by alloying iron (Fe) with chromium (Cr) and nickel (Ni) for improving a corrosion resistance of iron (Fe). The stainless steel may selectively include nickel (Ni) according to a kind thereof, and elasticity of the stainless steel may be adjusted according to a composition ratio of chromium (Cr). 
     The stainless steel may be classified into an iron-chromium group ferrite stainless steel and an iron-nickel-chromium group austenite stainless steel. 
     The amorphous metal may be formed by coagulating metallic atoms arranged formlessly like a glass or a liquid. When a metallic material is rapidly cooled down, the amorphous metal is formed due to obstruction of a crystal growth. The amorphous metal may have a corrosion resistance and an elasticity superior to the crystalline metal. 
     The amorphous silicon has an irregularity similar to the amorphous metal. The amorphous silicon may have an elasticity superior to the amorphous metal. 
     The invar is an alloy of iron (Fe) and nickel (Ni) and has an excellent corrosion resistance and an excellent tensile strength. The invar may be compared with the stainless steel according to whether chromium (Cr) is contained or not. 
     The stainless steel and the invar may have an excellent rigidity, and the amorphous metal and the amorphous silicon may have an excellent elasticity. The degree of internal bonding and the rigidity may be controlled by adjusting the composition ratio of the metallic alloy or the cooling speed. 
     The adhesive layer  144  may attach the plate top  142  and the plate bottom  146 . For example, the adhesive layer  144  may include a curable resin such as an epoxy resin, a silicon resin, a polyester resin, a polyurethane resin and/or a phenol resin. 
     For example, a second thickness t 2  of the adhesive layer  144  may be within a range of about 5 μm to about 100 μm (preferably, a range of about 15 μm to about 50 μm). 
     When the second thickness t 2  of the adhesive layer  144  is greater than about 100 μm, the stress of the plate top  142  is not alleviated in a reliable manner. When the second thickness t 2  of the adhesive layer  144  is smaller than about 5 μm, an adhesion between the plate top  142  and the plate bottom  146  is reduced and deterioration such as detachment due to the folding stress may occur. 
     The plate bottom  146  may include at least one groove  146   a  symmetrically disposed in a groove area GA with respect to the folding axis FX. The groove area GA may have a relatively low rigidity as compared with the non-folding area NFA due to the at least one groove  146   a  and may produce the out-folding area OFA which is bent during the out-folding operation. 
     Accordingly, an appearance (or visibility) of a border between the non-folding area NFA and the out-folding area OFA during the out-folding operation may be suppressed by adjusting a fourth width w 4  of the groove area GA, and an elasticity may be added to the back plate  140  during the out-folding operation by adjusting a fourth thickness t 4  of the plate bottom  146  of a central portion of the groove area GA. 
     The fourth width w 4  of the groove area GA along the direction perpendicular to the folding axis FX may be equal to or greater than the third width w 3  of the out-folding area OFA along the direction perpendicular to the folding axis FX and may be equal to or smaller than 1.2 times of the third width w 3  of the out-folding area OFA (w 3 ≤w 4 ≤(1.2)*w 3 ). A boundary of the groove area GA where the at least one groove  146   a  begins may be disposed in the non-folding area NFA. 
     When the fourth width w 4  of the groove area GA is smaller than the third width w 3  of the out-folding area OFA, the border between the non-folding area NFA and the out-folding area OFA may be visible or seen. When the fourth width w 4  of the groove area GA is greater than 1.2 times of the third width w 3  of the out-folding area OFA, an impact resistance property and a reliability may be deteriorated. 
     For example, the third thickness t 3  of the plate bottom  146  may be within a range of about 10 μm to about 200 μm (preferably, a range of about 50 μm to about 120 μm). 
     When the third thickness t 3  of the plate bottom  146  is greater than about 200 μm, a thickness of the foldable display device  110  increases. When the third thickness t 3  of the plate bottom  146  is smaller than about 10 μm, the border between the out-folding area OFA and the non-folding area NFA may be visible or seen. 
     The fourth thickness t 4  of the plate bottom  146  of the central portion of the groove area GA (a minimum thickness of the plate bottom  146 ) may be equal to or smaller than a fifth thickness t 5  of a maximum depth of the at least one groove  146   a  of the central portion of the groove area GA. (t 4 ≤t 5 ) 
     When the fourth thickness t 4  is greater than the fifth thickness t 5 , the border between the out-folding area OFA and the non-folding area NFA may be visible or seen due to the stress of the out-folding operation. 
     The plate bottom  146  may include a material having a relatively high elastic modulus. For example, the plate bottom  146  may include the same type of material as the plate top  142  but in a different alloy composition ratio. 
     The in-folding operation and the out-folding operation of the foldable display device  110  will be illustrated further with reference to drawings. 
       FIGS.  3 A and  3 B  are examples of cross-sectional views showing an in-folding operation and an out-folding operation, respectively, of a foldable display device according to a first example embodiment of the present disclosure. 
     In  FIGS.  3 A and  3 B , the foldable display device  110  according to a first example embodiment of the present disclosure may include the display panel  120 , the cover window  130 , the back plate  140 , a case  150  and first, second and third guides  152 ,  154  and  156 . 
     The first, second and third guides  152 ,  154  and  156  may be disposed between the back plate  140  and the case  150  and support the back plate  140  during the in-folding operation and the out-folding operation. 
     In  FIG.  3 A , during the in-folding operation where the display surface of the display panel  120  is hidden inside, the in-folding area IFA of the back plate  140  has a cross-section of an omega (Ω) shape due to the plurality of holes  142   a  of the plate top  142  of the back plate  140 . As a result, the cover window  130  and the display panel  120  have a cross-section of an omega shape. 
     The folding stress has a maximum value in the folding shoulder area FSA of the border between the in-folding area IFA where the deformation of the back plate  140  occurs and the non-folding area NFA where deformation of the back plate  140  does not occur. 
     The two second guides  154  may slide from the two first guides  152 , respectively, and contact both side surfaces of the omega shape of the in-folding area IFA of the back plate  140  to support the back plate  140 . 
     The third guide  156  may contact an inner surface of the case  150  and a lower surface of the omega shape of the in-folding area IFA of the back plate  140  to support the back plate  140 . 
     In  FIG.  3 B , during the out-folding operation where the display surface of the display panel  120  is exposed outside, the out-folding area OFA of the back plate  140  has a cross-section of a U shape due to the at least one groove  146   a  of the plate bottom  146  of the back plate  140 . As a result, the cover window  130  and the display panel  120  have a cross-section of a U shape. 
     The two second guides  154  may slide to the two first guides  152 , respectively, and may be separated from the back plate  140 . 
     The third guide  156  is separated from the inner surface of the case  150  and contacts the U shape of the out-folding area OFA of the back plate  140  to support the back plate  140 . 
     The plurality of holes  142   a  of the plate top  142  of the back plate  140  of the foldable display device  110  will be illustrated further with reference to drawings. 
       FIG.  4    is a plan view showing a plate top of a back plate of a foldable display device according to a first example embodiment of the present disclosure. 
     In  FIG.  4   , the plate top  142  of the back plate  140  of the foldable display device  110  according to a first example embodiment of the present disclosure may include the plurality of holes  142   a  disposed in the hole area HA. 
     The plurality of holes  142   a  provides flexibility to the plate top  142  during the folding operation such that the plate top  142  is flexibly bent. The plurality of holes  142   a  absorb a compressive stress and a tensile stress of the folding operation so that the plate top  142  can be easily restored to an original shape. 
     Each of the plurality of holes  142   a  may include a rectangular shape. The plurality of holes  142   a  may be separated from each other and disposed in matrix. The holes  142   a  in adjacent two rows may be disposed to partially overlap each other, and the holes  142   a  in adjacent two columns may be disposed to be separated from each other. 
     A first distance d 1  which is a width of a short side of the rectangular shape of each hole  142   a  may be equal to or smaller than a second distance d 2  which is a separation distance between long sides of the rectangular shapes of adjacent two holes  142   a.  (d 1 ≤d 2 ) A third distance d 3  which is a width of the long side of the rectangular shape of each hole  142   a  may be greater than a fourth distance d 4  which is a separation distance between the short sides of the rectangular shape of adjacent two holes  142   a.  (d 3 &gt;d 4 ) 
     For example, each of the first and second distances d 1  and d 2  may be within a range of about 20 μm to about 500 μm (preferably, a range of about 50 μm to about 300 μm), and each of the third and fourth distances d 3  and d 4  may be within a range of about 0.1 mm to about 10 mm (preferably, a range of about 0.5 mm to about 7 mm). 
     When one of the first and second distances d 1  and d 2  is smaller than about 20 μm or greater than about 500 μm and one of the third and fourth distances d 3  and d 4  is smaller than about 0.1 mm or greater than about 10 mm, the flexibility of the hole area HA of the plate top  142  is reduced. As a result, a folding property may be deteriorated, or the display panel  120  may become deformed. 
     Although the long side of the rectangular shape of each hole  142   a  is disposed along a direction parallel to the folding axis FX in  FIG.  4   , the long side of the rectangular shape of each hole  142   a  may be disposed along a direction perpendicular to the folding axis FX in another embodiment. 
     The plurality of holes may have various shapes and be disposed in various ways in another embodiment. 
       FIGS.  5  to  7    are examples of plan views showing a plate top of a back plate of a foldable display device according to second to fourth example embodiments, respectively, of the present disclosure. For the elements shown in  FIGS.  5  to  7    that are substantially the same or similar to the elements described with respect to the first example embodiment, their detailed descriptions may be omitted or shortened for brevity. 
     In  FIG.  5   , a plate top  242  of a back plate of a foldable display device according to a second example embodiment of the present disclosure may include a plurality of holes  242   a  in a hole area HA. 
     Each of the plurality of holes  242   a  may have a rectangular shape having round corners or an elliptical shape having a minor axis of a constant length. 
     The shape of the plurality of holes  242   a  and the relationship of the first to fourth distances of the second embodiment may be the same as or substantially similar to those of the first example embodiment. 
     In  FIG.  6   , a plate top  342  of a back plate of a foldable display device according to a third example embodiment of the present disclosure may include a plurality of holes  342   a  in a hole area HA. 
     Each of the plurality of holes  342   a  may have a lozenge shape, and the plurality of holes  342   a  may be disposed in matrix and separated from each other. The holes  342   a  in adjacent two rows may be disposed to be separated from each other, and the holes  342   a  in adjacent two columns may be disposed to be separated from each other. 
     Although a major axis of the lozenge shape of each hole  342   a  is disposed along a direction parallel to a folding axis FX in  FIG.  6   , the major axis of the lozenge shape of each hole  342   a  may be disposed along a direction perpendicular to the folding axis FX in another embodiment. 
     In  FIG.  7   , a plate top  442  of a back plate of a foldable display device according to a fourth example embodiment of the present disclosure may include a plurality of holes  442   a  in a hole area HA. 
     Each of the plurality of holes  442   a  may have a rectangular shape, and the plurality of holes  442   a  may be disposed in matrix and separated from each other. The holes  442   a  in adjacent two rows may be disposed to overlap each other, and the holes  442   a  in adjacent two columns may be disposed to be separated from each other. 
     Each of a second distance d 2  which is a separation distance between long sides of the rectangular shape of the adjacent two holes  442   a  and a fourth distance d 4  which is a separation distance between short sides of the rectangular shape of the adjacent two holes  442   a  may decrease from an edge portion to a central portion of the hole area HA along a direction perpendicular to a folding axis FX. Further, each of a first distance d 1  which is a width of a short side of the rectangular shape of each hole  442   a  and a third distance d 3  which is a width of a long side of the rectangular shape of each hole  442   a  may increase from the edge portion to the central portion of the hole area HA along the direction perpendicular to the folding axis FX. 
     The plurality of holes  442   a  may be disposed such that a rigidity of the plate top  442  decreases from the edge portion to the central portion of the hole area HA along the direction perpendicular to the folding axis FX. 
     The stress in the folding shoulder area may be reduced and the omega shape may be obtained by adjusting the width of the hole area in the first to fourth example embodiments. 
       FIG.  8    is a graph showing a strain of a cover window according to widths of a hole area of a back plate of a foldable display device according to a first example embodiment of the present disclosure, and  FIG.  9    is a table showing rendering of an omega shape according to widths of a hole area of a back plate of a foldable display device according to a first example embodiment of the present disclosure. 
     In  FIGS.  8  and  9   , a reference (Ref) sample includes a back plate without a plurality of holes, and A, B, C, D, E and F samples have the first widths w 1  of the hole area HA of about 1.1 mm, about 2.9 mm, about 4.3 mm, about 5.9 mm, about 9.89 mm and about 15.89 mm, respectively. 
     For the reference (Ref.) sample and the A, B, C, D, E and F samples, an in-folding operation where a radius of curvature of the in-folding area IFA is within a range of 1 R (1 mm) to 3 R (3 mm) and an out-folding operation where a radius of curvature of the out-folding area OFA is within a range of 3 R (3 mm) to 5 R (5 mm) are performed. During the in-folding operation, the cover window  130  of the reference (Ref.) sample and the A, B, C, D, E and F samples has a strain other than 0 in the in-folding area IFA, a strain of 0 in the non-folding area NFA and a negative strain of a maximum absolute value at the folding axis FX. 
     Further, the reference (Ref.) sample and the A, B, C, D, E and F samples have tensile strains of about 0.1%, about 0.04%, about 0%, about 0%, about 0%, about 0.05% and about 0.07%, respectively. 
     During the in-folding operation, the strain of the reference (Ref.) sample and the A, B, C, D, E and F samples decreases from the folding axis FX to the edge portion of the in-folding area IFA. 
     While the folding stress is focused on the folding shoulder area FSA in the reference (Ref.) sample and the A, E and F samples, the folding stress is not focused on the folding shoulder area FSA in the B, C and D samples and is minimized. 
     The folding shoulder area FSA of the cover window  130  of the reference (Ref.) sample and the A, B, C, D, E and F samples have the tensile strain of about 0.1%, about 0.04%, about 0%, about 0%, about 0%, about 0.05% and about 0.07%, respectively. 
     The transparent adhesive layer of the reference (Ref.) sample and the A, B, C, D, E and F samples have the maximum strain of about 133%, about 113%, about 112%, about 112%, about 113%, about 117% and about 119%, respectively. 
     During the in-folding operation, the reference (Ref.) sample does not produce an omega shape, the A, E and F samples do not produce a normal omega shape, and the B, C and D samples produce a normal omega shape. 
     As a result, in the foldable display device  110  according to a first example embodiment of the present disclosure, the first width w 1  of the hole area HA of the plate top  142  of the back plate  140  may be within a range of about 2.5 mm to about 6.5 mm. 
     When the first width w 1  of the hole area HA of the plate top  142  of the back plate  140  is within a range of about 2.5 mm to about 6.5 mm, the omega shape may be obtained without the first, second and third guides  152 ,  154  and  156  (of  FIG.  3 A ) during the in-folding operation. When the first width w 1  of the hole area HA of the plate top  142  of the back plate  140  is greater than about 6.5 mm, the omega shape may be obtained by using the first, second and third guides  152 ,  154  and  156  during the in-folding operation. 
     In the foldable display device  110  according to the first to fourth example embodiments of the present disclosure, the plurality of holes  142   a  may be disposed in the hole area HA of the plate top  142  of the back plate  140 , and the at least one groove  146   a  may be disposed in the groove area GA of the plate bottom  146  of the back plate  140 . As a result, a transfer of deformation due to the folding stress to the cover window  130  and the display panel  120  may be minimized, and the reproduction accuracy to the original shape may be improved. 
     In addition, the stress of the folding shoulder area FSA may be minimized by adjusting the width of the hole area HA of the plate top  142  of the back plate  140 . As a result, the normal omega shape may be obtained during the in-folding operation. 
     In another embodiment, a resin layer for absorbing an impact may be disposed in a groove of a plate bottom. 
       FIG.  10    is a cross-sectional view showing a foldable display device according to a fifth example embodiment of the present disclosure. For the elements shown in  FIG.  10    that are substantially the same or similar to the elements described with respect to the first example embodiment, their detailed descriptions may be omitted or shortened for brevity. 
     In  FIG.  10   , a foldable display device  510  according to a fifth example embodiment of the present disclosure may include a display panel  520 , a cover window  530 , a back plate  540  and a resin layer  548 . 
     The back plate  540  may support the display panel  520  and protect the display panel  520  from an external impact, an external moisture and an external heat. The back plate  540  may include a plate top  542 , an adhesive layer  544  and a plate bottom  546  sequentially disposed under the display panel  520 . 
     The plate top  542  may include a plurality of holes  542   a  symmetrically disposed with respect to the folding axis FX in a hole area HA. The hole area HA may produce the in-folding area IFA which has a rigidity relatively lower than the non-folding area NFA due to the plurality of holes  542   a  and is bent during the in-folding operation. 
     The plate bottom  546  may include at least one groove  546   a  symmetrically disposed in a groove area GA with respect to the folding axis FX. The groove area GA may have a relatively low rigidity as compared with the non-folding area NFA due to the at least one groove  546   a  and may produce the out-folding area OFA which is bent during the out-folding operation. 
     The plate bottom  546  may include a material having a relatively high elastic modulus. For example, the plate bottom  546  may include the same type of material as the plate top  542  but in a different alloy composition ratio. 
     The resin layer  548  may be disposed in the at least one groove  546   a  of the plate bottom  546 . The resin layer  548  may include a soft resin and absorb an external impact to protect the display panel  520  and the back plate  540 . 
     In the foldable display device  510  according to a fifth example embodiment of the present disclosure, the plurality of holes  542   a  may be disposed in the hole area HA of the plate top  542  of the back plate  540  and the at least one groove  546   a  may be disposed in the groove area GA of the plate bottom  546  of the back plate  540 . As a result, a transfer of deformation due to the folding stress to the cover window  530  and the display panel  520  may be minimized, and the reproduction accuracy to the original shape may be improved. 
     In addition, the stress of the folding shoulder area FSA may be minimized by adjusting the width of the hole area HA of the plate top  542  of the back plate  540 . As a result, the normal omega shape can be obtained during the in-folding operation. 
     Further, since the resin layer  548  of a soft resin is disposed in the at least one groove  546   a  of the plate bottom  546 , the display panel  520  and the back plate  540  may be protected from an external impact. 
     In another embodiment, a plurality of grooves may be formed in a plate bottom. 
       FIG.  11    is a cross-sectional view showing a foldable display device according to a sixth example embodiment of the present disclosure. For the elements shown in  FIG.  11    that are substantially the same or similar to the elements described with respect to the first example embodiment, their detailed descriptions may be omitted or shortened for brevity. 
     In  FIG.  11   , a foldable display device  610  according to a sixth example embodiment of the present disclosure may include a display panel  620 , a cover window  630 , and a back plate  640 . 
     The back plate  640  may support the display panel  620  and protect the display panel  620  from an external impact, an external moisture and an external heat. The back plate  640  may include a plate top  642 , an adhesive layer  644  and a plate bottom  646  sequentially disposed under the display panel  620 . 
     The plate top  642  may include a plurality of holes  642   a  symmetrically disposed with respect to the folding axis FX in a hole area HA. The hole area HA may produce the in-folding area IFA which has a rigidity relatively lower than the non-folding area NFA due to the plurality of holes  642   a  and is bent during the in-folding operation. 
     The plate bottom  646  may include a plurality of grooves  646   a  symmetrically disposed in a groove area GA with respect to the folding axis FX. The groove area GA may have a relatively low rigidity as compared with the non-folding area NFA due to the plurality of grooves  646   a  and may produce the out-folding area OFA which is bent during the out-folding operation. 
     The plurality of grooves  646   a  may have a same fifth thickness t 5  which is a depth thereof as each other. 
     The plurality of grooves  646   a  of the plate bottom  646  may have a shape corresponding to the plurality of holes  642   a  of the plate top  642 . 
     For example, each of the plurality of grooves  646   a  of the plate bottom  646  may have one of a rectangular shape, a rectangular shape having round corners or an elliptical shape having a minor axis of a constant length and a lozenge shape. A separation distance between the grooves  626   a  may decrease and a width of the groove  626   a  may increase from an edge portion to a central portion of the groove area GA. 
     The plate bottom  646  may include a material having a relatively high elastic modulus. For example, the plate bottom  646  may include the same type of material as the plate top  642  but in a different alloy composition ratio. 
     In the foldable display device  610  according to a sixth example embodiment of the present disclosure, the plurality of holes  642   a  may be disposed in the hole area HA of the plate top  642  of the back plate  640 , and the plurality of grooves  646   a  may be disposed in the groove area GA of the plate bottom  646  of the back plate  640 . As a result, a transfer of deformation due to the folding stress to the cover window  630  and the display panel  620  may be minimized, and the reproduction accuracy to the original shape may be improved. 
     In addition, the stress of the folding shoulder area FSA may be minimized by adjusting the width of the hole area HA of the plate top  642  of the back plate  640 . As a result, the normal omega shape may be obtained during the in-folding operation. 
     Consequently, in the foldable display device according to the first to sixth example embodiment of the present disclosure, since the plurality of holes are disposed in the plate top of the back plate and at least one groove is disposed in the plate bottom of the back plate, a transfer of deformation due to the folding stress to the display panel can be minimized, the reproduction accuracy to the original shape can be improved, and the bidirectional folding can be thus obtained. 
     Further, since the widths of the hole area, the in-folding area and the out-folding area of the back plate can be adjusted, the normal omega shape can be obtained during the in-folding operation and the stress in the folding shoulder area can be minimized. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope of the present disclosure. Thus, it is intended that the present disclosure covers the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.