Patent Publication Number: US-2023156933-A1

Title: Flexible display device and manufacturing method thereof

Description:
FIELD OF INVENTION 
     The present application relates to the field of flexible display technology and specifically to a flexible display device and manufacturing method thereof. 
     BACKGROUND OF INVENTION 
     With development of flexible display technology, development of flexible display devices that are thin and lightweight has become the mainstream in the industry. 
     However, the thinner a thickness of a flexible display device is, the greater a curvature of a bending part is, stress exerted on a connecting part of a bending part and a non-bending part becomes greater, and defects such as wiring breakage and delamination easily occur, significantly decreasing mass production yields of the flexible display device. With this limitation, so far a width of flexible display devices invariably cannot be further decreased in the industry, encountering a technical bottleneck when making flexible display devices lighter and thinner. 
     SUMMARY OF INVENTION 
     Embodiments of the present application provide a flexible display device and manufacturing method thereof to solve a technical problem that when a thickness of the flexible display device is excessively thin, wiring breaking and delamination occurs at a preset bending position of a flexible display panel due to excessive bending stress. 
     Embodiments of the present application provide a flexible display device that includes a flexible display panel, wherein the flexible display panel is bent at a preset position to form a first plane part, a bending part, and a second plane part, and the bending part is located between the first plane part and the second plane part; wherein the preset position comprises a first bending point located on a surface where the bending part meets the first plane part and a second bending point located on a surface where the bending part meets the second plane part, the first plane part lies on a first plane, and the first bending point and the second bending point lie on a second plane perpendicular to the first plane; wherein a distance between the first bending point and the second bending point is H, a bending axis of the bending part is perpendicular to the second plane, and a middle point of a line formed by connecting the first bending point and the second bending point is located on the bending axis; and wherein in the second plane, at least one radius of curvature of a point on the bending part is greater than H/2. 
     Optionally, in some embodiments of the present application, the second plane part is parallel to the first plane; wherein in the second plane, a distance between a curved-surface vertex of the bending part and the first plane part is equal to a distance between the curved-surface vertex and the second plane part. 
     Optionally, in some embodiments of the present application, a difference between a longest radius of curvature r of the bending part and a shortest radius of curvature H/2 is at least greater than 120 μm. 
     Optionally, in some embodiments of the present application, a bending axis of the bending part is perpendicular to the second plane, a middle point of a line formed by connecting the first bending point and the second bending point is located on the bending axis, and an angle between a plane formed by the curved-surface vertex and the bending axis and the first plane part ranges from 0 degrees to 30 degrees. 
     Optionally, in some embodiments of the present application, an angle between a plane formed by a curved-surface vertex of the bending part on the second plane and a bending axis of the bending part and the first plane part and/or the second plane part ranges from 0 degrees to 30 degrees. 
     Optionally, in some embodiments of the present application, the flexible display device further includes a backplate, wherein the flexible display panel is disposed on the backplate, a first backplate of the backplate is disposed corresponding to the first plane part, the first plane part is located on the first backplate, a groove part is defined at where the backplate corresponds to the bending part, the bending part is disposed corresponding to the groove part, a second backplate of the backplate is disposed corresponding to the second plane part, and the second plane part is located on the second backplate; wherein the second plane part is bent to one side of the first backplate away from the first plane part, and a design value of a length L of the groove part before bending satisfies following formula: 
         L=απ ( H−h )/2 
     wherein a is a bending process accuracy corrected value, H is a distance between the first bending point and the second bending point, and h is a thickness of the flexible display panel. 
     Optionally, in some embodiments of the present application, the bending process accuracy corrected value a satisfies following formula: 
       α=( A{circumflex over ( )} 2+ B{circumflex over ( )} 2+ C{circumflex over ( )} 2) 1/2  
 
     Wherein A is a manufacturing accuracy of the groove part, B is a lamination accuracy of the flexible display panel and the backplate, and C is a bending alignment accuracy of the first backplate and the second backplate. 
     Optionally, in some embodiments of the present application, the flexible display device includes a composite supporting structure, wherein the composite supporting structure is located between the first plane part and the second plane part after bending, the composite supporting structure separately abuts the first backplate and the second backplate, and the distance H between the first bending point and the second bending point satisfies following formula: 
         H= 2( h+a )+ b    
     Wherein h is a thickness of the flexible display panel, a is a thickness of the backplate, and b is a thickness of the composite supporting structure. 
     Optionally, in some embodiments of the present application, a shape of the bending part comprises an ellipse curved surface. 
     Optionally, in some embodiments of the present application, in the second plane, the bending part has a curved-surface vertex, a radius of curvature of the bending part gradually increases in an interval from the first bending point to the curved-surface vertex, and the radius of curvature of the bending part gradually decreases in an interval from the curved-surface vertex to the second bending point. 
     Optionally, in some embodiments of the present application, in the second plane, the radius of curvature of the bending part varies between a shortest radius of curvature H/2 and a longest radius of curvature r. 
     Embodiments of the present application further provides a manufacturing method of the above-described flexible display device that includes: 
     B1, obtaining a length L of a groove part trenched in a backplate corresponding to the bending part before bending according to following formula: 
         L=απ ( H−h )/2 
     Wherein a is a bending process accuracy corrected value, H is a target distance between the first bending point and the second bending point, and h is a thickness of the flexible display panel; 
     B2, presetting a bending trajectory of the bending part by a bending apparatus of the flexible display panel according to the length L of the groove part and the target distance H between the first bending point and the second bending point, such that a distance between a curved-surface vertex of the bending part in the second plane and the first plane part is equal to a distance between the curved-surface vertex and the second plane part; 
     B3, bending the flexible display panel by the bending apparatus according to the preset bending trajectory such that a difference between a longest radius of curvature r of the bending part and H/2 is at least greater than 120 μm. 
     Optionally, in some embodiments of the present application, step B2 further includes presetting the bending trajectory of the bending part such that a bending axis of the bending part is perpendicular to the second plane, a middle point of a line formed by connecting the first bending point and the second bending point is located on the bending axis, and an angle between a plane formed by the curved-surface vertex of the bending part after bending and the bending axis and the first plane part ranges from 0 degrees to 30 degrees. 
     Optionally, in some embodiments of the present application, the bending process accuracy corrected value a satisfies following formula: 
       α=( A{circumflex over ( )} 2+ B{circumflex over ( )} 2+ C{circumflex over ( )} 2) 1/2  
 
     Wherein A is a manufacturing accuracy of the groove part, B is a lamination accuracy of the flexible display panel and the backplate, and C is a bending alignment accuracy of the first backplate and the second backplate. 
     Optionally, in some embodiments of the present application, the flexible display device comprises a composite supporting structure, and step B3 includes wherein the first plane part is located on one side of the composite supporting structure, bending the second plane part to another side of the composite supporting structure, such that the composite supporting structure separately abuts a first backplate and a second backplate, and the distance H between the first bending point and the second bending point satisfies following formula: 
         H= 2( h+a )+ b    
     Wherein h is a thickness of the flexible display panel, a is a thickness of the backplate, and b is a thickness of the composite supporting structure. 
     Optionally, in some embodiments of the present application, the bending trajectory of the bending part comprises an ellipse curved surface. 
     Optionally, in some embodiments of the present application, in step B2, presetting the bending trajectory of the bending part such that a radius of curvature of the bending part gradually increases in an interval from the first bending point to the curved-surface vertex, and the radius of curvature of the bending part gradually decreases in an interval from the curved-surface vertex to the second bending point. 
     Optionally, in some embodiments of the present application, in step B2, presetting the bending trajectory of the bending part such that in the second plane, the radius of curvature of the bending part varies between a shortest radius of curvature H/2 and the longest radius of curvature r. 
     Optionally, in some embodiments of the present application, step B3 comprises bending the flexible display panel by the bending apparatus according to the preset bending trajectory such that the difference between the longest radius of curvature r of the bending part and H/2 is at least greater than 200 μm. 
     While decreasing a thickness of the entire flexible display device, a distance between the first bending point and the second bending point at two ends of the bending part is decreased, and through lengthening the greatest radius of curvature r of the bending part to be greater than half the distance between the first bending point and the second bending point, a curved surface of the bending part includes an ellipse curved surface, thereby decreasing bending stress at the first bending point and the second bending point, preventing wiring on the flexible display panel from defects such as breakage and delamination due to excessive bending stress, and significantly increasing mass production yield of the ultra-thin flexible display panel. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       The accompanying figures to be used in the description of embodiments of the present application will be described in brief to more clearly illustrate the technical solutions of the embodiments. The accompanying figures described below are only part of the embodiments of the present application, from which figures those skilled in the art can derive further figures without making any inventive efforts. 
         FIG.  1    is a schematic side view of a flexible display device according to a first embodiment of the present application. 
         FIG.  2    is a structural schematic diagram of a flexible display panel and a backplate in  FIG.  1    before bending. 
         FIG.  3    is a side view of a flexible display device according to a second embodiment of the present application. 
         FIG.  4    is an experimental test chart of the flexible display device according to the present application. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     For better clearness and definiteness of purpose, technical approach, and effect of the present application, the following further describes embodiments of the present application in detail with reference to accompanying drawings. It should be understood that the embodiments described here is merely for explaining the present application and not intended to limit the present application. 
     The present application provides a reference coordinate system, specifically including an XY-plane, an YZ-plane, and an XZ-plane defined by a first direction X, a second direction Y, and a third direction Z, for clearer elaborating a technical approach of the present application. 
     The present application provides a first embodiment as shown in  FIG.  1   , a flexible display device  100  that includes a flexible display panel  1 , a backplate  2 , a composite supporting structure  3 , a polarizer  4 , a cover plate  5 , and a protection layer  6 . 
     The flexible display panel  1  includes a display area AA for displaying images. The display area AA is on a plane that is parallel to the XY-plane. For decreasing a border width of the flexible display device  100  as much as possible, the flexible display panel  1  can be bent at a preset position. When the flexible display panel  1  is bent in a way shown in  FIG.  1   , the flexible display panel  1  can be divided into a first plane part  11 , a bending part  12 , and a second plane part  13  sequentially connected, and the bending part  12  is located between the first plane part  11  and the second plane part  13 . 
     The first plane part  11  lies on a first plane  110  that is parallel to the XY-plane, and the display area AA is located on the first plane part  11 . Pixels (not shown) for displaying images and signal lines (not shown) for transmitting driving signals are formed on the display area AA, conventional technology in the industry can be adopted for specific pixel and circuit structures, and they are not limited here. 
     The preset position includes a first bending point M located on a surface where the bending part  12  meets the first plane part  11  and a second bending point N located on a surface where the bending part  12  meets the second plane part  13 . The first bending point M is located on one surface of the first plane part  11  closer to the polarizer, and the second bending point N is located on one surface of the second plane part  13  away from the polarizer. Both the first bending point M and the second bending point N are on a second plane  130  perpendicular to the first plane  110 . The second plane  130  is parallel to the XZ-plane, and the second plane  130  is perpendicular to the first plane  110 .  FIG.  1    to  FIG.  3    are structural schematic diagrams of the flexible display device presented on the second plane  130 . 
     It can be understood that deformation of the first plane part  11  and the second plane part  13  is less than deformation of the bending part  12 , and the first plane part  11  and the second plane part  13  is not necessarily a flat surface. In other embodiments, the entire flexible display panel  1  can be bent or rolled. In other words, the bending part  12  is not limited to a part of the flexible display panel  1  that is bent as shown in the drawing, and any part of the flexible display panel  1 , when bent, can be at least one of the first plane part  11 , the bending part  12 , and the second plane part  13 . 
     On the second plane, a shape of the bending part  12  is elliptical, the first bending point M and the second bending point N are separately located at two ends of the bending part  12 , and a distance between the first bending point M and the second bending point N is H. Through forming a perpendicular line with respect to the second plane at a middle point of a line formed by connecting the first bending point M and the second bending point N, a bending axis P of the bending part  12  can be obtained. 
     On the second plane  130 , a radius of curvature of the bending part  12  can be defined as a shortest distance from the bending axis P to a designated point on a surface of the bending part  12  away from the bending axis P. For example, in the second plane  130  shown in  FIG.  1   , a radius of curvature of the bending part  12  at point M and point N is H/2, and a radius of curvature at a point O on the bending part  12  farthest from the bending axis P is r. In the second plane  130 , it can be observed that a distance from the bending axis P to a surface of the bending part  12  away from the bending axis P keeps varying between H/2 and r. That is, the bending part  12  has a radius of curvature that constantly varies between H/2 and r. When a shape of the bending part  12  is elliptical as shown in the drawing, a radius of curvature at a curved-surface vertex O is the longest radius of curvature r, and r is greater than H/2. 
     In the second plane  130 , the radius of curvature of the bending part  12  gradually increases in an interval from the first bending point M to the curved-surface vertex O. In the second plane  130 , the radius of curvature of the bending part  12  gradually decreases in an interval from the curved-surface vertex O to the second bending point N. 
     It can be understood that, in other embodiments of the present application, the bending part  12  can include a plurality of bending portions, and as long as a radius of curvature of at least one point on the bending part  12  is greater than H/2, stress at the first bending point M and the second bending point N can be decreased to a certain degree. 
     Also referring to  FIG.  2   , the backplate  2  includes a first backplate  21  corresponding to the first plane part  11  and a second backplate  23  corresponding to the second plane part  13 , and a groove part  22  corresponding to the bending part  12  is formed between the first backplate  21  and the second backplate  23 . The backplate  2  can be manufactured by adopting organic insulation materials capable of blocking water and oxygen such as polyimide (PI) and/or polyethylene terephthalate (PET). 
     The flexible display panel  1  is formed on the backplate  2 . Wherein, the first plane part  11  is located on the first backplate  21 , the bending part  12  is disposed corresponding to the groove part  22 , and the second plane part  13  is located on the second backplate  23 . In some embodiments, the groove part  22  can penetrate the backplate  2  in a thickness direction of the backplate  2  to expose a part of the flexible display panel  1 , so as to increase flexibility of the flexible display panel  1  at the bending part  12  as much as possible. 
     When a target thickness of the flexible display device  100  is decided, a target distance H between the first bending point M and the second bending point N can also be decided. Because a length of a curve segment formed by bending the bending part  12  and a length L of the groove part  22  are nearly identical, the longest radius of curvature r of the bending part  12  is decided by the length L of the groove part  22  before bending. 
     The present application provides a design formula for the length L of the groove part  22  before bending as follows: 
         L=απ ( H−h )/2 
     Wherein, α is a bending process accuracy corrected value, H is a distance between the first bending point M and the second bending point N, and h is a thickness of the flexible display panel  1 . 
     In actual production, the bending process accuracy corrected value a satisfies a following formula: 
       α=( A{circumflex over ( )} 2+ B{circumflex over ( )} 2+ C{circumflex over ( )} 2) 1/2  
 
     Wherein, A is a manufacturing accuracy of the groove part  22 , B is a lamination accuracy of the flexible display panel  1  and the backplate  2 , and C is a bending alignment accuracy of the first backplate  21  and the second backplate  23 . 
     When the length L of the groove part  22  before bending is calculated through the above-described formulas, a bending apparatus of the flexible display panel  1  can preset a bending trajectory of the bending part  12  according to the length L of the groove part obtained and the target distance H between the first bending point M and the second bending point N, obtaining an r value. When the display panel  1  is bent in a way shown in  FIG.  1   , so that a point O on the bending part  12  farthest from the bending axis P has an equal distance from the first plane part  11  and the second plane part  13 , the r value can be obtained through a commonly used perimeter formula for an ellipse: L=2r+H+πH/2 
     The composite supporting structure  3  is located between the first plane part  11  and the second plane part  13  after bending, the composite supporting structure  3  includes a supporting layer  31  and a lamination layer  32 , and the supporting layer  31  is disposed on the lamination layer  32 . 
     Wherein, the supporting layer  31  includes a composite structure of a metal heat dissipation layer (not shown), a foam supporting layer (not shown), etc. The first backplate  21  is located between the supporting layer  31  and the first plane part  11 , and one surface of the supporting layer  31  abuts the first backplate  21  and is fixedly laminated. The lamination layer  32  can use a common double-sided tape, and the lamination layer  32  fixedly laminates together the other surface of the supporting layer  31  and a surface of the second backplate  23 . 
     The protection layer  6  often adopts ultraviolet (UV) adhesive. The protection layer  6  is configured to protect the bending part  12  of the flexible display panel  1  from damage due to external force. One end of the protection layer  6  extends to a surface of the first plane part  11 , and the other end of the protection layer  6  extends to a surface of the second plane part  13 . 
     In the present embodiment, the distance H between the first bending point M and the second bending point N satisfies a following formula: 
         H= 2( h+a )+ b    
     Wherein, h is a thickness of the flexible display panel  1 , a is a thickness of the backplate  2 , and b is a thickness of the composite supporting structure  3 . 
     In conventional technology, a radius of curvature of a bending part is a fixed value and equals H/2. In order to fulfill a need of constantly decreasing a thickness of flexible display devices in the industry, a distance H between the first bending point M and the second bending point N is constantly decreased, and the radius of curvature of the bending part is also decreased correspondingly. However, the shorter the radius of curvature of the bending part is, the greater bending stress exerted at two stress concentration points of the first bending point M and the second bending point N of a flexible display panel is. When H is less than 0.5 mm, the bending stress exerted at the two stress concentration points of the first bending point M and the second bending point N of the flexible display panel is excessively great, and this easily causes defects such as wiring breaking and delamination, significantly decreasing manufacturing yields of flexible display devices. Embodiments of the present application provides an approach that while adaptively decreasing the H value, through lengthening the longest radius of curvature r of the bending part  12  to make the bending part  12  become an ellipse shape, a bending angle of the bending part  12  at the first bending point M and the second bending point N is decreased to decrease bending stress at the first bending point M and the second bending point N, thereby solving the above-described technical problem. 
     In embodiments shown in the drawings, the second plane part  13  lies on a plane that is parallel to the XY-plane and the first plane  110 . In the second plane  130  shown in the drawings, an end point N on a surface of the second plane part  13  aligns with an end point M on a surface of the first plane part  11  in a vertical direction, so that a distance between the curved-surface vertex O of the bending part  12  and the first plane part  11  is equal to a distance between the curved-surface vertex O and the second plane part  13 . At this time, the longest radius of curvature r of the bending part  12  at the curved-surface vertex O is greater than the shortest radius of curvature H/2 at the first bending point M and the second bending point N by at least 120 μm. In some embodiments of the present application, the longest radius of curvature r of the bending part  12  at the curved-surface vertex O is greater than the shortest radius of curvature H/2 at the first bending point M and the second bending point N by at least 200 μm. 
     It can be understood that, when designing a bending trajectory of the bending part  12 , the curved-surface vertex O of the bending part  12  can also be closer to the first plane part  11  or the second plane part  13  to adapt to different application situations or match different housings. In such embodiments, an angle between a plane formed by the curved-surface vertex O of the bending part  12  and the bending axis P and the XY-plane ranges from 0 degrees to 30 degrees. That is, the curved-surface vertex O of the bending part  12  can be shifted closer to the first plane part  11  or the second plane part  13  within an angle of 30 degrees. 
     It can be understood that the flexible display panel  1  can also have various bending angles. The drawings merely illustratively show a flexible display device whose first plane part  11  is parallel to the second plane part  13 . In other embodiments of the present application, an angle between a plane which the first plane part  11  lies on and a plane which the second plane part  13  lies on can be 60 degrees, 90 degrees, etc., and they are not limited here. 
     In the first embodiment, the longest radius of curvature r of the bending part  12  is lengthened, leading to a widened border width of the flexible display device  100 . As shown in  FIG.  1   , the border width of the flexible display device  100  includes a width W of a non-display area and the greatest radius of curvature r of the bending part  12 . 
     In order to realize a narrow border design of the flexible display device  100 , the present application provides a second embodiment as shown in  FIG.  3   . A width W′ of the non-display area in the second embodiment is less than the width W of the non-display area in the first embodiment. Through decreasing the width W′ of the non-display area, a sum of W′ and r is less than or equal to a sum of W and r in the first embodiment, thereby ensuring the border width of the flexible display device  100  to remain unchanged or become narrower, satisfying a current trend of narrow border design. 
     In the present embodiment, the width W′ of the non-display area can be realized by shrinking a wiring area of a fan-out area, adjusting a position of a driving control wiring, etc., and they are not limited here. 
     The present application further provides a manufacturing method of the above-described flexible display device  100 , including: 
     B1, obtaining a length L of the groove part  22  on the backplate  2  before bending according to a following formula: 
         L =απ( H−h )/2
 
     Wherein, H is a target distance between the first bending point M and the second bending point N, h is a thickness of the flexible display panel  1 , and a is a bending process accuracy corrected value. 
     In actual production, the bending process accuracy corrected value a satisfies a following formula: 
       α=( A{circumflex over ( )} 2+ B{circumflex over ( )} 2+ C{circumflex over ( )} 2) 1/2  
 
     Wherein, A is a manufacturing accuracy of the groove part  22 , B is a lamination accuracy of the flexible display panel  1  and the backplate  2 , and C is a bending alignment accuracy of the first backplate  21  and the second backplate  23 . 
     B2, presetting a bending trajectory of the bending part  12  by a bending apparatus of the flexible display panel  1  according to the length L of the groove part obtained in step B1 and the target distance H between the first bending point M and the second bending point N. 
     In embodiments shown in the drawings, the bending trajectory of the bending part  12  includes an ellipse curved surface, and a distance between the curved-surface vertex 0 of the bending part  12  and the first plane part  11  is equal to a distance between the curved-surface vertex O and the second plane part  13 . A radius of curvature r at the curved-surface vertex O is greater than H/2. 
     B3, bending the flexible display panel  1  by the bending apparatus according to the preset bending trajectory such that a difference between a longest radius of curvature r of the bending part  12  and H/2 is at least greater than 120 μm. 
       FIG.  4    shows an experimental test chart for stress of the flexible display device according to the present application. In the drawing, under an ideal condition where step of the backplate  2  is zero, i.e., when a dislocation distance between an edge of the first backplate  21  and an edge of the second backplate  23  after bending is zero, average stress at the first bending point M and the second bending point N simulated when the bending part  12  adopting a positive semicircle design is 1134 MPa, and average stress at the first bending point M and the second bending point N simulated when the bending part  12  adopting a non-perfect semicircle design is 1022 MPa. 112 MPa of stress can be decreased by using design of the present application. 
     Under real production test adjustment, an alignment error between the edge of the first backplate  21  and the edge of the second backplate  23  after bending is inevitable. In a real test, when step of the backplate  2  is 220 μm, i.e., when a dislocation distance between the edge of the first backplate  21  and the edge of the second backplate  23  after bending is 220 μm, average stress at the first bending point M and the second bending point N obtained when the bending part  12  adopting a positive semicircle design is 2858 MPa, and average stress at the first bending point M and the second bending point N obtained when the bending part  12  adopting a non-perfect semicircle design is 1769 MPa. 1089 MPa of stress can be decreased by using design of the present application. 
     In the present application, while decreasing a thickness of the entire flexible display device, a distance H between the first bending point M and the second bending point N at two ends of a surface of the bending part  12  is adaptively decreased, and through lengthening the greatest radius of curvature r of the bending part  12  to be greater than H/2, a curved surface of the bending part  12  includes an ellipse curved surface, thereby decreasing bending stress at the first bending point M and the second bending point N, preventing wiring on the flexible display panel from defects such as breakage and delamination due to excessive bending stress, and significantly increasing mass production yield of the ultra-thin flexible display panel. 
     The technical approach of the present application overcomes a technical bottleneck in conventional technology that the distance H between the first bending point M and the second bending point N is restricted by a radius of curvature R of the bending part and cannot be further decreased. In the conventional technology, a thickness of H ranges between 0.5 mm and 0.7 mm, and through improvement of the present application, the value of H can be decreased to less than 0.5 mm. While ensuring mass production yield, a further decrease of a thickness of the flexible display device becomes possible. 
     Although the present application has been explained in relation to its preferred embodiment, it does not intend to limit the present application. It will be apparent to those skilled in the art having regard to this present application that other modifications of the exemplary embodiments beyond these embodiments specifically described here may be made without departing from the spirit of the application. Accordingly, such modifications are considered within the scope of the application as limited solely by the appended claims.