JP2006330082A - Curvature-detecting apparatus and flexible apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a curvature-detecting apparatus for preventing excessive deformations which cause permanent deformations. <P>SOLUTION: A plurality of curvature detecting means 3, 6 and 7 detect whether the curvature of a base material 1 having flexibility reaches a reference value. When the curvature of the base material reaches the reference value, a warning generating means 9 imparts that the curvature reaches the reference value. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a bending amount detection device and a flexible device, and more particularly to a bending amount detection device and a flexible device that are effective in preventing the occurrence of excessive bending.

  Development of a wearable display device is expected for the realization of a ubiquitous society. The wearable display device has a structure in which a display element is formed on a flexible substrate (flexible substrate) and is bendable. Examples of such display elements include liquid crystal display elements, organic electroluminescence (organic EL) elements, paper-like displays (electronic paper), and the like.

  As the flexible substrate, for example, a polymer material such as polyimide is used. Such a substrate can be bent within the range of elastic deformation without causing plastic deformation (permanent deformation) up to a curvature radius of about several millimeters. However, if it is bent beyond the range of elastic deformation, permanent deformation is caused and the deformed portion becomes cloudy white.

  In many flexible display devices, an inorganic insulating film, an indium tin oxide (ITO) electrode, or the like is used. Further, in an active display device, a thin film transistor is formed over a flexible substrate. Therefore, when the flexible display device is bent beyond a certain radius of curvature, these insulating films and electrodes are destroyed.

  Patent Document 1 listed below discloses a flexible thin film circuit board having high resistance to bending stress. In the thin film circuit substrate disclosed in Patent Document 1, an inorganic insulating film patterned in an island shape is provided so as to cover an electronic element such as a thin film transistor. Further, the inorganic insulating film and the wiring are covered with a resin film.

  The inorganic insulating film having higher rigidity than the organic insulating film covers the electronic element to protect the electronic element. When bending stress is applied to the thin film circuit board, the inorganic insulating film is patterned in an island shape, so that the substrate and the resin film can be bent following the bending stress.

JP 2004-101976 A

  In the invention disclosed in Patent Document 1, an electronic element can be protected from bending stress, but a region where the electronic element is not formed may bend beyond the range of elastic deformation. This bent portion causes permanent deformation of the flexible substrate and causes white turbidity. Therefore, in order not to cause permanent deformation, the handler of the flexible substrate needs to handle with care to bend.

  An object of the present invention is to provide a bending amount detection device for preventing excessive deformation that causes permanent deformation. Another object of the present invention is to provide a flexible device that allows a handler to easily recognize the occurrence of excessive bending and prevent breakage due to bending.

  According to one aspect of the present invention, a base material having flexibility, a curvature amount detection means for detecting whether or not a curvature amount of the base material has reached a certain reference value, and the curvature amount detection means, When it is detected that the bending amount of the base material has reached the reference value, a bending amount detection device is provided that includes a warning generation means for notifying that the bending amount has reached the reference value.

  According to another aspect of the present invention, there is provided a functional panel that has flexibility and displays an image or emits illumination light, and the above-described bending amount detection device attached to the functional panel. The bending amount detecting device is provided with a flexible device that bends following the bending of the functional panel.

  Since a warning is issued when the amount of bending of the substrate reaches the reference value, the operator can easily recognize that the amount of bending has approached the allowable limit. Thereby, the damage of the functional panel resulting from excessive curvature can be prevented.

  FIG. 1A shows a plan view of the bending amount detection apparatus according to the first embodiment, and FIG. 1B shows a cross-sectional view taken along one-dot chain line B1-B1 in FIG. A plurality of convex portions 2 are provided on the surface of the base material 1 made of polyimide. The base material 1 has a rectangular shape with side lengths of 4 cm and 3 cm, for example. An XYZ orthogonal coordinate system is introduced in which the direction parallel to one side is the X axis and the direction perpendicular to the X axis is the Y axis.

  Each of the convex portions 2 extends in a direction parallel to the Y axis, and extends from the vicinity of one side parallel to the X axis to the vicinity of the side opposite to the one side. The plurality of convex portions 2 are arranged at a constant pitch in a direction parallel to the X axis. As an example, the height of the convex portion 2 is 1 mm, the width is 1 mm, and the interval between the convex portions 2 adjacent to each other is also 1 mm. The thickness of the base material 1 in a region where the convex portion 2 is not provided is, for example, 0.5 mm.

  The surface of the convex portion 2 is covered with a conductive film 3 such as aluminum (Al). The thickness of the conductive film 3 is, for example, 200 nm. The conductive films 3 covering the two convex portions 2 adjacent to each other are electrically insulated from each other.

  When serial numbers are assigned to the convex portions 2 from one end to the other end in the X-axis direction, the conductive film 3 that covers the odd-numbered convex portions 2 is connected to the wiring 4A, and the conductive film that covers the even-numbered convex portions 2. The film 3 is connected to another wiring 4B. The wirings 4 </ b> A and 4 </ b> B are made of a conductive film such as aluminum formed on the surface of the substrate 1. Terminals 5A and 5B are formed at the ends of the wirings 4A and 4B, respectively.

  An electric circuit in which a power source 6 and a current detector 7 are connected in series is connected to the terminals 5A and 5B. In a normal state, no current flows through the current detector 7. When the conductive films 3 covering the two adjacent protrusions 2 are in contact with each other, a current flows through the current detector 7.

  When detecting the current, the current detector 7 transmits a current detection signal to the driver circuit 8. When the driver circuit 8 receives the current detection signal from the current detector 7, the driver circuit 8 transmits a drive signal to the warning generator 9. The warning generator 9 is a light emitting element such as a light emitting diode, and emits light when a drive signal is received. As the warning generator 9, a sound generator, a vibration generator, a character display device, or the like may be used. When the sound generator receives the drive signal, the sound generator emits an alarm sound. When receiving the drive signal, the vibration generator generates vibration that can be recognized by the handler. When the character display device receives the drive signal, the character display device displays character information that alerts the operator. The power source 6, the current detector 7, the driver circuit 8, and the warning generator 9 are mounted on a substrate different from the base material 1, for example.

Next, with reference to FIG. 2, the manufacturing method of the bending amount detection apparatus by 1st Example is demonstrated.
As shown in FIG. 2A, a base material 1 having a plurality of convex portions 2 provided on the surface is prepared. The substrate 1 can be formed by injection molding polyimide. As shown in FIG. 2B, a conductive film 3 having a thickness of 200 nm is formed by vacuum-depositing aluminum on the surface on which the convex portions 2 are provided. As shown in FIG. 2C, a resist film R is formed by spin coating a photoresist on the surface of the conductive film 3.

  As shown in FIG. 2D, the resist film R is exposed and developed to form a resist pattern RP. The resist pattern RP covers the side surfaces and the upper surface of the protrusions 2 and further covers the regions where the wirings 4A and 4B and the terminals 5A and 5B are arranged.

  As shown in FIG. 2E, the conductive film 3 is etched using the resist pattern RP as an etching mask. As shown in FIG. 2F, the resist pattern RP is removed. Through the above steps, the conductive film 3 covering the upper surface and side surfaces of the convex portion 2 is formed, and the wirings 4A and 4B and the terminals 5A and 5B shown in FIG. 1A are formed.

  With reference to FIG. 3, the operation of the bending amount detection apparatus according to the first embodiment will be described. FIG. 3 shows a state in which the bending amount detection device is bent along a column surface parallel to the Y axis so that the surface on which the convex portion 2 is provided is on the inside. When the amount of curvature reaches a certain reference value, the conductive films 3a and 3b covering the surfaces of the two adjacent convex portions 2a and 2b come into contact with each other. In the case of the first embodiment, the reference value for the amount of bending corresponds to a curvature radius of 4 mm. When both come into contact with each other, a current flows through the current detector 7 shown in FIG. Thereby, the warning generator 9 emits light. Conversely, when no light emission occurs, it means that the bending amount has not reached the reference value. Thus, the convex part 2, the electrically conductive film 3, wiring 4A, 4B, the power supply 6, and the current detector 7 comprise a curvature amount detection means.

  The operator can easily recognize that the bending amount has reached the reference value by issuing a warning such as light emission. Thereby, generation | occurrence | production of the further curve can be prevented. By attaching this bending amount detection device to a flexible functional panel such as a flexible liquid crystal display device, a paper-like display, or a flexible lighting panel, the deformation amount of the flexible functional panel is close to the allowable limit. , You can make the handlers recognize.

FIG. 4 shows a plan view of a bending amount detection apparatus according to the second embodiment. Hereinafter, the description will be given focusing on differences from the bending amount detection apparatus according to the first embodiment shown in FIG.
In the first embodiment, the protrusions 2 that are long in the Y-axis direction are arranged at equal intervals in the X-axis direction. However, in the second embodiment, the plurality of protrusions 2 are arranged in a matrix, in the X-axis direction and They are arranged at equal intervals in the Y-axis direction. Each upper surface and side surface of the convex portion 2 are covered with the conductive film 3 as in the case of the first embodiment. When a row parallel to the X axis is a row and a row parallel to the Y axis is a column, one convex portion 2 can be specified by a combination of a row number and a column number.

  The conductive film covering the convex part 2 (the convex part hatched in FIG. 4) in which the sum of the row number and the column number is an even number is connected to the wiring 4A, and the sum of the row number and the column number is an odd number. The conductive film covering the convex portion 2 (the white convex portion in FIG. 4) is connected to the other wiring 4B. That is, voltages having opposite polarities are applied to the conductive film covering the convex portions 2 adjacent to each other in the X-axis direction and the Y-axis direction. Other configurations are the same as those of the bending amount detection apparatus according to the first embodiment.

  In the case of the first example, the base material 1 is easy to bend along the column surface parallel to the X axis, and is difficult to bend along the column surface parallel to the Y axis. In the case of the second embodiment, it is bent equally in any direction. Further, in any direction of bending, when the amount of bending reaches a reference value, a warning can be issued by light emission or the like.

  FIG. 5 shows a plan view of a bending amount detection apparatus according to the third embodiment. In the second embodiment shown in FIG. 4, the protrusions 2 are arranged at equal intervals in the X-axis direction and the Y-axis direction. However, in the third embodiment, the interval in the Y-axis direction is the same as that in the X-axis direction. It is wider than the interval. Other configurations are the same as those of the bending amount detection apparatus according to the second embodiment.

  In the third embodiment, the reference value of the amount of curvature when curved along the column surface parallel to the Y axis is greater than the reference value of the amount of curvature when curved along the column surface parallel to the X axis. Is also big. For this reason, when it is curved along a column surface parallel to the Y-axis, a warning such as light emission is issued at a time when the amount of bending is smaller than when it is curved along a column surface parallel to the X-axis. Be emitted.

  For example, a wiring made of a transparent conductive film such as indium tin oxide (ITO) is arranged in parallel to the first direction, and a metal wiring such as aluminum is arranged in parallel to a second direction orthogonal thereto. A case where the bending amount detection device according to the third embodiment is attached to the flexible function panel will be described. Wiring made of ITO or the like is more susceptible to damage due to deformation than metal wiring. The bending amount detection device is attached to the flexible function panel so that the X-axis direction of the bending amount detection device according to the third embodiment is parallel to the first direction in which the wiring made of ITO or the like extends.

  As a result, when the wiring made of ITO or the like curves in the direction of deformation, a warning is issued when the amount of bending is relatively small, and a relatively large curve can be allowed in the direction of deformation of the metal wiring. .

  In the first to third embodiments, the bending amount is detected by contact of the conductive film covering the adjacent convex portions. However, the bending amount can be detected by other methods. For example, a strain gauge whose electrical resistance changes by bending may be attached to the flexible substrate. Information relating to the amount of bending can be obtained by detecting a change in electrical resistance.

  Next, with reference to FIGS. 6A and 6B, a flexible image display apparatus according to a fourth embodiment will be described. The flexible image display apparatus according to the fourth embodiment is a flexible image display board, for example, a paper-like display using electrophoresis, and the flexibility according to the first, second, or third embodiment. A substrate is applied.

  FIG. 6A shows a cross-sectional view of two pixel portions of the flexible image display device according to the fourth embodiment, and FIG. 6B shows a cross-sectional view of many pixel portions. A switching element 13 such as a thin film transistor is formed for each pixel on the surface of the rear substrate 11. An interlayer insulating film 15 is formed on the rear substrate 11 so as to cover the switching element 13. A first electrode 14 is formed on the interlayer insulating film 15 for each pixel. The first electrode 14 is connected to the corresponding switching element 13 via a via hole penetrating the interlayer insulating film 15. An inorganic insulating film 16 made of silicon nitride or the like is formed so as to cover the first electrode 14.

  The display-side substrate 10 is disposed with a gap from the insulating film 16. A partition wall 12 is provided between the display side substrate 10 and the insulating film 16. The partition walls 12 are arranged in a lattice pattern on the surface of the insulating film 16, define pixels, and secure a certain gap between the display-side substrate 10 and the insulating film 16.

  A second electrode 17 made of aluminum or the like is disposed between the partition wall 12 and the insulating film 16. For example, the second electrode 17 may be embedded in the partition wall 12. A space surrounded by the insulating film 16, the display-side substrate 10, and the partition wall 12 is filled with an insulating liquid 20 such as silicon oil. A large number of charged electrophoretic particles 21 having a particle diameter of about 1 to 2 μm are dispersed in the insulating liquid 20. Examples of the material of the charged electrophoretic particles 21 include a polystyrene-polymethyl methacrylate copolymer resin containing carbon black.

  A paper-like display sandwiched between the rear substrate 11 and the display-side substrate 10 is configured. This paper-like display is disclosed in, for example, Japanese Patent Application Laid-Open No. 2003-161966.

  On the outer surface of the rear substrate 11, the bending amount detection device 30 is attached. The bending amount detection device 30 is the same as the bending amount detection device according to any one of the first to third embodiments described above. With respect to the in-plane direction, a plurality of pixels are arranged in one convex portion of the bending amount detection device 30.

  The operation of the flexible image display device according to the fourth embodiment will be described. The second electrode 17 is grounded. It is assumed that the charged electrophoretic particles 21 are positively charged. When a positive voltage is applied to the first electrode 14, the charged electrophoretic particles 21 gather near the second electrode 17 as shown in the pixel on the right side of FIG. If the surface of the first electrode 14 is white, this pixel is in a white display state. When a negative voltage is applied to the first electrode 14, the charged electrophoretic particles 21 spread on the first electrode 16 as shown in the left pixel of FIG. Thereby, this pixel is in a state where the black color of the charged electrophoretic particles 21 is observed.

  When the flexible image display device is bent so that the rear substrate 11 is on the inner side, and the bending amount reaches a reference value, the bending amount detection device 30 issues a warning. The operator recognizes the warning and suppresses the occurrence of further bending, thereby preventing the paper-like display from being destroyed.

  FIG. 7 is an exploded perspective view of a flexible image display device according to a modification of the fourth embodiment. On the paper-like display 35, an image display area 35A for displaying an image and a frame area 35B surrounding the image display area 35A are defined. A frame region 35B is bonded to the frame 36. The image display area 35A does not overlap the frame 36. For this reason, the image display area 35 </ b> A can be visually recognized without being blocked by the frame 36.

  A curvature amount detection device similar to any one of the curvature amount detection devices according to the first to third embodiments is bonded to one surface or both surfaces of the frame 36. When the paper-like display 35 is bent, the frame 36 is deformed accordingly. Since the warning is issued when the deformation amount of the frame 36 reaches the reference value, the destruction of the paper-like display 35 can be prevented.

  The frame 36 itself may be configured with a bending amount detection device. Further, the curvature amount detection device may be attached to both surfaces of the frame 36. When pasted on both sides, a warning can be issued even if any side is curved so as to be inside.

  As shown in FIG. 8, a convex portion 35C may be provided directly in the frame region 35B. The surface of the convex portion 35C is covered with a conductive film, and the bending amount detection device according to the first to third embodiments can be configured. For example, the frame area 35B is divided into four areas along the four sides of the image display area 35A. A convex portion 35C extending in the width direction is formed in each of the four regions. Since the convex portion 35C is not disposed in the image display area 35A, it is possible to view the image from both sides without being blocked by the bending amount detection device.

  The paper-like display is a generic name for thin displays such as paper, and as in the flexible image display device according to the fourth embodiment, the horizontal direction in which charged particles are moved in the horizontal direction (in-plane direction). In addition to the mobile electrophoretic display, various methods have been proposed. For example, a microcapsule type electrophoretic display, a twisting ball display, a field deposition / dissolution display, and the like have been proposed. The bending amount detection devices according to the first to third embodiments can be applied to these various paper-like displays.

  Next, a flexible image display device according to a fifth embodiment will be described with reference to FIGS. The flexible image display device according to the fifth embodiment is obtained by applying the bending amount detection device according to the first to third embodiments to an organic EL element.

  FIG. 9A shows a cross-sectional view of three pixel portions of the flexible image display device according to the fifth embodiment, and FIG. 9B shows a cross-sectional view of many pixel portions. A plurality of positive electrodes 41 made of ITO are formed on the surface of the transparent film 40 having flexibility. The positive electrodes 41 are arranged in parallel to each other (lateral direction in FIG. 9A) within the surface of the transparent film 40.

  A plurality of strip-shaped organic light emitting layers 42 are arranged on the transparent film 40 so as to intersect the positive electrode 41. Each of the organic light emitting layers 42 has a multilayer structure in which a hole transport layer 42A, a light emitting layer 42B, and an electron transport layer 42C are laminated in order from the transparent film 40 side. A negative electrode 45 such as aluminum is formed on the surface of the electron transport layer 42C. If necessary, a hole injection layer may be inserted between the positive electrode 41 and the hole transport layer 42A. Further, an electron injection layer may be inserted between the negative electrode 45 and the electron transport layer 42C. The intersection of the positive electrode 41 and the organic light emitting layer 42 functions as a pixel.

The structure from the transparent film 41 to the negative electrode 45 is disclosed by Unexamined-Japanese-Patent No. 2003-272864, for example.
The positive electrode 41, the organic light emitting layer 42, and the negative electrode 45 are sealed by bonding the bending amount detection device 50 to the transparent film 40 using an epoxy resin adhesive 48. The bending amount detection device 50 is the same as any one of the bending amount detection devices according to the first to third embodiments described above, and is bonded so that the surface with the convex portion faces outward.

  The light generated in the light emitting layer 42B passes through the transparent film 40 and is emitted to the outside. When the flexible image display device is bent so that the bending amount detection device 50 is on the inner side, and the bending amount reaches the reference value, the bending amount detection device 50 issues a warning. The operator can prevent the destruction of the organic EL element by recognizing the warning and suppressing the occurrence of further bending.

  Also in the fifth embodiment, as shown in FIG. 7, the bending amount detection device may be attached to the frame attached to the organic EL element, or as shown in FIG. A bending amount detection device may be formed by providing a portion.

  Next, a flexible image display device according to a sixth embodiment will be described with reference to FIGS. In the flexible image display device according to the sixth embodiment, any one of the bending amount detection devices according to the first to third embodiments is applied to a liquid crystal display element having flexibility.

  FIG. 10A shows a cross-sectional view of the vicinity of both ends of the flexible image display device according to the sixth embodiment, and FIG. 10B shows a cross-sectional view of a large number of pixel portions. A pair of flexible substrates 60 and 65 having a thickness of about 100 μm are arranged in parallel with a gap therebetween. Transparent electrodes 61 and 66 made of ITO or the like are formed on the inner surfaces of the substrates 60 and 65, respectively. The substrates 60 and 65 are generally based on PET, PC, PES, etc., and have a structure in which a gas barrier layer, a surface treatment layer, and the like are laminated. The transparent electrode 61 is composed of a plurality of strip-shaped electrodes extending in the horizontal direction in the figure, and the transparent electrode 66 is composed of a plurality of strip-shaped electrodes extending in a direction perpendicular to the drawing sheet. Pixels are defined at the intersections of the two. Alignment films 62 and 67 are formed on the substrates 60 and 65 so as to cover the transparent electrodes 61 and 66, respectively.

  A gap holding member 70 and a nematic liquid crystal 71 are filled between the two substrates 60 and 65. The substrates 60 and 65 are polymerized and bonded by the sealant 72. In addition, a color filter and a polarizing film are arrange | positioned as needed. This liquid crystal display element is disclosed in, for example, Japanese Patent Laid-Open No. 9-101509.

  A curvature amount detecting device 75 is attached to the outer surface of one substrate 60. The bending amount detection device 75 is the same as one of the bending amount detection devices according to the first to third embodiments described above. The surface to which the convex portion of the bending amount detection device 75 is attached faces outward.

  When the bending amount detection device 75 is bent so as to be inward, the bending amount detection device 75 issues a warning when the bending amount reaches a reference value. For this reason, generation | occurrence | production of the curve beyond it can be suppressed and destruction of a liquid crystal display element can be prevented.

  Also in the sixth embodiment, similarly to the structure shown in FIG. 7, the bending amount detection device may be attached to the frame attached to the liquid crystal display element, or the frame is similar to the structure shown in FIG. 8. A convex amount may be provided only in the region to form a bending amount detection device.

  In the flexible image display devices according to the fourth to sixth embodiments, irreversible degradation occurs when an image display plate, such as a paper-like display, an organic EL element, a liquid crystal display element, or the like is excessively deformed. The “irreversible deterioration” means deterioration that does not return the display quality even if the image display board is returned to its original shape. Examples of irreversible deterioration include cracks in electrodes such as ITO, and cloudiness of flexible films.

  It is preferable to set the bending amount reference value so that the bending amount detecting device issues a warning before the deformation that causes such irreversible deterioration occurs. The reference value of the amount of bending can be set to a desired value by adjusting the geometric shape, height, interval, etc. of the protrusions.

  Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

(A) is a top view of the bending amount detection apparatus by 1st Example, (B) is the sectional drawing. It is sectional drawing of the apparatus in the middle of manufacture for demonstrating the manufacturing method of the curvature amount detection apparatus by 1st Example. It is sectional drawing of the state which curved the curvature amount detection apparatus by 1st Example. It is a top view of the curvature amount detection apparatus by 2nd Example. It is a top view of the curvature amount detection apparatus by a 3rd Example. It is sectional drawing of the flexible image display apparatus by a 4th Example. It is sectional drawing of the flexible image display apparatus by the modification of a 4th Example. It is sectional drawing of the flexible image display apparatus by another modification of a 4th Example. It is sectional drawing of the flexible image display apparatus by a 5th Example. It is sectional drawing of the flexible image display apparatus by a 6th Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Base material 2 Convex part 3 Conductive film 4A, 4B Wiring 5A, 5B Terminal 6 Power supply 7 Current detector 8 Driver circuit 9 Warning generator 10 Display side substrate 11 Back substrate 12 Partition 13 Switching element 14 First electrode 15 Interlayer insulation film 16 Inorganic insulating film 17 Second electrode 20 Insulating liquid 21 Electrophoretic particles 30, 50, 75 Curvature detection device 35 Paper-like display 35A Image display area 35B Frame area 35C Convex part 36 Frame 40 Transparent film 41 Positive electrode 42 Organic light emitting layer 45 Negative electrode 48 Adhesive 60, 65 Substrate 61, 66 Transparent electrode 62, 67 Alignment film 70 Gap holding member 71 Liquid crystal 72 Sealing agent

Claims (5)

A flexible substrate;
A bending amount detecting means for detecting whether or not the bending amount of the base material has reached a certain reference value;
A bending amount detection apparatus comprising: a warning generation unit that notifies that the bending amount has reached the reference value when the bending amount detection unit detects that the bending amount of the base material has reached the reference value. A plurality of convex portions are provided on the surface of the base material,
The bending amount detection means includes a conductive film that covers at least a part of each surface of the convex portions of the base material, and a conductive film that covers the surfaces of two convex portions adjacent to each other among the convex portions. The voltage application means for applying a voltage to the conductive film and the conductive film covering the surfaces of the two adjacent protrusions are in contact with each other so that a voltage having the opposite polarity is applied, and a current flows between them. The bending amount detection device according to claim 1, further comprising: current detection means for detecting the current.   The bending amount detection device according to claim 1, wherein the warning generation unit notifies that the bending amount has reached a reference value by light emission, sound generation, generation of vibration, or display of characters. A functional panel having flexibility and displaying an image or emitting illumination light;
The bending amount detection device according to any one of claims 1 to 3 attached to the functional panel,
The bending amount detection device is a flexible device that bends following the bending of the functional panel.   The flexible device according to claim 4, wherein the reference value is set such that the amount of curvature of the base material reaches the reference value before irreversible deformation occurs in the functional panel.

Images