Patent Publication Number: US-2022240382-A1

Title: Flexible circuit board and method of manufacturing the same, and display module thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present disclosure is based on International Application No. PCT/CN2021/075800, filed on Feb. 7, 2021, which claims the benefit and priority of a Chinese patent application filed on Feb. 20, 2020, with the application number 202010103856.7, titled “FLEXIBLE CIRCUIT BOARD AND MANUFACTURING METHOD THEREFOR, AND DISPLAY MODULE”, and the entire content of the Chinese patent application is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to the field of display technology, in particular to a flexible circuit board, a method of manufacturing the same, and a display module thereof. 
     BACKGROUND 
     The technology of packaging a driver chip of display panel mainly includes COF packaging and COG packaging technology. COG packaging technology directly binds the driver chip to the display panel through anisotropic conductive adhesive film. COF packaging technology binds the driver chip to the flexible circuit board by utilizing the gold-tin eutectic principle, and the driver chip is connected to the display panel through the flexible circuit board. Compared with the COG packaging technology, the COF packaging technology can avoid setting an area of display panel for binding the driving chip, thereby further reducing the frame width of display panel. 
     However, due to the bonding characteristics of Au—Sn eutectic, a flip-chip film and the corresponding driver chip cannot be provided with high-density pins (too high pin density will cause short circuits between the pins), and due to the limitation of size of bonding area of driver chip, by using the flip-chip film as the flexible circuit board, fewer output pins can be provided. 
     The related technology usually solves the technical problem of flexible circuit board with fewer pins through a double-layer flip-chip film. However, the double-layer flip-chip film is expensive and costly. 
     It should be noted that the information disclosed in the background section above is only used to enhance the understanding of the background of the present disclosure, and therefore may include information that does not constitute prior art known to those of ordinary skill in the art. 
     SUMMARY 
     According to one aspect of the present disclosure, there is provided a flexible circuit board, and flexible circuit board includes: a flexible substrate, a lead layer, a first insulating layer, and a pin layer. The lead layer includes a plurality of leads, and is arranged on a side of the flexible substrate; the first insulating layer is arranged on a side of the lead layer distal to the flexible substrate, a plurality of via holes are provided in the first insulating layer, and orthographic projections of the via holes on the flexible substrate at least partially overlap with orthographic projections of the leads on the flexible substrate; and the pin layer includes a plurality of pins, and is provided on a side of the first insulating layer distal to the flexible substrate, the plurality of pins are arranged in a one-to-one correspondence with the plurality of leads, and the pins are connected to the corresponding leads through the via holes. 
     In an exemplary embodiment of the present disclosure, the flexible circuit board further includes a second insulating layer disposed between the flexible substrate and the lead layer. 
     In an exemplary embodiment of the present disclosure, the pin layer includes at least a pin group, each of the pin groups includes a plurality of rows of the pins, each row of the pins are arranged at intervals along a first direction, and two adjacent rows of the pins are staggered in the first direction. 
     In an exemplary embodiment of the present disclosure, the pin layer includes two rows of the pins. 
     In an exemplary embodiment of the present disclosure, the pin layer includes a plurality of the pin groups. 
     In an exemplary embodiment of the present disclosure, the orthographic projections of the plurality of leads on the flexible substrate are arranged at intervals along a first direction, and extend along a second direction, the first direction intersects with the second direction, the lead includes a wiring part and a connecting part, and an orthographic projection of the wiring part on the flexible substrate extends along the second direction; and the connecting part is connected to the wiring part, and a size of the connecting part in the first direction is larger than a size of the wiring part in the first direction; and the orthographic projection of the via hole on the flexible substrate is located on an orthographic projection of the corresponding connecting part on the flexible substrate, and the pin is connected to the corresponding connecting part through the via hole. 
     In an exemplary embodiment of the present disclosure, a size of the pin in the first direction is larger than a size of the connecting part of the corresponding lead in the first direction, and in the first direction, an orthographic projection of the pin on the flexible substrate cover the orthographic projection of the corresponding connecting part on the flexible substrate. 
     In an exemplary embodiment of the present disclosure, each of the pins is connected to the corresponding lead through a plurality of the via holes. 
     In an exemplary embodiment of the present disclosure, the orthographic projections of the plurality of via holes for connecting a same one of the leads, on the flexible substrate are arranged at intervals along the second direction. 
     In an exemplary embodiment of the present disclosure, the second insulating layer is formed of silicon oxide material. 
     According to one aspect of the present disclosure, there is provided a method of manufacturing a flexible circuit board, and the method of manufacturing a flexible circuit board includes: 
     forming a flexible substrate; 
     forming a lead layer on a side of the flexible substrate, wherein the lead layer includes a plurality of leads; 
     forming a first insulating layer on a side of the lead layer distal to the flexible substrate, and providing a plurality of via holes on the first insulating layer, wherein orthographic projections of the via holes on the flexible substrate at least partially overlap with orthographic projections of the leads on the flexible substrate; and 
     forming a pin layer on a side of the first insulating layer distal to the flexible substrate, wherein the pin layer includes a plurality of pins, and is are disposed on the side of the first insulating layer distal to the flexible substrate, the plurality of pins are arranged in a one-to-one correspondence with the plurality of leads, and the pins are connected to the corresponding leads through the via holes. 
     In an exemplary embodiment of the present disclosure, the method further includes: forming a second insulating layer between the flexible substrate and the lead layer. 
     In an exemplary embodiment of the present disclosure, the pin layer includes at least a pin group, each of the pin groups includes a plurality of rows of the pins, each row of the pins are arranged at intervals along a first direction, and two adjacent rows of the pins are staggered in the first direction. 
     In an exemplary embodiment of the present disclosure, the orthographic projections of the plurality of leads on the flexible substrate are arranged at intervals along a first direction, and extend along a second direction, the first direction intersects with the second direction, the lead includes: a wiring part and a connecting part, and an orthographic projection of the wiring part on the flexible substrate extends along the second direction; and the connecting part is connected to the wiring part, and a size of the connecting part in the first direction is larger than a size of the wiring part in the first direction; and the orthographic projection of the via hole on the flexible substrate is located on an orthographic projection of the corresponding connecting part on the flexible substrate, and the pin is connected to the corresponding connecting part through the via hole. 
     In an exemplary embodiment of the present disclosure, a size of the pin in the first direction is larger than a size of the connecting part of the corresponding lead in the first direction, and in the first direction, an orthographic projection of the pin on the flexible substrate cover the orthographic projection of the corresponding connecting part on the flexible substrate. 
     In an exemplary embodiment of the present disclosure, each of the pins is connected to the lead through a plurality of the via holes. 
     In an exemplary embodiment of the present disclosure, the orthographic projections of the plurality of via holes for connecting a same one of the leads, on the flexible substrate are arranged at intervals along the second direction. 
     In an exemplary embodiment of the present disclosure, the pin layer includes a plurality of pin groups. 
     According to one aspect of the present disclosure, there is provided a display module, and the display module includes: a display panel; the above flexible circuit board, a driving chip, and a control motherboard, the flexible circuit board is bound to the display panel through an anisotropic conductive adhesive film; the driving chip is bound to the flexible circuit board through an anisotropic conductive adhesive film; and the control motherboard is bound to the flexible circuit board through an anisotropic conductive adhesive film. 
     It should be understood that the above general description and the following detailed description are only exemplary and explanatory, and cannot limit the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings herein are incorporated into the specification and constitute a part of the specification, show embodiments consistent with the disclosure, and are used together with the specification to explain the principle of the disclosure. Apparently, the drawings in the following description are only some embodiments of the present disclosure. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative work. 
         FIG. 1  is a structural layout of an exemplary embodiment of the flexible circuit board of the present disclosure. 
         FIG. 2  is a cross-sectional view of the dashed line A-A in  FIG. 1 . 
         FIG. 3  is a structural layout of the lead layer in  FIG. 1 . 
         FIG. 4  is a structural layout of the pin layer in  FIG. 1 . 
         FIG. 5  is a cross-sectional view of another exemplary embodiment of the flexible circuit board of the present disclosure. 
         FIG. 6  is a schematic structural diagram of another exemplary embodiment of the flexible circuit board of the present disclosure. 
         FIG. 7  is a flowchart of an exemplary embodiment of a method of manufacturing of the flexible circuit board of the present disclosure. 
         FIG. 8  is a schematic structural diagram of an exemplary embodiment of the display module of the present disclosure. 
         FIG. 9  is a cross-sectional view of the dashed line B-B in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments will now be described more fully with reference to the accompanying drawings. However, the example embodiments can be implemented in various forms, and should not be construed as being limited to the examples set forth herein; on the contrary, the provision of these embodiments makes the present disclosure more comprehensive and complete, and fully conveys the concept of the example embodiments to those skilled in the art. The same reference numerals in the figures represent the same or similar structures, and thus their detailed descriptions will be omitted. 
     Although relative terms such as “upper” or “lower” are used in this specification to describe the relative relationship between one component and another component indicated in the drawings, these terms are used in this specification only for convenience, for example, the direction of the example described according to the drawings. It can be understood that, if the device indicated in the drawings is turned over to be upside down, the component described as “upper” will become the “lower” component. Other relative terms, such as “high”, “low”, “top”, “bottom”, “left”, “right”, etc., have similar meanings. When a certain structure is “on” another structure, it may mean that the certain structure is integrally formed on the said another structure, or that the certain structure is “directly” arranged on the said another structure, or that the certain structure is “indirectly” arranged on the said another structure through further another structure. 
     The terms “a”, “an”, and “the” are used to express the presence of one or more elements/components/etc.; the terms “include” and “have” are used to express the open-ended meaning of inclusion and refer to in addition to the listed elements/compositions/etc., there may be other elements/compositions/etc. 
     The exemplary embodiment provides a flexible circuit board, as shown in  FIGS. 1, 2, 3, and 4 ,  FIG. 1  is a structural layout of an exemplary embodiment of the flexible circuit board of the present disclosure,  FIG. 2  is a cross-sectional view of the dashed line A-A in  FIG. 1 ,  FIG. 3  is a structural layout of the lead layer in  FIG. 1 , and  FIG. 4  is a structural layout of the pin layer in  FIG. 1 . In the drawings, a flexible substrate is in a flattened state. The flexible circuit board includes: a flexible substrate  1 , a lead layer, a first insulating layer  3 , and a pin layer. The lead layer includes a plurality of leads  21 , and the lead layer may be arranged on a side of the flexible substrate  1 . The first insulating layer  3  may be arranged on a side of the lead layer distal to the flexible substrate  1 . A plurality of via holes  31  may be provided in the first insulating layer  3 , and orthographic projections of the via holes  31  on the flexible substrate may at least partially overlap with orthographic projections of the leads  21  on the flexible substrate. The pin layer may be provided on a side of the first insulating layer  3  distal to the flexible substrate  1 . The pin layer may include a plurality of pins  41 , the plurality of pins  41  are arranged in a one-to-one correspondence with the plurality of leads  21 , and the pins  41  can be connected to the corresponding leads  21  through the via holes  31 . 
     The pins in the flexible circuit board can be formed by a patterning process. For example, the flexible circuit board can be formed by a patterning device for manufacturing an array substrate in a display panel. Since the patterning process can reach a higher precision, the pins  41  can reach a higher density. In addition, the flexible circuit board can be bound with the driving chip (with a larger number of pins) in the COG packaging technology through an anisotropic conductive adhesive film. On the one hand, the flexible circuit board can output more pins in a limited size, on the other hand, the flexible circuit board has a simple structure and low cost. 
     As shown in  FIGS. 1-4 , in the exemplary embodiment, the orthographic projections of a plurality of leads  21  on the flexible substrate may be arranged at intervals along a first direction X, and extend along a second direction Y, and the first direction may intersect with the second direction, for example, the first direction is perpendicular to the second direction. The lead  21  may include a wiring part  211  and a connecting part  212 , an orthographic projection of the wiring part  211  on the flexible substrate may extend along the second direction Y, the connecting part  212  may be connected to the wiring part  211 , a size of the connecting part  212  in the first direction X may be larger than a size of the wiring part  211  in the first direction X. The orthographic projection of the via hole  31  on the flexible substrate may be located on an orthographic projection of the corresponding connecting part  212  on the flexible substrate, and the pin  41  may be connected to the corresponding connecting part  212  through the via hole  31 . Orthographic projections of the plurality of pins  41  on the flexible substrate may be arranged at intervals along the first direction X, and extend along the second direction Y, as well. 
     As shown in  FIGS. 1-4 , both the connecting part  212  and the pin  41  may be rectangular, and the via hole  31  may be circular. It should be understood that, in other exemplary embodiments, the connecting part  212 , the pin  41 , and the via hole  31  may have other shapes as well. For example, the connecting part  212 , the pin  41 , and the via hole  31  may be circular, trapezoidal, or the like. 
     As shown in  FIGS. 1-4 , in the exemplary embodiment, a size of the pin  41  in the first direction X may be larger than the size of the corresponding connecting part  212  (that is, the connecting part of the lead corresponding to the pin) in the first direction X, and in the first direction X, the orthographic projection of the pin  41  on the flexible substrate can cover the orthographic projection of the corresponding connecting part  212  on the flexible substrate. Wherein, as shown in  FIG. 1 , the pin  41  may include a side  411  and a side  412  that are disposed oppositely in the first direction X, the connecting part  212  may include a side  2121  and a side  2122  that are disposed oppositely in the first direction X. In the first direction X, the orthographic projection of the pin  41  on the flexible substrate can cover the orthographic projection of the corresponding connecting part  212  on the flexible substrate, which can be understood as: orthographic projections of the side  2121  and the side  2122  of the connecting part  212  on the flexible substrate are located between orthographic projections of the side  411  and the side  412  of the pin  41  on the flexible substrate. 
     As shown in  FIGS. 1-4 , in the exemplary embodiment, each pin  41  may be connected to the lead through a plurality of via holes  31 . The orthographic projections of the plurality of via holes  31  for connecting the same lead  21 , on the flexible substrate may be arranged at intervals along the second direction Y. For example, as shown in  FIGS. 1-4 , each pin  41  may be connected to the lead  21  through three via holes  31 . It should be understood that, in other exemplary embodiments, each of the pins  41  may be connected to the leads through other numbers of via holes  31 . The number of via holes corresponding to the respective pins  41  can be the same or different. 
     In the exemplary embodiment, the flexible substrate  1  may be composed of polyimide acid material, and the flexible substrate formed of the polyimide acid material has good bending properties. The lead layer may be made of metal material or alloy material. For example, the lead layer may be metal or alloy material such as Cu, Mo/Cu, NiCr/Cu, Ni/Cu, Cr/Cu, Ti/Al/Ti. The leads of lead layer can be used to form connecting wires. For example, when the flexible circuit board is used for packaging the driver chip of display panel, the leads can be used to connect the display panel and the driver chip, and to connect a control motherboard and the driver chip. The driving chip may include a source driving circuit for generating data signals, a touch signal driving circuit for generating touch signals, and the like. The control motherboard may include a clock control circuit for generating a clock signal and a control signal, and the clock signal and the control signal may be used to control driving circuits such as the source driving circuit, the touch signal driving circuit, etc., to generate corresponding driving signals. The pin layer may be composed of indium tin oxide material. The first insulating layer  3  may be composed of silicon oxide material, and the pins of pin layer are used to bind external components such as the driving chip, the display panel, and the control motherboard. One pin can be connected to the corresponding lead through a plurality of via holes to reduce resistance between the pin and the lead, and at the same time, the plurality of via holes are redundantly arranged to improve stability of the flexible circuit board. 
     In the exemplary embodiment, as shown in  FIG. 5 , it is a cross-sectional view of another exemplary embodiment of the flexible circuit board of the present disclosure. The flexible circuit board may further include a second insulating layer  5 , and the second insulating layer  5  may be disposed between the flexible substrate and the lead layer. In the exemplary embodiment, the second insulating layer  5  may be formed of silicon oxide (SiOx) material. The second insulating layer formed of the SiOx material has the effect of isolating water and oxygen, and can prevent organic matter and moisture in the flexible substrate from affecting the subsequent processes. It should be understood that the second insulating layer can be composed of other inorganic material as well. Inorganic material has better water and oxygen barrier properties. Any side of the inorganic layer can be further provided with an organic layer. The organic layer can perform a planarization process to the surface of the inorganic layer, so that the inorganic layer has a better effect of isolating water and oxygen. 
     In the exemplary embodiment, the pin layer may include a pin group, as shown in  FIG. 1 ,  FIG. 1  exemplarily shows a distribution diagram of a pin group, and the pin group includes two rows of pins  41 . Each row of pins  41  may be arranged at intervals along the first direction X, and two adjacent rows of pins  41  may be staggered in the first direction X. As shown in  FIG. 4 , two adjacent rows of pins  41  may include a first pin row  041  and a second pin row  042 . The first pin row  041  and the second pin row  042  respectively include a plurality of pins  41 . Areas covered by the plurality of pins  41  in the first pin row  041  moving in the second direction Y is a plurality of first strip-shaped areas  411  arranged at intervals along the first direction X and extending along the second direction Y. Areas covered by the plurality of pins  41  in the second pin row  042  moving in the second direction Y is a plurality of second strip-shaped areas  421  arranged at intervals along the first direction X and extending along the second direction Y. The two adjacent rows of pins  41  may be staggered in the first direction X, which can be understood as that the first strip-shaped areas  411  formed by the plurality of pins in the first pin row  041  and the second strip-shaped areas  421  formed by the plurality of pins in the second pin row  042  are alternately arranged in the first direction X and do not intersect each other. This arrangement can realize that a larger number of pins are formed on the flexible circuit board within in the limited size in the first direction X. It should be understood that the pin group may include other numbers of rows of pins as well, each row of pins are arranged at intervals along the first direction X, and two adjacent rows of pins  41  may be staggered in the first direction X. 
     In the exemplary embodiment, the pin layer may include a plurality of pin groups. For example, as shown in  FIG. 6 , which is a schematic structural diagram of another exemplary embodiment of the flexible circuit board of the present disclosure, the pin layer may include three pin groups  11 ,  12 , and  13 , each of which includes a plurality of pins. Each pin group is used to connect different external components. For example, the pin group  11  can be used to connect to the display panel, the pin group  12  can be used to connect to the driver chip, and the pin group  13  can be used to connect to the control motherboard. 
     The exemplary embodiment further provides a method of manufacturing a flexible circuit board. As shown in  FIG. 7 , it is a flowchart of an exemplary embodiment of the method of manufacturing a flexible circuit board of the present disclosure. And the method includes: 
     step S 1 : forming a flexible substrate; 
     step S 2 : forming a lead layer on a side of the flexible substrate, wherein the lead layer includes a plurality of leads; 
     step S 3 : forming a first insulating layer on a side of the lead layer distal to the flexible substrate, and providing a plurality of via holes on the first insulating layer, wherein orthographic projections of the via holes on the flexible substrate partially covers orthographic projections of the leads on the flexible substrate; and 
     step S 4 : forming a pin layer on a side of the first insulating layer distal to the flexible substrate, wherein the pin layer includes a plurality of pins, and is disposed on the side of the first insulating layer distal to the flexible substrate, and the pins are correspondingly connected with the leads through the via holes. 
     The above steps are described in detail below: 
     In step S 1 , since the flexible circuit board needs to be transferred a plurality of times among the respective devices during the manufacturing process, forming the flexible substrate may include forming the flexible substrate on a rigid substrate to facilitate the transfer of the flexible substrate. The rigid substrate can be a glass substrate. The flexible substrate may be formed through a coating process. 
     In step S 2 , forming the lead layer on a side of the flexible substrate, may include forming the lead layer through a patterning process. The patterning process may specifically include: firstly, forming an entire conductive layer on a side of the flexible substrate, wherein the entire conductive layer may be formed by metal sputtering; then, forming a required lead layer pattern by processes such as coating photoresist, masking, exposing, and etching, etc. The conductive layer may be a metal layer. 
     In step S 3 , forming the first insulating layer on the side of the lead layer distal to the flexible substrate, may include forming the first insulating layer on the side of the lead layer distal to the flexible substrate through a vapor deposition process. In addition, providing the plurality of via holes on the first insulating layer, can be formed by a patterning process as well. 
     In step S 4 , forming the pin layer on the side of the first insulating layer distal to the flexible substrate, may include: forming an entire conductive layer on the side of the first insulating layer distal to the flexible substrate through a sputtering process; then, forming the pin layer with a preset pattern from the conductive layer through a patterning process. The conductive layer may be an indium tin oxide layer. 
     A plurality of the flexible circuit boards may be formed in a flexible circuit motherboard at one time through the above steps, and the flexible circuit motherboard includes a plurality of flexible circuit boards. After the flexible circuit motherboard is formed, the flexible circuit motherboard can be divided into a plurality of flexible circuit boards through a cutting process. After the flexible circuit motherboard is divided into a plurality of flexible circuit boards, the flexible circuit boards can be coated with liquid solder resist, wherein the liquid solder resist can be green oil. The liquid solder resist can increase flexibility of the flexible circuit board, and the liquid solder resist can further protect the flexible circuit board. 
     In the exemplary embodiment, the pin layer may include at least one pin group, each of the pin groups includes a plurality of rows of pins, each row of the pins are arranged at intervals along the first direction, and two adjacent rows of pins are staggered in the first direction. The pin layer may include a plurality of pin groups. 
     In the exemplary embodiment, the method of manufacturing a flexible circuit board further includes: disposing a second insulating layer between the flexible substrate and the lead layer, wherein the second insulating layer may be formed of silicon oxide (SiOx) material. The second insulating layer formed of the SiOx material has the effect of isolating water and oxygen, and can prevent organic matter and moisture in the flexible substrate from affecting the subsequent processes. 
     In the exemplary embodiment, orthographic projections of the plurality of leads on the flexible substrate may be arranged at intervals along the first direction and extend along the second direction, wherein the first direction and the second direction may intersect. The lead may include: a wiring part and a connecting part, wherein an orthographic projection of the wiring part on the flexible substrate may extend along the second direction, the connecting part is connected to the wiring part, a size of the connecting part in the first direction may be larger than a size of the wiring part in the first direction, the orthographic projection of the via hole on the flexible substrate may be located on an orthographic projection of the connecting part on the flexible substrate, and the pin is connected to the connecting part through the via hole. 
     In the exemplary embodiment, a size of the pin in the first direction may be larger than the size of the connecting part of the corresponding lead in the first direction, and in the first direction, an orthographic projection of the pin on the flexible substrate can cover the orthographic projection of the corresponding connecting part on the flexible substrate. 
     In the exemplary embodiment, each of the pins may be connected to the lead through a plurality of via holes. 
     In the exemplary embodiment, the orthographic projections of the plurality of via holes used for connecting the same lead, on the flexible substrate, may be arranged at intervals along the second direction. 
     The method of manufacturing a flexible circuit board can form the above-mentioned flexible circuit board, and the method of manufacturing a flexible circuit board has the same technical features and technical effects as that of the above-mentioned flexible circuit board, which will not be repeated here. 
     The exemplary embodiment further provides a display module, as shown in  FIGS. 8 and 9 ,  FIG. 8  is a schematic structural diagram of an exemplary embodiment of the display module of the present disclosure, and  FIG. 9  is a cross-sectional view of the dashed line B-B in  FIG. 8 . The display module includes: a display panel  61 , an above-mentioned flexible circuit board  62 , a driving chip  63 , and a control motherboard  64 . The flexible circuit board  62  can be bound to the display panel  61  through an anisotropic conductive adhesive film. The driving chip  63  is bound to the flexible circuit board  62  through an anisotropic conductive adhesive film  65 . The control motherboard  64  is bound to the flexible circuit board  62  through an anisotropic conductive adhesive film. 
     As shown in  FIG. 9 , the anisotropic conductive adhesive film  65  contains conductive particles  651 , and the driving chip  63  can be connected to the pins  41  on the flexible circuit board  62  through the conductive particles  651 . 
     The driving chip may include a source driving circuit for generating data signals, a touch signal driving circuit for generating touch signals, and the like. The control motherboard may include a clock control circuit for generating a clock signal and a control signal. The clock signal and the control signal may be used to control a circuit such as the source drive circuit, the touch signal drive circuit, etc., to generate corresponding drive signals. By bending the flexible circuit board, the driver chip and the control motherboard can be encapsulated on the back of the display panel, thereby reducing the width of frame of the display panel. 
     Those skilled in the art will easily think of other embodiments of the present disclosure after considering the description of the present disclosure and practicing the disclosure disclosed herein. This application is intended to cover any variations, uses, or adaptive changes of the present disclosure. These variations, uses, or adaptive changes follow the general principles of the present disclosure and include common knowledge or conventional technical means in the technical field that are not disclosed in the present disclosure. The description and embodiments of the present disclosure are only regarded as exemplary, and the true scope and spirit of the present disclosure are pointed out by the appended claims. 
     It should be understood that the present disclosure is not limited to the precise structure that has been described above and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present disclosure is limited only by the appended claims.