Abstract:
A conductive trace design is described that minimizes the possibility of crack initiation and propagation in conductive traces during bending. The conductive trace design has a winding trace pattern that is more resistant to the formation of cracks at high stress points in the conductive traces. The conductive trace design includes a cap that helps ensure electrical connection of the conductive trace even though one or more cracks may begin to form in the conductive portion of the conductive trace.

Description:
BACKGROUND 
     Technical Field 
       [0001]    This relates generally to electronic devices, and more particularly, to electronic devices with a flexible display. 
       Description of the Related Art 
       [0002]    Electronic devices often include displays. For example, cellular telephones and portable computers include displays for presenting information to a user. Many electronic devices are now being built on flexible substrates rather than rigid circuit boards or glass. This allows electronic devices to be flexed or bent to some degree. Like conventional non-flexible devices, flexible electronic devices include conductive traces, typically made of metal, that are used to propagate signals within the electronic devices. However, the conventional designs of conductive traces are prone to cracking and/or delamination over repeated bending of the electronic devices, resulting in reduced performance and/or reliability. 
       SUMMARY 
       [0003]    The embodiments herein describe a winding conductive trace design that is resistant to cracking during bending and stretching stresses. The winding conductive trace design may be incorporated in any flexible electronic device such as a flexible display device, or in any electronic device that may not necessarily be flexible. In some embodiments, a winding conductive trace includes a cap located in a low stress region of the winding conductive trace. The width of a metal trace line located in the regions of the winding conductive trace that include caps is wider than the width of the metal trace line located in other regions of the winding conductive trace that lack the caps. The cap helps ensure electrical connection of the metal trace line even though one or more cracks may begin to form in the metal trace line. 
         [0004]    In one embodiment, an apparatus comprises a flexible substrate and a winding conductive trace formed over the flexible substrate. The winding conductive trace includes a plurality of alternating crests and troughs. Each crest and each trough has a first edge (e.g., an outer edge) and a second edge (e.g., an inner edge) positioned opposite the first edge. A first portion of the winding conductive trace located between each alternating crest and trough is smaller in width than a second portion of the winding conductive trace between the first edge and the second edge of each of the crests and troughs. 
         [0005]    In one embodiment, a winding conductive trace splits into multiple sub-traces which converge back into a single winding conductive trace at certain intervals to prevent or minimize severance of interconnections by cracks in the winding conductive trace. An apparatus including multiple sub-traces comprises a flexible substrate and a winding conductive trace formed over the flexible substrate. The winding conductive trace includes a first sub-trace and a second sub-trace that is symmetric to the first sub-trace. The first sub-trace and the second sub-trace are disposed in a mirrored shape and each includes a plurality of alternating crests and troughs that each has a first edge (e.g., an outer edge) and a second edge (e.g., an inner edge) positioned opposite the first edge. The first sub-trace and the second sub-trace split from the winding conductive trace and merge back together at a plurality of joints where each joint is located at a trough of the first sub-trace and a crest of the second sub-trace. A first portion of each first sub-trace and each second sub-trace located between each alternating crest and trough is smaller in width than a second portion of each first sub-trace and each second sub-trace between the first edge and the second edge of each of the crests and troughs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIGS. 1A, 1B, 2A, and 2B  are schematic views of an exemplary flexible display device according to one embodiment. 
           [0007]      FIGS. 3A and 3B  are respectively a schematic plane view and corresponding perspective view of a winding conductive trace of the exemplary flexible display device according to one embodiment. 
           [0008]      FIG. 4  is a detailed view of the winding conductive trace according to one embodiment. 
           [0009]      FIG. 5  is a cross-section view of the wire trace according to one embodiment. 
           [0010]      FIG. 6  is a detailed view of a winding conductive trace according to another embodiment. 
           [0011]      FIG. 7  is a detailed view of a winding conductive trace according to still another embodiment. 
           [0012]      FIG. 8A  a detailed view of mirrored conductive traces according to one embodiment. 
           [0013]      FIG. 8B  illustrates a staggered arrangement of multiple mirrored conductive traces according to one embodiment. 
           [0014]      FIG. 9  is a view of mirrored conductive traces according to another embodiment. 
       
    
    
     DETAILED DESCRIPTION 
     Flexible Display Device 
       [0015]      FIG. 1  illustrates an exemplar flexible display  100  which may be incorporated in electronic devices. The flexible display  100  includes at least one active area (i.e., display area), in which an array of display pixels are formed therein. Each pixel may be associated with a corresponding pixel circuit, which may be coupled to one or more signal lines for communicating with the driving circuits (e.g., a gate driver, a data driver, etc.) to activate the pixels. In way of an example, each pixel circuit may be electrically connected to a gate line and a data line. 
         [0016]    The flexible display  100  may include one or more inactive areas at the periphery of the active area. That is, the inactive area may be adjacent to one or more sides of the active area such that the active area may be partly or entirely surrounded by the inactive area. For instance, the active area of the flexible display  100  may have a square or a rectangular shape, and the inactive area of the flexible display  100  may surround the active area. However, it should be appreciated that the shapes of the active area and the inactive area are not particularly limited. The active area and the inactive area may be in any shape according to the design of the electronic device employing the flexible display  100 . Non-limiting examples of the active area shapes in the flexible display  100  include a pentagonal shape, a hexagonal shape, a circular shape, an oval shape, and more. 
         [0017]    The flexible display  100  can include various circuits, which may be used in generating various signals, for example, signals operating the pixels of the flexible display  100  and signals for sensing touch inputs from a user, and various other functionality associated with the flexible display  100  and/or the electronic device employing the flexible display  100 . 
         [0018]    Some of the circuits may be mounted on an external printed circuit and coupled to a connection interface (Pads/Bumps) disposed in the inactive area using flexible printed circuit board (FPCB), chip-on-film (COF), tape-carrier-package (TCP) or any other suitable technologies. Also, some of the circuits may be implemented with one or more transistors fabricated in the inactive area of the flexible display  100 . For example, one or more gate drivers may be implemented with transistors fabricated in the inactive area as depicted in  FIG. 1A . Such gate drivers may be referred to as a gate-in-panel (GIP). It should be appreciated that other types of driving circuits, including but not limited to, an inverter circuit, a multiplexer, data driver, source driver, electro static discharge (ESD) circuit and the like, may also be formed in the inactive area of the flexible display  100 . 
         [0019]    The transistors used in implementing such driving circuits in the inactive area are not particularly limited. That is, the type of transistors used in implementing the driving circuits in the inactive area need not be the same as the transistors used for implementing the pixel circuits in the active area. The type of transistors may be selected according to the operating conditions and/or requirements of the transistors in the corresponding circuit. 
         [0020]    In the embodiments herein, parts of the flexible display  100  may be defined by a central portion and a bend portion. One or more bend portions of the flexible display  100  can be bent away from the tangent plane at a certain bend angle and a bend radius around the betiding axis. A bend portion of the flexible display  100  can be bent away in an inclination angle or in a declination angle at a bend line BL. 
         [0021]    The bend line BL may extend horizontally (e.g., X-axis shown in  FIG. 1A ), vertically (e.g., Y-axis shown in  FIG. 1A ) or even diagonally in the flexible display  100 , or in any other direction of the flexible display  100 . Multiple portions of the flexible display  100  can be bent. Accordingly, one or more edges of the flexible display  100  can be bent away from the plane of the central portion along the several bend lines BL. While the bend line BL is depicted as being located towards the edges of the flexible display  100  in the example of  FIG. 1A , it should be appreciated that the location the bend lines BL is not limited as such. Any one or more corners of the flexible display  100  may be bent as well. The flexible display  100  can be bent in any combination of horizontal, vertical and/or diagonal directions based on the desired design of the flexible display  100 . The bend line BL may be run across the central portion of the flexible display  100  to provide a foldable display or a double-sided display having display pixels on both outer sides of a folded display. 
         [0022]    For example, the central portion of the flexible display  100  may be substantially flat, and one or more bend portions may be bent away from the tangent plane of the central portion. The size of each bend portion that is bent away from the central portion needs not be the same. 
         [0023]    In some embodiments, the radius of curvatures (i.e., bend radius) for the bend portions in the flexible display  100  may be between about 0.1 mm to about 10 mm, between about 0.1 mm to about 5 mm, or between about 0.1 mm to about 1 mm, or between about 0.1 mm to about 0.5 mm. The smallest bend radius of the bend portion of the flexible display  100  may be less than 0.5 mm. 
         [0024]    While the central portion of the flexible display  100  may have a flat surface, some embodiments may not have such a flat central portion. The central portion of the flexible display  100  can be curved-in or curved-out as depicted in  FIG. 19 , providing flexible display  100  with a concave or a convex central portion. Even in the embodiments with a convex or concave curved central portion, one or more bend portions of the flexible display  100  can be bent inwardly or outwardly along the bend line at a bend angle around a bend axis. 
         [0025]    In  FIG. 1A , the bend portion of the flexible display  100  may include an active area capable of displaying an image from the bend portion, which is referred herein after as the secondary active area. That is, the bend line can be positioned in the active area so that at least some display pixels of the active area is included in the bend portion that is bent away from the plane of the central portion of the flexible display  100 . In this case, the matrix of display pixels in the secondary active area of the bend portion may be continuously extended from the matrix of the display pixels in the active area of the central portion as depicted in  FIG. 2A . Alternatively, the secondary active area within the bend portion and the active area within the central portion of the flexible display  100  may be separated apart from each other by the outer bend radius as depicted in  FIG. 2B . 
         [0026]    The secondary active area in the bend portion may serve as a secondary display area in the flexible display  100 . The size of the secondary active area is not particularly limited. The size of the secondary active area may depend on its functionality within the electronic device. For instance, the secondary active area may be used to provide images and/or texts such a graphical user interface, buttons, text messages, and the like. In some cases, the secondary active area may be used to provide light of various colors for various purposes (e.g., status indication light), and thus, the size of the secondary active area need not be as large as the active area in the central portion of the flexible display  100 . 
         [0027]    The pixels in the secondary active area and the pixels in the central active area may be addressed by the driving circuits (e.g., gate driver, data driver, etc.) as if they are in a single matrix. In this case, the pixels of the central active area and the pixels of the secondary active area may be operated by the same set of signal lines (e.g., gate lines, emission lines, etc.). In way of example, the N th  row pixels of the central active area and the N th  row pixels of the secondary active area may be configured to receive a signal from the driving circuit via the conductive traces crossing over the bend allowance section as depicted in  FIG. 2B . 
         [0028]    In some embodiments, the pixels in the secondary active area may be driven discretely from the pixels in the central active area. That is, the pixels of the secondary active area may be recognized by the display driving circuits as being an independent matrix of pixels separate from the matrix of pixels in the central active area. In such cases, the pixels of the central active area and the pixels of the secondary active area may utilize different set of signal lines from each other. Further, the secondary active area may employ one or more display driving circuits discrete from the ones employed by the central active area. 
         [0029]    There are several conductive traces included in the flexible display  100  for electrical interconnections between various components therein. The circuits, for instance the ones fabricated in the active area and inactive area, may transmit various signals via the conductive traces to provide a number of functionalities in the flexible display  100 . As briefly discussed, some conductive traces may be used to provide electrical interconnections between the circuits and/or other components in the central portion and the bend portion of the flexible display  100 . 
         [0030]    In the embodiments herein, the conductive traces may include source/drain electrodes of the TFTs as well as the gate lines/data lines used in transmitting signals from some of the display driving circuits (e.g., gate driver-IC, data driver-IC) in the inactive area to the pixels in the active area. Likewise, some conductive traces like touch sensor electrodes or fingerprint sensor electrodes may provide signals for sensing touch input or recognizing fingerprints on the flexible display  100 . The conductive traces can also provide interconnections between the pixels of the active area in the central portion and the pixels of the secondary active area in the bend portion of the flexible display  100 . Still other conductive traces may be used to provide power (e.g., supply voltage) to circuit components within the flexible display  100 . Aforementioned uses of conductive traces are merely illustrative. As used herein, the conductive traces broadly refer to a conductive path for transmitting any type of electrical signals, power and/or voltages from one point to another point in the flexible display  100 . 
         [0031]    Some of the conductive traces may be extended from the central portion to the bend portion of the flexible display  100 . In such cases, some portions of the conductive traces may be configured differently from the other portions to withstand the bending stress. In particular, the portion of the conductive traces over at least the bend allowance section of the flexible display  100  may include several features that can reduce cracks and fractures of the conductive traces to maintain proper interconnection. 
         [0032]    At least some of the conductive traces may have a multi-layered structure, which may allow more stretching (or flexibility) with less chance of breakage and to reduce galvanic corrosion as will be further described below. 
       Conductive Traces 
       [0033]    In one embodiment, a conductive trace is multi-layered. The conductive trace may include a lower protection layer such as a passivation layer, a metal layer which is a metal trace line formed on the lower protection layer, and an upper protection layer such as a passivation layer that is formed on the metal layer, as will be further described below in more detail with respect to  FIG. 5 . 
         [0034]    The trace design is determined by considering the electrical requirements of the conductive trace as well as the type of signals transmitted on the conductive trace. Also, the properties of the materials (e.g., Young&#39;s modulus) used in forming the conductive trace can be considered in designing the traces. It should be noted that various other factors such as a thickness, a width, a length, a layout angle for a section as well as for the entirety of the metal trace line and the passivation layers may need to be considered to provide a trace design having sufficient electrical and mechanical reliability for use in the flexible display  100 . 
         [0035]    The conductive trace design may be specifically tailored for the conductive trace based on their placement and orientation in reference to the bending direction (i.e., tangent vector of the curve) of the flexible display  100 . A conductive trace will be subjected to more bending stress as the orientation in which the conductive trace extends is more aligned to the tangent vector of the curvature. In other words, a conductive trace will withstand better against the bending stress when the length of the conductive trace aligned to the tangent vector of the curvature is reduced. 
         [0036]    In order to reduce the length of the conductive trace portion being aligned to the tangent vector of the curvature, conductive traces of the flexible display  100  may employ any one or more winding designs as will be further described below. In such configurations, the bending stress may be distributed to the trace portions oriented in an angle shifted away from the tangent vector of the curvature. 
         [0037]      FIGS. 3A and 3B  respectively illustrate a plane view and a perspective view of a winding conductive trace  300  according to one embodiment. As shown in  FIGS. 3A and 3B , the winding conductive trace  300  is formed on a flexible substrate  302  and a cover layer  301  is formed on the substrate  302  to cover and protect the winding conductive trace  300  from external elements such as moisture or air that can degrade the winding conductive trace  300 . 
         [0038]    In one embodiment, the winding conductive traces  300 A,  300 B, and  300 C have a winding trace pattern that is curved and includes caps as will be further described with respect to  FIG. 4 . The winding trace pattern of the winding conductive trace  300  is more resistant to bending and stretching stresses compared to conventional trace patterns (e.g., straight line trace patterns or sign wave wire patterns) due to the caps included in the winding conductive traces  300 . In the example shown in  FIGS. 3A and 3B , the winding conductive traces  300  are subject to bending in either direction perpendicular to the substrate  300  (e.g., the “z” axis direction as shown in  FIG. 3B ) or in an angled direction, as shown with the arrows. The winding conductive trace  300  maintains integrity without cracking or delamination when the flexible display  100  is bent as will be further described below. 
         [0039]      FIG. 4  is a detailed plane view of a winding conductive trace  300  according to one embodiment. The winding conductive trace  300  resembles a temple gate structure. The width of the metal trace line  401  is represented by the dashed lines in  FIG. 4  and varies depending on the location on the winding conductive trace  300 . The passivation layer  403  generally has a width that is larger than that of the metal trace line  401  throughout the winding conductive trace  300 , and the width of the passivation layer  403  in various portions of the winding conductive trace  300  corresponds to the width of the metal trace line  401 . In other words, in a plane view as depicted in  FIG. 4 , the trace shape of the passivation layer  403  is substantially identical to the trace shape of the metal trace line  401 , but with a predetermined margin beyond the width of the metal trace line  401 . 
         [0040]    The metal trace line  401  may be formed of conductive materials such as copper, gold, silver, and other types of coated or printed materials. Although the term “metal trace line” is used, it should be noted that the metal trace line in the winding conductive trace  300  may be replaced with other types of conductive materials, such as carbon based materials (e.g., graphene, carbon nanotube), conducting polymers, and other non-metal based conductive materials. Furthermore, the metal trace line need not be straight. The metal trace line can be curved. With bend radius requirement at the bend allowance section of the flexible display  100 , however, the materials for forming the winding conductive trace  300  should meet the minimum mechanical requirement and the size requirement as well as the stringent electrical requirements of the flexible display  100 . The metal trace line  401  can comprise one or more metal layers such as aluminum and other metals. In some embodiments requiring less flexibility in the winding conductive trace  300 , molybdenum or other conductive materials discussed above may be used. 
         [0041]    The passivation layer  403  may be formed of inorganic materials which are generally less ductile than the metal trace line  401  of the winding conductive trace  300 . Examples of the materials used to form the passivation layer include inorganic materials such as silicon nitride, silicon dioxide and other dielectric materials commonly used in semiconductor device and other electronics processing. 
         [0042]    Given the same amount of bending stress, cracks generally initiate from the passivation layer of the winding conductive trace  300 . Even if the metal trace lines have sufficient strength to withstand the bending stress without cracking, cracks are generally initiated from the passivation layer and tend to grow and propagate into the metal trace lines of the winding conductive trace  300 , creating spots of poor electrical contacts that could render the flexible display  100  unusable. Accordingly, various bending stress reduction techniques are utilized in both the passivation layers and the metal trace line of the winding conductive traces  300 , according to the embodiments herein. 
         [0043]    As shown in  FIG. 4 , the winding conductive trace  300  includes a plurality of alternating crests  405  (e.g.,  405 A,  405 B, . . . ) and alternating troughs  407  (e.g.,  407 A,  407 B, . . . ). Each crest  405  includes a convex edge  408  (e.g.,  408 A) of the winding conductive trace  300  exhibiting the local maximum amounts of upward distance in the y-direction from the rest of the portions of the winding conductive trace  300 . In one embodiment, the convex edge  408  is a surface (e.g., a first edge) of the winding conductive trace  300  that is curved like the exterior of a trapezoid. Conversely, each trough  407  includes another convex edge  408  (e.g.,  408 B) of the winding conductive trace  300  exhibiting the local maximum of downward distance in the y-direction from the rest of the portions of the winding conductive trace  300 . 
         [0044]    The total distance from a convex edge of a crest (e.g., e.g.,  408 A) to a convex edge of a trough (e.g.,  408 B) of the winding conductive trace  300  in the y-direction represents the height of winding conductive trace  300 . The total distance from a first crest  405 A to an adjacent second crest  405 B in the x-direction represents the pitch (width) of the winding conductive trace  300 . The pitch of the winding conductive trace  300  is approximately one times (1×) the height of the winding conductive trace  300  in ne embodiment. However, other pitches may be used such as 0.3 times (0.3×) to 0.5 times (0.5×) the height of the winding conductive trace  300  or 2 times (2×) to 3 times (3×) the height of the winding conductive trace  300 . 
         [0045]    Additionally, each crest  405  and trough  407  of the winding conductive trace  300  includes a concave edge  412 , ( 412 A,  412 B, . . . ) located opposite to the convex edge  408  of each respective crest and trough. A concave edge  412  of each crest and trough is substantially rounded (e.g., like a half-circle) shape with a radius that is maximized according to the height and pitch of the winding conductive trace  300 . The concave edge  412  represents a surface (e.g., a second edge) of the winding conductive trace  300  that curves inward like the interior of a circle or sphere. It should be noted that the concave edges  412  may be other shapes other than a half circle. The trace of the concave edges  412  may simply be more rounded than the trace of the convex edges at the troughs and crests. 
         [0046]    During bending of the flexible display  100  (e.g., in the “z” axis direction), the concave edge  412  of the winding conductive trace  300  is an area of high stress whereas the convex edges  408  are areas of low stress. Cracks typically start to occur at the passivation layer  403  located at the concave edges  412  of the winding conductive trace  300  during bending of the flexible display  100 . The cracks tend to grow and propagate into the metal trace line  401  of the winding conductive trace  300  thereby creating spots of poor electrical contacts that could render the flexible display  100  unusable. 
         [0047]    In one embodiment, the winding conductive trace  300  includes caps  409  ( 409 A,  409 B  409 C . . . ) that are portions of the metal trace line  401  located between the convex edges  4085  and concave edges  412  of each crest  405  and trough  407  and extend beyond the width  413  of the metal trace line without the cap  409 . The width  411  of the metal trace line  401  located in the regions of the winding conductive trace  300  that include the caps  409  is wider than the width  413  of the metal trace line  401  located in intermediate portions  421  of the winding conductive trace  300  located between each alternating crest (e.g.,  405 A) and trough (e.g.,  407 A or  407 B). Due to the wider width  411  of the metal trace line  401  including the cap regions  409  (e.g.,  409 A,  409 B,  409 C . . . ), cracks in the passivation layers located at the concave edges  412  will not propagate across the entire width  411  of the metal trace line  401 , and thus electrical connections will be maintained even if cracks are formed in the concave edges  412  due to bending. For example, the metal trace line  401  in crest  405 A is provided with a cap  409 A. If a crack propagates across a portion of the width  411  of the metal trace line  401 , spots of poor electrical contact are prevented since the width  411  of the metal trace line  401  including cap  409 A is wider than the width  413  of the metal trace line  401  at the intermediate portions  421 . Furthermore, since the cap  409 A is located at a low stress region of the winding conductive trace  300  during bending of the flexible display  100 , it is unlikely for the crack to propagate across the width  411  of the metal trace line  401  including cap  409 A. In one embodiment, the cap  409  is similar to a trapezoid in shape as shown in  FIG. 4 . The trapezoidal shape of the cap  409  is defined by the convex edge  408  of the winding conductive trace  300 . 
         [0048]    In some embodiments, the width  411  of the metal trace line  401  is roughly three times the width  413  of the concentric circle segment (e.g., concave edge  412 ). Although the cap  409  is described as being trapezoidal in shape  300 , the cap  409  can be other shapes which can be chosen based on electric static discharge (ESD) requirements, stress requirements, and nesting requirements as will be further described below. 
         [0049]    In one embodiment, the intermediate portion  421  includes the passivation layers  403  and the metal trace line  401  that extend from the portion of the winding conductive trace  300  located between a convex  408  edge and a concave edge  412  located opposite the convex edge  408 . As shown in  FIG. 4 , the intermediate portion  421  lacks (i.e., does not have) the cap  409 . In other words, the width of the winding conductive trace  300  is at its minimum at the intermediate portion  421 . 
         [0050]    The width  411  of the metal trace line  401  located between the convex edge  408  and concave edge  412  of a crest/trough that includes a cap  409  is larger than the width  413  of the metal trace line  401  located at the intermediate portion  421  that lacks the cap  409 . In contrast, the width of the passivation layers  403  varies based on the position of the winding conductive trace  300 . That is, the width of the passivation layers  403  varies according to the width of the metal trace line  401 . For example, in one embodiment the width of the passivation layer  403  located in the intermediate portion  421  is smaller than the width of the passivation layer located between a convex edge  408  and a concave edge  412  opposite the convex edge of each crest and/or trough. 
         [0051]      FIG. 5  is a cross-section view of the winding conductive trace  300  along line A to A′ shown in  FIG. 4  according to one embodiment. As shown in  FIG. 5 , the winding conductive trace  300  includes a first passivation layer  501  formed on the substrate  302 . The first passivation layer  501  may be made of inorganic material such as silicon oxide (SiO2), silicon nitride (SiNx), and a combination of both. The substrate  302  may be made of flexible material such as polyimide. 
         [0052]    The metal trace line  401  is formed on the first passivation layer  501 . The metal trace line  401  may be made of aluminum or may be a combination of conductive material. A second passivation layer  505  is formed over the metal trace line  401 . The second passivation layer  505  can also be made of inorganic material such as SiO2 and/or SiNx. As shown in  FIG. 5 , the first passivation layer  501  and the second passivation layer  505  cover all sides of the metal trace line  401 . The first passivation layer  501  and the second passivation layer  505  form a protective layer around the metal trace line  401  that protects the metal trace line  401  from moisture and/or air. The first passivation layer  501 , the second passivation layer  505 , and the metal trace line  401  collectively represent the winding conductive trace  300  according to one embodiment. As shown in  FIG. 5 , a cover layer  301  is formed over the second passivation layer  505 , the metal trace line  401 , the first passivation layer  501 , and the substrate  300  to provide further protection from moisture and/or air. The cover layer  301  can be made of any polymer. The total height  509  of the winding conductive trace  300  may be 10 to 1000 nm in one embodiment. 
         [0053]    In one embodiment, the width of the first passivation layer  501  and the width of the second passivation layer  505  are designed to extend past edges  513  of the metal trace line  401 . The first passivation layer  501  and second passivation layer  505  are designed to be long enough to protect the metal trace  401  from moisture and/or air, but cannot be too long as the first passivation layer  501  and second passivation layer  505  may crack too easily when bent. In one embodiment, the portion  511 A of the first passivation layer  501  and the second passivation layer  501  extend past edge  513 A of the metal trace line  401  and portion  511 B of the first passivation layer  501  and the second passivation layer  501  extend past edge  513 B of the metal trace line  401 . The portions  511  that extend past the edges  513  of the metal trace line  401  are at most 10 μm in some embodiments. 
         [0054]    As mentioned previously, the width of the passivation layers vary based on the position of the winding conductive trace  300 . For example, the width of the first passivation layer  501  in the intermediate portion  421  has a width that is smaller than the width of the first passivation layer  501  in the portion of the winding conductive trace  300  between a convex edge  408  and concave edge  412  of each crest and trough. Similarly, the width of the second passivation layer  505  in the intermediate portion  421  has a width that is smaller than the width of the second passivation layer  505  in the portion of the trace  300  between a convex edge and concave edge. In one embodiment, the width of the first passivation layer  501  and the width of the second passivation layer  505  in the intermediate portion  421  are substantially the same and the width of the first passivation layer  501  and the width of the second passivation layer  505  in the portion of the winding conductive trace  300  between a convex edge and a concave edge of a crest and trough are substantially the same. 
         [0055]    In one embodiment, the target bending radii of curvature of the flexible display device  100  may be as small as 0.1 mm if a cover layer  301  is applied as shown in  FIG. 5 . The cover layer  301  is chosen to place the winding conductive trace  300  in or near the mechanical neutral plane of the flexible display device  100 . The design of the cover  301  is based on the thickness, modulus, and residual stress of the substrate  302  and cover layer  301 . Since the winding conductive trace  300  including the passivation layers  501 ,  505  and metal trace  401  is typically very thin compared to the substrate  302  and cover layer  301 , the thickness of the passivation layers  501 ,  503  and metal trace  401  can be ignored when designing the thickness of the cover layer  301 . In one embodiment, the neutral plane design of the cover layer  301  is calculated according to the following equation: 
         [0000]      substrateModulous·(substrate Thickness) 2 =coverlayerModulous·(coverlayerThickness) 2  
 
         [0056]    As shown above, the product of the modulus of the substrate  302  and the thickness of the substrate  302  squared is equivalent to the product of the modulus of the cover layer  301  and the thickness of the cover layer  301  squared in order to keep the substrate  302  and the cover layer  301  bending together without being delaminated when bent. Once the materials for the substrate  302  and cover layer  301  are known as well as the thickness of the substrate  302 , the thickness of the cover layer  301  can be determined. 
         [0057]      FIG. 6  illustrates a trace pattern of a winding conductive trace  600  according to another embodiment. As mentioned above,  FIGS. 3-4  illustrate a winding conductive trace with a trapezoidal shaped cap  409 . In contrast,  FIG. 6  illustrates a winding conductive trace  600  with rounded caps  601 . The rounded shape of the cap is defined by the convex edge  603  of the winding conductive trace  600 . The winding conductive trace  600  includes similar features as the winding conductive trace  300  described above, the description of which is omitted for brevity. 
         [0058]    The rounded cap  601  reduces the generation of electrical fields between adjacent winding conductive traces that use the rounded cap  601 . In contrast, winding conductive traces that use pointed caps such as trapezoidal caps  409  may increase the electrical field formed between two adjacent winding conductive traces. Strong electrical fields generated between adjacent traces may cause the passivation layers covering the metal trace to deteriorate. On the other hand, winding conductive traces  600  using rounded caps  601  may not be able to be packed as tightly together due to the larger surface area of the rounded cap  601 , compared to the trapezoidal shaped cap  409 . 
         [0059]      FIG. 7  illustrates a winding conductive trace  700  according to another embodiment. In particular,  FIG. 7  illustrates a winding conductive trace  700  with triangular caps  701 . The triangular shape of the cap  701  is defined by the convex edge  703  of the winding conductive trace  700 . The winding conductive trace  700  also includes similar features as the winding conductive trace  300  described above, the description of which is omitted for brevity. 
         [0060]    The triangular cap  701  improves the ability to tightly pack more winding conductive traces  700  together due to the smaller surface area of the triangular shaped cap  701  compared to the rounded cap  601 . However, the triangular cap  701  may increase the electrical field formed between adjacent winding conductive traces due to the pointed nature of the triangular cap  701 . The sharp points of the triangular cap  701  may increase the electrical field generated between adjacent winding conductive traces, which in turn may cause the passivation layers covering the metal wire trace to deteriorate. 
       Mirrored Wiring 
       [0061]    In order to prevent or minimize severance of interconnections by cracks in the winding conductive traces, the winding conductive traces may split into multiple sub-traces, which converge back into a single trace at certain intervals.  FIG. 8A  illustrates a detailed view of a mirrored trace  800  according to one embodiment. The mirrored trace  800  includes two symmetric winding conductive traces that are adjoined in a mirrored configuration and are symmetric with respect to the line of symmetry shown in  FIG. 8A . The mirrored trace  800  resembles a double temple gate structure. In the example shown in  FIG. 8A , the mirrored trace  800  is composed of two winding conductive traces each of which with a trapezoidal cap as shown in  FIG. 4 . The mirrored trace  800  is alternatively composed of two winding conductive traces each of which with a rounded cap as shown in  FIG. 6  or two winding conductive traces each of which with a triangular cap as shown in  FIG. 7 . 
         [0062]    In one embodiment, the mirrored trace  800  includes sub-trace A and sub-trace B, which merge back together at every joint  813 . The metal trace line of sub-trace A and sub-trace B at joint  813  may have a height in the y-direction of 17.48 μm. Each sub-trace A, B is multi-layered such that the trace of the passivation layer covers at least some part of the metal trace lines. The width of the mirrored trace  800  corresponds to the width  801  of the passivation layer and the width  803  is the width of the metal trace line that is at least partially covered by the passivation layer similar to the single winding conductive trace described above. In one embodiment, the width  803  of the metal trace line may be in a range of about 2 μm to about 3 μm. In one embodiment, the distance  814  that the passivation layer extends past the edge of the metal trace line may be in a range of about 1.0 μm to about 1.5 μm. 
         [0063]    As shown in  FIG. 8A , sub-trace A has a winding conductive trace pattern that includes a plurality of alternating crests (e.g., crest  805 A) and troughs (e.g., trough  807 A). Each crest  805  and trough  807  includes a convex edge  806  (e.g.,  806 A and  806 B) and a concave edge  812  (e.g.,  812 A and  812 B) similar to the description described above with respect to  FIG. 4 . Similarly, sub-trace B also has a winding conductive trace pattern that includes a plurality of alternating crests (e.g., crests  805 B) and troughs (e.g., trough  807 B). Each crest and trough of sub-trace B also includes a convex edge  806  and a concave edge  812  similar to the description described above with respect to  FIG. 4 . The total distance from the convex edge of crest  805 A of sub-trace A to the convex edge of trough  807 B of sub-trace B in the y-direction represents the height of the mirrored trace  800 . 
         [0064]    Sub-trace A includes a cap  809 A and sub-trace B includes a cap  809 B at low stress portions of the mirrored trace  800  to prevent cracks from propagating across the width  816  of the metal trace line that include the cap, in order to prevent poor electrical contact. The cap top  818  represents a substantially flat edge of the caps  809 . 
         [0065]    As mentioned above, the crest  805  of sub-trace A also has a concave edge  812 A and the trough  809 B of sub-trace B has a concave edge  812 B that are substantially rounded, like a half-circle in shape. During bending of the flexible display  100 , the concave edges  812  of the crests and troughs are areas of high stress whereas the convex portions (e.g.,  806 ) of the crests and troughs are areas of low stress. Cracks typically start to occur at the passivation layer located at the concave edges  812  of the crests and troughs of the mirrored trace  800  during bending of the flexible display  100 . The rounded shape of the concave edges  812  of sub-trace A and sub-trace B distribute the mechanical stress over the larger area of the concave edges  812  thereby reducing the onset of a crack generation. 
         [0066]    However, if cracks occur at the passivation layer located at the concave edges  812  of the mirrored trace  800 , they could grow and propagate into the metal trace lines of the mirrored traces  800 . Advantageously, if a crack propagates across a portion of the width  816  of the metal trace line, spots of poor electrical contact are prevented since the width  816  of the metal trace line include the cap region  809  that is wider than the width  803  of the metal trace line at the intermediate portions without the cap  809 . Furthermore, since the cap  809  is located at a low stress region, it is unlikely that cracks will propagate across the width  816  of the metal trace line that includes the cap  809  during bending of the flexible display  100 . 
         [0067]    In one embodiment, alternating concave edges of a sub-trace have different radii. For example, concave edge  812 A of sub-trace A may have a radius of 8.1 μm whereas concave edge  812 C of sub-trace A may have a radius of 9.46 μm. In other embodiments, alternating concave edges of the sub-trace may have substantially the same radii. 
         [0068]    Furthermore, by splitting the mirrored trace  800  into multiple sub-traces, a backup electrical pathway is provided in case one of the sub-traces is damaged by cracks. As such, the mirrored trace  800  can be used in the bend portion, and may be particularly helpful within the bend allowance section subjected to severe bending stress. 
         [0069]    Referring now to  FIG. 8B , a plurality of mirrored traces  800  are shown in a staggered configuration. In  FIG. 8B , the mirrored traces  800  are positioned adjacent to each other in a staggered configuration to maximize the number of wires in a given area. The mirrored traces  800  are staggered such that the convex edge of a given double-temple gate trace is placed in line with a concave edge of an adjacent mirrored trace. The size of the mirrored trace can be reduced or increased for more efficient use of the given space. Furthermore, the dimension of two adjacent mirrored traces can be different from each other. For example, the size of the concave edge of a first mirrored trace can be larger or smaller than the size of the concave edge of an adjacent second mirrored trace. 
         [0070]    For example,  FIG. 8B  includes a first mirrored trace  815 . A second mirrored trace  817  is adjacent to a first sub-trace of the first mirrored trace  815  and a third mirrored trace  819  is adjacent to a second sub-trace of the first mirrored trace  815 . In one embodiment, each convex edge  821  of the first sub-trace of the first mirrored trace  815  is positioned in line with a corresponding concave edge  823  of the second mirrored trace  817 . Similarly, each concave edge  825  of the second sub-trace of the first mirrored trace  815  is positioned in line with a corresponding convex edge  827  of the third mirrored trace  817 . By staggering the mirrored traces, more mirrored traces can be fit in a given area. 
         [0071]    In one embodiment, the mirrored traces split into additional number of sub-traces, creating a grid-like trace  900  in the bending area of the flexible display  100  as illustrated in  FIG. 9 . As an example, the sub-traces can be configured to form a web of mirrored traces  800  that resemble a grid  900 . Such a trace design may be useful for traces that transmit a common signal, for example supply voltage signals, VSS and VDD, for the display  100 . If one of the traces cracks due to bending, redundancy of the mirrored traces still allows for the transmission of the common signal. Neither the number of sub-traces nor the shape of the sub-traces forming the grid-like trace design are particularly limited to the example shown in  FIG. 9 . In some embodiments, the sub-traces may converge into a single trace past the bend allowance section of the flexible display  100 . 
         [0072]    As shown in  FIG. 9 , the grid-like trace  900  includes a first mirrored trace  901 , a second mirrored trace  903 , and a third mirrored trace  905 , and so on. In one embodiment, each convex edge of a mirrored trace is connected to a corresponding convex edge of an adjacent mirrored trace. For example, in  FIG. 9  each convex edge  907  of the right sub-trace of the first mirrored trace  901  is connected to a corresponding convex edge  909  of the left sub-trace of the second mirrored trace  903 . Similarly, each convex edge  911  of the right sub-trace of the second mirrored trace  903  is connected to a corresponding convex edge  913  of a left sub-trace of the third mirrored trace  905 . 
         [0073]    The strain reducing trace designs discussed above may be used for all or parts of the conductive trace. In some embodiments, the part of conductive trace in the bend portion of the flexible display  100  may adopt such a strain reducing trace design. The parts of a conductive trace prior to or beyond the part with the strain reducing trace design may have the same trace design or a difference trace design. If desired, the strain reducing trace designs may be applied to multiple parts of a conductive trace. 
         [0074]    Even with the strain reducing trace design, the inevitable bending stress remains at certain points of the trace (i.e., stress point). The location of stress point is largely dependent on the shape of the trace as well as the bending direction. It follows that, for a given bending direction, the trace of a wire and/or an insulation layer can be designed such that the remaining bending stress would concentrate at the desired parts of their trace, Accordingly, a crack resistance area can be provided in a trace design to reinforce the part of the wire trace where the bend stress concentrates. 
         [0075]    While the embodiments herein are described with respect to a flexible display, other flexible electronic devices can use the various trace designs described above. For example, the embodiments herein may be incorporated in wearable electronic device that are designed to be flexed and worn on surfaces of the human body such as flexible electronic watches. Other examples in which the embodiments herein may be incorporated are mobile phones that are bendable, rolled displays for use in electronic tangible media such as electronic newspapers, magazines, and books. The embodiments herein may also be incorporated in flexible display screens of televisions. In addition, while the benefits of the embodiments herein are better realized in flexible electronic devices, the winding conductive traces according to the embodiments herein may be used any type of electronic devices including non-flexible electronics that employ a rigid substrate. These various aspects, embodiments, implementations or features of the described embodiments can be used separately or in any combination. The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope of the invention.