Abstract:
A flexible wiring board is provided having a wiring structure which can reduce transmission loss by reducing impedance mismatching even if being folded in a three-dimensional manner. In a flexible wiring board  10  having a characteristic impedance control circuit  20,  the flexible wiring board has a planar projection shape of a folded spot  20 A in the characteristic impedance control circuit after folding in an arc state along a tangent.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a flexible wiring board and particularly to a characteristic impedance control circuit for a high-frequency signal. 
         [0003]    2. Related Art 
         [0004]    A flexible wiring board  10  has been incorporated in a small-sized electronic device as shown in  FIG. 7  in many cases, and particularly a cable portion is folded along a folding line shown by a broken line and incorporated in equipment. In this case, an impedance control line  20  is sharply folded at a portion of the folding line and the characteristic impedance is rapidly changed at this folded portion. 
         [0005]    That is, in the characteristic impedance control circuit of a high-frequency circuit formed on a flexible wiring board, electromagnetic-field distribution is distorted at a folded portion in the circuit (“microstrip line” in Patent Document 1), which might adversely affect circuit characteristics such as reflection and irradiation or malfunction of other circuits as described in [Background Art] in Patent Document 1. 
         [0006]    As a measure against that, it is possible to form the folded portion in a characteristic impedance control circuit as long as it is on a plane (Patent Document 1). 
         [0007]    However, when three-dimensional folding is to be performed at an assembling stage of the flexible wiring board into electronic equipments, the change will be made at a steep angle in the state where the characteristic impedance control circuit is folded as in Patent Document 1. 
         [0008]    As a result, the electromagnetic-field distribution is distorted and signal reflection or irradiation is generated, which might cause adverse effect of the circuit characteristic or malfunction of other circuits. 
         [0009]    Patent Document 1: Japanese Patent Laid-Open No. 2004-363535 
       SUMMARY OF THE INVENTION 
       [0010]    The characteristic impedance control circuit in a high-frequency circuit shows the best transmission characteristic in a straight and uniform state as above. 
         [0011]    However, in an actual product having a high-frequency circuit, in the case of a flexible wiring board, not only folding on a plane but three-dimensional folding spots are also generated. 
         [0012]    The present invention was made in view of the above points and has an object to provide a flexible wiring board having a wiring structure which can reduce transmission loss by reducing impedance mismatching even in the case of three-dimensional folding. 
         [0013]    In order to achieve the above object, the present invention provides a flexible wiring board, characterized in that in a fixable wiring board having a characteristic impedance control circuit, 
         [0014]    a planar projection shape of a folded spot in said characteristic impedance control circuit after folding is an arc state along a tangent. 
         [0015]    Since the present invention is configured so that the planar projection shape of a folded spot in a characteristic impedance control circuit after folding draws an arc along a tangent, local impedance mismatching in the characteristic impedance control circuit can be reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0016]      FIGS. 1A and 1B  are explanatory diagrams illustrating a configuration of a first embodiment of the present invention, in which  FIGS. 1A and 1B  show each state before and after folding; 
           [0017]      FIGS. 2A and 2B  are explanatory diagrams illustrating a configuration of a second embodiment of the present invention, in which  FIGS. 2A and 2B  show each state before and after folding; 
           [0018]      FIGS. 3A and 3B  are explanatory diagrams illustrating a configuration of a third embodiment of the present invention, in which  FIGS. 3A and 3B  show each state before and after folding; 
           [0019]      FIGS. 4A and 4B  are explanatory diagrams illustrating a configuration of a fourth embodiment of the present invention, in which  FIGS. 4A and 4B  show each state before and after folding; 
           [0020]      FIGS. 5A and 5B  are explanatory diagrams illustrating a configuration of a fifth embodiment of the present invention, in which  FIGS. 5A and 5B  show each state before and after folding; 
           [0021]      FIG. 6  is an explanatory diagram illustrating a sixth embodiment of the present invention; and 
           [0022]      FIG. 7  is an explanatory diagram illustrating a characteristic impedance control circuit of a conventional flexible wiring board. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    Embodiments of the present invention will be described below referring to the attached drawings. 
       Embodiment 1 
       [0024]      FIGS. 1A and 1B  illustrate a first embodiment of the present invention, in which  FIG. 1A  shows a state before folding and  FIG. 1B  shows a state after folding. 
         [0025]    In  FIGS. 1A and 1B , a flexible wiring board  10  has a characteristic impedance control circuit, that is, an impedance control line  20  having a folded portion  20 A extending in a straight state along the substantial center in its width direction and having a planar shape before folding in the S-shape at the substantial center in the longitudinal direction. 
         [0026]    As shown in  FIG. 1A , in the S-shaped portion constituting the folded portion  20 A, its center portion includes a portion parallel to a folding line (shown by an imaginary line) set in the direction diagonal to the flexible wiring board  10  and also crossing it. 
         [0027]    As a result, as shown in  FIG. 1B , when the flexible wiring board  10  is folded along the folding line, the planar projection mage of the folded portion  20 A is in relation to make an arc shape with the folding line as a tangent. 
         [0028]    Therefore, the entire planar projection image of the impedance control line  20  after the flexible wiring board  10  is folded is configured so that a portion tangent to the folding line is in an arc state and the ends of the impedance control line  20  arranged in the relation crossing each other are connected at the arc-state portion. 
         [0029]    Here, a connected portion between the straight portion and the arc-state portion of the impedance control line  20  is folded as smoothly as possible. For that purpose, such a measure may be taken that the position of the impedance control line  20  is brought close to one side in the width direction of the flexible wiring board  10 , the size of an arc of the folded portion  20 A is set appropriately or a curvature of the arc is varied continuously. 
         [0030]    That is, the arc state refers to a shape including a literal circle having a single curvature and also a case where circles with different curvatures are sequentially continued. Since the arc continues to the straight portion, it is needless to say that the connection portion has a curvature different from that of the arc. 
         [0031]    When the straight portions of the impedance control line are arranged at a right angle by folding the flexible board  10  along the folding line in this way, since the straight portions are connected by the arc-state portion, a smooth planar shape is obtained without making a sharp bending in its entirety. 
         [0032]    Since the flexible wiring board  10  is folded along the folding line and overlapped, it is actually folded also in the thickness direction, and the folded portion  20 A is folded in a three-dimensional manner. However, the folding in the thickness direction is slight, which can be ignored in practice. 
         [0033]    Thus, the impedance control line  20  is configured in a folded shape hardly affecting the entire characteristic impedance including the folded portion  20 A. 
       Embodiment 2 
       [0034]      FIGS. 2A and 2B  show a second embodiment of the present invention and an example in which the folding line crosses the flexible wiring board  10  in the width direction so that the flexible wiring board  10  is folded in the longitudinal direction. Here, the folded portion  20 A is configured as the S-shape formed by continuing substantial semi-circles in opposite directions so that a planar projection shape substantially becomes a circle (See  FIG. 2B ) after folding as shown in  FIG. 1A . 
         [0035]    In this case, too, it is configured so that a connection portion between the folded portion  20 A and the straight portion of the impedance control line  20  is folded smoothly. For that purpose, the position of the straight portion may be displaced to one side in the width direction of the flexible wiring board  10 . 
       Embodiment 3 
       [0036]      FIGS. 3A and 3B  show a third embodiment of the present invention, in which the S-shape of the folded portion  20 A is made small and the straight portions on both sides of the folded portion  20 A are displaced to one side in the width direction of the flexible wiring board  10  ( FIG. 3A ). 
         [0037]    By this configuration, referring to the state after folding shown in  FIG. 3B , as shown in  FIG. 3B , the straight portions crossing each other are connected by a substantial ¼ arc. Thus, the impedance control line  20  configures a planar projection shape including only one folding portion, including the folded portion  20 A. 
       Embodiment 4 
       [0038]      FIGS. 4A and 4B  show a fourth embodiment of the present invention, in which the S-shape of the folded portion  20 A is made small and the folding line is set so as to cross the width direction of the flexible wiring board  10  at a right angle as in the second embodiment. Here, the folded portion  20 A makes a semi circle connecting two straight portions arranged in parallel. 
       Embodiment 5 
       [0039]      FIGS. 5A and 5B  show a fifth embodiment of the present invention, in which in order to fold the impedance control line  20  in a three-dimensional manner, a reinforcing member  30  is provided inside the folded portion of the flexible wiring board  10  so as to realize gentle folding. 
         [0040]    By this configuration, the folding degree of the impedance control line . 20  becomes gentle, and local stress concentration to the flexible wiring board  10  and the impedance control line  20  can be prevented. 
       Embodiment 6 
       [0041]      FIGS. 6A and 6B  show a sixth embodiment of the present invention and a configuration of the folded portion  20 A when a plurality of impedance control lines  20  are arranged in parallel. 
         [0042]    By this configuration, even if the plurality of impedance control lines  20  are provided on a single flexible wiring board  10 , the characteristic impedance of each impedance control line  20  is hardly affected.