Abstract:
A method of manufacturing a flexible printed circuit board having an insulation layer, a first signal wiring layer including a microstrip line, a second signal wiring layer including a signal connection terminal for allowing the microstrip line to connect the exterior connector electrically, and a ground conductive section having a ground connection terminal for connecting the exterior connector. The microstrip line and the signal connection terminal are connected to each other by a wiring via hole. The wiring via hole passes through the insulation layer, the first signal wiring layer, and the second signal wiring layer. The microstrip line has a taper section which gradually enlarges a width of the microstrip line toward the wiring via hole in the vicinity of the wiring via hole. The ground conductive section that corresponds to the microstrip line has a taper section with a shape matching the taper section of the microstrip line.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The subject matter of application Ser. No. 11/586,209 is incorporated herein by reference. The present application is a Continuation of U.S. Ser. No. 11/586,209, filed Oct. 24, 2006, now U.S. Pat. No. 7,688,594 issued May 30, 2010, which claims priority to Japanese Patent Application JP 2005-317101 filed in the Japanese Patent Office on Oct. 31, 2005, the entire contents of which being incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a flexible printed circuit board, and to an optical transmitter-receiver module and an optical transmitter-receiver which are each equipped with such a flexible printed circuit board. 
     2. Description of Related Art 
     For electrically connecting a flexible printed circuit board with other types of board or the like, a method has been used to connect them by means of a connector for connecting the flexible printed circuit board. Further, for transmitting high-frequency signals over a printed circuit board, a printed circuit board with a microstrip line structure has been employed. 
       FIGS. 1-4  are explanatory illustrations of a structure of a flexible printed circuit board  50  as related art, which is electrically connected to a connector.  FIG. 1  is a plan view of the flexible printed circuit board  50  for showing an outline thereof, in which parts of its configuration is, for purposes of clarification, indicated in a transparent state by a broken line.  FIG. 2  is a sectional view of the flexible printed circuit board  50  taken along lines M-M of  FIG. 1  for illustrating an outline thereof.  FIG. 3  is a plan view of a first wiring layer  3 , described later, of the flexible printed circuit board  50 , as viewed from an upper side in  FIG. 2 .  FIG. 4  is a plan view of a second wiring layer  5 , described later, of the flexible printed circuit board  50 , as viewed from a lower side in  FIG. 2 . Further,  FIG. 5  is a cross-sectional view of the flexible printed circuit board  50 , a printed circuit board  18 , and a flexible printed circuit (FPC) connector  7  mounted on the printed circuit board  18  for illustrating a condition where the flexible printed circuit board  50  is connected to the FPC connector  7 . It is to be noted that the flexible printed circuit board  50  is illustrated as one shown in  FIG. 5  with it being taken along the lines M-M of  FIG. 1  and the printed circuit board  18  is shown only partially. 
     As shown in  FIGS. 1-4 , the flexible printed circuit board  50  is formed by alternately laminating vertically the first, second and third insulation layers  2 ,  4 ,  6  and the first and second wiring layers  3 ,  5 . The first insulation layer  2  and the third insulation layer  6  are made of, for example, a cover lay as a protection film. In a predetermined region of an end of the flexible printed circuit board  50 , for purposes of making electrical connection with the FPC connector  7 , the third insulation layer  6  is in an unformed state. On an upper surface of the first insulation layer  2 , a cover plate  53  is provided to prevent any damage from occurring at times of making connection with the FPC connector  7 . 
     As shown in  FIGS. 1 and 3 , on the first wiring layer  3 , a solid pattern ground layer  12   b  is formed. As shown in  FIGS. 1 and 4 , signal connection pads  52  for connection of a signal line  9 , which will be described later, are equipped at a portion of the second wiring layer  5  that is in the vicinity of the end of the board, as connection pads for electrical connection with the FPC connector  7 . Ground connection pads  51  for connection of the ground layer  12   b  are also equipped at a portion of the second wiring layer  5  that is in the vicinity of the end of the flexible printed circuit board  50 , as connection pads for electrical connection with the FPC connector  7 . Each of the ground connection pads  51  is connected to the ground layer  12   b  that is formed on the first wiring layer  3 , via a ground via hole  54  passing through the flexible printed circuit board  50 . Further, on the second wiring layer  5 , a pair of signal lines  9   c  and  9   d , which are microstrip lines, are wired in such a way that they can be respectively connected to the signal connection pad  52 . 
     Further, as shown in  FIG. 5 , the FPC connector  7  is mounted on the printed circuit board  18 , which includes a signal wiring layer  18   a , an insulation layer  18   b , and a ground layer  18   c , and is connected to the flexible printed circuit board  50 . The FPC connector  7  includes a resin-made housing  36  and a predetermined number of metal-made contacts  35  within the housing  36 , each contact being constituted of an FPC connection section  35   a  that comes into contact with the connection pads of the flexible printed circuit board  50 , a support section  35   c  for supporting the housing  36 , and a lead section  35   b  to be connected to the printed circuit board  18 . The metal-made contacts  35  are arranged in parallel to each other with predetermined spacing therebetween. Spacing between the connection pads of the flexible printed circuit board  50  correspond to spacing between the contacts  35 . 
     The contacts  35  of the FPC connector  7  are respectively soldered to connection pads, not shown, which are formed on the outermost signal wiring layer  18   a  of the printed circuit board  18  at a position of the lead section  35   b  of each of the contacts  35 , which is indicated by P. The connection pads of the printed circuit board  18  are respectively connected to a signal line or a ground pattern, neither of which is shown. The signal line or the ground pattern is formed on the signal wiring layer  18   a  of the printed circuit board  18 . 
     The FPC connector  7  shown in  FIG. 5  is of a so-called lower-contact type, so that the flexible printed circuit board  50  is thus connected to the FPC connector  7  in a condition where the connection pads of the flexible printed circuit board  50  are positioned toward a lower surface thereof, as indicated by O. 
     In such a configuration, a high-frequency signal current flows through the signal lines  9  on the flexible printed circuit board  50 , the contact  35  of the FPC connector  7  that corresponds to each of the signal lines  9 , and the signal line formed on the printed circuit board  18 . At this time, a feedback current for the signal current flows in an opposite direction thereto through the ground layer  12   b  on the flexible printed circuit board  50 , through the ground via hole  54 , through the contact  35  of the FPC connector  7  that corresponds to a ground line, and through the ground layer  18   c  on the printed circuit board  18 .  FIG. 6  is a plan view of the flexible printed circuit board  50  for showing the flows of the signal current and the feedback current therein, which are illustrated on the ground layer  12   b  provided on the first a wiring layer  3  and the signal lines  9  provided on the second wiring layer  5  in the flexible printed circuit board  50 . On the flexible printed circuit board  50 , as indicated by arrows Q in  FIG. 6 , if signal currents flow through the signal lines  9 , feedback currents flow through the ground layer  12   b  on the first wiring layer  3 , as indicated by arrows R. 
     Alternatively, a connector for flexible printed circuit board connection has been proposed which facilitates the insertion of a flexible printed circuit board with lower degree of insertion force (see, Japanese Patent Application Publication No. 2002-50423). 
     The connector for flexible printed circuit board connection disclosed in the above Japanese Patent Application Publication is equipped with a board insertion section into which a flexible printed circuit board is inserted and, on the opposite side thereof, a cover insertion section into which a slide cover is inserted in such a manner that it can be retreated. Further, this connector is also equipped with a contact that applies pressure to, and releases pressure from, a flexible printed circuit board inserted into the board insertion section, respectively in response to the insertion of a slide cover and to the withdrawal thereof. With such a configuration, it becomes possible to insert a flexible printed circuit board with lower degree of insertion force in condition where it does not interfere with any operations of the slide cover. 
     SUMMARY OF THE INVENTION 
     However, in such the flexible printed circuit board electrically connected to a connector as described in  FIGS. 1-6 , if the flexible printed circuit board  50  is connected to the FPC connector  7 , the second wiring layer  5  having signal lines  9  exists on the side of the printed circuit board  18 , as shown in  FIG. 5 . Accordingly, the signal lines  9  on the second wiring layer  5  and a portion, indicated by N, of the ground layer  18   c  on the printed circuit board  18  are coupled to each other to give rise to a degree of capacitance so that the electrical impedance characteristics of the signal lines  9  can be degraded. Thus, the electrical impedance characteristics of the signal lines  9  may fall short of a predetermined value, thereby leading to deterioration in transmission characteristics of a high-frequency signal. 
     Further, the connector for flexible printed circuit board connection disclosed in the above Japanese Patent Application Publication is of such a configuration that a flexible printed circuit board can be inserted with small degree of insertion force and, therefore, it is difficult to improve the transmission characteristics of a high-frequency signal. 
     It is thus desirable to provide a flexible printed circuit board that can improve the transmission characteristics of a high-frequency signal in the vicinity of a portion thereof, which is to be connected to a connector, and an optical transmitter-receiver module and an optical transmitter-receiver, which are each equipped with such a flexible printed circuit board. 
     According to a first embodiment of the invention, there is provided a flexible printed circuit board for connecting an exterior connector electrically. The flexible printed circuit board has an insulation layer, a first signal wiring layer, which is provided on one side of the insulation layer, including a microstrip line, and a second signal wiring layer, which is provided on the other side of the insulation layer. The second wiring layer includes a signal connection terminal for allowing the microstrip line to connect the exterior connector electrically and a ground conductive section having a ground connection terminal for connecting the exterior connector. The ground connection terminal is arranged away from the signal connection terminal at a predetermined position connection. The microstrip line and the signal connection terminal are connected to each other by a wiring via hole that passes through the insulation layer, the first signal wiring layer, and the second signal wiring layer. The microstrip line has a taper section which gradually enlarges a width of the microstrip line toward the wiring via hole in the vicinity of the wiring via hole. The ground conductive section that corresponds to the microstrip line has a taper section with a shape matching the taper section of the microstrip line. 
     In the flexible printed circuit board according to a first embodiment of the present invention, a high-frequency signal current flows through the microstrip line of the first signal wiring layer and the wiring via hole that connects the microstrip line and the signal connection terminal to each other. In this case, a feedback current for the signal current flows in an opposite direction thereto, through the ground conductive section that corresponds to the microstrip line. 
     In this embodiment, the microstrip line has a taper section which gradually enlarges a width thereof toward the wiring via hole in the vicinity of the wiring via hole. The ground conductive section also has a taper section with a shape matching that of the taper section of the microstrip line. 
     For this reason, it is possible to prevent any miss matching between the microstrip line and the ground conductive section in the vicinity of the connection between the microstrip line and the wiring via hole, thereby suppressing sudden changes in characteristic impedance of the transmission lines. This allows the transmission characteristics of high-frequency signal in the vicinity of the connection between the microstrip line and the connector to be improved 
     According to a second embodiment of the invention, there is provided an optical transmitter-receiver module having an optical transmitting-receiving circuit board, an optical transmitter module that converts an electrical signal into an optical signal and output the converted optical signal, and an optical receiver module the converts an optical signal into an electrical signal and outputs the converted electrical signal. The optical transmitter module and the optical receiver module are connected to the optical transmitting-receiving circuit board. The optical transmitting-receiving circuit board is electrically connected to a connector attached to another board by way of the above flexible printed circuit board as the first embodiment. 
     In the optical transmitter-receiver module according to the second embodiment of the present invention, a high-frequency signal current flows through the microstrip line of the first signal wiring layer and the wiring via hole that connects the microstrip line and the signal connection terminal to each other in the flexible printed circuit board. In this case, a feedback current for the signal current flows in an opposite direction thereto, through the ground conductive section that corresponds to the microstrip line. 
     In this embodiment, the microstrip line has a taper section which gradually enlarges a width thereof toward the wiring via hole in the vicinity of the wiring via hole. The ground conductive section also has a taper section with a shape matching that of the taper section of the microstrip line. 
     For this reason, it is possible to prevent any miss matching between the microstrip line and the ground conductive section in the vicinity of the connection between the microstrip line and the wiring via hole, thereby suppressing sudden changes in characteristic impedance of the transmission lines. This allows the transmission characteristics of high-frequency signal in the vicinity of the connection between the microstrip line and the connector to be improved, thereby enabling stable transmission and reception of data to be performed at high speed. 
     According to a third embodiment of the invention, there is provided an optical transmitter-receiver having the optical transmitter-receiver module as the second embodiment and a mother board to which the optical transmitter-receiver module is connected. 
     In the optical transmitter-receiver according to the third embodiment of the present invention, a high-frequency signal current flows through the microstrip line of the first signal wiring layer and the wiring via hole that connects the microstrip line and the signal connection terminal to each other in the flexible printed circuit board. In this case, a feedback current for the signal current flows in an opposite direction thereto, through the ground conductive section that corresponds to the microstrip line. 
     In this embodiment, the microstrip line has a taper section which gradually enlarges a width thereof toward the wiring via hole in the vicinity of the wiring via hole. The ground conductive section also has a taper section with a shape matching that of the taper section of the microstrip line. 
     For this reason, it is possible to prevent any miss matching between the microstrip line and the ground conductive section in the vicinity of the connection between the microstrip line and the wiring via hole, thereby suppressing sudden changes in characteristic impedance of the transmission lines. This allows the transmission characteristics of high-frequency signal in the vicinity of the connection between the microstrip line and the connector to be improved, thereby enabling stable transmission and reception of data to be performed at high speed. 
     The concluding portion of this specification particularly points out and directly claims the subject matter of the present invention. However, those skilled in the art will best understand both the organization and method of operation of the invention, together with further advantages and objects thereof, by reading the remaining portions of the specification in view of the accompanying drawing(s) wherein like reference characters refer to like elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a flexible printed circuit board as a related art; 
         FIG. 2  is a cross-sectional view of the flexible printed circuit board as a related art taken along lines M-M of  FIG. 1 ; 
         FIG. 3  is a plan view of a first wiring layer of the flexible printed circuit board as related art; 
         FIG. 4  is a plan view of a second wiring layer of the flexible printed circuit board as related art; 
         FIG. 5  is a cross-sectional view of the flexible printed circuit board as related art, a printed circuit board, and a flexible printed circuit (FPC) connector mounted on the printed circuit board for illustrating a condition where the flexible printed circuit board is connected to the connector; 
         FIG. 6  is a plan view of the flexible printed circuit board as related art for showing the flows of the signal current and the feedback current therein; 
         FIG. 7  is a plan view of a flexible printed circuit board according to a first embodiment of the present embodiment; 
         FIG. 8  is a cross-sectional view of the flexible printed circuit board according to the first embodiment of the present invention taken along lines A-A of  FIG. 7 ; 
         FIG. 9  is a plan view of a first wiring layer of the flexible printed circuit board according to the first embodiment of the present invention; 
         FIG. 10  is a plan view of a second wiring layer of the flexible printed circuit board according to the first embodiment of the present invention; 
         FIG. 11  is another plan view of the second wiring layer of the flexible printed circuit board according to the first embodiment of the present invention; 
         FIG. 12  is a cross-sectional view of the flexible printed circuit board according to the first embodiment of the present invention, a printed circuit board, and a flexible printed circuit (FPC) connector mounted on the printed circuit board for illustrating a condition where the flexible printed circuit board is connected to the connector; 
         FIG. 13  is a plan view of the flexible printed circuit board according to the first embodiment of the present invention for showing the flows of the signal current and the feedback current therein; 
         FIG. 14  is a graph for illustrating the results of measurement of reflection loss; 
         FIG. 15  is a graph for illustrating the results of measurement of transmission loss; 
         FIG. 16  is a plan view of an optical transmitter-receiver module and a network card as a first example of second and third embodiments of the present invention; 
         FIG. 17  is a cross-sectional view of the optical transmitter-receiver module and the network card as the first example of the second and third embodiments of the present invention; 
         FIG. 18  is a plan view of an optical transmitter-receiver module and a network card as a second example of the second and third embodiments of the present invention; and 
         FIG. 19  is a cross-sectional view of the optical transmitter-receiver module and the network card as the second example of the second and third embodiments of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following will describe preferred embodiments of a flexible printed circuit board, an optical transmitter-receiver module, and an optical transmitter-receiver in accordance with the present invention. First, a first embodiment of the flexible printed circuit board of the present invention will be described. 
     Configuration of the Flexible Printed Circuit Board According to the First Embodiment of the Present Embodiment 
       FIGS. 7-11  are explanatory views of a configuration of a flexible printed circuit board  100  according to a first embodiment of the present invention.  FIG. 7  is a plan view of the flexible printed circuit board  100  for showing an outline thereof, a part of which configuration is indicated, in a transparent condition, by a broken line for purposes of clarification.  FIG. 8  is a cross-sectional view of the flexible printed circuit board  100  according to the first embodiment of the present invention taken along lines A-A of  FIG. 7 .  FIG. 9  is a plan view of a first wiring layer  103 , which will be described later, of the flexible printed circuit board  100  according to the first embodiment of the present invention, as viewed from an upper side in  FIG. 8 .  FIGS. 10 and 11  are plan views of a second wiring layer  105  of the flexible printed circuit board  100  according to the first embodiment of the present invention, as viewed from a lower side in  FIG. 8 .  FIG. 10  illustrates portions that correspond to those of  FIGS. 7 and 9 .  FIG. 11  is a horizontal elongation of  FIG. 10 . 
       FIG. 12  is a cross-sectional view of the flexible printed circuit board  100  according to the first embodiment of the present invention, a printed circuit board  118 , and a flexible printed circuit (FPC) connector  107  mounted on the printed circuit board  118  for illustrating a condition where the flexible printed circuit board  100  is connected to the FPC connector  107 . It is to be noted that the flexible printed circuit board  100  in  FIG. 12  is illustrated as one shown in  FIG. 7  with it being taken along the lines A-A of  FIG. 7  and the printed circuit board  118  is shown only partially. 
     As shown in  FIGS. 7-11 , the flexible printed circuit board  100  is formed by alternately laminating vertically the first, second and third insulation layers  102 ,  104 ,  106  and the first and second wiring layers  103 ,  105 . The first and second wiring layers  103 ,  105  are made of a metal film such as a copper clad laminate (CCL). The first, second and third insulation layers  102 ,  104 ,  106  are made of an epoxy-based, or a polyimide-based, resin. The first insulation layer  102  and the third insulation layer  106  are made of, for example, a cover lay as a protection film. In a predetermined region of an end of the flexible printed circuit board  100 , for purposes of making electrical connection with the FPC connector  107 , the third insulation layer  106  is in an unformed state. On an upper surface of the first insulation layer  102 , a cover plate  108  is provided to prevent any damage from occurring at times of making connection with the FPC connector  107 . 
     As shown in  FIGS. 7 and 9 , on the first wiring layer  103 , signal lines  109   a  and  109   b  are formed as microstrip lines. At an end section of the second wiring layer  105  are formed a signal connection pad  110  for electrical connection between the FPC connector  107  and the signal lines  109 , and a ground connection pad  111  for connection between the FPC connector  107  and therein an additional plan view of a flow of a signal current and a feedback current at a time. Further, as shown in  FIGS. 7 ,  10 , and  11 , on the second wiring layer  105 , a ground layer  112   a  is formed. As indicated by L 16  in  FIG. 11 , the ground layer  112   a  is formed so as to have a predetermined width at a position thereof that corresponds to the signal lines  109   a  and  109   b . The width of the ground layer  112   a , indicated by L 16 , is determined by taking into account any influence on the surroundings by the resonance of the ground layer  112   a.    
     The signal lines  109  on the first wiring layer  103  and the signal connection pads  110  on the second wiring layer  105  are connected to each other by signal via holes  113  which passes through the first wiring layer  103 , the second insulation layer  104 , and the second wiring layer  105 . For purposes of reducing a difference in potential between the signal lines  109  and the signal connection pads  110 , they are connected to each other by, for example, the three signal via holes  113 . 
     Further, as illustrated in  FIG. 9 , in order to avoid intrusion of noise from the outside and interference between the signal lines, ground guard sections  114  are respectively formed at portions corresponding to positions of the ground connection pads  111 . To form the ground guard section  114 , at portions that correspond to the ground connection pads  111 , the first wiring layer  103  and the ground layer  112   a  on the second wiring layer  105  are connected to each other by ground via holes  115 . To reduce the difference in potential between the ground guard section  114  in the first wiring layer  103  and the second wiring layer  105 , the two are connected to each other by the ground via holes  115 , for example, of three or more. 
     As illustrated in  FIGS. 7 and 9 , each of the signal lines  109   a  and  109   b  has a signal line taper section  116  which enlarges gradually its width toward the signal via hole  113  in the vicinity of the signal via hole  113 . Further, as shown in  FIGS. 7 and 10 , the ground layer  112   a  on the second wiring layer  105  has ground layer taper sections  117 , a shape of each of which meets that of the signal line taper section  116  of each of the signal lines  109   a  and  109   b.    
     As shown in  FIG. 12 , the FPC connector  107  is mounted on a printed circuit board  118 , which includes a signal wiring layer  118   a , an insulation layer  118   b , and a ground layer  118   c , and is connected to the flexible printed circuit board  100 . The FPC connector  107  includes a resin-made housing  136  and a predetermined number of metal-made contacts  135  within the housing  136 , each contact being constituted of an FPC connection section  135   a  that comes into contact with the connection pads of the flexible printed circuit board  100 , a support section  135   c  for supporting the housing  136 , and a lead section  135   b  to be connected to the printed circuit board  118 . The metal-made contacts  135  are arranged in parallel to each other with predetermined spacing therebetween. Spacing between the connection pads of the flexible printed circuit board  100  corresponds to spacing between the contacts  135 . 
     The contacts  135  of the FPC connector  107  are respectively soldered to connection pads, not shown, which are formed on the outermost signal wiring layer  118   a  of the printed circuit board  118  at a position on the lead section  135   b  indicated by C. The connection pads of the printed circuit board  118  are respectively connected to a signal line or to a ground pattern, neither of which is shown. The signal line or the ground pattern is formed on the signal wiring layer  118   a  of the printed circuit board  118 . 
     The FPC connector  107  shown in  FIG. 12  is of a so-called lower contact type, so that the flexible printed circuit board  100  is thus connected to the FPC connector  107  in a condition where the connection pads are positioned toward a lower surface thereof, as indicated by D. 
     The flexible printed circuit board  100  of the present embodiment shown in  FIGS. 7-12  may be configured so that a single end mode signal may be transmitted through a single signal line  109 , or a differential signal may be transmitted through a pair of signal lines  109 . 
     Operations of the Flexible Printed Circuit Board According to the First Embodiment of the Present Invention 
     Next, the following will describe an example of the operations of the flexible printed circuit board  100  according to the first embodiment of the present invention. In the flexible printed circuit board  100  of the present embodiment, a signal is transmitted through the signal lines  109   a  and  109   b  and the signal via holes  113 . When the flexible printed circuit board  100  of the present embodiment is connected to the lower contact type FPC connector  107  as shown in  FIG. 12 , the first wiring layer  103  including the signal lines  9  is positioned at the opposite side of the printed circuit board  118  on which the FPC connector  107  is mounted. Accordingly, the signal lines  9  are coupled mainly with the ground layer  112   a  provided on the second wiring layer  105  of the flexible printed circuit board  100 , so that the signal lines  109  are not coupled with the ground layer  118   c  of the printed circuit board  118  indicated by B. This prevents electrical impedance characteristics of the signal lines  109  because of capacitance occurred as a result of coupling between the signal lines  109  and the ground layer  118   c  of the printed circuit board  118  from deteriorating. This also prevents any transmission characteristics of a high-frequency signal from deteriorating. 
       FIG. 13  is a plan view of the flexible printed circuit board  100  according to the first embodiment of the present invention for showing the flows of a signal current and a feedback current when a high-frequency signal is being transmitted through the signal line.  FIG. 13  shows the signal lines  109   a  and  109   b  formed on the first wiring layer  103 , and the ground layer  112   a  formed on the second wiring layer  105  on the flexible printed circuit board  100 . When the high-frequency signal is transmitted through the signal lines  109  on the flexible printed circuit board  100 , currents flow through the signal lines  109   a  and  109   b , as indicated by arrows E in  FIG. 13 . At the same time, feedback currents flow through the ground layer  112   a , as indicated by arrows F in  FIG. 13 . 
     In the flexible printed circuit board  100  according to the first embodiment of the present invention, each of the signal lines  109   a  and  109   b  provided on the first wiring layer  103   a  has a signal line taper section  116  which enlarges gradually a width of the signal line toward the signal via hole  113  in the vicinity of the signal via hole  113 . Further, the ground layer  112   a  has ground layer taper sections  117  in the vicinity of the ground connection pads  111  on the second wiring layer  5  so that a shape of each of ground layer taper sections  117  can meet that of each of the taper sections  116  of the signal lines  109   a  and  109   b.    
     Accordingly, in the vicinity of a connection between the signal lines  109   a  and  109   b  and the signal via holes  113 , it is possible to reinforce coupling between the signal lines  109   a  and  109   b  and the ground layer  112   a  on the second wiring layer  105 , thereby inhibiting sudden changes in the electrical impedance characteristics of the transmission line. Furthermore, as indicated by arrows F in  FIG. 13 , in the vicinity of bottom of each of the ground connection pads  111 , it is possible to suppress sudden changes in the routing of a feedback current. Thus, in places where the flexible printed circuit board  100  and the FPC connector  107  are connected to each other, it is possible to improve transmission characteristics of a high-frequency signal. 
       FIG. 14  shows the results of measuring a reflection loss (S 11 ) of signal currents through the signal lines  9  at various frequencies between the flexible printed circuit board  50  as related art, which is illustrated in  FIGS. 1-5  and the flexible printed circuit board  100  according to the first embodiment of the present invention, which is illustrated in  FIGS. 7-12 . In  FIG. 14 , G indicates the results of measurement of the flexible printed circuit board  100  and H indicates the results of measurement of the flexible printed circuit board  50 . 
       FIG. 15  shows the results of measuring a transmission loss (S 21 ) of signal currents through the signal lines  9  at various frequencies between the flexible printed circuit board  50  as related art, which is illustrated in  FIGS. 1-5  and the flexible printed circuit board  100  according to the first embodiment of the present invention, which is illustrated in  FIGS. 7-12 . In  FIG. 15 , I indicates the results of measurement of the flexible printed circuit board  100  and J indicates the results of measurement of the flexible printed circuit board  50 . 
     Measurements of various items incorporated into the measurement results are as follows. In the flexible printed circuit board  100  of the present embodiment, the diameters of the signal via hole  113  and the ground via hole  115  are 0.25 mm, respectively. Space between the signal via holes  113  or the ground via holes  115  indicated by L 4  in  FIG. 9  is 0.725 mm. Further, a width represented by L 7  is 0.95 mm and a width represented by L 8  is 0.65 mm. Further, the lengths represented by L 1  and L 2  are 0.5 mm, respectively. A length represented by L 3  is 2.0 mm. A width represented by L 17  is 0.25 mm. A length represented by L 5  is 3.0 mm. 
     Further, in the flexible printed circuit board  100  of the present embodiment, a width of the signal connection pad  110  represented by L 11  in  FIG. 10  is 0.95 mm and a width of the ground connection pad  111  represented by L 13  is 0.65 mm. Furthermore, a distance represented by L 9  is 0.35 mm. A distance represented by L 10  is 0.1025 mm. A distance represented by L 12  is 1.35 mm. Further, in  FIG. 11 , a length represented by L 14  is 1.5 mm. A length represented by L 15  is 2.0 mm. A width represented by L 16  is 2.5 mm. 
     Further, in the flexible printed circuit board  100  of the present embodiment, the second insulation layer  104  is made of a polyimide-based resin and has a dielectric constant value of 3.2 and a tan δ value of 0.005. Further, the flexible printed circuit board  100  has a thickness of 0.05 mm and is controlled so as to have an electrical impedance characteristic value of 50Ω. 
     To transmit a high-speed signal stably, it is demanded that at a frequency of a transmission data rate, a transmission channel have a reflection loss (S 11 ) of −10 dB or less and a transmission loss (S 21 ) of −3 dB or more. As shown in  FIGS. 14 and 15 , in the flexible printed circuit board  100  of the present embodiment, at a frequency of 10 GHz, its reflection loss is −16 dB or less and its transmission loss is −0.3 dB or more. Therefore, in the flexible printed circuit board  100  of the present embodiment, a high speed serial transmission of 10 Gbps can be carried out stably. From the above, it can be confirmed that in the flexible printed circuit board  100  of the present embodiment, the transmission characteristics of a high-frequency signal can be enhanced in the vicinity of the connection portion of the flexible printed circuit board  100  that is connected with the FPC connector  7 . 
     The following will describe an optical transmitter-receiver module and a network card as an optical transmitter-receiver module and an optical transmitter-receiver according to second and third embodiments of the present invention. It is to be noted that in the optical transmitter-receiver module and the network card, the flexible printed circuit board  100  of the first embodiment thereof is used. 
     Configuration Example of Optical Transmitter-Receiver Module and Network Card of the Second and Third Embodiments 
       FIGS. 16-19  are explanatory diagrams each for illustrating a configuration of an optical transmitter-receiver module  219  and a network card  220  according to the second and third embodiments of the present invention.  FIG. 16  is a plan view of an optical transmitter-receiver module  219  and a network card  220  as a first example of second and third embodiments of the present invention and  FIG. 17  is a cross-sectional view of the optical transmitter-receiver module  219  and the network card  220  for showing an outline thereof.  FIG. 18  is a plan view of an optical transmitter-receiver module  319  and a network card  320  as a second example of the second and third embodiments of the present invention and  FIG. 19  is a cross-sectional view of the optical transmitter-receiver module  312  and the network card  320  for showing an outline thereof. In  FIG. 17-19 , a bezel  224 , which will be described later, is not shown. 
     The network cards  220 ,  320  of the third embodiment of the invention include the optical transmitter-receiver modules  219 ,  319 , respectively, as the second embodiment of the invention. The network cards  220 ,  320  are respectively inserted into an expansion slot in an item of equipment such as a personal computer, to enable transmission of data to, or receipt of data from an item of equipment such as an external information communication device, to be implemented by way of an optical cable connected to optical cable connection connector  233  that will be described later. 
     The following will describe configurations of the optical transmitter-receiver modules  219 ,  319  and the network cards  220 ,  320 . 
     As shown in  FIGS. 16 and 17 , the network card  220  includes the optical transmitter-receiver module  219  having the optical cable connection connector  233 , an optical transmitting-receiving board connection FPC  221 , a host board  223  having an optical transmitting-receiving circuit section  222 , and a bezel  224  attached to an end section of the host board  223 . The optical transmitter-receiver module  219  is mounted on the host board  223  so that the optical cable connection connector  233  can protrude from the bezel  224 . Further, the host board  223  has card edge sections  225 . The network card  220  is mounted on an item such as a personal computer by inserting the card edge sections  225  into an expansion slot thereof. 
     The optical transmitter-receiver module  219  has an optical transmitter-receiver module cabinet  226 , a Transmitter Optical Sub-Assembly (TOSA)  227 , a Receiver Optical Sub-Assembly (ROSA)  228 , a TOSA connection FPC  230 , a ROSA connection FPC  229 , and an optical transmitting-receiving board  232  having an optical transmitting-receiving circuit section  231 . 
     The TOSA  227  and the ROSA  228  are arranged side by side at a position that corresponds to the optical cable connection connector  233  on the optical transmitter-receiver module cabinet  226 . The TOSA  227  is a transmission optical device equipped with a laser diode etc. and has an interface for a connector of an optical cable connected to the optical cable connection connector  233 , so as to convert an electrical signal into an optical signal and output it. The TOSA  227  is one example of an optical transmitter module. The ROSA  228  is a reception optical device equipped with a photodiode and the like and has an interface for the connector of the optical cable connected to the optical cable connection connector  233 , so as to convert an optical signal into an electrical signal and output it. The ROSA  228  is one example of an optical receiver module. 
     The TOSA  227  and the ROSA  228  are connected to the optical transmitting-receiving board  232  by means of respectively the TOSA connection FPC  230  and the ROSA connection FPC  229 . The optical transmitting-receiving board  232  is a rigid board and has the optical transmitting-receiving circuit section  231  connected to the TOSA  227  and the ROSA  228  through the TOSA connection FPC  230  and the ROSA connection FPC  229 , respectively. The optical transmitting-receiving circuit section  231  includes a drive circuit for the laser diode in the TOSA  227 , a post-amplifier circuit for an optical signal received by the photodiode in the ROSA  228 . 
     The optical transmitting-receiving board  232  is connected to the host board  223  by way of an optical transmitting-receiving board connection FPC  221 . Thus, circuits of the optical transmitting-receiving circuit section  231  are connected to circuits of an optical transmitting-receiving circuit section  222  by way of the optical transmitting-receiving board connection FPC  221 . The optical transmitting-receiving circuit section  222  is equipped with items such as, for example, a physical layer (PHY) chip and a media access control (MAC) chip. The optical transmitting-receiving board  232  is one example of an optical transmitting-receiving circuit board and the host board  223  is one example of a mother board. The flexible printed circuit board  100  of the first embodiment illustrated in  FIGS. 7-12  is applied to the optical transmitting-receiving board connection FPC  221 . 
     In the optical transmitter-receiver module  219 , the TOSA connection FPC  230 , the ROSA connection FPC  229 , the optical transmitting-receiving board connection FPC  221 , and the optical transmitting-receiving board  232  are soldered to connection portions of the boards indicated by K in  FIGS. 16 and 17 . Accordingly, in contrast to circumstances where the TOSA connection FPC  230 , the ROSA connection FPC  229 , the optical transmitting-receiving board connection FPC  221 , and the optical transmitting-receiving board  232  are integrally manufactured into a flex-rigid board, these FPC and board can be manufactured separately from each other. It is thus possible to manufacture these FPC and board inexpensively. Furthermore, since these FPC and board are manufactured separately, if a design of, for example, only the optical transmitting-receiving board connection FPC  221  is changed, it is possible to correspond to this change by changing only the process of manufacturing the optical transmitting-receiving board connection FPC  221 , thereby limiting the effects of changes in design to a smaller range thereof. 
     The optical transmitter-receiver module  319  and the network card  320  as shown in  FIGS. 18 and 19  has a similar configuration to the optical transmitter-receiver module  219  and the network card  220  as shown in  FIGS. 16 and 17  without the following items. Therefore, like reference characters of the first example of the second and third embodiments shown in  FIGS. 16 and 17  refer to like elements of the second example thereof shown in  FIGS. 18 and 19  without differences. In the optical transmitter-receiver module  319  of the second example shown in  FIGS. 18 and 19 , the TOSA connection FPC  330 , the ROSA connection FPC  329 , the optical transmitting-receiving board connection FPC  321 , and the optical transmitting-receiving board  332  are each constituted of a flex-rigid board. Accordingly, in contrast to a configuration in which the flexible printed circuit boards of the TOSA connection FPC  330 , the ROSA connection FPC  329 , and the optical transmitting-receiving board connection FPC  321  are soldered onto the optical transmitting-receiving board  332 , a soldering operation in manufacture is rendered unnecessary. Accordingly, it is possible to abbreviate periods of manufacturing operations and, furthermore, to prevent occurrences of poor soldering, and occurrences of deficiencies in manufacturing caused by the adverse effects of heat generated on peripheral components during soldering. 
     Furthermore, in the optical transmitting-receiving modules  219 ,  319  and the network cards  220 ,  320  according to the second and third embodiments of the present invention illustrated in  FIGS. 16-19 , each of the optical transmitting-receiving connection FPCs  221 ,  321  is connected to an FPC connector  234  attached to the host board  223 . This allows each of the optical transmitting-receiving board connection FPCs  221 ,  321  to be easily mounted onto the host board  223 . 
     Further, in the optical transmitter-receiver modules  219 ,  319  and the network cards  220 ,  320  of the second and third embodiments, the TOSAs  227 , the ROSAs  228 , the optical transmitting-receiving boards  232 ,  332  and the host boards  223  are respectively connected to the corresponding flexible printed circuit boards. Accordingly, within a length of each of these flexible printed circuit boards, positions of various members can be changed. For example, after each of the members has been connected to each of the flexible printed circuit boards, a position of the bezel  224  can be adjusted to align it with an end face of the optical transmitter-receiver module cabinet  226  on which each of the optical transmitting-receiving boards  232 ,  332  is mounted. 
     Moreover, in the optical transmitter-receiver modules  219 ,  319  and the network cards  220 ,  320  of the second and third embodiments, a part of the modules and the circuits for optical transmission and reception is configured as an optical transmitter-receiver module. Accordingly, it is possible to provide common specifications for an optical transmitter-receiver such as other network cards and an optical transmitter-receiver module so that any optical transmitter-receiver module having specifications that are identical to those of the optical transmitter-receiver such as other network cards can be used. This enables to reduce any costs in both of the design and manufacture thereof. 
     Operations of the Optical Transmitter-Receiver Modules and the Network Cards According to the Second and Third Embodiments of the Present Invention 
     The following will describe an example of operations of the optical transmitter-receiver module  219  and the network card  220  illustrated in  FIGS. 16A and 17 . The optical transmitter-receiver module  219  and the network card  220  are inserted into an expansion slot in an item of equipment such as a personal computer. The optical transmitter-receiver module  219  and the network card  220  transmits and receives data to and from an item of equipment such as an external information communication device via an optical cable connected to the optical cable connection connector  233 , which will be described below. 
     Data is transmitted to the equipment such as the external information communication device as follows. Information that is necessary for data transmission is input as an electrical signal to the optical transmitting-receiving circuit section  222  via the card edge sections  225  inserted into the expansion slot in the item of equipment such as personal computer. The information that is necessary for data transmission input to the optical transmitting-receiving circuit sections  222  as an electrical signal is processed by a chip such as a MAC chip or a PHY chip and input as an electrical signal onto the optical transmitting-receiving circuit section  231  on the optical transmitting-receiving board  232  by way of the optical transmitting-receiving board connection FPC  221 . Then, on the basis of the information input into the optical transmitting-receiving circuit section  231 , an electrical signal is used to drive the laser diode in the TOSA  227  by way of the TOSA connection FPC  230  so that data may be transmitted as an optical signal to the external information communication device by means of an optical cable. 
     The data is received from the item of equipment such as the external information communication device as follows. The data from the external information communication device is input as an optical signal by means of the optical cable onto the photodiode in the ROSA  228 . The optical signal input onto the photodiode in the ROSA  228  is converted into an electrical signal, which is in turn input by way of the ROSA connection FPC  229  onto the optical transmitting-receiving circuit section  231  of the optical transmitting-receiving board  232 . The electrical signal that has been input onto the optical transmitting-receiving circuit section  231  is processed by equipment such as the post-amplifier circuit and input onto the optical transmitting-receiving circuit section  222  on the host board  223  by way of the optical transmitting-receiving board connection FPC  221 . The electrical signal that has been input onto the optical transmitting-receiving circuit section  222  is processed by a chip such as a PHY chip or a MAC chip, and output as received data to the item of equipment such as the personal computer by way of the card edge sections  225 . 
     Thus, as described above, in the course of the transmission of data to and reception of data from the external information communication device via the optical cable, a high-frequency electrical signal is transmitted through the signal lines and the connections on the TOSA connection FPC  230 , the ROSA connection FPC  229 , the optical transmitting-receiving board connection FPC  221 , the optical transmitting-receiving board  232 , and the host board  223 . For example, in circumstances where serial data is transmitted at a high speed such as 10G bits/s, it is necessary to correspond to a signal having a high level of frequency in excess of 10 GHz. 
     Operations similar to the above operations of the optical transmitter-receiver module  219  and the network card  220  as the first example of the second and third embodiments of the invention is also performed in the optical transmitter-receiver module  319  and the network card  320  as the second example thereof, which are illustrated in  FIGS. 18 and 19 . 
     In the optical transmitter-receiver modules  219 ,  319  and the network cards  220 ,  320  of the second and third embodiments, the flexible printed circuit board  100  of the first embodiment of the present invention illustrated in  FIGS. 7-12  can be applied to the optical transmitting-receiving board connection FPCs  221 ,  321 . Accordingly, by performing high-speed data transmission and reception, a high-quality signal can be transmitted through the connections between the signal lines of the flexible printed circuit board and the FPC connector even if a high-frequency signal is transmitted, thereby facilitating stability in the transmission and reception of data. 
     Although the flexible printed circuit board connected to the FPC connector, and an optical transmitter-receiver module and an optical transmitter-receiver, which are each equipped with such a flexible printed circuit board, has been described, this invention is not limited thereto. For example, this invention can be applied to a flexible printed circuit board connected to another connector. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alternations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.