Patent Publication Number: US-2023133827-A1

Title: Transmission module, electronic unit, and electronic device

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to a technique of transmitting a digital signal. 
     Description of the Related Art 
     An electronic device includes an electronic unit including a transmission module and two electronic modules that communicate data in the form of a digital signal via the transmission module. Since flexibility is required for the transmission module in the case where the electronic unit is disposed in a casing of an electronic device, a flexible printed wiring board is used for the transmission module. The flexible printed wiring board and a rigid printed wiring board of each electronic module are interconnected via a connector. That is, a connector on the flexible printed wiring board is attached to a connector on the rigid printed wiring board. The flexible printed wiring board of this kind has less strength than the rigid printed wiring board. Therefore, Japanese Patent Laid-Open No. 2009-135285 discloses a configuration in which a reinforcing member is disposed at a position opposing a connector with the flexible printed wiring board therebetween. 
     Incidentally, there is an increasing tendency in the transmission speed of the digital signal transmitted via the flexible printed wiring board. Accompanied by the increase in the transmission speed, reflection of the digital signal in the signal line of the flexible printed wiring board that has not been an issue has come to affect the quality of the digital signal more. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a transmission module includes a flexible printed wiring board including a signal line, a connector mounted on the flexible printed wiring board, and a reinforcing member disposed at a position opposing the connector with the flexible printed wiring board therebetween. The signal line includes a pad connected to a terminal of the connector. The reinforcing member includes a first portion disposed in a region including at least part of the pad as viewed in a direction perpendicular to a main surface of the flexible printed wiring board, and a second portion disposed around the first portion as viewed in the direction perpendicular to the main surface. A member constituting the first portion is a member having a nature that reduces a characteristic impedance of the pad more than a member constituting the second portion does. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an explanatory diagram of a digital camera serving as an example of an electronic device according to a first embodiment. 
         FIG.  2 A  is an explanatory diagram of an electronic unit according to a first embodiment. 
         FIG.  2 B  is an explanatory diagram of the electronic unit according to the first embodiment. 
         FIG.  3    is an explanatory diagram of comparison between two digital signals. 
         FIG.  4 A  is a plan view of a transmission module according to the first embodiment. 
         FIG.  4 B  is a longitudinal section view of the transmission module according to the first embodiment. 
         FIG.  5 A  is a partial plan view of the transmission module according to the first embodiment. 
         FIG.  5 B  is a partial section view of the transmission module according to the first embodiment. 
         FIG.  6 A  is a cross-section view of the transmission module according to the first embodiment. 
         FIG.  6 B  is a cross-section view of the transmission module according to the first embodiment. 
         FIG.  6 C  is a cross-section view of the transmission module according to the first embodiment. 
         FIG.  7 A  is a plan view of a transmission module of a comparative example. 
         FIG.  7 B  is a longitudinal section view of the transmission module of the comparative example. 
         FIG.  8 A  is a cross-section view of the transmission module of the comparative example. 
         FIG.  8 B  is a cross-section view of the transmission module of the comparative example. 
         FIG.  8 C  is a cross-section view of the transmission module of the comparative example. 
         FIG.  9 A  is a plan view of a transmission module according to a second embodiment. 
         FIG.  9 B  is a longitudinal section view of the transmission module according to the second embodiment. 
         FIG.  10 A  is a cross-section view of the transmission module according to the second embodiment. 
         FIG.  10 B  is a cross-section view of the transmission module according to the second embodiment. 
         FIG.  10 C  is a cross-section view of the transmission module according to the second embodiment. 
         FIG.  11 A  is a plan view of a transmission module according to a third embodiment. 
         FIG.  11 B  is a longitudinal section view of the transmission module according to the third embodiment. 
         FIG.  12 A  is a cross-section view of the transmission module according to the third embodiment. 
         FIG.  12 B  is a cross-section view of the transmission module according to the third embodiment. 
         FIG.  12 C  is a cross-section view of the transmission module according to the third embodiment. 
         FIG.  13 A  is a plan view of a transmission module according to a fourth embodiment. 
         FIG.  13 B  is a longitudinal section view of the transmission module according to the fourth embodiment. 
         FIG.  14 A  is a cross-section view of the transmission module according to the fourth embodiment. 
         FIG.  14 B  is a cross-section view of the transmission module according to the fourth embodiment. 
         FIG.  14 C  is a cross-section view of the transmission module according to the fourth embodiment. 
         FIG.  15 A  is a plan view of a transmission module according to a fifth embodiment. 
         FIG.  15 B  is a longitudinal section view of the transmission module according to the fifth embodiment. 
         FIG.  16 A  is a cross-section view of the transmission module according to the fifth embodiment. 
         FIG.  16 B  is a cross-section view of the transmission module according to the fifth embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present disclosure will be described in detail below with reference to drawings. 
     First Embodiment 
       FIG.  1    is an explanatory diagram of a digital camera  600  that is an image pickup apparatus serving as an example of an electronic device according to a first embodiment. The digital camera  600  that is an image pickup apparatus is a digital camera of a lens-replacing type, and includes a camera body  601 . A lens unit (lens barrel)  602  including a lens is attachable to and detachable from the camera body  601 . The camera body  601  includes a casing  611 , and an electronic unit  500  disposed inside the casing  611 . 
     The electronic unit  500  includes an image pickup module  200  serving as an example of a first electronic module, an image processing module  300  serving as an example of a second electronic module, and a transmission module  100 . The image pickup module  200  transmits a digital signal D 2  indicating an image signal to the image processing module  300  via the transmission module  100 . The image pickup module  200  transmits the digital signal D 2  to the image processing module  300  via the transmission module  100  by multilevel transmission of 3 or more levels, which is 4-level transmission in the first embodiment. As a result of this, the transmission speed of data can be increased. 
       FIGS.  2 A and  2 B  are explanatory diagrams of an electronic unit  500  according to the first embodiment.  FIG.  2 A  is a schematic plan view of the electronic unit  500 , and  FIG.  2 B  is a schematic side view of the electronic unit  500 . In  FIGS.  2 A and  2 B , the transmission module  100  is stretched straight. 
     The image pickup module  200  serves as an example of a printed circuit board, and also serves as an example of a semiconductor module. The image pickup module  200  includes a printed wiring board  201 , an image sensor  202  serving as an example of a semiconductor device, a conversion circuit  203  serving as an example of a semiconductor device, and a connector  204 . 
     The printed wiring board  201  is a rigid printed wiring board. The image sensor  202 , the conversion circuit  203 , and the connector  204  are mounted on the printed wiring board  201 . 
     The image sensor  202  is, for example, a complementary metal oxide semiconductor CMOS image sensor, or a charge coupled device: CCD image sensor. The image sensor  202  includes a circuit that converts light incident via the lens unit  602  into an analog signal that is an electric signal, and a circuit that converts the analog signal into a digital signal D 1 . As a result of this, the image sensor  202  outputs a digital signal D 1 . The digital signal D 1  is an image signal. In the first embodiment, the digital signal D 1  is a binary signal. 
     The conversion circuit  203  converts the digital signal D 1  that is a binary signal to a multilevel signal, which is the digital signal D 2  that is a 4-level signal in the first embodiment. As described above, the conversion circuit  203  modulates the digital signal D 1  that is a binary signal to the digital signal D 2  that is a 4-level signal, and outputs the digital signal D 2  to the connector  204  subsequent thereto. 
     The connector  204  is an interface through which the digital signal D 2  is output from the conversion circuit  203  to the transmission module  100 , and is electrically connected to the conversion circuit  203 . 
     To be noted, although a case where the conversion circuit  203  is constituted by a semiconductor device different from the image sensor  202  has been described, the configuration is not limited to this, and the image sensor  202  may be configured to output the digital signal D 2  that is a 4-level signal as an image signal. For example, the conversion circuit  203  may be included in the image sensor  202 . 
     The image processing module  300  serves as an example of a printed circuit board, and also serves as an example of a semiconductor module. The image processing module  300  includes a printed wiring board  301 , and as examples of semiconductor devices, an image processing device  302 , a memory device  303 , and a conversion circuit  304 . 
     The printed wiring board  301  is a rigid printed wiring board. The image processing device  302 , the memory device  303 , and the conversion circuit  304  are mounted on the printed wiring board  301 . 
     A connector  305  is an interface through which input of the digital signal D 2  from the transmission module  100  is received, and is electrically connected to the conversion circuit  304  subsequent thereto. In the first embodiment, the connector  305  has substantially the same configuration as the connector  204 . 
     The conversion circuit  304  converts the digital signal D 2  that is a 4-level signal into the digital signal D 1  that is a binary signal, and outputs the digital signal D 1  to the image processing device  302 . That is, the conversion circuit  304  demodulates the digital signal D 2  that is a 4-level signal into the digital signal D 1  that is a binary signal. 
     The image processing device  302  is, for example, a digital signal processor, obtains the digital signal D 1 , and performs correction processing on the digital signal D 1  to generate image data. The memory device  303  stores the image data. 
     To be noted, although a case where the conversion circuit  304  is constituted by a semiconductor device different from the image processing device  302  has been described, the configuration is not limited to this, and the conversion circuit  304  may be included in the image processing device  302 . That is, the image processing device  302  may be configured to obtain the digital signal D 2  that is a 4-level signal. 
     The transmission module  100  is an example of a flexible printed circuit board. The transmission module  100  is used for transmitting the digital signal D 2  from the image pickup module  200  to the image processing module  300 . The digital signal D 2  is preferably a differential signal that enables high-speed transmission. 
     The transmission module  100  includes a flexible printed wiring board  101 , and connectors  109  and  120  mounted on the flexible printed wiring board  101 . The connectors  109  and  120  are each electrically connected to the flexible printed wiring board  101 . The connector  109  is detachably attached to the connector  204 , and the connector  120  is detachably attached to the connector  305 . The connector  109  is electrically connected to the connector  204  when attached to the connector  204 . In addition, the connector  120  is electrically connected to the connector  305  when attached to the connector  305 . In the first embodiment, the connector  120  has substantially the same configuration as the connector  109 . 
     As a result of the configuration described above, the image sensor  202  is capable of communicating data with the image processing device  302  via the conversion circuit  203 , the connector  204 , the connector  109 , the flexible printed wiring board  101 , the connector  120 , the connector  305 , and the conversion circuit  304 . 
     Here, in the case of transmitting a digital signal by multilevel transmission, the transmission speed is improved but the S/N ratio to the noise of the same voltage amplitude becomes low as compared with a case of transmitting a digital signal by binary transmission.  FIG.  3    is an explanatory diagram comparing a case where a digital signal DA is transmitted by binary transmission and a case where a digital signal DB is transmitted by 4-level transmission. The maximum voltage amplitudes of the digital signals DA and DB are set to be equal. In addition, the voltage amplitudes of a noise N superimposed on the digital signals DA and DB are also set to be equal. Even in the case where the noise N of the same amplitude is superimposed on the digital signals DA and DB, the S/N ratio of the digital signal DB that is a 4-level signal is lower than the S/N ratio of the digital signal DA that is a binary signal. One cause of the noise N is inconsistency of a characteristic impedance. When there is inconsistency of the characteristic impedance, a reflection wave of the signal is generated as the noise N at an inconsistent portion. 
       FIG.  4 A  is a plan view of the transmission module  100  according to the first embodiment.  FIG.  4 B  is a longitudinal section view of the transmission module  100  according to the first embodiment.  FIGS.  4 A and  4 B  schematically illustrate the transmission module  100 . To be noted, in  FIGS.  4 A and  4 B , the flexible printed wiring board  101  is stretched straight. 
     The flexible printed wiring board  101  includes a plurality of signal lines  110  used for transmission of the digital signal D 2 . Further, the flexible printed wiring board  101  may include lines other than the signal line  110  such as a control line, a power line, and a ground line. Among the plurality of signal lines  110 , pairs of adjacent signal lines  110  each constitute a differential line pair  111  that is a transmission path used for transmitting a differential signal. In the example of  FIG.  4 A , eight signal lines  110  constitute four differential line pairs  111 . Due to increase in the size of the image data, the digital signal D 2  is transmitted at a transmission speed of 10 Gbps or more per one differential line pair  111 . Gbps stands for giga bits per second. The signal lines  110  are each formed from a metal foil such as a copper foil. 
       FIG.  5 A  is a partial plan view of the transmission module  100  according to the first embodiment.  FIG.  5 B  is a partial section view of the transmission module  100  according to the first embodiment. To be noted, in  FIG.  5 A , illustration of the connector  109  is omitted.  FIG.  6 A  is a cross-section view of the transmission module  100  taken along a line VIA-VIA of  FIG.  4 A .  FIG.  6 B  is a cross-section view of the transmission module  100  taken along a line VIB-VIB of  FIG.  4 A .  FIG.  6 C  is a cross-section view of the transmission module  100  taken along a line VIC-VIC of  FIG.  4 A . To be noted, in  FIG.  6 C , illustration of the connector  109  is omitted. 
     The flexible printed wiring board  101  includes an insulating layer  1014  that is electrically insulating and supports the plurality of signal lines  110 . The insulating layer  1014  includes a base layer  1011  and a cover layer  1013 . The plurality of signal lines  110  are disposed in a conductor layer  1012  on the base layer  1011 . The conductor layer  1012  is covered by the cover layer  1013 . The base layer  1011  and the cover layer  1013  are formed from, for example, polyimide. 
     The transmission module  100  includes a reinforcing member  130  disposed at a position opposing the connector  109  with the flexible printed wiring board  101  therebetween. In addition, the transmission module  100  includes a reinforcing member  140  disposed at a position opposing the connector  120  with the flexible printed wiring board  101  therebetween. The reinforcing member  130  includes an insulating layer  135  that is electrically insulating. The reinforcing member  140  includes an insulating layer  145  that is electrically insulating. The reinforcing member  130  is a member for reinforcing the flexible printed wiring board  101  to suppress breakage of the signal lines  110  when attaching or detaching the connector  109  to or from the connector  204 . Therefore, the insulating layer  135  is thicker than the flexible printed wiring board  101 . Similarly, the reinforcing member  140  is a member for reinforcing the flexible printed wiring board  101  to suppress breakage of the signal lines  110  when attaching or detaching the connector  120  to or from the connector  305 . Therefore, the insulating layer  145  is thicker than the flexible printed wiring board  101 . As viewed in a Z direction perpendicular to a main surface  1010  of the flexible printed wiring board  101 , the reinforcing member  130  is disposed in a region including the entirety of the connector  109 . In addition, as viewed in the Z direction, the reinforcing member  140  is disposed in a region including the entirety of the connector  120 . 
     Here, a transmission module of a comparative example will be described.  FIG.  7 A  is a plan view of a transmission module  100 X of a comparative example.  FIG.  7 B  is a longitudinal section view of the transmission module IMX of the comparative example.  FIGS.  7 A and  7 B  schematically illustrate the transmission module  100 X.  FIG.  8 A  is a cross-section view of the transmission module  100 X taken along a line VIIIA-VIIIA of  FIG.  7 A   FIG.  8 B  is a cross-section view of the transmission module  100 X taken along a line VIIIB-VIIIB of  FIG.  7 A .  FIG.  8 C  is a cross-section view of the transmission module  100 X taken along a line VIIIC-VIIIC of  FIG.  7 A . To be noted, in  FIG.  8 C , illustration of the connector  109  is omitted. 
     The transmission module  100 X includes a flexible printed wiring board  101 X. The flexible printed wiring board  101 X includes a plurality of signal lines  110 X. The plurality of signal lines  110 X are disposed in one conductor layer  1012 X. Among the plurality of signal lines  110 X, a pair of adjacent signal lines  110 X constitute a differential line pair  111 X that is a transmission path used for transmitting a differential signal. The flexible printed wiring board  101 X includes an insulating layer  1014  that has a configuration having substantially the same configuration as in the first embodiment and supports the plurality of signal lines  110 X. The insulating layer  1014  includes the base layer  1011  and the cover layer  1013 . 
     In addition, the transmission module  100 X includes the connector  109  mounted on the flexible printed wiring board  101 X and having substantially the same configuration as in the first embodiment, and a reinforcing member  130 X disposed at a position opposing the connector  109  with the flexible printed wiring board  101 X therebetween. The reinforcing member  130 X is constituted by only an insulating layer having substantially the same configuration as the insulating layer  135  of the first embodiment. The flexible printed wiring board  101 X is a one-sided flexible printed wiring board including one conductor layer  1012 X. Therefore, there is no planar ground conductor having a stable potential around the plurality of signal lines  110 X. 
     The signal lines  110 X each include a pad  104 X bonded to a terminal  1091  of the connector  109 , and wiring portions  102 X and  103 X. As viewed in the Z direction, the pad  104 X and the wiring portion  103 X overlap the reinforcing member  130 X, and the wiring portion  102 X does not overlap the reinforcing member  130 X. A width W 103 X of the wiring portion  103 X is equal to a width W 102 X of the wiring portion  102 X. In addition, in the differential line pair  111 X, a distance S 103 X between two adjacent wiring portions  103 X is equal to a distance S 102 X between two adjacent wiring portions  102 X A width W 104 X of the pad  104 X is larger than each of the width W 102 X of the wiring portion  102 X and the width W 103 X of the wiring portion  103 X. In addition, in the differential line pair  111 X, a distance S 104 X between two adjacent pads  104 X is larger than each of the distance S 102 X between two adjacent wiring portions  102 X and the distance S 103 X between two adjacent wiring portions  103 X. 
     Here, a differential signal is transmitted through the pair of signal lines  110 X of the differential line pair  111 X. Therefore, a characteristic impedance Z 1 X of the wiring portion  102 X described below is a differential impedance of the pair of wiring portions  102 X in the differential line pair  111 X. In addition, a characteristic impedance Z 2 X of the wiring portion  103 X is a differential impedance of the pair of wiring portions  103 X in the differential line pair  111 X. In addition, a characteristic impedance Z 3 X of the pad  104 X is a differential impedance of the pair of pads  104 X in the differential line pair  111 X. 
     In the configuration described above, the characteristic impedance Z 3 X of the pad  104 X is higher than the characteristic impedance Z 1 X of the wiring portion  102 X, and the characteristic impedance Z 2 X of the wiring portion  103 X is lower than the characteristic impedance Z 1 X of the wiring portion  102 X. Specifically, the characteristic impedance Z 2 X of the wiring portion  103 X overlapping the reinforcing member  130 X having a higher relative permittivity than the air, is lower than the characteristic impedance Z 1 X of the wiring portion  102 X not overlapping the reinforcing member  130 X. In addition, since the distance S 104 X between the two pads  104 X is larger than each of the distance S 102 X between the two wiring portions  102 X and the distance S 103 X between the two wiring portions  103 X, the characteristic impedance Z 3 X of the pad  104 X is higher than each of the characteristic impedances Z 1 X and Z 2 X. Therefore, there is a difference between the characteristic impedances Z 1 X and Z 2 X, and there is a difference between the characteristic impedances Z 2 X and Z 3 X. Due to these differences between the characteristic impedances, particularly the difference between the characteristic impedances Z 2 X and Z 3 X, a reflection wave of the digital signal is generated as a noise in the signal line  110 X. That is, a slight difference between the widths W 103 X and W 104 X of the signal line  110 X, a slight difference between the distances S 103 X and S 104 X between a pair of the signal lines  110 X, difference in the relative permittivity around the signal line  110 X, and the like make the characteristic impedance of the signal line  110 X inconsistent. 
     When the characteristic impedance is inconsistent in the signal line  110 X, a reflection wave, that is, a noise is generated in the signal line  110 X, and thus the quality of the digital signal transmitted through the signal line  110 X is likely to deteriorate. Further, as the transmission speed of the digital signal increases, the deterioration of the quality of the digital signal transmitted through the signal line  110 X becomes greater. 
     Therefore, in the first embodiment, the reinforcing member  130  is configured in a different manner from the reinforcing member  130 X of the comparative example, and the signal line  110  is configured in a different manner from the signal line  110 X of the comparative example. 
     With reference to  FIGS.  4 A to  6 C , the signal line  110  includes wiring portions  102  and  103  as a main line, and a pad  104 . The wiring portion  102  serves as an example of a first wiring portion, and is disposed at a position not overlapping the reinforcing member  130  as viewed in the Z direction. The wiring portion  103  serves as a second wiring portion, and is disposed between the wiring portion  102  and the pad  104 . The wiring portion  103  and the pad  104  are disposed in a region overlapping the reinforcing member  130  as viewed in the Z direction. The pad  104  is bonded to the terminal  1091  of the connector  109  via solder or the like. 
     In addition, the signal line  110  includes a wiring portion  105  and a pad  106 . The wiring portion  105  is disposed between the wiring portion  102  and the pad  106 . The wiring portion  105  and the pad  106  are disposed in a region overlapping the reinforcing member  140  as viewed in the Z direction. The pad  106  is bonded to a terminal  1201  of the connector  120  via solder or the like. 
     In the first embodiment, the reinforcing member  130  includes a conductive member  136  disposed on the insulating layer  135 . In addition, in the first embodiment, the reinforcing member  140  includes a conductive member  146  disposed on the insulating layer  145 . 
     The configuration of the reinforcing member  140  is substantially the same as the reinforcing member  130 . In addition, the positional relationship of the reinforcing member  140  with the connector  120 , the wiring portion  105 , and the pad  106  is substantially the same as the positional relationship of the reinforcing member  130  with the connector  109 , the wiring portion  103 , and the pad  104 . Therefore, detailed description of the reinforcing member  140  will be omitted. 
     The insulating layer  135  of the reinforcing member  130  is formed in a uniformly constant thickness in a direction parallel to the main surface  1010 . Examples of the material of the insulating layer  135  of the reinforcing member  130  include resins such as polyimide, polyethylene terephthalate: PET, and glass epoxy, and among the resins, glass epoxy, which has high rigidity, is particularly preferable. The conductive member  136  of the reinforcing member  130  is disposed on the insulating layer  135 . The conductive member  136  is a metal foil such as a copper foil. The conductive member  136  may be electrically connected to an unillustrated ground terminal of the connector  109 . 
     Among the plurality of pads  104 , description will be given focusing on one pad  104 . As viewed in the Z direction, the reinforcing member  130  includes a first portion P 1  disposed in a region including at least part of the pad  104 , and a second portion P 2  disposed around the first portion P 1  as viewed in the Z direction. It is preferable that the region of the first portion P 1  includes 90% or more of the area of the pad  104  as viewed in the Z direction. In the first embodiment, as viewed in the Z direction, the first portion P 1  is disposed in a region including the entirety of the pad  104 . 
     Focusing on the plurality of the pads  104 , that is, all the pads  104 , the first portion P 1  is disposed in a region including the entirety of the plurality of pads  104  as viewed in the Z direction. Further, the second portion P 2  is disposed around the first portion P 1  so as to surround the first portion P 1  as viewed in the Z direction. 
     Here, a differential signal is transmitted through the pair of signal lines  110  of the differential line pair  111 . Therefore, the characteristic impedance Z 1  of the wiring portion  102  described below is a differential impedance of the pair of wiring portions  102  in the differential line pair  111 . In addition, the characteristic impedance Z 2  of the wiring portion  103  is a differential impedance of the pair of wiring portions  103  in the differential line pair  111 . In addition, the characteristic impedance Z 3  of the pad  104  is a differential impedance of the pair of pads  104  in the differential line pair  111 . 
     In the first embodiment, a member constituting the first portion P 1  is a member having a nature that reduces the characteristic impedance Z 3  of the pad  104  more than a member constituting the second portion P 2  does. 
     Specifically, the first portion P 1  is constituted by an insulating member  1351  that is part of the insulating laver  135 , and the conductive member  136  disposed on the insulating member  1351 . As viewed in the Z direction, the insulating member  1351  and the conductive member  136  each have the same shape and size as the first portion P 1 . In addition, the second portion P 2  is constituted by an insulating member  1352  that is part of the insulating layer  135  and disposed around the insulating member  1351 . As viewed in the Z direction, the insulating member  1352  has the same shape and size as the second portion P 2 . The insulating member  1351  serves as an example of a first insulating member. The insulating member  1352  serves as an example of a second insulating member. The insulating member  1351  is formed from the same material as the insulating member  1352  and in the same thickness as the insulating member  1352 , and has the same relative permittivity as the insulating member  1352 . 
     As described above, in the first embodiment, the insulating member  1351  and the conductive member  136  are members constituting the first portion P 1 . In addition, in the first embodiment, the insulating member  1352  having the same relative permittivity and the same thickness as the insulating member  1351  is a member constituting the second portion P 2 . The member constituted by the insulating member  1351  and the conductive member  136  has a nature that reduces the characteristic impedance of an opposing conductor more than the member constituted by the insulating member  1352  does. Since the reinforcing member  130 X of the comparative example has substantially the same configuration as the insulating layer  135 , the characteristic impedance Z 3  of the first embodiment is reduced more than the characteristic impedance Z 3 X of the comparative example. That is, since the conductive member  136  is disposed to oppose the pad  104  with the insulating member  1351  therebetween, the characteristic impedance Z 3  of the pad  104  is reduced. As a result of this, the absolute value of the difference (Z 3 -Z 2 ) between the characteristic impedance Z 2  of the wiring portion  103  and the characteristic impedance Z 3  of the pad  104  can be reduced. Therefore, in the signal line  110 , generation of the reflection wave of the digital signal D 2 , that is, generation of the noise can be reduced, and thus the quality of the digital signal D 2  transmitted through the signal line  110  can be improved. 
     A width W 104  of the pad  104  is preferably larger than each of a width W 102  of the wiring portion  102  and a width W 103  of the wiring portion  103  for bonding the terminal  1091  of the connector  109  thereto. In addition, a distance S 104  between the pair of pads  104  is preferably larger than each of a distance S 102  between a pair of wiring portions  102  and a distance S 103  between a pair of wiring portions  103  for bonding the terminal  1091  of the connector  109  thereto. 
     In addition, the width W 103  of the wiring portion  103  is preferably equal to or less than the width W 102  of the wiring portion  102 . As viewed in the Z direction, the wiring portion  103  overlaps the second portion P 2  of the reinforcing member  130  having a higher relative permittivity than the air. Therefore, the width W 103  of the wiring portion  103  may be equal to the width W 102  of the wiring portion  102  not overlapping the reinforcing member  130 , but is preferably smaller than the width W 102 . As a result of this, the characteristic impedance Z 2  of the wiring portion  103  is higher than the characteristic impedance Z 2 X of the wiring portion  103 X of the comparative example. Therefore, the absolute value of the difference (Z 2 -Z 1 ) between the characteristic impedance Z 1  of the wiring portion  102  and the characteristic impedance Z 2  of the wiring portion  103  can be reduced. In addition, the absolute value of the difference (Z 3 -Z 2 ) between the characteristic impedance Z 2  of the wiring portion  103  and the characteristic impedance Z 3  of the pad  104  can be reduced. Therefore, in the signal line  110 , generation of the reflection wave of the digital signal D 2 , that is, generation of the noise can be more effectively reduced, and the quality of the digital signal D 2  transmitted through the signal line  110  can be more effectively improved. 
     In addition, the distance S 103  between a pair of the wiring portions  103  is preferably equal to or larger than the distance S 102  between a pair of the wiring portions  102 . As viewed in the Z direction, the pair of the wiring portions  103  overlaps the second portion P 2  of the reinforcing member  130  having a higher relative permittivity than the air. Therefore, the distance S 103  between the pair of the wiring portions  103  may be equal to the distance S 102  of the pair of the wiring portions  102  not overlapping the reinforcing member  130 , but is preferably larger than the distance S 102 . As a result of this, the characteristic impedance Z 2  is higher than the characteristic impedance Z 2 X of the comparative example. Therefore, the absolute value of the difference (Z 2 -Z 1 ) between the characteristic impedance Z 1  and the characteristic impedance Z 2  and the absolute value of the difference (Z 3 -Z 2 ) between the characteristic impedance Z 2  and the characteristic impedance Z 3  can be reduced. Therefore, in the signal line  110 , generation of the reflection wave of the digital signal D 2 , that is, generation of the noise can be more effectively reduced, and the quality of the digital signal D 2  transmitted through the signal line  110  can be more effectively improved. 
     In addition, as viewed in the Z direction, although the wiring portion  103  may partially overlap the first portion P 1 , since the first portion P 1  has a nature that reduces the characteristic impedance of an opposing conductor, it is preferable that the wiring portion  103  does not overlap the first portion P 1 . As a result of this, reduction of the characteristic impedance Z 2  of the wiring portion  103  can be suppressed, and the absolute value of the difference (Z 2 -Z 1 ) and the absolute value of the difference (Z 3 -Z 2 ) can be reduced. Therefore, in the signal line  110 , generation of the reflection wave of the digital signal D 2 , that is, generation of the noise can be more effectively reduced, and the quality of the digital signal D 2  transmitted through the signal line  110  can be more effectively improved. 
     In addition, although the reinforcing member  130  has been described, since the reinforcing member  140  has substantially the same configuration as the reinforcing member  130 , the quality of the digital signal D 2  transmitted through the signal line  110  can be more effectively improved. 
     Example 1 
     Simulation of differential impedance was performed for the transmission module  100  according to the first embodiment. HyperLynx available from Mentor Graphics was used for the simulation of the differential impedance. 
     The thickness of the base layer  1011  is denoted by T 1011 , the thickness of the conductor layer  1012  is denoted by T 1012 , the thickness of a portion of the cover layer  1013  overlapping the signal line  110  on the conductor layer  1012  is denoted by T 1013 . In addition, the thickness of the insulating laver  135  is denoted by T 105 , and the thickness of the conductive member  136  is denoted by T 106 . In the simulation, parameter values of the respective thicknesses were as follows: T 1011 =12.5 μm; T 1012 =12 μm; T 1013 =27.5 μm; T 105 =265 μm and T 106 =115 μm. To be noted, the thickness T 105  of the insulating layer  135  includes a thickness of 15 μm of an adhesive between the insulating layer  135  and the base layer  1011 . In addition, the thickness T 106  of the conductive member  136  includes a thickness of 15 μm of an adhesive between the conductive member  136  and the insulating layer  135 . The relative permittivity of the base layer  1011  was set to 3.3, the relative permittivity of the cover layer  1013  was set to 3.6, the relative permittivity of the insulating layer  135  of the reinforcing member  130  was set to 4.7, and the relative permittivity of the adhesive was set to 4.0. The conductivity of the signal line  110  and the conductivity of the conductive member  136  were set to 1.724×10 −8  Ωm. 
     In addition, in the simulation, the parameter values of the width W 104  and the distance S 104  were as follows: W 104 =250 μm; and S 104 =150 μm. 
     As Example 1, simulation was performed for three patterns while changing the magnitude relationship between the width W 102  and the width W 103 , and the magnitude relationship between the distance S 102  and the distance S 103 . The simulation results of the three patterns are shown in the following Table 1 as Examples 1-1, 1-2, and 1-3. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Z1 = 103.8Ω 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 MAGNITUDE 
                   
                   
                   
                   
                   
               
               
                   
                 RELATIONSHIP 
                 W102 
                 W103 
                 S102 
                 S103 
                 Z2 
               
               
                   
               
               
                 Example 1-1 
                 W102 &gt; W103 
                 150 μm 
                 130 μm 
                 45 μm 
                 65 μm 
                 100.5Ω 
               
               
                   
                 S102 &lt; S103 
                   
                   
                   
                   
                 Z2 − Z1 = −3.3Ω 
               
               
                 Example 1-2 
                 W102 &gt; W103 
                 150 μm 
                  65 μm 
                 45 μm 
                 45 μm 
                 100.1Ω 
               
               
                   
                 S102 = S103 
                   
                   
                   
                   
                 Z2 − Z1 = −3.7Ω 
               
               
                 Example 1-3 
                 W102 = W103 
                 150 μm 
                 150 μm 
                 45 μm 
                 70 μm 
                 100.3Ω 
               
               
                   
                 S102 &lt; S103 
                   
                   
                   
                   
                 Z2 − Z1 = −3.5Ω 
               
               
                   
               
            
           
         
       
     
     To be noted, in Example 1-1, W 104 &gt;W 102 &gt;W 103  and S 104 &gt;S 103 &gt;S 102  held. In Example 1-2, W 104 &gt;W 102 &gt;W 103  and S 104 &gt;S 103 =S 102  held. In Example 1-3, W 104 &gt;W 102 =W 103  and S 104 &gt;S 103 &gt;S 102  held. 
     In each of Examples 1-1, 1-2, and 1-3, the characteristic impedance (differential impedance) Z 1  of the wiring portion  102  was 103.8Ω The characteristic impedance (differential impedance) Z 3  of the pad  104  was 102.2Ω. 
     Comparative Example 1 
     In addition, the simulation of differential impedance was also performed for the transmission module  100 X of the comparative example illustrated in  FIGS.  7 A to  8 C . HyperLynx available from Mentor Graphics was used for the simulation of the differential impedance. 
     The thickness of the base layer  1011  is denoted by T 1011 X, the thickness of the conductor layer  1012 X is denoted by T 1012 X, the thickness of a portion of the cover layer  1013  overlapping the signal line  110 X on the conductor layer  1012 X is denoted by T 1013 X In addition, the thickness of the reinforcing member  130 X is denoted by T 105 X. In the simulation, parameter values of the respective thicknesses were as follows, similarly to Example 1: T 1011 X=12.5 μm; T 1012 X=12 μm; T 1013 X=27.5 μm; and T 105 X=265 μm. To be noted, the thickness T 105 X of the reinforcing member  130 X includes a thickness of 15 μm of an adhesive between the reinforcing member  130 X and the base layer  1011 . The relative permittivity of the base layer  1011  was set to 3.3, the relative permittivity of the cover layer  1013  was set to 3.6, the relative permittivity of the reinforcing member  130 X was set to 4.7, and the relative permittivity of the adhesive was set to 4.0. 
     The simulation results of the differential impedance of Comparative Example 1 will be described. The characteristic impedance (differential impedance) Z 1 X of the wiring portion  102 X was 103.8Ω. The characteristic impedance (differential impedance) Z 2 X of the wiring portion  103 X was 85.5Ω. The characteristic impedance (differential impedance) Z 3 X of the pad  104 X was 118.2Ω. 
     The distance S 104 X between a pair of the pads  104 X is larger than each of the distance S 102 X between a pair of the wiring portions  102 X and the distance S 103 X between a pair of the wiring portions  103 X. Therefore, in the configuration of Comparative Example 1 not including the conductive member  136 , the characteristic impedance (differential impedance) Z 3 X of the pad  104 X was higher than the characteristic impedance Z 2 X of the wiring portion  103 X. The difference (Z 3 X-Z 2 X) between the characteristic impedances was 32.7Ω. 
     In contrast, in Examples 1-1, 1-2, and 1-3, the difference (Z 3 -Z 2 ) in the characteristic impedance were respectively 1.7 Ω, 2.1Ω, and 1.9Ω. Therefore, in all of Examples 1-1, 1-2, and 1-3, the absolute value |Z 3 -Z 2 | of the difference in the characteristic impedance was smaller than the absolute value |Z 3 X-Z 2 X| of the difference in the characteristic impedance of Comparative Example 1. Therefore, in each of Examples 1-1, 1-2, and 1-3, the characteristic impedance was more consistent than in Comparative Example 1. Therefore, in Examples 1-1, 1-2, and 1-3, the generation of the reflection wave can be reduced 
     Particularly, in Example 1-1, the absolute value |Z 3 -Z 2 | of the difference in the characteristic impedance was smaller than in Examples 1-2 and 1-3. Therefore, in Example 1-1, the generation of the reflection wave can be reduced as compared with Examples 1-2 and 1-3. 
     In addition, in Example 1-3, the absolute value |Z 3 -Z 2 | of the difference in the characteristic impedance was smaller than in Example 1-2. Therefore, in Example 1-3, the generation of the reflection wave can be reduced as compared with Example 1-2. 
     In addition, the effective permittivity of the surroundings of the wiring portion  103 X was higher than the effective permittivity of the surroundings of the wiring portion  102 X. Therefore, the characteristic impedance Z 2 X of the wiring portion  103 X was lower than the characteristic impedance Z 1 X of the wiring portion  102 X, and the difference (Z 2 X-Z 1 X) in the characteristic impedance was −18.3Ω. 
     Meanwhile, as shown in Table 1, the difference (Z 2 -Z 1 ) in the characteristic impedance in Examples 1-1, 1-2, and 1-3 were respectively −3.3 Ω, −3.7Ω, and −3.5Ω. Therefore, in all of Examples 1-1, 1-2, and 1-3, the absolute value |Z 2 -Z 1 | of the difference in the characteristic impedance was smaller than the absolute value |Z 2 X-Z 1 X| of the difference in the characteristic impedance of Comparative Example 1. Therefore, in each of Examples 1-1, 1-2, and 1-3, the characteristic impedance was more consistent than in Comparative Example 1. Therefore, in Examples 1-1, 1-2, and 1-3, the generation of the reflection wave can be effectively reduced. 
     Particularly, in Example 1-1, the absolute value |Z 2 -Z 1 | of the difference in the characteristic impedance was smaller than in Examples 1-2 and 1-3. Therefore, in Example 1-1, the generation of the reflection wave can be reduced as compared with Examples 1-2 and 1-3. 
     In addition, in Example 1-3, the absolute value |Z 2 -Z 1 | of the difference in the characteristic impedance was smaller than in Example 1-2. Therefore, in Example 1-3, the generation of the reflection wave can be reduced as compared with Example 1-2. 
     In Example 1, the effect of the consistency of the characteristic impedance increases as the transmission speed increases. For example, in the case where the length of the pads  104  and  104 X in the wiring direction is 1 mm, the transmission time of the signal is about 7 ps. In the case where the transmission speed is 5 Gbps (signal period: 200 ps), the rising time of the signal is about 40 ps to 66 ps (about ⅕ to ⅓ of the period). Therefore, in the pads  104  and  104 X, the rising time of the signal is longer than the transmission time of the signal. Therefore, even in the case of Comparative Example 1, the deterioration of the signal waveform caused by the inconsistency of the impedance in the pad  104 X is small. 
     However, in the case where the transmission speed is 10 Gbps (signal period: 100 ps), the rising time of the signal is about 20 ps to 33 ps. Therefore, in Comparative Example 1, deterioration of the signal waveform caused by the inconsistency of the impedance of the pad  104  starts becoming apparent in Comparative Example 1. In the case where the transmission speed is 20 Gbps (signal period: 50 ps), the rising time of the signal is about 10 ps to 17 ps. Therefore, in Comparative Example 1, the deterioration of the signal wavelength caused by the inconsistency of the impedance of the pad  104 X becomes prominent. 
     In addition, in the case of multilevel transmission such as 4-level or 16-level transmission, waveforms of different signal amplitudes are mixed, and therefore the S/N ratio of a signal of a low amplitude is lower than the S/N ratio of a signal of a high amplitude. Therefore, the deterioration of the waveform caused by the inconsistency of the impedance is likely to occur in a signal of a low amplitude. 
     In contrast, in Example 1, since the impedance is consistent between the pad  104  and the wiring portion  103 , the deterioration of the signal waveform is less likely to occur no matter whether the transmission speed of the signal is 10 Gbps or 20 Gbps, and the quality of the signal is improved. 
     Second Embodiment 
     Next, a transmission module of a second embodiment will be described.  FIG.  9 A  is a plan view of a transmission module  100 A according to the second embodiment.  FIG.  9 B  is a longitudinal section view of the transmission module  100 A according to the second embodiment.  FIGS.  9 A and  9 B  schematically illustrate the transmission module  100 A. In the second embodiment, the transmission module  100 A is applied to the electronic unit  500  in place of the transmission module  100  of the first embodiment. Therefore, description of elements substantially the same as in the first embodiment will be omitted. 
     The transmission module  100 A of the second embodiment includes the flexible printed wiring board  101 , the connector  109 , and the connector  120  described in the first embodiment. To be noted, in  FIGS.  9 A and  9 B , the flexible printed wiring board  101  is stretched straight.  FIG.  10 A  is a cross-section view of the transmission module  100 A taken along a line XA-XA of  FIG.  9 A .  FIG.  10 B  is a cross-section view of the transmission module  100 A taken along a line XB-XB of  FIG.  9 A .  FIG.  10 C  is a cross-section view of the transmission module  100 A taken along a line XC-XC of  FIG.  9 A . To be noted, in  FIG.  10 C , illustration of the connector  109  is omitted. 
     The flexible printed wiring board  101  includes a plurality of signal lines  110  used for transmission of the digital signal D 2 . Among the plurality of signal lines  110 , pairs of adjacent signal lines  110  each constitute a differential line pair  111  that is a transmission path used for transmitting a differential signal. The signal lines  110  each include the wiirng portion  102 , the wiring portion  103 , the pad  104 , the wiring portion  105 , and the pad  106 . 
     The transmission module  100 A of the second embodiment includes a reinforcing member  130 A disposed at a position opposing the connector  109  with the flexible printed wiring board  101  therebetween. In addition, the transmission module  100 A includes a reinforcing member  140 A disposed at a position opposing the connector  120  with the flexible printed wiring board  101  therebetween. 
     The reinforcing member  130 A includes insulating members  1351 A and  1352 A that are electrically insulating. The relative permittivity of the insulating member  1351 A is higher than the relative permittivity of the insulating member  1352 A. The reinforcing member  140 A includes insulating members  1451 A and  1452 A that are electrically insulating. The relative permittivity of the insulating member  1451 A is higher than the relative permittivity of the insulating member  1452 A. 
     The reinforcing member  130 A is a member for reinforcing the flexible printed wiring board  101  to suppress breakage of the signal lines  110  when attaching or detaching the connector  109  to or from the connector  204 . Therefore, the reinforcing member  130 A is thicker than the flexible printed wiring board  101 . Similarly, the reinforcing member  140 A is a member for reinforcing the flexible printed wiring board  101  to suppress breakage of the signal lines  110  when attaching or detaching the connector  120  to or from the connector  305 . Therefore, the reinforcing member  140 A is thicker than the flexible printed wiring board  101 . As viewed in the Z direction perpendicular to the main surface  1010  of the flexible printed wiring board  101 , the reinforcing member  130 A is disposed in a region including the entirety of the connector  109 . In addition, as viewed in the Z direction, the reinforcing member  140 A is disposed in a region including the entirety of the connector  120 . 
     The configuration of the reinforcing member  140 A is substantially the same as the configuration of the reinforcing member  130 A. In addition, the positional relationship of the reinforcing member  140 A with the connector  120 , the wiring portion  105 , and the pad  106  is substantially the same as the positional relationship of the reinforcing member  130 A with the connector  109 , the wiring portion  103 , and the pad  104 . Therefore, detailed description of the reinforcing member  140 A will be omitted. 
     The insulating member  1351 A of the reinforcing member  130 A serves as an example of a first insulating member. The insulating member  1352 A of the reinforcing member  130 A serves as an example of a second insulating member. 
     The insulating member  1352 A is formed in a uniformly constant thickness in a direction parallel to the main surface  1010 . Examples of the material of the insulating member  1352 A include resins such as polyimide, PET, and glass epoxy, and among the resins, glass epoxy, which has high rigidity, is particularly preferable. The insulating member  1351 A is formed in the same thickness as the insulating member  1352 A. The material of the insulating member  1351 A is, for example, alumina. 
     Among the plurality of pads  104 , description will be given focusing on one pad  104 . As viewed in the Z direction, the reinforcing member  130 A includes a first portion P 1 A disposed in a region including at least part of the pad  104 , and a second portion P 2 A disposed around the first portion P 1 A as viewed in the Z direction. It is preferable that the region of the first portion P 1 A includes 90% or more of the area of the pad  104  as viewed in the Z direction. In the second embodiment, as viewed in the Z direction, the first portion P 1 A is disposed in a region including the entirety of the pad  104 . 
     Focusing on the plurality of the pads  104 , that is, all the pads  104 , the first portion P 1 A is disposed in a region including entirety of the plurality of pads  104  as viewed in the Z direction. Further, the second portion P 2 A is disposed around the first portion PIA so as to surround the first portion P 1 A as viewed in the Z direction. 
     Here, a differential signal is transmitted through the pair of signal lines  110  of the differential line pair  111 . Therefore, a characteristic impedance Z 1 A of the wiring portion  102  described below is a differential impedance of the pair of wiring portions  102  in the differential line pair  111 . In addition, a characteristic impedance Z 2 A of the wiring portion  103  is a differential impedance of the pair of wiring portions  103  in the differential line pair  111 . In addition, a characteristic impedance Z 3 A of the pad  104  is a differential impedance of the pair of pads  104  in the differential line pair  111 . 
     In the second embodiment, a member constituting the first portion P 1 A is a member having a nature that reduces the characteristic impedance Z 3 A of the pad  104  more than a member constituting the second portion P 2 A does. 
     Specifically, the first portion P 1 A is constituted by the insulating member  1351 A described above. As viewed in the Z direction, the insulating member  1351 A has the same shape and size as the first portion P 1 A. In addition, the second portion P 2 A is constituted by the insulating member  1352 A disposed around the insulating member  1351 A. As viewed in the Z direction, the insulating member  1352 A has the same shape and size as the second portion P 2 A. The insulating member  1351 A is formed from a different material from the insulating member  1352 A but in the same thickness as the insulating member  1352 A, and has a higher relative permittivity than the insulating member  1352 A. 
     As described above, in the second embodiment, the insulating member  1351 A is a member constituting the first portion P 1 A. In addition, in the second embodiment, the insulating member  1352 A formed from a different material from the insulating member  1351 A is a member constituting the second portion P 2 A. The insulating member  1351 A has a nature that reduces the characteristic impedance of an opposing conductor more than the insulating member  1352 A does. The reinforcing member  130 X of the comparative example is formed from the same material as and in the same thickness as the insulating member  1352 A. Therefore, the characteristic impedance Z 3 A of the second embodiment is reduced more than the characteristic impedance Z 3 X of the comparative example. That is, since the insulating member  1351 A is disposed to oppose the pad  104 , the characteristic impedance Z 3 A of the pad  104  is reduced. As a result of this, the absolute value of the difference (Z 3 A-Z 2 A) between the characteristic impedance Z 2 A of the wiring portion  103  and the characteristic impedance Z 3 A of the pad  104  can be reduced. Therefore, in the signal line  110 , generation of the reflection wave of the digital signal D 2 , that is, generation of the noise can be reduced, and thus the quality of the digital signal D 2  transmitted through the signal line  110  can be improved. 
     A width W 204  of the pad  104  is preferably larger than each of A width W 202  of the wiring portion  102  and a width W 203  of the wiring portion  103  for bonding the terminal  1091  of the connector  109  thereto. In addition, a distance S 204  between the pair of pads  104  is preferably larger than each of a distance S 202  between a pair of wiring portions  102  and a distance S 203  between a pair of wiring portions  103  for bonding the terminal  1091  of the connector  109  thereto. 
     In addition, the width W 203  of the wiring portion  103  is preferably equal to or less than the width W 202  of the wiring portion  102 . As viewed in the Z direction, the wiring portion  103  overlaps the second portion P 2 A of the reinforcing member  130 A having a higher relative permittivity than the air. Therefore, the width W 203  of the wiring portion  103  may be equal to the width W 202  of the wiring portion  102  not overlapping the reinforcing member  130 A, but is preferably smaller than the width W 202 . As a result of this, the characteristic impedance Z 2 A of the wiring portion  103  is higher than the characteristic impedance Z 2 X of the wiring portion  103 X of the comparative example. Therefore, the absolute value of the difference (Z 2 A-ZIA) between the characteristic impedance Z 1 A of the wiring portion  102  and the characteristic impedance Z 2 A of the wiring portion  103  can be reduced. In addition, the absolute value of the difference (Z 3 A-Z 2 A) between the characteristic impedance Z 2 A of the wiring portion  103  and the characteristic impedance Z 3 A of the pad  104  can be reduced. Therefore, in the signal line  110 , generation of the reflection wave of the digital signal D 2 , that is, generation of the noise can be more effectively reduced, and the quality of the digital signal D 2  transmitted through the signal line  110  can be more effectively improved. 
     In addition, the distance S 203  between a pair of the wiring portions  103  is preferably equal to or larger than the distance S 202  between a pair of the wiring portions  102 . As viewed in the Z direction, the pair of the wiring portions  103  overlaps the second portion P 2 A of the reinforcing member  130 A having a higher relative permittivity than the air. Therefore, the distance S 203  between the pair of the wiring portions  103  may be equal to the distance S 202  of the pair of the wiring portions  102  not overlapping the reinforcing member  130 A, but is preferably larger than the distance S 202 . As a result of this, the characteristic impedance Z 2 A is higher than the characteristic impedance Z 2 X of the comparative example. Therefore, the absolute value of the difference (Z 2 A-Z 1 A) between the characteristic impedance Z 1 A and the characteristic impedance Z 2 A and the absolute value of the difference (Z 3 A-Z 2 A) between the characteristic impedance Z 2 A and the characteristic impedance Z 3 A can be reduced. Therefore, in the signal line  110 , generation of the reflection wave of the digital signal D 2 , that is, generation of the noise can be more effectively reduced, and the quality of the digital signal D 2  transmitted through the signal line  110  can be more effectively improved. 
     In addition, as viewed in the Z direction, although the wiring portion  103  may partially overlap the first portion P 1 A, since the first portion P 1 A has a nature that reduces the characteristic impedance of an opposing conductor, it is preferable that the wiring portion  103  does not overlap the first portion P 1 A. As a result of this, reduction of the characteristic impedance Z 2 A of the wiring portion  103  can be suppressed, and the absolute value of the difference (Z 2 A-Z 1 A) and the absolute value of the difference (Z 3 A-Z 2 A) can be reduced. Therefore, in the signal line  110 , generation of the reflection wave of the digital signal D 2 , that is, generation of the noise can be more effectively reduced, and the quality of the digital signal D 2  transmitted through the signal line  110  can be more effectively improved. 
     To be noted, although the reinforcing member  130 A has been described, since the reinforcing member  140 A has substantially the same configuration as the reinforcing member  130 A, the quality of the digital signal D 2  transmitted through the signal line  110  can be more effectively improved. 
     In addition, the first portion PIA may further include the conductive member  136  having substantially the same configuration as in the first embodiment. 
     Example 2 
     Simulation of differential impedance was performed for the transmission module  100 A according to the second embodiment. HyperLynx available from Mentor Graphics was used for the simulation of the differential impedance. 
     The thickness of the base layer  1011  is denoted by T 2011 , the thickness of the conductor layer  1012  is denoted by T 2012 , and the thickness of a portion of the cover layer  1013  overlapping the signal line  110  on the conductor layer  1012  is denoted by T 2013 . In addition, the thickness of the reinforcing member  130 A, that is, the thickness of the insulating member  1351 A and  1352 A is denoted by T 205 . In the simulation, parameter values of the respective thicknesses were as follows: T 2011 =12.5 μm; T 2012 =12 μm; T 2013 =27.5 μm; and T 205 =430 μm. To be noted, the thickness  1205  of the reinforcing member  130 A includes a thickness of 30 μm of an adhesive between the reinforcing member  130 A and the base layer  1011 . The relative permittivity of the base layer  1011  was set to 3.3, the relative permittivity of the cover layer  1013  was set to 3.6, the relative permittivity of the insulating member  1352 A of the reinforcing member  130 A was set to 4.7, and the relative permittivity of the adhesive was set to 4.0. The relative permittivity of the insulating member  1351 A was set to 9.8. The conductivity of the signal line  110  was set to 1.724×10 −8  Ωm. 
     The width of the wiring portion  102  is denoted by W 202 , the width of the wiring portion  103  is denoted by W 203 , and the width of the pad  104  is denoted by W 204 . In addition, the distance between a pair of the wiring portions  102  in the differential line pair  111  is denoted by S 202 , the distance between a pair of the wiring portions  103  in the differential line pair  111  is denoted by S 203 , and the distance between a pair of the pads  104  in the differential line pair  111  is denoted by S 204 . In the simulation, the values of the widths and the distances were as follows: W 202 =150 μm; S 202 =45 μm; W 203 =120 μm; S 203 =75 μm; W 204 =250 μm: and S 204 =150 μm. As described above, in Example 2, W 204 &gt;W 202 &gt;W 203  and S 204 &gt;S 203 &gt;S 202  hold. 
     In Example 2, the characteristic impedance (differential impedance) ZIA of the wiring portion  102  was 103.8Ω. The characteristic impedance (differential impedance) Z 2 A of the wiring portion  103  was 99.2Ω. The characteristic impedance (differential impedance) Z 3 A of the pad  104  was 100.8Ω. 
     In Comparative Example 1, the difference (Z 3 X-Z 2 X) in the characteristic impedance was 32.7Ω. In contrast, in Example 2, the difference (Z 3 A-Z 2 A) in the characteristic impedance was 1.6Ω. Therefore, the absolute value |Z 3 A-Z 2 A| of the difference in the characteristic impedance of Example 2 was smaller than the absolute value |Z 3 X-Z 2 X| of the difference in the characteristic impedance of Comparative Example 1, which indicates that the characteristic impedance was more consistent in Example 2 than in Comparative Example 1. Therefore, in Example 2, generation of the reflection wave can be reduced. 
     In Comparative Example 1, the difference (Z 2 X-Z 1 X) in the characteristic impedance was −18.3Ω. In contrast, in Example 2, the difference (Z 2 A-Z 1 A) in the characteristic impedance was −4.6Ω. Therefore, the absolute value |Z 2 A-Z 1 A| of the difference in the characteristic impedance of Example 2 was smaller than the absolute value |Z 2 X-Z 1 X| of the difference in the characteristic impedance of Comparative Example 1, which indicates that the characteristic impedance was more consistent in Example 2 than in Comparative Example 1. Therefore, in Example 2, generation of the reflection wave can be reduced. 
     Third Embodiment 
     Next, a transmission module of a third embodiment will be described.  FIG.  11 A  is a plan view of a transmission module  100 B according to the third embodiment.  FIG.  11 B  is a longitudinal section view of the transmission module  100 B according to the third embodiment.  FIGS.  11 A and  11 B  schematically illustrate the transmission module  100 B. In the third embodiment, the transmission module  100 B is applied to the electronic unit  500  in place of the transmission module  100  of the first embodiment. Therefore, description of elements substantially the same as in the first embodiment will be omitted. 
     The transmission module  10 B of the third embodiment includes the flexible printed wiring board  101 , the connector  109 , and the connector  120  described in the first embodiment. To be noted, in  FIGS.  11 A and  11 B , the flexible printed wiring board  101  is stretched straight.  FIG.  12 A  is a cross-section view of the transmission module  100 B taken along a line XIIA-XIIA of  FIG.  11 A .  FIG.  12 B  is a cross-section view of the transmission module  100 B taken along a line XIIB-XIIB of  FIG.  1 A .  FIG.  12 C  is a cross-section view of the transmission module  100 B taken along a line XIIC-XIIC of  FIG.  11 A . To be noted, in  FIG.  12 C , illustration of the connector  109  is omitted. 
     The flexible printed wiring board  101  includes a plurality of signal lines  110  used for transmission of the digital signal D 2 . Among the plurality of signal lines  110 , a pair of adjacent signal lines  110  constitute a differential line pair III that is a transmission path used for transmitting a differential signal. The signal lines  110  each include the wiring portion  102 , the wiring portion  103 , the pad  104 , the wiring portion  105 , and the pad  106 . 
     The transmission module  100 B of the third embodiment includes a reinforcing member  130 B disposed at a position opposing the connector  109  with the flexible printed wiring board  101  therebetween. In addition, the transmission module  100 B includes a reinforcing member  140 B disposed at a position opposing the connector  120  with the flexible printed wiring board  101  therebetween. 
     The reinforcing member  130 B includes insulating members  1351 B,  1352 B, and  1353 B that are electrically insulating. The relative permittivity of the insulating member  1351 B is higher than the relative permittivity of the insulating member  1352 B. The insulating member  1353 B is formed from the same material as the insulating member  1352 B and has the same relative permittivity as the insulating member  1352 B, but is thinner than the insulating member  1352 B. 
     The reinforcing member  140 B includes insulating members  1451 B,  1452 B, and  1453 B that are electrically insulating. The relative permittivity of the insulating member  1451 B is higher than the relative permittivity of the insulating member  1452 B. The insulating member  1453 B is formed from the same material as the insulating member  1452 B and has the same relative permittivity as the insulating member  1452 B, but is thinner than the insulating member  1452 B. 
     The reinforcing member  130 B is a member for reinforcing the flexible printed wiring board  101  to suppress breakage of the signal lines  110  when attaching or detaching the connector  109  to or from the connector  204 . Therefore, the reinforcing member  130 B is thicker than the flexible printed wiring board  101 . Similarly, the reinforcing member  140 B is a member for reinforcing the flexible printed wiring board  101  to suppress breakage of the signal lines  110  when attaching or detaching the connector  120  to or from the connector  305 . Therefore, the reinforcing member  140 B is thicker than the flexible printed wiring board  101 . As viewed in the Z direction perpendicular to the main surface  1010  of the flexible printed wiring board  101 , the reinforcing member  130 B is disposed in a region including the entirety of the connector  109 . In addition, as viewed in the Z direction, the reinforcing member  140 B is disposed in a region including the entirety of the connector  120 . 
     The configuration of the reinforcing member  140 B is substantially the same as the configuration of the reinforcing member  130 B. In addition, the positional relationship of the reinforcing member  140 B with the connector  120 , the wiring portion  105 , and the pad  106  is substantially the same as the positional relationship of the reinforcing member  130 B with the connector  109 , the wiring portion  103 , and the pad  104 . Therefore, detailed description of the reinforcing member  140 B will be omitted. 
     The insulating member  1351 B of the reinforcing member  130 B serves as an example of a first insulating member. The insulating member  1352 B of the reinforcing member  130 B serves as an example of a second insulating member. The insulating member  1353 B of the reinforcing member  130 B serves as an example of a third insulating member. 
     The insulating member  1352 B is formed in a uniformly constant thickness in a direction parallel to the main surface  1010 . Examples of the material of the insulating member  1352 B include resins such as polyimide, PET, and glass epoxy, and among the resins, glass epoxy, which has high rigidity, is particularly preferable. The material of the insulating member  1351 B is, for example, titanium oxide. The material of the insulating member  1353 B is different from that of the insulating member  1351 B, and is the same as that of the insulating member  1352 B. 
     Among the plurality of pads  104 , description will be given focusing on one pad  104 . As viewed in the Z direction, the reinforcing member  130 B includes a first portion P 1 B disposed in a region including at least part of the pad  104 , and a second portion P 2 B disposed around the first portion P 1 B as viewed in the Z direction. It is preferable that the region of the first portion P 1 B includes 90% or more of the area of the pad  104  as viewed in the Z direction. In the third embodiment, as viewed in the Z direction, the first portion P 1 B is disposed in a region including the entirety of the pad  104 . 
     Focusing on the plurality of the pads  104 , that is, all the pads  104 , the first portion P 1 B is disposed in a region including entirety of the plurality of pads  104  as viewed in the Z direction. Further, the second portion P 2 B is disposed around the first portion P 1 B so as to surround the first portion P 1 B as viewed in the Z direction. 
     Here, a differential signal is transmitted through the pair of signal lines  110  of the differential line pair  111 . Therefore, the characteristic impedance Z 1 B of the wiring portion  102  described below is a differential impedance of the pair of wiring portions  102  in the differential line pair  111 . In addition, the characteristic impedance Z 2 B of the wiring portion  103  is a differential impedance of the pair of wiring portions  103  in the differential line pair  111 . In addition, the characteristic impedance Z 3 B of the pad  104  is a differential impedance of the pair of pads  104  in the differential line pair  111 . 
     In the third embodiment, a member constituting the first portion P 1 B is a member having a nature that reduces the characteristic impedance Z 3 B of the pad  104  more than a member constituting the second portion P 2 B does. 
     Specifically, the first portion P 1 B is constituted by the insulating members  1351 B and  1353 B described above. As viewed in the Z direction, the insulating members  1351 B and  1353 B are laminated in the thickness direction of the flexible printed wiring board  101 , that is, the Z direction. As viewed in the Z direction, the insulating members  1351 B and  1353 B each have the same shape and size as the first portion P 1 B. 
     In addition, the second portion P 2 B is constituted by the insulating member  1352 B disposed around the insulating member  1351 B. As viewed in the Z direction, the insulating member  1352 B has the same shape and size as the second portion P 2 B. The relative permittivity of the insulating member  1353 B is equal to the relative permittivity of the insulating member  1352 B, and is different from the relative permittivity of the insulating member  1351 B. In the third embodiment, the insulating member  1351 B has a higher relative permittivity than the insulating members  1352 B and  1353 B. 
     As described above, in the third embodiment, the insulating members  1351 B and  1353 B formed from different materials are members constituting the first portion P 1 B. In addition, in the third embodiment, the insulating member  1352 B formed from a different material from the insulating member  1351 B is a member constituting the second portion P 2 B. The insulating member  1351 B has a nature that reduces the characteristic impedance of an opposing conductor more than the insulating member  1352 B does. The reinforcing member  130 X of the comparative example is formed from the same material as and in the same thickness as the insulating member  1352 B. Therefore, a characteristic impedance Z 3 B of the third embodiment is reduced more than the characteristic impedance Z 3 X of the comparative example. That is, since the laminate of the insulating members  1351 B and  1353 B is disposed to oppose the pad  104 , the characteristic impedance Z 3 B of the pad  104  is reduced. As a result of this, the absolute value of the difference (Z 3 B-Z 2 B) between a characteristic impedance Z 2 B of the wiring portion  103  and the characteristic impedance Z 3 B of the pad  104  can be reduced. Therefore, in the signal line  110 , generation of the reflection wave of the digital signal D 2 , that is, generation of the noise can be reduced, and thus the quality of the digital signal D 2  transmitted through the signal line  110  can be improved. 
     A width W 304  of the pad  104  is preferably larger than each of a width W 302  of the wiring portion  102  and a width W 303  of the wiring portion  103  for bonding the terminal  1091  of the connector  109  thereto. In addition, a distance S 304  between the pair of pads  104  is preferably larger than each of a distance S 302  between a pair of wiring portions  102  and a distance S 303  between a pair of wiring portions  103  for bonding the terminal  1091  of the connector  109  thereto. 
     In addition, the width W 303  of the wiring portion  103  is preferably equal to or less than the width W 302  of the wiring portion  102 . As viewed in the Z direction, the wiring portion  103  overlaps the second portion P 2 B of the reinforcing member  130 B having a higher relative permittivity than the air. Therefore, the width W 303  of the wiring portion  103  may be equal to the width W 302  of the wiring portion  102  not overlapping the reinforcing member  130 B, but is preferably smaller than the width W 302 . As a result of this, the characteristic impedance Z 2 B of the wiring portion  103  is higher than the characteristic impedance Z 2 X of the wiring portion  103 X of the comparative example. Therefore, the absolute value of the difference (Z 2 B-Z 1 B) between a characteristic impedance Z 1 B of the wiring portion  102  and the characteristic impedance Z 2 B of the wiring portion  103  can be reduced. In addition, the absolute value of the difference (Z 3 B-Z 2 B) between the characteristic impedance Z 2 B of the wiring portion  103  and the characteristic impedance Z 3 B of the pad  104  can be reduced. Therefore, in the signal line  110 , generation of the reflection wave of the digital signal D 2 , that is, generation of the noise can be more effectively reduced, and the quality of the digital signal D 2  transmitted through the signal line  110  can be more effectively improved. 
     In addition, the distance S 303  between a pair of the wiring portions  103  is preferably equal to or larger than the distance S 302  between a pair of the wiring portions  102 . As viewed in the Z direction, the pair of the wiring portions  103  overlaps the second portion P 2 B of the reinforcing member  130 B having a higher relative permittivity than the air. Therefore, the distance S 303  between the pair of the wiring portions  103  may be equal to the distance S 302  of the pair of the wiring portions  102  not overlapping the reinforcing member  130 B, but is preferably larger than the distance S 302 . As a result of this, the characteristic impedance Z 2 B is higher than the characteristic impedance Z 2 X of the comparative example. Therefore, the absolute value of the difference (Z 2 B-Z 1 B) between the characteristic impedance Z 1 B and the characteristic impedance Z 2 B and the absolute value of the difference (Z 3 B-Z 2 B) between the characteristic impedance Z 2 B and the characteristic impedance Z 3 B can be reduced. Therefore, in the signal line  110 , generation of the reflection wave of the digital signal D 2 , that is, generation of the noise can be more effectively reduced, and the quality of the digital signal D 2  transmitted through the signal line  110  can be more effectively improved. 
     In addition, as viewed in the Z direction, although the wiring portion  103  may partially overlap the first portion P 1 B, since the first portion P 1 B has a nature that reduces the characteristic impedance of an opposing conductor, it is preferable that the wiring portion  103  does not overlap the first portion P 1 B. As a result of this, reduction of the characteristic impedance Z 2 B of the wiring portion  103  can be suppressed, and the absolute value of the difference (Z 2 B-Z 1 B) and the absolute value of the difference (Z 3 B-Z 2 B) can be reduced. Therefore, in the signal line  110 , generation of the reflection wave of the digital signal D 2 , that is, generation of the noise can be more effectively reduced, and the quality of the digital signal D 2  transmitted through the signal line  110  can be more effectively improved. 
     To be noted, although the reinforcing member  130 B has been described, since the reinforcing member  140 B has substantially the same configuration as the reinforcing member  130 B, the quality of the digital signal D 2  transmitted through the signal line  110  can be more effectively improved. 
     In addition, the first portion P 1 B may further include the conductive member  136  having substantially the same configuration as in the first embodiment. 
     Example 3 
     Simulation of differential impedance was performed for the transmission module  100 B according to the third embodiment. HyperLynx available from Mentor Graphics was used for the simulation of the differential impedance. 
     The thickness of the base layer  1011  is denoted by T 3011 , the thickness of the conductor layer  1012  is denoted by T 3012 , and the thickness of a portion of the cover layer  1013  overlapping the signal line  110  on the conductor layer  1012  is denoted by T 3013 . In addition, the thickness of the reinforcing member  130 B, that is, the thickness of the insulating member  1352 B is denoted by T 305 . The thickness of the insulating member  1351 B is denoted by T 3051 , and the thickness of the insulating member  1353 B is denoted by T 3053 . The sum of the thickness T 3051  and the thickness T 3053  equals to the thickness T 305 . In the simulation, parameter values of the respective thicknesses were as follows: T 3011 =12.5 μm; T 3012 =12 μm; T 3013 =27.5 μm; T 305 =415 μm; T 3053 =100 μm; and T 3051 =315 μm. To be noted, the thickness T 305  of the reinforcing member  130 B and the thickness T 3053  of the insulating member  1353 B includes a thickness of 15 μm of an adhesive between the reinforcing member  130 B and the base layer  1011 . The relative permittivity of the base layer  1011  was set to 3.3, the relative permittivity of the cover layer  1013  was set to 3.6, the relative permittivity of the insulating members  1352 B and  1353 B was set to 4.7, and the relative permittivity of the adhesive was set to 4.0. The relative permittivity of the insulating member  1351 B was set to 30. The conductivity of the signal line  110  was set to 1.724×10 −8  Ωm. 
     The width of the wiring portion  102  is denoted by W 302 , the width of the wiring portion  103  is denoted by W 303 , and the width of the pad  104  is denoted by W 304 . In addition, the distance between a pair of the wiring portions  102  in the differential line pair  111  is denoted by S 302 , the distance between a pair of the wiring portions  103  in the differential line pair  111  is denoted by S 303 , and the distance between a pair of the pads  104  in the differential line pair  111  is denoted by S 304 . In the simulation, the values of the widths and the distances were as follows: W 302 =150 μm; S 302 =45 μm: W 303 =130 μm: S 303 =65 μm; W 304 =250 μm; and S 304 =150 μm. As described above, in Example 3. W 304 &gt;W 302 &gt;W 303  and S 304 &gt;S 303 &gt;S 302  hold. 
     In Example 3, the characteristic impedance (differential impedance) Z 1 B of the wiring portion  102  was 103.8Ω. The characteristic impedance (differential impedance) Z 2 B of the wiring portion  103  was 100.0Ω. The characteristic impedance (differential impedance) Z 3 B of the pad  104  was 100.8Ω. 
     In Comparative Example 1, the difference (Z 3 X-Z 2 X) in the characteristic impedance was 32.7Ω. In contrast, in Example 3, the difference (Z 3 B-Z 2 B) in the characteristic impedance was 0.8Ω. Therefore, the absolute value |Z 3 B-Z 2 B| of the difference in the characteristic impedance of Example 3 was smaller than the absolute value |Z 3 X-Z 2 X| of the difference in the characteristic impedance of Comparative Example 1, which indicates that the characteristic impedance was more consistent in Example 3 than in Comparative Example 1. Therefore, in Example 3, generation of the reflection wave can be reduced. 
     In Comparative Example 1, the difference (Z 2 X-Z 1 X) in the characteristic impedance was −18.3Ω. In contrast, in Example 3, the difference (Z 2 B-Z 1 B) in the characteristic impedance was −3.8Ω. Therefore, the absolute value |Z 2 B-Z 1 B| of the difference in the characteristic impedance of Example 3 was smaller than the absolute value |Z 2 X-Z 1 X| of the difference in the characteristic impedance of Comparative Example 1, which indicates that the characteristic impedance was more consistent in Example 3 than in Comparative Example 1. Therefore, in Example 3, generation of the reflection wave can be reduced. 
     Fourth Embodiment 
     Next, a transmission module of a fourth embodiment will be described.  FIG.  13 A  is a plan view of a transmission module  100 C according to the fourth embodiment.  FIG.  13 B  is a longitudinal section view of the transmission module  100 C according to the fourth embodiment.  FIGS.  13 A and  13 B  schematically illustrate the transmission module  100 C. In the fourth embodiment, the transmission module  100 C is applied to the electronic unit  500  instead of the transmission module  100  of the first embodiment. Therefore, description of elements substantially the same as in the first embodiment will be omitted. 
     The transmission module  100 C of the fourth embodiment includes the flexible printed wiring board  101 , the connector  109 , and the connector  120  described in the first embodiment. To be noted, in  FIGS.  13 A and  13 B , the flexible printed wiring board  101  is stretched straight.  FIG.  14 A  is a cross-section view of the transmission module  100 C taken along a line XIVA-XIVA of  FIG.  13 A .  FIG.  14 B  is a cross-section view of the transmission module  100 C taken along a line XIVB-XIVB of  FIG.  13 A .  FIG.  14 C  is a cross-section view of the transmission module  100 C taken along a line XIVC-XIVC of  FIG.  13 A . To be noted, in  FIG.  14 C , illustration of the connector  109  is omitted. 
     The flexible printed wiring board  101  includes a plurality of signal lines  110  used for transmission of the digital signal D 2 . Among the plurality of signal lines  110 , pairs of adjacent signal lines  110  each constitute a differential line pair  111  that is a transmission path used for transmitting a differential signal. The signal lines  110  each include the wiring portion  102 , the wiring portion  103 , the pad  104 , the wiring portion  105 , and the pad  106 . 
     The transmission module  100 C of the fourth embodiment includes a reinforcing member  130 C disposed at a position opposing the connector  10 ) with the flexible printed wiring board  101  therebetween. In addition, the transmission module  100 C includes a reinforcing member  140 C disposed at a position opposing the connector  120  with the flexible printed wiring board  101  therebetween. 
     The reinforcing member  130 C includes insulating members  1351 C and  1352 C that are electrically insulating. The relative permittivity of the insulating member  1351 C is equal to the relative permittivity of the insulating member  1352 C. The insulating member  1351 C is disposed at a position opposing the plurality of pads  104 . The insulating member  1351 C is thicker than the insulating member  1352 C. 
     The reinforcing member  140 C includes insulating members  1451 C and  1452 C that are electrically insulating. The relative permittivity of the insulating member  1451 C is equal to the relative permittivity of the insulating member  1452 C. The insulating member  1451 C is disposed at a position opposing the plurality of pads  106 . The insulating member  1451 C is thicker than the insulating member  1452 C. 
     The reinforcing member  130 C is a member for reinforcing the flexible printed wiring board  101  to suppress breakage of the signal lines  110  when attaching or detaching the connector  109  to or from the connector  204 . Therefore, the insulating member  1352 C of the reinforcing member  130 C is thicker than the flexible printed wiring board  101 . Similarly, the reinforcing member  140 C is a member for reinforcing the flexible printed wiring board  101  to suppress breakage of the signal lines  110  when attaching or detaching the connector  120  to or from the connector  305 . Therefore, the insulating member  1452 C of the reinforcing member  140 C is thicker than the flexible printed wiring board  101 . As viewed in the Z direction perpendicular to the main surface  1010  of the flexible printed wiring board  101 , the reinforcing member  130 C is disposed in a region including the entirety of the connector  109 . In addition, as viewed in the Z direction, the reinforcing member  140 C is disposed in a region including the entirety of the connector  120 . 
     The configuration of the reinforcing member  140 C is substantially the same as the configuration of the reinforcing member  130 C. In addition, the positional relationship of the reinforcing member  140 C with the connector  120 , the wiring portion  105 , and the pad  106  is substantially the same as the positional relationship of the reinforcing member  130 C with the connector  109 , the wiring portion  103 , and the pad  104 . Therefore, detailed description of the reinforcing member  140 C will be omitted. 
     The insulating member  1351 C of the reinforcing member  130 C serves as an example of a first insulating member. The insulating member  1352 C of the reinforcing member  130 C serves as an example of a second insulating member. 
     The insulating member  1352 C is formed in a uniformly constant thickness in a direction parallel to the main surface  1010 . Examples of the material of the insulating member  1352 C include resins such as polyimide. PET, and glass epoxy, and among the resins, glass epoxy, which has high rigidity, is particularly preferable. The insulating member  1351 C is formed from the same material as the insulating member  1352 C. By using the same material for the insulating members  1351 C and  1352 C, the manufacturing cost can be reduced. 
     Among the plurality of pads  104 , description will be given focusing on one pad  104 . As viewed in the Z direction, the reinforcing member  130 C includes a first portion P 1 C disposed in a region including at least part of the pad  104 , and a second portion P 2 C disposed around the first portion P 1 C as viewed in the Z direction. It is preferable that the region of the first portion P 1 C includes 90% or more of the area of the pad  104  as viewed in the Z direction. In the fourth embodiment, as viewed in the Z direction, the first portion P 1 C is disposed in a region including the entirety of the pad  104 . 
     Focusing on the plurality of the pads  104 , that is, all the pads  104 , the first portion P 1 C is disposed in a region including the entirety of the plurality of pads  104  as viewed in the Z direction. Further, the second portion P 2 C is disposed around the first portion P 1 C so as to surround the first portion P 1 C as viewed in the Z direction. 
     Here, a differential signal is transmitted through the pair of signal lines  110  of the differential line pair  111 . Therefore, a characteristic impedance Z 1 C of the wiring portion  102  described below is a differential impedance of the pair of wiring portions  102  in the differential line pair  111 . In addition, a characteristic impedance Z 2 C of the wiring portion  103  is a differential impedance of the pair of wiring portions  103  in the differential line pair  111 . In addition, a characteristic impedance Z 3 C of the pad  104  is a differential impedance of the pair of pads  104  in the differential line pair  111 . 
     In the fourth embodiment, a member constituting the first portion P 1 C is a member having a nature that reduces the characteristic impedance Z 3 C of the pad  104  more than a member constituting the second portion P 2 C does. 
     Specifically, the first portion P 1 C is constituted by the insulating member  1351 C described above. As viewed in the Z direction, the insulating member  1351 C has the same shape and size as the first portion P 1 C. In addition, the second portion P 2 C is constituted by the insulating member  1352 C disposed around the insulating member  1351 C. As viewed in the Z direction, the insulating member  1352 C has the same shape and size as the second portion P 2 C. The insulating member  1351 C has the same relative permittivity as the insulating member  1352 C. 
     To be noted, although the insulating member  1351 C may be formed integrally with the insulating member  1352 C, the insulating member  1351 C may be divided into two portions  1351 C- 1  and  1351 C- 2  in view of ease of manufacture thereof. In this case, the portions  1351 C- 1  and  1351 C- 2  may be joined using an adhesive. In addition, in this case, the insulating member  1352 C may be integrally formed with the portion  1351 C- 1 . 
     As described above, in the fourth embodiment, the insulating member  1351 C is a member constituting the first portion P 1 C. In addition, in the fourth embodiment, the insulating member  1352 C is a member constituting the second portion P 2 C. In addition, the insulating member  1351 C is thicker than the insulating member  1352 C. Therefore, the insulating member  1351 C has a nature that reduces the characteristic impedance of an opposing conductor more than the insulating member  1352 C does. The reinforcing member  130 X of the comparative example is formed from the same material as and in the same thickness as the insulating member  1352 C. Therefore, the characteristic impedance Z 3 C of the fourth embodiment is reduced more than the characteristic impedance Z 3 X of the comparative example. That is, since the insulating member  1351 C is disposed to oppose the pad  104 , the characteristic impedance Z 3 C of the pad  104  is reduced. As a result of this, the absolute value of the difference (Z 3 C-Z 2 C) between the characteristic impedance Z 2 C of the wiring portion  103  and the characteristic impedance Z 3 C of the pad  104  can be reduced. Therefore, in the signal line  110 , generation of the reflection wave of the digital signal D 2 , that is, generation of the noise can be reduced, and thus the quality of the digital signal D 2  transmitted through the signal line  110  can be improved. 
     A width W 404  of the pad  104  is preferably larger than each of a width W 402  of the wiring portion  102  and a width W 403  of the wiring portion  103  for bonding the terminal  1091  of the connector  109  thereto. In addition, a distance S 404  between the pair of pads  104  is preferably larger than each of a distance S 402  between a pair of wiring portions  102  and a distance S 403  between a pair of wiring portions  103  for bonding the terminal  1091  of the connector  109  thereto. 
     In addition, the width W 403  of the wiring portion  103  is preferably equal to or less than the width W 402  of the wiring portion  102 . As viewed in the Z direction, the wiring portion  103  overlaps the second portion P 2 C of the reinforcing member  130 C having a higher relative permittivity than the air. Therefore, the width W 403  of the wiring portion  103  may be equal to the width W 402  of the wiring portion  102  not overlapping the reinforcing member  130 C, but is preferably smaller than the width W 402 . As a result of this, the characteristic impedance Z 2 C of the wiring portion  103  is higher than the characteristic impedance Z 2 X of the wiring portion  103 X of the comparative example. Therefore, the absolute value of the difference (Z 2 C-Z 1 C) between the characteristic impedance Z 1 C of the wiring portion  102  and the characteristic impedance Z 2 C of the wiring portion  103  can be reduced. In addition, the absolute value of the difference (Z 3 C-Z 2 C) between the characteristic impedance Z 2 C of the wiring portion  103  and the characteristic impedance Z 3 C of the pad  104  can be reduced. Therefore, in the signal line  110 , generation of the reflection wave of the digital signal D 2 , that is, generation of the noise can be more effectively reduced, and the quality of the digital signal D 2  transmitted through the signal line  110  can be more effectively improved. 
     In addition, the distance S 403  between a pair of the wiring portions  103  is preferably equal to or larger than the distance S 402  between a pair of the wiring portions  102 . As viewed in the Z direction, the pair of the wiring portions  103  overlaps the second portion P 2 C of the reinforcing member  130 C having a higher relative permittivity than the air. Therefore, the distance S 403  between the pair of the wiring portions  103  may be equal to the distance S 402  of the pair of the wiring portions  102  not overlapping the reinforcing member  130 C, but is preferably larger than the distance S 402 . As a result of this, the characteristic impedance Z 2 C is higher than the characteristic impedance Z 2 X of the comparative example. Therefore, the absolute value of the difference (Z 2 C-Z 1 C) between the characteristic impedance Z 1 C and the characteristic impedance Z 2 C and the absolute value of the difference (Z 3 C-Z 2 C) between the characteristic impedance Z 2 C and the characteristic impedance Z 3 C can be reduced. Therefore, in the signal line  110 , generation of the reflection wave of the digital signal D 2 , that is, generation of the noise can be more effectively reduced, and the quality of the digital signal D 2  transmitted through the signal line  110  can be more effectively improved. 
     In addition, as viewed in the Z direction, although the wiring portion  103  may partially overlap the first portion P 1 C, since the first portion P 1 C has a nature that reduces the characteristic impedance of an opposing conductor, it is preferable that the wiring portion  103  does not overlap the first portion P 1 C. As a result of this, reduction of the characteristic impedance Z 2 C of the wiring portion  103  can be suppressed, and the absolute value of the difference (Z 2 C-Z 1 C) and the absolute value of the difference (Z 3 C-Z 2 C) can be reduced. Therefore, in the signal line  110 , generation of the reflection wave of the digital signal D 2 , that is, generation of the noise can be more effectively reduced, and the quality of the digital signal D 2  transmitted through the signal line  110  can be more effectively improved. 
     To be noted, although the reinforcing member  130 C has been described, since the reinforcing member  140 C has substantially the same configuration as the reinforcing member  130 C, the quality of the digital signal D 2  transmitted through the signal line  110  can be more effectively improved. 
     In addition, the first portion P 1 C may further include the conductive member  136  having substantially the same configuration as in the first embodiment. In addition, part or the entirety of the insulating member  1351 C included in the first portion P 1 C may be formed from a material having a higher relative permittivity than the insulating member  1352 C. 
     Example 4 
     Simulation of differential impedance was performed for the transmission module  100 C according to the fourth embodiment. HyperLynx available from Mentor Graphics was used for the simulation of the differential impedance. 
     The thickness of the base layer  1011  is denoted by T 4011 , the thickness of the conductor layer  1012  is denoted by T 4012 , and the thickness of a portion of the cover layer  1013  overlapping the signal line  110  on the conductor layer  1012  is denoted by T 4013 . In addition, the thickness of the insulating member  1352 C of the reinforcing member  130 C is denoted by T 405 . The thickness of the portion  1351 C- 1  is also denoted by T 405 . The thickness of the insulating member  1351 C is denoted by T 4051 . The thickness of the portion  1351 C- 2 , which is a projecting portion, that is obtained by subtracting the thickness T 405  from the thickness T 4051  of the insulating member  1351 C is denoted by T 406 . In the simulation, parameter values of the respective thicknesses were as follows: T 4011 =12.5 μm; T 4012 =12 μm; T 4013 =27.5 μm; T 405 =415 μm, and T 406 =415 μm. To be noted, the thickness T 405  of the insulating member  1352 C includes a thickness of 15 μm of an adhesive between the insulating member  1352 C and the base layer  1011 . The thickness T 406  of the portion  1351 C- 2  includes a thickness of 15 μm between the portion  1351 C- 1  and the portion  1351 C- 2 . The relative permittivity of the base layer  1011  was set to 3.3, the relative permittivity of the cover layer  1013  was set to 3.6, the relative permittivity of the portions  1351 C- 1  and  1351 C- 2  and the insulating member  1352 C was set to 4.7, and the relative permittivity of the adhesive was set to 4.0. The conductivity of the signal line  110  was set to 1.724×10 −8  Ωm. 
     The width of the wiring portion  102  is denoted by W 402 , the width of the wiring portion  103  is denoted by W 403 , and the width of the pad  104  is denoted by W 404 . In addition, the distance between a pair of the wiring portions  102  in the differential line pair  111  is denoted by S 402 , the distance between a pair of the wiring portions  103  in the differential line pair  111  is denoted by S 403 , and the distance between a pair of the pads  104  in the differential line pair  111  is denoted by S 404 . In the simulation, the values of the widths and the distances were as follows: W 402 =150 μm: S 402 =45 μm; W 403 =130 μm; S 403 =65 μm; W 404 =290 μm: and S 404 =110 μm. As described above, in Example 4, W 404 &gt;W 402 &gt;W 403  and S 404 &gt;S 403 &gt;S 402  hold. 
     In Example 4, the characteristic impedance (differential impedance) Z 1 C of the wiring portion  102  was 103.8Ω. The characteristic impedance (differential impedance) Z 2 C of the wiring portion  103  was 100.0Ω. The characteristic impedance (differential impedance) Z 3 C of the pad  104  was 99.7Ω. 
     In Comparative Example 1, the difference (Z 3 X-Z 2 X) in the characteristic impedance was 32.7Ω. In contrast, in Example 4, the difference (Z 3 C-Z 2 C) in the characteristic impedance was −0.3Ω. Therefore, the absolute value |Z 3 C-Z 2 C| of the difference in the characteristic impedance of Example 4 was smaller than the absolute value |Z 3 X-Z 2 X| of the difference in the characteristic impedance of Comparative Example 1, which indicates that the characteristic impedance was more consistent in Example 4 than in Comparative Example 1. Therefore, in Example 4, generation of the reflection wave can be reduced. 
     In Comparative Example 1, the difference (Z 2 X-Z 1 X) in the characteristic impedance was −18.3Ω. In contrast, in Example 4, the difference (Z 2 C-Z 1 C) in the characteristic impedance was −3.8Ω. Therefore, the absolute value |Z 2 C-Z 1 C| of the difference in the characteristic impedance of Example 4 was smaller than the absolute value |Z 2 X-Z 1 X| of the difference in the characteristic impedance of Comparative Example 1, which indicates that the characteristic impedance was more consistent in Example 4 than in Comparative Example 1. Therefore, in Example 4, generation of the reflection wave can be reduced. 
     Fifth Embodiment 
     Next, a transmission module of a fifth embodiment will be described.  FIG.  15 A  is a plan view of a transmission module  100 D according to the fifth embodiment.  FIG.  15 B  is a longitudinal section view of the transmission module  100 D according to the fifth embodiment.  FIGS.  15 A and  15 B  schematically illustrate the transmission module  100 D. In the fifth embodiment, the transmission module  100 D is applied to the electronic unit  500  instead of the transmission module  100  of the first embodiment. Therefore, description of elements substantially the same as in the first embodiment will be omitted. 
     The transmission module  100 D of the fifth embodiment includes a flexible printed wiring board  101 D, and the connector  109  and the connector  120  described in the first embodiment. To be noted, in  FIGS.  15 A and  15 B , the flexible printed wiring board  101 D is stretched straight.  FIG.  16 A  is a cross-section view of the transmission module  100 D taken along a line XVIA-XVIA of  FIG.  15 A .  FIG.  16 B  is a cross-section view of the transmission module  100 D taken along a line XVIB-XVIB of  FIG.  15 A . To be noted, in  FIG.  16 B , illustration of the connector  109  is omitted. 
     The flexible printed wiring board  101 D includes a plurality of signal lines  110 D used for transmission of the digital signal D 2 . Among the plurality of signal lines  110 D, pairs of adjacent signal lines  110 D each constitute a differential line pair  111 D that is a transmission path used for transmitting a differential signal. Due to increase in the size of the image data, the digital signal D 2  is transmitted at a transmission speed of 10 Gbps or more per one differential line pair  111 D. The signal lines  110 D are each formed from a metal foil such as a copper foil. 
     The flexible printed wiring board  101 D includes the insulating layer  1014  that is described in the first embodiment that supports the plurality of signal lines  110 D. The insulating layer  1014  includes the base layer  1011  and the cover layer  1013 . The plurality of signal lines  110 D are disposed in a conductor layer  1012 D on the base layer  1011 . The base layer  1011  and the cover layer  1013  are formed from, for example, polyimide. 
     The transmission module  100 D of the fifth embodiment includes a reinforcing member  130 D disposed at a position opposing the connector  109  with the flexible printed wiring board  101 D therebetween. In addition, the transmission module  100 D includes a reinforcing member  140 D disposed at a position opposing the connector  120  with the flexible printed wiring board  101 D therebetween. The reinforcing member  130 D includes an insulating layer  135 D that is electrically insulating. The reinforcing member  140 D includes an insulating layer  145 D that is electrically insulating. The reinforcing member  130 D is a member for reinforcing the flexible printed wiring board  101 D to suppress breakage of the signal lines  110 D when attaching or detaching the connector  109  to or from the connector  204 . Therefore, the insulating layer  135 D of the reinforcing member  130 D is thicker than the flexible printed wiring board  101 D. Similarly, the reinforcing member  140 D is a member for reinforcing the flexible printed wiring board  101 D to suppress breakage of the signal lines  110 D when attaching or detaching the connector  120  to or from the connector  305 . Therefore, the insulating layer  145 D of the reinforcing member  140 D is thicker than the flexible printed wiring board  101 D. As viewed in the Z direction perpendicular to a main surface  1010 D of the flexible printed wiring board  101 D, the reinforcing member  130 D is disposed in a region including the entirety of the connector  109 . In addition, as viewed in the Z direction, the reinforcing member  140 D is disposed in a region including the entirety of the connector  120 . 
     The signal line  110 D includes a wiring portion  102 D as a main line, and a pad  104 D connected to the wiring portion  102 D. The wiring portion  102 D serves as an example of a first wiring portion, and is disposed at a position not overlapping the reinforcing member  130 D as viewed in the Z direction. The pad  104 D is disposed in a region overlapping the reinforcing member  130 D as viewed in the Z direction. The pad  104 D is bonded to the terminal  1091  of the connector  109  via solder or the like. 
     In addition, the signal line  110 D includes a pad  106 D connected to the wiring portion  102 D. The pad  106 D is disposed in a region overlapping the reinforcing member  140 D as viewed in the Z direction. The pad  106 D is bonded to the terminal  1201  of the connector  120  via solder or the like. 
     In the fifth embodiment, the reinforcing member  130 D includes a conductive member  136 D disposed on the insulating layer  135 D. In addition, in the fifth embodiment, the reinforcing member  140 D includes a conductive member  146 D disposed on the insulating layer  145 D. 
     The configuration of the reinforcing member  140 D is substantially the same as the reinforcing member  130 D. In addition, the positional relationship of the reinforcing member  140 D with the connector  120 , the wiring portion  102 D, and the pad  106 D is substantially the same as the positional relationship of the reinforcing member  130 D with the connector  109 , the wiring portion  102 D, and the pad  104 D. Therefore, detailed description of the reinforcing member  140 D will be omitted. 
     The insulating layer  135 D of the reinforcing member  130 D is formed in a uniformly constant thickness in a direction parallel to the main surface  1010 D. Examples of the material of the insulating layer  135 D include resins such as polyimide, PET, and glass epoxy, and among the resins, glass epoxy, which has high rigidity, is particularly preferable. The conductive member  136 D of the reinforcing member  130 D is disposed on the insulating layer  135 D. The conductive member  136 D is a metal foil such as a copper foil. The conductive member  136 D may be electrically connected to an unillustrated ground terminal of the connector  109 . 
     Among the plurality of pads  104 D, description will be given focusing on one pad  104 D. As viewed in the Z direction, the reinforcing member  130 D includes a first portion P 1 D disposed in a region including at least part of the pad  104 D, and a second portion P 2 D disposed around the first portion P 1 D as viewed in the Z direction. It is preferable that the region of the first portion PID includes 90% or more of the area of the pad  104 D as viewed in the Z direction. In the fifth embodiment, as viewed in the Z direction, the first portion P 1 D is disposed in a region including the entirety of the pad  104 D. 
     Focusing on the plurality of the pads  104 D, that is, all the pads  104 D, the first portion P 1 D is disposed in a region including entirety of the plurality of pads  104 D as viewed in the Z direction. Further, the second portion P 2 D is disposed around the first portion P 1 D so as to surround the first portion P 1 D as viewed in the Z direction. 
     Here, a differential signal is transmitted through the pair of signal lines  110 D of the differential line pair  111 D. Therefore, a characteristic impedance Z 1 D of the wiring portion  102 D described below is a differential impedance of the pair of wiring portions  102 D in the differential line pair  111 D. In addition, a characteristic impedance Z 3 D of the pad  104 D is a differential impedance of the pair of pads  104 D in the differential line pair  111 D. 
     In the fifth embodiment, a member constituting the first portion P 1 D is a member having a nature that reduces the characteristic impedance Z 3 D of the pad  104 D more than a member constituting the second portion P 2 D does. 
     Specifically, the first portion P 1 D is constituted by an insulating member  1351 D that is part of the insulating layer  135 D, and the conductive member  136 D disposed on the insulating member  1351 D. As viewed in the Z direction, the insulating member  1351 D and the conductive member  136 D each have the same shape and size as the first portion P 1 D. In addition, the second portion P 2 D is constituted by an insulating member  1352 D that is part of the insulating layer  135 D and disposed around the insulating member  1351 D. As viewed in the Z direction, the insulating member  1352 D has the same shape and size as the second portion P 2 D. The insulating member  1351 D serves as an example of a first insulating member. The insulating member  1352 D serves as an example of a second insulating member. The insulating member  1351 D is formed from the same material as the insulating member  1352 D and in the same thickness as the insulating member  1352 D, and has the same relative permittivity as the insulating member  1352 D. 
     As described above, in the fifth embodiment, the insulating member  1351 D and the conductive member  136 D are members constituting the first portion PID. In addition, in the fifth embodiment, the insulating member  1352 D having the same relative permittivity and the same thickness as the insulating member  1351 D is a member constituting the second portion P 2 D. The member constituted by the insulating member  1351 D and the conductive member  136 D has a nature that reduces the characteristic impedance of an opposing conductor more than the member constituted by the insulating member  1352 D does. Since the reinforcing member  130 X of the comparative example has substantially the same configuration as the insulating laver  135 D, the characteristic impedance Z 3 D of the fifth embodiment is reduced more than the characteristic impedance Z 3 X of the comparative example. That is, since the conductive member  136 D is disposed to oppose the pad  104 D with the insulating member  1351 D therebetween, the characteristic impedance Z 3 D of the pad  104 D is reduced. As a result of this, the absolute value of the difference (Z 3 D-Z 1 D) between the characteristic impedance Z 1 D of the wiring portion  102 D and the characteristic impedance Z 3 D of the pad  104 D can be reduced. Therefore, in the signal line  110 D, generation of the reflection wave of the digital signal D 2 , that is, generation of the noise can be reduced, and thus the quality of the digital signal D 2  transmitted through the signal line  110 D can be improved. 
     A width W 504  of the pad  104 D is preferably larger than the width W 502  of the wiring portion  102 D for bonding the terminal  1091  of the connector  109  thereto. In addition, a distance S 504  between the pair of pads  104 D is preferably larger than a distance S 502  between a pair of wiring portions  102 D for bonding the terminal  1091  of the connector  109  thereto. 
     To be noted, although a case where the first portion P 1 D of the fifth embodiment has substantially the same configuration as the first portion P 1  of the first embodiment has been described, the configuration is not limited to this. For example, the first portion P 1 D of the fifth embodiment may be configured in substantially the same manner as one of the first portions P 1 A to P 1 C of the second to fourth embodiments. 
     In addition, whereas the reinforcing member  130 D has been described, the reinforcing member  140 D has substantially the same configuration as the reinforcing member  130 D, and therefore the quality of the digital signal D 2  transmitted through the signal line  110 D can be more effectively improved. 
     Example 5 
     Simulation of differential impedance was performed for the transmission module  100 D according to the fifth embodiment. HyperLynx available from Mentor Graphics was used for the simulation of the differential impedance. 
     The thickness of the base layer  1011  is denoted by T 5011 , the thickness of the conductor layer  1012 D is denoted by T 5012 , the thickness of a portion of the cover layer  1013  overlapping the signal line  110 D on the conductor layer  1012 D is denoted by T 5013 . In addition, the thickness of the insulating layer  135 D of the reinforcing member  130 D is denoted by T 505 , and the thickness of the conductive member  136 D is denoted by T 506 . In the simulation, parameter values of the respective thicknesses were as follows: T 5011 =12.5 μm; T 5012 =12 μm; T 5013 =27.5 μm: T 505 =265 μm: and T 506 =115 μm. To be noted, the thickness T 505  of the insulating layer  135 D includes a thickness of 15 μm of an adhesive between the insulating layer  135 D and the base layer  1011 . In addition, the thickness T 506  of the conductive member  136 D includes a thickness of 15 μm of an adhesive between the conductive member  136 D and the insulating layer  135 D. The relative permittivity of the base layer  1011  was set to 3.3, the relative permittivity of the cover layer  1013  was set to 3.6, the relative permittivity of the insulating layer  135 D of was set to 4.7, and the relative permittivity of the adhesive was set to 4.0. The conductivity of the signal line  110 D and the conductivity of the conductive member  136 D were set to 1.724×10 −8  Ωm. 
     The width of the wiring portion  102 D is denoted by W 502 , and the width of the pad  104 D is denoted by W 504 . In addition, the distance between a pair of the wiring portions  102 D in the differential line pair  111 D is denoted by S 502 , and the distance between a pair of the pads  104 D in the differential line pair  111 D is denoted by S 504 . In the simulation, the values of the widths and the distances were as follows: W 502 =150 μm; S 502 =45 μm; W 504 =250 μm; and S 504 =150 μm. As described above, in Example 5, W 504 &gt;W 502  and S 504 &gt;S 502  hold. 
     In Example 5, the characteristic impedance (differential impedance) Z 1 D of the wiring portion  102 D was 103.8Ω. The characteristic impedance (differential impedance) Z 3 D of the pad  104 D was 102.2Ω. 
     In Comparative Example 1, the difference (Z 3 X-Z 2 X) in the characteristic impedance was 32.7Ω. In addition, in Comparative Example 1 the difference (Z 2 X-Z 1 X) in the characteristic impedance was −18.3Ω. In contrast, in Example 5, the difference (Z 3 D-Z 1 D) in the characteristic impedance was −1.6Ω. Therefore, the absolute value |Z 3 D-Z 1 D| of the difference in the characteristic impedance of Example 5 was smaller than the absolute values |Z 3 X-Z 2 X| and |Z 2 X-Z 1 X| of the difference in the characteristic impedance of Comparative Example 1, which indicates that the characteristic impedance was more consistent in Example 5 than in Comparative Example 1. Therefore, in Example 5, generation of the reflection wave can be reduced. 
     As described above, according to the present disclosure, the quality of the digital signal that is transmitted is improved. 
     The present invention is not limited to the embodiments described above, and can be modified in many ways within the technical concept of the present disclosure. In addition, the effects described in the embodiments are merely enumeration of the most preferable effects that can be achieved by the present invention, and the effects of the present invention are not limited to those described in the embodiments. 
     Although the digital signal D 2  is a 4-level signal in the first to fifth embodiments, the configuration is not limited to this. In addition, a configuration in which the image signal transmitted from the image sensor  202  to the image processing device  302  as the digital signal D 1  that is a binary signal is not transmitted through the conversion circuits  203  and  204  may be employed. In the case of transmitting a binary signal, the conversion circuits  203  and  204  can be omitted. Even in these cases, the present disclosure is applicable when the digital signals D 1  and D 2  are transmitted at a high speed. 
     Although a case where the electronic unit of the present disclosure is applied to an image pickup apparatus such as a digital camera has been described in the first to fifth embodiments, the configuration is not limited to this. For example, the electronic unit of the present disclosure is applicable to electronic devices capable of incorporating the electronic unit, such as mobile communication devices, wearable devices, and image forming apparatuses. Examples of the mobile communication devices include devices such as smartphone, tablet PCs, and gaming devices. Examples of the image forming apparatuses include printers, copiers, facsimile machines, and multifunctional apparatuses having functions of these. 
     In addition, although a case where the first electronic module is configured to transmit a digital signal to the second electronic module via the transmission module has been described in the first to fifth embodiments, the configuration is not limited to this. Further, the second electronic module may be configured to transmit a digital signal to the first electronic module via the transmission module. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2021-178495, filed Nov. 1, 2021, which is hereby incorporated by reference herein in its entirety.