Patent Publication Number: US-2019182949-A1

Title: Flexible printed circuit board and optical module

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of Japanese Application No. JP2017-238686 filed on Dec. 13, 2017, the disclosures of which are hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     This disclosure relates to a flexible printed circuit board and an optical module. 
     BACKGROUND 
     Japanese Unexamined Patent Publication No. 2015-216385 describes a technology for a printed interconnection board. This printed interconnection board includes a base substrate, a plurality of pads for electrical connection arranged on one surface side of the base substrate and a plurality of interconnections respectively connected to the plurality of pads. A plurality of pads are arranged in a staggered arrangement of two rows back and forth. 
     Japanese Unexamined Patent Publication No. 2016-46338 describes a technology of a flexible printed circuit board. This flexible printed circuit board includes a film-like insulating base material, a first terminal formed on an end of a film-like insulating base material, a second terminal formed on the other end of the film-like insulating base material, a conductor pattern formed on the film-like insulating and electrically connecting the first terminal with the second terminal, and a film-like insulating coat film covering a predetermined portion of the conductor pattern. The first terminal and the second terminal are formed on both surfaces of the film-like insulating base material, and the terminals on both surfaces are electrically connected with each other through a via hole penetrating the film-like insulating base material. 
     Japanese Unexamined Patent Publication No. 2014-96424 describes a light-emitting module having an external connecting terminal connected with a flexible printed circuit board. 
     SUMMARY 
     In recent years, a flexible printed circuit board has been used for various applications. One of those applications is an application for transmitting a high frequency signal. For example, a flexible printed circuit board, which connects with a light-emitting module or light-receiving module in an optical communication system, transmits a high frequency transmission or reception signal. In the application of transmitting the high frequency signal, it is important to suppress attenuation of the signal as much as possible. For this reason, it is effective to provide a signal line on one surface of a flexible board and a ground pattern on the other surface to configure a microstrip line. Since a connecting part of the flexible printed circuit board on only any one surface of the board is connected with an optical module or the like, it is effective to configure a so-called coplanar line in which ground terminals (ground lines) are arranged both sides of a signal terminal (signal line). 
     One the other hand, as electrical equipment is reduced in size and complexed in recent years, an interconnection density of the signal lines in the flexible printed circuit board is increasing. For example, in the light-emitting module or light-receiving module used in the optical communication system, the number of electrical signals input to or output from one module is increasing because of multiplexing of optical signals associated with increase in an information and communication amount. Therefore, also in the flexible printed circuit board connected with the light-emitting module or light-receiving module, an interconnection density of signal lines transmitting transmission signals or received signals is increasing. 
     However, the signal terminal and the ground terminal are arranged to be aligned at the connecting part of the flexible printed circuit board as described above, and therefore, if the density of the signal lines is increased, a distance between the signal terminal and the ground terminal is narrowed. If the distance between the signal terminal and the ground terminal is narrowed, a coupling capacitance of the signal terminal and the ground terminal increases to lead to decrease in a characteristic impedance. If the characteristic impedance is decreased, a high frequency characteristic of the flexible printed circuit board is degraded to attenuate the signal. 
     One aspect of this disclosure relates to a flexible printed circuit board includes: a board having a top surface and a back surface; a signal line provided on the top surface of the board; a ground line provided on the back surface and overlapping with the signal line; a first signal terminal extending along a first direction in the top surface of the board, the first signal terminal including a first via-hole and electrically connected with the signal line; a first ground terminal provided next to the first signal terminal along a second direction intersecting the first direction on the top surface of the board, the first ground terminal including a second via-hole electrically connected with the ground line in the back surface of the board; and a second ground terminal provided next to the first signal terminal in a side opposite to the first ground terminal along the second direction on the top surface of the board, the second ground terminal including a third via-hole electrically connected with the ground line in the back surface of the board, wherein the first to third via holes are staggeringly disposed along the second direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other purposes, aspects and advantages will be better understood from the following detailed description of embodiments of the invention with reference to the drawings, in which: 
         FIG. 1  is a plan view illustrating an end portion of a flexible printed circuit board according to an embodiment of the disclosure; 
         FIG. 2  is a bottom plan view illustrating the end portion of the flexible printed circuit board; 
         FIG. 3  is a perspective view enlargedly illustrating the end portion of the flexible printed circuit board, where illustrated is an outer appearance of the flexible printed circuit board seen from a top surface side; 
         FIG. 4  is a perspective view enlargedly illustrating the end portion of the flexible printed circuit board, where illustrated is an outer appearance of the flexible printed circuit board seen from a back surface side; 
         FIG. 5A  is a cross-sectional view along a line Va-Va illustrated in  FIG. 4 , and  FIG. 5B  is a cross-sectional view along a line Vb-Vb illustrated in  FIG. 4 ; 
         FIG. 6  is a diagram enlargedly illustrating a land and a surrounding thereof; 
         FIG. 7  is a partial enlarged view of  FIG. 2 ; 
         FIG. 8  is a plan view of an optical module including the flexible printed circuit board; 
         FIG. 9  is a plan view illustrating an inner configuration of the optical module without an upper lid of a chassis; 
         FIG. 10  is a cross-sectional view along a line X-X illustrated in  FIG. 9 ; 
         FIG. 11  is a plan view enlargedly illustrating a connecting portion of the flexible printed circuit board and a feed-through; 
         FIG. 12  is a plan view illustrating an end portion (connecting part) of a flexible printed circuit board of prior art; and 
         FIG. 13  is a diagram illustrating a state where a width of conductive films for each of a signal terminal and a ground terminal are thinned to widen a distance between the signal terminal and the ground terminal. 
     
    
    
     DETAILED DESCRIPTION 
     Description of Embodiment of the Disclosure 
     Firstly, a description is given of contents of embodiments of the present disclosure in a listing manner. A flexible printed circuit board according to an embodiment comprises a substrate which is flexible and insulative, and has a top surface and back surface, a signal line provided on a top surface, a ground pattern provided on the back surface at a position opposed to the signal line, a first signal terminal extending in a first direction on the top surface and being electrically connected with the signal line, a second signal terminal on the back surface being electrically connected with the first signal terminal through one or more first via holes, first and second ground terminals provided respectively on both sides of the second signal terminal in a second direction intersecting the first direction on the back surface, and extending in the first direction and being electrically connected with the ground pattern, and third and fourth ground terminals provided respectively on both sides of the second signal terminal in the second direction on the top surface, and being electrically connected with the first and second ground terminals through one or more second via holes, wherein a position of the first via hole on the first and second signal terminals and positions of the second via holes on the first to fourth ground terminals are displaced from each other in the first direction, and interconnection widths in the second direction of the first and second signal terminals and the first to fourth ground terminals are thinner than an outer diameter of a land provided around each of the first and second via holes. 
     In general, the via hole needs to have a certain size of inner diameter so that a metal as the conductive film is easily inserted into an inner side thereof. An area (the land) having a certain size of width from the inner wall of the via hole is secured for the conductive film provided around the via hole. In order to prevent a coupling capacitance of the signal terminal and the ground terminal from increasing, it is effective to thin the interconnection widths of the signal terminal and the ground terminal to widen a distance between the signal terminal and the ground terminal. However, if a distance between the land of the signal terminal and the land of the ground terminal is shorter, the whole coupling capacitance cannot be sufficiently suppressed due to capacitive coupling of these lands. 
     In order to address such a problem, in the above flexible printed circuit board, the position of the first via hole on the first and second signal terminals and the positions of the second via holes on the first to fourth ground terminals are displaced from each other in the first direction. This can make the distance between the land of the signal terminal and the land of the ground terminal be longer to decrease the coupling capacitance of the lands. Therefore, according to the above flexible printed circuit board, it is possible to prevent the increase in the coupling capacitances of the signal terminal and the ground terminal which is involved by the increase in the interconnection density of the signal lines, and effectively suppress decrease in the characteristic impedance. 
     In the above flexible printed circuit board, the first via hole on the first and second signal terminals and the second via holes on the first to fourth ground terminals may be alternately arranged in the first direction. This allows the land of the signal terminal and the land of the ground terminal to be arranged away from each other to prevent a short caused by contacting. In this case, the distances between the lands of the first and second signal terminals and the lands of the first to fourth ground terminals next to the former lands may be equal to each other. 
     In the above flexible printed circuit board, the number of first via holes on the first and second signal terminals may be less than the number of via holes on the first and third ground terminals and the number of via holes on the second and fourth ground terminals. 
     In the above flexible printed circuit board, the position of the second via holes on the first and third ground terminals in the first direction may match the position of the second via holes on the second and fourth ground terminals in the first direction. 
     An optical module according to another embodiment includes any flexible printed circuit board described above, and a chassis having an external board in which an optical element is mounted and a plurality of interconnections being electrically connected with the optical element are provided. A plurality of interconnections on the external board electrically connect with the first signal terminal on the top surface, and the third and fourth ground terminals. According to the optical module, since any flexible printed circuit board described above is included, it is possible to prevent the increase in the coupling capacitances of the signal terminal and the ground terminal which is involved by the increase in the interconnection density of the signal lines, and effectively suppress decrease in the characteristic impedance. 
     Detailed Description of Embodiment of the Disclosure 
     Next, embodiment according to the present disclosure will be described as referring to accompanying drawings. The present disclosure, however, is not restricted to the embodiment and has a scope defined in claims attached hereto and all changes and/or modifications with the scope and equivalent there to. In the description of the drawings, numerals or symbols same with or similar to each other will refer to elements same with or similar to each other without overlapping explanations. 
       FIG. 1  is a plan view illustrating an end portion of a flexible printed circuit board  1 A according to an embodiment of the disclosure.  FIG. 2  is a bottom plan view illustrating the end portion of the flexible printed circuit board  1 A.  FIG. 3  is a perspective view enlargedly illustrating the end portion of the flexible printed circuit board  1 A, where illustrated is an outer appearance of the flexible printed circuit board  1 A seen from a side of a top surface  10   a .  FIG. 4  is a perspective view enlargedly illustrating the end portion of the flexible printed circuit board  1 A, where illustrated is an outer appearance of the flexible printed circuit board  1 A seen from a side of a back surface  10   b .  FIG. 5A  is a cross-sectional view along a line Va-Va illustrated in  FIG. 4 .  FIG. 5B  is a cross-sectional view along a line Vb-Vb illustrated in  FIG. 4 . 
     As illustrated in  FIG. 1  to  FIG. 5B , the flexible printed circuit board  1 A in the embodiment comprises a plate  10 , N signal lines  20  (N is an integer equal to or more than 2, and the embodiment illustrates a case of N=8), a ground pattern  30 , N first signal terminals  41 , N second signal terminals  42 , (N+1) ground terminals  51 , (N+1) ground terminals  52 , and overlays  81  and  82 . 
     The plate  10  consists of a flexible and insulative material, for example, a dielectric material such as polyimide. The plate  10  extends like a flat substrate along a plane including a first direction A 1  and a second direction A 2  intersecting (e.g., perpendicular to) each other, and has a flat top surface  10   a  and a flat back surface  10   b  positioned opposite to the top surface  10   a . A planar shape seen in a thickness direction of the plate  10  is substantially rectangular, and the plate  10  further has a linear end edge  10   c  extending along the second direction A 2 , and a pair of side edges  10   d  and  10   e  extending from both ends of the end edge  10   c  along the first direction A 1 . 
     The signal line  20  is a conductive film provided on the top surface  10   a . As illustrated in  FIG. 1  and  FIG. 3 , N signal lines  20  respectively extend in the first direction A 1  and are arranged parallel to each other in the second direction A 2 . The signal line  20  consists of a metal material, for example, copper (Cu). The signal line  20  extends from one end portion to the other end portion of the flexible printed circuit board  1 A in the first direction A 1 . The signal line  20  transmits a high frequency signal which is output from or input to electrical equipment connected to the end portion of the flexible printed circuit board  1 A. A frequency of the high frequency signal is 40 GHz or more, for example. 
     The ground pattern  30  is a conductive film provided on the back surface  10   b . The ground pattern  30  is provided on the back surface  10   b  at a position opposed to the N signal lines  20 . In other words, the ground pattern  30  is layered with N signal lines  20  when seen in the thickness direction of the plate  10 . As illustrated in  FIG. 2 , the ground pattern  30  in the embodiment is formed as one film covering from one side edge  10   d  to the other side edge  10   e  of the plate  10  on the back surface  10   b . The ground pattern  30  consists of a metal material, for example, copper (Cu). The ground pattern  30  and the signal lines  20  constitute microstrip lines. 
     Each of N signal terminals  41  is a conductive film provided on the top surface  10   a . N signal terminals  41  extend in the first direction A 1  on the top surface  10   a  and are aligned in the second direction A 2 . N signal terminals  41  electrically connect with N signal lines  20  respectively. N signal terminals  41  are provided to electrically connect N signal lines  20  of the flexible printed circuit board  1 A with N signal lines of the electrical equipment via an electrically conductive adhesive such as solder. As illustrated in  FIG. 1  and  FIG. 3 , each signal terminal  41  continuously extends from the corresponding signal line  20  in the first direction A 1 . Each signal terminal  41  consists of the same metal material as the signal line  20 , for example, and is integrally formed with the corresponding signal line  20 . Each signal terminal  41  has one end connected to the signal line  20 , and the other end reaching the end edge  10   c  of the plate  10 . 
     Each of N signal terminals  42  is a conductive film provided on the back surface  10   b . N signal terminals  42  extend in the first direction A 1  on the back surface  10   b  and are aligned in the second direction A 2 . As illustrated in  FIG. 2  and  FIG. 4 , each signal terminal  42  is provided on a position opposed to each signal terminal  41 . In other words, the signal terminal  41  and the signal terminal  42  are layered with each other when seen in the thickness direction of the plate  10 . The signal terminal  42  consists of the same metal material as the signal terminal  41 , for example. The signal terminal  42  has one end facing the ground pattern  30 , and the other end reaching the end edge  10   c  of the plate  10 . 
     The signal terminal  42  electrically connects with signal terminal  41  through one or more first via holes  43  (one via hole in the embodiment). The via hole  43  is a hole for electrically connecting the signal terminal  41  and the signal terminal  42  with each other, and penetrates the plate  10  from the top surface  10   a  to the back surface  10   b . A planar shape of the via hole  43  is a circle, for example. On an inner wall of the via hole  43 , a metal film electrically connecting the signal terminal  41  with the signal terminal  42  is formed. As illustrated in  FIG. 3 , a land  44  is provided surrounding the via hole  43  on the top surface  10   a . As illustrated in  FIG. 4 , a land  45  is provided surrounding the via hole  43  on the back surface  10   b . The lands  44  and  45  are metal films formed on the top surface  10   a  and the back surface  10   b , respectively. A planar shape of each of lands  44  and  45  is a circular annular shape centered on the via hole  43 , for example. The lands  44  and  45  are respectively and integrally formed with the signal terminals  41  and  42 . 
       FIG. 6  is a diagram enlargedly illustrating the land  44  ( 45 ) and a surrounding thereof. As illustrated in  FIG. 6 , interconnection widths of the respective signal terminals  41  and  42  in a direction (i.e., the second direction A 2 ) intersecting a longitudinal direction are thinner than outer diameters of the respective lands  44  and  45 . In an example, a width W 1  from the inner wall of the via hole  43  to an outer perimeter of the land  44  or  45  is 60 μm, a diameter W 2  of the via hole  43  is 100 μm, and an outer diameter W 3  of the land  44  or  45  is 220 μm. 
     The plate  10  has a semi-circular cutout  46  on the end edge  10   c  (see  FIG. 3  and  FIG. 4 ) when seen in the thickness direction of the plate  10 . The cutout  46  is a portion for electrically connecting the signal terminal  41  with the signal terminal  42 , and extends from the top surface  10   a  to the back surface  10   b . On an inner wall of the cutout  46 , a metal film electrically connecting the signal terminal  41  with the signal terminal  42  is formed. As illustrated in  FIG. 3 , a land  47  is provided surrounding the cutout  46  on the top surface  10   a . As illustrated in  FIG. 4 , a land  48  is provided surrounding the cutout  46  on the back surface  10   b . The lands  47  and  48  are metal films formed on the top surface  10   a  and the back surface  10   b , respectively. A planar shape of each of lands  47  and  48  is a semi-circular annular shape centered on the cutout  46 , for example. The lands  47  and  48  are respectively and integrally formed with the signal terminals  41  and  42 . 
     The ground terminal  51  is a conductive film provided on the top surface  10   a . The ground terminal  51  is provided on both sides of each signal terminal  41  in the second direction A 2  on the top surface  10   a . In the embodiment, (N+1) ground terminals  51  are aligned alternately with N signal terminals  41  in the second direction A 2 . One of the ground terminals  51  provided on both sides of a signal terminal  41  corresponds to a third ground terminal in the embodiment, and the other corresponds to a fourth ground terminal in the embodiment. A distance between centers (pitch) of the signal terminal  41  and the ground terminal  51  is 380 μm or less, for example. These ground terminals  51  extend along the first direction A 1 . The ground terminal  52  is a conductive film provided on the back surface  10   b . The ground terminal  52  is provided on both sides of each signal terminal  42  in the second direction A 2  on the back surface  10   b . In the embodiment, (N+1) ground terminals  52  are aligned alternately with N signal terminals  42  in the second direction A 2 . One of the ground terminals  52  provided on both sides of a signal terminal  42  corresponds to a first ground terminal in the embodiment, and the other corresponds to a second ground terminal in the embodiment. A distance between centers (pitch) of the signal terminal  42  and the ground terminal  52  is 380 μm or less, for example. These ground terminals  52  extend along the first direction A 1 , and electrically connect with ground pattern  30 . The ground terminal  52  electrically connects with the ground terminal  51  through one or more second via holes  53  (four via holes in the embodiment). The ground terminals  51  and  52  are provided to electrically connect with the ground pattern  30  of the flexible printed circuit board  1 A with a ground interconnection of the electrical equipment via an electrically conductive adhesive such as solder. 
     As illustrated in  FIG. 2  and  FIG. 4 , the ground terminals  52  continuously extend from the ground pattern  30  in the first direction A 1 . The ground terminals  52  consist of the same metal material as the ground pattern  30 , for example, and are integrally formed with the ground pattern  30 . Each ground terminal  52  has one end connected to the ground pattern  30 , and the other end reaching the end edge  10   c  of the plate  10 . The ground terminals  52  are aligned alternately with the signal terminals  42  in the second direction A 2 . Specifically, one ground terminal  52  is arranged between the signal terminals  42  next to each other, and one signal terminal  42  is arranged between the ground terminals  52  next to each other. 
     As illustrated in  FIG. 1  and  FIG. 3 , each ground terminal  51  is provided on a position opposed to each ground terminal  52 . In other words, the ground terminal  52  and the ground terminal  51  are layered with each other when seen in the thickness direction of the plate  10 . The ground terminals  51  extend along the first direction A 1 . The ground terminal  51  consists of the same metal material as the ground terminal  52 , for example. Each ground terminal  51  has one end on a side opposite to the end edge  10   c  of the plate  10 , and the other end reaching the end edge  10   c . One end of each ground terminal  51  is positioned between the signal lines  20 . The ground terminals  51  are aligned alternately with the signal terminals  41  in the second direction A 2 . Specifically, one ground terminal  51  is arranged between the signal terminals  41  next to each other, and one signal terminal  41  is arranged between the ground terminals  51  next to each other. 
     The via hole  53  is a hole for electrically connecting the ground terminal  51  and the ground terminal  52  with each other, and penetrates the plate  10  from the top surface  10   a  to the back surface  10   b . A planar shape of the via hole  53  is a circle, for example. On an inner wall of the via hole  53 , a metal film electrically connecting the ground terminal  51  with the ground terminal  52  is formed. As illustrated in  FIG. 3 , a land  54  is provided surrounding the via hole  53  on the top surface  10   a . As illustrated in  FIG. 4 , a land  55  is provided surrounding the via hole  53  on the back surface  10   b . The lands  54  and  55  are metal films formed on the top surface  10   a  and the back surface  10   b , respectively. A planar shape of each of lands  54  and  55  is a circular annular shape centered on the via hole  53 , for example. The lands  54  and  55  are respectively and integrally formed with the ground terminals  51  and  52 . 
     A shape of the lands  54  and  55 , and surrounding thereof is the same as the shape of the lands  44  and  45 , and surrounding thereof illustrated in  FIG. 6 . Specifically, widths of the ground terminals  51  and  52  in the direction (i.e., the second direction A 2 ) intersecting the longitudinal direction are thinner than outer diameters of the lands  54  and  55 , respectively. In an example, a width from the inner wall of the via hole  53  to an outer perimeter of the land  54  or  55  is 60 μm, a diameter of the via hole  53  is 100 μm, and an outer diameter of the land  54  or  55  is 220 μm. 
     The plate  10  further has a semi-circular cutout  56  on the end edge  10   c  (see  FIG. 3  and  FIG. 4 ) when seen in the thickness direction of the plate  10 . The cutout  56  is a portion for electrically connecting the ground terminal  51  with the ground terminal  52 , and extends from the top surface  10   a  to the back surface  10   b . On an inner wall of the cutout  56 , a metal film electrically connecting the ground terminal  51  with the ground terminal  52  is formed. As illustrated in  FIG. 3 , a land  57  is provided surrounding the cutout  56  on the top surface  10   a . As illustrated in  FIG. 4 , a land  58  is provided surrounding the cutout  56  on the back surface  10   b . The lands  57  and  58  are metal films formed on the top surface  10   a  and the back surface  10   b , respectively. A planar shape of each of lands  57  and  58  is a semi-circular annular shape centered on the cutout  56 , for example. The lands  57  and  58  are respectively integrally formed with the ground terminals  51  and  52 . 
     In the embodiment, as illustrated in  FIG. 1  to  FIG. 4 , positions of the via hole  43  and the via hole  53  which are next to each other are displaced from each other in the first direction A 1 . In other words, the via hole  43  and the via hole  53  are alternately arranged (alternated with each other) in the first direction A 1  to configure a staggered arrangement. Such a form is further described with reference to  FIG. 7  that is a partial enlarged view of  FIG. 2 . 
     The configuration describe above may be considered as that in which a first ground terminal  52 A and a second ground terminal  52 B arranged on both sides of each signal terminal  42  in the second direction A 2 . Assuming that the number of via holes  43  formed on each signal terminal  42  is K and the number of via holes  53  formed on each of the ground terminals  52 A and  52 B is Q (where, K is an integer equal to or more than 1 and Q is an integer equal to or more than 2), Q&gt;K holds. As an example, a case of K=1 and Q=4 is illustrated in  FIG. 7 . Positions where the via holes  53  are formed on the first ground terminal  52 A in the first direction A 1  respectively match (coincide with) positions where the via holes  53  are formed on the second ground terminal  52 B in the first direction A 1 . In other words, the positions where the via holes  53  are formed on the first ground terminal  52 A and the positions where the via holes  53  are formed on the second ground terminal  52 B are line-symmetric with respect to a center line of the signal terminal  42 . 
     Here, assume that distances between the land  45  on the signal terminal  42  and two lands  55  on the first ground terminal  52 A next to the land  45  are D 1  and D 2 , respectively. Assume that distances between the land  45  on the signal terminal  42  and two lands  55  on the second ground terminal  52 B next to the land  45  are D 3  and D 4 , respectively. In the embodiment, the distances D 1  to D 4  are equal to each other. Specifically, the distances between the land  45  and all the lands  55  next to the land  45  and of the first and second ground terminals  52 A and  52 B are equal to each other. This also holds for the land  44  on the signal terminal  42  and the lands  54  next to the land  44  and of the first and second ground terminals  52 A and  52 B. 
     The overlays  81  and  82  are films made of a resin (e.g., a resist). The overlay  81  is provided on the top surface  10   a  of the plate  10  to collectively cover N signal lines  20 . The overlay  82  is provided on the back surface  10   b  of the plate  10  to cover entirely the ground pattern  30 . 
     Next, a description is given of an example of an optical module including the flexible printed circuit board  1 A described above. The embodiment illustrates a TOSA (Transmit Optical Sub-Assembly) type light-emitting module as an optical module.  FIG. 8  is a plan view of an optical module  100  including the flexible printed circuit board  1 A.  FIG. 9  is a plan view illustrating an inner configuration of the optical module  100  without an upper lid of a chassis  2 .  FIG. 10  is a cross-sectional view along a line X-X illustrated in  FIG. 9 . Note that an XYZ orthogonal coordinate system is illustrated in each drawing for easy understanding. An X-axis is along the direction A 2  and a Y-axis is along the direction A 1 . As illustrated in  FIG. 8  to  FIG. 10 , the optical module  100  according to the embodiment includes, besides the flexible printed circuit board  1 A, a chassis  2 , an optical output port  21  (see  FIG. 8 ), a feed-through  60 , and a flexible printed circuit board  90  (see  FIG. 10 ). 
     The chassis  2  has a rectangular parallelepiped shape extending in a Y direction. The chassis  2  has side walls  2   a  and  2   b  facing each other in the Y direction, and a bottom surface  2   c  intersecting a Z direction. The side walls  2   a  and  2   b  are along an X direction. The bottom surface  2   c  is in contact with lower ends of the side walls  2   a  and  2   b  in the Z direction. The chassis  2  houses N/2 (e.g., four) laser diodes (LDs)  3  as optical elements. The LDs  3  are arranged on the bottom surface  2   c  to be aligned along the X direction. For example, in a case that each LD  3  is a direct modulation type LD, optical intensity modulation of a laser light is performed depending on a modulated current signal applied via the flexible printed circuit board  1 A. A wavelength of each laser light consists of four different wavelengths standardized by the MSA. 
     The chassis  2  houses a thermal electric cooler (TEC)  4 , a base  5 , carriers  6  and  7 , a thermistor  8 , an optical multiplexer  11 , and (N/2) lenses  12 . The TEC  4  is mounted on the bottom surface  2   c . A voltage applied to the TEC  4  is controlled such that a value of resistance obtained from the thermistor  8  is a constant value, and such that temperatures of the LDs  3  are constant to control the wavelengths of the laser lights generated in the LDs  3 . The base  5  is constituted by a material having high thermal conductivity, for example, aluminum nitride, with which heat influx by the TEC  4  can be efficiently performed. The base  5  is mounted on the TEC  4 . The carriers  6  and  7  are mounted on the base  5 . The carrier  6  is an external board in the embodiment, and is constituted by a material having high thermal conductivity, for example, aluminum nitride, with which a heat generated in the LD  3  can be efficiently diffused. The carrier  7 , which mounts thereon parts not involving heat generation such as the lens  12  and the optical multiplexer  11 , is constituted by aluminum nitride or alumina, for example. The LDs  3  and the thermistor  8  are mounted on the carrier  6 . The thermistor  8  detects a surrounding temperature of each LD  3 . On the carrier  6 , mounted are a plurality of interconnections electrically connecting respectively with a plurality of LDs  3  through a bonding wire, for example. A plurality of interconnections are provided to transmit modulated signals applied through the flexible printed circuit board  1 A to a plurality of LDs  3 . 
     The carrier  7  is arranged between the carrier  6  and the side wall  2   a  in the Y direction. The lenses  12  and the optical multiplexer  11  are mounted on the carrier  7 . The lenses  12  are arranged to correspond respectively to the LDs  3 , and optically coupled with the LDs  3 . Each lens  12  parallelizes a laser light output from each LD  3 . The optical multiplexer  11  is arranged on a light path between the lenses  12  and the optical output port  21  in the Y direction. The optical multiplexer  11  is optically coupled with the lenses  12 , and multiplexes (N/2) laser lights different in the wavelength which are parallelized by the lenses  12 . The laser lights (parallel lights) multiplexed by the optical multiplexer  11  are output outside of the optical module  100  via the optical output port  21 . 
     The optical output port  21  has a columnar shape extending along the Y direction. A part of the optical output port  21  is embedded in the side wall  2   a . In that part, an optical window  22  is housed as illustrated in  FIG. 10 . The optical window  22  is optically coupled with the optical multiplexer  11 . The rest of the optical output port  21  is positioned outside of the side wall  2   a  in the Y direction. That rest holds a lens and an optical fiber which are optically coupled with the optical window  22 . The laser lights (parallel lights) output from the optical multiplexer  11  are collected by a lens arranged before the optical fiber and input to the optical fiber. The laser lights are supplied to the outside of optical module  100  through the optical fiber. 
     The feed-through  60  is provided to electrically connect an inside of the side wall  2   b  with an outside thereof. The feed-through  60  is arranged on the bottom surface  2   c , and extends from the inside to the outside of the side wall  2   b  along the Y direction. The feed-through  60  is constituted by ceramic containing alumina, for example. The feed-through  60  has an inner structure  61  provided inside the side wall  2   b , an outer structure  65  provided outside the side wall  2   b , and a ground pattern  75  which is formed on from the inner structure  61  to the outer structure  65  in an XY plane and defines a reference potential. 
     The inner structure  61  includes top surfaces  62  and  63  intersecting the Z direction. The top surfaces  62  and  63  are positioned inside the side wall  2   b  in the Y direction. The top surface  62  includes a transmission line  62 A to transmit the modulated signal applied through the flexible printed circuit board  1 A to each LD  3  (see  FIG. 9 ). The transmission line  62 A includes N signal interconnections  62   a  electrically connecting with the LDs  3 , and (N+1) ground interconnections  62   b  defining the reference potential. The signal interconnections  62   a  and the ground interconnections  62   b  extend along the Y direction. The signal interconnections  62   a  and the ground interconnections  62   b  are alternately aligned along the X direction. In the embodiment, one differential signal interconnection is configured every two signal interconnections  62   a . The differential signal interconnection is provided to correspond to each LD  3 . The differential signal interconnections and the ground interconnections  62   b  connect with the interconnections on the carrier  6  through a bonding wire, for example. Therefore, each of the differential signal interconnections electrically connects with the corresponding LD  3  through the bonding wire and the interconnections on the carrier  6 . The top surface  63  includes a plurality of terminals  63   a  including a power interconnection and an analog signal interconnection. The terminals  63   a  electrically connect with the thermistor  8 , the TEC  4 , and the like through the bonding wire. 
     The outer structure  65  includes a top surface  66  perpendicular to the Z direction, a back surface  67  opposed to the top surface  66  in the Z direction, a rear surface  70  arranged opposite to the inner structure  61  with respect to the side wall  2   b  in the Y direction, and a rear surface  71  arranged between a plane including the rear surface  70  and a plane including the side wall  2   b  in the Y direction. 
     The top surface  66  is positioned outside the side wall  2   b  in the Y direction. The top surface  66  includes N signal lines  66   a  and (N+1) ground lines  66   b . The signal lines  66   a  and the ground lines  66   b  extend along the Y direction. The signal line  66   a  and the ground lines  66   b  are alternately aligned along the X direction. Specifically, two ground lines  66   b  are arranged on both sides of one signal line  66   a  in the X direction. The signal lines  66   a  electrically connect with the signal interconnections  62   a  on the top surface. The ground lines  66   b  define the reference potential and electrically connect with the ground interconnections  62   b.    
     The back surface  67  includes a plurality of pads  67   a  including a power pad and an analog signal pad (see  FIG. 10 ). Each pad  67   a  electrically connects with each terminal  63   a  through a via, for example. The ground pattern  75  includes a portion positioned between the top surface  66  and the back surface  67  in the Z direction, and are along the top surface  66  and the back surface  67 . The ground pattern  75  electrically connects with ground lines  66   b  through a via. 
       FIG. 11  is a plan view enlargedly illustrating a connecting portion of the flexible printed circuit board  1 A and the feed-through  60 . As illustrated in  FIG. 10  and  FIG. 11 , the top surface  10   a  of the flexible printed circuit board  1 A is opposed to the top surface  66  in the Z direction. Then, the signal terminals  41  and the ground terminals  51  of the flexible printed circuit board  1 A, and the signal lines  66   a  and the ground lines  66   b  of the feed-through  60  stack one another in the Z direction. The signal terminals  41  connect with the signal lines  66   a  via an electrically conductive adhesive such as solder paste. The ground terminals  51  connect with the ground line  66   b  via an electrically conductive adhesive such as solder paste. The flexible printed circuit board  1 A applies the modulated signals to the LDs  3  through the signal lines  20  and the signal terminals  41 . 
     As illustrated in  FIG. 10 , the flexible printed circuit board  90  has a flexible substrate  91 , a plurality of interconnections  92  provided on one surface of the substrate  91 , and a plurality of interconnections  93  provided on the other surface of the substrate  91 . One surface of the flexible printed circuit board  90  is opposed to the back surface  67  in the Z direction. A plurality of interconnections  92  and  93  are connected respectively to a plurality of terminals provided to one end of the substrate  91 , and the terminals connect with the pads  67   a  via an electrically conductive adhesive such as solder paste. The flexible printed circuit board  1 A applies the modulated signals to the LDs  3  through N signal lines  20 . The flexible printed circuit board  90  applies control signals to the thermistor  8 , the TEC  4 , and the like through a plurality of interconnections  92  and  93 . 
     A description is given of effects obtained by the flexible printed circuit board  1 A according to the embodiment.  FIG. 12  is a plan view illustrating an end portion (connecting part) of a flexible printed circuit board  200  of prior art. In the connecting part of the flexible printed circuit board  200 , signal terminals  240  and ground terminals  250  are alternately arranged to be aligned in the second direction A 2 . Each signal terminal  240  has a plurality of via holes  212  arranged along the first direction A 1 , and each ground terminal  250  has a plurality of via holes  213  arranged along the first direction A 1 . If an interconnection density of the signal lines in the flexible printed circuit board  200  increases as the electrical equipment is reduced in size and complexed, a distance Da between the signal terminal  240  and the ground terminal  250  in the connecting part is narrowed. If the distance Da between the signal terminal  240  and the ground terminal  250  is narrowed, a coupling capacitance of the signal terminal  240  and the ground terminal  250  increases to lead to decrease in a characteristic impedance. If the characteristic impedance is decreased, a high frequency characteristic of the flexible printed circuit board  200  is degraded to attenuate the signal. 
     In order to prevent the coupling capacitance from increasing while a pitch (distance between centers) of the signal terminal  240  and the ground terminal  250  is maintained, it is effective to thin a width of a conductive film for each of the signal terminal  240  and the ground terminal  250  to widen the distance Da between the signal terminal  240  and the ground terminal  250 , as illustrated in  FIG. 13 . However, the via holes  212  and  213  need to have a certain size of inner diameter so that a metal as the conductive film is easily inserted into an inner side. An area having a certain size of width (land  215  or  217 ) from the inner wall of the via hole  212  or  213  is secured for the conductive film provided around the via hole  212  or  213 . If a distance between the land  215  on the signal terminal  240  and the land  217  on the ground terminal  250  is shorter, the whole coupling capacitance cannot be sufficiently suppressed due to capacitive coupling of these lands  215  and  217 . 
     It can be considered that the signal terminal and the ground terminal are arranged in a staggered arrangement of two rows back and forth as in Japanese Unexamined Patent Publication No. 2015-216385. However, such an arrangement may be difficult since a dimension in back and forth directions of a connected target of the flexible printed circuit board (the optical module  100  in the embodiment) is restricted, for example. 
     In order to address such a problem, in the flexible printed circuit board  1 A in the embodiment, positions of the via hole  43  on the signal terminals  41  and  42  and the via hole  53  on the ground terminals  51  and  52  are displaced from each other in the first direction A 1 . This can make the distances D 1  to D 4  between the lands  44  and  45 , and the lands  54  and  55  be longer to decrease the coupling capacitance of the lands. Therefore, according to the flexible printed circuit board  1 A in the embodiment, it is possible to prevent the increase in the coupling capacitances of the signal terminals  41  and  42 , and the ground terminals  51  and  52  which is involved by the increase in the interconnection density of the signal lines  20 , and effectively suppress decrease in the characteristic impedance. 
     The via hole  43  on the signal terminals  41  and  42 , and the via hole  53  on the ground terminals  51  and  52  may be alternately arranged (alternated with each other) in the first direction A 1 . This allows the lands  44  and  45  and the lands  54  and  55  to be arranged away from each other to prevent a short caused by contacting. Moreover, in this case, the distances D 1  to D 4  between the land  44  ( 45 ) and the land  54  ( 55 ) next to the land  44  ( 45 ) may be equal to each other. This makes the distances between the lands be equal to each other and values of the capacitances be equal to each other, and then, the high frequency characteristic can be improved. 
     As in the embodiment, the number of via holes  43  on the signal terminal  42  may be less than the number of via holes  53  on the first ground terminal  52 A and the number of via holes  53  on the second ground terminal  52 B. In the signal line, the smaller a change in the thickness, the more a variation in the characteristic impedance is suppressed. Specifically, the fewer the number of via holes  43 , the better, in order to bring the thickness of the signal terminals  41  and  42  closer substantially to a constant. On the other hand, on the ground terminals  51  and  52 , the more the number of via holes  53 , the better, in order to steady potentials of the ground terminals  51  and  52 . 
     The position of the via hole  53  on the first ground terminal  52 A in the first direction A 1  may match the position of the via hole  53  on the second ground terminal  52 B in the first direction A 1 . This makes the distances between the respective via holes  53  on the first ground terminal  52 A and second ground terminal  52 B and the via hole  43  on the signal terminal  42  be equal to each other and values of the capacitances be equal to each other, and then, the preferable characteristic in terms of a high frequency can be obtained. 
     The plate  10  may have the cutouts  46  and  56 . Since a fillet of an electrically conductive adhesive is formed on the end edge  10   c  of the plate  10  when the flexible printed circuit board  1 A is connected to the feed-through  60 , an adhesive strength between the flexible printed circuit board  1 A and the feed-through  60  is increased. This can improve a reliability of the optical module  100 . 
     The flexible printed circuit board and the optical module according to the disclosure are not limited to the embodiments described above, and other various modifications may be adopted. For example, the above embodiments illustrate the case that the number of via holes on each signal terminal is one, but the number of via holes on each signal terminal may be two or more. The above embodiments illustrate the light-emitting module as the application of the flexible printed circuit board, but the flexible printed circuit board can be applied to the light-receiving module and other various electronic equipment. The above embodiments illustrate the case that the light-receiving module includes two flexible printed circuit boards (i.e., the flexible printed circuit boards  1 A and  90 ), but the electronic equipment such as the light-receiving module may include only one flexible printed circuit board. Accordingly, the present disclosure has a scope defined in the claims attached below and all modifications and the changes for elements recited in the claims and equivalents thereto.