Patent Publication Number: US-11659658-B2

Title: Multilayer board

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2017-027026 filed on Feb. 16, 2017 and is a Continuation Application of PCT Application No. PCT/JP2018/003800 filed on Feb. 5, 2018. The entire contents of each application are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a multilayer board. 
     2. Description of the Related Art 
     A flat cable including multiple signal lines arranged in a width direction is attracting attention as a transmission line transmitting a high frequency signal in an electronic device. For example, WO 2014/115607 describes a transmission line including a flat dielectric element body, a signal conductor built into the dielectric element body and extending along a transmission direction, a reference ground conductor, an auxiliary ground conductor, and a thickness-direction connecting conductor, and it is described that the transmission line enables transmission with high isolation ensured between multiple high frequency signals and can be formed compact and thin. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide multilayer boards each including a laminated insulating body including a plurality of insulating base material layers that are laminated; three or more signal conductors provided inside the laminated insulating body and extending in a transmission direction of a signal along the insulating base material layers; and a plurality of ground conductors sandwiching each of the signal conductors in a lamination direction via the insulating base material layers. The multilayer board includes a parallel extending portion in which the signal conductors extend parallel or substantially parallel to transmit a high frequency signal. The parallel extending portion includes two or more signal conductors arranged separately from each other in a direction orthogonal or substantially orthogonal to the transmission direction in a planar view in the lamination direction, and a signal conductor including an overlap with the signal conductor in a planar view in the lamination direction and arranged separately from the signal conductor in the lamination direction. The parallel extending portion includes a first region and at least one second region respectively including the signal conductors arranged separately from each other in a direction orthogonal or substantially orthogonal to the transmission direction in a planar view in the lamination direction. The first region includes a larger number of overlapping signal conductors in the lamination direction than the second region. The first region includes a portion in which an interval between the ground conductors sandwiching the signal conductor is smaller than a minimum value of an interval between the ground conductors sandwiching the signal conductor in the second region. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a plan view of a layer L 1  of a multilayer board according to a first preferred embodiment of the present invention. 
         FIG.  1 B  is a plan view of a layer L 2  of the multilayer board according to the first preferred embodiment of the present invention. 
         FIG.  1 C  is a plan view of a layer L 3  of the multilayer board according to the first preferred embodiment of the present invention. 
         FIG.  1 D  is a plan view of a layer L 4  of the multilayer board according to the first preferred embodiment of the present invention. 
         FIG.  1 E  is a plan view of a layer L 5  of the multilayer board according to the first preferred embodiment of the present invention. 
         FIG.  2    is an exploded perspective view showing a structure of the multilayer board according to the first preferred embodiment of the present invention. 
         FIG.  3    is a transparent plan view of the multilayer board according to the first preferred embodiment of the present invention. 
         FIG.  4    is an exemplary cross-sectional view in a parallel extending portion of the multilayer board according to the first preferred embodiment of the present invention. 
         FIG.  5    is another exemplary cross-sectional view in the parallel extending portion of the multilayer board according to the first preferred embodiment of the present invention. 
         FIG.  6    is an exemplary transparent plan view of the multilayer board including auxiliary ground conductors in the parallel extending portion. 
         FIG.  7    is another exemplary transparent plan view of the multilayer board including auxiliary ground conductors in the parallel extending portion. 
         FIG.  8    is a partially transparent plan view of the multilayer board including a resist on a mounting surface as viewed from the mounting surface side. 
         FIG.  9    is an exemplary cross-sectional view of the multilayer board including the resist on the mounting surface. 
         FIG.  10    is another exemplary cross-sectional view of the multilayer board including the resist on the mounting surface. 
         FIG.  11    is a schematic for explaining a mounting method of a multilayer board including a connector on the mounting surface. 
         FIG.  12    is a schematic for explaining a mounting method of the multilayer board including the resist on the mounting surface. 
         FIG.  13    is a plan view showing an example of a mounting board with the multilayer board mounted thereon. 
         FIG.  14    is a side view showing another example of a mounting board with the multilayer board mounted thereon. 
         FIG.  15    is a transparent plan view of a multilayer board according to a second preferred embodiment of the present invention. 
         FIG.  16    is an exemplary cross-sectional view in the parallel extending portion of the multilayer board according to the second preferred embodiment of the present invention. 
         FIG.  17    is an exemplary transparent plan view of the multilayer board including auxiliary ground conductors in the parallel extending portion. 
         FIG.  18    is a transparent plan view of a multilayer board according to a third preferred embodiment of the present invention. 
         FIG.  19    is an exemplary cross-sectional view in the parallel extending portion of the multilayer board according to the third preferred embodiment of the present invention. 
         FIG.  20    is an exemplary cross-sectional view in an end portion of the parallel extending portion of the multilayer board according to the third preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A low-loss transmission line including more signal lines is required due to downsizing of electronic devices in which the transmission line is used. However, if a large number of signal transmission portions including signal conductors each sandwiched by ground conductors are arranged in the width direction of the transmission line, the transmission lines are increased in width, which may make it difficult to sufficiently meet a requirement for miniaturization. According to preferred embodiments of the present disclosure, multilayer boards which are each usable as a transmission line including multiple signal transmission portions and capable of reducing a length in the width direction are able to be provided. 
     A multilayer board according to a first preferred embodiment includes a laminated insulating body including a plurality of insulating base material layers that are laminated; three or more signal conductors provided inside the laminated insulating body and extending in a transmission direction of a signal along the insulating base material layers; and a plurality of ground conductors sandwiching each of the signal conductors in a lamination direction via the insulating base material layers. The multilayer board includes a parallel extending portion in which the signal conductors extend parallel or substantially parallel to transmit a high frequency signal. The parallel extending portion includes two or more signal conductors arranged separately from each other in a direction orthogonal or substantially orthogonal to the transmission direction in a planar view in the lamination direction, and a signal conductor including an overlap with the signal conductor in a planar view in the lamination direction and arranged separately from the signal conductor in the lamination direction. The parallel extending portion includes a first region and at least one second region respectively including the signal conductors arranged separately from each other in a direction orthogonal or substantially orthogonal to the transmission direction in a planar view in the lamination direction. The first region includes a larger number of the signal conductors arranged to overlap in the lamination direction than the second region, and the first region includes a portion in which an interval between the ground conductors sandwiching the signal conductor is smaller than a minimum value of an interval between the ground conductors sandwiching the signal conductor in the second region. 
     Since the first region includes a larger number of the signal conductors than the second region and the signal conductors are arranged such that the first region includes a portion in which an interval between the ground conductors sandwiching the signal conductor is smaller than a minimum value of an interval between the ground conductors sandwiching the signal conductor in the second region, the length in the width direction of the multilayer board orthogonal or substantially orthogonal to the lamination direction and the transmission direction are able to be reduced while reducing or preventing crosstalk between the signal conductor included in the first region and the signal conductor included in the second region. 
     The second region may include a signal conductor that is wider than the signal conductor included in the first region. When the second region includes a wider signal conductor, even when the ground conductor interval in the second region is wide, impedance is able to be easily matched between signal transmission portions, and a transmission loss of the signal transmission portions is able to be reduced. 
     The parallel extending portion may include a curved portion in which the transmission direction is bent along a plane orthogonal or substantially orthogonal to the lamination direction, and the first region may be disposed at a position on the inner side relative to the second region in the curved portion. When the first region is disposed in a portion on the inner side of the curved portion, the line lengths of more signal conductors are able to be shortened, and the transmission loss of the entire transmission line is able to be reduced. 
     Lead-out conductors respectively connected to the signal conductors and led out toward a mounting surface in the lamination direction may be provided at an end portion in the transmission direction, and the signal conductors that overlap in the lamination direction may be structured such that a total length of the lead-out conductors is shorter as compared to when the signal conductors are disposed at equal or substantially equal intervals in the lamination direction. Providing more signal conductors on the mounting surface side further reduces the transmission loss due to the lead-out conductors in the lamination direction connected to the signal conductors. 
     The multilayer board may preferably further include at least one interlayer connection conductor connecting the ground conductors in the lamination direction, between the signal conductor included in the first region and the signal conductor included in the second region. This improves isolation between the signal conductor included in the first region and the signal conductor included in the second region, so that crosstalk between the signal conductors is reduced or prevented. A plurality of interlayer connection conductors may be provided along the transmission direction. The interlayer connection conductor may be made of a conductive paste disposed through the insulating base material layer or may be defined by a through hole disposed in the multilayer board. 
     The multilayer board may preferably further include at least one interlayer connection conductor connecting the ground conductors in the lamination direction, in an outer edge portion of the parallel extending portion. This reduces or prevents unnecessary radiation from the signal conductors to the outside. A plurality of interlayer connection conductors may be provided along the transmission direction. 
     The multilayer board may preferably further include an auxiliary ground conductor disposed along the transmission direction and connected to the ground conductor, between the signal conductor included in the first region and the signal conductor included in the second region. This improves isolation between the signal conductor included in the first region and the signal conductor included in the second region, so that crosstalk between the signal conductors is effectively reduced or prevented. The auxiliary ground conductor is connected to the ground conductor by the interlayer connection conductor, for example. The auxiliary ground conductor may preferably be a flat plate-shaped conductor, for example. The auxiliary ground conductor may be defined by a continuous flat plate-shaped conductor extending along the transmission direction or may be defined by a plurality of flat plate-shaped conductors separated from each other along the transmission direction. The auxiliary ground conductor may be disposed in the same or substantially the same layer as the signal conductor in the lamination direction or may be disposed in a layer different from the signal conductor. When the auxiliary ground conductor is disposed in a layer different from the signal conductor in the lamination direction, a plurality of auxiliary ground conductors may be disposed in upper and lower layers sandwiching the layer in which the signal conductor is disposed, for example. 
     The multilayer board may preferably further include an auxiliary ground conductor disposed along the transmission direction and connected to the ground conductor, in the outer edge portion of the parallel extending portion. This reduces or prevents unnecessary radiation from the signal conductors to the outside. The auxiliary ground conductor is preferably, for example, connected to the ground conductor by the interlayer connection conductor. The auxiliary ground conductor is preferably connected to the ground conductor by the interlayer connection conductor, for example. The auxiliary ground conductor may preferably be a flat plate-shaped conductor, for example. The auxiliary ground conductor may be defined by a continuous flat plate-shaped conductor extending along the transmission direction or may be defined by a plurality of flat plate-shaped conductors separated from each other along the transmission direction. The auxiliary ground conductor may be disposed in the same or substantially the same layer as the signal conductor in the lamination direction or may be disposed in a layer different from the signal conductor. When the auxiliary ground conductor is disposed in a layer different from the signal conductor in the lamination direction, a plurality of auxiliary ground conductors may be disposed in upper and lower layers sandwiching the layer in which the signal conductor is disposed, for example. 
     Preferred embodiments of the present invention will now be described with reference to the drawings. It is noted that the preferred embodiments described below are examples of multilayer boards to describe the technical ideas and advantages of the present invention, and the present invention is not limited to the multilayer boards described below. The members described in claims are not limited to the members of the preferred embodiments in any way. Particularly, the dimensions, materials, shapes, relative arrangements, and structures of the elements and portions described in the preferred embodiments are merely illustrative examples and are not intended to limit the scope of the present invention only thereto unless otherwise specified. In the drawings, the same or similar portions are denoted by the same reference numerals. Although the preferred embodiments are separately described for convenience to facilitate explanation and/or understanding of main points, configurations described in different preferred embodiments may be partially replaced or combined. In second and subsequent preferred embodiments, matters common to the first preferred embodiment will not be described, and only the differences will be described. Particularly, the same advantageous effects according to the same or similar configuration will not be described in each preferred embodiment. 
     First Preferred Embodiment 
       FIGS.  1 A to  1 E  are plan views of respective layers L 1  to L 5  of a multilayer board according to a first preferred embodiment as viewed from the side of a surface defining and functioning as a mounting surface. The layer L 1  is on the mounting surface side, and the layers L 1  to L 5  are laminated in this order to provide the multilayer board. Each of the layers L 1  to L 5  includes a region corresponding to a parallel extending portion disposed with a ground conductor and/or a signal conductor, and a region corresponding to a lead-out portion extending from the region corresponding to the parallel extending portion to a connection terminal portion  17 . 
     In the layer L 1  of  FIG.  1 A , a ground conductor  12 A is provided on an insulating base material layer  11 A, and the connection terminal portion  17  is provided with a terminal electrode  18 . For example, a connector may be connected to the terminal electrode  18  by using solder, for example, or the terminal electrode  18  may be directly connected to a mounting electrode on a mounting board by solder, for example. The ground conductor  12 A disposed in the layer L 1  covers the regions corresponding to the parallel extending portion and the lead-out portion and a region of the connection terminal portion  17  excluding a portion around the terminal electrode  18 . The terminal conductor  18  is connected to a signal conductor built into the parallel extending portion via a lead-out conductor in the transmission direction of the layer L 2  and a first interlayer connection conductor  14 . If the connection conductor connecting the terminal electrode  18  and the signal conductor is provided on a side surface of the multilayer board, the connection conductor allowing passage of a signal is exposed, which may adversely affect surrounding devices due to unnecessary radiation. In the multilayer board, the terminal electrode  18  and the signal conductor are connected via the first interlayer connection conductor  14  disposed in the connection terminal portion  17 , and the periphery of the first interlayer connection conductor is surrounded by ground conductors and second interlayer connection conductors  15   f  connecting between the ground conductors. By connecting the signal conductor and the terminal electrode  18  through the first interlayer connection conductor  14  disposed inside the multilayer board in this manner, the unnecessary radiation is able to be reduced or prevented. In this case, the first interlayer connection conductor may preferably have a smaller cross-sectional area than a connection conductor defined by a planar conductor. Furthermore, when the first interlayer connection conductor is defined by a via conductor, for example, the via conductor is preferably made of a material that is reactive with and capable of being joined to a planar conductor disposed on the insulating base material layer. For example, if the planar conductor is made of copper, the via conductor is preferably made of a copper-tin-based material having a larger conductor loss than copper. From the above, the first interlayer connection conductor with a short line length is preferably provided. In the layer L 1 , the ground conductor  12 A does not cover an end surface portion of the insulating base material layer  11 A. Therefore, when the multilayer board is made, the ground conductor  12 A is not exposed on the side surface of the multilayer board and is built into a laminated insulating body. 
     The layer L 2  of  FIG.  1 B  includes a signal conductor  13 A disposed on the region corresponding to the parallel extending portion of an insulating base material layer  11 B and lead-out conductors  16 A to  16 C in the transmission direction disposed on the region corresponding to the lead-out portion. One end portion of each of the lead-out conductors  16 A to  16 C in the transmission direction is connected to the terminal electrode  18  via the first interlayer connection conductor  14  disposed in the layer L 1  through the insulating base material layer  11 A. The lead-out conductor  16 A is integrally provided with the signal conductor  13 A. The other end portion of each of the lead-out conductors  16 B,  16 C is provided with a connection portion for a signal conductor  13 B or  13 C, and the first interlayer connection conductor  14  penetrating the insulating base material layer  11 B is connected to the connection portion. 
     The region corresponding to the parallel extending portion of insulating base material layer  11 B includes second interlayer connection conductors  15   a ,  15   b , and  15   d  penetrating the insulating base material layer  11 B and connecting the ground conductors of the layers L 1 , L 3  to each other. A plurality of the second interlayer connection conductors  15   a  are provided along the transmission direction in an outer edge portion of the region corresponding to the parallel extending portion. A plurality of the second interlayer connection conductors  15   b  are provided along the transmission direction between the signal conductor  13 A in the region corresponding to the parallel extending portion and a region corresponding to the signal conductor  13 B. The second interlayer connection conductors  15   d  are provided at both end portions in the transmission direction of the region corresponding to the parallel extending portion. The isolation of the signal conductor  13 A is able to be improved by arranging the second interlayer connection conductors  15   a ,  15   b , and  15   d  to surround the signal conductor  13 A. A plurality of second interlayer connection conductors  15   c  are provided along the transmission direction in an outer edge portion of the region corresponding to the parallel extending portion, and second interlayer connection conductors  15   e  are also provided at both end portions in the transmission direction of the region corresponding to the parallel extending portion, so that the ground conductor  12 A of the layer L 1  and a ground conductor  12 C of the layer L 5  are connected via the second interlayer connection conductors  15   c  or  15   e  disposed in the layer L 3  and the layer L 4 . Since the ground conductors are connected through a plurality of the second interlayer connection conductors  15   a  to  15   e , a ground state of the multilayer board is more stabilized, and the isolation of the signal conductor is improved. 
     The second interlayer connection conductors  15   f  are provided in an outer circumferential portion of the connection terminal portion  17  of the layer L 2  and surrounds one end portion of each of the lead-out conductors  16 A to  16 C in the transmission direction. The second interlayer connection conductors  15   f  connect the ground conductor  12 A of the layer L 1  and the ground conductor  12 C of the layer L 5  via the second interlayer connection conductors  15   f  provided in the layer L 3  and the layer L 4 . Since the one end portion of each of the lead-out conductors  16 A to  16 C in the transmission direction is surrounded by the second interlayer connection conductors  15   f  connected to the ground conductors  12 A and  12 C, unnecessary radiation from the first interlayer connection conductor  14  connected to the terminal electrode  18  is reduced or prevented. The other end portions of the lead-out conductors  16 B and  16 C in the transmission direction are interposed between the second interlayer connection conductors  15   c ,  15   d , and  15   e  connected to the ground conductors  12 A and  12 C. This reduces or prevents unnecessary radiation from the first interlayer connection conductor  14  connecting the signal conductor  13 B and the lead-out conductor  16 B as well as the first interlayer connection conductor  14  connecting the signal conductor  13 C and the lead-out conductor  16 C. 
     In the layer L 3  of  FIG.  1 C , the signal conductor  13 B is provided at a position not overlapping in the lamination direction with the signal conductor  13 A of the layer L 2  on the region corresponding to the parallel extending portion of an insulating base material layer  11 C, and a ground conductor  12 B is provided at a position overlapping in the lamination direction with the signal conductor  13 A of the layer L 2 . The signal conductor  13 B is connected to the lead-out conductor  16 B via the first interlayer connection conductor  14  provided through the insulating base material layer  11 B in the other end portion of the lead-out conductor  16 B of the layer L 2 . As with the layer L 2 , a plurality of the second interlayer connection conductors  15   a  to  15   f  connecting between the ground conductors are provided through the insulating base material layer  11 C. 
     In the layer L 4  of  FIG.  1 D , the signal conductor  13 C is provided at a position corresponding to the signal conductor  13 A on the region corresponding to the parallel extending portion of an insulating base material layer  11 D. An end portion of the signal conductor  13 C is connected to the lead-out conductor  16 C via the first interlayer connection conductor  14  provided through the insulating base material layer  11 C in the layer L 3  and the first interlayer connection conductor  14  provided through the insulating base material layer  11 B in the other end portion of the lead-out conductor  16 C of the layer L 2 . As with the layers L 2  and L 3 , the plurality of the second interlayer connection conductors  15   a  to  15   f  connecting the ground conductors are provided through the insulating base material layer  11 D. 
     In the layer L 5  of  FIG.  1 E , the ground conductor  12 C is provided on an insulating base material layer  11 E, covering the regions corresponding to the parallel extending portion and the lead-out portion as well as the connection terminal portion  17 . The ground conductor  12 C is connected to the ground conductor  12 A of the layer L 1  via the second interlayer connection conductors  15   a  to  15   f  provided in each of the layers L 2  to L 4 . 
     The insulating base material layers  11 A to  11 E are preferably made of a thermoplastic resin, such as a liquid crystal polymer (LCP), for example. The ground conductors  12 A to  12 C, the signal conductor  13 A, the signal conductor  13 B, and the signal conductor  13 C are preferably made by, for example, a patterning process of a copper foil into a desired shape on a single-sided copper-clad base material including the copper foil affixed to one entire or substantially one entire surface of an insulating base material layer. The first interlayer connection conductor  14  and the second interlayer connection conductors  15   a  to  15   f  may penetrate the insulating base material layer in the thickness direction by, for example, forming a through-hole by a method such as irradiation with a laser beam from the surface of the single-sided copper-clad base material not covered with the copper foil, filling the through-hole with a conductive paste, and solidifying the paste by heating. 
       FIG.  2    is an exploded perspective view showing a state of connection between the layers through the first and second interlayer connection conductors in the multilayer board according to the first preferred embodiment as viewed from the side of the surface defining and functioning as the mounting surface. As shown in  FIG.  2   , the layers L 1  to L 5  are laminated such that the corresponding first interlayer connection conductors in the layers are connected to each other and that the corresponding second interlayer connection conductors in the layers are connected to each other, and are heated and pressurized in a lamination direction by a heating press, for example, to provide the multilayer board including a laminated insulating body integrated with the conductors. In  FIG.  2   , the ground conductors  12 A,  12 B, and  12 C are interconnected via the plurality of the second interlayer connection conductors  15   a  to  15   e  provided in the layers. To the terminal electrode  18  disposed in the connection terminal portion  17  of the layer L 1 , one end portion of each of the lead-out conductors  16 A,  16 B, and  16 C provided in the connection terminal portion  17  of the layer L 2  is connected via the first interlayer connection conductor  14 . To the other end portion of the lead-out conductor  16 B of the layer L 2 , the end portion of the signal conductor  13 B of the layer L 3  is connected via the first interlayer connection conductor  14  provided in the layer L 2 . To the other end portion of the lead-out conductor  16 C of the layer L 2 , the end portion of the signal conductor  13 C of the layer L 4  is connected via the first interlayer connection conductor  14  provided in the layer L 2  and the first interlayer connection conductor  14  provided in the layer L 3 . In  FIG.  2   , the state of connection of the second interlayer connection conductors  15   f  in the connection terminal portion  17  is not shown. Although the ground conductor  12 A is exposed on the mounting surface side in  FIG.  2   , a resist covering the ground conductor  12 A may preferably be further provided. When the multilayer board includes a resist, the ground conductor  12 A is protected from an external environment, and unnecessary connection to the mounting board is reduced or prevented. The resist preferably includes an insulating resin, for example. 
       FIG.  3    is a transparent plan view of a multilayer board  10  according to the first preferred embodiment as viewed from the mounting surface side. In  FIG.  3   , for simplicity, the ground conductors are not shown. The multilayer board  10  includes a laminated insulating body  19  including the insulating base material layers that are laminated, and the laminated insulating body  19  includes a parallel extending portion in which the signal conductors  13 A,  13 B, and  13 C are provided, and a lead-out portion including each of the lead-out conductors  16 A,  16 B and  16 C built therein and extending from the parallel extending portion to the connection terminal portion  17 . In the parallel extending portion, the signal conductors  13 A,  13 B, and  13 C each extend in the signal transmission direction. The signal conductors  13 A and  13 B are disposed separately from each other in the width direction of the parallel extending portion orthogonal or substantially orthogonal to the signal transmission direction as viewed in the lamination direction. The signal conductor  13 C overlaps with the signal conductor  13 A in the lamination direction as viewed in the lamination direction and is disposed separately from the signal conductor  13 A in the lamination direction. In  FIG.  3   , a large portion of the signal conductor  13 C is hidden behind the signal conductor  13 A. In the parallel extending portion, a first region A including the signal conductors  13 A and  13 C and a second region B including the signal conductor  13 B are spaced apart from each other in the width direction of the parallel extending portion along the transmission direction. 
     The signal conductor  13 A is integrated with the lead-out conductor  16 A in the transmission direction on the same surface of the insulating base material layer. The signal conductor  13 B is connected at each of the end portions in the transmission direction via the first interlayer connection conductor  14  to the end portion of the lead-out conductor  16 B in the parallel extending portion. The signal conductor  13 C includes a lead-out portion in the width direction of the parallel extending portion at each end portion in the transmission direction and is connected via the first interlayer connection conductor  14  in the parallel extending portion to the end portion of the lead-out conductor  16 C in the lead-out portion. The end portion of each of the lead-out conductors  16 A,  16 B and  16 C in the connection terminal portion  17  is connected to the terminal electrode  18  via the first interlayer connection conductor  14  (not shown). The ground conductors  12 A,  12 B, and  12 C not shown are interconnected via the second interlayer connection conductors  15   a  to  15   f , and the second interlayer connection conductors  15   a  to  15   f  provided in each of the layers are respectively connected in the lamination direction to the second interlayer connection conductors  15   a  to  15   f  disposed at corresponding positions in adjacent layers. The plurality of the second interlayer connection conductors  15   a  to  15   f  interconnecting the ground conductors  12 A,  12 B, and  12 C are provided in the outer edge portion of the parallel extending portion, between the signal conductors  13 A and  13 B, and in the outer circumferential portion of the connection terminal portion  17 , so that the ground state of the multilayer board used as a high frequency signal transmission path is stabilized. 
     In the multilayer board  10 , the plurality of the second interlayer connection conductors  15   a  interconnecting the ground conductors  12 A,  12 B, and  12 C is provided along the transmission direction in the outer edge portion of the first region A of the parallel extending portion. Although the four second interlayer connection conductors  15   a  are provided in  FIG.  3   , five or more interlayer connection conductors may be provided. In the multilayer board  10 , the plurality of the second interlayer connection conductors  15   c  interconnecting the ground conductors  12 A,  12 B, and  12 C is provided along the transmission direction in the outer edge portion of the second region B of the parallel extending portion. Although the four second interlayer connection conductors  15   c  are provided in  FIG.  3   , five or more interlayer connection conductors may be provided. In the multilayer board  10 , the second interlayer connection conductors  15   d  interconnecting the ground conductors  12 A,  12 B, and  12 C and the second interlayer connection conductors  15   e  interconnecting the ground conductors  12 A and  12 C are provided at both end portions in the transmission direction of the parallel extending portion. Since the signal conductor  13 A is surrounded by the ground conductors  12 A and  12 B as well as the second interlayer connection conductors  15   a  and  15   b , the isolation of the signal conductor  13 A is able to be further improved. Additionally, since the signal conductor  13 B is surrounded by the ground conductors  12 A and  12 C as well as the second interlayer connection conductors  15   b  and  15   c , the isolation of the signal conductor  13 B is able to be improved. Furthermore, since the signal conductor  13 C is surrounded by the ground conductors  12 B and  12 C as well as the second interlayer connection conductors  15   a  and  15   b , the isolation of the signal conductor  13 C is able to be improved. 
     The second interlayer connection conductors  15   f  are provided in the outer circumferential portion of the connection terminal portion  17  of the multilayer board  10  and surround the terminal electrode  18  and one end portion of each of the lead-out conductors  16 A to  16 C in the transmission direction. The second interlayer connection conductors  15   f  connect the ground conductor  12 A and the ground conductor  12 C in the lamination direction. Since the one end portion of each of the lead-out conductors  16 A to  16 C in the transmission direction is surrounded by the second interlayer connection conductors  15   f  connected to the ground conductors  12 A and  12 C, the unnecessary radiation from the first interlayer connection conductor  14  connected to the terminal electrode  18  is reduced or prevented. Additionally, the other end portions of the lead-out conductors  16 B and  16 C in the transmission direction are interposed between the second interlayer connection conductors  15   c ,  15   d , and  15   e  connected to the ground conductors  12 A and  12 C. This reduces or prevents the unnecessary radiation from the first interlayer connection conductor  14  connecting the signal conductor  13 B and the lead-out conductor  16 B as well as the first interlayer connection conductor  14  connecting the signal conductor  13 C and the lead-out conductor  16 C. 
     In the multilayer board  10 , the plurality of the second interlayer connection conductors  15   b  interconnecting the ground conductors  12 A,  12 B, and  12 C are provided along the transmission direction between the signal conductors  13 A,  13 C and the signal conductor  13 B. Although the two second interlayer connection conductors  15   b  are provided in  FIG.  3   , three or more second interlayer connection conductors  15   b  may be provided along the transmission direction. The plurality of the second interlayer connection conductors  15   b  may preferably be provided in a direction orthogonal or substantially orthogonal to the transmission direction between the signal conductors  13 A,  13 C and the signal conductor  13 B. This further improves both the isolation between the signal conductor  13 A and the signal conductor  13 B and the isolation between the signal conductor  13 C and the signal conductor  13 B. In addition to the second interlayer connection conductors  15   b , an auxiliary ground conductor (not shown) extending in the transmission direction may preferably be provided between the signal conductors  13 A,  13 C and the signal conductor  13 B. For example, the auxiliary ground conductor is preferably defined by, for example, a flat plate-shaped conductor along the transmission direction. The auxiliary ground conductor may be provided on the layer L 2  or L 4  in which the signal conductor  13 A or  13 C is provided or may be provided between the signal conductors  13 A and  13 C and/or between the signal conductors  13 B and  13 C in the lamination direction. A plurality of auxiliary ground conductors may preferably be arranged separately from each other along the transmission direction. 
     In the multilayer board  10 , the signal conductors and the lead-out conductors  16 A to  16 C in the transmission direction built into the lead-out portions are connected at the end portions of the parallel extending portion. The end portion of each of the lead-out conductors  16 A to  16 C on the connection terminal portion  17  side is connected to the terminal electrode  18  via the first interlayer connection conductor  14 . As shown in  FIGS.  10  and  11   , a connector may be connected to the terminal electrode  18  using a connecting material such as solder, for example, and may be connected to a connector on the mounting board. Alternatively, the terminal electrode  18  may be directly connected to a mounting electrode on the mounting board by a connecting material, such as solder, for example. 
       FIG.  4    is a cross-sectional view of the multilayer board  10  showing a cross section of the parallel extending portion taken along a cutting line a-a′ of  FIG.  3    with the mounting surface facing downward. In  FIG.  4   , a resist  19   a  covering the ground conductor  12 A is provided on the mounting surface side of the multilayer board. The multilayer board  10  includes, in the order from the side closer to the mounting surface, the signal conductor  13 A (hereinafter also referred to as a first signal conductor), the conductor  13 B (hereinafter also referred to as a second signal conductor) provided separately from the signal conductor  13 A in the width direction of the parallel extending portion as viewed in the lamination direction, and the signal conductor  13 C (hereinafter also referred to as a third signal conductor) overlapping with the signal conductor  13 A as viewed in the lamination direction and provided separately therefrom in the lamination direction, disposed inside the laminated insulating body  19 . In the multilayer board  10 , the signal conductors  13 A,  13 B, and  13 C are each sandwiched between two ground conductors via the insulating base material layer. Specifically, the signal conductor  13 A is sandwiched between the ground conductors  12 A and  12 B via the insulating base material layer, the signal conductor  13 B is sandwiched between the ground conductors  12 A and  12 C via the insulating base material layer, and the signal conductor  13 C is sandwiched between the ground conductors  12 B and  12 C via the insulating base material layer. The ground conductors  12 B and  12 C are built into the laminated insulating body  19 . The ground conductor  12 A is disposed on the mounting surface side of the laminated insulating body  19  and is covered with the resist  19   a.    
     The parallel extending portion of the multilayer board  10  is sectioned along the transmission direction into the first region A including a larger number of laminated signal conductors that overlap as viewed in the lamination direction and the second region B including a smaller number of laminated signal conductors than the first region A. Although the number of signal conductors arranged separately from each other in the width direction of the parallel extending portion is two in  FIG.  4   , three or more signal conductors may be arranged separately from each other in the width direction. In this case, the first region A is a region including the largest number of laminated signal conductors that overlap, and the second region B is each of the regions including the signal conductors arranged separately in the width direction from the signal conductors included in the first region A. In  FIG.  4   , the first signal conductor  13 A and the third signal conductor  13 C are arranged such that a width of an overlapping portion coincides with the respective line widths as viewed in the lamination direction. However, the signal conductors may be structured such that the width of the overlapping portion is narrower than the line width of any of the signal conductors. 
     In the multilayer board  10 , an interval between the ground conductors  12 A and  12 B sandwiching the first signal conductor  13 A and an interval between the ground conductors  12 B and  12 C sandwiching the third signal conductor  13 C included in the first region A are narrower than an interval between the ground conductors  12 A and  12 C sandwiching the second signal conductor  13 B in the second region B. In the multilayer board  10 , if the second region B includes a plurality of second signal conductors  13 B that overlap in the lamination direction, the first region A includes a signal conductor sandwiched by ground conductors at an interval narrower than the minimum value of the interval of the ground conductors sandwiching the second signal conductors  13 B. 
     In the multilayer board  10 , the line width of the signal conductor  13 B included in the second region B is wider than the line width of any of the first signal conductor  13 A and the third signal conductor  13 B included in the first region A. By providing the wider line width of the signal conductor  13 B sandwiched by the ground conductors at a wider interval, impedance is able to be easily matched with a signal transmission portion having a narrow ground conductor interval. The multilayer board  10  used as a transmission line is generally designed with a characteristic impedance of about 50Ω, for example. By making the line width of the second signal conductor  12 B wider, the same characteristic impedance of about 50Ω is able to be achieved in a first signal transmission portion including the first signal conductor  13 A, the ground conductor  12 A, and the ground conductor  12 B and a third signal transmission portion including the third signal conductor  13 C, the ground conductor  12 B, and the ground conductor  12 C, as well as a second signal transmission portion including the second signal conductor  13 B, the ground conductor  12 A, and the ground conductor  12 C. A signal passing through the second signal conductor  13 B having the larger line width is reduced in conductor loss as compared to a signal passing through the first signal conductor  13 A or the third signal conductor  13 C. Therefore, for example, if a signal of about 600 MHz to about 900 MHz band or about 2 GHz band, for example, used for cellular phones is assigned to a first signal transmission portion or a third signal transmission portion, for example, a signal of about 5 GHz band used for WiFi is preferably assigned to a signal transmission portion. In other words, it is preferable to assign a signal of a high frequency band, which is more significantly affected by transmission loss, to a second signal transmission portion including the signal conductor with a wider line width. Instead of making the line width of the signal conductor wider, the signal conductor may be made thicker to adjust the characteristic impedance. For example, the second signal conductor  13 B may be thicker than the first signal conductor  13 A or the third signal conductor  13 C to match the impedance. However, a process of manufacturing the multilayer board  10  used as a transmission line may be simplified by making the second signal conductor  13 B wider to match the impedance. 
     In the multilayer board  10 , the ground conductors  12 A,  12 B, and  12 C are provided along the transmission direction of the signal conductors. The ground conductors  12 A,  12 B, and  12 C are connected to each other since the second interlayer connection conductors  15   a  to  15   f  provided through the insulating base material layer are connected in the lamination direction. In  FIG.  4   , the ground conductors  12 A and  12 C are connected to each other via the second interlayer connection conductors  15   a  and  15   c  provided in the outer edge portion of the parallel extending portion and the signal interlayer connection conductor  15   b  provided between the first signal conductor and the third signal conductor. The ground conductor  12 B is connected to the ground conductors  12 A and  12 C via the second interlayer connection conductor  15   a  provided in the outer edge portion of the parallel extending portion and the second interlayer connection conductor  15   b  provided between the first signal conductor and the third signal conductor. 
       FIG.  5    is a cross-sectional view of the multilayer board  10  including auxiliary ground conductors extending in the transmission direction provided between the signal conductors  13 A,  13 C and the signal conductor  13 B, showing a cross section of the parallel extending portion corresponding to the cutting line a-a′ of  FIG.  3    with the mounting surface facing downward. In  FIG.  5   , the resist  19   a  covering the ground conductor  12 A is disposed on the mounting surface side of the multilayer board. In  FIG.  5   , an auxiliary ground conductor  12 D connected to the second interlayer connection conductor  15   b  is disposed between the signal conductors  13 A and  13 B separately from each of the signal conductors  13 A and  13 B in the width direction of the parallel extending portion and the lamination direction. An auxiliary ground conductor  12 E connected to the second interlayer connection conductor  15   b  is disposed between the signal conductors  13 B and  13 C separately from the signal conductors  13 B and  13 C in the width direction of the parallel extending portion and the lamination direction. By providing the auxiliary ground conductor  12 D, the isolation between the signal conductors  13 A and  13 B is able to be further improved. By providing the auxiliary ground conductor  12 E, the isolation between the signal conductors  13 C and  13 B is able to be further improved. The auxiliary ground conductors  12 D and  12 E preferably have a flat plate shape extending in the transmission direction, for example. 
       FIG.  6    is an exemplary transparent plan view of the parallel extending portion of the multilayer board  10  including the auxiliary ground conductors  12 D and  12 E connected to the second interlayer connection conductor  15   b  as viewed from the mounting surface side. In  FIG.  6   , for simplicity, the ground conductors  12 A,  12 B, and  12 C are not shown. In  FIG.  6   , the auxiliary ground conductor  12 D connected to the second interlayer connection conductors  15   b  is provided inside the laminated insulating body  19  defined by a flat plate-shaped conductor continuously extended in the transmission direction between the signal conductors  13 B and  13 A. The auxiliary ground conductor  12 E is hidden behind the auxiliary ground conductor  12 D. The auxiliary ground conductor  12 D is connected to the second interlayer connection conductor  15   a  via the ground conductors  12 A,  12 B, and  12 C not shown and is connected to the second interlayer connection conductors  15   c ,  15   d , and  15   e  via the ground conductors  12 A and  12 C. In a planar view, the signal conductor  13 A is surrounded by the auxiliary ground conductor  12 D and the second interlayer connection conductors  15   a  and  15   b , and the signal conductor  13 B is surrounded by the auxiliary ground conductor  12 D and the second interlayer connection conductors  15   b  and  15   c . The auxiliary ground conductors  12 D and  12 E may have the same width or different widths. 
       FIG.  7    is another exemplary transparent plan view of the parallel extending portion of the multilayer board  10  including the auxiliary ground conductors  12 D and  12 E connected to the second interlayer connection conductors  15   b  as viewed from the mounting surface side. In  FIG.  7   , for simplicity, the ground conductors  12 A,  12 B, and  12 C are not shown. In  FIG.  7   , the auxiliary ground conductors  12 D respectively connected to the second interlayer connection conductors  15   b  are provided inside the laminated insulating body  19  defined by rectangular or substantially rectangular flat plates provided separately from each other in the transmission direction between the signal conductors  13 B and  13 A. The auxiliary ground conductors  12 E are hidden behind the auxiliary ground conductors  12 D. In a planar view, the signal conductor  13 A is surrounded by the auxiliary ground conductors  12 D and the second interlayer connection conductors  15   a  and  15   b , and the signal conductor  13 B is surrounded by the auxiliary ground conductors  12 D and the second interlayer connection conductors  15   b  and  15   c . The auxiliary ground conductors  12 D preferably have a rectangular or substantially rectangular shape in  FIG.  7    or may have a polygonal shape, a circular shape, an elliptical shape, an oval shape, etc. The auxiliary ground conductors  12 D and  12 E may have the same shape or different shapes. 
       FIG.  8    is a partially transparent plan view of the multilayer board  10  on which the resist  19   a  covering the ground conductor  12 A is provided on the mounting surface side as viewed from the mounting surface side. The ground conductors  12 A and  12 C are not shown in a transparent portion. In  FIG.  8   , in the connection terminal portion  17 , the terminal electrode  18 , and ground electrodes  18   a  surrounding the terminal electrode  18  are exposed from opening portions of the resist  19   a . The ground electrodes  18   a  are exposed surfaces of the ground conductor  12 A in the opening portions of the resist  19   a , for example. 
       FIG.  9    is a cross-sectional view of the multilayer board  10  including the resist  19   a  on the mounting surface side taken along a line a-a′ of  FIG.  8    and is a cross-sectional view taken with the mounting surface facing upward. In  FIG.  9   , the ground conductor  12 C built into the laminated insulating body  19  and the ground conductor  12 A disposed on the surface of the laminated insulating body  19  on the mounting surface side are continuously disposed in the lead-out portion and the parallel extending portion. The resist  19   a  is disposed on the surface of the ground conductor  12 A on the mounting surface side. The ground conductors  12 A and  12 C are connected, together with the ground conductor  12 B disposed therebetween, in the lamination direction via the second interlayer connection conductor  15   b . The ground conductor  12 B is provided in the same layer as the signal conductor  13 B (not shown) included in the second region. The ground conductor  12 B and the ground conductor  12 A sandwich the signal conductor  13 A provided in the same layer as the lead-out conductor  16 C in the transmission direction in the first region. The ground conductor  12 B and the ground conductor  12 C sandwich the signal conductor  13 C in the first region. This improves the isolation between the signal conductors and reduces or prevents crosstalk. 
     The lead-out conductor  16 C in the transmission direction disposed in the lead-out portion is sandwiched between the ground conductors  12 A and  12 C, so that unnecessary radiation to the outside is reduced or prevented. The end portion of the lead-out conductor  16 C on the connection terminal portion  17  side is connected to the terminal electrode  18  disposed on the mounting surface via the first interlayer connection conductor  14  disposed through the insulating base material layer. The resist  19   a  is not disposed on a portion of the surface of the terminal electrode  18  on the mounting surface side, and the surface is exposed on the mounting surface side. The ground electrodes  18   a  are provided around the terminal electrode  18  as exposed portions of the ground conductor  12 A in the opening portions of the resist  19   a . The other end portion of the lead-out conductor  16 C on the side opposite to the connection terminal portion  17  is connected to the end portion of the signal conductor  12 C via the first interlayer connection conductor  14  disposed in the lamination direction through the insulating base material layer. 
       FIG.  10    is a cross-sectional view of the multilayer board  10  including connectors  18   b  on the connection terminal portion  17  corresponding to the line a-a′ of  FIG.  8    and is a cross-sectional view taken with the mounting surface facing upward. In  FIG.  10   , the resist  19   a  covering the ground conductor  12 A is disposed on the mounting surface side. The connectors  18   b  are each disposed on the resist  19   a  of the connection terminal portion  17 , and the terminal electrode  18  exposed from the resist  19   a  and the exposed portions of the ground conductor  12 A in the opening portions of the resist  19   a  are connected respectively via connection materials  18   c  to the connector  18   b . For example, solder may preferably be used for the connection materials  18   c . The connector  18   b  is connected via the connection material  18   c , the terminal electrode  18 , the first interlayer connection conductor  14 , and the lead-out conductor  16 C to the signal conductor  13 C and is connected via the connection material  18   c  to the ground conductor  12 A. 
       FIG.  11    is a schematic cross-sectional view for explaining a method of mounting the multilayer board  10  including the connectors  18   b  in the connection terminal portion  17  onto a mounting board  100 . Each of the connectors  18   b  of the multilayer board  10  is connected via the connection material  18   c , the terminal electrode  18 , the first interlayer connection conductor  14 , and the lead-out conductor  16 C to the signal conductor  13 C. Each of the connectors  18   b  is connected via the connection material  18   c  to the exposed portions of the ground conductor  12 A in the opening portions of the resist  19   a . The mounting board  100  includes a resist  102  disposed on an insulating base material layer  101  and partially exposing terminal portions  103   a  and  103   b . The terminal portions  103   a  and  103   b  are connected via connection materials not shown to connectors  104   a  and  104   b , respectively. The terminal portions  103   a  and  104   b  each include a signal terminal and a ground terminal, for example. The two connectors  18   b  of the multilayer board  10  are respectively connected to the connectors  104   a  and  104   b  of the mounting board  100 , so that the multilayer board  10  is mounted on the mounting board  100 . When the multilayer board  10  is mounted on the mounting board  100 , the signal terminal of  103   a  of the mounting board  100  is connected sequentially via the connector  104   a , the connector  18   b , the connection material  18   c , the terminal electrode  18 , the first interlayer connection conductor  14 , the lead-out conductor  16 C, and the first interlayer connection conductor  14  to one end portion of the signal conductor  13 C. The other end portion of the signal conductor  13 C is connected sequentially via the first interlayer connection conductor  14 , the lead-out conductor  16 C, the first interlayer connection conductor  14 , the terminal electrode  18 , the connection material  18   c , the connector  18   b , and the connector  104   b  to the signal terminal of  103   b  of the mounting board  100 . As a result, the signal terminal of the terminal portion  103   a  and the signal terminal of the terminal portion  103   b  of the mounting board  100  are connected to the multilayer board  10 . The ground terminals of  103   a  and  103   b  of the mounting board  100  are connected sequentially via the connector  104   a , the connector  18   b , and the connection material  18   c  to the ground electrode  18   a  integrally formed with the ground conductor  12 A. Therefore, a signal is transmitted between the terminal portions  103   a  and  103   b  of the mounting board  100  to the multilayer board  10 . 
       FIG.  12    is a schematic cross-sectional view for explaining another example of the method of mounting the multilayer board  10  onto the mounting board  100 . The mounting board  100  includes the resist  102  disposed on the insulating base material layer  101  and partially exposing signal terminals  103   a   1  and  103   a   2  and ground terminals  103   b   1  and  103   b   2 . Connection materials  105  are respectively disposed on the signal terminals  103   a   1  and  103   a   2  and the ground terminals  103   b   1  and  103   b   2 . For example, solder is preferably used for the connection materials  105 . The terminal electrodes  18  and the ground electrodes  18   a  of the connection terminal portions  17  are connected via the connection materials  105  to the signal terminals and the ground terminals of the mounting board, so that the multilayer board  10  is mounted on the mounting board. When the multilayer board  10  is mounted on the mounting board  100 , the signal terminal of  103   a   1  of the mounting board  100  is connected sequentially via the connection material  105 , the terminal electrode  18 , the first interlayer connection conductor  14 , the lead-out conductor  16 C, and the first interlayer connection conductor  14  to one end portion of the signal conductor  13 C. The other end portion of the signal conductor  13 C is connected sequentially via the first interlayer connection conductor  14 , the lead-out conductor  16 C, the first interlayer connection conductor  14 , the terminal electrode  18 , and the connection material  105  to the signal terminal  103   a   2  of the mounting board  100 . As a result, the signal terminal  103   a   1  and the signal terminal  103   a   2  of the mounting board  100  are connected to the multilayer board  10 . The ground terminals  103   b   1  and  103   b   2  of the mounting board  100  are respectively connected via the connection materials  105  to the ground electrodes  18   a  that are exposed portions of the ground conductor  12 A in the opening portions of the resist  19   a  of the multilayer board  10 . 
       FIG.  13    is a plan view schematically showing a mounting state of the multilayer board  10  on the mounting board  100 . The multilayer board  10  and other electronic components  110  are provided on the mounting board  100 . The electronic components  110  include chip components, such as integrated circuits (ICs), resistors, capacitors, and inductors, for example. The multilayer board  10  includes connection terminal portions  17   a   1 ,  17   b   1 ,  17   c   1 ,  17   a   2 ,  17   b   2 , and  17   c   2  respectively connected to terminal portions on the mounting board and is thus mounted on the mounting board. The connection terminal portions are connected to the terminal portions on the mounting board through connectors or a connection material, such as solder, for example. For example, the terminal portion connected to the terminal portion  17   a   1  of the multilayer board  10  is connected to the terminal portion connected to the terminal portion  17   a   2  via the multilayer board  10 , and the terminal portion connected to the terminal portion  17   b   1  of the multilayer board  10  is connected to the terminal portion  17   b   2  via the multilayer board  10 , and the terminal portion connected to the terminal portion  17   c   1  of the multilayer board  10  is connected to the terminal portion connected to the terminal portion  17   c   2  via the multilayer board  10 . 
       FIG.  14    is a plan view schematically showing the mounting board  100  including the multilayer board  10  mounted thereon and housed in a housing  120 . In  FIG.  14   , the connectors  18   b  provided on the connection terminal portions  17  of the multilayer board  10  are respectively connected to the connectors  104   a  and  104   b  provided on the mounting board  100 , so that the multilayer board  10  is mounted on the mounting board  100  over the electronic component  110 . The multilayer board  10  is then housed in the housing  120 . The multilayer board  10  includes the signal conductors provided in the first and second regions in the width direction of the parallel extending portion. As a result, the thickness of the entire multilayer board  10  is able to be thinner as compared to when all of the signal conductors are provided in the lamination direction. Therefore, by using a flexible base material, such as a liquid crystal polymer (LCP), for example, for the insulating base material layer of the multilayer board  10 , the multilayer board  10  is able to be bent and disposed depending on a shape of a gap between the mounting board  100  including the electronic components  110  provided thereon and the housing  120 . 
     Second Preferred Embodiment 
       FIG.  15    is a transparent plan view of a multilayer board  20  according to a second preferred embodiment of the present invention as viewed from the mounting surface side. In  FIG.  15   , for simplicity, the ground conductors are not shown. The multilayer board  20  of the second preferred embodiment has the same or substantially the same configuration as the multilayer board of the first preferred embodiment except that the multilayer board includes a curved portion C with a transmission direction that is bent along a plane orthogonal or substantially orthogonal to the lamination direction in the parallel extending portion and that the first region is disposed at an inner position in the curved portion C. The multilayer board having the curved portion in the parallel extending portion is mounted to detour around the electronic component  110  of  FIG.  13   , for example. 
     In the multilayer board  20 , signal conductors  23 A,  23 B, and  23 C extend in the transmission direction inside a laminated insulating body  29 . The signal conductors  23 A and  23 B are arranged separately from each other in the width direction of the parallel extending portion orthogonal or substantially orthogonal to the signal transmission direction. The signal conductor  23 C overlaps with the signal conductor  23 A in the lamination direction and is disposed separately from the signal conductor  23 A in the lamination direction. In the multilayer board  20 , the first region including a larger number of signal conductors laminated to overlap as viewed in the lamination direction is disposed at a position on the inner side relative to the second region in the curved portion C. In  FIG.  15   , the first region includes the signal conductors  23 A and  23 C, and the second region includes the signal conductor  23 B. The first region and the second region are sectioned in the width direction of the parallel extending portion along the transmission direction. 
     In  FIG.  15   , a plurality of second interlayer connection conductors  25   b  interconnecting ground conductors not shown are provided along the transmission direction between the signal conductors  23 A,  23 C and the signal conductor  23 B. Additionally, a plurality of second interlayer connection conductors  25   a  and  25   c  interconnecting the ground conductors are provided along the transmission direction in the outer edge portion of the parallel extending portion. By providing the plurality of the second interlayer connection conductors  25   a  and  25   c  along the transmission direction in the outer edge portion of the parallel extending portion, unnecessary radiation from the signal conductors to the outside is able to be effectively reduced or prevented. Particularly, since the plurality of the second interlayer connection conductors  25   a  are provided in the outer edge portion of the parallel extending portion on the inner side of the curved portion C of the multilayer board  20 , an influence of crosstalk, for example, is able to be more effectively reduced or prevented from occurring due to unnecessary radiation between different positions of the adjacent signal conductors on the inner side of the curved portion C. Although the same numbers of the second interlayer connection conductors  25   a ,  25   b , and  25   c  are provided along the transmission direction in  FIG.  15   , the second interlayer connection conductors  25   a ,  25   b , and  25   c  may be provided in respective different numbers. For example, the number of the second interlayer connection conductors  25   a  provided on the inner side of the curved portion C may be larger or smaller than the number of the second interlayer connection conductors  25   c  provided on the outer side of the curved portion C. The second interlayer connection conductors  25   a ,  25   b , and  25   c  may respectively be arranged at equal or substantially equal intervals. 
     Furthermore, the multilayer board  20  may include an auxiliary ground conductor (not shown) extending in the transmission direction, in addition to the ground conductors and the second interlayer connection conductors  25   a  to  25   c . By including the auxiliary ground conductor, the isolation between the signal conductors is able to be further improved. For example, the auxiliary ground conductor may preferably be a continuous flat plate-shaped conductor along the transmission direction or may be flat plate-shaped conductors separated from each other along the transmission direction. 
     A cross-sectional view of the multilayer board  20  showing a cross section of the parallel extending portion taken along a cutting line b-b′ of  FIG.  15    with the mounting surface facing downward is the same as  FIG.  4   . In the multilayer board  20 , an interval between the two ground conductors sandwiching the signal conductor included in the first region A is preferably smaller than an interval between the two ground conductors sandwiching the signal conductor in the second region B. Since the signal conductors included in the first region A on the inner side of the curved portion C are sandwiched by the ground conductors at narrow intervals, an influence of crosstalk, for example, is able to be effectively reduced or prevented from occurring due to unnecessary radiation between different positions of the adjacent signal conductors on the inner side of the curved portion C. 
       FIG.  16    is a cross-sectional view of the multilayer board  20  including auxiliary ground conductors extending in the transmission direction disposed between the signal conductors  23 A,  23 C and the signal conductor  23 B and in the outer edge portion of the parallel extending portion, showing a cross section of the parallel extending portion corresponding to the cutting line b-b′ of  FIG.  15    with the mounting surface facing downward. The multilayer board  20  includes, in the order from the side closer to the mounting surface, the signal conductor  23 A, the conductor  23 B disposed separately from the signal conductor  23 A in the width direction of the parallel extending portion as viewed in the lamination direction, and the signal conductor  23 C including an overlap with the signal conductor  23 A as viewed in the lamination direction and disposed separately therefrom in the lamination direction, provided inside the laminated insulating body  29 . In the multilayer board  20 , the signal conductor  23 A is sandwiched between ground conductors  22 A and  22 B via the insulating base material layer, the signal conductor  23 B is sandwiched between ground conductors  22 A and  22 C via the insulating base material layer, and the signal conductor  23 C is sandwiched between the ground conductors  22 B and  22 C via the insulating base material layer. The ground conductors  22 B and  22 C are built into the laminated insulating body  29 , and the ground conductor  22 A is disposed on the surface on the mounting surface side of the laminated insulating body  29 . In  FIG.  16   , a resist  29   a  covering the ground conductor  22 A is provided on the mounting surface side of the laminated insulating body  29 . 
     In  FIG.  16   , an auxiliary ground conductor  22 D 2  connected to the second interlayer connection conductor  25   b  is disposed between the signal conductors  23 A and  23 B separately from the signal conductors  23 A and  23 B in the width direction of the parallel extending portion and the lamination direction. An auxiliary ground conductor  22 E 2  connected to the second interlayer connection conductor  25   a  is disposed between the signal conductors  23 B and  23 C separately from the signal conductors  23 B and  23 C in the width direction of the parallel extending portion and the lamination direction. In the outer edge portion of the first region A, auxiliary ground conductors  22 D 1  and  22 E 1  connected to the second interlayer connection conductor  25   a  are provided with the ground conductor  22 B interposed between the signal conductors  23 A and  23 C. In the outer edge portion of the second region B, auxiliary ground conductors  22 D 3  and  22 E 3  connected to the second interlayer connection conductor  25   c  are disposed separately from each other in the lamination direction. Since the auxiliary ground conductors  22 D 1 ,  22 E 1 ,  22 D 3 , and  22 E 32  are provided in the outer edge portion of the parallel extending portion, the unwanted radiation from the signal conductors  23 A,  23 B, and  23 C to the outside is able to be more effectively reduced or prevented. Particularly, since the auxiliary ground conductors  22 D 1  and  22 E 1  are provided on the inner side of the curved portion C, an influence of crosstalk, for example, is able to be effectively reduced or prevented from occurring due to unnecessary radiation between different positions of the adjacent signal conductors on the inner side of the curved portion C. 
       FIG.  17    is an exemplary transparent plan view of the parallel extending portion of the multilayer board  20  including the auxiliary ground conductors  22 D 1  to  22 D 3  and  22 E 1  to  22 E 3  connected to the second interlayer connection conductor  25   b  as viewed from the mounting surface side. In  FIG.  17   , for simplicity, the ground conductors  12 A,  12 B, and  12 C are not shown. In  FIG.  17   , the auxiliary ground conductors  22 D 1  connected to the second interlayer connection conductor  25   a  are preferably oblong flat plate-shaped conductors separately from each other along the transmission direction in the outer edge portion of the parallel extending portion inside the laminated insulating body  29 . The auxiliary ground conductors  22 E 1  are hidden behind the auxiliary ground conductors  22 D 2 . The auxiliary ground conductors  22 D 2  connected to the second interlayer connection conductor  25   a  are preferably oblong flat plate-shaped conductors provided separately from each other along the transmission direction between the signal conductors  23 B and  23 A. The auxiliary ground conductors  22 E 2  are hidden behind the auxiliary ground conductors  22 D 2 . The auxiliary ground conductors  22 D 3  connected to the second interlayer connection conductor  25 C are preferably oblong flat plate-shaped conductors separately from each other along the transmission direction in the outer edge portion of the parallel extending portion. The auxiliary ground conductors  22 D 1 ,  22 D 2 , and  22 D 3  are interconnected via the ground conductors  22 A,  22 B, and  22 C not shown. In a planar view, the signal conductor  23 A is surrounded by the auxiliary ground conductors  22 D 1  and  22 D 2 , and the signal conductor  13 B is surrounded by the auxiliary ground conductors  22 D 2  and  22 D 3 . The auxiliary ground conductors  22 D 1  and  22 D 3  preferably have the same or substantially the same width, which is different from the width of the auxiliary ground conductor  22 D 2 . 
     Third Preferred Embodiment 
       FIG.  18    is a transparent plan view of a multilayer board  30  according to a third preferred embodiment of the present invention as viewed from the mounting surface side. In  FIG.  18   , for simplicity, the ground conductors are not shown. The multilayer board  30  of the third preferred embodiment has the same or substantially the same configuration as the multilayer board of the first preferred embodiment except that the first region includes four signal conductors while the second region includes three signal conductors and that signal conductors that overlap in the lamination direction are provided such that a total length of lead-out conductors in the lamination direction is shorter as compared to when the signal conductors are arranged at equal or substantially equal intervals in the lamination direction. 
     The parallel extending portion of the multilayer board  30  is sectioned in the width direction of the parallel extending portion along the transmission direction into a first region including signal conductors  33 A,  33 C 1 ,  33 C 2 , and  33 C 3  and a second region including signal conductors  33 B 1 ,  33 B 2 , and  33 B 3 . 
     The signal conductor  33 A is integral with a lead-out conductor in the transmission direction on the same surface of the insulating base material layer. The signal conductors  33 B 1  to  33 B 3  are connected at respective end portions in the transmission direction via first interlayer connection conductors  34  respectively to end portions of lead-out conductors in the parallel extending portion. The signal conductors  33 C 1  to  33 C 3  include lead-out portions in the width direction of the parallel extending portion at respective end portions in the transmission direction and are connected at the lead-out portions via the first interlayer connection conductors  34  to the end portions of lead-out conductors in the parallel extending portion. 
       FIG.  19    is a cross-sectional view of the multilayer board  30  showing a cross section of the parallel extending portion taken along a cutting line c-c′ of  FIG.  18    with the mounting surface facing downward. In  FIG.  19   , no resist covering the ground conductor  32 A is provided on the mounting surface side of the laminated insulating body. The multilayer board  30  includes the first region A in which the signal conductor  33 A (hereinafter also referred to as a first signal conductor) and the signal conductors  33 C 1  to  33 C 3  (hereinafter all also referred to as third signal conductors) arranged separately from each other in the lamination direction are provided in the order from the side closer to the mounting surface inside the laminated insulating body  19 . In the second region B, the signal conductors  33 B 1  to  33 B 3  (hereinafter all also referred to as second signal conductors) are provided in the order from the side closer to the mounting surface inside the laminated insulating body  19 . The second signal conductors  33 B 1  to  33 B 3  are arranged separately from the signal conductor  33 A in the width direction of the parallel extending portion when viewed in the lamination direction. The third signal conductors  33 C 1  to  33 C 3  overlap with the signal conductor  33 A when viewed in the lamination direction. 
     In  FIG.  19   , an interval between the ground conductors  32 A and  32 B sandwiching the first signal conductor  33 A and an interval between the ground conductors  32 B and  32 C sandwiching the third signal conductor  33 C 1  are narrower than an interval between the ground conductors  32 A and  32 C sandwiching the second signal conductor  33 B 1 . The line widths of the second signal conductors  33 B 1  to  33 B 3  included in the second region B are wider than the line widths of the first signal conductor  33 A and the third signal conductors  33 C 1  to  33 C 3  included in the first region A. 
     In the first region A of the multilayer board  30 , the signal conductors arranged along the lamination direction are provided at a higher density on the mounting surface side. Specifically, when the multilayer board  30  is divided into two regions by a plane passing through a middle point of the thickness thereof and orthogonal or substantially orthogonal to the thickness direction, the number of signal conductors included in the region on the mounting surface side is larger than a number of signal conductors included in the region on the side opposite to the mounting surface side. 
       FIG.  20    is a cross-sectional view of the multilayer board  30  showing a cross section of the parallel extending portion taken along a cutting line d-d′ of  FIG.  18    with the mounting surface facing downward. In  FIG.  20   , no resist covering the ground conductor  32 A is provided on the mounting surface side of the laminated insulating body. In the multilayer board  30 , the end portions of the signal conductors  33 B 1  to  33 B 3  and  33 C 1  to  33 C 3  are respectively connected via the first interlayer connection conductors  34  extending along the lamination direction from the signal conductors toward the mounting surface to the end portion of the lead-out conductors  36 B 1  to  36 B 3  and  36 C 1  to  36 C 3  in the transmission direction in the parallel extending portion. The first interlayer connection conductors  34  are provided such that the first interlayer connection conductors provided through the insulating base material layers are connected between the layers in the lamination direction. 
     In the second region B of  FIG.  20   , the signal conductors  33 B 1  to  33 B 3  are arranged at equal or substantially equal intervals in the lamination direction. In the first region A, the signal conductors  33 A and  33 C 1  to  33 C 3  are provided such that the total length of the first interlayer connection conductors  34  is shorter as compared to when the signal conductors are arranged at equal or substantially equal intervals in the lamination direction. 
     The ground conductors are connected by the second interlayer connection conductors provided through the insulating base material layers in the preferred embodiments described above or may be connected by through-holes, instead of the interlayer connection conductors. The ground conductors may be connected by a side conductor layer disposed along the lamination direction at end portions in the width direction in the parallel extending portion. The side conductor layer is disposed at both end portions in the width direction, for example. The ground conductors and the side conductor layer may be connected by extending the ground conductors to the end portions in the width direction or may be connected by providing lead-out conductors from the ground conductors to the end portions in the width direction. The side conductor layer may be provided by plating on the side surface of the multilayer board, for example. Furthermore, the side conductor layer may be provided on the entire or substantially the entire side surface of the parallel extending portion along the transmission direction or may be defined by a plurality of side conductor layers arranged separately from each other along the transmission direction and extend along the lamination direction. 
     The multilayer boards according to the preferred embodiments described above are each used as a transmission line for high frequency signals. Although the application of the first and third signal conductors and the second signal conductor included in the multilayer board is not particularly limited, for example, the second signal conductor may be made wider than the first and third signal conductors and is therefore suitable for signal transmission in which a lower loss is required, for example, signal transmission in a higher frequency band. On the other hand, the first signal conductor and the third signal conductor are sandwiched between the ground conductors at a narrower interval and are therefore suitable for signal transmission in which more isolation is required, for example, signal transmission in a lower frequency band. 
     All the documents, patent applications, and technical standards described in this description are hereby incorporated by reference to the same extent as if each of the documents, patent applications, and technical standards is specifically and individually described as being incorporated by reference. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.