Patent Publication Number: US-2022232708-A1

Title: Transmission line, method of manufacturing transmission line, and electronic apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2019-206607 filed on Nov. 15, 2019 and Japanese Patent Application No. 2020-059348 filed on Mar. 30, 2020, and is a Continuation application of PCT Application No. PCT/JP2020/041422 filed on Nov. 5, 2020. The entire contents of each application are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a transmission line for transmitting a high-frequency signal, a method of manufacturing a transmission line, and an electronic apparatus. 
     2. Description of the Related Art 
     As a transmission line according to the related art, for example, a signal transmission line is disclosed in International Publication No. 2017/130731. The signal transmission line includes a laminate having a hollow portion, a signal conductor, a plurality of ground conductors, and an interlayer connection conductor. The laminate is formed by laminating resin base materials. The signal conductor, the ground conductors, and the interlayer connection conductor are formed in the laminate. The signal conductor is disposed so as to be exposed to the hollow portion. The ground conductors are electrically connected by the interlayer connection conductor. 
     SUMMARY OF THE INVENTION 
     In the signal transmission line disclosed in International Publication No. 2017/130731, resin base materials formed with predetermined conductors are laminated and hot-pressed to join the resin base materials together and join the conductors together. Further, the hollow portion is formed by joining the resin base materials to each other. In this case, in the above laminating and hot-pressing processes, it is difficult to join the resin base materials together and join the conductors together, while ensuring a distance between a signal line and the ground conductor. Accordingly, the distance between the signal line and the ground conductor varies, and variations easily occur in transmission characteristics of the signal transmission line. 
     Preferred embodiments of the present invention provide methods of manufacturing transmission lines each having a hollow structure in which variations in transmission characteristics are reduced, and also provide electronic apparatuses. 
     A transmission line according to a preferred embodiment of the present invention includes a first structure including a first resin base material having flexibility, and a first ground conductor on the first resin base material, a second structure including a second resin base material having flexibility, and a first signal line and an interlayer connection conductor in or on the second resin base material, a first spacer between the first structure and the second structure, and a first metal joining material to join the first structure and the second structure with the first spacer interposed therebetween, wherein a first hollow portion is between the first structure and the second structure joined to each other with the first spacer interposed therebetween, the first signal line and the first ground conductor face each other in a joining direction with the first hollow portion interposed therebetween, the first resin base material and the second resin base material are not in contact with each other, and the first metal joining material has a melting point lower than that of the interlayer connection conductor. 
     A method of manufacturing a transmission line according to a preferred embodiment of the present invention includes forming a first structure by forming a first ground conductor on a first resin base material having flexibility, forming a second structure by forming a first signal line, and an interlayer connection conductor in or on a second resin base material having flexibility, and joining the first structure and the second structure by a metal joining material with a spacer interposed therebetween such that the first signal line and the first ground conductor face each other with a hollow portion interposed therebetween, wherein in the joining the first structure and the second structure, the first resin base material and the second resin base material are not in contact with each other, and the metal joining material has a melting point lower than that of the interlayer connection conductor. 
     According to preferred embodiments of the present invention, variations in transmission characteristics of a transmission line having a hollow structure can be reduced. 
     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  is an external perspective view of a transmission line  10  according to a first preferred embodiment of the present invention. 
         FIG. 2  is a plan view of each layer of the transmission line  10 . 
         FIG. 3  is a sectional view of the transmission line  10  taken along line A-A. 
         FIGS. 4A to 4F  are sectional views illustrating a method of manufacturing the transmission line  10 . 
         FIG. 5  is a conceptual side view of a mounting structure of the transmission line  10 . 
         FIG. 6  is a sectional view of a transmission line  50  according to a second preferred embodiment of the present invention. 
         FIG. 7  is a sectional view of a transmission line  60  according to a third preferred embodiment of the present invention. 
         FIG. 8  is a sectional view of a transmission line  70  according to a modification of the third preferred embodiment of the present invention. 
         FIG. 9  is a sectional view of a transmission line  80  according to a fourth preferred embodiment of the present invention. 
         FIG. 10  is a sectional view of a transmission line  90  according to a fifth preferred embodiment of the present invention. 
         FIG. 11  is a sectional view of a transmission line  100  according to a sixth preferred embodiment of the present invention. 
         FIG. 12  is a plan view of each layer of a transmission line  110  according to a seventh preferred embodiment of the present invention. 
         FIG. 13  is a sectional view of a transmission line  120  according to an eighth preferred embodiment of the present invention. 
         FIG. 14  is a sectional view of a transmission line  130  according to a ninth preferred embodiment of the present invention. 
         FIG. 15  is a sectional view of a transmission line  140  according to a tenth preferred embodiment of the present invention. 
         FIG. 16  is a sectional view of a transmission line  150  according to an eleventh preferred embodiment of the present invention. 
         FIG. 17  is a side view of a transmission line  160  according to a twelfth preferred embodiment of the present invention. 
         FIG. 18  is a plan view of each layer of a transmission line  180  according to a thirteenth preferred embodiment of the present invention. 
         FIG. 19A  is a partial plan view of a transmission line  60  according to a preferred embodiment of the present invention, and  FIGS. 19B and 19C  are sectional views of the transmission line  60 . 
         FIG. 20A  is a sectional view illustrating a state in which a connector  21  is mounted on the transmission line  60  having a configuration of the present invention, and  FIG. 20B  is a sectional view illustrating a state in which the connector  21  is mounted on a transmission line  60 X of a comparative example. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described. Each preferred embodiment is an example, and it is possible to partially replace or combine configurations illustrated in different preferred embodiments. In each preferred embodiment, points different from points described before the preferred embodiments will be described. In particular, similar effects due to a similar configuration will not be repeatedly described for each preferred embodiment. 
     First Preferred Embodiment 
       FIG. 1  is an external perspective view of a transmission line  10  according to a first preferred embodiment of the present invention.  FIG. 2  is a plan view of each layer of the transmission line  10 .  FIG. 3  is a sectional view of the transmission line  10  taken along line A-A. 
     As illustrated in  FIG. 1  and  FIG. 2 , the transmission line  10  includes structures  11 A and  11 B, a joining electrode  12 A, a first metal joining material  13 A, and a connector  21 . The structure  11 A is an example of a “first structure”. The structure  11 B is an example of a “second structure”. The structures  11 A and  11 B each have a planar shape, and extend in one direction. The structure  11 A and the structure  11 B are laminated such that respective longitudinal directions coincide with each other. The connectors  21  are provided on an upper surface of the structure  11 A at respective ends in the longitudinal direction of the structure  11 A. 
     Note that, in the present specification, the terms “upper surface” and “lower surface” are used for convenience to distinguish a main surface on one side from a main surface on another side. Similarly, terms “upper side” and “lower side” are used for convenience to distinguish one side from another side. 
     As illustrated in  FIG. 2  and  FIG. 3 , the structure  11 A and the structure  11 B are joined to each other with a metal joining material  13 A with a joining electrode  12 A forming a spacer interposed therebetween. The joining electrode  12 A is an example of a “first spacer”. The metal joining material  13 A is an example of a “first metal joining material”. A hollow portion  14 A is located between the structure  11 A and the structure  11 B, by joining the structure  11 A and the structure  11 B with the joining electrode  12 A interposed therebetween. The hollow portion  14 A is an example of a “first hollow portion”. 
     The structure  11 A includes a resin base material  15 A and a ground conductor  17 A. The resin base material  15 A is an example of a “first resin base material”. The ground conductor  17 A is an example of a “first ground conductor”. The structure  11 B includes a resin base material  15 B, a signal line  16 , ground conductors  17 B 1  and  17 B 2 , and interlayer connection conductors  18 B 1  and  18 B 2 . The resin base material  15 B is an example of a “second resin base material”. The ground conductors  17 B 1  and  17 B 2  are examples of a “second ground conductor”. The signal line  16  and the ground conductor  17 A face each other in a joining direction (a direction in which the structure  11 A and the structure  11 B are joined to each other) with the hollow portion  14 A interposed therebetween. The resin base material  15 A and the resin base material  15 B are not in contact with each other. 
     The resin base materials  15 A and  15 B each have flexibility, and contain, for example, a liquid crystal polymer (LCP) as a main component. The resin base material  15 B includes integrated resin layers  15 B 1 ,  15 B 2 , and  15 B 3 . The resin layers  15 B 1 ,  15 B 2 , and  15 B 3  are disposed in this order from an upper side to a lower side. 
     Note that, the resin base material  15 A of the structure  11 A may have a plurality of resin layers, similarly to the resin base material  15 B of the structure  11 B. 
     The resin base materials  15 A and  15 B are formed of materials of the same kind. Accordingly, the structures  11 A and  11 B can be integrated with each other in a state in which distortion due to a difference in physical properties between the resin base materials  15 A and  15 B is less likely to occur. 
     The resin base material  15 A and the resin base material  15 B may be formed of materials having characteristics different from each other. For example, the resin base material  15 A disposed on an outer side of the transmission line  10  may be formed of a material having excellent weather resistance (environmental resistance) or mechanical properties. Weather resistance is a property with which deformation, alteration, deterioration, and the like against changes in temperature, humidity, and the like are less likely to occur. Mechanical properties include strength such as bending strength, hardness, toughness, and the like. The resin base material  15 B disposed inside the transmission line  10  may be formed of a material for which importance is placed on electrical characteristics. For example, when characteristic impedance of the transmission line  10  is set to a desired value, by forming the resin base material  15 B with a material having a low relative dielectric constant, a line width of the signal line  16  can be increased, thus reducing a conductor loss caused in the signal line  16 . Alternatively, the resin base material  15 A and the resin base material  15 B may be formed of materials having different hues, respectively. This makes it easy to identify the base materials by image recognition in a process of manufacturing the transmission line  10 . 
     The joining electrode  12 A is located on an upper surface of the resin layer  15 B 1 . The joining electrode  12 A is disposed between the structure  11 A and the structure  11 B. The joining electrode  12 A includes a conductor made of the same material as that of a conductor defining the signal line  16 . The joining electrode  12 A may include a conductor thicker than the conductor defining the signal line  16 , in order to realize a predetermined thickness of the hollow portion  14 A. The joining electrode  12 A extends in a longitudinal direction of the structure  11 B at both ends in a width direction of the structure  11 B. The ground conductor  17 A is located on substantially an entire lower surface of the resin base material  15 A. The signal line  16 , the ground conductors  17 B 1  and  17 B 2 , and the interlayer connection conductors  18 B 1  and  18 B 2  are formed in the resin base material  15 B. The signal line  16  and the ground conductor  17 B 1  are disposed on an upper surface of the resin layer  15 B 2 . The signal line  16  extends in the longitudinal direction of the structure  11 B at a center in the width direction of the structure  11 B. The ground conductor  17 B 1  extends in the longitudinal direction of the structure  11 B at both ends in the width direction of the structure  11 B. The ground conductor  17 B 2  is disposed on substantially an entire lower surface of the resin layer  15 B 3 . The joining electrode  12 A, the signal line  16 , and the ground conductors  17 B 1  and  17 B 2  are conductor patterns formed of, for example, Cu foil. 
     Note that, instead of extending in the longitudinal direction of the structure  11 B, the joining electrodes  12 A and the ground conductors  17 B 1  may be disposed at intervals along the longitudinal direction of the structure  11 B. 
     Further, a plurality of the signal lines  16  may be provided. In this case, in order to ensure isolation among the signal lines  16 , a ground conductor and an interlayer connection conductor may be provided between the signal lines  16 . 
     The signal line  16  and the ground conductor  17 A face each other with the resin base material  15 B, in addition to the hollow portion  14 A, interposed therebetween. However, the signal line  16  and the ground conductor  17 A may face each other with at least one of the hollow portion  14 A and the resin base materials  15 A and  15 B, interposed therebetween by appropriately changing the disposition of the signal line  16 , the ground conductor  17 A, and the like. 
     The joining electrode  12 A is joined to and electrically connected to the ground conductor  17 B 1  with the interlayer connection conductor  18 B 1  interposed therebetween. The ground conductor  17 B 1  is joined to and electrically connected to the ground conductor  17 B 2  with the interlayer connection conductor  18 B 2  interposed therebetween. 
     The metal joining materials  13 A are disposed at intervals along a longitudinal direction of the transmission line  10 , at both ends in a width direction of the transmission line  10 . The metal joining material  13 A joins and electrically connects the joining electrode  12 A and the ground conductor  17 A to each other. Accordingly, the metal joining material  13 A joins the structure  11 A and the structure  11 B to each other with the joining electrode  12 A defining a spacer interposed therebetween. The metal joining material  13 A is made of a material having a melting point lower than those of the interlayer connection conductors  18 B 1  and  18 B 2 . For example, the metal joining material  13 A is made of solder, and the interlayer connection conductors  18 B 1  and  18 B 2  are made of a Cu—Sn alloy. Note that, the interlayer connection conductors  18 B 1  and  18 B 2  need not overlap with the metal joining material  13 A, when viewed in a top-bottom direction. 
     The hollow portion  14 A is defined by the structures  11 A and  11 B, the joining electrode  12 A, and the first metal joining material  13  as described above, and extends in the longitudinal direction of the transmission line  10 . The hollow portion  14 A is surrounded by a lower surface of the ground conductor  17 A, an upper surface of the resin base material  15 B, and an end surface inside the joining electrode  12 A. As described above, the hollow portion  14 A is between the signal line  16  and the ground conductor  17 A. Accordingly, generation of a dielectric loss is reduced or prevented, so that a transmission loss of the transmission line  10  is reduced. Further, when the characteristic impedance of the transmission line  10  is set to a desired value, the line width of the signal line  16  can be increased, thus reducing a conductor loss caused in the signal line  16 . 
     A protection layer  19 A is provided on a lower surface of the resin base material  15 B. The protection layer  19 A is disposed on an entirety of the lower surface of the resin base material  15 B so as to cover the ground conductor  17 B 2 . 
     Note that, a conductive protection film made of Ni/Au or the like having excellent oxidation resistance may be provided, for example, by plating, on a surface of the joining electrode  12 A and the ground conductor  17 A exposed to the hollow portion  14 A, or to an outside of the transmission line  10 . 
     Mounting electrodes  22  are provided on a lower surface side of the resin base material  15 A at both ends in a longitudinal direction of the resin base material  15 A. The mounting electrode  22  is connected to an end portion of the signal line  16 , with a metal joining material  23 , an internal electrode  24 , and an interlayer connection conductor  25  interposed therebetween. Openings  26  are provided at both ends in the longitudinal direction of the resin base material  15 A so as to expose a portion of the ground conductor  17 A and the mounting electrodes  22 . The connector  21  is electrically connected to the ground conductor  17 A and the mounting electrode  22  that are exposed through the opening  26 . 
     In the first preferred embodiment, the structure  11 A and the structure  11 B are joined to each other by the metal joining material  13 A with the joining electrode  12 A interposed therebetween, thus forming the hollow portion  14 A. Accordingly, it is easy to hold the hollow portion  14 A in a process of joining the structure  11 A and the structure  11 B to each other. As a result, a distance between the ground conductor  17 A and the signal line  16  is less likely to vary, and variations in transmission characteristics of the transmission line  10  are reduced. 
       FIGS. 4A to 4F  are sectional views illustrating a method of manufacturing the transmission line  10 . 
     First, as illustrated in  FIG. 4A , the ground conductor  17 A and the mounting electrode  22  (see  FIG. 2 ) are formed by patterning metal foil affixed to a resin base material  45 A by photolithography or the like. Further, the opening  26  (see  FIG. 2 ) is formed in the resin base material  45 A by using a laser or the like. Thus, a structure  41 A including a plurality of the structures  11 A is formed. 
     In addition, as illustrated in  FIG. 4B , the joining electrode  12 A, the signal line  16 , the ground conductors  17 B 1  and  17 B 2 , and the internal electrode  24  (see  FIG. 2 ) are formed, by patterning metal foil affixed to the resin base materials  45 B 1 ,  45 B 2 , and  45 B 3  by photolithography or the like. Further, a through-hole is formed in each of the resin base materials  45 B 1 ,  45 B 2 , and  45 B 3  using a laser or the like, and the through-hole is filled with a conductive paste  48 . 
     Next, as illustrated in  FIG. 4C , the resin base materials  45 B 1 ,  45 B 2 , and  45 B 3  are laminated and hot-pressed. Thus, the resin base materials  45 B 1 ,  45 B 2 , and  45 B 3  are integrated to form the resin base material  45 B, and the conductive paste  48  is solidified to form the interlayer connection conductors  18 B 1  and  18 B 2  and the interlayer connection conductor  25  (see  FIG. 2 ). In this manner, a structure  41 B including a plurality of the structures  11 B is formed. 
     Next, as illustrated in  FIG. 4D , for example, after a solder paste is applied to a surface of the joining electrode  12 A, the structures  41 A and  41 B are laminated and heated, with the structures  41 A and  41 B affixed to each other. Accordingly, the structure  41 A and the structure  41 B are joined to each other by the metal joining material  13 A with the joining electrode  12 A interposed therebetween, such that the signal line  16  and the ground conductor  17 A face each other with the hollow portion  14 A interposed therebetween. The resin base material  45 A and the resin base material  45 B are not in contact with each other. A heating temperature during joining is higher than a melting point of a material forming the metal joining material  13 A, and lower than a melting point of a material of the interlayer connection conductors  18 B 1  and  18 B 2 . 
     As described above, by joining the structure  41 A and the structure  41 B with the structure  41 A and the structure  41 B affixed to each other, the hollow portion  14 A can be secured between the signal line  16  and the ground conductor  17 A, and a variation in height of the hollow portion  14 A can be reduced. 
     Next, as illustrated in  FIG. 4E , a protection layer  49 A is formed on a lower surface of the structure  41 B by printing or the like, thus forming a collective board  40  including a plurality of the transmission lines  10 . 
     Finally, as illustrated in  FIG. 4F , the collective board is separated into individual pieces to obtain individual transmission lines  10 . 
       FIG. 5  is a side view of an electronic apparatus  1  including the transmission line  10 . The electronic apparatus  1  includes the transmission line  10 , a circuit board  31 , and a connector  32 . As illustrated in  FIG. 5 , the transmission line  10  can also be mounted on the circuit board  31  including a step. The connectors  21  of the transmission line  10  are connected to the respective connectors  32  of the circuit board  31 . The transmission line  10  is bent along the step of the circuit board  31 . The metal joining material  13 A disposed on a bent portion BP of the transmission line  10  is bent by plastic deformation. In a process of bending the transmission line  10 , the metal joining material  13 A is plastically deformed by heating. To be specific, after the metal joining material  13 A is melted or softened and deformed by heating, the deformed metal joining material  13 A is solidified by cooling. Accordingly, it is possible to obtain the transmission line  10  in which a bent shape is maintained. 
     Note that, by using a thermoplastic resin as the material of the resin base materials  15 A and  15 B of the transmission line  10 , the bent shape of the transmission line  10  can also be maintained when the resin base materials  15 A and  15 B are plastically deformed. 
     Note that, the hollow portion  14 A need not be provided in the bent portion BP. In this case, the hollow portion  14 A is not deformed at the bent portion BP, and the transmission characteristics of the transmission line  10  are less likely to change. 
     Second Preferred Embodiment 
     In a second preferred embodiment, a hollow portion is located on each of an upper side and a lower side of a signal line. 
       FIG. 6  is a sectional view of a transmission line  50  according to the second preferred embodiment of the present invention. 
     The transmission line  50  includes the structures  11 A,  51 B, and  11 C. The structure  11 C is an example of a “third structure”. The structures  11 A,  51 B, and  11 C are disposed in this order from an upper side to a lower side. The structure  11 A and the structure  51 B are joined to each other by the metal joining material  13 A with the joining electrode  12 A defining a spacer interposed therebetween. The structure  51 B and the structure  11 C are joined to each other by a metal joining material  13 B with a joining electrode  12 B forming a spacer interposed therebetween. The joining electrode  12 B is an example of a “second spacer”. The metal joining material  13 B is an example of a “second metal joining material”. 
     The structure  51 B includes a resin base material  55 B, the signal line  16 , and the interlayer connection conductor  18 B 1 . The resin base material  55 B has flexibility. The signal line  16  is disposed on an upper surface of the resin base material  55 B. The signal line  16  and the ground conductor  17 A face each other with the hollow portion  14 A interposed therebetween in a joining direction. 
     The joining electrode  12 A is provided on the upper surface of the resin base material  55 B. The joining electrode  12 A is disposed between the structure  11 A and the structure  51 B. The joining electrode  12 B is provided on a lower surface of the resin base material  55 B. The joining electrode  12 B is disposed between the structure  55 B and the structure  11 C. The joining electrodes  12 A and  12 B extend in a longitudinal direction of the structure  51 B at both ends in a width direction of the structure  51 B. The joining electrode  12 A is joined and electrically connected to the joining electrode  12 B with the interlayer connection conductor  18 B 1  interposed therebetween. 
     The structure  11 C includes a resin base material  15 C and a ground conductor  17 C. The resin base material  15 C is an example of a “third resin base material”. The ground conductor  17 C is an example of a “third ground conductor”. The resin base material  15 C has flexibility. The resin base material  55 B and the resin base material  15 C may be formed of materials of the same kind, or may be formed of materials having characteristics different from each other. The ground conductor  17 C is disposed on substantially an entire upper surface of the resin base material  15 C. In other words, the structure  11 C is similar to the structure  11 A. The structure  11 C is disposed on a lower side of the structure  51 B with a surface on which the ground conductor  17 C is disposed facing upward. 
     The ground conductor  17 A is disposed inside the resin base material  15 A, and the ground conductor  17 C is disposed inside the resin base material  15 C. This makes protection layers to protect the ground conductors  17 A and  17 C unnecessary. 
     The structure  51 B and the structure  11 C are joined to each other with the joining electrode  12 B interposed therebetween to define a hollow portion  14 B. The hollow portion  14 B is an example of a “second hollow portion”. The signal line  16  and the ground conductor  17 C face each other with the hollow portion  14 B and the resin base material  55 B interposed therebetween in the joining direction. The resin base material  55 B and the resin base material  15 C are not in contact with each other. 
     The metal joining material  13 B joins and electrically connects the joining electrode  12 B and the ground conductor  17 C to each other. The metal joining material  13 B is made of, for example, solder, and has a melting point lower than that of the interlayer connection conductor  18 B 1 . 
     In the second preferred embodiment, the hollow portion  14 B is also located between the signal line  16  and the ground conductor  17 C. Thus, a transmission loss of the transmission line  10  is further reduced. 
     Third Preferred Embodiment 
     In a third preferred embodiment, a hollow portion is located on each of an upper side and a lower side of a signal line, and the signal line faces a ground conductor with a resin base material and the hollow portion interposed therebetween. 
       FIG. 7  is a sectional view of a transmission line  60  according to the third preferred embodiment of the present invention. 
     The transmission line  60  includes structures  61 A,  61 B, and  61 C. The structures  61 A,  61 B, and  61 C are disposed in this order from an upper side to a lower side. The structure  61 A and the structure  61 B are joined to each other by the metal joining material  13 A with the joining electrode  12 A and a joining electrode  12 C that define a spacer interposed therebetween. The structure  61 B and the structure  61 C are joined to each other by the metal joining material  13 B with the joining electrode  12 B and a joining electrode  12 D that define a spacer interposed therebetween. 
     The structure  61 A includes the resin base material  15 A, the ground conductor  17 A, and an interlayer connection conductor  18 A. The ground conductor  17 A is disposed on substantially an entire upper surface of the resin base material  15 A. The structure  61 B includes the resin base material  15 B, the signal line  16 , the ground conductor  17 B 1 , and interlayer connection conductors  18 B 1  and  18 B 2 . The signal line  16  and the ground conductor  17 B 1  are provided in an inner layer of the resin base material  15 B as in the first preferred embodiment. The structure  61 C includes the resin base material  15 C, the ground conductor  17 C, and an interlayer connection conductor  18 C. The ground conductor  17 C is disposed on substantially an entire lower surface of the resin base material  15 C. The structure  61 C is similar to the structure  61 A. The structure  61 C is disposed on a lower side of the structure  61 B with a surface on which the ground conductor  17 C is disposed facing downward. 
     The joining electrodes  12 A and  12 B are provided on an upper surface and a lower surface of the resin base material  15 B, respectively. The joining electrode  12 C is provided on a lower surface of the resin base material  15 A. The joining electrode  12 D is provided on an upper surface of the resin base material  15 C. 
     The joining electrodes  12 A and  12 B are joined to and electrically connected to the ground conductor  17 B 1  with the interlayer connection conductors  18 B 1  and  18 B 2  interposed therebetween, respectively. The joining electrode  12 C is joined to and electrically connected to the ground conductor  17 A with the interlayer connection conductor  18 A interposed therebetween. The joining electrode  12 D is joined to and electrically connected to the ground conductor  17 C with the interlayer connection conductor  18 C interposed therebetween. The joining electrode  12 A and the joining electrode  12 C are joined and electrically connected to each other with the metal joining material  13 A interposed therebetween. The joining electrode  12 B and the joining electrode  12 D are joined and electrically connected to each other with the metal joining material  13 B interposed therebetween. 
     The structure  61 A and the structure  61 B are joined to each other with the joining electrodes  12 A and  12 C interposed therebetween, to form the hollow portion  14 A. The structure  61 B and the structure  61 C are joined to each other with the joining electrodes  12 B and  12 D interposed therebetween to form the hollow portion  14 B. The signal line  16  and the ground conductor  17 A face each other with the hollow portion  14 A and the resin base materials  15 A and  15 B interposed therebetween in a joining direction. The signal line  16  and the ground conductor  17 C face each other with the hollow portion  14 B and the resin base materials  15 B and  15 C interposed therebetween in the joining direction. 
     The protection layer  19 A and a protection layer  19 B are provided on the upper surface of the resin base material  15 A and the lower surface of the resin base material  15 C, respectively. 
       FIG. 8  is a sectional view of a transmission line  70  according to a modification of the third preferred embodiment of the present invention. The transmission line  70  is different from the transmission line  60  (see  FIG. 7 ) in the following points. The transmission line  70  includes the structures  11 A and  11 C instead of the structures  61 A and  61 C, the joining electrodes  12 C and  12 D, and the protection layers  19 A and  19 B. The structures  11 A,  61 B, and  11 C are disposed in this order from an upper side to a lower side. The structure  11 A and the structure  61 B are joined to each other by the metal joining material  13 A with the joining electrode  12 A defining a spacer interposed therebetween. The structure  61 B and the structure  11 C are joined to each other by the metal joining material  13 B with the joining electrode  12 B defining a spacer interposed therebetween. 
     In the third preferred embodiment, the signal line  16  faces the ground conductor  17 A with the hollow portion  14 A and the resin base material interposed therebetween. Accordingly, even when the hollow portion  14 A is deformed, a short circuit between the signal line  16  and the ground conductor  17 A is prevented by the resin base material disposed between the signal line  16  and the ground conductor  17 A. Similarly, the signal line  16  faces the ground conductor  17 C with the hollow portion  14 B and the resin base material interposed therebetween. Accordingly, even when the hollow portion  14 B is deformed, a short circuit between the signal line  16  and the ground conductor  17 C is prevented by the resin base material disposed between the signal line  16  and the ground conductor  17 C. 
     Note that, in the third preferred embodiment, it is preferable that the structure  11 A and the structure  11 C be harder than the structure  61 B. For example, it is preferable that Young&#39;s modulus of each of the resin base material  15 A of the structure  11 A and the resin base material  15 C of the structure  11 C be higher than Young&#39;s modulus of the resin base material  15 B of the structure  61 B. In addition, it is preferable that a coefficient of linear expansion CTE of each of the resin base material  15 A of the structure  11 A and the resin base material  15 C of the structure  11 C be larger than a CTE of the resin base material  15 B of the structure  61 B. Thus, unnecessary deformation of the hollow portion  14 A and the hollow portion  14 B is reduced or prevented. 
     Furthermore, it is preferable that the resin base material  15 A of the structure  11 A and the resin base material  15 C of the structure  11 C be made of the same material, and this makes it possible to reduce or prevent warpage of the transmission line  70 . 
     Fourth Preferred Embodiment 
     In a fourth preferred embodiment, at a position where a signal line faces a ground conductor, a thickness of a resin base material in which the signal line is located is reduced such that a ratio of a resin portion positioned between the signal line and the ground conductor decreases in a joining direction. 
       FIG. 9  is a sectional view of a transmission line  80  according to the fourth preferred embodiment of the present invention. The transmission line  80  is different from the transmission line  60  (see  FIG. 7 ) according to the third preferred embodiment in the following points. The transmission line  80  includes a structure  81 B including a resin base material  85 B instead of the structure  61 B including the resin base material  15 B. 
     The resin base material  85 B includes a portion BM at a position where the signal line  16  faces the ground conductors  17 A and  17 C. The portion BM of the resin base material  85 B is thinner than a portion of the resin base material  85 B that does not face the hollow portions  14 A and  14 B, in other words, thinner than an end portion of the resin base material  85 B in a short direction. The portion BM of the resin base material  85 B is thinned by forming a recess on each of an upper surface and a lower surface of the resin base material  85 B. That is, the transmission line  80  includes a portion in which a thickness of the resin base material  85 B is reduced, at a position where the signal line  16  faces the ground conductors  17 A and  17 C such that a ratio of a resin portion positioned between the signal line  16  and each of the ground conductors  17 A and  17 C decreases in a joining direction. 
     The portion BM of the resin base material  85 B extends along the signal line  16 . A width of the portion BM of the resin base material  85 B may be slightly larger than a width of the signal line  16 , the same as the width of the signal line  16 , or slightly smaller than the width of the signal line  16 . 
     The portion BM of the resin base material  85 B may be formed by digging (countersinking) the resin base material by plasma processing or the like. Alternatively, the portion BM of the resin base material  85 B may be formed by laminating resin base materials each having an opening formed in a portion thereof. 
     In the fourth preferred embodiment, a thickness of the resin base material  85 B is reduced, at a position where the signal line  16  faces the ground conductors  17 A and  17 C such that the ratio of the resin portion positioned between the signal line  16  and each of the ground conductors  17 A and  17 C decreases in the joining direction. As a result, a relative dielectric constant decreases between a layer in which the signal line  16  is disposed and layers in which the ground conductors  17 A and  17 C are disposed. Accordingly, when characteristic impedance of the transmission line  60  is set to a desired value, a line width of the signal line  16  can be increased, thus reducing a conductor loss caused in the signal line  16 . 
     Fifth Preferred Embodiment 
     In a fifth preferred embodiment, a joining electrode and a metal block define a spacer. 
       FIG. 10  is a sectional view of a transmission line  90  according to the fifth preferred embodiment of the present invention. 
     The transmission line  90  includes the structure  61 A and the structure  11 B. The structure  61 A and the structure  11 B are joined to each other by the metal joining material  13 A with the joining electrodes  12 A and  12 C and a metal block  33  interposed therebetween. The protection layers  19 A and  19 B are provided on an upper surface of the structure  61 A and a lower surface of the structure  11 B, respectively. 
     The joining electrodes  12 A and  12 C and the metal block  33  define a spacer. The metal block  33  is thicker than each of the signal line  16 , the ground conductors  17 A,  17 B 1 , and  17 B 2 , and the joining electrodes  12 A and  12 C. That is, the spacer includes a conductor thicker than the conductor defining the signal line  16 . 
     The metal block  33  is disposed between the joining electrode  12 A and the joining electrode  12 C. The metal blocks  33  are disposed at intervals along a longitudinal direction of the transmission line  90 , at both ends in a width direction of the transmission line  10 . The metal block  33  is joined and electrically connected to each of the joining electrodes  12 A and  12 C by the metal joining material  13 A. 
     In the fifth preferred embodiment, the joining electrodes  12 A and  12 C and the metal block  33  define the spacer. Accordingly, the hollow portion  14 A between the structure  61 A and the structure  11 B can be appropriately increased. 
     Sixth Preferred Embodiment 
     In a sixth preferred embodiment, a joining electrode and a resin base material define a spacer. 
       FIG. 11  is a sectional view of a transmission line  100  according to the sixth preferred embodiment of the present invention. The transmission line  100  is different from the transmission line  90  (see  FIG. 10 ) according to the fifth preferred embodiment in the following points. The transmission line  100  includes a resin base material  15 D having flexibility and a metal joining material  103 A, instead of the metal block  33  and the metal joining material  13 A. 
     The joining electrodes  12 A and  12 C and the resin base material  15 D define a spacer. The resin base material  15 D is disposed between the joining electrode  12 A and the joining electrode  12 C. The resin base material  15 D extends in a longitudinal direction of the transmission line  100  at both ends in a width direction of the transmission line  100 . The resin base material  15 D includes through-holes at intervals along a longitudinal direction of the resin base material  15 D. The through-holes in the resin base material  15 D are filled with the metal joining material  103 A which is made of, for example, solder. The metal joining material  103 A joins and electrically connects the joining electrode  12 A and the joining electrode  12 C to each other. Accordingly, the metal joining material  103 A joins the structure  61 A and the structure  11 B to each other with the joining electrodes  12 A and  12 C and the resin base material  15 D interposed therebetween. 
     In the sixth preferred embodiment, a position of the metal joining material  103 A is fixed by filling the through-hole of the resin base material  15 D with the metal joining material  103 A. 
     Note that, unlike the sixth preferred embodiment, after an interlayer connection conductor is formed in the resin base material  15 D with a conductive material having a relatively high melting point such as a Cu—Sn alloy, the interlayer connection conductor may be joined to each of the joining electrodes  12 A and  12 C using a metal joining material having a relatively low melting point such as solder. Note that, in the transmission line  100 , the joining electrodes  12 A and  12 C are not necessarily provided. In this case, the metal joining material  103 A is joined to the interlayer connection conductors  18 A and  18 B 1 . In addition, in the transmission line  100 , a joining electrode may further be provided on the resin base material  15 D. In this case, the joining electrode on the resin base material  15 D is joined to the joining electrode  12 C with a metal joining material interposed therebetween. Similarly, in the transmission line  100 , a joining electrode may further be provided under the resin base material  15 D. In this case, the joining electrode under the resin base material  15 D is joined to the joining electrode  12 A with a metal joining material interposed therebetween. 
     Seventh Preferred Embodiment 
     In a seventh preferred embodiment, in the transmission line  10 , the ground conductor  17 A is provided with slits SL and the joining electrode  12 A is divided into multiple pieces. 
       FIG. 12  is a plan view of each layer of a transmission line  110  according to the seventh preferred embodiment of the present invention. As illustrated in  FIG. 12 , the joining electrode  12 A is divided into rectangular-shaped small joining electrodes  112 A (small spacers). The ground conductor  17 A is provided with a plurality of slits SLs. By providing the plurality of slits SLs, a plurality of rectangular-shaped connection regions A 1  is provided. Each of the connection regions A 1  overlaps with a corresponding one of the small joining electrodes  112 A when viewed in a top-bottom direction. Thus, the metal joining material  13 A is prevented from spreading out on the joining electrode  12 A and the ground conductor  17 A. This makes it easy to keep the metal joining material  13 A uniform in thickness, and a variation in thickness of the hollow portion  14 A is reduced or prevented. 
     Eighth Preferred Embodiment 
     In an eighth preferred embodiment, in the transmission line  10 , a metal ball  113 A is present in the metal joining material  13 A. 
       FIG. 13  is a sectional view of a transmission line  120  according to the eighth preferred embodiment of the present invention. The transmission line  120  is provided with a plurality of the metal balls  113 A. The metal balls  113 A are spheres. Further, the metal balls  113 A are uniform in size. The plurality of metal balls  113 A is provided in the metal joining material  13 A. The metal balls  113 A are manufactured using Ni, which has a melting point higher than that of the metal joining material  13 A (that is, solder). Thus, a thickness of the metal joining material  13 A can be made larger than a thickness of the joining electrode  12 A. As a result, the hollow portion  14 A can be increased. 
     Ninth Preferred Embodiment 
     In a ninth preferred embodiment, a protection film  300  is provided in the transmission line  50 . 
       FIG. 14  is a sectional view of a transmission line  130  according to the ninth preferred embodiment of the present invention. The transmission line  130  further includes the protection film  300 . The protection films  300  cover respective surfaces of the resin base material  15 A and the resin base material  55 B, the surfaces facing each other. To be specific, the protection film  300  is provided on each of a lower surface of the resin base material  15 A, an upper surface of the resin base material  55 B, a lower surface of the resin base material  55 B, and an upper surface of the resin base material  15 C. However, when a dielectric constant of the protection film  300  is higher than a dielectric constant of each of the resin base materials  15 A,  55 B, and  15 C, characteristics of high-frequency characteristics of the transmission line  80  are affected. Thus, a thickness of the protection film  300  is reduced. The thickness of the protection film  300  is smaller than, for example, a thickness of the resin base material  55 B. Since the protection film  300  covers each of the joining electrodes  12 A and  12 B, the metal joining materials  13 A and  13 B are prevented from spreading out. 
     Tenth Preferred Embodiment 
     In a tenth preferred embodiment, the transmission line  10  includes a multi-core structure. 
       FIG. 15  is a sectional view of a transmission line  140  according to the tenth preferred embodiment of the present invention. The transmission line  140  further includes a signal line  116 . The signal line  116  is provided in the resin base material  15 B so as to extend parallel to the signal line  16 . Additionally, the joining electrode  12 A, the ground conductor  17 B 1 , and an interlayer connection conductor  18 B 1 ,  18 B 2  are provided between the signal line  16  and the signal line  116 . This ensures isolation between the signal line  16  and the signal line  116 . 
     Eleventh Preferred Embodiment 
     In an eleventh preferred embodiment, the transmission line  10  includes a multi-core structure. 
       FIG. 16  is a sectional view of a transmission line  150  according to the eleventh preferred embodiment of the present invention. The transmission line  150  further includes the signal line  116 . The signal line  116  is provided in the resin base material  15 B so as to extend parallel to the signal line  16 . However, the joining electrode  12 A, the ground conductor  17 B 1 , and the interlayer connection conductor  18 B 1  and  18 B 2  are not provided between the signal line  16  and the signal line  116 . In such a transmission line  140 , the signal line  16  and the signal line  116  form a differential transmission line. 
     Twelfth Preferred Embodiment 
     In a twelfth preferred embodiment, the connector  21  is mounted on one of two surfaces of a transmission line  160 , the one surface being spaced away from the hollow portion  14 A. 
       FIG. 17  is a side view of the transmission line  160  according to the twelfth preferred embodiment of the present invention. In the transmission line  160 , the connector  21  is mounted on one of the two surfaces of the transmission line  160 , the one surface being away from the hollow portion  14 A. The surface of the transmission line  160  away from the hollow portion  14 A is less likely to be deformed, compared to a surface of the transmission line  160  close to the hollow portion  14 A. Thus, the connector  21  is stably mounted on the transmission line  160 . 
     Thirteenth Preferred Embodiment 
     In a thirteenth preferred embodiment, the metal joining material  13 A has a rectangular shape in the transmission line  10 . 
       FIG. 18  is a plan view of each layer of a transmission line  180  according to the thirteenth preferred embodiment of the present invention. The metal joining material  13 A has a rectangular shape having a long side extending in an extending direction of the signal line  16 . Thus, the metal joining material  13 A effectively defines and functions as a shield. Furthermore, deformation of the transmission line  180  can be reduced or prevented more effectively. 
     Specific Example of Terminal Portion 
       FIG. 19A  is a partial plan view of a transmission line  60  according to a preferred embodiment of the present invention, and  FIGS. 19B and 19C  are sectional views of the transmission line  60 .  FIG. 19B  is a sectional view taken along line B-B in  FIGS. 19B and 19C  is a sectional view taken along line C-C in  FIG. 19A . 
     As illustrated in  FIGS. 19A to 19C , a terminal portion of the transmission line  60  includes four sets of the joining electrode  12 A, the joining electrode  12 B, the joining electrode  12 C, the joining electrode  12 D, the metal joining material  13 A, the metal joining material  13 B, the ground conductor  17 B 1 , the interlayer connection conductor  18 A, the interlayer connection conductor  18 B 1 , the interlayer connection conductor  18 B 2 , and the interlayer connection conductor  18 C. 
     In each set, in a thickness direction of the transmission line  60 , from the ground conductor  17 A toward the ground conductor  17 C, the interlayer connection conductor  18 A, the joining electrode  12 C, the metal joining material  13 A, the joining electrode  12 A, the interlayer connection conductor  18 B 1 , the ground conductor  17 B 1 , the interlayer connection conductor  18 B 2 , the joining electrode  12 B, the metal joining material  13 B, the joining electrode  12 D, and the interlayer connection conductor  18 C are arranged in this order, and overlap with each other in plan view. That is, a columnar body having a predetermined strength is defined by these components. 
     In other words, in the terminal portion of the transmission line  60 , a positional relationship among the structure  61 A, the structure  61 B, and the structure  61 C is maintained by these four columnar bodies. To be more specific, in the terminal portion of the transmission line  60 , a structure including the hollow portion  14 A between the structure  61 A and the structure  61 B and including the hollow portion  14 B between the structure  61 B and the structure  61 C is maintained by these four columnar bodies. These four columnar bodies are disposed so as to define a rectangular shape in a region Re 22  in plan view. 
     The mounting electrode  22 , a mounting auxiliary electrode  22 A, and the interlayer connection conductor  25 A are provided in or on the resin base material  15 A. Additionally, an opening  171  is provided in the ground conductor  17 A on the resin base material  15 A. 
     The opening  171  is a region where no conductor is located in the ground conductor  17 A on an upper surface of the resin base material  15 A. The opening  171  is disposed in the region Re 22  described above. 
     The mounting auxiliary electrode  22 A is disposed on an upper surface side of the resin base material  15 A, and is disposed in the opening  171 . Thus, the mounting auxiliary electrode  22 A is disposed in the region Re 22 . Further, the mounting auxiliary electrode  22 A is disposed at a position overlapping with the mounting electrode  22  in plan view. The mounting auxiliary electrode  22 A is connected to the mounting electrode  22  with the interlayer connection conductor  25 A interposed therebetween. The mounting electrode  22  is connected to the internal electrode  24  with the metal joining material  23  interposed therebetween, and the internal electrode  24  is connected to the signal line  16  with the interlayer connection conductor  25  interposed therebetween. 
     In this configuration, in plan view, a portion connected to the signal line  16  from the mounting auxiliary electrode  22 A is surrounded by the four columnar bodies. 
     In such a configuration, when the connector  21  is mounted on the transmission line  60 , a solder bump  170  is formed by providing an opening in each of a portion of the mounting auxiliary electrode  22 A in the protection layer  19 B and a portion of the ground conductor  17 A. Positions where the respective solder bumps  170  are provided on the ground conductor  17 A are positions overlapping with the four columnar bodies. By using the solder bumps  170 , the mounting auxiliary electrode  22 A and four portions of the ground conductor  17 A are joined to terminals of the connector  21 . 
     In this case, by using the above-described structure of the terminal portion, the following effects can be achieved. 
       FIG. 20A  is a sectional view illustrating a state in which the connector  21  is mounted on the transmission line  60  having the configuration according to the present preferred embodiment.  FIG. 20B  is a sectional view illustrating a state in which the connector  21  is mounted on a transmission line  60 X of a comparative example. The transmission line  60 X of the comparative example is not provided with the above-described columnar body that connects the ground conductor  17 A and the ground conductor  17 C, on a side closer to a center of the transmission line  60 X than the mounting electrode  22  in a longitudinal direction. 
     As illustrated in  FIG. 20A , in the transmission line  60 , the connector  21  is mounted on a portion supported by four columnar bodies. Accordingly, in a case of mounting the connector  21 , even when the connector  21  is mounted on the transmission line  60  while a predetermined pressure is applied, the connector  21  is supported by the four columnar bodies. Thus, even when the transmission line  60  includes the hollow portion  14 A, deformation of the transmission line  60 , more specifically, deformation in which the structure  61 A is recessed (see  FIG. 20B ) can be suppressed. 
     On the other hand, as illustrated in  FIG. 20B , in the transmission line  60 X of the comparative example, the connector  21  pushes the structure  61 A at a connecting portion with the ground conductor  17 A on a side closer to a center than the mounting auxiliary electrode  22 A, and there is a high possibility that the structure  61 A is recessed and deformed. When the structure  61 A is recessed, the connector  21  is inclined, and there is a high possibility that a joining failure occurs. 
     In this manner, by using the structure of the terminal portion described above, the connector  21  can be reliably mounted even in the structure in which the transmission line  60  includes the hollow portion  14 A. 
     Further, in this configuration, the columnar body is realized by the structure in which the ground conductor  17 A is connected to the ground conductor  17 C. Thus, a shape of the terminal portion of the transmission line  60  can be made smaller, compared to a case where a columnar body only for supporting the connector  21  is separately formed. In addition, by using the ground conductor  17 A and the ground conductor  17 C each having a larger area than that of other conductor as a portion of the columnar body, strength of the columnar body can be increased, which is more preferable. 
     Note that, in the above description, the mounting electrode  22 , the mounting auxiliary electrode  22 A, and the internal electrode  24  are disposed at a center of the region Re 22  surrounded by the four columnar bodies. However, the mounting electrode  22 , the mounting auxiliary electrode  22 A, and the internal electrode  24  are preferably included in the region Re 22 , and it is sufficient that at least a portion of the mounting electrode  22 , the mounting auxiliary electrode  22 A, and the internal electrode  24  is included in the region Re 22 . 
     In addition, in the above description, the region Re 22  is a rectangle, but is not limited to the rectangle, and it is sufficient that the region Re 22  is formed in a polygon of a triangle or above-described shapes. 
     In addition, in the above-described configuration, it is preferable that a melting point of solder forming the solder bump  170  have the following relationship with a melting point of the metal joining material and a melting point of the interlayer connection conductor. 
     (melting point of solder)&lt;(melting point of the metal joining material)&lt;(melting point of the interlayer connection conductor) 
     According to this relationship, remelting of the interlayer connection conductor can be reduced or prevented at the time of joining using the metal joining material, and remelting of the metal joining material and the interlayer connection conductor can be reduced or prevented at the time of joining using solder. 
     In addition, in each of the above-described preferred embodiments, a protection film may be patterned on the joining electrode in order to easily fix a position of the metal joining material. 
     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.