Patent Publication Number: US-2023158775-A1

Title: Resin multilayer substrate and method for manufacturing resin multilayer substrate

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2020-124365 filed on Jul. 21, 2020 and is a Continuation Application of PCT Application No. PCT/JP2021/026064 filed on Jul. 12, 2021. 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 resin multilayer substrate including a metal foil layer and a resin layer. 
     2. Description of the Related Art 
     As an invention related to an existing resin multilayer substrate, for example, a multilayer circuit substrate described in International Publication No. 2011/007659 is known. The multilayer circuit substrate includes an upper resin layer, a lower resin layer, and a metal foil layer. The upper resin layer is stacked on the lower resin layer. The metal foil layer is arranged between the upper resin layer and the lower resin layer. An upper main surface of the metal foil layer is in contact with the upper resin layer. A lower main surface of the metal foil layer is in contact with the lower resin layer. A surface roughness of the lower main surface of the metal foil layer is greater than a surface roughness of the upper main surface of the metal foil layer. Accordingly, the strength of the metal foil layer in close contact with the lower resin layer is stronger than the strength of the metal foil layer in close contact with the upper resin layer. As a result, peeling of the metal foil layer from the lower resin layer is prevented. 
     SUMMARY OF THE INVENTION 
     However, in the multilayer circuit substrate described in International Publication No. 2011/007659, the metal foil layer and the upper resin layer may be separated from each other. In this case, the upper resin layer and the lower resin layer may be separated from each other. 
     Preferred embodiments of the present invention provide resin multilayer substrates in each of which an upper resin layer and a lower resin layer are not easily separated from each other. 
     The inventor of the present application studied the following method to prevent the upper resin layer and the lower resin layer from being separated from each other. Both the main surfaces of the metal foil layer are roughened and both the main surfaces of the metal foil layer are fixed to the resin multilayer substrate. However, the inventor of the present application discovered that in a multilayer circuit substrate using a metal foil layer as a transmission line, when both the main surfaces of the metal foil layer in contact with a resin substrate layer are roughened, transmission loss is more likely to occur in an electric signal flowing through the metal foil layer than when one main surface of the metal foil layer is roughened. 
     Therefore, the inventor of the present application has studied a method of making it difficult for the upper resin layer and the lower resin layer to be separated from each other and reducing or preventing an increase in the transmission loss, and has conceived of and developed the following preferred embodiments of the present invention. 
     A resin multilayer substrate according to a preferred embodiment of the present invention includes a metal foil layer, an upper resin layer, and a lower resin layer, the metal foil layer including a metal foil layer upper main surface, a metal foil layer lower main surface having a surface roughness greater than a surface roughness of the metal foil layer upper main surface, and a metal foil layer side surface that is a surface of the metal foil layer excluding the metal foil layer upper main surface and the metal foil layer lower main surface, the metal foil layer upper main surface being in contact with the upper resin layer, the metal foil layer lower main surface is in contact with the lower resin layer, a portion of a lower main surface of the upper resin layer and a portion of an upper main surface of the lower resin layer being in contact with each other to define a layer interface, in a cross section of the metal foil layer parallel or substantially parallel to a multilayer body up-down direction, a direction from an inner side of a region surrounded by a surface of the metal foil layer toward an outer side portion of a region surrounded by the surface of the metal foil layer of directions perpendicular or substantially perpendicular to the multilayer body up-down direction is defined as an inside-outside direction, in a cross section of the metal foil layer parallel or substantially parallel to a multilayer body up-down direction, the metal foil layer side surface includes an outer point and an inner point located at a portion where the upper resin layer is in contact with the side surface, and the inner point is located below the outer point and is located inward of the outer point in the inside-outside direction. 
     Hereinafter, definitions of terms in the present specification will be described. In this specification, an axis or a member extending in a front-back direction does not necessarily indicate only an axis or a member parallel or substantially parallel to the front-back direction. The axis or the member extending in the front-back direction is an axis or a member inclined in a range of about ±45° with respect to the front-back direction. Similarly, an axis or a member extending in the up-down direction is an axis or a member inclined in a range of about ±45° with respect to the up-down direction. An axis or a member extending in a left-right direction is an axis or a member inclined in a range of about ±45° with respect to the left-right direction. 
     Hereinafter, a first member is a member or the like included in a high-frequency signal transmission line. In the present specification, unless otherwise specified, each portion of the first member is defined as follows. A front portion of the first member is a front half of the first member. A back portion of the first member is a back half of the first member. A left portion of the first member is a left half of the first member. A right portion of the first member is a right half of the first member. An upper portion of the first member is an upper half of the first member. A lower portion of the first member is a lower half of the first member. A front end of the first member is an end in a forward direction of the first member. A back end of the first member is an end in a backward direction of the first member. A left end of the first member is an end in a leftward direction of the first member. A right end of the first member is an end in a rightward direction of the first member. An upper end of the first member is an end in an upward direction of the first member. A lower end of the first member is an end in a downward direction of the first member. A front end portion of the first member is the front end of the first member and the vicinity thereof. A back end portion of the first member is the back end of the first member and the vicinity thereof. A left end portion of the first member is the left end of the first member and the vicinity thereof. A right end portion of the first member is the right end of the first member and the vicinity thereof. An upper end portion of the first member is the upper end of the first member and the vicinity thereof. A lower end portion of the first member is the lower end of the first member and the vicinity thereof. 
     According to the resin multilayer substrates of preferred embodiments of the present invention, the upper resin layer and the lower resin layer are not easily separated from each other. 
     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 high-frequency signal transmission line  10  including a bent portion to connect two circuits. 
         FIG.  2    is a side view of the high-frequency signal transmission line  10  including a bent portion to connect two circuits. 
         FIG.  3    is an external perspective view of the high-frequency signal transmission line  10  including a bent portion. 
         FIG.  4    is an external perspective view of the high-frequency signal transmission line  10  that is not bent. 
         FIG.  5    is an exploded perspective view of the high-frequency signal transmission line  10  that is not bent. 
         FIG.  6    is a cross-sectional view taken along line A-A of  FIG.  4   . 
         FIG.  7    is a cross-sectional view of the high-frequency signal transmission line  10  including an adhesive layer formed of an adhesive. 
         FIG.  8    is a cross-sectional view of a high-frequency signal transmission line  10   a  according to a first modification of a preferred embodiment of the present invention taken along line A-A. 
         FIG.  9    is a cross-sectional view of a high-frequency signal transmission line  10   b  according to a second modification of a preferred embodiment of the present invention taken along line A-A. 
         FIG.  10    is a cross-sectional view of a high-frequency signal transmission line  10   b   2  according to the second modification taken along line A-A. 
         FIG.  11    is a cross-sectional view of a high-frequency signal transmission line  10   c  according to a third modification of a preferred embodiment of the present invention taken along line A-A. 
         FIG.  12    is a cross-sectional view of a high-frequency signal transmission line  10   d  according to a fourth modification of a preferred embodiment of the present invention taken along line A-A. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A high-frequency signal transmission line  10  according to a first preferred embodiment of the present invention will be described with reference to the drawings.  FIG.  1    is an external perspective view of the high-frequency signal transmission line  10  including a bent portion to connect two circuits.  FIG.  2    is a side view of the high-frequency signal transmission line  10  including a bent portion connecting two circuits.  FIG.  3    is an external perspective view of the high-frequency signal transmission line  10  including a bent portion.  FIG.  4    is an external perspective view of the high-frequency signal transmission line  10  that is not bent.  FIG.  5    is an exploded perspective view of the high-frequency signal transmission line  10  that is not bent.  FIG.  6    is a cross-sectional view taken along line A-A illustrated in  FIG.  4   . Note that in  FIG.  1    and  FIG.  2   , the high-frequency signal transmission line  10  is indicated by a dot pattern. In addition, in  FIG.  2   , components provided at an upper end of a circuit A other than the high-frequency signal transmission line  10  are not shown. Further, in  FIG.  3   , illustration of a circuit to which the high-frequency signal transmission line  10  is connected is omitted. 
     In addition, in the present specification, directions are defined as follows. A stacking direction of an element body  15  of the high-frequency signal transmission line  10  is defined as a multilayer body up-down direction. A longitudinal direction of the element body  15  is defined as a multilayer body front-back direction. A lateral direction of the element body  15  is defined as a multilayer body left-right direction. The multilayer body up-down direction, the multilayer body front-back direction, and the multilayer body left-right direction are perpendicular or substantially perpendicular to each other. Note that the definitions of the direction and the stacking direction in the present specification are examples. Therefore, the direction in actual use of the high-frequency signal transmission line  10  does not need to coincide with the direction in the present specification. 
     The high-frequency signal transmission line  10  (corresponding to a resin multilayer substrate) is used to connect two circuits in an electronic device such as a mobile phone. The high-frequency signal transmission line  10  has flexibility. Therefore, the high-frequency signal transmission line  10  can be used in an electronic device in a bent state in the up-down direction (planar direction). For example, as illustrated in  FIG.  1    and  FIG.  2   , the high-frequency signal transmission line  10  can connect the circuit A and a circuit B in the bent state in the up-down direction. Note that the high-frequency signal transmission line  10  having a bent shape also has flexibility. Therefore, the high-frequency signal transmission line  10  in the bent state can be further bent. 
     As illustrated in  FIG.  2    and  FIG.  3   , the high-frequency signal transmission line  10  includes non-curved sections A 1  and A 3  and a curved section A 2 . The non-curved sections A 1  and A 3  are sections in which the high-frequency signal transmission line  10  is not bent in the up-down direction. The curved section A 2  is a section in which the high-frequency signal transmission line  10  includes a portion that is bent in the up-down direction. The non-curved section A 1  is adjacent to the curved section A 2 . The non-curved section A 1  is located to the left of the curved section A 2 . The non-curved section A 3  is adjacent to the curved section A 2 . The non-curved section A 3  is located to the right of the curved section A 2 . 
     In this case, as illustrated in  FIG.  2    and  FIG.  3   , the multilayer body up-down direction and the multilayer body front-back direction differ depending on the position of the high-frequency signal transmission line  10 . In the non-curved section A 1  where the high-frequency signal transmission line  10  is not bent (for example, at a position ( 1 ) in  FIG.  2   ), the multilayer body up-down direction and the multilayer body front-back direction coincide with the up-down direction and the front-back direction, respectively. On the other hand, in the curved section A 2  where the high-frequency signal transmission line  10  is bent (for example, at a position ( 2 ) in  FIG.  2   ), the multilayer body up-down direction and the multilayer body front-back direction do not coincide with the up-down direction and the front-back direction, respectively. 
     Next, the structure of the high-frequency signal transmission line  10  will be described with reference to  FIG.  4    and  FIG.  5   . Note that  FIG.  4    and  FIG.  5    illustrate the state where the high-frequency signal transmission line  10  is not bent. In this case, the high-frequency signal transmission line  10  includes only a non-curved section that is not bent. Therefore, in the high-frequency signal transmission line  10  illustrated in  FIG.  4    and  FIG.  5   , the multilayer body up-down direction and the multilayer body front-back direction coincide with the up-down direction and the front-back direction, respectively. As illustrated in  FIG.  4    and  FIG.  5   , the high-frequency signal transmission line  10  includes a resin layer  16   a  (corresponding to an upper resin layer), a resin layer  16   b  (corresponding to a lower resin layer), resist layers  17   a  and  17   b  (corresponding to a protective film), a signal conductor layer  18  (corresponding to a metal foil layer), a first ground conductor layer  20  (corresponding to a metal foil layer), a second ground conductor layer  22  (corresponding to a metal foil layer), outer electrodes  24  and  26 , a plurality of first interlayer connection conductors v 1 , a plurality of second interlayer connection conductors v 2 , and interlayer connection conductors v 11  and v 12 . 
     Note that in  FIG.  5   , representative interlayer connection conductors among the plurality of first interlayer connection conductors v 1  and the plurality of second interlayer connection conductors v 2  are denoted by reference numerals. 
     As illustrated in  FIG.  5   , each of the resin layers  16   a  and  16   b  has a rectangular or substantially rectangular shape having long sides extending in the front-back direction when viewed in the up-down direction. Therefore, the length of the resin layers  16   a  and  16   b  in the front-back direction is longer than the length of the resin layers  16   a  and  16   b  in the left-right direction. The length of the resin layers  16   a  and  16   b  in the front-back direction is longer than the length of the resin layers  16   a  and  16   b  in the up-down direction. The resin layers  16   a  and  16   b  have flexibility. The resin layers  16   a  and  16   b  are flexible dielectric sheets. The resin layers  16   a  and  16   b  are made of thermoplastic resin such as polyimide and liquid crystal polymer. Such resin layers  16   a  and  16   b  define the element body  15 . More specifically, the element body  15  has a structure in which the resin layers  16   a  and  16   b  are stacked in the up-down direction. The resin layers  16   a  and  16   b  are stacked so as to be arranged in this order from top to bottom. 
     Thus, the element body  15  has a plate shape. As illustrated in  FIG.  5   , the element body  15  has a rectangular or substantially rectangular shape having longer sides extending in the front-back direction when viewed in the up-down direction. Therefore, the length of the element body  15  in the front-back direction is longer than the length of the element body  15  in the left-right direction. The length of the element body  15  in the front-back direction is longer than the length of the element body  15  in the up-down direction. 
     The signal conductor layer  18  is provided on an upper main surface of the resin layer  16   b . Thus, the signal conductor layer  18  is provided in the element body  15 . As illustrated in  FIG.  5   , the signal conductor layer  18  has a linear shape extending in the front-back direction. The signal conductor layer  18  is arranged at the center in the left-right direction of the upper main surface of the resin layer  16   b . A front end of the signal conductor layer  18  is located at a front end portion of the resin layer  16   b . A back end of the signal conductor layer  18  is located at the back end portion of the resin layer  16   b . In the high-frequency signal transmission line  10 , the signal conductor layer  18  is a metal foil layer. The signal conductor layer  18  is a type of circuit pattern. Therefore, the circuit pattern is defined by the metal foil layer. A high-frequency signal is transmitted to the signal conductor layer  18 . 
     The first ground conductor layer  20  is provided on an upper main surface of the resin layer  16   a . Thus, the first ground conductor layer  20  is located on the signal conductor layer  18 . In the present specification, the state “the first ground conductor layer  20  is located on the signal conductor layer  18  in the up-down direction” is defined as follows. At least a portion of the first ground conductor layer  20  is located in a region through which the signal conductor layer  18  passes when moving in parallel or substantially parallel in the upward direction. Thus, the first ground conductor layer  20  may be within the region through which the signal conductor layer  18  passes when moving in parallel or substantially parallel in the upward direction, or may project from the region through which the signal conductor layer  18  passes when moving in parallel or substantially parallel. In the present preferred embodiment, the first ground conductor layer  20  projects from the region through which the signal conductor layer  18  moves in parallel or substantially parallel. 
     As illustrated in  FIG.  5   , the first ground conductor layer  20  has a rectangular or substantially rectangular shape having long sides extending in the front-back direction when viewed in the up-down direction. The first ground conductor layer  20  has a shape that substantially coincides with the shape of the resin layer  16   a  when viewed in the up-down direction. However, the first ground conductor layer  20  is slightly smaller than the resin layer  16   a  when viewed in the up-down direction. A ground is connected to the first ground conductor layer  20 . 
     The second ground conductor layer  22  is provided on a lower main surface of the resin layer  16   b . Thus, the second ground conductor layer  22  is located under the signal conductor layer  18 . As illustrated in  FIG.  5   , the second ground conductor layer  22  has a rectangular or substantially rectangular shape having longer sides extending in the front-back direction when viewed in the up-down direction. The second ground conductor layer  22  has a shape that substantially coincides with the shape of the resin layer  16   b  when viewed in the up-down direction. However, the second ground conductor layer  22  is slightly smaller than the resin layer  16   b  when viewed in the up-down direction. A ground is connected to the second ground conductor layer  22 . The first ground conductor layer  20  and the second ground conductor layer  22  described above have a strip line structure. 
     The outer electrode  24  is provided at the front end portion of the lower main surface of the resin layer  16   b . The outer electrode  24  has a rectangular or substantially rectangular shape when viewed in the up-down direction. The outer electrode  24  overlaps a front end portion of the signal conductor layer  18  when viewed in the up-down direction. The second ground conductor layer  22  is not provided around the outer electrode  24  so that the outer electrode  24  is insulated from the second ground conductor layer  22 . The outer electrode  26  has a structure symmetrical to the outer electrode  24  in the front-back direction. Therefore, the description of the outer electrode  26  is omitted. The signal conductor layer  18 , the first ground conductor layer  20 , the second ground conductor layer  22 , and the outer electrodes  24  and  26  described above are formed by, for example, etching a copper foil provided on the upper main surface or the lower main surface of each of the resin layers  16   a  and  16   b.    
     The plurality of first interlayer connection conductors v 1  is provided in the element body  15  so as to be located on the left of the signal conductor layer  18 . The plurality of first interlayer connection conductors v 1  is arranged in a row at equal or substantially equal intervals in the front-back direction. The plurality of first interlayer connection conductors v 1  penetrates the resin layers  16   a  and  16   b  in the up-down direction. Upper ends of the plurality of first interlayer connection conductors v 1  are connected to the first ground conductor layer  20 . Lower ends of the plurality of first interlayer connection conductors v 1  are connected to the second ground conductor layer  22 . Thus, the plurality of first interlayer connection conductors v 1  electrically connects the first ground conductor layer  20  and the second ground conductor layer  22 . 
     The plurality of second interlayer connection conductors v 2  is provided in the element body  15  so as to be located on the right of the signal conductor layer  18 . The plurality of second interlayer connection conductors v 2  is arranged in a row at equal or substantially equal intervals in the front-back direction. The plurality of second interlayer connection conductors v 2  penetrates the resin layers  16   a  and  16   b  in the up-down direction. The upper ends of the plurality of second interlayer connection conductors v 2  are connected to the first ground conductor layer  20 . The lower ends of the plurality of second interlayer connection conductors v 2  are connected to the second ground conductor layer  22 . Thus, the plurality of second interlayer connection conductors v 2  electrically connects the first ground conductor layer  20  and the second ground conductor layer  22 . 
     The interlayer connection conductor v 11  is provided at a front end portion of the element body  15 . The interlayer connection conductor v 11  penetrates the resin layer  16   b  in the up-down direction. An intermediate portion of the interlayer connection conductor v 11  is connected to the front end portion of the signal conductor layer  18 . A lower end of the interlayer connection conductor v 11  is connected to the outer electrode  24 . Thus, the interlayer connection conductor v 11  electrically connects the signal conductor layer  18  and the outer electrode  24 . Note that the interlayer connection conductor v 12  has a structure symmetrical with the interlayer connection conductor v 11  in the front-back direction. Therefore, description of the interlayer connection conductor v 12  will be omitted. The interlayer connection conductors v 1 , v 2 , v 11 , and v 12  are through-hole conductors. The through-hole conductors are formed by plating through-holes formed in the resin layers  16   a  and  16   b . Note that the interlayer connection conductors v 1 , v 2 , v 11 , and v 12  may be via-hole conductors. 
     The resist layers  17   a  and  17   b  have the same rectangular or substantially rectangular shape as the element body  15  when viewed in the up-down direction. The resist layers  17   a  and  17   b  are not portion of the element body  15 . The material of the resist layers  17   a  and  17   b  is different from the material of the element body  15 . That is, the resist layers  17   a  and  17   b  are layers different from the element body  15 . As illustrated in  FIG.  5    and  FIG.  6   , the resist layer  17   a  covers the surface of the element body  15 . To be specific, the resist layer  17   a  is provided on an upper main surface of the element body  15 . The resist layer  17   a  is a protective film and covers the first ground conductor layer  20 . Thus, the resist layer  17   a  protects the first ground conductor layer  20 . However, since a lower main surface of the first ground conductor layer  20  is fixed to the element body  15 , the lower main surface is not covered with the resist layer  17   a . The resist layer  17   a  like this is a resist, a coverlay, or the like. For example, when the resist layer  17   a  is a resist, the upper main surface of the resin layer  16   a  is coated with the resist. When the resist layer  17   a  is a coverlay, the resin layer  16   a  and the resist layer  17   a  are bonded to each other by an adhesive (not shown) or the like. 
     As illustrated in  FIG.  5    and  FIG.  6   , the resist layer  17   b  covers the surface of the element body  15 . To be specific, the resist layer  17   b  is provided under a lower main surface of the element body  15 . The resist layer  17   b  covers the second ground conductor layer  22 . Thus, the resist layer  17   b  protects the second ground conductor layer  22 . However, since an upper main surface of the second ground conductor layer  22  is fixed to the element body  15 , the upper main surface is not covered with the resist layer  17   b . Similar to the resist layer  17   a , the resist layer  17   b  is a resist, a coverlay, or the like. 
     Openings h 11  to h 18  are provided in the resist layer  17   b . When the resist layer  17   b  is viewed in the downward direction, the opening h 11  overlaps the outer electrode  24 . When the resist layer  17   b  is viewed in the downward direction, the opening h 15  overlaps the outer electrode  26 . Thus, the outer electrodes  24  and  26  are exposed to the outside from the high-frequency signal transmission line  10  through the openings h 11  and h 15 , respectively. The opening h 12  is provided to the right of the opening h 11 . The opening h 13  is provided in front of the opening h 11 . The opening h 14  is provided to the left of the opening h 11 . Thus, the second ground conductor layer  22  is exposed to the outside from the high-frequency signal transmission line  10  through the openings h 12  to h 14 . Note that the openings h 16  to h 18  have structures symmetrical to the openings h 12  to h 14  in the front-back direction. Therefore, description of the openings h 16  to h 18  will be omitted. 
     Openings h 19  and h 20  are provided in the second ground conductor layer  22 . The diameters of the openings h 19  and h 20  are larger than the diameters of the outer electrodes  24  and  26 . Therefore, when viewed in the up-down direction, the outer electrode  24  and the outer electrode  26  are not inscribed in the openings h 19  and h 20 , respectively. Thus, the outer electrodes  24  and  26  and the second ground conductor layer  22  are not short-circuited. 
     Meanwhile, the high-frequency signal transmission line  10  has a structure that prevents the resin layer  16   a  and the resin layer  16   b  from being separated from each other. This structure will be described in more detail below. 
     As illustrated in  FIG.  6   , the signal conductor layer  18  includes a signal conductor layer upper main surface S 1  (metal foil layer upper main surface), a signal conductor layer lower main surface S 2  (metal foil layer lower main surface), and a signal conductor layer side surface S 3  (metal foil layer side surface). In the present specification, the signal conductor layer upper main surface S 1  is a portion that is visible when the signal conductor layer  18  is viewed in the downward direction. The signal conductor layer lower main surface S 2  is a portion that is visible when the signal conductor layer  18  is viewed in the upward direction. In addition, the signal conductor layer side surface S 3  is a surface of the signal conductor layer  18  excluding the signal conductor layer upper main surface S 1  and the signal conductor layer lower main surface S 2 . Note that the signal conductor layer  18  may have any shape as long as the shape is defined by the signal conductor layer upper main surface S 1 , the signal conductor layer lower main surface S 2 , and the signal conductor layer side surface S 3 . In the present preferred embodiment, the signal conductor layer  18  has a rectangular or substantially rectangular shape defined by one signal conductor layer upper main surface S 1 , one signal conductor layer lower main surface S 2 , and four signal conductor layer side surfaces S 3 . 
     The signal conductor layer lower main surface S 2  of the signal conductor layer  18  is in contact with the resin layer  16   b . More precisely, the signal conductor layer lower main surface S 2  of the signal conductor layer  18  is in contact with the upper main surface of the resin layer  16   b . The signal conductor layer lower main surface S 2  of the signal conductor layer  18  is roughened. Therefore, it is possible to prevent separation between the signal conductor layer lower main surface S 2  of the signal conductor layer  18  and the resin layer  16   b . In addition, the signal conductor layer upper main surface S 1  of the signal conductor layer  18  is in contact with the resin layer  16   a . More precisely, the signal conductor layer upper main surface S 1  of the signal conductor layer  18  is in contact with the lower main surface of the resin layer  16   a . The upper main surface of the signal conductor layer  18  is not roughened to prevent an increase in transmission loss of electric signals. Therefore, the resin layer  16   a  is not fixed to the signal conductor layer  18 . Thus, the close-contact strength of the signal conductor layer  18  to the resin layer  16   b  is stronger than the close-contact strength of the signal conductor layer  18  to the resin layer  16   a.    
     The signal conductor layer  18  is provided between the resin layer  16   a  and the resin layer  16   b . As such, the resin layer  16   a  and the resin layer  16   b  are not in contact with each other over the entire lower main surface of the resin layer  16   a  and the entire upper main surface of the resin layer  16   b . Therefore, a portion of the lower main surface of the resin layer  16   a  is in contact with a portion of the upper main surface of the resin layer  16   b . Thus, a layer interface RI between the resin layer  16   a  and the resin layer  16   b  is formed. The layer interface RI is provided at a portion where the lower main surface of the resin layer  16   a  and the upper main surface of the resin layer  16   b  are in contact with each other. Therefore, the layer interface RI is not located in a portion where the signal conductor layer  18  is provided. 
     Here, the shape of the side surface of the signal conductor layer  18  will be described in more detail. Hereinafter, attention is paid to the right side surface of the signal conductor layer  18 . First, in a cross section parallel or substantially parallel to the multilayer body up-down direction (i.e., the cross section of  FIG.  6   ), a direction from an inner side of a region surrounded by the surfaces of the signal conductor layer  18  toward an outer side portion of the region surrounded by the surfaces of the signal conductor layer  18  among directions perpendicular or substantially perpendicular to the multilayer body up-down direction is defined as an inside-outside direction. Specifically, in a cross section parallel or substantially parallel to the multilayer body up-down direction, a point located at the inner side of the region surrounded by the surfaces of the signal conductor layer  18  is defined as a point X. A point located at the outer side portion of the region surrounded by the surfaces of the signal conductor layer  18  is defined as a point Y. The position of the point X in the multilayer body up-down direction is the same as the position of the point Y in the multilayer body up-down direction. At this time, the inside-outside direction is a direction from the point X toward the point Y (see  FIG.  6   ). On the right side surface of the signal conductor layer  18 , the inside-outside direction is the rightward direction. 
     The side surface of the signal conductor layer  18  has a cross-sectional shape obtained by rotating the V-shape by 90°. Specifically, the side surface of the signal conductor layer  18  includes an inclined surface extending outward and upward from the center of the side surface of the signal conductor layer  18  in the multilayer body up-down direction and an inclined surface extending outward and downward from the center of the side surface of the signal conductor layer  18  in the multilayer body up-down direction. As described above, since the side surface of the signal conductor layer  18  has the inclined surface extending outward and upward from the center of the side surface of the signal conductor layer  18  in the multilayer body up-down direction, the signal conductor layer  18  has a structure described below. 
     The side surface of the signal conductor layer  18  includes an outer point OP and an inner point IP in a cross section parallel or substantially parallel to the multilayer body up-down direction. The outer point OP is located at a portion where the resin layer  16   a  is in contact with the side surface. As illustrated in  FIG.  6   , the right side surface of the signal conductor layer  18  includes the outer point OP in a cross section perpendicular or substantially perpendicular to the front-back direction. The side surface of the signal conductor layer  18  includes the inner point IP in a cross section parallel or substantially parallel to the multilayer body up-down direction. The inner point IP is located at a portion where the resin layer  16   a  is in contact with the side surface. The inner point IP is located below the outer point OP and is located inward of the outer point OP in the inside-outside direction. On the right side surface of the signal conductor layer  18  illustrated in  FIG.  6   , the inside-outside direction is the rightward direction. Therefore, the right side surface of the signal conductor layer  18  includes the inner point IP located below and to the left of the outer point OP in the cross section perpendicular or substantially perpendicular to the front-back direction. 
     The outer point OP and the inner point IP are located at portions where the side surface of the signal conductor layer  18  and the resin layer  16   a  are in contact with each other. Therefore, the outer point OP and the inner point IP are not located in a portion where the side surface of the signal conductor layer  18  and the resin layer  16   b  are in contact with each other. In other words, the outer point OP and the inner point IP are not located below the layer interface RI. Note that since the left side surface of the signal conductor layer  18  has a bilaterally symmetrical structure with respect to the right side surface of the signal conductor layer  18 , description thereof will be omitted. 
     According to the high-frequency signal transmission line  10 , the resin layer  16   a  is prevented from being peeled off from the resin layer  16   b . More specifically, in a cross section of the signal conductor layer  18  parallel or substantially parallel to the multilayer body up-down direction, the side surface of the signal conductor layer  18  has the outer point OP and the inner point IP located at a portion where the resin layer  16   a  is in contact with the side surface of the signal conductor layer  18 . Further, the inner point IP is located below the outer point OP and is located inward of the outer point OP in the inside-outside direction. Therefore, the resin layer  16   a  is located under a portion P 0  between the outer point OP and the inner point IP on the side surface of the signal conductor layer  18 . As such, even when the resin layer  16   a  is about to peel off from the signal conductor layer  18 , the resin layer  16   a  is caught by the portion P 0 . Thus, the resin layer  16   a  is prevented from being peeled off from the signal conductor layer  18 . As a result, peeling of the resin layer  16   a  from the resin layer  16   b  due to peeling of the resin layer  16   a  from the signal conductor layer  18  is prevented. 
     In addition, since only the lower main surface of the signal conductor layer  18  is roughened, it is also possible to prevent or reduce an increase in transmission loss of an electric signal. 
     Method of Manufacturing High-Frequency Signal Transmission Line  10   
     Hereinafter, a non-limiting example of a method of manufacturing the high-frequency signal transmission line  10  will be described with reference to  FIG.  6   . 
     First, in a first step, the resin layer  16   a  including the metal foil layer attached to the upper main surface and the resin layer  16   b  including the metal foil layer attached to the upper main surface and the lower main surface are prepared. 
     Next, in a second step, the metal foil layer attached to the upper main surface of the resin layer  16   a  is etched to form the first ground conductor layer  20 . The signal conductor layer  18 , the second ground conductor layer  22 , and the outer electrodes  24  and  26  are formed by etching the metal foil layers attached to the upper main surface and the lower main surface of the resin layer  16   b . In the etching process, for example, a mask having the same shape as the first ground conductor layer  20  is formed on the metal foil layer attached to the upper main surface of the resin layer  16   a , and etching treatment is applied on the metal foil layer on which the mask is formed. Thus, the first ground conductor layer  20  is formed. Similarly, the metal foil layers attached to the upper main surface and the lower main surface of the resin layer  16   b  are etched and the signal conductor layer  18 , the second ground conductor layer  22 , and the outer electrodes  24  and  26  are formed. 
     Next, in a third step, the signal conductor layer  18  is deformed and processed, thus forming a side surface of the signal conductor layer  18  including the outer point OP located on the outermost side and the inner point IP located below the outer point OP and located on the inner side in the inside-outside direction. The third step is, for example, a wet blasting method. 
     Next, as a fourth step, the resin layer  16   a  is stacked on the signal conductor layer  18  and the resin layer  16   b . To be specific, the resin layer  16   a  is hot-pressed from above the signal conductor layer  18  and the resin layer  16   b . The resin layer  16   a  is made of thermoplastic resin (for example, liquid crystal polymer). Therefore, the resin layer  16   a  softens and flows during hot pressing. The fluidized resin layer  16   a  enters under the portion P 0  between the outer point OP and the inner point IP. Then, the temperature of the resin layer  16   a  that has entered under the portion P 0  returns to room temperature and the resin layer  16   a  solidifies. At this time, the solidified resin layer  16   a  is caught by the portion P 0 . Therefore, the resin layer  16   a  is prevented from being peeled off from the resin layer  16   b . That is, in the present step, when the outer point OP and the inner point IP are formed such that the fluidized resin layer  16   a  enters under the portion P 0  between the outer point OP and the inner point IP, the resin layer  16   a  is prevented from being peeled off from the resin layer  16   b . As described above, in the fourth step, by using the resin layer  16   a  made of thermoplastic resin, it is possible to cause the resin layer  16   a  to easily enter under the portion P 0 . 
     Note that the high-frequency signal transmission line  10  may include an adhesive layer between the resin layer  16   a  and the resin layer  16   b . To be specific, as illustrated in  FIG.  7   , in the high-frequency signal transmission line  10 , instead of the resin layer  16   a , an adhesive layer  19  may be stacked on the signal conductor layer  18  and the resin layer  16   b . The adhesive layer  19  is formed of a material that solidifies at a temperature lower than the melting temperature of the resin layer  16   b . The adhesive layer  19  is, for example, a thermoplastic material such as LCP or fluorine resin having a melting temperature lower than that of the resin layer  16   b . Alternatively, the adhesive layer  19  is made of, for example, a thermosetting material such as an elastomer material or an epoxy resin having high fluidity even at a temperature lower than the melting temperature of the resin layer  16   b . At this time, the resin layer  16   a  is stacked on the adhesive layer  19 . In this case, the adhesive layer  19  corresponds to the upper resin layer. 
     When the resin layer  16   a  and the resin layer  16   b  are made of the same material, there is a possibility that the resin layer  16   b  fixed to the signal conductor layer  18  may also flow and change in shape during hot pressing, as with the resin layer  16   a . When the resin layer  16   b  flows and changes in shape, the electrical characteristics of the high-frequency signal transmission line  10  may change. On the other hand, the adhesive layer  19  solidifies at a temperature lower than the melting temperature of the resin layer  16   b . Therefore, by performing hot pressing at a temperature at which the resin layer  16   b  does not flow and the adhesive layer  19  flows, it is possible to reduce the possibility that the shape of the resin layer  16   b  changes. As a result, when the adhesive layer  19  is used, it is possible to manufacture the high-frequency signal transmission line  10  while reducing the possibility of a change in electrical characteristics as compared with the case where the resin layer  16   a  is used. 
     Next, through-holes are formed by irradiating the resin layers  16   a  and  16   b  with a laser beam. Then, the interlayer connection conductors v 1 , v 2 , v 11 , and v 12  are formed by forming conductors in the through-holes by plating. Note that the through-hole may be formed by a drill. 
     Finally, the openings h 11  to h 18  and the resist layers  17   a  and  17   b  are formed. Through the above steps, the high-frequency signal transmission line  10  is completed. 
     According to the non-limiting example of a manufacturing method of the high-frequency signal transmission line  10 , it is possible to easily manufacture the high-frequency signal transmission line  10 . More specifically, by stacking the resin layer  16   a  on the signal conductor layer  18  and the resin layer  16   b  formed in the second step, at least a portion of the resin layer  16   b  enters under the side surface of the signal conductor layer  18  where the outer point OP is formed. As a result, the high-frequency signal transmission line  10  can be easily manufactured. 
     In addition, according to the wet blasting method, the outer point OP and the inner point IP can be easily formed on all the side surfaces of the signal conductor layer  18 . More specifically, the upper main surface of the signal conductor layer  18  is crushed by the wet blasting method. When the upper main surface of the signal conductor layer  18  is crushed, the side surface of the signal conductor layer  18  protrudes in the inside-outside direction, and the outer point OP and the inner point IP can be formed on all the side surfaces of the signal conductor layer  18 . 
     First Modification 
     Hereinafter, a high-frequency signal transmission line  10   a  according to a first modification of a preferred embodiment of the present invention will be described with reference to the drawings.  FIG.  8    is a cross-sectional view of the high-frequency signal transmission line  10   a  taken along line A-A. Note that  FIG.  4    is referred to for the external perspective view of the high-frequency signal transmission line  10   a.    
     As illustrated in  FIG.  8   , the high-frequency signal transmission line  10   a  is different from the high-frequency signal transmission line  10  in that the shape of a signal conductor layer  18   a  is different from the shape of the signal conductor layer  18 . Note that since other configurations of the high-frequency signal transmission line  10   a  are the same as those of the high-frequency signal transmission line  10 , descriptions thereof will be omitted. 
     The side surface of the signal conductor layer  18   a  has a cross-sectional shape that can be defined by one oblique line. To be specific, the side surface of the signal conductor layer  18   a  includes an inclined surface extending straight from an outer point OP 1  located at an upper end of the side surface of the signal conductor layer  18  in the multilayer body up-down direction to an inner point IP 1  located below the outer point OP 1  and located inside the outer point OP 1 . In this case, the resin layer  16   a  is located below a portion P 1  between the outer point OP 1  and the inner point IP 1  on the side surface of the signal conductor layer  18   a . Therefore, the resin layer  16   a  is caught by the portion P 1 . Accordingly, the resin layer  16   a  is prevented from being peeled off from the resin layer  16   b.    
     Second Modification 
     Hereinafter, a high-frequency signal transmission line  10   b  according to a second modification of a preferred embodiment of the present invention will be described with reference to the drawings.  FIG.  9    is a cross-sectional view of the high-frequency signal transmission line  10   b  taken along line A-A.  FIG.  10    is a cross-sectional view of a high-frequency signal transmission line  10   b   2  taken along line A-A. Note that  FIG.  4    is referred to for the external perspective view of the high-frequency signal transmission line  10   b.    
     As illustrated in  FIG.  9   , the high-frequency signal transmission line  10   b  is different from the high-frequency signal transmission line  10  in that the shape of a signal conductor layer  18   b  is different from the shape of the signal conductor layer  18 . Note that since other configurations of a high-frequency signal transmission line  10   b  are the same as those of the high-frequency signal transmission line  10 , descriptions thereof will be omitted. 
     A protrusion protruding outward is provided in the vicinity of the center of the right side surface of the signal conductor layer  18   b . An outer point OP 2  is located at the protrusion. An inner point IP 2  is located at a portion where the right side surface of the signal conductor layer  18   b  and the layer interface RI are in contact with each other. Also in the present modification, the resin layer  16   a  is located below a portion P 2  between the outer point OP 2  and the inner point IP 2 . Therefore, the resin layer  16   a  is caught by the protrusion. Accordingly, the resin layer  16   a  is prevented from being peeled off from the resin layer  16   b.    
     Note that the shape of the protrusion may be any shape. In addition, the protrusion does not necessarily extend in a direction parallel or substantially parallel to the multilayer body left-right direction. In the example illustrated in  FIG.  9   , the protrusion has a rectangular or substantially rectangular shape extending in a multilayer body rightward direction. However, for example, as illustrated in  FIG.  10   , the protrusion may have a triangular prism shape extending in the multilayer body rightward direction. In this case, as illustrated in  FIG.  10   , the protrusion has a triangular shape when viewed in the front-back direction. To be specific, the protrusion includes a first surface SS 1  and a second surface SS 2 . The first surface SS 1  is located above the second surface SS 2 . The second surface SS 2  extends in the inside-outside direction when viewed in the front-back direction. When viewed in the front-back direction, the first surface SS 1  extends in the inside-outside direction and in the multilayer body downward direction. When viewed in the front-back direction, the first surface SS 1  and the second surface SS 2  are in contact with each other at the outer point OP 2 . Even in this case, the resin layer  16   a  is located below the portion P 2  between the outer point OP 2  and the inner point IP 2 . Therefore, the resin layer  16   a  is prevented from being peeled off from the resin layer  16   b.    
     Third Modification 
     Hereinafter, the high-frequency signal transmission line  10   c  according to a third modification of a preferred embodiment of the present invention will be described with reference to the drawings.  FIG.  11    is a cross-sectional view of the high-frequency signal transmission line  10   c  taken along line A-A. Note that  FIG.  4    is referred to for the external perspective view of the high-frequency signal transmission line  10   c.    
     As illustrated in  FIG.  11   , the high-frequency signal transmission line  10   c  is different from the high-frequency signal transmission line  10  in that the shape of a first ground conductor layer  20   b  is different from the shape of the first ground conductor layer  20  and the shape of a second ground conductor layer  22   b  is different from the shape of the second ground conductor layer  22 . Note that since other configurations of the high-frequency signal transmission line  10   c  are the same as those of the high-frequency signal transmission line  10 , descriptions thereof will be omitted. 
     The side surface of the first ground conductor layer  20   b  includes an outer point OP 3  located outermost in a portion where the resist layer  17   a  is in contact with the side surface in the inside-outside direction. In addition, the side surface of the first ground conductor layer  20   b  includes an inner point IP 3  located below the outer point OP 3  and inside the outer point OP 3  in a portion where the resist layer  17   a  is in contact with the side surface in the inside-outside direction. The resist layer  17   a  is located under the portion P 3  between the outer point OP 3  and the inner point IP 3  on the side surface of the first ground conductor layer  20   b . Therefore, the resist layer  17   a  is caught by the portion P 3 . Thus, the resist layer  17   a  is prevented from being peeled off from the resin layer  16   a . In this case, the first ground conductor layer  20   b  corresponds to the metal foil layer, the resist layer  17   a  corresponds to the upper resin layer, and the resin layer  16   a  corresponds to the lower resin layer. 
     Further, when the high-frequency signal transmission line  10   c  is viewed in the downward direction, the second ground conductor layer  22   b  corresponds to the metal foil layer, the resist layer  17   b  corresponds to the upper resin layer, and the resin layer  16   b  corresponds to the lower resin layer. Therefore, the resist layer  17   b  is prevented from being peeled off from the resin layer  16   b.    
     As described above, the high-frequency signal transmission line  10   c  includes a plurality of sets of the upper resin layer, the lower resin layer, and the metal foil layer. More specifically, the high-frequency signal transmission line  10   c  includes three sets each having the upper resin layer, the lower resin layer, and the metal foil layer. 
     Fourth Modification 
     Hereinafter, a high-frequency signal transmission line  10   d  according to a fourth modification of a preferred embodiment of the present invention will be described with reference to the drawings.  FIG.  12    is a cross-sectional view of the high-frequency signal transmission line  10   d  taken along line A-A. 
     As illustrated in  FIG.  12   , the high-frequency signal transmission line  10   d  is different from the high-frequency signal transmission line  10  in that a signal conductor layer  18   d  having a shape different from that of the signal conductor layer  18  is provided. Note that since other configurations of the high-frequency signal transmission line  10   d  are the same as those of the high-frequency signal transmission line  10 , descriptions thereof will be omitted. 
     As illustrated in  FIG.  12   , the shape of the side surface of the signal conductor layer  18   d  is different from the shape of the side surface of the signal conductor layer  18 . To be specific, a portion located below an inner point IP 4  in the side surface of the signal conductor layer  18   d  is parallel or substantially parallel to the multilayer body up-down direction. Also in this case, similar to the high-frequency signal transmission line  10 , the resin layer  16   a  is located below a portion P 4  (see  FIG.  12   ). Therefore, in the high-frequency signal transmission line  10   d , the resin layer  16   a  is prevented from being peeled off from the resin layer  16   b  for the same reason as that of the high-frequency signal transmission line  10 . In other words, when the resin layer  16   a  is located below the portion P 4 , the resin layer  16   a  is prevented from being peeled off from the resin layer  16   b  regardless of the shape of the side surface of the signal conductor layer  18   d  located below the inner point IP 4 . 
     Other Preferred Embodiments 
     It is not limited to the high-frequency signal transmission lines  10  and  10   a  to  10   d  according to the above-described preferred embodiments of the present invention, and it can be changed within the scope of the gist thereof. Further, it is possible to combine the configurations of the high-frequency signal transmission lines  10  and  10   a  to  10   d.    
     In the high-frequency signal transmission lines  10  and  10   a  to  10   d , the resin layer  16   a  and the resin layer  16   b  are not necessarily made of the same material, and may be made of different materials. 
     In the high-frequency signal transmission lines  10 ,  10   b ,  10   b   2 ,  10   c , and  10   d , the outer points OP, OP 2 , OP 3 , and OP 4  do not necessarily need to be located above the center in the multilayer body up-down direction on the side surfaces of the signal conductor layers  18  and  18   b  and the first ground conductor layer  20   b . In addition, in the high-frequency signal transmission lines  10 ,  10   b ,  10   b   2 ,  10   c , and  10   d , the inner points Ip, IP 2 , IP 3 , and IP 4  do not necessarily need to be located below the center in the multilayer body up-down direction on the side surfaces of the signal conductor layers  18   a  and  18   b  and the first ground conductor layer  20   b . In the high-frequency signal transmission lines  10 ,  10   b ,  10   c , and  10   d , and the side surface of the first ground conductor layer  20   b , the outer points OP, OP 2 , OP 3 , and OP 4  are located above the center of the side surfaces of the signal conductor layers  18   a  and  18   b  and the first ground conductor layer  20   b  in the multilayer body up-down direction, and the inner points IP, IP 2 , IP 3 , and IP 4  may be located below the center of the side surfaces in the multilayer body up-down direction. 
     In the high-frequency signal transmission lines  10  and  10   a  to  10   d , the resin layers  16   a  and  16   b  are not necessarily be made of thermoplastic resin. 
     In the high-frequency signal transmission lines  10  and  10   a  to  10   d , the signal conductor layer  18  may be fixed to the resin layer  16   a.    
     In the high-frequency signal transmission lines  10  and  10   a  to  10   d , all the side surfaces of the signal conductor layer  18 ,  18   a ,  18   b , and  18   d  may include the outer points OP and OP 1  to OP 4 , and the inner points Ip and IP 1  to IP 4 . In this case, in all cross sections of the signal conductor layer  18  parallel or substantially parallel to the multilayer body up-down direction, the outer points OP and OP 1  to OP 4  are located above the inner points IP and IP 1  to IP 4  and the layer interface RI. Alternatively, in the high-frequency signal transmission lines  10  and  10   a  to  10   d , a partial side surface of the signal conductor layer  18  may include the outer points OP and OP 1  to OP 4 , and the inner points IP and IP 1  to IP 4 . 
     The left side surface of the signal conductor layer  18  may have a left-right asymmetric structure with respect to the right side surface of the signal conductor layer  18 . Similarly, the left side surface of the signal conductor layer  18   d  may have a left-right asymmetric structure with respect to the right side surface of the signal conductor layer  18   d.    
     The left side surface of the first ground conductor layer  20   b  may have a left-right asymmetric structure with respect to the right side surface of the first ground conductor layer  20   b . Similarly, the left side surface of the second ground conductor layer  22   b  may have a left-right asymmetric structure with respect to the right side surface of the second ground conductor layer  22   b.    
     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.