Patent Publication Number: US-2023156913-A1

Title: Circuit board

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2020-122128 filed on Jul. 16, 2020 and is a Continuation Application of PCT Application No. PCT/JP2021/024760 filed on Jun. 30, 2021. The entire contents of each application are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a circuit board including a first signal conductor layer and a second signal conductor layer. 
     2. Description of the Related Art 
     Regarding inventions related to existing circuit boards, a transmission line described in International Publication No. 2015/186720 is known. The transmission line includes a dielectric body, a first signal conductor, a second signal conductor, a first-signal-conductor ground portion, a second-signal-conductor ground portion, and an intermediate portion. The dielectric body includes plural dielectric layers that are stacked in an up-down direction. The first signal conductor and the second signal conductor are arranged in a left-right direction. The first-signal-conductor ground portion is disposed above the first signal conductor. The second-signal-conductor ground portion is disposed below the second signal conductor. The intermediate portion connects the first-signal-conductor ground portion and the second-signal-conductor ground portion to one another. Thus, the intermediate portion is disposed between the first signal conductor and the second signal conductor. As a result, the first signal conductor and the second signal conductor are prevented from being coupled to one another. 
     As described above, in such a transmission line including plural signal conductors, it is desirable to prevent the coupling between the plural signal conductors. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide circuit boards with each of which a first signal conductor layer and a second signal conductor layer are able to be prevented from being coupled to one another. 
     A circuit board according to a preferred embodiment of the present invention includes a board body including a plurality of insulator layers that are stacked, a first signal conductor layer in the board body and including a first section, and a second signal conductor layer in the board body and including a third section. A line width direction of the first signal conductor layer is defined as a signal-conductor-layer left-right direction. A direction in which the first signal conductor layer extends is defined as a signal-conductor-layer front-back direction. When viewed in a stacking direction of the plurality of insulator layers, the first section is on the left of the third section in the signal-conductor-layer left-right direction and extends, in parallel or substantially in parallel with the third section, in the signal-conductor-layer front-back direction. The first section includes a plurality of first thin line portions and a plurality of first thick line portions, each of the plurality of first thin line portions has a line width, and each of the plurality of first thick line portions has a line width greater than the line width of the plurality of first thin line portion. The plurality of first thin line portions and the plurality of first thick line portions are alternately arranged in the signal-conductor-layer front-back direction. In the signal-conductor-layer left-right direction, center lines of the plurality of first thin line portions are positioned leftward relative to center lines of the plurality of first thick line portions. 
     Hereinafter, the terms used in the description of preferred embodiments of the present description will be defined. Hereinafter, a first member, a second member, and a third member mean, for example, members provided for the circuit board. In the present description, except where specifically described, portions of the first member will be defined as follows. A front portion of the first member means the front half of the first member. A rear portion of the first member means the rear half of the first member. A left portion of the first member means the left half of the first member. A right portion of the first member means the right half of the first member. An upper portion of the first member means the upper half of the first member. A lower portion of the first member means the lower half of the first member. A front end of the first member means the forward end of the first member. A rear end of the first member means the rearward end of the first member. A left end of the first member means the leftward end of the first member. A right end of the first member means the rightward end of the first member. An upper end of the first member means the upward end of the first member. A lower end of the first member means the downward end of the first member. A front end portion of the first member means the front end of the first member and the vicinity of the front end. A rear end portion of the first member means the rear end of the first member and the vicinity of the rear end. A left end portion of the first member means the left end of the first member and the vicinity of the left end. A right end portion of the first member means the right end of the first member and the vicinity of the right end. An upper end portion of the first member means the upper end of the first member and the vicinity of the upper end. A lower end portion of the first member means the lower end of the first member and the vicinity of the lower end. 
     When any two members described in the present description are defined as the first member and the second member, the relationship between the two selected members is as follows. In the present description, a state in which the first member is supported by the second member includes a state in which the first member is attached to the second member so as not to move relative to the second member (that is, the first member is fixed to the second member) and a state in which the first member is attached to the second member so as to move relative to the second member. In addition, the state in which the first member is supported by the second member includes a state in which the first member is directly attached to the second member and a state in which the first member is attached to the second member with the third member interposed therebetween. 
     In the present description, the state in which the first member is fixed to the second member includes a state in which the first member is attached to the second member so as not to move relative to the second member but does not include a state in which the first member is attached to the second member so as to move relative to the second member. In addition, the state in which the first member is fixed to the second member includes a state in which the first member is directly attached to the second member and a state in which the first member is attached to the second member with the third member interposed therebetween. 
     In the present description, the phrase “the first member and the second member are electrically connected to one another” means that a direct current can flow between the first member and the second member. Thus, the first member and the second member may be in contact with one another, or the first member and the second member are not necessarily in contact with one another. When the first member and the second member are not in contact with one another, the third member having electrical conductivity is disposed between the first member and the second member. 
     With circuit boards according to preferred embodiments of the present invention, the first signal conductor layer and the second signal conductor layer are able to be prevented from being coupled to one another. 
     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 an electronic device  1  including a circuit board  10  according to a preferred embodiment of the present invention. 
         FIG.  2    is an exploded perspective view of the circuit board  10 . 
         FIG.  3    illustrates, in top view, a first signal conductor layer  22 , a second signal conductor layer  24 , and an insulator layer  16   b  of the circuit board  10 . 
         FIG.  4    is a top view of a first signal conductor layer  22 , a second signal conductor layer  24 , and an insulator layer  16   b  of a circuit board  10   a  according to a preferred embodiment of the present invention. 
         FIG.  5    illustrates, in top view, a first signal conductor layer  22 , a second signal conductor layer  24 , and an insulator layer  16   b  of a circuit board  10   b  according to a preferred embodiment of the present invention. 
         FIG.  6    illustrates, in top view, a first signal conductor layer  22 , a second signal conductor layer  24 , a third signal conductor layer  32 , and an insulator layer  16   b  of a circuit board  10   c  according to a preferred embodiment of the present invention. 
         FIG.  7    is a top view of a first signal conductor layer  22 , a second signal conductor layer  24 , a third signal conductor layer  32 , a fourth signal conductor layer  34 , and an insulator layer  16   b  of a circuit board  10   d  according to a preferred embodiment of the present invention. 
         FIG.  8    is a top view of a first signal conductor layer  22 , a second signal conductor layer  24 , and an insulator layer  16   b  of a circuit board  10   e  according to a preferred embodiment of the present invention. 
         FIG.  9    is an enlarged view of a first signal conductor layer  22  of a circuit board according to a preferred embodiment of the present invention. 
         FIG.  10    is an enlarged view of a first signal conductor layer  22  of a circuit board according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. 
     Preferred Embodiment 
     Hereinafter, a circuit board  10  according to a preferred embodiment of the present invention will be described with reference to the drawings. 
       FIG.  1    is an external perspective view of an electronic device  1  including the circuit board  10 . In  FIG.  1   , only a representative electronic component  4  of a plurality of electronic components  4  is denoted by a reference numeral.  FIG.  2    is an exploded perspective view of the circuit board  10 .  FIG.  3    illustrates, in top view, a first signal conductor layer  22 , a second signal conductor layer  24 , and an insulator layer  16   b  of the circuit board  10 . 
     In the present description, directions are defined as follows. A stacking direction of a resist layer  17   a , an insulator layer  16   a , the insulator layer  16   b , and a resist layer  17   b  is defined as an up-down direction. A longitudinal direction of a board body  12  is defined as a front-back direction. A lateral direction of the board body  12  is defined as a left-right direction. The up-down direction, the front-back direction, and the left-right direction are orthogonal or substantially orthogonal to one another. The definitions of the directions in the present description are examples. Thus, the directions regarding the board body  12  when in actual use do not necessarily coincide with the directions given in the present description. In addition, the up-down direction may be inverted in the drawings. Similarly, the left-right direction may be inverted in the drawings. The front-back direction may be inverted in the drawings. 
     The electronic device  1  is, for example, a portable communication terminal device, such as a smartphone. As  FIG.  1    illustrates, the electronic device  1  includes a motherboard  2 , a plurality of electronic components  4 , and the circuit board  10 . The motherboard  2  has a plate shape. Electric circuits are provided at a surface and in an inner portion of the motherboard  2 . A plurality of outer electrodes (not illustrated) for mounting the electronic components  4  and the circuit board  10  are provided on an upper main surface of the motherboard  2 . 
     The plurality of electronic components  4  include, for example, a chip electronic component and a semiconductor integrated circuit. The plurality of electronic components  4  are mounted on the upper main surface of the motherboard  2 . 
     The circuit board  10  is a radio-frequency-signal transmission line for electrically connecting two electric circuits, in the electronic device  1 . In the present preferred embodiment, the circuit board  10  electrically connects two spots of the motherboard  2 . A plurality of outer electrodes (not illustrated) are provided on a front end portion and a rear end portion of a lower main surface of the circuit board  10 . Each of the plurality of outer electrodes provided on the front end portion and the rear end portion of the lower main surface of the circuit board  10  is fixed with solder to a corresponding one of the plurality of outer electrodes provided on the upper main surface of the motherboard  2 . As  FIG.  2    illustrates, the circuit board  10  includes the board body  12 , the first signal conductor layer  22 , the second signal conductor layer  24 , a first ground conductor layer  26 , and a second ground conductor layer  28 . 
     The board body  12  has a plate shape. The board body  12  extends in the front-back direction. However, as  FIG.  1    and  FIG.  2    illustrate, in the left-right direction, the width of an intermediate portion, in the left-right direction, of the board body  12  is narrower than the widths of portions of the board body  12  other than the left-right-direction intermediate portion of the board body  12 . Thus, as  FIG.  1    illustrates, the circuit board  10  is prevented from being in contact with the electronic component  4 . 
     The board body  12  includes the resist layer  17   a , the insulator layers  16   a  and  16   b , and the resist layer  17   b  that are stacked in the up-down direction. In the present preferred embodiment, the resist layer  17   a , the insulator layers  16   a  and  16   b , and the resist layer  17   b  are stacked in this order from top to bottom. The insulator layers  16   a  and  16   b  are prepared by using an insulating material. Examples of the insulating material used for the insulator layers  16   a  and  16   b  include insulating resins such as a liquid-crystal polymer (LCP) and polyimide. The resist layers  17   a  and  17   b  will be described later. The above-described board body  12  has flexibility. Thus, by bending the board body  12  upward or downward, two substrates can be connected to one another with a connector. 
     The first signal conductor layer  22  is provided in the board body  12 . The first signal conductor layer  22  is provided on an upper main surface of the insulator layer  16   b . The first signal conductor layer  22  extends in the front-back direction. Hereinafter, the line width direction of the first signal conductor layer  22  is defined as a signal-conductor-layer left-right direction. In addition, the direction in which the first signal conductor layer  22  extends is defined as a signal-conductor-layer front-back direction. The reason for selectively using the two kinds of directions is described below. 
     The first signal conductor layer  22  is bent at a right angle at four spots when viewed in the up-down direction. Thus, the first signal conductor layer  22  includes, at two locations, two sections  22   d  and  22   e  extending in the left-right direction. In the sections  22   d  and  22   e , the line width direction of the first signal conductor layer  22  is the front-back direction, thereby not coinciding with the left-right direction. Similarly, in the sections  22   d  and  22   e , the extending direction of the first signal conductor layer  22  is the left-right direction, thus not coinciding with the front-back direction. Thus, in the present description, the left-right direction and the signal-conductor-layer left-right direction are selectively used. In addition, the front-back direction and the signal-conductor-layer front-back direction are selectively used. 
     The first signal conductor layer  22  includes a second section  22   a , a first section  22   b , and a second section  22   c . The first section  22   b  is an intermediate portion of the first signal conductor layer  22 . The intermediate portion of the first signal conductor layer  22  is a portion not including the left end or the right end of the first signal conductor layer  22 . The second sections  22   a  and  22   c  are sections other than the first section  22   b . The second section  22   a , the first section  22   b , and the second section  22   c  are arranged in this order from front to back in the signal-conductor-layer front-back direction and connected to one another. 
     The second signal conductor layer  24  is provided in the board body  12 . The second signal conductor layer  24  is provided on the upper main surface of the insulator layer  16   b . The second signal conductor layer  24  has a linear shape extending in the left-right direction. The second signal conductor layer  24  is disposed on the right of the first signal conductor layer  22 . In the present description, the state in which the second signal conductor layer  24  is disposed on the right of the first signal conductor layer  22  is as follows. At least a portion of the second signal conductor layer  24  is disposed in a region through which the first signal conductor layer  22  passes when translated rightward. Thus, the second signal conductor layer  24  may be within such a region through which the first signal conductor layer  22  passes when translated rightward, or a portion of the second signal conductor layer  24  may extend beyond the region through which the first signal conductor layer  22  passes when translated rightward. Although illustration is omitted, in the present preferred embodiment, the front end of the first signal conductor layer  22  and the front end of the second signal conductor layer  24  overlap one another when viewed in the left-right direction. Similarly, the rear end of the first signal conductor layer  22  and the rear end of the second signal conductor layer  24  overlap one another when viewed in the left-right direction. Thus, no portion of the second signal conductor layer  24  extends beyond the region through which the first signal conductor layer  22  passes when translated rightward. Such a positional relationship between the first and second signal conductor layers  22  and  24  may be applied to the positional relationship between members other than the first signal conductor layer  22  and the second signal conductor layer  24 . 
     The second signal conductor layer  24  includes a fourth section  24   a , a third section  24   b , and a fourth section  24   c . The third section  24   b  is an intermediate portion of the second signal conductor layer  24 . The fourth sections  24   a  and  24   c  are sections other than the third section  24   b . The fourth section  24   a , the third section  24   b , and the fourth section  24   c  are arranged in this order from front to back in the signal-conductor-layer front-back direction and connected to one another. 
     Here, the first section  22   b  and the third section  24   b  will be described in further detail. The first section  22   b  is disposed on the left of the third section  24   b  in the signal-conductor-layer left-right direction when viewed in the up-down direction (stacking direction). The first section  22   b  extends, in parallel or substantially in parallel with the third section  24   b , in the signal-conductor-layer front-back direction. In the present preferred embodiment, each of the first section  22   b  and the third section  24   b  has a linear shape extending in the left-right direction. Moreover, the first section  22   b  and the third section  24   b  are provided on an upper main surface of the insulator layer  16   b , thus being disposed at the same position in the up-down direction. Thus, the first section  22   b  is disposed on the left of the third section  24   b . The first section  22   b  extends, in parallel or substantially in parallel with the third section  24   b , in the front-back direction. 
     Next, a distinction between the first section  22   b  and the second sections  22   a  and  22   c  and a distinction between the third section  24   b  and the fourth sections  24   a  and  24   c  will be described. As  FIG.  3    illustrates, a distance d 2  between the first section  22   b  and the third section  24   b  is shorter than a distance d 1  between the second section  22   a  and the fourth section  24   a . As  FIG.  3    illustrates, the distance d 2  between the first section  22   b  and the third section  24   b  is shorter than a distance d 3  between the second section  22   c  and the fourth section  24   c . The distance d 2  between the first section  22   b  and the third section  24   b  is the shortest distance between the first section  22   b  and the third section  24   b.    
     Thus, in the first signal conductor layer  22 , a section whose distance from the second signal conductor layer  24  is relatively short is defined as the first section  22   b . More specifically, in the first signal conductor layer  22 , a section including a section whose distance from the second signal conductor layer  24  is shortest is defined as the first section  22   b . On the other hand, in the first signal conductor layer  22 , sections whose distances from the second signal conductor layer  24  are relatively long are defined as the second sections  22   a  and  22   c.    
     In the present description, the two sections  22   d  and  22   e , in the first signal conductor layer  22 , extending in the left-right direction are portions included in the respective second sections  22   a  and  22   c.    
     In addition, in the second signal conductor layer  24 , a section whose distance from the first signal conductor layer  22  is relatively short is defined as the third section  24   b . More specifically, in the second signal conductor layer  24 , a section including a section whose distance from the first signal conductor layer  22  is shortest is defined as the third section  24   b . On the other hand, in the second signal conductor layer  24 , sections whose distances from the first signal conductor layer  22  are relatively long are defined as the fourth sections  24   a  and  24   c.    
     Next, the structure of the first section  22   b  will be described with reference to  FIG.  3   . The first section  22   b  includes a plurality of first thin line portions  222  and a plurality of first thick line portions  224 . In  FIG.  3   , only a representative first thin line portion  222  in the plurality of first thin line portions  222  and a representative first thick line portion  224  in the plurality of first thick line portions  224  are denoted by reference numerals. Each of the plurality of first thin line portions  222  has a line width w 1 , and each of the plurality of first thick line portions  224  has a line width w 2  greater than the line width w 1 . In the signal-conductor-layer front-back direction, the first thin line portion  222  has a length  11  smaller than or equal to a length  12  of the first thick line portion  224 . In addition, the plurality of first thin line portions  222  and the plurality of first thick line portions  224  are alternately arranged in the signal-conductor-layer front-back direction. Thus, the line width of the first section  22   b  changes periodically. The total of the length  11  of the first thin line portion  222  and the length  12  of the first thick line portion  224  in the signal-conductor-layer front-back direction is less than or equal to half the wavelength of a radio-frequency signal that is transmitted through the first signal conductor layer  22 . That is, the length of the first section  22   b  per period of change in the line width of the first section  22   b  is less than or equal to half the wavelength of a radio-frequency signal that is transmitted through the first signal conductor layer  22 . 
     In addition, the left ends of the plurality of first thin line portions  222  coincide with the left ends of the plurality of first thick line portions  224  in the signal-conductor-layer left-right direction. Thus, the left edge of the first section  22   b  is even. In the present preferred embodiment, the left edge of the first section  22   b  has a linear shape extending in the front-back direction. On the other hand, the right ends of the plurality of first thin line portions  222  are positioned leftward relative to the right ends of the plurality of first thick line portions  224  in the signal-conductor-layer left-right direction. Thus, the right edge of the first section  22   b  is uneven. The right edge of the first section  22   b  has a zigzag shape (whose turns are not necessarily sharp) extending in the left-right direction. As a result, in the signal-conductor-layer left-right direction, center lines L 1  of the plural first thin line portions  222  are positioned leftward relative to center lines L 2  of the plurality of first thick line portions  224 . Thus, a center line La of the first section  22   b  in the signal-conductor-layer left-right direction has a zigzag shape extending in the left-right direction. 
     The line width of the third section  24   b  is uniform. Thus, each of the left edge and the right edge of the third section  24   b  has a linear shape extending in the left-right direction. 
     As described above, in the left-right direction, the width of the intermediate portion, in the left-right direction, of the board body  12  is narrower than the widths of portions of the board body  12  other than the left-right-direction intermediate portion of the board body  12  as  FIG.  3    illustrates. More specifically, in the signal-conductor-layer left-right direction, a width W 2  of the board body  12  in the first section  22   b  is narrower than a width W 1  and a width W 3  of the board body  12  in the respective second sections  22   a  and  22   c . However, in the signal-conductor-layer left-right direction, the width of a portion, of the board body  12 , including a front end portion of the first section  22   b  is the width W 1  greater than the width W 2 . Similarly, in the signal-conductor-layer left-right direction, the width of a portion, of the board body  12 , including a rear end portion of the first section  22   b  is the width W 3  greater than the width W 2 . 
     The first ground conductor layer  26  is provided on the upper main surface of the insulator layer  16   a . The first ground conductor layer  26  has a planar shape. The first ground conductor layer  26  covers the entire or substantially the entire upper main surface of the insulator layer  16   a . Thus, the first ground conductor layer  26  is disposed above the first signal conductor layer  22  and the second signal conductor layer  24 . In addition, the first ground conductor layer  26  overlaps the first signal conductor layer  22  and the second signal conductor layer  24  when viewed in the up-down direction. 
     The second ground conductor layer  28  is provided on a lower main surface of the insulator layer  16   b . The second ground conductor layer  28  has a planar shape. The second ground conductor layer  28  covers the entire or substantially the entire lower main surface of the insulator layer  16   b . Thus, the second ground conductor layer  28  is disposed below the first signal conductor layer  22  and the second signal conductor layer  24 . In addition, the second ground conductor layer  28  overlaps the first signal conductor layer  22  and the second signal conductor layer  24  when viewed in the up-down direction. The first signal conductor layer  22 , the second signal conductor layer  24 , the first ground conductor layer  26 , and the second ground conductor layer  28  that are described above define a strip-line structure. The first signal conductor layer  22 , the second signal conductor layer  24 , the first ground conductor layer  26 , and the second ground conductor layer  28  are made of, for example, metal foil such as copper foil that is subjected to patterning. 
     The resist layer  17   a  covers the first ground conductor layer  26 . The resist layer  17   a  defines and functions as a protective layer for the first ground conductor layer  26 . The resist layer  17   b  covers the second ground conductor layer  28 . The resist layer  17   b  defines and functions as a protective layer for the second ground conductor layer  28 . The resist layers  17   a  and  17   b  are prepared by, for example, printing an insulating material. 
     The circuit board  10  includes a via hole conductor and an outer electrode, which are not illustrated. Such a via hole conductor defines and functions as an interlayer connection conductor electrically connecting the first ground conductor layer  26  and the second ground conductor layer  28  to one another. In addition, a via hole conductor defines and functions as an interlayer connection conductor electrically connecting the first signal conductor layer  22  and an outer electrode to one another. Moreover, a via hole conductor defines and functions as an interlayer connection conductor electrically connecting the second signal conductor layer  24  and an outer electrode to one another. However, because the via hole conductors and the outer electrodes of the circuit board  10  have typical structures, the description thereof are thus omitted. 
     Advantageous Effects 
     With the circuit board  10 , the first signal conductor layer  22  and the second signal conductor layer  24  can be prevented from being coupled to one another even when the distance between the first signal conductor layer  22  and the second signal conductor layer  24  is reduced. More specifically, as  FIG.  1    illustrates, there may be an intention to prevent the circuit board  10  from being in contact with the electronic component  4 . In such a case, in the left-right direction, the width of the intermediate portion, in the left-right direction, of the board body  12  is narrower than the widths of portions of the board body  12  other than the left-right-direction intermediate portion of the board body  12 . Thus, the distance between the first signal conductor layer  22  and the second signal conductor layer  24  is reduced in the intermediate portion, in the left-right-direction, of the board body  12 . That is, the first section  22   b  and the third section  24   b  extend in the signal-conductor-layer front-back direction in parallel or substantially in parallel with one another in the intermediate portion, in the left-right-direction, of the board body  12 . In this case, the first section  22   b  and the third section  24   b  are likely to be coupled to one another. Thus, the degree of isolation between the first signal conductor layer  22  and the second signal conductor layer  24  is likely to be reduced. 
     Thus, the first section  22   b  includes the plurality of first thin line portions  222  having the line width w 1  and the plurality of first thick line portions  224  having the line width w 2  greater than the line width w 1 . The plurality of first thin line portions  222  and the plurality of first thick line portions  224  are alternately arranged in the signal-conductor-layer front-back direction. In the signal-conductor-layer left-right direction, the center lines L 1  of the plurality of first thin line portions  222  are positioned leftward relative to the center lines L 2  of the plurality of first thick line portions  224 . Thus, a distance d 4  between the first thin line portion  222  and the third section  24   b  is greater than the distance d 2  between the first thick line portion  224  and the third section  24   b . As a result, the average of the distances between the first section  22   b  and the third section  24   b  is greater than the distance d 2 . The first section  22   b  and the third section  24   b  are prevented from being coupled to one another. As described above, with the circuit board  10 , the first signal conductor layer  22  and the second signal conductor layer  24  can be prevented from being coupled to one another even when the distance between the first signal conductor layer  22  and the second signal conductor layer  24  is reduced. 
     In addition, with the circuit board  10 , the insertion loss of the first signal conductor layer  22  can be reduced or prevented from increasing. More specifically, for example, the line width of the first signal conductor layer  22  in the entire first section  22   b  may be reduced to prevent the first signal conductor layer  22  and the second signal conductor layer  24  from being coupled to one another. Consequently, the distance between the first section  22   b  and the third section  24   b  can be increased. However, the insertion loss of the first signal conductor layer  22  increases. Thus, in the circuit board  10 , the first section  22   b  includes the plurality of first thin line portions  222  having the line width w 1  and the plurality of first thick line portions  224  having the line width w 2  greater than the line width w 1 . Accordingly, with the circuit board  10 , the insertion loss of the first signal conductor layer  22  is reduced or prevented from increasing. 
     In addition, according to the circuit board  10 , the total of the length  11  of the first thin line portion  222  and the length  12  of the first thick line portion  224  in the signal-conductor-layer front-back direction is less than or equal to half the wavelength of a radio-frequency signal that is transmitted through the first signal conductor layer  22 . Thus, degradation of the characteristics of the circuit board  10  due to generation of a standing wave in the first thin line portions  222  and the first thick line portions  224  are reduced or prevented from occurring. As a result, the transmission quality of the first signal conductor layer  22  can be reduced or prevented from being degraded. 
     In addition, according to the circuit board  10 , in the signal-conductor-layer front-back direction, the first thin line portion  222  has the length  11  smaller than or equal to the length  12  of the first thick line portion  224 . In particular, in the signal-conductor-layer front-back direction, the length  11  of the first thin line portion  222  is preferably equal or substantially equal to the length  12  of the first thick line portion  224 . Due to the configuration, the proportion of the first thick line portions  224  to the first section  22   b  increases. As a result, the first signal conductor layer  22  and the second signal conductor layer  24  can be prevented from being coupled to one another while the insertion loss of the first signal conductor layer  22  is reduced or prevented from increasing. 
     First Modification 
     Hereinafter, a circuit board  10   a  according to a first modification of a preferred embodiment of the present invention will be described with reference to the drawings.  FIG.  4    is a top view of a first signal conductor layer  22 , a second signal conductor layer  24 , and an insulator layer  16   b  of the circuit board  10   a.    
     The circuit board  10   a  differs from the circuit board  10  in the shape of the first section  22   b . In the circuit board  10 , the left edge of the first section  22   b  has a linear shape extending in the left-right direction. On the other hand, in the circuit board  10   a , the left edge of a first section  22   b  has a zigzag shape extending in the left-right direction. That is, the left ends of the plurality of first thin line portions  222  are positioned rightward relative to the left ends of the plurality of first thick line portions  224  in the signal-conductor-layer left-right direction. The structures of the other portions of the circuit board  10   a  are the same or substantially the same as those of the circuit board  10 , and the descriptions thereof are thus omitted. 
     In the signal-conductor-layer left-right direction, the first thick line portions  224  of the circuit board  10   a  have a line width greater than the line width of the first thick line portions  224  of the circuit board  10 . Thus, in the circuit board  10   a , the insertion loss of the first signal conductor layer  22  is further reduced or prevented from increasing compared with the circuit board  10 . In addition, in the circuit board  10   a , both the left edge and the right edge of the first section  22   b  have a zigzag shape. Accordingly, with the circuit board  10   a , the first section  22   b  can be prevented from being coupled to a circuit, a component, a housing, or other portions that are positioned on the right of the first section  22   b.    
     Second Modification 
     Hereinafter, a circuit board  10   b  according to a second modification of a preferred embodiment of the present invention will be described with reference to the drawings.  FIG.  5    illustrates, in top view, a first signal conductor layer  22 , a second signal conductor layer  24 , and an insulator layer  16   b  of the circuit board  10   b.    
     The circuit board  10   b  differs from the circuit board  10  in the shape of the third section  24   b . In the circuit board  10 , the left edge of the third section  24   b  has a linear shape extending in the left-right direction. On the other hand, in the circuit board  10   b , the left edge of a third section  24   b  has a zigzag shape extending in the left-right direction. 
     Next, the structure of the third section  24   b  will be described with reference to  FIG.  5   . The third section  24   b  includes a plurality of second thin line portions  242  and a plurality of second thick line portions  244 . Each of the plurality of second thin line portions  242  has a line width w 3 , and each of the plurality of second thick line portions  244  has a line width w 4  greater than the line width w 3 . In addition, the plurality of second thin line portions  242  and the plurality of second thick line portions  244  are alternately arranged in the signal-conductor-layer front-back direction. Thus, the line width of the third section  24   b  changes periodically. The total of a length  13  of the second thin line portion  242  and a length  14  of the second thick line portion  244  in the signal-conductor-layer front-back direction is less than or equal to half the wavelength of a radio-frequency signal that is transmitted through the second signal conductor layer  24 . That is, the length of the third section  24   b  per period of change in the line width of the third section  24   b  is less than or equal to half the wavelength of a radio-frequency signal that is transmitted through the second signal conductor layer  24 . 
     In addition, the right ends of the plurality second thin line portions  242  coincide with the right ends of the plurality second thick line portions  244  in the signal-conductor-layer left-right direction. Thus, the right edge of the third section  24   b  is even. In the present preferred embodiment, the right edge of the third section  24   b  has a linear shape extending in the left-right direction. On the other hand, the left ends of the plurality of second thin line portions  242  are positioned rightward relative to the left ends of the plurality of second thick line portions  244  in the signal-conductor-layer left-right direction. Thus, the left edge of the third section  24   b  is uneven. The left edge of the third section  24   b  has a zigzag shape extending in the front-back direction. As a result, in the signal-conductor-layer left-right direction, center lines L 3  of the plurality of second thin line portions  242  are positioned rightward relative to center lines L 4  of the plurality of second thick line portions  244 . Thus, a center line Lb of the third section  24   b  in the signal-conductor-layer left-right direction has a zigzag shape extending in the left-right direction. 
     In addition, the total of a length  11  of a first thin line portion  222  and a length of 12 of a first thick line portion  224 , in the signal-conductor-layer front-back direction, differs from the total of the length  13  of the second thin line portion  242  and the length  14  of the second thick line portion  244 , in the signal-conductor-layer front-back direction. In the present preferred embodiment, the total of the length  11  of the first thin line portion  222  and the length of 12 of the first thick line portion  224  in the signal-conductor-layer front-back direction is smaller than the total of the length  13  of the second thin line portion  242  and the length  14  of the second thick line portion  244  in the signal-conductor-layer front-back direction. This is because the wavelength of a radio-frequency signal that is transmitted through the first signal conductor layer  22  is smaller than the wavelength of a radio-frequency signal that is transmitted through the second signal conductor layer  24 . That is, when the frequency of a radio-frequency signal that is transmitted through the first signal conductor layer  22  differs from the frequency of a radio-frequency signal that is transmitted through the second signal conductor layer  24 , the periodic zigzag pattern of the first section  22   b  differs from that of the third section  24   b.    
     With the circuit board  10   b , the first signal conductor layer  22  and the second signal conductor layer  24  can further be prevented from being coupled to one another. More specifically, the third section  24   b  includes the plurality of second thin line portions  242  having the line width w 3  and the plurality of second thick line portions  244  having the line width w 4  greater than the line width w 3 . The plurality of second thin line portions  242  and the plurality of second thick line portions  244  are alternately arranged in the signal-conductor-layer front-back direction. In the signal-conductor-layer left-right direction, the center lines L 3  of the plurality of second thin line portions  242  are positioned rightward relative to the center lines L 4  of the plurality of second thick line portions  244 . Thus, the average of the distances between the first section  22   b  and the third section  24   b  in the circuit board  10   b  is greater than the average of the distances between the first section  22   b  and the third section  24   b  in the circuit board  10 . As a result, the first section  22   b  and the third section  24   b  are further prevented from being coupled to one another. 
     Third Modification 
     Hereinafter, a circuit board  10   c  according to a third modification of a preferred embodiment of the present invention will be described with reference to the drawings.  FIG.  6    illustrates, in top view, a first signal conductor layer  22 , a second signal conductor layer  24 , a third signal conductor layer  32 , and an insulator layer  16   b  of the circuit board  10   c.    
     The circuit board  10   c  differs from the circuit board  10   b  in the shape of the third section  24   b  and the presence or absence of the third signal conductor layer  32 . Hereinafter, such differences will be described. 
     In the circuit board  10   b , the right edge of the third section  24   b  has a linear shape extending in the left-right direction. On the other hand, in the circuit board  10   c , the right edge of a third section  24   b  has a zigzag shape extending in the left-right direction. That is, the right ends of the plurality of second thin line portions  242  are positioned leftward relative to the right ends of the plurality of second thick line portions  244  in the signal-conductor-layer left-right direction. 
     The circuit board  10   c  further includes the third signal conductor layer  32 . The third signal conductor layer  32  includes a sixth section  32   a , a fifth section  32   b , and a sixth section  32   c . The fifth section  32   b  is an intermediate portion of the third signal conductor layer  32 . The sixth sections  32   a  and  32   c  are sections other than the fifth section  32   b . The sixth section  32   a , the fifth section  32   b , and the sixth section  32   c  are arranged in this order from front to back in the signal-conductor-layer front-back direction and connected to one another. The fifth section  32   b  is disposed on the right of the third section  24   b  in the signal-conductor-layer left-right direction when viewed in the up-down direction (stacking direction). The fifth section  32   b  extends, in parallel or substantially in parallel with the third section  24   b , in the signal-conductor-layer front-back direction. 
     Next, the structure of the fifth section  32   b  will be described with reference to  FIG.  6   . The fifth section  32   b  includes a plurality of third thin line portions  322  and a plurality of third thick line portions  324 . Each of the plurality of third thin line portions  322  has a line width w 5 , and each of the plurality of third thick line portions  324  has a line width w 6  greater than the line width w 5 . In addition, the plurality of third thin line portions  322  and the plurality of third thick line portions  324  are alternately arranged in the signal-conductor-layer front-back direction. Thus, the line width of the fifth section  32   b  changes periodically. The total of a length  15  of the third thin line portion  322  and a length  16  of the third thick line portion  324  in the signal-conductor-layer front-back direction is less than or equal to half the wavelength of a radio-frequency signal that is transmitted through the third signal conductor layer  32 . That is, the length of the fifth section  32   b  per period of change in the line width of the fifth section  32   b  is less than or equal to half the wavelength of a radio-frequency signal that is transmitted through the third signal conductor layer  32 . 
     In addition, the right ends of the plurality of third thin line portions  322  coincide with the right ends of the plurality of third thick line portions  324  in the signal-conductor-layer left-right direction. Thus, the right edge of the fifth section  32   b  is even. In the present preferred embodiment, the right edge of the fifth section  32   b  has a linear shape extending in the left-right direction. On the other hand, the left ends of the plurality of third thin line portions  322  are positioned rightward relative to the left ends of the plurality of third thick line portions  324  in the signal-conductor-layer left-right direction. Thus, the left edge of the fifth section  32   b  is uneven. The left edge of the fifth section  32   b  has a zigzag shape extending in the left-right direction. As a result, in the signal-conductor-layer left-right direction, center lines L 5  of the plurality of third thin line portions  322  are positioned rightward relative to center lines L 6  of the plurality of third thick line portions  324 . Thus, a center line Lc of the fifth section  32   b  in the signal-conductor-layer left-right direction has a zigzag shape extending in the left-right direction. The structures of the other portions of the circuit board  10   c  are the same or substantially the same as those of the circuit board  10   b , and the descriptions thereof are thus omitted. 
     With the circuit board  10   c , the first signal conductor layer  22  and the second signal conductor layer  24  can be prevented from being coupled to one another, and the second signal conductor layer  24  and the third signal conductor layer  32  can also be prevented from being coupled to one another. 
     Fourth Modification 
     Hereinafter, a circuit board  10   d  according to a fourth modification of a preferred embodiment of the present invention will be described with reference to the drawings.  FIG.  7    is a top view of a first signal conductor layer  22 , a second signal conductor layer  24 , a third signal conductor layer  32 , a fourth signal conductor layer  34 , and an insulator layer  16   b  of the circuit board  10   d.    
     The circuit board  10   d  differs from the circuit board  10   b  in the presence or absence of the third signal conductor layer  32  and the fourth signal conductor layer  34 . More specifically, the circuit board  10   d  further includes the third signal conductor layer  32  and the fourth signal conductor layer  34 . In addition, first differential signals are transmitted to the first signal conductor layer  22  and the fourth signal conductor layer  34 . Second differential signals are transmitted to the second signal conductor layer  24  and the third signal conductor layer  32 . 
     The fourth signal conductor layer  34  is provided in the board body  12 . The fourth signal conductor layer  34  is disposed on the left of the first signal conductor layer  22 . The fourth signal conductor layer  34  and the first signal conductor layer  22  are symmetrical or substantially symmetrical on the left and right sides. More specifically, the fourth signal conductor layer  34  includes an eighth section  34   a  (not illustrated), a seventh section  34   b , and an eighth section  34   c  (not illustrated). The seventh section  34   b  is an intermediate portion of the fourth signal conductor layer  34 . The eighth sections  34   a  and  34   c  are sections other than the seventh section  34   b . The eighth section  34   a , the seventh section  34   b , and the eighth section  34   c  are arranged in this order from front to back in the signal-conductor-layer front-back direction and connected to one another. The seventh section  34   b  is disposed on the left of a first section  22   b  in the signal-conductor-layer left-right direction when viewed in the up-down direction (stacking direction). The seventh section  34   b  extends, in parallel or substantially in parallel with the first section  22   b , in the signal-conductor-layer front-back direction. 
     Next, the structure of the seventh section  34   b  will be described with reference to  FIG.  7   . The seventh section  34   b  includes a plurality of fourth thin line portions  342  and a plurality of fourth thick line portions  344 . Each of the plurality of fourth thin line portions  342  has a line width w 7 , and each of the plurality of fourth thick line portions  344  has a line width w 8  greater than the line width w 7 . In addition, the plurality of fourth thin line portions  342  and the plurality of fourth thick line portions  344  are alternately arranged in the signal-conductor-layer front-back direction. Thus, the line width of the seventh section  34   b  changes periodically. The total of a length  17  of the fourth thin line portion  342  and a length  18  of the fourth thick line portion  344  in the signal-conductor-layer front-back direction is less than or equal to half the wavelength of a radio-frequency signal that is transmitted through the fourth signal conductor layer  34 . That is, the length of the seventh section  34   b  per period of change in the line width of the seventh section  34   b  is less than or equal to half the wavelength of a radio-frequency signal that is transmitted through the fourth signal conductor layer  34 . In addition, the total of a length of a first thin line portion  222  and a length of a first thick line portion  224  in the signal-conductor-layer front-back direction is equal or substantially equal to the total of a length of the fourth thin line portion  342  and a length of the fourth thick line portion  344  in the signal-conductor-layer front-back direction. 
     In addition, the right ends of the plurality of fourth thin line portions  342  coincide with the right ends of the plurality of fourth thick line portions  344  in the signal-conductor-layer left-right direction. Thus, the right edge of the seventh section  34   b  is even. In the present preferred embodiment, the right edge of the seventh section  34   b  has a linear shape extending in the left-right direction. On the other hand, the left ends of the plurality of fourth thin line portions  342  are positioned rightward relative to the left ends of the plurality of fourth thick line portions  344  in the signal-conductor-layer left-right direction. Thus, the left edge of the seventh section  34   b  is uneven. The left edge of the seventh section  34   b  has a zigzag shape extending in the left-right direction. As a result, in the signal-conductor-layer left-right direction, center lines L 7  of the plurality of fourth thin line portions  342  are positioned rightward relative to center lines L 8  of the plurality of fourth thick line portions  344 . Thus, a center line Ld of the seventh section  34   b  in the signal-conductor-layer left-right direction has a zigzag shape extending in the left-right direction. 
     The third signal conductor layer  32  is provided in the board body  12 . The third signal conductor layer  32  is disposed on the right of the second signal conductor layer  24 . The third signal conductor layer  32  includes a sixth section  32   a  (not illustrated), a fifth section  32   b , and a sixth section  32   c  (not illustrated). The fifth section  32   b  is an intermediate portion of the third signal conductor layer  32 . The sixth sections  32   a  and  32   c  are sections other than the fifth section  32   b . The sixth section  32   a , the fifth section  32   b , and the sixth section  32   c  are arranged in this order from front to back in the signal-conductor-layer front-back direction and connected to one another. The fifth section  32   b  is disposed on the right of a third section  24   b  in the signal-conductor-layer left-right direction when viewed in the up-down direction (stacking direction). The fifth section  32   b  extends, in parallel or substantially in parallel with the third section  24   b , in the signal-conductor-layer front-back direction. 
     Next, the structure of the fifth section  32   b  will be described with reference to  FIG.  7   . The fifth section  32   b  includes a plurality of third thin line portions  322  and a plurality of third thick line portions  324 . Each of the plurality of third thin line portions  322  has a line width w 5 , and each of the plurality of third thick line portions  324  has a line width w 6  greater than the line width w 5 . In addition, the plurality of third thin line portions  322  and the plurality of third thick line portions  324  are alternately arranged in the signal-conductor-layer front-back direction. Thus, the line width of the fifth section  32   b  changes periodically. The total of a length  15  of the third thin line portion  322  and a length  16  of the third thick line portion  324  in the signal-conductor-layer front-back direction is less than or equal to half the wavelength of a radio-frequency signal that is transmitted through the third signal conductor layer  32 . That is, the length of the fifth section  32   b  per period of change in the line width of the fifth section  32   b  is less than or equal to half the wavelength of a radio-frequency signal that is transmitted through the third signal conductor layer  32 . In addition, the total of a length of the second thin line portion  242  and a length of the second thick line portion  244  in the signal-conductor-layer front-back direction is equal or substantially equal to the total of a length of the third thin line portion  322  and a length of the third thick line portion  324  in the signal-conductor-layer front-back direction. 
     In addition, the left ends of the plurality of third thin line portions  322  coincide with the left ends of the plurality of third thick line portions  324  in the signal-conductor-layer left-right direction. Thus, the left edge of the fifth section  32   b  is even. In the present preferred embodiment, the left edge of the fifth section  32   b  has a linear shape extending in the left-right direction. On the other hand, the right ends of the plurality of third thin line portions  322  are positioned leftward relative to the right ends of the plurality of third thick line portions  324  in the signal-conductor-layer left-right direction. Thus, the right edge of the fifth section  32   b  is uneven. The right edge of the fifth section  32   b  has a zigzag shape extending in the left-right direction. As a result, in the signal-conductor-layer left-right direction, center lines L 5  of the plurality of third thin line portions  322  are positioned leftward relative to center lines L 6  of the plurality of third thick line portions  324 . Thus, a center line Lc of the fifth section  32   b  in the signal-conductor-layer left-right direction has a zigzag shape extending in the left-right direction. The structures of the other portions of the circuit board  10   d  are the same or substantially the same as those of the circuit board  10   b , and the descriptions thereof are thus omitted. 
     In the above-described circuit board  10   d , the first signal conductor layer  22  and the second signal conductor layer  24  can be prevented from being coupled to one another. 
     In addition, the fourth signal conductor layer  34  and the first signal conductor layer  22  are symmetrical or substantially symmetrical on the left and right sides. Thus, the radio-frequency-signal transmission characteristics of the first signal conductor layer  22  are the same as or similar to those of the fourth signal conductor layer  34 . Accordingly, the first differential signals can be transmitted to the first signal conductor layer  22  and the fourth signal conductor layer  34 . Due to the same or similar reason, the second differential signals can be transmitted to the second signal conductor layer  24  and the third signal conductor layer  32 . 
     Fifth Modification 
     Hereinafter, a circuit board  10   e  according to a fifth modification of a preferred embodiment of the present invention will be described with reference to the drawings.  FIG.  8    is a top view of a first signal conductor layer  22 , a second signal conductor layer  24 , and an insulator layer  16   b  of the circuit board  10   e.    
     The circuit board  10   e  differs from the circuit board  10  in the structures of the plurality of first thick line portions  224 . A line width w 2  of first thick line portions  224  becomes narrower approaching both ends (the front end and the rear end) of a first section  22   b . The structures of the other portions of the circuit board  10   e  are the same or substantially the same as those of the circuit board  10 , and the descriptions thereof are thus omitted. 
     With the circuit board  10   e , the characteristic impedance generated in the first signal conductor layer  22  is reduced or prevented from changing abruptly. As a result, in the first signal conductor layer  22 , a radio-frequency signal is reduced or prevented from being reflected, and the insertion loss of the first signal conductor layer  22  is reduced. 
     Other Preferred Embodiments 
     The circuit boards according to preferred embodiments of the present invention are not limited to any one of the circuit boards  10  and  10   a  to  10   e  and can be changed without departing from the spirit of the present invention. In addition, any of the configurations of the circuit boards  10  and  10   a  to  10   e  may be combined. 
     In the circuit boards  10  and  10   a  to  10   e , the first signal conductor layer  22 , the second signal conductor layer  24 , the third signal conductor layer  32 , and the fourth signal conductor layer  34  may be bent in the left-right direction when viewed in the up-down direction. 
     In the circuit boards  10  and  10   a  to  10   e , in the signal-conductor-layer left-right direction, the width of the board body  12  in the first section  22   b  may be greater than or equal to the widths of the board body  12  in the second sections  22   a  and  22   c.    
     In the circuit boards  10  and  10   a  to  10   e , the total of the length  11  of the first thin line portion  222  and the length  12  of the first thick line portion  224  in the signal-conductor-layer front-back direction is preferably less than or equal to half the wavelength of a radio-frequency signal that is transmitted through the first signal conductor layer  22 . However, the total of the length  11  of the first thin line portion  222  and the length  12  of the first thick line portion  224  in the signal-conductor-layer front-back direction may be greater than half the wavelength of a radio-frequency signal that is transmitted through the first signal conductor layer  22 . 
     In the circuit boards  10  and  10   a  to  10   e , in the signal-conductor-layer front-back direction, the length  11  of the first thin line portion  222  is preferably smaller than or equal to the length  12  of the first thick line portion  224 . However, in the signal-conductor-layer front-back direction, the length  11  of the first thin line portion  222  may be greater than the length  12  of the first thick line portion  224 . 
     In the circuit boards  10  and  10   a  to  10   e , the first ground conductor layer  26  and the second ground conductor layer  28  are not necessarily provided. Thus, the circuit boards  10  and  10   a  to  10   e  do not necessarily include the first ground conductor layer  26  or the second ground conductor layer  28  or do not necessarily include one of the first ground conductor layer  26  and the second ground conductor layer  28 . In the case in which the circuit boards  10  and  10   a  to  10   e  do not include the first ground conductor layer  26  or the second ground conductor layer  28 , the first section  22   b  and the third section  24   b  do not overlap a ground conductor layer when viewed in the up-down direction (stacking direction). The structure of the first section  22   b  in this case will be described with reference to the drawings. Each of  FIG.  9    and  FIG.  10    is an enlarged view of a first signal conductor layer  22  of a circuit board according to a preferred embodiment of the present invention. 
     As  FIG.  9    illustrates, a line width w 2  of a first thick line portion  224  is preferably less than or equal to twice of a distance d 10  between the right end of the first thick line portion  224  and the right end of a first thin line portion  222  in the signal-conductor-layer left-right direction. That is, the depth (distance d 10 ) of a recess defined by the first thin line portion  222  and the first thick line portions  224  is preferably greater than or equal to half the line width w 2  of the first thick line portion  224 . Due to the configuration, unnecessary radiation from the first signal conductor layer  22  can be reduced or prevented. 
     The first thick line portion  224  may be tapered as  FIG.  10    illustrates. In such a case, a length  11  of the first thin line portion  222  is the average of a length  111  and a length  112 . In addition, the distance d 10  and the widths w 1  and w 2  are as illustrated in  FIG.  10   . As described above, with the first signal conductor layer  22  having any one of the structures illustrated in  FIG.  9    and  FIG.  10   , unnecessary radiation from the first signal conductor layer  22  can be reduced or prevented. Thus, when including the first signal conductor layer  22  having any one of the structures illustrated in  FIG.  9    and  FIG.  10   , the circuit boards  10  and  10   a  to  10   e  do not necessarily include the first ground conductor layer  26  or the second ground conductor layer  28 . 
     In the circuit boards  10  and  10   a  to  10   e , the positions of the first signal conductor layer  22  and the second signal conductor layer  24  in the up-down direction may differ from one another. Thus, the first signal conductor layer  22  and the second signal conductor layer  24  may be provided on different insulator layers. 
     In the circuit boards  10  and  10   a  to  10   e , the first section  22   b  and the third section  24   b  have been described as the intermediate portions of the first signal conductor layer  22  and the second signal conductor layer  24 , respectively. However, the first section  22   b  may be a portion other than the intermediate portion of the first signal conductor layer  22 , and the third section  24   b  may be a portion other than the intermediate portion of the second signal conductor layer  24 . For example, the first section  22   b  may include a left end portion or a right end portion of the first signal conductor layer  22 , and the third section  24   b  may include a left end portion or a right end portion of the second signal conductor layer  24 . 
     The left edge and/or the right edge of the third section  24   b  of the circuit board  10   c  does not necessarily have a zigzag shape. 
     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.