Patent Publication Number: US-2017373396-A1

Title: Coil device and electronic device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of International application No. PCT/JP2016/056088, filed Feb. 29, 2016, which claims priority to Japanese Patent Application No. 2015-046119, filed Mar. 9, 2015, the entire contents of each of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a coil device and an electronic device having the coil device. 
     BACKGROUND ART THE INVENTION 
     Helical coils, such as the coil device disclosed in Unexamined Japanese Patent Publication No. 2005-26384 or 2009-289995, are known. In these disclosed structures the helical coil is mounted on a circuit substrate and made of wire. 
     This structure is disadvantageous because the distance between adjacent conductors can vary due to a change in shape of the wire. These changes cause a variation in magnetic field distribution generated by the helical coil and there is a variation in its inductance value. When this coil conductor is used in a wireless communication device as an antenna, variations in the magnetic field distribution of the helical coil result in a variation of the communicating distance of the coil. Therefore, the communication distance can vary with a production lot and a tuning element is needed to correct a variation in resonant frequency of the antenna. One object of the present invention is to provide a helical coil which suppresses these variations in magnetic field distribution and inductance values. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect of the invention, a coil device comprises: 
     a plate-like element body having a main surface, a back surface, and an end surface; 
     a helical coil having a coil axis extending between the main surface and the back surface of the element body, the helical coil including a first plurality of conductive portions located in the element body and arranged at spaced locations along the coil axis, each of the first plurality of conductive portions being a conductive pin; and 
     a connection terminal provided on at least one of the back surface and the end surface of the element body and connected to the helical coil. 
     The helical coil can further comprise a second plurality of conductive portions, the first plurality of conductive portions lying in a first plane and the second plurality of conductive portions lying in a second plane spaced from the first plane. Each of the second plurality of conductive portions is a preferably conductive pin. The first and second planes are preferably parallel to one another and the main and back surfaces of the element body. 
     In one aspect of the invention, the first plurality of conductive portions are spaced apart from each other by a first distance as measured along the coil axis and the second plurality of conductive portions are spaced apart from each other by a second distance as measured along the coil axis, the first and second distances being different than each other. Preferably the first distance is smaller than the second distance. 
     In another aspect of the invention, each of the conductive portions of the first and second plurality of conductive portions are circular in cross-section. 
     In another aspect of the invention, the conductive portions of the first plurality of conductive portions have a larger diameter than the conductive portions of the second plurality of conductive portions. 
     In accordance with a further aspect of the invention, the first plurality of conductive portions lie in a plane that is parallel to the coil axis and include first and second outer conductive portions with the remaining conductive portions of the first plurality of conductive portions being located between the first and second outer conductive portions. The second plurality of conductive portions also lie in a plane that is parallel to the coil axis and include third and fourth outer conductive portions with the remaining conductive portions of the second plurality of conductive portions being located between the third and fourth outer conductive portions. The distance between the first and second outer conductive portions, as measured along the coil axis, is less than the distance between the third and fourth outer conductive portions as measured along the coil axis. 
     In yet another aspect of the invention, each conductive portion of the second plurality of conductive portions is located closer to the back surface of the element body than to the main surface of the element body and each of the conductive portions of the first plurality of conductive portions is located closer to the main surface of the element body than to the back surface of the element body. The second plurality of conductive portions include first and second outer conductive portions, the remaining conductive portions of the second plurality of conductive portions being located between the first and second outer conductive portions, at least a portion of the first and second outer conductive portions being exposed at the back side of the element body and acting as a connection terminal portion. In a preferred embodiment, the first and second conductive portions are semicircular in cross-section and the remaining conductive portions of the second plurality of conductive portions are circular in cross-section. More preferably, the remaining conductive portions of the second plurality of conductive portions are metal pins. 
     In a further aspect of the invention, the second plurality of conductive portions are located on the back surface of the element body and have a rectangular cross-section. 
     In another aspect of the invention, coil device includes a magnetic body disposed in the helical coil. 
     In some embodiments the helical coil includes conductive connecting portions which connect pins of the first plurality of conductive portions to the conductive portions of the second plurality of conductive portions, the conductive connecting portions being located on the end surface of the element body. 
     The invention is also directed towards a wireless communication device including the forgoing coil device and a circuit substrate having at least one wiring pattern, the coil device being mounted on the circuit substrate and being electrically coupled to the wiring pattern either the back surface or the end surface of the element body. The coil device is preferably electrically coupled to the wiring pattern via at last one connection terminal located on the back surface of the element body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial schematic perspective view of a wireless communication device having a coil device as an antenna in an embodiment 1 of the present invention. 
         FIG. 2  is a perspective view showing an inside of the coil device in the embodiment 1. 
         FIG. 3  is a cross-sectional view taken along a line Q-Q in  FIG. 2 . 
         FIG. 4  is a view showing a magnetic field distribution generated by the coil device. 
         FIG. 5A  is a view to explain a step in one example of a method for manufacturing the coil device in the embodiment 1. 
         FIG. 5B  is a view to explain a step subsequent to the step in  FIG. 5A . 
         FIG. 5C  is a view to explain a step subsequent to the step in  FIG. 5B . 
         FIG. 5D  is a view to explain a step subsequent to the step in  FIG. 5C . 
         FIG. 5E  is a view to explain a step subsequent to the step in  FIG. 5D . 
         FIG. 6  is a partial cross-sectional view of a wireless communication device having a coil device as an antenna in an embodiment 2 of the present invention. 
         FIG. 7  is a partial cross-sectional view of a wireless communication device having a coil device as an antenna in an embodiment 3 of the present invention. 
         FIG. 8  is a partial cross-sectional view of a wireless communication device having a coil device as an antenna in an embodiment 4 of the present invention. 
         FIG. 9  is a partial schematic perspective view of a wireless communication device having a coil device as an antenna in an embodiment 5 of the present invention. 
         FIG. 10  is a view to explain a step in one example of a method for manufacturing the coil device in the embodiment 5. 
         FIG. 11  is a partial side view of a wireless communication device having a coil device as an antenna in an embodiment 6 of the present invention. 
         FIG. 12  is a perspective view of a wireless communication device having a coil device as an antenna in an embodiment 7 of the present invention. 
         FIG. 13  is a cross-sectional view of the wireless communication device in the embodiment 7. 
         FIG. 14  is an exploded perspective view of a booster antenna. 
         FIG. 15  is a circuit diagram of the booster antenna. 
         FIG. 16  is a perspective view of a coil device in an embodiment 8 of the present invention. 
         FIG. 17  is a view to explain a step in one example of a method for manufacturing the coil device in the embodiment 8. 
         FIG. 18  is an exploded perspective view of a wireless communication device having a coil device as an antenna in an embodiment 9 of the present invention. 
         FIG. 19  is a circuit diagram of the wireless communication device in the embodiment 9. 
         FIG. 20  is an exploded perspective view of a wireless communication device having a surface mount coil as an antenna in an embodiment 10 of the present invention. 
         FIG. 21  is a circuit diagram of the wireless communication device in the embodiment 10. 
         FIG. 22  is an exploded perspective view of a DC-DC converter device having a coil device as a choke coil in an embodiment 11. 
         FIG. 23  is a circuit diagram of a DC-DC converter module in the embodiment 11. 
         FIG. 24  is an exploded perspective view of a wireless communication device having a coil device as an antenna in an embodiment 12. 
         FIG. 25  is an exploded perspective view of a wireless communication device having a coil device as an antenna in an embodiment 13. 
     
    
    
     Embodiment 1 
       FIG. 1  is a view schematically showing a wireless communication device serving as one example of an electronic device having a coil device as an antenna in the embodiment 1 of the present invention.  FIG. 2  is a perspective view showing an inside of the coil device.  FIG. 3  is a cross-sectional view taken along a line Q-Q in  FIG. 2 . In the above drawings, an X-Y-Z orthogonal coordinate system, and a s-t-u orthogonal coordinate system are shown, but these are provided to easily understand the embodiments of the present invention and do not limit the present invention. 
     In this embodiment, and as shown in  FIG. 1 , the wireless communication device  10  has a circuit substrate  12  and a coil device  14 , which serves as an antenna and is mounted on a main surface  12   a  of the circuit substrate  12 . The coil device  14  is shown as having a cuboidal shape (block shape) in the drawing for ease of illustration, but preferably has a plate-like shape which is as thin as possible. That is, it has a chip-like shape in its length L, as measured along the X axis extending along the axis CA of the helical coil  16  formed in the helical device  14 , and its width W, as measured along the Y direction perpendicular to the axis CA and are greater than its thickness T as measured along the Z direction. In this embodiment, the coil device  14  is preferably an antenna element which has a carrier frequency in a HF band and is used, for example, in a NFC (Near Field Communication) system. 
     The circuit substrate  12  is preferably a mother substrate such as printed wiring board and has a wiring pattern (not shown) made of conductive material (such as copper material) on the main surface  12   a  for mounting the coil device  14 . The circuit substrate  12  preferably has an RFIC chip and a surface mount type capacitor on its main surface  12   a  and these components are connected to the coil device  14  through the wiring pattern. Furthermore, as shown in  FIG. 3 , the circuit substrate  12  preferably has an internal conductive ground layer  12   b.  Alternatively, the ground layer  12   b  may be provided on a back surface  12   c  of the circuit substrate  12  as a ground pattern. The ground layer  12   b  is preferably formed along almost the entirety of the circuit substrate  12 . 
     As shown in  FIG. 2 , the coil device  14  has a helical coil  16  and a plate-like binder member (element body)  18  having a mount surface (back surface)  18   a  which faces the main surface  12   a  of the circuit substrate  12 , a top surface (main surface)  18   d  opposing the mount surface  18   a,  two end surfaces  18   e  and  18   f  intersecting the coil axis CA and end surfaces  18   b  and  18   c  extending parallel to the coil axis CA. The mount surface  18   a  serves as the mount surface when the coil device is mounted on the mother substrate. The mount surface  18   a  and the top surface  18   d  are preferably larger in area than the end surfaces  18   b,    18   c,    18   e  and  18   f.  That is, the element body  18  is formed into a plate-like shape (thin plate-like shape) in which its thickness T is smaller than its length L and its width W. In this embodiment, the area of its end surfaces  18   b  and  18   c  are larger than the areas of its end surfaces  18   e  and  18   f  However, the end surfaces  18   e  and  18   f  may be larger in area than the end surfaces  18   b  and  18   c.    
     As shown in  FIG. 1 , the coil axis CA of the helical coil  16  extends between the mount surface  18   a  and the top surface  18   d  of the element body  18  parallel to the main surface  12  of the circuit substrate. However, the invention is not limited to this arrangement and the coil axis CA and the plane of the main surface  18  need not be parallel to one another. 
     As shown in  FIG. 2 , the helical coil  16  is composed of first to fourth sets of pluralities of conductors  16   a  to  16   d.  More specifically, as shown in  FIG. 2 , the plurality of second conductors  16   a  are arranged at regular intervals along the coil axis CA (the X-axis direction) at a position closer to the mount surface  18   a  than the top surface  18   d.  In this embodiment 1, the plurality of second conductors  16   a  are arranged parallel to each other in a common plane and each of them is composed of a respective metal pin extending parallel to the mount surface  18   a  (that is, the circuit substrate  12 ) (in the Y-axis direction) and having a circular cross-section. The metal pin is preferably a columnar metal member made of, for example, copper material. Furthermore, the diameter of the circular cross-section of the metal pin is, for example, 30 μm to 1 mm. 
     A plurality of first conductors  16   b  are arranged at regular intervals along the coil axis at a position closer to the top surface  18   d  of the element body  18  than the plurality of second conductors  16   a  and are therefore further from the main surface  12   a  of the circuit substrate  12  than the plurality of second conductors  16   a.  In embodiment 1, the plurality of first conductors  16   b  are located in a common plane and are spaced from and extend parallel to each other and each of them is preferably composed of a metal pin extending parallel to the Y-axis direction and having a circular cross-section. The metal pin is preferably a columnar metal member made of, for example, copper material. The diameter of the circular cross-section surface of the metal pin is, for example, 30 μm to 1 mm. In embodiment 1, the plurality of first conductors  16   a  and the plurality of first conductors  16   b  are preferably composed of the same metal pins. Thus, compared with a case where the first conductor  16   a  and the second conductor  16   b  are composed of different metal pins, a manufacture cost of the coil device  14  can be reduced. 
     Each of a plurality of third conductors  16   c  is located on the side surface  18   b  of the element body  18  and connects one end of a respective first conductor  16   a  to one end of a respective first conductor  16   b.  Each of the plurality of fourth conductors  16   d  is located on the side surface  18   c  of the element body and connects one end of a respective second conductor  16   a  to one end of a respective first conductor  16   b.    
     The plurality of first and second conductors  16   b  and  16   a  are located inside of the element body  18 . The element body  18  preferably has the plate-like shape and is preferably made of resin material such as epoxy resin. Because the plurality of second conductors  16   a  are located internally of the element body  18 , they are stably arranged at regular intervals along the direction of the coil axis CA (the X-axis direction). Similarly, the plurality of first conductors  16   b  are stably arranged at regular intervals along the direction of the coil axis CA. 
     As noted above, the plurality of third and fourth conductors  16   c  and  16   d  are respectively provided on the end surfaces  18   b  and  18   c  of the element body  18  and are preferably composed of conductive patterns formed on the end surfaces  18   b  and  18   c  of the element body  18 . 
     The end surfaces  18   b  and  18   c  of the element body  18  have respective connection terminals  20 , each of which is electrically connected to a respective terminal  12   d  (one of which is shown in  FIG. 1 ) on the main surface  12   a  of the circuit substrate  12  through a conductive bonding material  22  such as solder. Each of the connection terminals  20  is connected to a respective end (this end is not connected to the third conductor  16   c  and the fourth conductor  16   d ) of one of the second conductors  16   a ′ provided at opposite ends of the plurality of first conductors  16   a  in the extending direction of the coil axis CA (X-axis direction). See  FIG. 2 . That is, one connection terminal  20  is connected to one terminal of the helical coil  16 , while the other connection terminal  20  is connected to the other terminal of the helical coil  16 . 
     The coil device  14  having the helical coil  16  is preferably mounted at or near an end (edge) of the main surface  12   a  of the circuit substrate  12  so that one axial coil opening of the helical coil  16  faces inwardly of the circuit substrate  12  while the other faces outwardly of the circuit substrate  12 . In embodiment 1, the coil device  14  is mounted on the main surface  12   a  of the circuit substrate  12  such that the coil axis CA of the helical coil  16  intersects with a side between the main surface  12   a  and an end surface  12   e  (an end surface in the X-axis direction) of the circuit substrate  12 . 
     As shown in  FIG. 4 , when the coil device  14  is disposed as described above, a magnetic field (broken lines) generated from the helical coil  16  expands upwardly from the main surface  12   a  of the circuit substrate  12 . At the same time, the magnetic field expands outwardly away from the end surface  12   e.  As a result, the magnetic field from the helical coil  16  is not easily canceled by the ground layer  12   b.  Thus, compared with a case where the coil device  14  is provided on a center of the main surface  12   a  of the circuit substrate  12 , a communicatable range of the wireless communication device  10  can be enlarged and a communication distance thereof can be increased. 
     As shown in  FIG. 4 , the magnetic field generated from the helical coil  16  of the coil device  14  is blocked by the ground layer  12   b  in the circuit substrate  12  and by the wiring pattern (not shown) on the main surface  12   a,  so that it hardly expands toward the back surface  12   c  of the circuit substrate  12 . That is, the communication distance can be increased by using a metal body such as the ground layer provided in the circuit substrate  12 . 
     Hereinafter, one example of a method for manufacturing the coil device  14  in the embodiment 1 will be described. 
     First, as shown in  FIG. 5A , a plurality of metal pins  50  having the same shape are set in a pin stand  52  so as to be arranged in rows in a direction (t-axis direction) perpendicular to a longitudinal direction (u-axis direction) of the metal pins. Each metal pin  50  may be fixed in the pin stand  52  with a bonding agent such as epoxy resin. 
     After that, as shown in  FIG. 5B , a resin block  54  is formed on the pin stand  52  so as to internally contain the plurality of metal pins  50 . The resin block  54  is preferably formed by pouring an uncured resin onto the pin stand  52  and heat curing the resin. 
     Subsequently, as shown in  FIG. 5C , upper and lower sides of the resin block  54  (as viewed in  FIG. 5 c   ) are polished so that opposite ends of the metal pins  50  in the longitudinal direction (u-axis direction in  FIG. 5 c   ) are exposed from the resin block  54 . Thus, the upper and lower surfaces are planarized and the ends of the metal pins are exposed, whereby the plurality of second conductors  16   a  and the plurality of first conductors  16   b  are manufactured so as to be internally contained in the resin block  54 . In  FIG. 5C , the metal pins  50  are not shown for the sake of simplification, and only the exposed end surfaces are shown. 
     Subsequently, as shown in  FIG. 5D , the plurality of third conductors  16   c  and at least one connection terminal  20  are patterned and formed on the polished surface  54   a  of the resin block  54 . Similarly, the plurality of fourth conductors  16   d  and at least one connection terminal  20  are patterned and formed on the other polished surface  54   b  of the resin block  54 . For example, the third conductors  16   c,  the fourth conductors  16   d,  and the connection terminals  20  are formed as metal layers (such as copper layers) on the polished surfaces  54   a  and  54   b  of the resin block  54  and then the metal layers are patterned by photo-etching. Alternatively, they are formed such that conductive paste is formed into a predetermined pattern by screen printing and then subjected to a heat treatment. After the patterning, the surface of the metal layer may be plated with, for example, nickel-gold or tin. 
     Then, as shown in  FIG. 5E , the resin block  54 , including the third conductors  16   c,  the fourth conductors  16   d,  and the connection terminals  20 , is cut into a plurality of individual parts, whereby the plurality of coil devices  14  are manufactured. 
     According to embodiment 1, most of the helical coil  16  (more particularly the first and second conductors  16   b,    16   a ) are composed of the metal pins which are more rigid than wire. Thus, compared with the case where the helical coil is entirely composed of wire, a variation in magnetic field distribution and a variation in inductance value can be suppressed. When the coil device  14  is used as the antenna, the following effects can be provided in the wireless communication device  10 . 
     First, since the plurality of first conductors  16   b  are provided at a position relatively distant from the mount surface  18   a  it is possible to prevent pure resistance (DC resistance; Rdc) from being increased due to skin effect and, at the same time, suppress a variation in magnetic field distribution. As a result, a variation in communication distance of the coil device  14  serving as the antenna can be suppressed. 
     In use, when the coil device  14  is used as an antenna and receives or transmits a high-frequency signal, skin effect is generated in the helical coil  16  of the coil device  14 . The skin effect is a phenomenon where alternating current tends to avoid passing through the center of a solid conductor, limiting itself to conduction near the surface. This effectively limits the cross-sectional area of the conductor which is available to carry alternating current flow, increasing the resistance of the conductor above what it would normally be for direct current. As a result of skin effect, current flows in a portion extending from the outer surface of the conductor inwardly to a predetermined depth (skin depth). 
     The skin depth varies as a function of the conductor material and the current frequency. As the frequency becomes high, the skin depth becomes small and the effective resistance of the conductor becomes high. The skin depth also varies as a function of the material used for the conductor. The skin depth is larger, and the effective resistance is lower, when copper is used rather than silver. The skin depth is larger, and the effective resistance is lower, when gold is used rather than copper. 
     As the aspect ratio (horizontal to vertical ratio) of the cross-sectional shape of the conductor approaches 1, and as the cross-sectional shape becomes less angular, the current can flow throughout the cross-section of the conductor. For example, in a case where the cross-sectional surface of the conductor is rectangular, the current intensively flows near the surface on a short side. Furthermore, in a case where the cross-sectional surface is angular (it is triangle, for example), the current intensively flows in an angular portion. Therefore, in order to make the current flow throughout the cross-section of the conductor, that is, in order to prevent the effective resistance of the conductor from being increased due to the skin effect, the cross-sectional surface of the conductor is preferably circular. 
     In order to minimize the skin effect, it is preferably that the plurality of conductor portions located remotely from the mount surface  18   a,  that is, the plurality of first conductors  16   b  largely contributing to the communication, are composed of the plurality of metal pins having the circular cross-sectional surface. 
     As shown in  FIG. 4 , the magnetic field generated by the coil device  14  extends upwardly and away from the main surface  12   a  of the circuit substrate  12 . The first conductors  16   b  which are relatively far from the mount surface  18   a  (that is, circuit substrate  12 ) contributes more to the magnetic field distribution than the other conductors making up the helical coil  16 . By using metal pins having a circular cross section for the first conductors  16   b  (which are the primary contributor to the magnetic field), the effective resistance can be prevented from being increased due to the skin effect and the signal decay and the power loss can be reduced in the first conductor  16   b.    
     Furthermore, when the first conductor  16   b  is composed of the rigid metal pin instead of flexible metal material such as wire it is unlikely to change in shape compared with the case where it is composed of the wire. Therefore, the variation of the spacing between the first conductors  16   b  is small and the magnetic field distribution and a self-resonant frequency of the helical coil  16  are unlikely to vary. As a result, when the helical coil  16  is used as an antenna, its communication distance and frequency characteristic will have small variations. 
     Furthermore, because the first conductors  16   b  are internally contained in the element body  18 , they do not change shape and the space between them can be stably maintained. Therefore, variations in the magnetic field distribution of the helical coil  16  can be reduced. If the coil made of wire is to be sealed with resin, due to resin flow at the time of sealing, the distance between segments of the wire is likely to vary and disconnection could occur in some cases. 
     In addition, similar to the first conductor  16   b,  the second conductor  16   a  is composed of a metal pin having a circular cross-sectional. Therefore, its effective resistance will not be increased due to skin effect. As a result, signal decay and power loss can be also reduced in the first conductor  16   a.    
     Still furthermore, because the element body  18  has the plate-like (flat) cuboidal shape, the first and second conductors  16   b  and  16   a  are the longest conductors among the first to fourth conductors  16   a  to  16   d  in the helical coil  16 , the helical coil  16  can be mostly composed of the metal pins and the coil device can function as an antenna having small losses and in a high communication distance. 
     As shown in  FIG. 3 , a distance D 2  from the metal pin  16   b  at one end of the coil axis to the metal pin  16   b  at the other end of the coil axis is smaller than a distance D 1  from the metal pin  16   a  at one end of the coil axis to the metal pin  16   a  at the other side of the coil axis. Therefore, compared with a case where the distances D 1  is equal to the distance D 2 , a coil opening of the helical coil  16  can be larger in area, and the coil opening can face a top surface side, so that the communication distance in a top surface direction can be increased. 
     Embodiment 2 
     A wireless communication device in embodiment 2 differs from the wireless communication device in embodiment 1 by the second conductor in the helical coil of the coil device. The wireless communication device in the embodiment 2 will be described with a focus on this difference. 
       FIG. 6  is a partial cross-sectional view of the wireless communication device having a coil device used as an antenna. As shown in  FIG. 6 , a helical coil  116  of a coil device  114  has a plurality of second conductors  116   a  and a plurality of first conductors  116   b  internally contained in an element body  118 . 
     The plurality of second conductors  116   a  are preferably located in a common plane and are spaced at regular intervals along the coil axis CA (X-axis direction), more specifically, at a predetermined pitch interval p 1 . The plurality of first conductors  116   b  are also preferably located in a common plane and are spaced at regular intervals along the coil axis CA at regular intervals, more specifically, at a predetermined pitch interval p 2 . In embodiment 2, the pitch interval pl is preferably equal to the pitch interval p 2 . 
     While pitch intervals p 1  and p 2  are equal in length, the distance (space) g 1  between the adjacent first conductors  116   a  along the coil axis CA (X-axis direction) is preferably different than a distance (space) g 2  between the adjacent second conductors  116   b  along the coil axis CA. More specifically, the space g 2  of the first conductors  116   b  is preferably smaller than the space g 1  of the second conductors  116   a.    
     Because the space g 2  of the first conductors  116   b  is smaller than the space g 1  of the second conductors  116   a,  a length of a cross-sectional surface (that is, a diameter d 2 ) of the first conductor  116   b  in the X-axis direction of the coil axis CA is larger than a length (diameter d 1 ) of the second conductor  116   a  in the X-axis direction of the coil axis CA. Stated otherwise, the metal pins used for the first conductors  116   b  are thicker than the metal pin used for the second conductors  116   a.    
     In this way, when the plurality of first conductors  116   b  which primarily contribute to the magnetic field distribution have larger cross-section and are more closely spaced, the magnetic field of the helical coil  116  can expand greatly. More particularly, when the space g 2  is small, a magnetic flux generated from one of the adjacent first conductors  116   b  across the space g 2  and passing through the space g 2  is prevented from being canceled by a magnetic flux generated from the other and passing through the space g 2 . As a result, a minor loop can be prevented from being generated in a portion of the helical coil  116  which mainly contributes to magnetic field coupling, with a coil (antenna) on a communication partner side, so that a power supplied to the helical coil  116  can be effectively used for forming the magnetic field for the wireless communication. As a result, the coil device  114  having the helical coil  116  is high in energy efficiency. 
     Like embodiment 1, the plurality of first conductors  116   b  of embodiment 2 reduce skin effect and prevent the effective resistance from being increased while at the same time suppressing variations in magnetic field distribution. Furthermore, the coil device  114  can form the magnetic field for the wireless communication energetically high in efficiency. 
     Embodiment 3 
     A wireless communication device in embodiment 3 differs from the wireless communication device in embodiment 1 by the second conductor in the coil device. The wireless communication device in embodiment 3 will be described with a focus on this difference. 
     As shown in  FIG. 7 , a helical coil  216  of a coil device  214  has a plurality of second conductors  216   a  and a plurality of first conductors  216   b  internally contained in an element body  218 , similar to the helical coil  16  in embodiment 1. 
     The plurality of second conductors  216   a  arranged at regularly spaced locations along the coil axis CA (X-axis direction), more specifically, at a predetermined pitch interval p 1 ′. The plurality of first conductors  216   b  are similarly arranged at regular spaced locations along the coil axis CA, more specifically, at a predetermined pitch interval p 2 ′. 
     In embodiment 3, the diameter d 1 ′ of the first conductor  216   a  is equal to the diameter d 2 ′ of the first conductor  216   b.  However, a distance (space) g 1 ′ between the adjacent first conductors  216   a  along the coil axis CA (X-axis direction) differs from a distance (space) g 2 ′ between adjacent first conductors  216   b  along the coil axis CA. More specifically, the space g 2 ′ is smaller than the space g 1   
     Because the space g 2 ′ is smaller than the space g 1 ′, the pitch interval p 2 ′ of the plurality of first conductors  116   b  is smaller than the pitch interval p 1 ′ of the plurality of second conductors  216   a.  Thus, when the plurality of first conductors  216   b  (which largely contribute to the magnetic field distribution toward an upper part of the main surface  212   a  of the circuit substrate  212 ) have the small space g 2 ′, the magnetic field of the helical coil  216  can be significantly enlarged. This is because when the space g 2 ′ is small, the magnetic flux generated from one of two adjacent first conductors  216   b  (and passing through the space g 2 ′) is prevented from being canceled by the magnetic flux generated from the other of the two adjacent first conductors  216   b  and (also passing through the space g 2 ′). Thus, power supplied to the helical coil  216  can be effectively used for forming the magnetic field for the wireless communication. As a result, the coil device  214  having the helical coil  216  is high in energy efficiency. 
     According to embodiment 3, the effective resistance of the plurality of first conductors  216   b,  which are the primary contributors to the communication (and are distant from the mount surface) can be prevented from being increased due to the skin effect and at the same time, a variation in magnetic field distribution can be suppressed. Furthermore, the coil device  214  can form the magnetic field for the wireless communication energetically high in efficiency. 
     Embodiment 4 
     The wireless communication device in embodiment 4 differs from the wireless communication device in embodiment 1 by presence of a magnetic body  330 . The wireless communication device in the embodiment 4 will be described with a focus on this difference. 
     As shown in  FIG. 8 , the coil device  314  has a magnetic body  330  unlike the coil device  14  in embodiment 1. The magnetic body  330  is preferably a magnetic body made of ferrite ceramics, or a magnetic body in which ferrite powder is dispersed in a resin and has a plate-like shape. The magnetic body  330  is internally contained in an element body  318  within a helical coil  316 . That is, the magnetic body  330  is disposed between a plurality of second conductors  316   a  and a plurality of first conductors  316   b  of the helical coil  316  and held by the plurality of second conductors  316   a  and the plurality of first conductors  316   b.    
     When the magnetic body  330  is disposed in the helical coil  316 , a magnetic field generated by the helical coil  316  can largely expand. As a result, a communicatable distance of a wireless communication device  310  having the coil device  314  as an antenna can be increased. 
     According to embodiment 4, the skin effect in the plurality of first conductors  316   b  (which are the primary contributors to the communication) can be lowered and the effective resistance of the helical coil  316  can be prevented from being increased. At the same time, a variation in magnetic field distribution can be suppressed. Furthermore, the wireless communication device  310  can be long in communicatable distance. 
     Furthermore, since the plate-like magnetic body  330  is held by the plurality of second conductors  316   a  and the plurality of first conductors  316   b,  the magnetic body  330  is not likely to be moved due to resin flow generated when an element body  318  is formed, so that the coil device can have small manufacturing variations. 
     Embodiment 5 
     A wireless communication device in embodiment 5 differs from the wireless communication device in embodiment 1 by the connection between the coil device and the terminal on the circuit substrate. The wireless communication device in embodiment 5 will be described with a focus on this difference. 
     In embodiment 1 (shown in  FIG. 1 ) the coil device  14  has respective connection terminals  20  for electrically connecting the helical coil  16  to respective terminals  12   d  on the circuit substrate  12 , on each of the end surfaces  18   b  and  18   c  which do not face the main surface  12   a  of the circuit substrate  12 . The connection terminal  20  is electrically connected to the terminal  12   d  of the circuit substrate  12  through the conductive bonding material  22  such as solder. 
     In contrast, as shown in  FIG. 9 , a coil device  414  in embodiment 5 has a connection terminal  420  which is located on a mount surface  418   a  facing a main surface  412   a  of a circuit substrate  412 . More specifically, each of the two connection terminals  420  is preferably composed of a second conductor  416   a ′ provided at each opposite ends of the plurality of second conductors  416   a  relative to the coil axis CA (X-axis direction). 
     Each of the second conductors  416   a ′, located at opposite ends of the coil axis CA (X-axis direction), are composed of a metal pin which is thicker than the other second conductors  416   a  and has a roughly semicircular cross-section with, for example, a planar surface facing the main surface  412   a  of the circuit substrate  412 . Each planar surface is exposed on an outside of the element body  418 , more particularly on the mount surface  418   a  of the element body  418 , and functions as a respective connection terminal  420  of the coil device  414 . A plated film is preferably formed on a surface of the connection terminal  420 . 
     A terminal  412   d  is provided on the main surface  412   a  of the circuit substrate  412  facing the connection terminal  420  (the planar surface of the second conductor  416   a ′) of the coil device  414 . Therefore, when the coil device  414  is mounted on the main surface  412   a  of the circuit substrate  412 , the connection terminal  420  of the coil device  414  comes in contact with the terminal  412   d  of the circuit substrate  412 . As a result, an LGA type terminal electrode can be formed and the helical coil  416  of the coil device  414  can be connected to the terminal  412   d  of the circuit substrate  412  through conductive bonding material such as solder. 
     The connection terminal  420  composed of the planar surface of the second conductor  416   a ′ is preferably manufactured such that, as shown in  FIG. 10 , a resin block  454  internally containing the second conductors  416   a  ( 416   a ′) and the first conductor  416   b  is cut across the first conductor  416   a ′, for example. That is, the cut surface of the resin block  454  becomes the mount surface  418   a  of the element body  418 , and the cut surface of the metal pin becomes the terminal surface. 
     According to embodiment 5, the plurality of first conductors  416   b  are the primary contributors to the communication. Since they are circular in cross-section an increase in the effective resistance due to the skin effect can be mitigated and, at the same time, a variation in magnetic field distribution can be suppressed. Furthermore, the helical coil  416  of the coil device  414  can be easily connected to the terminal  412   d  on the circuit substrate  412 . 
     In this embodiment, the metal pins constituting the second conductors  416   a ′ have a diameter which is larger than the diameter of the metal pins constituting the other first conductors  416   a,  but the diameter may be equal to each other. 
     Embodiment 6 
     A wireless communication device in embodiment 6 differs from the wireless communication device in embodiment 1 by the first conductor of the coil device. The wireless communication device in embodiment 6 will be described with a focus on this difference. 
     As shown in  FIG. 11 , the coil device  514  has a plurality of second conductors  516   a  (forming part of a helical coil  516 ) which are composed of a metal member having a rectangular cross-section. Each of the second conductors  516   a  is provided on the mount surface  518   a  (an outer surface) of the element body  518  facing a main surface  512   a  of the circuit substrate  512  instead of being provided inside the element body  518 . The second conductor  516   a  is preferably a conductive pattern formed on the mount surface  518   a  of the element body  518 , for example. 
     Each of the first conductors  516   b  have a circular cross-section and are regularly spaced along the axial direction of coil axis CA (X-axis direction) of the helical coil  516  at a position remote from the circuit substrate  512 . The first conductors  516   b  are the primary contributor to an magnetic field distribution extending upwardly from the main surface  512   a  of the circuit substrate  512 . Therefore, even though the plurality of second conductors  516   a  have a rectangular cross-section and the effective resistance of the second conductors  516   a  is high compared with a circular cross-section, and even through they are provided outside the element body  518 , a significant negative effect is avoided. 
     An advantage of embodiment 6 is that because the plurality of second conductors  516   a  are provided outside element body  518  and have a rectangular cross-section, a method for manufacturing the coil device has a high degree of freedom. For example, the plurality of second conductors  516   a  of the helical coil  516  may be formed on the main surface  512   a  of the circuit substrate  512  instead of being formed on the element body  518  of the coil device  514 . 
     In this case, the circuit substrate  512  has the plurality of second conductors  516   a  arranged in parallel (in the X-axis direction) on the main surface  512   a.  On the other hand, the element body  518  has the plurality of first conductors  516   b,  a plurality of third conductors  516   c,  and a plurality of fourth conductors. That is, the first conductor  516   b,  the third conductor  516   c,  and the fourth conductor constitute a semi-ring-shaped conductor having an opening on a side of the mount surface  518   a , and a plurality of semi-ring-shaped conductors are arranged in parallel along the parallel direction of the first conductors  516   a  (X-axis direction). 
     The element body  518  is mounted on the circuit substrate  512  such that the plurality of third conductors  516   c  and the plurality of fourth conductors on the element body  518  are connected to the plurality of first conductors  516   a  on the circuit substrate  512 . Thus, a coil device  514  having the helical coil  516  is constituted. That is, the element body  518  having the plurality of semi-ring-shaped conductors serves as a surface mount type component which is mounted on the circuit substrate  512  as part of the coil device  514 . Thus, a wireless communication device  510  is composed of the element body  518  and the circuit substrate  512 . In addition, the plurality of second conductors  516   a  on the circuit substrate  512  are connected to the plurality of third conductors  516   c  and the plurality of fourth conductors on the element body  518  through conductive bonding material such as solder. 
     According to embodiment  6 , it is possible to minimize the skin effect in the plurality of first conductors  516   b  due to their circular cross-section. Because the first conductors  516   b  are located remotely from the circuit subtract  512  and are the primary contributors to the magnetic field, a variation in magnetic field distribution can be suppressed. Furthermore, a method for manufacturing the coil device has a high degree of freedom. 
     In addition, the plurality of second conductors  516   a  may be formed on the side of the mount surface of the element body  518 . In this case, conductive paste may be patterned by screen printing, or a metal film may be entirely patterned by etching. 
     Embodiment 7 
     The coil device used in the wireless communication device of embodiment 7 is the same as the coil device of embodiment 1. Therefore, a detailed description for a constitution of the coil device is omitted. 
       FIG. 12  shows a wireless communication device  600  serving as a mobile terminal having the coil device  14  in the embodiment 1 used as an antenna, for example.  FIG. 13  is a cross-sectional view of the wireless communication device  600 . 
     As shown in  FIG. 12 , the wireless communication device  600  has a casing  602  which accommodates the coil device  14  and a circuit substrate  604 . As best shown in  FIG. 13 , battery  605  for driving the wireless communication device  600  and a component  606  for receiving and transmitting a signal having a frequency in the HF band through the coil device  14  are mounted on the circuit substrate  604  together with the coil device  14 . Furthermore, the casing  602  also accommodates a booster antenna (coil antenna)  608  having a resonant frequency in the HF band. 
     As shown in  FIG. 14 , the booster antenna  608  has a first coil pattern  610 , a second coil pattern  612 , and an insulating plate  614  which is interposed between them to support them. Each of the first coil pattern  610  and the second coil pattern  612  has a rectangular spiral shape and are formed on the insulating plate  614 , for example. Furthermore, openings of the first and second coil patterns  610  and  612  are larger in area than the opening of the helical coil in the coil device  14 . 
     The first and second coil patterns  610  and  612  are configured to be capacitively coupled with each other when a current flows in the same direction, or when a current flows clockwise at viewing in a direction (Z-axis direction) perpendicular to the insulating plate  614 , for example. Therefore, floating capacitance is formed between the first coil pattern  610  and the second coil pattern  612 . Thus, as shown in  FIG. 15 , a resonant circuit is composed of an inductance L 1  of the first coil pattern  610 , an inductance L 2  of the second coil pattern  612 , and floating capacitances C 1  and C 2  between terminals of the first and second coil patterns  610  and  612 . The booster antenna  608  is configured such that a resonance frequency of the resonant circuit matches with the frequency in the HF band of the signal received by and transmitted from the coil device  14  such as 13.56 MHz. 
     The booster antenna  608  is accommodated in the casing  602  in such a manner that it does not overlap the battery  605 , and the first and second coil patterns  610  and  612  are partially located in the magnetic field (broken line) generated from the coil device  14 . Thus, magnetic coupling is generated between the coil device  14  (the helical coil in it) and the booster antenna  608 , and a current flows in a circuit of the booster antenna  608 . Since the openings of the first and second coil patterns  610  and  612  of the booster antenna  608  are larger in area than the opening of the helical coil in the coil device  14 , a large magnetic field is formed compared with a case where the coil device  14  is only provided. As a result, a communicatable distance of the wireless communication device  600  can be increased. 
     Embodiment 8 
     Embodiment 8 is an improved embodiment of embodiment 5. Therefore, embodiment 8 will be described with a focus on the point of difference from embodiment 5. 
     As shown in  FIGS. 9 and 10 , the coil device  414  of embodiment 5 includes a pair of second conductors  416   a ′ composed of the metal pin having a circular cross-section which has been cut along the planar surface including a center axis of the metal pin, and its rectangular cut surface is used as the connection terminal  420 . However, since the second conductor  416 ′ is the metal pin having the circular cross-sectional surface, its cut surface (the terminal surface of the connection terminal  420 ) can vary in size. That is, when the metal pin is not cut along the center axis of the metal pin, the cut surface varies in size depending on a distance from the center axis. When the cut surface (the terminal surface of the connection terminal  420 ) varies in size, the impedance of the connection terminal  420  varies, and as a result, communication characteristics of the wireless communication device also varies. 
     To avoid this problem, in the coil device  714  in embodiment  8 , as shown in  FIGS. 16 and 17 , a cuboid-shaped (rectangular cross-section) metal block  756  is provided in a resin block  754  and is cut in half to make two second conductors  716   a ′. Thus, a cut surface, that is, a connection terminal  720  is formed. Thus, when the cuboid-shaped metal block  756  is cut, the cut surface is constant in size even when the location of the cut varies. Therefore, the terminal surface of the connection terminal  720  does not vary in size and has a constant size. As a result, a variation in communication characteristics of a wireless communication device can be suppressed. 
     Embodiment 9 
     In the above embodiments, in embodiment 1 for example, and as shown in  FIG. 1 , the coil device  14  is mounted on the circuit substrate  12  which is larger than the coil device  14 . Thus, the wireless communication device  10  is relatively large in size. 
     In embodiment 9, a coil device is mounted on a circuit substrate having the same or smaller size. In other words, the circuit substrate is mounted on the coil device, and a wireless communication device is relatively small in size. 
       FIG. 18  shows a wireless communication device  810  in accordance with embodiment 9. The wireless communication device  810  is a RFID (Radio Frequency Identification) tag. As shown in  FIG. 18 , the wireless communication device  810  has the coil device  414  of embodiment 5 and a circuit substrate  830  mounted on it. The circuit substrate  830  has a flexible substrate  832  made of thermoplastic resin, an RFIC (Radio Frequency Integrated Circuit) element  834  mounted on a main surface  832   a  of the substrate  832 , and two capacitor elements  836  and  838  also mounted on the main surface  832   a  of the substrate  832 . As shown in  FIG. 19 , a RFID circuit is composed of the RFIC element  834 , the capacitor element  836 , the capacitor element  838 , and the helical coil  416  of the helical coil  414 . 
     Furthermore, as shown in  FIG. 18 , a back surface  832   b  of the substrate  832  is bonded to the mount surface  418   a  of the coil device  414  (the back surface of the element body  418 ). At this time, the connection terminal  420  on the mount surface  418   a  of the coil device  414  is electrically connected to a connection terminal  832   c  on the back surface  832   b  of the substrate  832 . As shown in  FIG. 19 , the connection terminal  832   c  is connected to the RFIC element  834 . Furthermore, instead of the capacitor elements  836  and  838 , a capacitor pattern may be provided on the substrate  832 . 
     Embodiment 10 
     A wireless communication device in embodiment 10 is a RFID tag similar to embodiment 9. Therefore, it will be described with a focus on a point different from embodiment 9. 
     As shown in  FIG. 20 , a wireless communication device  910  of embodiment 10 has the coil device  414  of embodiment 5 and a circuit substrate  930  mounted on it. The circuit substrate  930  has a flexible substrate  932  made of thermoplastic resin, an RFIC (Radio Frequency Integrated Circuit) element  934  incorporated in the substrate  932  and two capacitor elements  936  and  938  also incorporated in the substrate  932 . As shown in  FIG. 21 , an RFID circuit is composed of the RFIC element  934 , the capacitor element  936 , the capacitor element  938 , and the helical coil  416  of the coil device  414 . 
     Furthermore, the circuit substrate  930  has a plurality of connection terminals  932   d  to  932   g  to electrically connect the RFIC element  934  with an external control circuit or power supply circuit. The plurality of connection terminals  932   d  to  932   g  are provided on the main surface  932   a  of the substrate  932 . 
     A back surface  932   b  of the substrate  932  is bonded to the mount surface  418   a  of the coil device  414  (the back surface of the element body  418 ). At this time, the connection terminal  420  on the mount surface  418   a  of the coil device  414  is electrically connected to a connection terminal  932   c  on the back surface  932   b  of the substrate  932 . As shown in  FIG. 21 , the connection terminal  932   c  is connected to the RFIC element  934 . 
     Furthermore, instead of the capacitor elements  936  and  938 , a capacitor pattern may be provided on the substrate  932 . 
     Embodiment 11 
     In the above embodiments, the coil device is used as the antenna in the wireless communication device. In the embodiment 11, the electronic device has a coil device which is used for a purpose other than the antenna. 
       FIG. 22  shows a DC-DC converter module serving as one example of the electronic device in the embodiment 11 of the present invention. As shown in  FIG. 22 , a DC-DC converter module  1010  has the coil device  414  in the embodiment 5 and a circuit substrate  1030  mounted on it. 
     As shown in  FIG. 22 , the circuit substrate  1030  has a flexible substrate  1032  made of thermoplastic resin, a switching IC element  1034  incorporated in the substrate  1032 , and two capacitor elements  1036  and  1038  also incorporated in the substrate  1032 . As shown in  FIG. 23 , a DC-DC converter circuit is composed of the switching IC element  1034 , the capacitor element  1036 , the capacitor element  1038 , and the helical coil  416  of the coil device  414 . The helical coil  416  functions as a choke coil. 
     In addition, the circuit substrate  1030  has a plurality of connection terminals  1032   d  to  1032   j  to ground the switching IC element  1034 , or electrically connect it with an external control circuit or power supply circuit. The plurality of connection terminals  1032   d  to  1032   j  are provided on the main surface  1032   a  of the substrate  1032 . 
     Furthermore, as shown in  FIG. 22 , a back surface  1032   b  of the substrate  1032  is bonded to the mount surface  418   a  of the coil device  414  (the back surface of the element body  418 ). At this time, the connection terminal  420  on the mount surface  418   a  of the coil device  414  is electrically connected to a connection terminal  1032   c  on the back surface  1032   b  of the substrate  1032 . As shown in  FIG. 23 , the connection terminal  1032   c  is connected to the switching IC element  1034 . 
     Furthermore, instead of the capacitor elements  1036  and  1038 , a capacitor pattern may be provided on the substrate  1032 . 
     Embodiment 12 
     In the above plurality of embodiments, the back surface of the coil device is mounted on the circuit substrate. That is, the coil device is mounted on the circuit substrate through the relatively large surface (compared with the end surface) of the plate-like element body. In contrast, in embodiment 12, a coil device is mounted on (bonded to) a circuit substrate through its end surface. That is, the end surface of the plate-like element body is used as a mount surface of the coil device. More specifically, a wireless communication device  1110  in the embodiment 12 of the present invention shown in  FIG. 24  is an RFID tag having the same RFID circuit (refer to  FIG. 19 ) as in embodiment 9. 
     As shown in  FIG. 24 , the wireless communication device  1110  in the embodiment 12 of the present invention has a coil device  1114  having a helical coil and a circuit substrate  1130  mounted on it. The coil device  1114  has a connection terminal  1120  connected to the helical coil at each end, on its end surface  1118   b , instead of a back surface  1118   a  or a main surface  1118   d  of a plate-like element body  1118 . The circuit substrate  1130  has a flexible substrate  1132  made of thermoplastic resin, an RFIC element  1134  mounted on a main surface  1132   a  of the substrate  1132 , and two capacitor elements  1136  and  1138  also mounted on the main surface  1132   a  of the substrate  1132 . 
     A back surface  1132   b  of the substrate  1132  is bonded to the mount surface  1118   b  of the coil device  1114  (end surface  1118   b  of the element body  1118 ). At this time, the connection terminal  1120  on the mount surface  1118   b  of the coil device  1114  is electrically connected to a connection terminal  1132   c  on the back surface  1132   b  of the substrate  1132 . The connection terminal  1132   c  is connected to the RFIC element  1134 . 
     Embodiment 13 
     A wireless communication device in embodiment 13 is a RFID tag including the same RFID circuit (refer to  FIG. 21 ) as in embodiment 10. However, a coil device is bonded to a circuit substrate through its end surface, similar to embodiment 12. 
     As shown in  FIG. 25 , a wireless communication device  1210  in the embodiment 13 has a coil device  1214  and a circuit substrate  1230  mounted on it. The coil device  1214  has a connection terminal  1220  connected to the helical coil at each end, on its mount surface  1218   b  (an end surface of an element body  1218 ). The circuit substrate  1230  has a flexible substrate  1232  made of thermoplastic resin, an RFIC element  1234  mounted on a main surface  1232   a  of the substrate  1232 , and two capacitor elements  1236  and  1238  also mounted on the main surface  1232   a  of the substrate  1232 . 
     A back surface  1232   b  of the substrate  1232  is bonded to the mount surface  1218   b  of the coil device  1214  (the end surface of the element body  1218 ). At this time, the connection terminal  1220  on the mount surface  1218   b  of the coil device  1214  is electrically connected to a connection terminal  1232   c  on the back surface  1232   b  of the substrate  1232 . The connection terminal  1232   c  is connected to the RFIC element  1234 . 
     In embodiment 13, a plurality of connection terminals  1232   d  to  1232   g  which electrically connect the RFIC element  1234  with an external control circuit or power supply circuit are provided on the back surface  1232   b  of the substrate  1232 . A plurality of conductors  1222  connected to the respective connection terminals  1232   d  to  1232   g  are provided in the coil device  1214 . 
     Each of the plurality of conductors  1222  is a conductor layer formed from the back surface  1218   a  and extending along the end surface  1218   b  of the element body  1218 . In addition, each of the plurality of conductors  1222  is a metal pin internally contained in the element body  1218  and exposed on an outside of the back surface  1218   a  and the end surface  1218   b  of the element body  1218 , for example. 
     Embodiment 13 is useful when the end surface  1218   b  of the coil device  1214  is very small, that is, when the main surface  1232   a  of the substrate  1232  having the mounted RFIC element  1234  has no space for the connection terminal to be externally connected. 
     The present invention has been described with the above-described plurality of embodiments, but the present invention is not limited to the embodiments. 
     For example, in the above plurality of embodiments, the helical coil of the coil device is composed of the first to fourth conductors. However, an embodiment of the present invention is not limited to this. More broadly, the coil device in the embodiment of the present invention has the plate-like element body having the main surface, the back surface, and the end surface, the helical coil provided in the element body and having the coil axis extending between the main surface and the back surface, and the connection terminal provided on the back surface or the end surface of the element body and connected to the helical coil, in which as for the conductor constituting the helical coil, the plurality of conductor portions extending along the main surface are composed of the plurality of metal pins, respectively. 
     Furthermore, in the above plurality of embodiments, as for the wireless communication device serving as one example of the electronic device, the coil device serving as the antenna is mounted at the end of the main surface of the circuit substrate. Instead of this, it may be mounted at another place such as center of the main surface of the circuit substrate. 
     Furthermore, the first to fourth conductors of the helical coil may be made of the same material or different material. For example, in order to prevent the increase in pure resistance of the second conductor which largely contributes to the magnetic field distribution, the metal pin of the second conductor may be made of material in which a skin depth is large. For example, when the metal pin of the second conductor is made of gold and the other conductor is made of copper, the pure resistance of the second conductor can be prevented from being increased and at the same time, the helical coil can be manufactured at low cost (compared with a case where all of the conductors of the helical coil are made of gold). 
     Still furthermore, as shown in  FIG. 2 , for example, when the first conductor and the second conductor are composed of the metal pin common to each other, the respective first and second conductors are preferably parallel to each other. The reason for this is that as shown in  FIG. 5A , the plurality of metal pins can be easily set when the coil device is manufactured. 
     As for the first and second conductors, in the above plurality of embodiments, as shown in  FIG. 3 , for example, the first conductors arranged in the extending direction of the coil axis (X-axis direction) at the position close to the mount surface (circuit substrate) are not disposed face-to-face with the second conductors arranged in the extending direction of the coil axis at the position distant from the mount surface (circuit substrate), in the direction (Z-axis direction) perpendicular to the circuit substrate. Instead of this, the helical coil may be configured such that the first conductor is disposed face-to-face with the second conductor in the direction perpendicular to the circuit substrate. 
     Furthermore, the coil device serving as the antenna in the above embodiments of the present invention is not limited to be used to transmit and receive the signal having a frequency in the HF band, it can be used to transmit and receive a signal having a frequency in various bands. The coil device serving as the antenna in the above embodiments of the present invention may be used to transmit and receive a signal having a frequency in a UHF band, for example. 
     Finally, a new embodiment can be provided by combining at least one of the characteristics in any of the above embodiments, in the other embodiment. For example, when the magnetic body  330  of the coil device  314  in the embodiment 4 is disposed in the coil device  14  in the embodiment 1, a new embodiment can be provided. In addition, when the coil device  114  in the embodiment 2 is applied to the wireless communication device  600  in the embodiment 7, a new embodiment can be provided. 
     The coil device in the present invention is applicable not only to the wireless communication device and the DC-DC converter module, but also to other devices using a coil.