Abstract:
Disclosed herein is a coil component that includes: a drum core including a first flange portion, a second flange portion and a winding core portion positioned between the first and second flange portions; a plurality of coated conductive wires forming a first winding layer wound around the winding core portion and a second winding layer wound around the winding core portion with an intervention of the first winding layer; and a resin coating layer covering the coated conductive wires. A maximum space between the coated conductive wires in the first winding layer is narrower than a diameter of the coated conductive wires.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Field of the Invention 
         [0002]    The present invention relates to a coil component and a manufacturing method of the coil component, and particularly to a coil component that uses a drum core and a manufacturing method thereof. 
         [0003]    Description of Related Art 
         [0004]    In recent years, electronic components that are used in information terminal devices such as smartphones have been strongly required to be smaller in size and lower in height. Therefore, as for coil components such as pulse transformers, surface-mount coil components that use drum cores instead of toroidal cores have been frequently used. For example, Japanese Patent Application Laid-Open No. 2012-119568 discloses a step-up transformer of a surface-mount type that uses a drum core. 
         [0005]    The coil components that use drum cores have been required to be even smaller in size and lower in height. The size of a winding core portion has been decreasing from year to year. In order to secure a required inductance, a coated conductive wire that is thinner in diameter needs to be used. 
         [0006]    However, the coated conductive wire that is thin in diameter is low in dielectric strength voltage. Accordingly, coil components that need to insulate primary and secondary windings, such as pulse transformers, may be insufficient in dielectric strength voltage. In particular, if wires are connected to terminal electrodes by thermo-compression bonding or laser bonding, heat that is applied at the time of wire connection is conveyed via core material of the coated conductive wire, and the coating film would be degraded. Therefore, the problem is that the component is likely to be insufficient in dielectric strength voltage. 
         [0007]    The inventors hereof have studied the use of conductive wires coated with resin having a low melting point in order to provide a coil component having a high dielectric breakdown voltage. This is because scars and cracks, if any, in the resin coating, can be filled with the resin when the coated conductive wire is heated. This prevents a decrease in the dielectric breakdown voltage of the coated conductive wire. 
         [0008]    Our study reveals, however, that if the resin film is too thick, it applies a high stress on the conductive wire as it is cooled. Consequently, the conductor filaments are greatly displaced. 
         [0009]    The moving of coated conductive wires used does not influence the basic property (e.g., inductance) of the coil component. However, the study of the inventors hereof shows that if the coated conductive wires move much, the dielectric breakdown voltage of the coated conductive wire will more decreases a little than otherwise. This is probably because the distance between any adjacent coated conductive wires becomes short at some positions as the coated conductive wires move, intensifying the electric field between the primary winding and the secondary winding. 
       SUMMARY 
       [0010]    It is therefore an object of this invention to provide a coil component having conductive wires coated with resin, and a method of fabricating this coil component, particularly to provide a coil component having high dielectric breakdown voltage and a method of fabricating the same. 
         [0011]    A coil component according to one aspect of the present invention includes: a drum core including a first flange portion, a second flange portion and a winding core portion positioned between the first and second flange portions; a plurality of coated conductive wires forming a first winding layer wound around the winding core portion and a second winding layer wound around the winding core portion with an intervention of the first winding layer; and a resin coating layer covering the coated conductive wires. A maximum space between the coated conductive wires in the first winding layer is narrower than a diameter of the coated conductive wires. 
         [0012]    The study of the inventors hereof shows that the breakdown voltage of a coil component using wires coated with a resin film decreases if the pace made in the first winding layer is narrower than the diameter of the coated conductive wires. In the coil component according to this aspect of the present invention, the coated conductive wires are inhibited from moving, and the maximum space between the coated conductive wires in the first winding layer is therefore less than the diameter of the coated conductive wires. Hence, the coil component can acquire a high breakdown voltage. 
         [0013]    It is preferable that a maximum space between the coated conductive wires in the second winding layer is narrower than the diameter of the coated conductive wires. According to this feature, the coated conductive wires are strongly inhibited from moving, and the coil component can acquire a high breakdown voltage. 
         [0014]    It is preferable that each of the first and second flange portions has a plurality of connection portions, and each end of the coated conductive wires is connected to an associated one of the connection portions. In this case, it is more preferable that the coated conductive wires include a primary and secondary windings insulated from each other. This is because most coil components of this type must have a high breakdown voltage. 
         [0015]    It is preferable that each of the connection portions is substantially free from the resin coating layer. If the resin coating layer does not cover the connecting parts, it will not cause insufficient electrical connection or inadequate solder wettability. 
         [0016]    In a method of manufacturing a coil component according to another aspect of the present invention, the method includes: winding a plurality of coated conductive wires around a winding core portion of a drum core to form a first winding layer wound around the winding core portion and a second winding layer wound around the winding core portion with an intervention of the first winding layer, each of the coated conductive wires including a core member, a coating film covering the core member, and a resin film covering the coating film; connecting both ends of the coated conductive wires to connection portions provided on the first and second flange portions of the drum core; and melting the resin film to form a resin coating layer covering the coated conductive wires. A maximum space between the coated conductive wires in the first winding layer is narrower than a diameter of the coated conductive wires. 
         [0017]    According this aspect of the present invention, the resin coating layer is formed as the resin film is melted. Scars, if any, are removed from the resin film, enhancing the dielectric breakdown voltage of the coil component. Moreover, the number of steps does not increase because any resin need not be applied after the coated conductive wires have been wound. Further, the coil component can acquire a high breakdown voltage because the coated conductive wires are inhibited from moving as the resin coating layer shrinks. 
         [0018]    According to the present invention, the connecting is preferably carried out by thermo-compression bonding or laser bonding. The reason is that, if the wire is connected by thermo-compression bonding or laser bonding, the dielectric strength voltage tends to become insufficient due to the heat applied at the time of the wire connection. 
         [0019]    In this case, the coated conductive wires preferably include a first coated conductive wire that is located in the first winding layer in the winding core portion and a second coated conductive wire that is located in a second or subsequent winding layer in the winding core portion, and the connecting includes a step of connecting the first coated conductive wire to the wire connection portion and then the second coated conductive wire to the wire connection portion. The reason is that, if the wire connection work is carried out multiple times on the same wire connection portions as described above, the effects of the heat become more significant. 
         [0020]    The method of producing the coil component of the present invention preferably further includes bonding a plate core to the first and second flange portions, wherein the resin film melts due to heat applied at the bonding step. According to this method, the step of bonding the plate core and the step of melting the resin film can be performed at the same time. 
         [0021]    Thus, the present invention can provide a coil component having wires coated with resin, and a method of producing this coil component, particularly a coil component having high dielectric breakdown voltage and a method of fabricating the same. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The above features and advantages of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which: 
           [0023]      FIG. 1  is a schematic perspective view showing the appearance structure of a coil component according to a first embodiment of the present invention; 
           [0024]      FIG. 2  shows an equivalent circuit of the coil component shown in  FIG. 1 ; 
           [0025]      FIG. 3  is a cross-sectional view taken along line A-A′ shown in  FIG. 1 ; 
           [0026]      FIG. 4  is an enlarged cross-sectional view of a part of first and second winding layers; 
           [0027]      FIG. 5  is an enlarged cross-sectional view of a part of first and second winding layers; 
           [0028]      FIG. 6  is a cross-sectional view of a coated conductive wire; 
           [0029]      FIG. 7A  is a schematic plan view indicating a state where two coated conductive wires are wound around a winding core portion in a first layer; 
           [0030]      FIG. 7B  is a schematic plan view indicating a state where another two coated conductive wires are further wound around the winding core portion in a second layer; 
           [0031]      FIG. 8  is a schematic plan view showing the configuration of a coil component according to a second embodiment of the present invention; 
           [0032]      FIG. 9  is a cross-sectional view showing one example of an xz cross-section of a winding core portion of a drum core; 
           [0033]      FIG. 10  is a graph indicating measurement results of a maximum space; 
           [0034]      FIG. 11  is a graph indicating measurement results of a breakdown voltage; 
           [0035]      FIG. 12A  shows a cross section of the sample A; and 
           [0036]      FIG. 12B  shows a cross section of the sample B. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0037]    Preferred embodiments of the present invention will be explained below in detail with reference to the accompanying drawings. 
         [0038]      FIG. 1  is a schematic perspective view showing the appearance structure of a coil component  10  according to the first embodiment of the present invention. 
         [0039]    The coil component  10  of the present embodiment is a pulse transformer of a surface-mount type. As shown in  FIG. 1 , the coil component  10  includes a drum core  11 , a plate core  12  that is bonded to the drum core  11 , and coated conductive wires S 1  to S 4  that are wound around a winding core portion  11   a  of the drum core  11 . The coil component of the present invention is not limited to the pulse transformer. The coil component of the present invention may be any other transformer component such as a balun transformer or step-up transformer, or may be a filter component such as a common mode choke coil. 
         [0040]    The drum core  11  and the plate core  12  are made of a magnetic material that is relatively high in magnetic permeability such as a sintered composite of Ni—Zn ferrite or Mn—Zn ferrite, for example. Incidentally, the magnetic material that is high in magnetic permeability such as Mn—Zn ferrite is usually low in specific resistance and electrically conductive. 
         [0041]    The drum core  11  includes the rod-shaped winding core portion  11   a , and first and second flange portions  11   b  and  11   c  that are provided at both ends in y-direction of the winding core portion  11   a . The winding core portion  11   a  and flange portions  11   b  and  11   c  are integrally formed. The coil component  10  is a component that is mounted on a surface of a printed circuit board at the time of actual use. The coil component  10  is mounted in such a way that z-direction upper surfaces  11   bs  and  11   cs  of the flange portions  11   b  and  11   c  face the printed circuit board. To the opposite sides, or lower surfaces, of the flange portions  11   b  and  11   c  from the upper surfaces  11   bs  and  11   cs , the plate core  12  is bonded with an adhesive. According to this structure, a closed magnetic circuit is formed by the drum core  11  and the plate core  12 . 
         [0042]    On the upper surface  11   bs  of the first flange portion  11   b , three wire connection portions E 1  to E 3  that serve as terminal electrodes are provided. On the upper surface  11   cs  of the second flange portion  11   c , three wire connection portions E 4  to E 6  that serve as terminal electrodes are provided. The wire connection portions E 1  to E 6  include L-shaped terminal metal fittings that are attached to the corresponding flange portions  11   b  and  11   c . However, the terminal metal fittings are not necessarily required to be used. The wire connection portions E 1  to E 6  may be formed by conductor film that is burned into the surfaces of the corresponding flange portions  11   b  and  11   c . The wire connection portions E 1  to E 3  are arranged in this order from one end side in x-direction as shown in  FIG. 1 . Similarly, the wire connection portions E 4  to E 6  are arranged in this order from one end side in x-direction. Ends of the coated conductive wires S 1  to S 4  are connected to the wire connection portions E 1  to E 6  by thermo-compression bonding or laser bonding. 
         [0043]    As shown in  FIG. 1 , the distance between the wire connection portions E 2  and E 3  is designed in such a way as to be greater than the distance between the wire connection portions E 1  and E 2 . Similarly, the distance between the wire connection portions E 4  and E 5  is designed in such a way as to be greater than the distance between the wire connection portions E 5  and E 6 . This configuration is intended to improve the withstand voltage between a primary winding that is formed by the coated conductive wires S 1  and S 2  and a secondary winding that is formed by the coated conductive wires S 3  and S 4 . 
         [0044]    The coated conductive wires S 1  to S 4  include a core material (metal core) that is made of a good conductor, and an insulating coating film that covers the core material. The coated conductive wires S 1  to S 4  are wound around the winding core portion  11   a  in a double-layered structure. While the details will be described later, the coated conductive wires S 1  and S 4  are wound around the winding core portion  11   a  in a bifilar winding pattern in order to form a first winding layer, and the coated conductive wires S 2  and S 3  are wound around the winding core portion  11   a  in a bifilar winding pattern in order to form a second winding layer. The numbers of turns of the coated conductive wires S 1  to S 4  may be equal. 
         [0045]    The winding direction of the coated conductive wires S 1  to S 4  is different between the first and second winding layers. When the winding direction from the first flange portion  11   b  to the second flange portion  11   c  is seen from the flange portion  11   b &#39;s side, the winding direction of the coated conductive wires S 1  and S 4  is counterclockwise, and the winding direction of the coated conductive wires S 2  and S 3  is clockwise. In this manner, the winding direction of the coated conductive wires S 1  and S 4  is opposite to the winding direction of the coated conductive wires S 2  and S 3 . 
         [0046]    One end S 1   a  and the other end S 1   b  of the coated conductive wire S 1  are connected to the wire connection portions E 1  and E 4 , respectively. One end S 4   a  and the other end S 4   b  of the coated conductive wire S 4  are connected to the wire connection portions E 3  and E 6 , respectively. One end S 2   a  and the other end S 2   b  of the coated conductive wire S 2  are connected to the wire connection portions E 4  and E 2 , respectively. One end S 1   a  and the other end S 1   b  of the coated conductive wire S 3  are connected to the wire connection portions E 5  and E 3 , respectively. 
         [0047]      FIG. 2  shows an equivalent circuit of the coil component  10  according to the present embodiment. 
         [0048]    As shown in  FIG. 2 , the wire connection portions E 1  and E 2  are used as balanced-input positive terminal IN+ and negative terminal IN−, respectively. The wire connection portions E 5  and E 6  are used as balanced-output positive terminal OUT+ and negative terminal OUT−, respectively. The wire connection portions E 3  and E 4  are used as output-side center tap CT and input-side center tap CT, respectively. The coated conductive wires S 1  and S 2  constitute the primary winding of the pulse transfer. The coated conductive wires S 3  and S 4  constitute the secondary winding of the pulse transfer. 
         [0049]      FIG. 3  is a cross-sectional view taken along line A-A′ shown in  FIG. 1 . 
         [0050]    As shown in  FIG. 3 , the coated conductive wires S 1  and S 4  are wound as the first winding layer on the winding core portion  11   a  of the drum core  11 . The coated conductive wires S 2  and S 3  are wound as the second winding layer on the first winding layer. That is, the coated conductive wires S 1  to S 4  that are wound around the winding core portion  11   a  have a double-layered structure. At least the surfaces of the coated conductive wires S 1  and S 4  that are located in the first winding layer are covered with a resin coating layer  20 . The resin coating layer  20  is made of an insulating resin material that is low in melting point, such as polyester, for example. The resin coating layer  20  preferably cover the coated conductive wires S 2  and S 3  that are located in the second winding layer. According to the present embodiment, particularly the upper surfaces of the coated conductive wires S 2  and S 3  that are located in the second winding layer may be partially covered due to a production method described later. 
         [0051]      FIG. 4  is an enlarged cross-sectional view of a part of first and second winding layers. 
         [0052]    As shown in  FIG. 4 , the coated conductive wires S 1  to S 4  have the structure in which the core material (metal core)  31  is covered with a coating film (insulating film)  32 . The resin coating layer  20  is provided in such a way as to cover the coating film  32  of the coated conductive wires S 1  to S 4 . As for the coated conductive wires S 1  and S 4  that are located in the first winding layer, almost no area of the coating film  32  is exposed, and almost the entire area is covered with the resin coating layer  20 . As for the coated conductive wires S 2  and S 3  that are located in the second winding layer, it is preferable that almost no area of the coating film  32  is exposed, and almost the entire area may be covered with the resin coating layer  20 . 
         [0053]    In that manner, in the coil component  10  of the present embodiment, the coated conductive wires S 1  to S 4  are covered with the resin coating layer  20 . Therefore, defective portions of the coating film  32 , such scratches and cracks, can be filled with the resin coating layer  20 . Accordingly, it is possible to prevent a decline in dielectric strength voltage associated with the defective portions, and to secure a high dielectric strength voltage. 
         [0054]    It is preferable that the resin coating layer  20  exists only on the winding core portion  11   a  of the drum core  11 . In other words, it is preferable that no resin coating layer  20  exists on the flange portions  11   b  and  11   c . This means that no resin coating layer  20  may exist between the flange portions  11   b  and  11   c  and the plate core  12 , and that the wire connection portions E 1  to E 6  may be not covered with the resin coating layer  20 . 
         [0055]    As shown in  FIG. 4 , the coated conductive wires S 1  and S 4  are alternately arranged in the first winding layer, and the coated conductive wires S 2  and S 3  are alternately arranged in the second winding layer. As the resin coating layer  20  molten due to the thermal load applied during the production and mounting is cooled, a stress is applied to the coated conductive wires S 1  to S 4 . The coated conductive wires S 1  to S 4  aligned one with another may therefore move in part as shown in  FIG. 5 . Consequently, the space between the adjacent coated conductive wires S 1  and S 4  changes, and so does the space between the adjacent coated conductive wires S 2  and S 3 . 
         [0056]    In the coil component  10  according to this embodiment, however, the coated conductive wires S 1  to S 4  are inhibited from moving. Therefore, the maximum space W 1  between the coated conductive wires S 1  and S 4  is less than the diameter φ of the coated conductive wires S 1  to S 4 . In other words, no spaces equal to or larger than the diameter φ exist in the first winding layer. It is desired that the maximum space W 2  between the coated conductive wires S 2  and S 3  should also be less than the diameter φ of the coated conductive wires S 1  to S 4 . It is also desired that the maximum space W 1  is less than the maximum space W 2 . 
         [0057]    In the instance of  FIG. 5 , a relatively large space W 2  exists between the adjacent coated conductive wires S 2  and S 3 , and a void V lies between these wires. The void V may reach the first winding layer. Even in this case, the maximum space W 1  in the first winding layer should preferably less than the diameter φ of the coated conductive wires. 
         [0058]    The reason why the maximum space W 1  in the first winding layer should less than the diameter φ of the coated conductive wires is as follows. 
         [0059]    As will be described later in detail, the resin coating layer  20  is made of a resin film (molten layer) applied to the surfaces of the coated conductive wires S 1  to S 4 . The amount of resin used can be adjusted in accordance with the thickness of the resin coating layer  20 . If the resin film is too thick, however, the coated conductive wires S 1  to S 4  aligned well by virtue of the stress contracting the resin coating layer  20  will move much as the molten resin is cooled and solidified. The coated conductive wires S 1  to S 4  are therefore no longer be aligned with one another. As a result, the adjacent coated conductive wires S 1  and S 4  or the adjacent coated conductive wires S 2  and S 3  may contact to each other at a specific part, inevitably decreasing the breakdown voltage. In addition, the resin coating layer  20 , which is excessively thick, intensifies the electric field between any two adjacent coated conductive wires, further decreasing the breakdown voltage. 
         [0060]    The breakdown voltage is lowed very much if the maximum space W 1  between the coated conductive wires S 1  and S 4  constituting the first winding layer increases to a value equal to or greater than diameter φ of the coated conductive wires S 1  and S 4 . That is, if the space W 1 , which has an initial value of less than diameter φ, increases to a value equal to or greater than diameter φ, the breakdown voltage will decrease. This is a sign of decreasing a breakdown voltage. It is therefore necessary to reduce the thickness of the resin film to such a value as would not decrease the breakdown voltage. 
         [0061]    A manufacturing method of the coil component  10  according to the present embodiment will be described. 
         [0062]    As shown in  FIG. 6 , the coated conductive wires S 1  to S 4  of a three-layer structure that includes the core material  31 , the coating film  32 , and a resin film  33  are prepared. The core material  31  is made of a good conductor such as copper (Cu), and the surface thereof is covered with the coating film  32 . The coating film  32  is made of insulating material such as imide-modified polyurethane, and the surface thereof is covered with the thin resin film  33 . The resin film  33  is made of insulating resin material such as polyester. The material of the resin film  33  is selected in such a way as to have a melting point that is sufficiently lower than that of the coating film  32 . In one example, the melting point of imide-modified polyurethane is about 260 degrees Celsius, while the melting point of polyester is about 70 degrees Celsius. A thickness of the resin film  33  is designed to be sufficiently thin as long as defective portions of the coating film  32  can be properly repaired. 
         [0063]    As shown in  FIG. 7A , the coated conductive wires S 1  and S 4  are wound around the winding core portion  11   a  in a bifilar winding pattern, and both ends of each of the coated conductive wires S 1  and S 4  are connected to the corresponding wire connection portions E 1 , E 3 , E 4 , and E 6  in order to form the first winding layer. More specifically, one ends S 1   a  and S 4   a  of the coated conductive wires S 1  and S 4  are connected by thermo-compression bonding or laser bonding to the wire connection portions E 1  and E 3 , respectively. Then, the drum core  11  is rotated in one direction in order to wound the coated conductive wires S 1  and S 4  around the winding core portion  11   a . After the rotation of the drum core  11  is stopped, the other ends S 1   b  and S 4   b  of the coated conductive wires S 1  and S 4  are connected by thermo-compression bonding or laser bonding to the wire connection portions E 4  and E 6 , respectively. During this process, the heat generated by the thermo-compression bonding or laser bonding is conveyed via the core material  31 . Accordingly, in portions close to the ends, the coating film  32  of the coated conductive wires S 1  and S 4  might be degraded, and defective portions, such as scratches or cracks, could emerge. Furthermore, due to mechanical stress that occurs at the time of winding, the coating film  32  could become defective. Moreover, when the thermo-compression bonding or laser bonding is carried out, the resin film  33  that exists at the one ends S 1   a  and S 4   a  of the coated conductive wires S 1  and S 4  and at the other ends S 1   b  and S 4   b  would change in quality due to the heat. According to the present invention, the resin that has changed in quality due to the heat at the time of wire connection is not part of the resin coating layer  20 . 
         [0064]    Immediately after the coated conductive wires S 1  and S 4  are wound, they should better be aligned, closely positioned to each other with the resin coating layer  20  interposed between them, though it is not absolutely necessary to do so. According to this structure, a maximum density in the first winding layer can be obtained, and the maximum numbers of turns can be obtained. Nonetheless, it is not absolutely required that all turns of the coated conductive wire S 1  contact all turns of the coated conductive wire S 4 , respectively, immediately after the coated conductive wires S 1  and S 4  are wound. Some turns of the coated conductive wire S 1  may be spaced apart from the adjacent coated conductive wire S 4 . Even in this case, the maximum space W 1  between the coated conductive wires S 1  and S 4  must be less than the diameter φ of the coated conductive wires. If the maximum space W 1  is equal to or larger than the diameter φ, the coated conductive wires S 2  and S 3  forming the second winding layer cannot be properly formed. 
         [0065]    Then, as shown in  FIG. 7B , the coated conductive wires S 2  and S 3  are wound around the winding core portion  11   a  in a bifilar winding pattern, and both ends of each of the coated conductive wires S 2  and S 3  are connected to the corresponding wire connection portions E 2 , E 3 , E 4 , and E 5  in order to form the second winding layer. More specifically, the other ends S 2   b  and S 1   b  of the coated conductive wires S 2  and S 3  are connected by thermo-compression bonding or laser bonding to the wire connection portions E 2  and E 3 , respectively. Then, the drum core  11  is rotated in the opposite direction in order to wound the coated conductive wires S 2  and S 3  around the winding core portion  11   a . After the rotation of the drum core  11  is stopped, one ends S 2   a  and S 1   a  of the coated conductive wires S 2  and S 3  are connected by thermo-compression bonding or laser bonding to the wire connection portions E 4  and E 5 , respectively. 
         [0066]    If the coated conductive wires S 1  and S 4  forming the first winding layer contact each other or if the maximum space is less than diameter φ immediately after the coated conductive wires S 1  and S 4  are wound, the coated conductive wires S 2  and S 3  can be correctly wound on the first winding layer so that they may constitute the second winding layer. Conversely, if a space equal to or larger than the diameter φ exists between the coated conductive wires S 1  and S 4  immediately after the coated conductive wires S 1  and S 4  are wound, the coated conductive wire S 2  or S 3  falls into this space. In this case, the second winding layer cannot be correctly formed. This is why the maximum space W 1  between the coated conductive wires S 1  and S 4  is less than the diameter φ of the coated conductive wires immediately the coated conductive wires S 1  and S 4  are wound. 
         [0067]    When the coated conductive wire S 2  is connected, at one end, to the connecting part E 2 , and at the other end, to the connecting part E 3 , and the coated conductive wire S 3  is connected, at one end, to the connecting part E 4 , and at the other end, to the connecting part E 5 , those parts of the resin film  33  existing at the ends of the coated conductive wires S 2  and S 3 , respectively, are affected by the heat applied to them. Further, those parts of the coat film  32 , which are close to the ends of the coated conductive wires S 1  to S 4 , are deteriorated because heat is conveyed to the coated conductive wires S 1  to S 4  via the core member  31  during the thermo-compression bonding or laser bonding. 
         [0068]    The coated conductive wires S 1  and S 4  suffer thermal damage twice, from the heat generated by the thermo-compression bonding or laser bonding during the formation of the first winding layer and from the heat generated by the thermo-compression bonding or laser bonding during the formation of the second winding layer. Therefore, the coating film.  32  is likely to degrade. That is, the coated conductive wires S 1  and S 4  that constitute the first winding layer suffers greater damage than the coated conductive wires S 2  and S 3  that constitutes the second winding layer. Therefore, defective portions such as scratches or cracks are more likely to emerge in the coating film  32  of the coated conductive wires S 1  and S 4 . 
         [0069]    After the work to wind the coated conductive wires S 1  to S 4  is completed, the plate core  12  is bonded to the drum core  11 . More specifically, a small amount of adhesive is applied to the flange portions  11   b  and  11   c  of the drum core  11 . Then, the plate core  12  is placed on the flange portions  11   b  and  11   c  of the drum core  11 . Then, thermal treatment is carried out to solidify the adhesive, and the plate core  12  is firmly fixed to the drum core  11  as a result. This thermal treatment is carried out at 150 degrees Celsius for about one hour, for example. 
         [0070]    The resin film  33  that exists on the surfaces of the coated conductive wires S 1  to S 4  melts during the thermal treatment, and is infiltrated into gaps between the coated conductive wires S 1  to S 4 . If defective portions F such as scratches or cracks exist on the coating film  32 , the defective portions are filled with the resin coating layer  20  which is the melted resin film  33 . The resin coating layer  20  which is the melted resin film  33  gathers around the coated conductive wires S 1  and S 4  located in the first winding layer because of capillarity. Therefore, at least almost the entire area of the first layer is covered with the resin coating layer  20 . On the other hand, mainly the upper surface of the second winding layer may not be covered with the resin coating layer  20 , and the coating film  32  is sometimes being exposed. Incidentally, the resin film  33  that exists in the wire connection portions E 1  to E 6  has already changed in quality due to the heat at the time of wire connection. The resin film  33  therefore does not melt during the thermal treatment. 
         [0071]    When the heating is terminated, the resin coating layer  20  molten is cooled and solidifies. The stress generated as the resin coating layer  20  solidifies moves the coated conductive wires S 1  to S 4  move out of mutual alignment, generating a space between the any adjacent coated conductive wires. In this embodiment, however, the resin film.  33  formed on the wires S 1  to S 4  is thin, and the resin coating layer  20  is not excessively thick. Hence, the maximum space W 1  in at least the first winding layer can be reduced to less than diameter φ of the coated conductive wires. In other words, in the first winding layer, the maximum space W 1  which is less than the diameter φ immediately after winding the coated conductive wires S 1  and S 4  remains less than the diameter φ, never increasing over diameter φ. Preferably, the maximum space W 2  remains less than the diameter φ, never increasing over diameter φ, also in the second winding layer. 
         [0072]    As seen from the above, a phenomenon that the maximum space W 1  in the first winding layer increases to or over the diameter φ is a sign that the breakdown voltage at both the primary winding and the secondary winding will decrease. In view of this, in order not to appear the sign, the resin film  33  is thin enough to prevent the breakdown voltage from decreasing in the primary winding or the secondary winding. 
         [0073]    Through the steps described above, the coil component  10  of the present embodiment is completed. 
         [0074]    As described above, according to the present embodiment, the coated conductive wires S 1  to S 4  whose surface is covered with the resin film  33  are used. Then, thermal treatment is carried out so that the resin film  33  melts. In this manner, the resin coating layer  20  is formed. As a result, at least the surfaces of the coated conductive wires S 1  and S 4  that are located in the first layer are automatically covered with the resin coating layer  20 . As described above, the coated conductive wires S 1  and S 4  that are located in the first winding layer suffer thermal damage twice, and defective portions are likely to emerge in the coating film  32 . However, according to the present embodiment, the surfaces of the coated conductive wires S 1  and S 4  that are located in the first winding layer are automatically covered with the resin coating layer  20 . Therefore, it is possible to ensure that defective portions that emerge in the coating film  32  in the first winding layer are filled with the resin coating layer  20 . Even if defective portions emerge in the coating film  32 , it is possible to secure a sufficient dielectric strength voltage. 
         [0075]    Another possible method is to coat with the resin material after the coated conductive wires S 1  to S 4  are wound around the winding core portion  11   a  in order to improve the dielectric strength voltage. However, if the viscosity of the resin material is high, the coated conductive wires S 1  to S 4  cannot be sufficiently coated. If the viscosity of the resin material is low, the resin material can get into the flange portions  11   b  and  11   c  of the drum core  11  because of capillarity. Particularly in the case of a coil component that is low in height with a small difference in height between the winding core portion  11   a  and the flange portions  11   b  and  11   c , the inflow of the resin material inevitably occurs due to capillarity. 
         [0076]    If the resin material flows to the lower surfaces of the flange portions  11   b  and  11   c , the flow of the resin material creates a gap between the flange portions  11   b  and  11   c  and the plate core  12 , resulting in a decrease in magnetic properties. If the resin material flows to the upper surfaces  11   bs  and  11   cs  of the flange portions  11   b  and  11   c , the wire connection portions E 1  to E 6  that are terminal electrodes may be partially covered with the resin material, leading to a decrease in solder wettability at the time of implementation. 
         [0077]    According to the present embodiment, the coated conductive wires S 1  to S 4  that are wound are not coated later with the resin material. The winding work is performed with the use of the coated conductive wires S 1  to S 4  on the surfaces of which the resin film  33  is provided in advance. After that, the resin film  33  is melted to form the resin coating layer  20 , thereby eliminating the risk that the resin material could flow into the flange portions  11   b  and  11   c . Furthermore, it is possible to ensure that the resin coating layer  20  covers the first winding layer constituted of the coated conductive wires S 1  and S 4  in which defective portions are more likely to occur. 
         [0078]    As has been described, the coated conductive wires S 1  and S 4  are covered with the resin coating layer  20  in the coil component  10  according to this embodiment. Hence, the coil component can have sufficient dielectric breakdown voltage even if the coated conductive wires used have a small diameter. Further, neither the magnetic property nor the solder wettability are degraded, because the resin coating layer  20  never reach the flange parts  11   b  and  11   c.    
         [0079]    Moreover, the resin coating layer  20  would not become excessively thick in this embodiment. This is because the resin film  33  the coated conductive wires S 1  to S 4  have a thin resin film  33 . Therefore, the sign of decreasing the breakdown voltage does not appear. 
         [0080]      FIG. 8  is a schematic plan view showing the configuration of a coil component  13  according to the second embodiment of the present invention, showing the configuration of a bottom surface side. 
         [0081]    As shown in  FIG. 8 , the coil component  13  of the second embodiment is characterized in that the number of wire connection portions provided in each of the flange portions  11   b  and  11   c  is not 3 but 4. In the flange portion  11   b , four wire connection portions E 1 , E 2 , E 3   a , and E 3   b  are provided. In the flange portion  11   c , four wire connection portions E 4   a , E 4   b , E 5 , and E 6  are provided. An electrical connection between the other end S 1   b  of the coated conductive wire S 1  and one end S 2   a  of the coated conductive wire S 2  is achieved by a wiring pattern or land pattern on a printed circuit board at a time when the coil component  13  is mounted. Similarly, an electrical connection between the other end S 1   b  of the coated conductive wire S 3  and one end S 4   a  of the coated conductive wire S 4  is achieved by a wiring pattern or land pattern on a printed circuit board at a time when the coil component  13  is mounted. The rest of the configuration is the same as that of the coil component  10  of the first embodiment. Therefore, the same components will be represented by the same reference symbols, and will not be described again. 
         [0082]    In that manner, in the coil component  13  of the present embodiment, the two wire connection portions E 3   a  and E 3   b  are short-circuited on the printed circuit board. Furthermore, the two wire connection portions E 4   a  and E 4   b  are short-circuited on the printed circuit board. Accordingly, it is possible to realize the same structure as that of the coil component  10  of the first embodiment. Thus, it is possible to achieve the same operation and advantageous effects as the first embodiment. 
         [0083]      FIG. 9  is a cross-sectional view showing one example of an xz cross-section of a winding core portion  11   a  of a drum core  11 . 
         [0084]    In the example shown in  FIG. 9 , an upper surface  14  and lower surface  15  of the winding core portion  11   a  are arc-shaped. If the winding core portion  11   a  that has such an arc-shaped cross-section is used, the melted resin film  33  is infiltrated into the corners of the winding core portion  11   a  more easily than when a winding core portion  11   a  that is rectangular in cross-section is used. As a result, it is possible to ensure that the resin coating layer  20  covers the coated conductive wires S 1  and S 4  that are located at the corners of the winding core portion  11   a . If the winding core portion  11   a  is elliptical or circular in cross-section, there are no corners. Therefore, it is possible to ensure that the resin coating layer  20  covers the coated conductive wires S 1  and S 4 . 
         [0085]    It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention. 
         [0086]    In the embodiments described above, the coated conductive wires are wound around the winding core, forming two winding layers. The coil component according to this invention is not limited to this configuration, nevertheless. The coated conductive wires may be wound around the winding core to form three or more winding layers. 
         [0087]    Further, the method of winding the coated conductive wires is not limited to a particular one. Both the wires on the first winding layer and the wires on the second winding layer may be wound by a bifilar winding such as the embodiments described above. Alternatively, the coated conductive wires may be wound, one by one. 
       Examples 
       [0088]    A drum core  11  was prepared, 4.5 mm long in the x direction, 3.2 mm wide in the y direction and 2.9 mm high in the z direction. Further, coated conductive wires S 1  to S 4  were prepared, each comprising a core member  31  having a diameter of 40 μm, a coat film  32  having thickness of 10 μm and a resin film  33  having thickness of 1 μm or 3.5 μm. The coated conductive wires S 1  to S 4  were wound around the drum core  11 , by using the method described with reference to  FIG. 7 . However, connecting parts E 1  to E 6  were not formed, making the coated conductive wires S 1  to S 4  open at both ends. Thus, samples A and B of the coil component were produced. The sample A has coated conductive wires S 1  to S 4 , each comprising a resin film  33  having thickness of 1 μm. The sample B has coated conductive wires S 1  to S 4 , each comprising a resin film  33  having thickness of 3.5 μm. 
         [0089]    Next, a thermal load was applied to the resin film  33 , melding the resin film  33 . Then, the resin film  33  was cooled, thereby forming a resin coating layer  20 . The thermal load was applied twice, first in such a way as in the adhering the plate-shaped core, and then in such away as in the re-flowing to mount the coil component. Then, the maximum space W 1  in the first winding layer was measured. The measuring results were as shown in  FIG. 10 . 
         [0090]    As seen from  FIG. 10 , the maximum space W 1  in the first winding layer was 20 μm to 56 μm in the sample A having coated conductive wires S 1  to S 4 , each having a resin film  33  having thickness of 1 μm. In the sample B having coated conductive wires S 1  to S 4 , each having a resin film  33  having thickness of 3.5 μm, the maximum space W 1  in the first winding layer was 61 μm to 107 μm. Thus, the maximum space W 1  in the first winding layer did not exceed the diameter φ (i.e., 60 μm) of the coated conductive wires in the sample A even after the coated conductive wires S 1  to S 4  have moved due to the thermal load, but exceeded the diameter φ (i.e., 60 μm) of the coated conductive wires in the sample A after the coated conductive wires S 1  to S 4  have moved due to the thermal load. 
         [0091]    Next, in both samples A and B, the end S 1   a  of the coated conductive wire S 1  and the end S 2   b  of the coated conductive wire S 2  were short-circuited to each other and were connected to one test terminal (+) of a tester, and the end S 1   b  of the coated conductive wire S 3  and the end S 4   a  of the coated conductive wire S 4  were short-circuited to each other and were connected to the other test terminal (−) of a tester. Then, a 50-Hz AC voltage of was applied between the test terminals for 60 seconds, and the samples A and B were examined for dielectric breakdown. The voltage was set to initial value of 1.5 kV. If the sample was not dielectrically broken down, the voltage was raised by 0.1 kV and applied to the sample again. The voltage at which the sample reaches the dielectric breakdown was plotted. The result was as sown in  FIG. 11 . 
         [0092]    As seen from  FIG. 11 , the sample A underwent dielectric breakdown when applied with voltage of 4.7 kV to 5.0 kV, and the sample B underwent dielectric breakdown when applied with voltage of 4.0 kV to 4.7 kV. Thus, the sample A had a higher breakdown voltage than the sample B. 
         [0093]    Then, the samples A and B were cut, exposing their yz-faces, which were examined by using a scanning electron microscope (SEM).  FIG. 12A  shows a cross section of the sample A, and  FIG. 12B  shows a cross section of the sample B. 
         [0094]    As seen from  FIG. 12A , the coated conductive wires S 1  to S 4  moved but a little in the sample A, no large voids V were not made in the resin coating layer  20 . By contrast, as seen from  FIG. 12B , the coated conductive wires S 1  to S 4  greatly moved in the sample B, large voids V were made in the resin coating layer  20 , each void reaching the winding core part  11   a . The large voids V spaced the coated conductive wires S 1  and S 4  constituting the first winding layer, from each other, by a distance larger than the diameter φ of the coated conductive wires.