Patent Publication Number: US-11640869-B2

Title: Bonding structure of a sheet core and a pair of flange parts of a coil component

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/925,583, filed Mar. 19, 2018, which claims priority to Japanese Patent Application No. 2017-071219, filed, Mar. 31, 2017, the disclosure of which is incorporated herein by reference in its entirety including any and all particular combinations of the features disclosed therein. 
    
    
     The applicant herein explicitly rescinds and retracts any prior disclaimers or disavowals or any amendment/statement otherwise limiting claim scope made in any parent, child or related prosecution history with regard to any subject matter supported by the present application. 
     BACKGROUND 
     Field of the Invention 
     The present invention relates to a winding-type common mode choke coil used in various electronic devices, as well as a method for manufacturing such common mode choke coil, and a circuit board. 
     Description of the Related Art 
     There has been a demand, in recent years, that common mode choke coils mounted on the circuit boards of mobile devices such as onboard devices have high impact resistance to withstand the impact of dropping, etc. For example, a winding-type common mode choke coil, which is structurally constituted by a drum core and a sheet core, requires stronger bonding of the drum core and the sheet core in order to achieve high impact resistance. 
     Patent Literature 1 and Patent Literature 2 each disclose a common mode choke coil whose structure is characterized by a drum core and a sheet core bonded together using an adhesive. In Patent Literature 1, grooves are provided on the contact surface between the flange and the sheet core, and an epoxy resin, which serves as an adhesive, is filled in the grooves. These grooves and adhesive allow the cores to securely adhere together, and improve the bonding strength. In Patent Literature 2, a tapered part is provided on the top face of the flange part so that the space between the sheet core and the drum core can be filled with the minimum required amount of adhesive, thereby achieving high bonding strength with a small amount of adhesive. 
     BACKGROUND ART LITERATURES 
     
         
         [Patent Literature 1] Japanese Patent Laid-open No. 2009-224649 
         [Patent Literature 2] Japanese Patent Laid-open No. 2014-99587 
       
    
     SUMMARY 
     To achieve high impact resistance in a winding-type common mode choke coil structurally constituted by a drum core and a sheet core, the bonding strength between the bonding surfaces of the drum core and the sheet core must be increased more effectively. And, to effectively increase the bonding strength between the bonding surfaces of the drum core and the sheet core, it is considered that not only the shape of the bonding surface such as grooves and tapering, but also the properties of the bonding surface such as surface roughness and surface waviness, must be improved. 
     Here, examples of bonding surfaces on the drum core and the sheet core of a conventional common mode choke coil are shown in  FIGS.  10 A and  10 B . 
       FIG.  10 A  shows that, if both the top face  110   t  of the flange part constituting the drum core, and the bottom face  100   b  of the sheet core, have low surface roughness, then the amount of adhesive  120  between the bonding surfaces becomes small, which may cause the bonding strength to drop and impact resistance to decrease. Furthermore, decreasing the amount of adhesive for fear that extra adhesive may ooze out of the bonding surfaces, creates unbonded areas where there is insufficient adhesive, and this causes the bonding strength to drop. On the other hand,  FIG.  10 B  shows that, if both the top face  110   t  of the flange part and the bottom face  100   b  of the sheet core have high surface roughness, then it becomes difficult to achieve a constant thickness of the adhesive  120  between the bonding surfaces. For the reasons mentioned above, the reliability of mechanical strength in terms of impact resistance, and the electrical characteristics such as impedance, both of which are considered important in mobile devices such as onboard devices, may drop. 
     Accordingly, it is considered necessary to improve not only the shapes, but also the properties, of the bonding surfaces in order to effectively increase the bonding strength between the bonding surfaces of the drum core and the sheet core. It is clear that the object of Patent Literature 1 and Patent Literature 2, where there is no mention of the surface properties of the bonding surfaces of the sheet core and the drum core, is not to improve the bonding strength by means of the surface properties of both cores. Also, increasing the bonding strength alone is not enough, and the electrical characteristics must also be maintained/improved. 
     An object of the present invention is to provide a common mode choke coil capable of achieving both high bonding strength and excellent inductance characteristics, as well as a method for manufacturing such common mode choke coil. 
     Any discussion of problems and solutions involved in the related art has been included in this disclosure solely for the purposes of providing a context for the present invention, and should not be taken as an admission that any or all of the discussion were known at the time the invention was made. 
     In some embodiments, the common mode choke coil proposed by the present invention is characterized in that it comprises: a shaft part; a pair of flange parts formed on both ends of the shaft part to constitute a drum core together with the shaft part; a sheet core bonded, in a manner connecting the pair of flange parts across the shaft part, to the top faces of the flange parts facing away from the bottom faces of the flange parts to be mounted on a circuit board; a coil-shaped conductor constituted by sheathed conductive wires wound around the shaft part; and electrode terminals formed on the flange parts and electrically connected to the ends of the sheathed conductive wires; wherein the bonding surfaces of each of the flange parts and the sheet core have multiple contact areas (predetermined contact areas) where the flange part makes direct contact (no or substantially no materially intervening adhesive or other material therebetween) with the sheet core, as well as adhesive areas (predetermined adhesive areas) between the contact areas where an adhesive is disposed. 
     The method for manufacturing the common mode choke coil proposed by the present invention is characterized in that it includes: a step to form a drum core and a sheet core; a step to form electrode terminals on the drum core; a step to wind sheathed conductive wires around the drum core to form a coil-shaped conductor; a step to connect both ends of the sheathed conductive wires and the electrode terminals; and a step to apply an adhesive to the sheet core on a jig, install the drum core on the sheet core to which the adhesive has been applied, and stack the jig on which the drum core has been installed with other such jigs, and thereby apply pressure while the adhesive is cured. 
     According to the present invention, a common mode choke coil capable of achieving both high bonding strength and excellent inductance characteristics, as well as a method for manufacturing such common mode choke coil, can be provided. Additionally, a circuit board using such common mode choke coil can be provided. 
     For purposes of summarizing aspects of the invention and the advantages achieved over the related art, certain objects and advantages of the invention are described in this disclosure. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein. 
     Further aspects, features and advantages of this invention will become apparent from the detailed description which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of this invention will now be described with reference to the drawings of preferred embodiments which are intended to illustrate and not to limit the invention. The drawings are greatly simplified for illustrative purposes and are not necessarily to scale. 
         FIG.  1    is a perspective view of an example of a common mode choke coil pertaining to the first embodiment of the present invention. 
         FIG.  2    is a drawing explaining an example of external dimensions in the first embodiment. 
         FIG.  3 A  is a drawing explaining an example of external dimensions in the first embodiment. 
         FIG.  3 B  is an enlarged view of  FIG.  3 A , explaining examples of radius dimensions of the flange part and sheet core in the first embodiment. 
         FIG.  4    is a drawing showing an example of bonding surfaces pertaining to the first embodiment. 
         FIG.  5    is a drawing showing an example of a sheet core pertaining to a variation example of the first embodiment. 
         FIG.  6    is a perspective view showing an example of a common mode choke coil pertaining to a variation example of the first embodiment. 
         FIG.  7 A  is a drawing showing an example of bonding surfaces pertaining to the second embodiment. 
         FIG.  7 B  is an enlarged view of  FIG.  7 A , showing an example of bonding surfaces pertaining to the second embodiment. 
         FIG.  8    is a drawing explaining an example of a bonding method as part of the manufacturing method. 
         FIG.  9    is a drawing explaining an example of a pressure method as part of the manufacturing method. 
         FIG.  10 A  is a drawing showing an example of conventional bonding surfaces. 
         FIG.  10 B  is a drawing showing an example of conventional bonding surfaces. 
     
    
    
     DESCRIPTION OF THE SYMBOLS 
     
         
         
           
               10  Common mode choke coil 
               12  Shaft part 
               14  Flange part 
               14   b  Bottom face of flange part 
               14   t  Top face of flange part (including a bonding surface) 
               15  Contact area 
               15   a  First contact area 
               15   aa  First contact-area center point 
               15   ab  Contacting region 
               15   b  Second contact area 
               15   ba  Second contact-area center point 
               16  Drum core 
               18  Sheet core 
               18   b  Bottom face of sheet core (including a bonding surface) 
               20  Coil-shaped conductor 
               22   a ,  22   b  Electrode terminal 
               28  Sheathed conductive wire 
               28   a ,  28   b  Lead part 
               30  Adhesive 
               30   a  Adhesive area 
               30   b  First adhesive area 
           
         
       
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The common mode choke coils pertaining to embodiments of the present invention are explained below by referring to the drawings. It should be noted that these are only examples and the present invention is not limited to the embodiments illustrated. Characteristic portions of the invention may be emphasized in the drawings, so the scale of each part of the drawings may not necessarily be accurate. 
     Also, the drawings indicate the xyz coordinate system being a three-dimensional orthogonal coordinate system, as deemed appropriate. In the xyz coordinate system, the x-axis direction represents the circumferential direction of the coil-shaped conductor, running in parallel with the bottom faces of the flange parts to be mounted on a circuit board as shown in  FIG.  1   . The y-axis direction represents the axial direction of the coil-shaped conductor as shown in  FIG.  1   . The z-axis direction represents the direction orthogonal to the bottom faces of the flange parts as shown in  FIG.  1   , which is the direction orthogonal to both the x-axis direction and the y-axis direction. 
     First Embodiment 
     &lt;Overall Configuration&gt; 
       FIG.  1    is a perspective view of an example of a common mode choke coil  10  pertaining to the first embodiment of the present invention. The common mode choke coil  10  in this embodiment has: a drum core  16  constituted by a shaft part  12  and flange parts  14 ; a sheet core  18 ; a coil-shaped conductor  20 ; and electrode terminals  22   a . Each constitutional member is explained below in detail. 
     &lt;Drum Core  16  and Sheet Core  18 &gt; 
     As shown in  FIG.  1   , the drum core  16  comprises the shaft part  12  and the pair of flange parts  14  provided on both ends thereof. The sheet core  18  is provided in a manner connecting the pair of flange parts across the shaft part  12 . 
     The shaft part  12  is not limited to any particular shape so long as there is a region around which the below-mentioned sheathed conductive wires  28  can be wound, but preferably it has a cylindrical shape, prism shape, or other solid shape with a long axis. The flange parts  14  are provided on both ends of the shaft part  12 , respectively, and have a quadrangular, sheet-shaped structure of a specified thickness. Preferably the quadrangular shape is a rectangle with long sides and short sides. The flange parts  14  each have a bottom face  14   b  to be mounted on a circuit board (not illustrated), side faces joined to the bottom face  14   b , and a top face  14   t  facing the sheet core  18  and bonded to it by an adhesive  30 . 
     Preferably both long-axis ends of the shaft part  12  abut the centers of the quadrangular shapes constituting the flange parts  14 . It should be noted that the shaft part  12  and flange parts  14  may be constituted as one piece. 
     The sheet core  18  is not limited to any particular shape, but preferably it has a quadrangular, sheet-shaped structure of a specified thickness. Also, preferably the quadrangular shape is a rectangle with long sides and short sides. The sheet core  18  has a bottom face  18   b  facing the top faces  14   t  of the flange parts and bonded to them by an adhesive  30 . 
     The bonding surfaces on the top face  14   t  of the flange part and the bottom face  18   b  of the sheet core have multiple contact areas  15  which are provided on the bonding surface on the flange part side (top face  14   t  of the flange part) and where the bottom face  18   b  of the sheet core makes direct contact with the top face  14   t  of the flange part, as well as adhesive areas  30   a  which are provided on the bonding surface excluding the contact areas  15  and where an adhesive  30  is disposed; the details of these areas are described separately in detail below. 
     For the material of the drum core  16  and sheet core  18 , Ni—Zn—Cu ferrite, Mn—Zn—Cu ferrite, etc., may be used, for example. Choices are not limited to the foregoing, however, and any alloy-based magnetic material (such as Fe—Cr—Si alloy, Fe—Al—Si alloy, etc.) may also be used. The drum core  16  and sheet core  18  may each be formed by mixing any such magnetic material with a binder and then pressure-molding the mixture into a drum shape using dies, followed by sintering, etc. For the modes, manufacturing methods, and other aspects of the drum core  16  and sheet core  18  in this embodiment, any prior art or the below-mentioned example may be referenced as deemed appropriate. An example of a manufacturing method is also described below. 
       FIG.  2    presents a drawing explaining an example of external dimensions of the common mode choke coil  10 . The external dimensions of the sheet core  18  are not limited in any way, but preferably, as shown in  FIG.  2   , the external dimension L 1  of the sheet core  18  in the long direction (y-axis direction) of the sheet core  18  is greater than the external dimension L 2  of the drum core  16 . For example, the external dimension L 1  of the sheet core  18  in the long direction is greater than the external dimension L 2  of the drum core  16  by approx. 0.1 to 0.2 mm. 
     This constitution allows the sheet core  18  to absorb any displacement resulting from its bonding with the drum core  16 , and thus eliminates any negative effect attributable to the accuracy of the bonded positions, or in other words suppresses any change in the size of the bonding surface, of the sheet core  18  and the drum core  16 , which in turn makes the electrical characteristics of the common mode choke coil  10  stable. In addition, the foregoing constitution also minimizes any negative effect attributable to the forming accuracy of the drum core  16  and sheet core  18  by, for example, reducing any negative effect from burrs that generate when the drum core  16  and sheet core  18  are formed. This means that, even when burrs generated at the time of forming remain on the sheet core  18 , or even when the edges of the bottom face  18   b  of the sheet core are rounded, the areas of the bonding surfaces of the drum core  16  and the sheet core  18  can be made constant. It should be noted that, while  FIG.  2    explains the external dimensions L 1 , L 2  of the sheet core and drum core in the long direction of the sheet core, respectively, the external dimension of the sheet core  18  in the short direction (x-axis direction in  FIG.  1   ) of the sheet core  18  may be greater than the external dimension of the drum core  16  (not illustrated). This is because the same operations and effects achieved in the long direction, can also be achieved in the short direction, of the sheet core  18 . Furthermore, the external dimensions of the sheet core  18  in both the long direction and short direction may be greater than the external dimensions of the drum core  16  in the same directions. 
       FIG.  3 A  presents a drawing that explains an example of external dimensions of the common mode choke coil  10 .  FIG.  3 B  shows an enlarged view of the area in  FIG.  3 A  surrounded by the dashed-dotted line. 
     As shown in  FIG.  3 A , when the external dimension L 1  of the sheet core  18  in the long direction of the sheet core is roughly the same as the external dimension L 2  of the drum core  16 , preferably the corners at the end of the top face  14   t  of the flange part and the end of the bottom face  18   b  of the sheet core are both rounded, where the radius dimension R 1  of the rounded shape of the top face  14   t  of the flange part in the long direction of the sheet core is greater than the radius dimension R 2  of the rounded shape of the bottom face  18   b  of the sheet core. It should be noted that, in this Specification, the term “rounded shape” indicates a curved shape of a corner in a cross-sectional view. Also, the term “radius dimension” indicates the radius dimension of a curved line on a curved plane. More preferably the difference between the external dimensions of the sheet core  18  and drum core  16  is greater than the radius dimension R 1  of the rounded shape of the flange part. 
     This constitution allows the adhesive  30  bonding the top face  14   t  of the flange part and the bottom face  18   b  of the sheet core, to wet and spread over the rounded shapes in a manner preventing the adhesive  30  from oozing out, the result of which is that, even when the external dimension of the sheet core varies relative to the external dimension of the drum core due to a manufacturing error, etc., the sheet core  18  and the drum core  16  flange can still be maintained in a well-bonded state without an excessive amount of adhesive  30  remaining and generating magnetic gaps in between, or an excessive amount of adhesive  30  attaching to the sheathed conductive wires  28  and causing the stray capacitance between the conductive wires to vary. For the modes, manufacturing methods, and other aspects of the rounded shapes in this embodiment, any prior art or the below-mentioned example may be referenced as deemed appropriate. For example, the rounded shapes in this embodiment may be formed by grinding down the corners on the outer periphery of the core, or the like. Also, they can be formed by pre-shaping the corners into arc-shaped curved lines when the core is formed, for example. 
     &lt;Coil-Shaped Conductor  20 &gt; 
     The coil-shaped conductor  20  is provided on the outer periphery of the shaft part  12  and constituted by two sheathed conductive wires  28  wound in the same winding direction by the same number of turns. At the ends of the sheathed wires  28  are lead parts  28   a  that have been led out from the coil. For the method for winding the sheathed conductive wires  28 , any generally used winding method, such as bifilar winding or layer winding, may be selected as deemed appropriate. 
     Preferably the coil-shaped conductor  20  in this embodiment is such that the sheathed conductive wires  28  constituting the coil-shaped conductor  20  are separate from the bonding surfaces of the drum core  16  and the sheet core  18 . In other words, the adhesive  30  between the bonding surfaces should be positioned away from the sheathed conductive wires  28  by a sufficient distance to prevent contact. 
     This constitution prevents any negative effect the adhesive  30  may have on the sheathed conductive wires  28 . Here, “negative effect” means stressing of the sheathed conductive wires  28  due to volume shrinkage of the adhesive  30  when the adhesive is cured, or change in the stray capacitance between the sheathed conductive wires due to a chemical reaction of the components of the adhesive and sheathed conductive wire and due to the adhesive  30 , for example. 
     For the specific manufacturing method and other aspect of the coil-shaped conductor  20 , any prior art or the descriptions below may be referenced as deemed appropriate. An example of a manufacturing method is also described below. 
     &lt;Electrode Terminal  22   a&gt;   
     Two electrode terminals  22   a  are provided, one on each flange part  14 , and electrically connected to the lead parts  28   a  at the ends of the sheathed conductive wires  28 . In  FIG.  1   , the electrode terminals  22   a  are provided on the bottom faces  14   b  of the flange parts; however, the present invention is not limited to the foregoing, and they may also be provided on side faces of the flange parts  14  as described in the variation examples below. 
     For the specific shape, manufacturing method, and other aspects of the electrode terminals  22   a , any prior art or the description below may be referenced as deemed appropriate, and a typical manufactured method is based on plating. An example of a manufacturing method is also described below. 
     (Bonding Surfaces in First Embodiment) 
     The bonding surfaces of the drum core  16  and the sheet core  18  in this embodiment are explained below. As described above, the drum core  16  and the sheet core  18  are bonded together by the adhesive  30 , with the top faces  14   t  of the flange parts constituting the drum core facing the bottom face  18   b  of the sheet core. 
       FIG.  4    shows an example of bonding surfaces on the top face  14   t  of one flange part, and the bottom face  18   b  of the sheet core, of the common mode choke coil  10  in  FIG.  1    as viewed from the direction of arrow B. As shown in  FIG.  4   , the bonding surfaces on the top face  14   t  of the flange part and the bottom face  18   b  of the sheet core have multiple, or at least two, contact areas  15  which are provided on the bonding surface on the flange part side and where the top face  14   t  of the flange part makes direct contact with the bottom face  18   b  of the sheet core, as well as adhesive areas  30   a  which are provided on the bonding surface excluding the contact areas  15  and where an adhesive  30  is disposed. It should be noted that the bonding surface of the top face  14   t  of the other flange part and that of the bottom face  18   b  of the sheet core have the same structure (not illustrated) as the one shown in  FIG.  4   . 
     Each contact area  15  has a projecting shape that projects from the top face  14   t  of the flange part, as shown in  FIG.  4   ; however, the contact area is not limited to this shape and can have any other shape so long as the bottom face  18   b  of the sheet core is contacted. As for the adhesive area  30   a , the shape of the adhesive area  30   a  generates according to the projecting shape of the top face  14   t  of the flange part in the contact area  15 , and in this embodiment, the shape of the adhesive area  30   a  is not limited in any way. 
       FIG.  4    shows three contact areas  15 ; however, the number of contact areas  15  is not limited to three, so long as there are at least two such areas on the bonding surface of the flange part  14 . This constitution having multiple contact areas  15  improves/stabilizes the bonding strength compared to the conventional common mode choke coils shown in  FIGS.  10 A and  10 B  where the adhesive is applied over the entire surface. The common mode choke coil shown in  FIG.  10 A  has lower bonding strength because the adhesive  120  is applied by a smaller amount. Also, when the adhesive  120  is applied by an amount just sufficient to not generate excess, it may turn out that the adhesive  120  is insufficient and unbonded areas may be created as a result. With the common mode choke coil shown in  FIG.  10 B , controlling the amount of adhesive to an optimal level is difficult because the thickness of the adhesive area varies locally. For this reason, the amount of adhesive  120  tends to become excessive in some areas and insufficient in other areas. As a result, stable bonding strength is not achieved in  FIG.  10 A  or  FIG.  10 B . Furthermore, the constitution having multiple contact areas  15  reduces the negative effect caused by varying magnetic gaps due to the adhesive layer, compared to the conventional common mode choke coils shown in  FIGS.  10 A and  10 B  where the adhesive is applied over the entire surface. This means that the common mode choke coil  10  in this embodiment offers superior inductance characteristics. 
     The multiple contact areas  15  in this embodiment may be formed by pre-forming tapered or other projecting areas on the top faces  14   t  of the flange parts when the drum core  16  is formed; however, preferably they are formed by grinding the top faces  14   t  of the flange parts, etc. Here, in this Specification, the surface properties of the drum core  16  and sheet core  18  are expressed by surface roughness Ra (arithmetic average roughness) and surface waviness Wa (arithmetic average waviness). 
     Surface roughness Ra (arithmetic average roughness) and surface waviness Wa (arithmetic average waviness) are specified in JIS B 0601, respectively. In this Specification, surface roughness Ra is defined as a surface property associated with an amplitude value of less than 10 μm, while surface waviness Wa is defined as a surface property associated with an amplitude value of 10 μm or more, with a cutoff value of 200 μm. 
     Presence of contact areas  15  can be confirmed by grinding a cross-section and observing it with an optical microscope, etc., and distances can also be measured using any length measurement function as deemed appropriate. Contact areas  15  may be specified using a three-dimensional X-ray inspection machine, etc., or the ground cross-section may be determined using such machine in a supplementary manner. 
     Preferably the surface waviness Wa of the bonding surface of the flange part  14  is greater than the surface waviness Wa of the bonding surface of the sheet core  18 . This constitution allows the bonding surfaces, or specifically the top face  14   t  of the flange part and the bottom face  18   b  of the sheet core, to make direct contact with each other in a reliable manner via multiple contact areas  15 . This makes it possible to control the amount of adhesive  30  and thereby improve/stabilize the bonding strength by applying more adhesive in a stable manner, whereas, heretofore, the amount of adhesive  30  could not be increased in the interest of preventing excess adhesive  30 . In addition, preferably the bonding surface of the sheet core  18  does not have surface waviness Wa. Furthermore, preferably the surface waviness of the bonding surface of the flange part  14  is 25 μm or less. When the surface waviness of the bonding surface of the flange part  14  is 25 μm or less, effectively no magnetic gaps will form and the inductance characteristics will improve further. 
     According to the conventional structure where the bonding surfaces, or specifically the top face  14   t  of the flange part and the bottom face  18   b  of the sheet core, do not make direct contact with each other in a reliable manner via multiple contact areas  15 , controlling the thickness of adhesive is difficult because it cannot be determined by the shapes of members and is instead affected by the amount of adhesive, surface roughness, waviness and other surface irregularities that vary in each circumstance. If the thickness of adhesive exceeds 25 μm even in only some areas, it effectively serves as a magnetic gap and the inductance characteristics fluctuate as a result. On the other hand, this structure where the bonding surfaces, or specifically the top face  14   t  of the flange part and the bottom face  18   b  of the sheet core, make direct contact with each other in a reliable manner via multiple contact areas  15 , the thickness of adhesive can be controlled by applying proper pressure when the drum core  16  and sheet core  18  are bonded together, and thereby forming contact areas  15  in a reliable manner. This makes it possible to design a thickness of adhesive to become 25 μm or less so as to effectively create substantially no magnetic gaps, which in turn achieves good inductance characteristics. 
     Variation Examples of First Embodiment 
     (Variation Example Characterized by Grooves Provided in Sheet Core  18 ) 
     The common mode choke coil  10  pertaining to the first embodiment, as shown in  FIG.  1   , represents an example where the bonding surface of the sheet core  18  is roughly flat. However, the sheet core  18  is not limited to this structure and, for example, one or more grooves  40  may be provided in the bonding surface of the sheet core  18 , as shown in  FIG.  5   . The groove  40  may be a straight groove or circular groove. In other words, the grooves  40  need to provide only enough space to accommodate the excess adhesive  30   c  not involved in the bonding by the adhesive  30 , and the number of grooves and shape of grooves are not limited. Preferably the volume of the grooves  40  is the same as or greater than the volume of the adhesive that has cured on the bonding surface. Also, the pitch, dimensions, etc., of the grooves  40  need not be constant and, for example, the grooves  40  may be made shallower toward the edges of the sheet core  18 . 
     According to this variation example, the amount of adhesive involved in the bonding becomes always constant without being excessive or insufficient because, even when the adhesive  30  is applied by an excessive amount in the below-mentioned step to apply the adhesive  30 , the excess adhesive  30  will flow into the grooves  40  and the bonding strength will improve/stabilize as a result. Furthermore, any negative effect of the adhesive  30  attaching to the sheathed conductive wires  28  can be avoided. Here, “negative effect” means stressing of the sheathed conductive wires  28  due to volume shrinkage of the adhesive  30  when the adhesive is cured, or change in the stray capacitance between the sheathed conductive wires due to a chemical reaction of the components of the adhesive and sheathed conductive wire and due to the adhesive  30 , for example. As described above, preferably the volume of the grooves  40  is the same as or greater than the volume of the adhesive that has cured on the bonding surface. This constitution achieves the aforementioned effects in a more reliable manner. It should be noted that, while the illustrated example explains forming of grooves  40  in the bonding surface of the sheet core  18 , grooves  40  may also be formed in the bonding surface of the flange part  14 , or grooves  40  may even be formed in both the bonding surfaces of the sheet core  18  and the flange part  14 . 
     (Variation Example Characterized by Electrode Terminals  22   b  Provided in Side Grooves  32  of Flange Parts  14 ) 
     The common mode choke coil  10  pertaining to the first embodiment, as shown in  FIG.  1   , represents an example where the electrode terminals  22   a  are provided on the bottom faces  14   b  of the flange parts. However, the present invention is not limited to this structure and, for example, the electrode terminals  22   b  may be provided in side grooves  32 , which are grooves formed on the side faces of the flange parts  14 , as shown in  FIG.  6   . 
     The positions of the side grooves  32  are not limited in any way, and may be provided roughly at the centers of the flange parts in the z-axis direction, as shown in  FIG.  6   , for example. In this variation example, the electrode terminals  22   b  are positioned inside the side grooves  32  and connected to the lead parts  28   b  at the ends of the sheathed conductive wires  28 . It should be noted that, since the ends of the lead parts  28   b  are connected to the electrode terminals  22   b  by means of thermal bonding, etc.,  FIG.  6    shows connection parts  34  where the electrode terminals  22   b  are connected to the ends of the lead parts  28   b.    
     According to this variation example, contact between the lead parts  28   b  and the adhesive  30  can be prevented because the lead parts  28   b  are away from the adhesive  30  on the bonding surface. Also, the stray capacitance of the common mode choke coil  10  is suppressed and thermal stress is not applied to the lead parts  28   b . Furthermore, traveling of the adhesive  30  along the lead parts  28   b  can be prevented. 
     Also, the center position of the side groove  32  in the z-axis direction may be the same as the center height position of the flange part  14 . This way, the top side, and the bottom side, of the side face of the flange part  14 , except for the side groove  32 , have the same dimensions, and consequently mechanical strength can be achieved in these areas. Furthermore, the center position of the shaft part  12  in the z-axis direction can also be the same as the center height position of the flange part  14 . This way, any risk of damage to the coil-shaped conductor  20  due to handling after the sheathed conductive wires  28  have been wound, can be reduced. 
     Second Embodiment 
     Next, the common mode choke coil pertaining to the second embodiment of the present invention is explained.  FIG.  7 A  shows an example of bonding surfaces on the top face  14   t  of one flange part, and the bottom face  18   b  of the sheet core, of the common mode choke coil  10  pertaining to the second embodiment.  FIG.  7 B  is an enlarged view of the area in  FIG.  7 A  surrounded by the dashed-dotted line. It should be noted that the bonding surface of the top face  14   t  of the other flange part and that of the bottom face  18   b  of the sheet core have the same structure (not illustrated). 
     As is evident from  FIG.  7 A , the number and layout of the contact areas  15  between the bonding surfaces are different from the first embodiment shown in  FIG.  4   . The following explains primarily the differences from the first embodiment. With the common mode choke coil  10  pertaining to this embodiment, those constitutions identical to the corresponding constitutions of the common mode choke coil  10  pertaining to the first embodiment are denoted by the same symbols and are not explained. 
     The contact areas  15  pertaining to this embodiment include at least two contact areas  15 , or specifically a first contact area  15   a  and a second contact area  15   b . Between the first contact area  15   a  and the second contact area  15   b  is a first adhesive area  30   b . In this embodiment, the first contact area  15   a  has a contacting region  15   ab  of the flange part  14  and the sheet core  18 , as shown in  FIG.  7 B . The shape of the contacting region  15   ab  is not limited in any way, and it may be a roughly planar or arc-shaped curved surface, etc., for example. When the contacting region  15   ab  is an arc-shaped curved surface, this arc-shaped curved surface contacts linearly (makes line contact) with the bottom face  18   b  of the sheet core on the bonding surface. It should be noted that, similarly, the second contact area  15   b  also has a contacting region of the flange part  14  and the sheet core  18  (not illustrated). Furthermore, the first contact area  15   a  has a first contact-area center point  15   aa  representing the center of the contacting region  15   ab  in the x-axis direction. Similarly, the second contact area  15   b  has a second contact-area center point  15   ba  representing the center of the contacting region in the x-axis direction. The distance d 1 , in the x-axis direction, from the first contact-area center point  15   aa  to the center axis of flange part A-A′ representing the center axis of the top face  14   t  of the flange part, is at least 0.25 times the width of the x-axis direction side of the top face  14   t  of the flange part. Similarly, in the second contact area  15   b , the distance d 2 , in the x-axis direction, from the second contact-area center point  15   ba  to the center axis of flange part A-A′ is at least 0.25 times the width of the x-axis direction side of the top face  14   t  of the flange part. More preferably the ratio of the distance from the first contact-area center point  15   aa , and the distance from the second contact-area center point  15   ba , to the center axis of flange part A-A′ in between, is 0.7 or more but no more than 0.9. A greater distance between the two contact points improves the dynamic stability of the flange part  14  and the sheet core  18  owing to their position relationship when pressure is applied. 
     According to this embodiment, the bonding surfaces on the top face  14   t  of one flange part and the bottom face  18   b  of the sheet core contact each other via two or more areas, and their respective contact-area center points  15   aa ,  15   ba  have specified distances d 1 , d 2  between them over the rough center part of the bonding surfaces, and this constitution achieves stable bonding, and thus stable bonding strength, of the common mode choke coil  10  owing to its shape. Also, the effective adhesive area is larger and the bonding strength is greater over the entire bonding surfaces compared to a conventional common mode choke coil with adhesive applied over the entire surfaces. In other words, the common mode choke coil  10  in this embodiment has improved/stable bonding strength. Furthermore, the common mode choke coil  10  in this embodiment, because of its constitution to have the first adhesive area  30   b , offers superior inductance characteristics compared to a conventional common mode choke coil with adhesive  30  applied over the entire surfaces, as any negative effect of varying magnetic gaps due to the adhesive layer is reduced. 
     It should be noted that, while  FIGS.  7 A and  7 B  explained this embodiment with a focus on the x-axis direction of the bonding surfaces, this embodiment is not limited in scope to the x-axis direction and, for example, the bonding surfaces may also be constituted as above in the y-axis direction, to achieve the same effects (not illustrated). In other words, it suffices that the multiple contact areas  15  include at least a first contact area  15   a  and a second contact area  15   b  on the top face  14   t  of each flange part, that there is a specified distance from the center of the first contact area to the center of the second contact area in a cross-section which is orthogonal to the top face  14   t  of the flange part and passing through the center of the first contact area and the center of the second contact area, and that the center axis of the cross-section exists within this distance (not illustrated). It should be noted that the center axis of the cross-section represents the center axis of the cross-section in the direction extending from the center of the first contact area toward the center of the second contact area. The aforementioned effects are achieved by this constitution. 
     &lt;Manufacturing Method&gt; 
     The following explains an example of how the common mode choke coil  10  proposed by the present invention is manufactured. It should be noted, however, that the present invention is not limited to the example described below. 
     For the magnetic material of the drum core  16  and sheet core  18 , a Ni—Zn ferrite material is used, for example. The magnetic permeability (μ) of the magnetic material only needs to be between 400 and 1000. First, the Ni—Zn ferrite material is mixed with a binder and the mixture is compression-molded into a drum shape using molding dies. Here, preferably the fill ratio of magnetic material is different between the flange part  14  and the sheet core  18 . When the flange part  14  and sheet core  18  have rounded corners, as shown in  FIG.  3 B , adjusting the radius dimensions becomes easy if the fill ratio of magnetic material is different between the flange part  14  and the sheet core  18 . Furthermore, preferably the fill ratio of magnetic material of the sheet core  18  and that of the shaft part  12  are higher than the fill ratio of magnetic material of the flange part  14 . This way, the mechanical strength can be increased and the common mode choke coil  10  can be made smaller. 
     Next, as necessary, the surface of the molding is ground to the required surface roughness Ra or surface waviness Wa. The grinding method, abrasive agent, etc., are not limited in any way, and any prior art may be used as deemed appropriate. From the viewpoint of controlling the surface roughness Ra or surface waviness Wa, preferably the grinding is performed using an automatic grinding machine, etc., for example. It should be noted that a step to increase the surface roughness Ra or surface waviness Wa of the molding by means of grinding may also be implemented. It should also be noted that, because the molding often has molding burrs, the burrs are removed by means of barreling, etc., for example. Agitation using an abrasive agent, or sandblasting, may be used. Preferably the surface roughness Ra of the bonding surface of the sheet core  18  is lower than the surface roughness Ra of the bonding surface of the flange part  14 . Now, the drum core  16  has a more complex shape compared to the sheet core  18 , so it has more burrs and thus requires more barreling. For this reason, the side of the molding having a higher surface roughness Ra can be used for the drum core  16 , while the side having a lower surface roughness Ra can be used as the sheet core  18 , so that a common mode choke coil  10  that can be produced affordably and offers excellent electrical characteristics, can be obtained. 
     Thereafter, the molding is sintered at the required sintering temperature to obtain a magnetic body that constitutes a drum core  16  having a shaft part  12  and flange parts  14 . Similarly, a sheet core  18  is also molded into a sheet shape using dies, and sintered. 
     Thereafter, an Ag paste is roller-transferred onto specified areas of the flange parts  14  and then thermally treated, and plated with Ni and Sn, to form electrode terminals  22   a . For example, Ni plating and Sn plating are combined to form electrode terminals  22   a  with a thickness of approx. 10 μm. Then, sheathed conductive wires  28  are wound around the outer periphery of the shaft part  12 , to form a coil-shaped conductor  20 . For the sheathed conductive wires  28 , UEWs (polyurethane copper wires) of Ø50 μm may be used, for example. Here, preferably the surface roughness Ra of the shaft part  12  is lower than the surface roughness Ra of the flange part. This way, any negative effect of the surface irregularities of the shaft part  12  can be reduced and the sheathed conductive wires  28  can be wound in a stable state. Thereafter, the drum core  16  and sheet core  18  are bonded at their bonding surfaces, and the adhesive  30  is cured under pressure. The bonding method is explained below. 
       FIG.  8    is a drawing explaining an example of a bonding method as part of the manufacturing method. When the drum core  16  is bonded with the sheet core  18 , first the sheet core  18  is stored inside a jig  60  with the bottom face  18   b  of the sheet core facing up, after which an adhesive  30  is applied by a specified amount using a dispenser, etc., at specified positions on the bottom face  18   b  of the sheet core, as shown in  FIG.  8   . The inner diameter dimension of the jig  60  is effectively equal to the external dimension of the sheet core  18 , so the sheet core  18  is secured as the sheet core  18  is stored inside the jig  60 . The jig  60  is not limited to any particular shape, so long as it has an opening in which the sheet core  18  can be stored. The material of the jig  60  is not limited in any way, either. Thereafter, the top faces  14   t  of the flange parts are adhered to the specified positions on the bottom face  18   b  of the sheet core where the adhesive  30  has been applied. 
     For the adhesive  30 , an epoxy (specification having a grass transition temperature Tg of 125° C.) may be used, for example. The application amounts and application positions of the adhesive  30  are adjusted in such a way that, when the top faces  14   t  of the flange parts are adhered to the bottom face  18   b  of the sheet core, the compressed adhesive  30  will not ooze out of the edges of the top faces  14   t  of the flange parts and the adhesive  30  will reach the outer lines of the bottoms of the adhesive areas  30   a ,  30   b . Also, with a constitution where the bottom face  18   b  of the sheet core has grooves  40 , as is the case in the variation example shown in  FIG.  5   , the application amounts of adhesive  30  are adjusted to not exceed the volumes of the grooves  40 , and the application amounts and positions are adjusted so that the compressed adhesives  30   c  will reach the grooves  40 . The respective dimensions of the drum core  16 , sheet core  18 , and jig  60  are controlled, and by handling the drum core  16  and the sheet core  18  as they are bonded together and by also positioning the adhesive  30  entirely inside the jig  60 , the number of times the product is handled can be reduced compared to when any prior art is used. In addition, magnetic gaps that generate between the drum core  16  and the sheet core  18  can be minimized regardless of the sizes and weights of the drum core  16  and sheet core  18 . Particularly when a small, lightweight drum core  16  is used, its movement, and consequent shifting, can be prevented as the adhesive  30  is cured. Thereafter, the adhesive  30  is thermally cured while pressure is applied according to the below-mentioned pressurization method, to bond the drum core  16  and the sheet core  18 . The pressurization method is explained below. 
       FIG.  9    is a drawing explaining an example of a pressurization method as part of the manufacturing method. As shown in  FIG.  9   , multiple jigs  60 , each storing a product, are stacked and the adhesive  30  is thermally cured while pressure is applied with a thermal press. A curing temperature is selected according to the glass transition temperature Tg of the adhesive. Preferably this temperature is higher than the glass transition temperature Tg, but no higher than Tg+50° C. Since the adhesive is cured under pressure, the positions of the drum core  16  and sheet core  18 , as they are bonded, do not shift but remain stable in the vertical direction. A flexible sheet  70  is provided at the bottom of each jig  60 . The sheet  70  may be a synthetic rubber or silicone rubber sheet, for example, but other sheet may be used so long as it has the flexibility to apply pressure to the products almost uniformly without damaging the cores, etc., and its shape and material are not limited in any way. By providing a flexible sheet  70  at the bottom of each jig  60 , the required bonding pressure can be applied to each individual product inside the jigs  60  where multiple products are arranged, and the uncured adhesive  30  can be spread uniformly and thinly. The specific pressure required is between 0.1 MPa and 1 MPa in equivalent pressure relative to the area of contact. During manufacture, pressure can be applied to multiple products all at once using a heat press. Presence of the contact areas  15  eliminates the need to finely control the required pressure per product, compared to when a prior art without contact areas  15  is used. This is because the contact areas  15  prevent the bonding thickness from decreasing further. Furthermore, the adhesive  30  can be cured in a uniformly and thinly spread state, which reduces the negative effect of magnetic gaps which would otherwise generate due to the amount of adhesive  30  becoming uneven in some areas and thus the thickness of the adhesive layer becoming uneven; and consequently, a common mode choke coil  10  offering excellent inductance characteristics can be obtained. In addition, implementing the main curing inside the jig  60  prevents the product from moving while the adhesive  30  is cured. Furthermore, implementing the main curing inside the jig  60  makes a transfer step unnecessary, compared to when a prior art is used that requires a separate main curing step after a preliminary curing, and this ensures high productivity while also reducing damage the product would otherwise suffer due to transfer. 
     The common mode choke coil  10 , thus obtained, is mounted in an electronic component, etc., with the electrode terminals  22   a  soldered to a circuit board. 
     To give an example of external dimensions of the common mode choke coil  10  thus obtained, the product size is 3.2 mm in length, 2.5 mm in width, and 2.5 mm in height. Also, the dimensions of the drum core  16  are such that its external shape is 2.9 mm long, 2.5 mm wide, and 2.1 mm high. Its shaft part  12  is 1.1 mm wide and 0.8 mm high, while its flange parts  14  are each 0.3 mm thick. In addition, the sheet core  18  has an external shape of 3.2 mm in length, 2.5 mm in width, and 0.4 mm in height. Also, in the case of the variation example shown in  FIG.  6   , the side grooves are each 0.3 mm wide and 0.2 mm deep. 
     The foregoing explained several embodiments of the present invention; however, these embodiments were presented only as examples and they are not intended to limit the scope of the invention. Various changes can be added to these embodiments so long as doing so does not deviate from the key points of the present invention. For example, the shapes and external dimensions shown in the aforementioned embodiments are only examples, and may be changed as necessary and deemed appropriate. Also, the materials of the respective parts shown in the aforementioned embodiments are also examples, and various known materials may be used instead. Furthermore, the manufacturing procedure shown in the aforementioned example is also an example, and may be changed as deemed appropriate to the extent that the same effects can be achieved. Moreover, the common mode choke coil  10  proposed by the present invention is favorably used in mobile devices such as onboard devices that require impact resistance, or high-frequency components of such devices; however, it can also be applied to all other known applications. 
     In the present disclosure where conditions and/or structures are not specified, a skilled artisan in the art can readily provide such conditions and/or structures, in view of the present disclosure, as a matter of routine experimentation. Also, in the present disclosure including the examples described above, any ranges applied in some embodiments may include or exclude the lower and/or upper endpoints, and any values of variables indicated may refer to precise values or approximate values and include equivalents, and may refer to average, median, representative, majority, etc. in some embodiments. Further, in this disclosure, “a” may refer to a species or a genus including multiple species, and “the invention” or “the present invention” may refer to at least one of the embodiments or aspects explicitly, necessarily, or inherently disclosed herein. The terms “constituted by” and “having” refer independently to “typically or broadly comprising”, “comprising”, “consisting essentially of”, or “consisting of” in some embodiments. In this disclosure, any defined meanings do not necessarily exclude ordinary and customary meanings in some embodiments. 
     It will be understood by those of skill in the art that numerous and various modifications can be made without departing from the spirit of the present invention. Therefore, it should be clearly understood that the forms of the present invention are illustrative only and are not intended to limit the scope of the present invention.