Patent Publication Number: US-2023154663-A1

Title: Coil component

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation of U.S. patent application Ser. No. 16/712,759 filed on Dec. 12, 2019, which claims benefit of priority to Japanese Patent Application No. 2018-248775, filed Dec. 29, 2018, the entire content of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a wire-wound coil component having a structure in which a wire is wound around a substantially drum-shaped core, and more particularly to a structure of a connection portion in which the wire and a terminal electrode are connected to each other. 
     Background Art 
     For example, Japanese Unexamined Patent Application Publication No. 2006-286807 or Japanese Unexamined Patent Application Publication No. 2011-216681 describes a wire-wound coil component in which end portions of a wire are connected, by thermocompression bonding, to terminal electrodes that are provided on flange portions of a substantially drum-shaped core, the flange portions being positioned at the opposite ends of the substantially drum-shaped core. 
       FIG.  8    is an enlarged cross-sectional view of a portion of a flange portion  2  that is positioned at one end of a substantially drum-shaped core  1 . The flange portion  2  illustrated in  FIG.  8    has a mounting surface  3  that is directed to a mounting substrate at the time of mounting a coil component, and a terminal electrode  4  is provided on the mounting surface  3 . For example, the terminal electrode  4  includes a conductor film that serves as a base and that is formed by baking an electrically conductive paste containing silver as an electrically conductive component and a plating film that is made of Ni, Cu, Sn, or the like and that is formed on the conductor film. Note that the surface of the terminal electrode  4  is formed of a tin-plated film in order to achieve favorable solderability at the time of mounting the coil component. 
     In contrast, an end portion of a wire  6  that is wound around a winding core portion  5  of the substantially drum-shaped core  1  in a substantially helical manner is connected to the above-mentioned terminal electrode  4  by thermocompression bonding. For example, the wire  6  is formed of a copper wire and is coated with an insulating film made of a resin such as polyurethane or a polyimide. The insulating film made of the resin is decomposed by, for example, heat, which is applied when thermocompression bonding is performed, and removed. 
     SUMMARY 
     In order to sufficiently and properly achieve the thermocompression bonding of the wire  6  to the terminal electrode  4  as illustrated in  FIG.  8   , it is necessary to apply heat at a relatively high temperature, which is, for example, about 300° C. to about 500° C., and a relatively high pressure to the wire  6  so as to cause moderate plastic deformation of the wire  6 . 
     However, the inventor of the present disclosure has discovered that, as a result of applying pressure in the above-mentioned thermocompression bonding process, a stress is concentrated at a portion  8  of the wire  6  that is located in the vicinity of a ridge line where the mounting surface  3  of the flange portion  2  and an inner end surface  7  of the flange portion  2  intersect each other, so that the wire  6  may sometimes be easily broken at the portion  8 . More specifically, it was found that the above-mentioned breakage of the wire  6  occurs when, for example, the diameter of the wire  6  is reduced to about 15 μm to about 100 μm with the reduction in the size of the coil component or when the coil component is exposed to at a high temperature of about 120° C. to about 150° C. 
     Accordingly, the present disclosure provides a coil component having a structure capable of simultaneously achieving prevention of wire breakage and stable thermocompression bonding. 
     First and second embodiments of the present disclosure are each directed to a coil component that includes a substantially drum-shaped core including a winding core portion and a flange portion that is provided at an end portion of the winding core portion, a wire wound around the winding core portion, and a terminal electrode to which an end portion of the wire is connected. 
     The flange portion has an inner end surface that faces the winding core portion and that positions the end portion of the winding core portion, an outer end surface that is opposite to the inner end surface and that faces outward, and a mounting surface that connects the inner end surface and the outer end surface to each other and that is directed to a mounting substrate at the time of mounting the coil component. In addition, the terminal electrode is disposed on the mounting surface of the flange portion. 
     In the coil component according to the first embodiment, a rounded surface is formed on the mounting surface, the rounded surface having a central axis that linearly extends in a widthwise direction parallel to the mounting surface and the outer end surface and having a curvature radius that is longer than a distance between the inner end surface and the outer end surface, and the end portion of the wire extends along the rounded surface from a side on which the inner end surface is present toward a side on which the outer end surface is present. 
     According to the second embodiment, a recess and a flat surface, which is different from the recess, are formed in and on the mounting surface, the recess being open on a side on which the inner end surface is present. The end portion of the wire is received in the recess from the side on which the inner end surface is present toward a side on which the outer end surface is present and has a thickness that continuously changes such that the thickness is relatively thin on the side on which the outer end surface is present and is relatively thick on the side on which the inner end surface is present. 
     The coil component according to preferred embodiments of the present disclosure can simultaneously achieve prevention of wire breakage and stable thermocompression bonding. 
     Other features, elements, characteristics and advantages of the present disclosure will become more apparent from the following detailed description of embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view illustrating the appearance of a coil component according to a first embodiment of the present disclosure while a surface of the coil component that is to be directed to a mounting substrate faces upward; 
         FIG.  2    is a perspective view illustrating the appearance of a substantially drum-shaped core that is included in the coil component illustrated in  FIG.  1    while surfaces of the substantially drum-shaped core that are to be directed to the mounting substrate face upward; 
         FIG.  3    is an enlarged cross-sectional view of a portion of the substantially drum-shaped core illustrated in  FIG.  2   ; 
         FIG.  4    is for describing a thermocompression bonding process and is an enlarged cross-sectional view illustrating a portion of the substantially drum-shaped core and a portion of a wire that are included in the coil component illustrated in  FIG.  1   ; 
         FIG.  5    is an enlarged view of a portion of a substantially drum-shaped core and a portion of the wire that are included in a coil component according to a second embodiment of the present disclosure and corresponds to  FIG.  4   ; 
         FIG.  6    is an enlarged view of a portion of a substantially drum-shaped core included in a coil component according to a third embodiment of the present disclosure and corresponds to  FIG.  3   ; 
         FIG.  7    is an enlarged view of a portion of a substantially drum-shaped core included in a coil component according to a fourth embodiment of the present disclosure and corresponds to  FIG.  3   ; and 
         FIG.  8    is an enlarged cross-sectional view of a portion of a substantially drum-shaped core and a portion of a wire that are included in a coil component of the related art. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    illustrates the appearance of a coil component  11  according to a first embodiment of the present disclosure. In  FIG.  1   , a surface of the coil component  11  that is to be directed to a mounting substrate faces upward. 
     Referring to  FIG.  1   , the coil component  11  includes a substantially drum-shaped core  15  that includes a winding core portion  12 , a first flange portion  13 , and a second flange portion  14 . The first flange portion  13  and the second flange portion  14  are respectively provided on a first end portion and a second end portion of the winding core portion  12  that oppose each other. The substantially drum-shaped core  15  is made of, for example, alumina, ferrite, or the like. In addition, the substantially drum-shaped core  15  has a dimension of, for example, about 0.4 mm to about 4.5 mm in the longitudinal direction thereof. The substantially drum-shaped core  15  is solely illustrated in  FIG.  2   . 
     Referring to  FIG.  1    and  FIG.  2   , the first flange portion  13  has an inner end surface  17 , an outer end surface  19 , and a mounting surface  21 , and the second flange portion  14  has an inner end surface  18 , an outer end surface  20 , and a mounting surface  22 . The inner end surface  17  faces the winding core portion  12  and positions the first end portion of the winding core portion  12 , and the inner end surface  18  faces the winding core portion  12  and positions the second end portion of the winding core portion  12 . The outer end surface  19  is opposite to the inner end surface  17  and faces outward, and the outer end surface  20  is opposite to the inner end surface  18  and faces outward. The mounting surface  21  connects the inner end surface  17  and the outer end surface  19  to each other and is directed to the mounting substrate at the time of mounting the coil component  11 , and the mounting surface  22  connects the inner end surface  18  and the outer end surface  20  to each other and is directed to the mounting substrate at the time of mounting the coil component  11 . 
     The coil component  11  further includes a wire  23  that is wound around the winding core portion  12  of the substantially drum-shaped core  15 , a first terminal electrode  25 , and a second terminal electrode  26 . A first end portion and a second end portion of the wire  23  are respectively connected to the first terminal electrode  25  and the second terminal electrode  26 . The regions in which the terminal electrodes  25  and  26  are formed are illustrated by half-tone shading in  FIG.  1    and  FIG.  2   . The first terminal electrode  25  is formed so as to cover the entire mounting surface  21  of the first flange portion  13 , and the second terminal electrode  26  is formed so as to cover the entire mounting surface  22  of the second flange portion  14 . 
     For example, the terminal electrodes  25  and  26  are formed as follows. An electrically conductive paste in which a resin binder contains silver as an electrically conductive component and glass fit as an adhesive component is applied to the mounting surfaces  21  and  22  by a dip method and then baked, so that a conductor film that serves as a base is formed. Subsequently, the conductor film serving as a base is plated with Ni, Cu, Sn, or the like. As a result of performing the above-mentioned dip method, the terminal electrodes  25  and  26  are formed in such a manner as to extend from their respective mounting surfaces  21  and  22  to portions of surfaces adjacent to the mounting surfaces  21  and  22 . Note that it is preferable that a surface of each of the terminal electrodes  25  and  26  be formed of a tin-plated film in order to achieve favorable solderability at the time of mounting the coil component  11 . In addition, the plating film may be formed only around the portions of the terminal electrodes  25  and  26  to each of which the wire  23  is connected. 
     The above-mentioned wire  23  is formed of, for example, a core wire that is made of Cu and that has a diameter of about 15 μm to about 200 μm and has a structure in which the core wire is coated with an insulating film that is made of a resin such as polyurethane or a polyimide and that has a thickness of about a few μm. Thermocompression bonding is employed for connecting the end portions of the wire  23  to the terminal electrodes  25  and  26 . For example, the insulating film coating each of the end portions of the wire  23  is removed as a result of being decomposed by heat that is applied thereto when the thermocompression bonding is performed or by being irradiated with laser. 
     The mounting surface  21  of the first flange portion  13  and the mounting surface  22  of the second flange portion  14  are each provided with a characteristic shape such as that described later. Note that such a characteristic shape is not provided only by, for example, performing barrel polishing or the like and is ultimately obtained as a result of being provided during a process of forming the substantially drum-shaped core  15 , and such a characteristic shape is a configuration that clearly remains after post-processes, such as firing and barrel polishing, have been performed on the substantially drum-shaped core  15 . 
     The circumstances under which the inventor of the present disclosure has adopted characteristic configurations according to embodiments of the present disclosure will be described below. 
     Referring to  FIG.  8   , which has been described above, the inventor of the present disclosure presumes that copper that forms a central conductor of the wire  6 , which has undergone thermocompression bonding, generates an alloy between tin forming the surface of the terminal electrode  4  and tin contained in solder used at the time of mounting the coil component and becomes brittle under a severe temperature condition, and as a result, the wire  6  is easily broken at the portion  8 , which is located in the vicinity of the ridge line and where a stress is particularly likely to be concentrated. Note that the inventor of the present disclosure also found that breaking of the wire  6  as a result of such a series of events is more likely to occur as the wire  6  becomes thicker. 
     Although the substantially drum-shaped core  1  is made of, for example, alumina, ferrite, or the like, barrel polishing is performed on the substantially drum-shaped core  1  after the substantially drum-shaped core  1  has been formed and fired. In this barrel polishing, a ridge line portion between the mounting surface  3  and the inner end surface  7  of the flange portion  2  and a ridge line portion between the mounting surface  3  and an outer end surface  9  are round-chamfered. Note that the state in which the ridge line portions are round-chamfered is not illustrated in  FIG.  8   . It is easily conceivable that the degree of progress of the round chamfering is lower in the ridge line portion between the mounting surface  3  and the inner end surface  7  than in the ridge line portion between the mounting surface  3  and an outer end surface  9 . This is because the collision probability with a granular abrasive material is lower at the ridge line portion between the mounting surface  3  and the inner end surface  7  than at the ridge line portion between the mounting surface  3  and an outer end surface  9 . Thus, the ridge line portion between the mounting surface  3  and the inner end surface  7  remains sharper than the ridge line portion between the mounting surface  3  and an outer end surface  9 , and the radius of curvature of the ridge line portion between the mounting surface  3  and the inner end surface  7  decreases to be smaller than that of the ridge line portion between the mounting surface  3  and an outer end surface  9 . This is also presumed to be one of the factors that cause the wire  6  to easily break at the portion  8 , which is located in the vicinity of the ridge line. 
     In order to reduce the probability of occurrence of the above-mentioned breakage of the wire  6 , it may be considered to reduce the pressure that is applied to the wire  6  at the time of thermocompression bonding so as to reduce the degree of plastic deformation that occurs in the wire  6 . However, from the standpoint of the adhesion strength between the wire  6  and the terminal electrode  4 , it is essential to apply a sufficient pressure when the thermocompression bonding is performed while maintaining the temperature sufficiently high, and in order to sufficiently and properly achieve the thermocompression bonding, the pressure cannot be simply reduced. In any case, it is important to adjust the pressure applied at the time of performing the thermocompression bonding. On the other hand, adjustment of the pressure is extremely subtle, and it is undeniable that stable setting of an appropriate pressure is difficult to achieve. 
     Under the above-mentioned circumstances, it was found that it is preferable that the embodiments of the present disclosure have the following features. 
     The mounting surface  21  has a recess  27  that is open on the side on which the inner end surface  17  is present and a flat surface  29  that is formed in a region other than the recess  27 , and the mounting surface  22  has a recess  28  that is open on the side on which the inner end surface  18  is present and a flat surface  30  that is formed in a region other than the recess  28 . The recess  27  extends from the side on which the inner end surface  17  is present toward the side on which the outer end surface  19  is present, and the recess  28  extends from the side on which the inner end surface  18  is present toward the side on which the outer end surface  20  is present. As illustrated in detail in  FIG.  3   , which illustrates the recess  27 , at least a portion of the bottom surface of the recess  27  forms a rounded surface  31 , and at least a portion of the bottom surface of the recess  28  forms a rounded surface  32 . In the present embodiment illustrated in  FIG.  3   , the entire bottom surfaces of the recesses  27  and  28  are respectively formed of the rounded surfaces  31  and  32 . 
     Note that the terminal electrodes  25  and  26  are respectively formed over the entire mounting surfaces  21  and  22 . Consequently, the recesses  27  and  28 , the flat surfaces  29  and  30 , and the rounded surfaces  31  and  32 , which have been described above, are shapes provided by the mounting surfaces  21  and  22 . However, the recesses  27  and  28 , the flat surfaces  29  and  30 , and the rounded surfaces  31  and  32 , will hereinafter sometimes be described as shapes provided by the terminal electrodes  25  and  26 . 
     A configuration on the side on which the first flange portion  13  illustrated in  FIG.  3    and  FIG.  4    is present will be described below. A configuration on the side on which the second flange portion  14  is present is symmetrical to the configuration on the side on which the first flange portion  13  is present, and thus, the description thereof will be omitted. 
     As illustrated in  FIG.  3   , the rounded surface  31  forming the bottom surface of the recess  27 , which is formed in the mounting surface  21  of the first flange portion  13 , has a central axis CA linearly extending in a widthwise direction parallel to the mounting surface  21  and the outer end surface  19  and has a curvature radius r that is longer than the distance between the inner end surface  17  and the outer end surface  19 , that is, the thickness of the first flange portion  13 . Note that the curvature radius r may be infinite. Although the central axis CA is not necessarily parallel to a ridge line L, it is preferable that the central axis CA extend parallel to or approximately parallel to the ridge line L. In addition, as may be inferred from  FIG.  3   , it is preferable that the central axis CA be located on a plane in which the outer end surface  19  extends or be located outside the plane in which the outer end surface  19  extends. With such a configuration, the highest point of the rounded surface  31  that is illustrated in  FIG.  3    can be located at the position of the ridge line L. This is advantageous for performing a thermocompression bonding process, which will be described later with reference to  FIG.  4   . 
     In addition, as a result of forming the rounded surface  31  as described above, the bottom surface of the recess  27  forms a slope that is relatively shallow on the side on which the outer end surface  19  is present and that is relatively deep on the side on which the inner end surface  17  is present when seen as a whole. In other words, the recess  27  becomes deeper from the side on which the outer end surface  19  is present to the side on which the inner end surface  17  is present. In addition, the bottom surface of the recess  27  has a portion that is positioned such that the difference between the heights of the portion and the flat surface  29  is equal to or smaller than the diameter of the wire  23 . This is also advantageous for performing the thermocompression bonding process, which will be described later with reference to  FIG.  4   . In the embodiment illustrated in  FIG.  3   , the bottom surface of the recess  27  excluding a limited portion thereof that is located in the vicinity of the inner end surface  17  is positioned such that the difference between the heights of the bottom surface of the recess  27  and the flat surface  29  is equal to or smaller than the diameter of the wire  23 . Here, to be exact, the diameter of the wire  23  is the diameter of a portion of the wire  23  that is wound around the winding core portion  12  (a portion of the wire  23  that is not pressed and deformed in the radial direction thereof). 
     Note that the axial direction of the wire  23  is not necessarily parallel to the flat surface  29 , and thus, it should be understood that the difference between the heights of the bottom surface of the recess  27  and the flat surface  29  refers to the difference between the heights that is measured in the radial direction of the wire  23 , and more precisely, the diameter of the portion of the wire  23  that is wound around the winding core portion  12 . 
     In addition, the flat surface  29  is positioned such that the recess  27  is sandwiched by portions of the flat surface  29  as illustrated in  FIG.  1    and  FIG.  2   . This is also advantageous for performing the thermocompression bonding process, which will be described later with reference to  FIG.  4   . 
     As illustrated in  FIG.  4   , when the thermocompression bonding process is performed, the end portion of the wire  23  that is connected to the terminal electrode  25  is received in the recess  27  and positioned along the rounded surface  31  while extending from the side on which the inner end surface  17  is present toward the side on which the outer end surface  19  is present. In  FIG.  4   , a portion of the wire  23  that has not yet undergone the thermocompression bonding is indicated by a dashed line. 
     Next, the thermocompression bonding process is started, and in the thermocompression bonding process, a thermocompression-bonding head  33  moves downward in the direction of arrows  34  toward the mounting surface  21  of the flange portion  13  and applies pressure to the end portion of the wire  23  while heating the end portion of the wire  23  so as to cause the wire  23  to become plastically deformed as indicated by a solid line. Here, although the thermocompression bonding temperature varies depending on the material of the insulating film of the wire  23 , the thermocompression bonding temperature is selected from, for example, the range of about 300° C. to about 500° C. In addition, the thermocompression bonding pressure is selected from, for example, the range of about several tens kgf to about several kgf when the diameter of the wire  23  is about 15 μm to about 200 μm. 
     The insulating film coating the end portion of the wire  23  is removed, at a relatively early stage of the above-described thermocompression bonding process, as a result of being decomposed by heat that is applied thereto by the thermocompression-bonding head  33 . 
     Subsequently, the thermocompression-bonding head  33  further moves downward in the direction of arrows  34 , and the end portion of the wire  23  is pressed between the thermocompression-bonding head  33  and the rounded surface  31  so as to deform in the radial direction. As a result, as indicated by a solid line in  FIG.  4   , the end portion of the wire  23  is plastically deformed and heated so as to be bonded to the terminal electrode  25 . At the same time, the wire  23  is cut off at an edge of a ridge line where the outer end surface  19  of the flange portion  13  and the rounded surface  31  intersect each other. The thermocompression bonding process is completed in the manner described above.  FIG.  1    illustrates the state in which the thermocompression bonding process has been completed and in which the end portions of the wire  23  have been bonded to the terminal electrodes  25  and  26 . 
     As illustrated in  FIG.  4   , in a state where the wire  23  has become plastically deformed after being subjected to the above-described thermocompression bonding process, one of the end portions of the wire  23  has a thickness that continuously changes such that the thickness is relatively thin on the side on which the outer end surface  19  is present and is relatively thick on the side on which the inner end surface  17  is present. Particularly, in the present embodiment, the entire bottom surface of the recess  27  is not positioned such that the difference between the heights of the entire bottom surface of the recess  27  and the flat surface  29  is equal to or smaller than the diameter of the wire  23 , and the difference between the heights of the limited portion of the bottom surface of the recess  27 , which is located in the vicinity of the inner end surface  17 , and the flat surface  29  is larger than the diameter of the wire  23 . Therefore, in the above-mentioned limited portion that is located in the vicinity of the inner end surface  17 , there is a region  35  where no plastic deformation occurs in the wire  23 . 
     There is a region  36  that is next to, on the side on which the outer end surface  19  is present, the above-mentioned region  35  and in which the thickness of the end portion of the wire  23  continuously changes so as to be gradually decrease. Such continuous changes in the thickness are caused by the rounded surface  31 . In the region  36  where the thickness of the end portion of the wire  23  continuously changes, a region is obtained in which the pressure that is applied to the wire  23  by the thermocompression-bonding head  33  continuously changes in the lengthwise direction of the wire  23  such that the pressure gradually increases from a position where the wire is relatively weakly pressed by the thermocompression-bonding head  33  to a position where the wire  23  is relatively strongly pressed by the thermocompression-bonding head  33 . By increasing the above-mentioned curvature radius r of the rounded surface  31  such that the curvature radius r becomes longer than the distance between the inner end surface  17  and the outer end surface  19 , more gradual changes in the pressure can be achieved over a wider area in the region  36 . 
     More specifically, as illustrated in  FIG.  4   , in the state where the thermocompression bonding process has been performed, significant plastic deformation of the wire  23  is caused on the side on which the outer end surface  19  of the flange portion  13  is present so that the wire  23  is bonded to the terminal electrode  25 . In this case, a relatively strong pressure is applied to the wire  23 , and thus, the wire  23  has a relatively large peel strength with respect to the terminal electrode  25 . In contrast, the pressure applied to the wire  23  is relatively low on the side on which the inner end surface  17  of the flange portion  13  is present, and thus, the wire  23  has only a relatively small peel strength with respect to the terminal electrode  25 . Instead, the strength of the wire  23  itself is maintained with almost no decrease because the amount of deformation of the wire  23  is relatively small. 
     Here, assume that a force that tries to peel off the wire  23  is applied to the wire  23  from the outside. In this case, there are two conceivable types of breakage. One is that the wire  23  becomes separated from the terminal electrode  25  due to insufficient press bonding, and the other is that the wire  23  breaks because the wire  23  has become too thin as a result of undergoing the thermocompression bonding. 
     In the region  36  where the thickness of the end portion of the wire  23  continuously changes as illustrated in  FIG.  4   , an area that is located on the side on which the outer end surface  19  is present and where the press-bonding strength is relatively large while the thickness of the wire  23  is small and an area where the press-bonding strength is relatively small while the strength of the wire  23  is relatively large because the amount of deformation of the wire  23  is small are smoothly continuous with each other along the rounded surface  31 . Thus, when a peeling force is applied to the wire  23  from the outside, an area where the wire  23  is less likely to be peeled off against the peeling force and is less likely to break, that is, an area where an appropriate pressure is applied to the wire  23  inevitably appears somewhere midway in the region  36 . Therefore, accidental breakage of the wire  23  is prevented from occurring, and sufficient and proper thermocompression bonding can be stably achieved. 
     When the above-described thermocompression bonding is performed, the flat surface  29 , which is formed on the portion of the mounting surface  21  of the flange portion  13  excluding the recess  27 , serves to prevent application of excessive pressure by the thermocompression-bonding head  33 . In other words, termination of the downward movement of the thermocompression-bonding head  33  in the direction of arrows  34  is provided by the flat surface  29 . As in the present embodiment, by positioning the flat surface  29  such that the recess  27  is sandwiched by the portions of the flat surface  29 , the termination of the downward movement of the thermocompression-bonding head  33  can be stably provided. Note that, regarding the widthwise dimension of the substantially drum-shaped core  15 , it is preferable that the recess  27  be about 1.0 times or more of the wire  23  and be about ⅔ or less of the flange portion  13 . 
     A second embodiment of the present disclosure will now be described with reference to  FIG.  5   .  FIG.  5    corresponds to  FIG.  4    and illustrates a portion of a substantially drum-shaped core  15   a  and a portion of the wire  23  that are included in a coil component. In  FIG.  5   , components that correspond to those illustrated in  FIG.  4    are denoted by the same reference signs, and repeated descriptions thereof will be omitted. 
     In the second embodiment illustrated in  FIG.  5   , the entire bottom surface of the recess  27  is positioned such that the difference between the heights of the entire bottom surface of the recess  27  and the flat surface  29  is equal to or smaller than the diameter of the portion of the wire  23  that is wound around the winding core portion  12 . In this case, it is preferable that the difference between the heights of an end portion of the bottom surface of the recess  27  that is located on the side on which the inner end surface  17  is present and the flat surface  29  be about 0.5 to about 1.0 times the diameter of the wire  23 . 
     According to the present embodiment, there is substantially no region  35  (see  FIG.  4   ), in which plastic deformation does not occur in the wire  23 , and a region  36  where the thickness of the end portion of the wire  23  continuously changes so as to be gradually decrease is present throughout the recess  27 . 
     In the second embodiment, as described above, since there is substantially no region  35 , in which plastic deformation does not occur in the wire  23 , there is a possibility that the variation width of the thickness of the wire  23  in the region  36 , where the thickness of the end portion of the wire  23  continuously changes so as to be gradually decrease, may be narrower than that in the above-described first embodiment. Thus, in order to always form an area where the wire  23  is less likely to be peeled off against a peeling force applied from the outside and is less likely to break, that is, an area where an appropriate pressure is applied to the wire  23  in the region  36 , it is necessary to increase the difference in the heights the end portion of the bottom surface of the recess  27  that is located on the side on which the inner end surface  17  is present and the flat surface  29  to a certain extent or more, and thus, it is preferable that the difference in the heights be about 0.5 times or more the diameter of the wire  23  as mentioned above. 
     A third embodiment of the present disclosure will now be described with reference to  FIG.  6   .  FIG.  6    corresponds to  FIG.  3    and illustrates a portion of a substantially drum-shaped core  15   b  that is included in a coil component. In  FIG.  6   , components that correspond to those illustrated in  FIG.  3    are denoted by the same reference signs, and repeated descriptions thereof will be omitted. 
     In the third embodiment illustrated in  FIG.  6   , the rounded surface  31  does not form the entire bottom surface of the recess  27  and does not extend on the end portion of the bottom surface of the recess  27  that is located on the side on which the inner end surface  17  is present. 
     As in the present embodiment, advantageous effects similar to those in the above-described embodiments may be obtained not only in the case where the entire bottom surface of the recess  27  is formed of the rounded surface  31 . 
     A fourth embodiment of the present disclosure will now be described with reference to  FIG.  7   .  FIG.  7    corresponds to  FIG.  3    and illustrates, in a further enlarged manner than  FIG.  3    does, a portion of a substantially drum-shaped core  15   c  included in a coil component. In  FIG.  7   , components that correspond to those illustrated in  FIG.  3    are denoted by the same reference signs, and repeated descriptions thereof will be omitted. 
     Similar to the third embodiment illustrated in  FIG.  6   , in the fourth embodiment illustrated in  FIG.  7   , the rounded surface  31  does not form the entire bottom surface of the recess  27 . In the fourth embodiment illustrated in  FIG.  7   , the rounded surface  31  does not extend on the end portion of the bottom surface of the recess  27  that is located on the side on which the outer end surface  19  is present, and a portion of the flat surface  29  is positioned between the recess  27  and the outer end surface  19  with a step portion D interposed between the portion of the flat surface  29  and the recess  27 . In addition, an angular edge  38  is formed at an end edge formed of the step portion D, which is formed at the portion of the flat surface  29  that is located on the side on which the outer end surface  19  is present. 
     It is preferable that the above-mentioned step portion D be about 1/10 or more and about ⅓ or less (i.e., from about 1/10 to about ⅓) the diameter of the portion of the wire  23  that is wound around the winding core portion  12 . According to the present embodiment, when the wire  23  is bonded to the terminal electrode  25  by thermocompression bonding, a stress is concentrated at the edge  38 . Thus, an unnecessary end portion of the wire  23  can be cut off with a small pressing force, and regardless of the dimensions and the shape of the thermocompression-bonding head  33  (see  FIG.  4   ) used for the thermocompression bonding and the application position of the thermocompression-bonding head  33 , the thermocompression bonding can be more reliably controlled such that the amount of deformation of the wire  23  will not become excessive. 
     Although the second to fourth embodiments have been described with reference to  FIG.  5    to  FIG.  7   , only the configuration on the side on which the first flange portion  13  is present is illustrated in  FIG.  5    to  FIG.  7   , and only the configuration on the side on which the first flange portion  13  is present has been described. Although description of the configuration on the side on which the second flange portion  14  is present is omitted, the configuration on the side on which the second flange portion  14  is present is symmetrical to the configuration on the side on which the first flange portion  13  is present. 
     According to the above-described embodiments, the end portions of the wire  23  that are connected to the terminal electrodes  25  and  26  can each have a portion where the pressure that is applied to the wire  23  in the thermocompression bonding process continuously changes such that the pressure becomes stronger with decreasing distance from the end of the wire  23 , that is, a portion where the pressure continuously changes such that the pressure becomes weaker with increasing distance from the end of the wire  23 . 
     Thus, in a certain length range of each of the end portions of the wire  23 , which are connected to the terminal electrodes  25  and  26 , the state in which the pressure applied in thermocompression bonding continuously changes in the lengthwise direction of the wire  23  is obtained. Thus, in this length range, there is always a portion having press-bonding strength that is further improved than that in a structure of the related art. As a result, in the coil component  11 , prevention of breakage of the wire  23  and stable thermocompression bonding can be simultaneously achieved. 
     Although the present disclosure has been described above in connection with the embodiments illustrated in the drawings, the present disclosure can employ other various embodiments within the scope of the present disclosure. 
     For example, although not illustrated, a substantially plate-shaped core may be provided so as to connect two surfaces of the substantially drum-shaped core  15  to each other, each of the two surfaces opposing to one of the mounting surface  21  of the first flange portion  13  and the mounting surface  22  of the second flange portion  14 . In the case where both the substantially drum-shaped core  15  and the substantially plate-shaped core are made of a magnetic material, the substantially drum-shaped core  15  and the substantially plate-shaped core form a closed magnetic circuit. 
     In addition, although each of the above-described embodiments relates to a coil component that includes a single wire, for example, the present disclosure can also be applied to a coil component that includes a plurality of wires, such as a coil component that forms a common-mode choke coil or a coil component that forms a transformer. Thus, the number of wires is changed in accordance with the function of the coil component, and accordingly, the number of terminal electrodes provided at each flange portion is not limited to one and may be plural. In the case where the number of terminal electrodes provided at each flange portion is plural, a plurality of terminal electrodes are arranged in the widthwise direction of each flange portion so as to be electrically isolated from each other. Therefore, a plurality of recesses are provided so as to be arranged in the widthwise direction of each flange portion. 
     In addition, the rounded surfaces  31  and  32  may be respectively formed on the entire bottom surfaces of the recesses  27  and  28  or may be respectively formed on only portions of the bottom surfaces of the recesses  27  and  28 . As in the latter case, when the rounded surfaces  31  and  32  are respectively formed only on portions of the bottom surfaces of the recesses  27  and  28 , the rounded surfaces  31  and  32  may be formed in one of the following patterns. As a first pattern, the rounded surface  31  may be formed only in a region of the flange portion  13  that is located on the side on which the inner end surface  17  is present, and the rounded surface  32  may be formed only in a region of the flange portion  14  that is located on the side on which the inner end surface  18  is present. As a second pattern, the rounded surface  31  may be formed only in a region of the flange portion  13  that is located on the side on which the outer end surface  19  is present, and the rounded surface  32  may be formed only in a region of the flange portion  14  that is located on the side on which the outer end surface  20  is present. As a third pattern, the rounded surfaces  31  and  32  may be respectively formed only at central portions of the recesses  27  and  28 . In the above three patterns, a portion of the bottom surface of the recess  27  that is not the rounded surface  31  may be a surface having a slope with respect to the mounting surface  21  or may be a surface parallel to the mounting surface  21 , and a portion of the bottom surface of the recess  28  that is not the rounded surface  32  may be a surface having a slope with respect to the mounting surface  22  or may be a surface parallel to the mounting surface  22 . 
     In addition, although it is preferable that the rounded surfaces  31  and  32  be formed in a convex manner as in the embodiments illustrated in the drawings because the press-bonding of the wire  23  may be easily performed with the rounded surfaces  31  and  32  formed in a convex manner, the rounded surfaces  31  and  32  may be formed in a recessed manner. 
     The scope of the present disclosure is not limited to the above-described embodiments and includes other embodiments that are obtained by partially replacing or combining the configurations according to the above-described different embodiments with one another. 
     While some embodiments of the disclosure have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the disclosure. The scope of the disclosure, therefore, is to be determined solely by the following claims.