TECHNICAL FIELD 
     The present invention relates to a method of producing a surface-mount inductor and, more particularly, to a method of forming an external electrode of the surface-mount inductor. 
     BACKGROUND ART 
     Heretofore, there has been employed a surface-mount inductor in which an external electrode is formed on a chip-like element body by using an electrically-conductive paste. For example, JP 2005-116708A discloses a method which comprises: applying an electrically-conductive paste on a surface of a resin-molded chip encapsulating a winding wire; then curing the electrically-conductive paste to form an underlying electrode; and further subjecting the underlying electrode to plating to form an external electrode. 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     Generally, in the conventional surface-mount inductor, as the electrically-conductive paste, there has been used a type in which metal particles such as Ag are dispersed in a thermosetting resin such as an epoxy resin. In this type of electrically-conductive paste, shrinkage stress arising from curing of the thermosetting resin is utilized to cause the metal particles dispersed in the resin to come contact with each other or with an electrically-conductive wire to thereby obtain electrical conductivity. 
     Meanwhile, the resign in the electrically-conductive paste tends to be degraded in a high-humidity environment. In the case where the surface-mount inductor as disclosed in JP 2005-116708A is formed using a conventional electrically-conductive paste, there is a problem that a bonding strength between the element body and the external electrode becomes degraded under a moisture resistance test, causing peeling of the external electrode. 
     As an alternative electrode forming method, there has been known a method which comprises sintering a metal powder contained in an electrically-conductive paste to form an underlying electrode, as disclosed in JP 10-284343A. As the electrically-conductive paste, it is possible to use a type obtained by kneading a metal powder such as an Ag powder, an inorganic binder such as glass frit, and an organic vehicle. This electrically-conductive paste is applied to a chip-like element body, and then sintered by heating at a temperature of 600 to 1000° C. to form the underlying electrode. When this method is used, metal particles of the metal powder are mutually sintered, and baked onto the element body, so that it becomes possible to increase the bonding strength between the element body and the external electrode. However, this method is required to allow an inorganic binder such as glass frit in the electrically-conductive paste to be melted, so that it is necessary to subject the electrically-conductive paste to a heat treatment at a high temperature of 600° C. or more. Thus, for production of a surface-mount inductor configured such that a winding wire formed by winding an electrically-conductive wire is encapsulated therein with a sealant primarily comprising a magnetic powder and a resin, the above method cannot be employed, because, if the sealant and the winding wire are subjected to a heat treatment at a temperature greater than 250° C., the resin in the sealant or a self-bonding coating of the electrically-conductive wire will be degraded. 
     It is therefore an object of the present invention to provide a method of producing a surface-mount inductor which comprises an external electrode having high fixing strength with respect to an element body even in a high-humidity environment. 
     Solution to Problem 
     To accomplish the above object, the method of producing a surface-mount inductor according to the present invention comprises the steps of: winding an electrically-conductive wire to form a coil; forming a core using a sealant primarily containing a metal magnetic powder and a resin in such a manner as to encapsulate the coil in the sealant while allowing each of opposite ends of the coil to be at least partially exposed on a surface of the core; reducing smoothness of a surface of at least a part of a portion of the core on which an external electrode is formed as compared to a surface therearound; and forming the external electrode on the core in such a manner as to be electrically conducted with the coil. 
     Effect of Invention 
     The present invention makes it possible to produce a surface-mount inductor which comprises an external electrode having high fixing strength with respect to an element body even in a high-humidity environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an air-cored coil for use in a first embodiment of the present invention. 
         FIG. 2  is a perspective view of a core according to the first embodiment, of the present invention. 
         FIG. 3  is a perspective view of the core in a fabricated state according to the first embodiment of the present invention. 
         FIG. 4  is a perspective view of the core to which an electrically-conductive paste is applied, according to the first embodiment of the present invention. 
         FIG. 5  is a perspective view of a surface-mount inductor produced by a method according to the first embodiment of the present invention. 
         FIG. 6  is a perspective view of a core according to a second embodiment of the present invention. 
         FIG. 7  is a perspective view of the core in a fabricated state according to the second embodiment of the present invention. 
         FIG. 8  is a perspective view of the core to which an electrically-conductive paste is applied, according to the first embodiment of the present invention. 
         FIG. 9  is a perspective view of a surface-mount inductor produced by a method according to the second embodiment of the present invention. 
         FIG. 10  is a perspective view of a core according to a third embodiment of the present invention. 
         FIG. 11  is a perspective view of the core in a fabricated state according to the third embodiment of the present invention. 
         FIG. 12  is a perspective view of the core to which an electrically-conductive paste is applied, according to the third embodiment of the present invention. 
         FIG. 13  is a perspective view of a surface-mount inductor produced by a method according to the third embodiment of the present invention. 
         FIG. 14  is a perspective view of a core according to a fourth embodiment of the present invention. 
         FIG. 15  is a perspective view of the core in a fabricated state according to the fourth embodiment of the present invention. 
         FIG. 16  is a perspective view of the core to which an electrically-conductive paste is applied, according to the fourth embodiment of the present invention. 
         FIG. 17  is a perspective view of a surface-mount inductor produced by a method according to the fourth embodiment of the present invention. 
         FIG. 18  is a perspective view of the core illustrating another fabricated state according to the present invention. 
         FIG. 19  is a bottom view of the core illustrating yet another fabricated state according to the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the method of producing a surface-mount inductor according to the present invention, a surface of at least a part of a portion of the core, for allowing an external electrode to be formed thereon is caused to be increased in roughness as compared to a surface therearound. This makes it possible to allow an electrically-conductive paste to be entered into concave portions on the surface of the core while increasing a contact area between the external electrode and an element body. Further, in the method of producing a surface-mount inductor according to the present invention, an electrically-conductive paste containing metal fine particles having a sintering temperature of 250° C. or less is applied onto the surface of the core. This makes it possible to allow the metal fine particles contained in the electrically-conductive paste to be easily entered into concave portions on the surface of the core while increasing a contact area between the external electrode and the element body. Furthermore, by using an electrically-conductive paste containing metal fine particles having a sintering temperature of 250° C. or less, the metal fine particles are sintered with each other or with an internal electrical conductor at a low temperature, so that the DC resistance is not degraded even in a high-humidity environment. 
     Embodiments 
     With reference to the drawings, a surface-mount inductor production method of the present invention will now be described. 
     With reference to  FIGS. 1 to 5 , a surface-mount inductor production method according to a first embodiment of the present invention will be described.  FIG. 1  illustrates a perspective view of an air-cored coil for use in a first embodiment of the present invention.  FIG. 2  illustrates a perspective view of a core of a surface-mount inductor according to the first embodiment of the present invention.  FIG. 3  illustrates a perspective view of the core in a fabricated state according to the first embodiment of the present invention.  FIG. 4  illustrates a perspective view of the core to which an electrically-conductive paste is applied, according to the first embodiment of the present invention.  FIG. 5  illustrates a perspective view of a surface-mount inductor produced by a method according to the first embodiment of the present invention. 
     Firstly, an electrically-conductive wire having a rectangular cross-section provided with a self-bonding coating is used to form a coil. As illustrated in  FIG. 1 , the electrically-conductive wire is wound in two-tiered outward spiral pattern in such a manner as to allow its opposite ends  1   a  to be positioned on an outermost periphery to form a coil  1 . As the electrically-conductive wire for use in this embodiment, a type is used which has an imide-modified polyurethane layer as the self-bonding coating. Alternatively, the self-bonding coating may be of polyamide series or polyester series, preferably having a higher heatproof temperature. Further, the electrically-conductive wire used in this embodiment has a rectangular cross-section. Alternatively, it is also possible to use around wire or a wire having a polygonal cross-section. 
     Next, as a sealant, a type is used in which iron-based metal magnetic powders and an epoxy resin are mixed and granulated into powders to form a core  2  encapsulating the coil as illustrated in  FIG. 2  by a compressing molding process. In this process, each of the opposite ends  1   a  of the coil is allowed to be exposed on a surface of the core  2 . In this embodiment, the core is formed by the compressing molding process. Alternatively, it is also possible to form the core by other molding process such as a powder compacting molding process. 
     Then, after removing the coating on a surface of the exposed opposite ends  1   a  by mechanical stripping, a treatment such as lasering, sandblasting or polishing is applied to the entire portion of the core  2  for allowing an external electrode to be formed thereon to remove components such as a resin component present on its surface to roughen the surface, thereby causing the surface of the entire portion of the core  2 , for allowing the external electrode to be formed thereon, to be increased in roughness as compared to a surface therearound, as illustrated in  FIG. 3 . As a result, the surface of the entire portion of the core  2  for allowing the external electrode to be formed thereon is reduced in smoothness as compared to a surface therearound. 
     Next, as illustrated in  FIG. 4 , an electrically-conductive paste  3  is applied by a dip process on the portion of the core  2  for allowing the external electrode to be formed thereon. In this embodiment, as the electrically-conductive paste, a type is used in which metal particles such as Ag are dispersed in a thermosetting resin such as an epoxy resin. Further, the dip process is used in this embodiment as a process for applying the electrically-conductive paste. Alternatively, it is also possible to use other process such as a printing process or a potting process. 
     The core  2  on which the electrically-conductive paste  3  is applied is subjected to a heat treatment at 200° C., thereby to cause the core  2  and the thermosetting resin in the electrically-conductive paste to be cured. In this way, shrinkage stress arising from curing of the thermosetting resin is utilized to cause the metal particles dispersed in the resin of the electrically-conductive paste to come contact with each other or with an electrically-conductive wire to thereby obtain electrical conductivity. Further, the electrically-conductive paste  3  is fixed to the core  2 , with the thermosetting resin and the metal particles in the electrically-conductive paste being entered into the concave portions on the surface of the core, formed in the roughened portion of the surface of the core  2 . 
     Finally, the core  2  is subjected to plating to form an external electrode  4  on the surface of the electrically-conductive paste, thereby to obtain a surface-mount inductor as illustrated in  FIG. 5 . The electrode formed by the plating may be formed by appropriately selecting one or more from materials such as Ni, Sn, Cu, Au and Pd. 
     (Second Embodiment) 
     With reference to  FIGS. 6 to 9 , a surface-mount inductor production method according to a second embodiment of the present invention will be described.  FIG. 6  illustrates a perspective view of a core according to a second embodiment of the present invention.  FIG. 7  illustrates a perspective view of the core in a fabricated state according to the second embodiment of the present invention.  FIG. 8  illustrates a perspective view of the core to which an electrically-conductive paste is applied, according to the first embodiment of the present invention.  FIG. 9  illustrates a perspective view of a surface-mount inductor produced by a method according to the second embodiment of the present invention. 
     Firstly, the electrically-conductive wire used in the first embodiment is wound in two-tiered outward spiral pattern in such a manner as to allow its opposite ends  11   a  to be positioned on an outermost periphery to form a coil  11 . In this embodiment, the opposite ends  11   a  of the coil  11  are led out to be opposed to each other across the wound portion of the coil  11 . Next, a sealant having the same composition as that used in the first embodiment is used to form a core  12  encapsulating the coil  11  as illustrated in  FIG. 6  by a compressing molding process. In this process, each of the opposite ends  11   a  of the coil is allowed to be exposed on respective one of opposed lateral surfaces of the core  12 . 
     Then, after removing the coating on a surface of the exposed opposite ends  11   a  by mechanical stripping, a treatment such as lasering, sandblasting or polishing is applied to the entire portion of the core  12  for allowing an external electrode to be formed thereon to remove components such as a resin component present on its surface to roughen the surface, thereby causing the surface of the entire portion of the core  12 , for allowing the external electrode to be formed thereon, to be increased in roughness as compared to a surface therearound, as illustrated in  FIG. 7 . As a result, the surface of the entire portion of the core  12  for allowing the external electrode to be formed thereon is reduced in smoothness as compared to a surface therearound. 
     Next, as illustrated in  FIG. 8 , an electrically-conductive paste  13  used in the first embodiment is applied by a printing process in an L-shape on the portion of the core  2  for allowing the external electrode to be formed thereon. The core  12  on which the electrically-conductive paste  13  is applied is subjected to a heat treatment at 200° C., thereby to cause the core  12  and the thermosetting resin in the electrically-conductive paste to be cured. In this way, shrinkage stress arising from curing of the thermosetting resin is utilized to cause the metal particles dispersed in the resin of the electrically-conductive paste to come contact with each other or with an electrically-conductive wire to thereby obtain electrical conductivity. Further, the electrically-conductive paste  13  is fixed to the core  12 , with the thermosetting resin and the metal particles in the electrically-conductive paste being entered into the concave portions on the surface of the core, formed in the roughened portion of the surface of the core  12 . 
     Finally, the core  12  is subjected to plating to form an external electrode  14  on the surface of the electrically-conductive paste, thereby to obtain a surface-mount inductor comprising an L-shaped external electrode  14  as illustrated in  FIG. 9 . 
     (Third Embodiment) 
     With reference to  FIGS. 10 to 13 , a surface-mount inductor production method according to a third embodiment of the present invention will be described.  FIG. 10  illustrates a perspective view of a core according to a third embodiment of the present invention.  FIG. 11  illustrates a perspective view of the core in a fabricated state according to the third embodiment of the present invention.  FIG. 12  illustrates a perspective view of the core to which an electrically-conductive paste is applied, according to the third embodiment of the present invention.  FIG. 13  illustrates a perspective view of a surface-mount inductor produced by a method according to the third embodiment of the present invention. 
     Firstly, an electrically-conductive wire having a rectangular cross-section provided with a self-bonding coating is wound in two-tiered outward spiral pattern in such a manner as to allow its opposite ends  21   a  to be positioned on an outermost periphery to form a coil  21 . Next, as a sealant, a type is used in which iron-based metal magnetic powders and an epoxy resin are mixed and granulated into powders to form a core  22  encapsulating the coil as illustrated in  FIG. 10  by a compressing molding process. In this process, each of the opposite ends  21   a  of the coil is allowed to be exposed on a surface of the core  22 . 
     Then, after removing the coating on a surface of the exposed opposite ends  21   a  by mechanical stripping, a treatment such as lasering, sandblasting or polishing is applied to the entire portion of the core  22  for allowing an external electrode to be formed thereon to remove components such as a resin component present on its surface to roughen the surface, thereby causing the surface of the entire portion of the core  22 , for allowing the external electrode to be formed thereon, to be increased in roughness as compared to a surface therearound, as illustrated in  FIG. 11 . As a result, the surface of the entire portion of the core  22  for allowing the external electrode to be formed thereon is reduced in smoothness as compared to a surface therearound. 
     Next, as illustrated in  FIG. 12 , an electrically-conductive paste  23  is applied by a dip process on the portion of the core  22  for allowing the external electrode to be formed thereon. In this embodiment, as the electrically-conductive paste, a type is used in which Ag fine particles having a particle size of 10 nm or less and solvent such as organic solvent are mixed and pasted. Metals will have a lowered sintering temperature or melting temperature due to size effect when the particle size thereof is reduced below 100 nm. In particular, the sintering temperature or the melting temperature is significantly lowered with a size less than 10 nm. In this embodiment, the Ag fine particle is used. Alternatively, it is also possible to use Au or Cu. Further, the dip process is used in this embodiment as a process for applying the electrically-conductive paste. Alternatively, it is also possible to use other process such as a printing process or a potting process. 
     The core  22  on which the electrically-conductive paste  23  is applied is then subjected to a heat treatment at 200° C., thereby to sinter the Ag fine particles in the electrically-conductive paste  23  while curing the core  22 . Since the Ag fine particle has a particle size of 10 nm or less, it can be easily sintered at this level of temperature. By causing the metal fine particles to be sintered, bonding between metals becomes stronger than the case of causing the metal particles to come contact with each other or with the electrically-conductive wire as in the first and second embodiments, so that electrical conduction with high connection reliability can be obtained between the coil and the electrically-conductive paste. Even when metal powders having a particle size of greater than 100 nm are mixed, the metal fine particles come into a sintered or molten state, so that it becomes possible to have bonding between metals stronger than the case of merely causing the metal fine particles to come contact with each other. Then, a heat treatment only at 250° C. or less is required in this process, so that damage to the core or the coating of the electrically-conductive wire is reduced. Further, the electrically-conductive paste is fixed to the core  22 , with the Ag fine particles in the electrically-conductive paste  23  being entered into the concave portions on the surface of the core, formed in the roughened portion of the surface of the core  22 . A content of a metal in the electrically-conductive paste fixed to the core  22  was in the range of 85 to 98%. 
     Finally, the core  22  is subjected to plating to form an external electrode  24  on the surface of the electrically-conductive paste, thereby to obtain a surface-mount inductor as illustrated in  FIG. 13 . The electrode formed by the plating may be formed by appropriately selecting one or more from materials such as Ni, Sn, Cu, Au and Pd. 
     (Fourth Embodiment) 
     With reference to  FIGS. 14 to 17 , a surface-mount inductor production method according to a fourth embodiment of the present invention will be described.  FIG. 14  illustrates a perspective view of a core according to a fourth embodiment of the present invention.  FIG. 15  illustrates a perspective view of the core in a fabricated state according to the fourth embodiment of the present invention.  FIG. 16  illustrates a perspective view of the core to which an electrically-conductive paste is applied, according to the fourth embodiment of the present invention.  FIG. 17  illustrates a perspective view of a surface-mount inductor produced by a method according to the fourth embodiment of the present invention. 
     Firstly, the electrically-conductive wire used in the third embodiment is wound in two-tiered outward spiral pattern in such a manner as to allow its opposite ends  31   a  to be positioned on an outermost periphery to form a coil  31 . In this embodiment, the opposite ends  31   a  of the coil  31  are led out to be opposed to each other across the wound portion of the coil  31 . Next, a sealant having the same composition as that used in the third embodiment is used to form a core  32  encapsulating the coil  31  as illustrated in  FIG. 14  by a compressing molding process. In this process, each of the opposite ends  31   a  of the coil is allowed to be exposed on respective one of opposed lateral surfaces of the core  32 . 
     Then, after removing the coating on a surface of the exposed opposite ends  31   a  by mechanical stripping, a treatment such as lasering, sandblasting or polishing is applied to the entire portion of the core  32  for allowing an external electrode to be formed thereon to remove components such as a resin component present on its surface to roughen the surface, thereby causing the surface of the entire portion of the core  32 , for allowing the external electrode to be formed thereon, to be increased in roughness as compared to a surface therearound, as illustrated in  FIG. 15 . As a result, the surface of the entire portion of the core  32  for allowing the external electrode to be formed thereon is reduced in smoothness as compared to a surface therearound. 
     Next, as illustrated in  FIG. 16 , an electrically-conductive paste  33  is applied by a printing process in an L-shape on the portion of the core  32  for allowing the external electrode to be formed thereon. In this embodiment, as the electrically-conductive paste, a type is used in which Ag fine particles having a particle size of 10 nm or less, Ag particles having a particle size of 0.1 to 10 μm, and an epoxy resin are mixed and pasted. The electrically-conductive paste is prepared such that a ratio of the Ag particles having a particle size of 0.1 to 10 μm contained in the electrically-conductive paste to a sum of the Ag fine particles having a particle size of 10 nm or less and the Ag particles having a particle size of 0.1 to 10 μm is 30 wt %. Containing a 30 to 50 wt % of metal particles having a particle size of 0.1 to 10 μm provides an effect of reducing heat shrinkage at the time of thermal curing as compared to the case of only containing metal fine particles having a particle size of less than 100 nm. Further, the small amount of metal fine particles can also promise reduction in the material cost. 
     The core  32  on which the electrically-conductive paste  33  is applied is then subjected to a heat treatment at 200° C., thereby to sinter the Ag fine particles in the electrically-conductive paste  33  while curing the core  32 , In this process, the electrically-conductive paste is fixed to the core  32 , with the Ag fine particles in the electrically-conductive paste  32  being entered into the concave portions on the surface of the core, formed in the roughened portion of the surface of the core  32 . A content of a metal in the electrically-conductive paste fixed to the core  22  was in the range of 85 to 98%. 
     Finally, the core  32  is subjected to plating to form an external electrode  34  on the surface of the electrically-conductive paste, thereby to obtain a surface-mount inductor as illustrated in  FIG. 17 . 
     In the above embodiments, as a sealant, a type is used in which iron-based metal magnetic powders as the magnetic powder and an epoxy resin as the resin are mixed. Alternatively, the magnetic powder for use in the sealant may be, for example, a ferritic magnetic powder or a magnetic powder that is subjected to surface modification such as insulation coating formation or surface oxidation. It is also possible to add an inorganic material such as a glass powder. Further, the resin for use in the sealant may be other thermosetting resin such as a polyimide resin or a phenol resin, or may be a thermoplastic resin such as a polyethylene resin or a polyamide resin. 
     In the above embodiments, as a coil, a type of being wound in two-tiered spiral pattern is used. Alternatively, the coil may be a type of being wound in edgewise winding or aligned winding pattern, or in a circular, rectangular, trapezoidal, semicircular shape, or combination thereof, in addition to an elliptic shape. 
     In the above embodiments, mechanical stripping is used as a method of stripping the coating on the surface of the ends of the coil. Alternatively, it is also possible to use other stripping methods. In addition, the coating on the ends may be stripped in advance prior to forming the core. 
     In the above embodiments, a treatment such as lasering, sandblasting or polishing is applied to the entire portion of the core for allowing an external electrode to be formed thereon to remove components such as a resin component present on its surface to roughen the surface, thereby causing the surface of the entire portion of the core, for allowing the external electrode to be formed thereon, to be reduced in smoothness as compared to a surface therearound. Alternatively, in the first and third embodiments, for example, it is also possible to cause a surface of a portion of only the upper and lower surfaces of the core, for allowing the external electrode to be formed thereon, to be reduced in smoothness as compared to a surface therearound, as illustrated in  FIG. 18 . Further, in the first to fourth embodiments, it is also possible to cause a surface of a part of a portion of the bottom surface of the core, for allowing an external electrode to be formed thereon, to be reduced in smoothness as compared to a surface therearound, as illustrated in  FIG. 19 . Furthermore, it is also possible to cause the entire bottom surface of the core to be reduced in smoothness as compared to other surfaces to thereby form an external electrode on the core. 
     EXPLANATION OF CODES 
     
         
           1 ,  11 ,  21 ,  31 : coil ( 1   a ,  11   a ,  21   a ,  31   a : end) 
           2 ,  12 ,  22 ,  32 : core 
           3 ,  13 ,  23 ,  33 : electrically-conductive paste 
           4 ,  14 ,  24 ,  34 : external electrode
A method of producing a surface-mount inductor including an external electrode having high fixing strength with respect to an element body even in a high-humidity environment. The method includes the steps of: winding an electrically-conductive wire to form a coil; forming a core using a sealant primarily containing a metal magnetic powder and a resin in such a manner as to encapsulate the coil in the sealant while allowing each of opposite ends of the coil to be at least partially exposed on a surface of the core; reducing smoothness of a surface of at least a part of a portion of the core on which an external electrode is formed as compared to a surface therearound; and forming the external electrode on the core in such a manner as to be electrically conducted with the coil.