Patent Publication Number: US-2021193362-A1

Title: Magnetic base body containing metal magnetic particles and electronic component including the same

Description:
TECHNICAL FIELD 
     This application is based on and claims the benefit of priority from Japanese Patent Application Serial No. 2019-230336 (filed on Dec. 20, 2019), the contents of which are hereby incorporated by reference in its entirety. 
     The present invention relates to a magnetic base body containing metal magnetic particles and an electronic component including the magnetic base body. 
     BACKGROUND 
     A metal composite type magnetic base body is known as a magnetic substrate for electronic components such as inductors. In the metal composite type magnetic base body, a large number of metal magnetic particles are bonded to each other by a binder made of a resin material. The metal composite magnetic base body is produced by, for example, making a slurry by mixing and kneading magnetic particles and a resin material, pouring the slurry into a mold, and applying pressure to the slurry in the mold. 
     Magnetic base bodies for electronic components are required to have a high magnetic permeability. Efforts have been made to improve the magnetic permeability of the magnetic base bodies. For example, Japanese Patent Application Publication No. 2016-208002 (“the &#39;002 Publication”) proposes that two or more types of metal magnetic particles having different average particle sizes may be mixed together to increase the magnetic particle filling factor (filling density) in the metal composite type magnetic base body. 
     Japanese Patent Application Publication No. 2018-041955 (“the &#39;955 Publication”) discloses a metal composite type magnetic base body containing metal magnetic particles whose surfaces are coated with a resin layer. The resin layer coating the metal magnetic particle is a single layer made of a macromolecular material and thus serves as an insulator, a binder and a hardener. According to the disclosure of the &#39;002 Publication, the resin layer is in direct contact with the metal magnetic particle which allows any magnetic materials to be used to form the metal magnetic particles. The &#39;002 Publication claims that an inductor with a high magnetic permeability can be consequently provided. The &#39;002 Publication also describes that the metal magnetic particles may be a particle mixture obtained mixing together two or more types of metal magnetic particles having different average particle sizes. 
     Surface of the metallic magnetic particles described in the &#39;002 Publication are not covered with an insulating film. Thus, if the filling factor of the metal magnetic particles in the magnetic substrate is increased, the adjacent metal magnetic particles are likely to come into contact with each other. When the adjacent metal magnetic particles come into contact with each other in the magnetic base body of an inductor. Consequently a magnetic flux concentrates at the position where the metal magnetic particles contact each other when a current flowing through the coil changes. This leads to a local magnetic saturation near the contact position, which is undesirable. Moreover, such a local magnetic saturation in the magnetic base body deteriorates a DC bias characteristic of the inductor. 
     By coating the surface of each metal magnetic particle with an insulating film, it is possible to prevent the adjacent metal magnetic particles from electrically contacting each other. However, as described in the &#39;955 Publication, when all of the two or more types of the metal magnetic particles having different particle sizes coated with the insulating film, the filling factor of the metal magnetic particles in the magnetic base body is decreased by the amount of the insulating film. In other words, the insulating film that coats the metal magnetic particles hampers improvement in the filling factor of the metal magnetic particles. 
     SUMMARY 
     One object of the present invention is to overcome at least a part of the above drawback. One specific object of the invention is to provide a magnetic base body having a high filling factor of metal magnetic particles and a uniform magnetic flux distribution. The challenges achieved by the invention disclosed herein will be apparent through the description of the entire specification. The invention disclosed herein may solve an object other than the above. 
     A magnetic base body according to one aspect of the invention includes a plurality of first metal magnetic particles made of a first magnetic material and having a first average particle size, a coating film that is made of an insulating material having a magnetic permeability lower than the first magnetic material and covers each of the plurality of first metal magnetic particles, a plurality of second metal magnetic particles having a second average particle size smaller than the first average particle size, and a binder that is made of a resin material having a magnetic permeability lower than the first magnetic material and binds the plurality of first metal magnetic particles and the plurality of second metal magnetic particles. Each of the plurality of second metal magnetic particles directly contacts with the binder on its outer surface. 
     In the magnetic base body, the plurality of second metal magnetic particles may be arranged at positions that do not overlap with a virtual center line connecting centers of two adjacent first metal magnetic particles among the plurality of first metal magnetic particles. 
     In the magnetic base body, at least some of the plurality of second metal magnetic particles may be in contact with the coating film. 
     In the magnetic base body, an aspect ratio of the second metal magnetic particle may be in a range of 1.0 to 1.4. 
     In the magnetic base body, the average particle size of the plurality of first metal magnetic particles may be in a range of 10 to 50 μm. 
     In the magnetic base body, the average particle size of the plurality of second metal magnetic particles may be in a range of 0.1 to 5 μm. 
     In the magnetic base body, the average particle size of the plurality of second metal magnetic particles may be 10% or less of the average particle size of the plurality of first metal magnetic particles. 
     In the magnetic base body, the distance between two adjacent particles among the plurality of first metal magnetic particles may be 10 times or less the average particle size of the plurality of second metal magnetic particles. 
     In the magnetic base body, the distance between the two adjacent particles among the plurality of first metal magnetic particles may be in a range of 10 nm to 15 μm. 
     In the magnetic base body, a filling factor of the plurality of first metal magnetic particles and the plurality of second metal magnetic particles together in the magnetic base body may be 85% or more. 
     An electronic component according to another aspect of the invention may include the above magnetic base body and a coil conductor embedded in the magnetic base body. 
     According to the present disclosure it is possible to provide a magnetic base body having a high filling factor of metal magnetic particles and a uniform magnetic flux distribution. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a coil component including a magnetic base body relating to an embodiment of the invention. 
         FIG. 2  is a view schematically showing a cross section of the coil component of  FIG. 1  cut along the line I-I. 
         FIG. 3A  is an enlarged schematic view of a region A of the magnetic base body of  FIG. 2 . 
         FIG. 3B  is an enlarged schematic view of a region corresponding to the region A of the magnetic base body in another embodiment of the invention. 
         FIG. 4  is an enlarged sectional view of a second metal magnetic particle shown in  FIG. 3 . 
         FIG. 5  is a perspective view of a coil element according to another embodiment of the invention. 
         FIG. 6  is a front view of the coil component according to another embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Various embodiments of the present invention will be hereinafter described with reference to the accompanying drawings. Reference characters designating corresponding components are repeated as necessary throughout the drawings for the sake of consistency and clarity. For convenience of explanation, the drawings are not necessarily drawn to scale. 
     An electronic component including a magnetic base body  10  according to one embodiment of the invention will be hereinafter described with reference to  FIGS. 1 to 3 . These drawings show a coil component  1  as an example of the electronic component including the magnetic base body  10 .  FIG. 1  is a perspective view of a coil component according to one embodiment of the invention,  FIG. 2  is a schematic sectional view of the coil component along the line I-I in  FIG. 1 , and  FIG. 3  schematically illustrates a captured image of a region A of the section of  FIG. 2 . 
     In this specification, a “length” direction, a “width” direction, and a “thickness” direction of the coil component  1  correspond to the “L axis” direction, the “W axis” direction, and the “T axis” direction in  FIG. 1 , respectively, unless otherwise construed from the context. 
     The coil component  1  is, for example, an inductor. The inductor is an example of the coil component to which the invention can be applied to. The invention can also be applied to transformers, filters, reactors, and various any other coil components. Advantageous effects of the invention will be more remarkably exhibited if the invention is applied to coil components and any other electronic components to which large current is applied. An inductor used in a DC-DC converter is an example of a coil component to which large current is applied. The invention may be also applied to coupled inductors, choke coils, and any other magnetically coupled coil components, in addition to the inductors used in DC-DC converters. Applications of the coil component  1  are not limited to those explicitly described herein. 
     As illustrated in the accompanying drawings, the coil component  1  includes a metal composite type magnetic base body  10 , a coil conductor  25  embedded in the magnetic base body, an external electrode  21  electrically connected to one end of the coil conductor  25 , and an external electrode  22  electrically connected to the other end of the coil conductor  25 . 
     The illustrated coil component  1  is mounted on a mounting substrate  102   a . The mounting substrate  102   a  may have land portions  103  provided thereon. In the case where the coil component  1  includes two external electrodes  21  and  22 , the mounting substrate  102   a  is provided with two landing portions  103  correspondingly. The coil component  1  may be mounted on the mounting substrate  102   a  by joining the external electrodes  21 ,  22  to the corresponding land portions  103  of the mounting substrate  102   a . A circuit board  102  according to one embodiment includes the mounting substrate  102   a  and the coil component  1  mounted on the mounting substrate  102   a . The circuit board  102  can be installed in various electronic devices. Electronic devices in which the circuit board  102  may be installed include smartphones, tablets, game consoles, electrical components of automobiles, and various other electronic devices. The coil component  1  may be embedded in the mounting substrate  102   a.    
     The magnetic base body  10  has a substantially rectangular parallelepiped shape. In one embodiment of the invention, the magnetic base body  10  has a length (the dimension in the direction L) of 1.0 to 2.6 mm, a width (the dimension in the direction W) of 0.5 to 2.1 mm, and a thickness (the dimension in the direction T) of 0.5 to 1.0 mm. The length of the magnetic base body may be 0.3 to 1.6 mm, the width may be 0.1 to 0.8 mm, and the the thickness may be 0.1 to 0.8 mm. The top surface and the bottom surface of the magnetic base body  10  may be covered with a cover layer. 
     The magnetic base body  10  has a first principal surface  10   a , a second principal surface  10   b , a first end surface  10   c , a second end surface  10   d , a first side surface  10   e , and a second side surface  10   f . The outer surface of the magnetic base body  10  may be defined by these six surfaces. The first principal surface  10   a  and the second principal surface  10   b  are opposed to each other, the first end surface  10   c  and the second end surface  10   d  are opposed to each other, and the first side surface  10   e  and the second side surface  10   f  are opposed to each other. As shown in  FIG. 1 , the first principal surface  10   a  lies on the top side in the magnetic base body  10 , and therefore, the first principal surface  10   a  may be herein referred to as “the top surface.” Similarly, the second principal surface  10   b  may be referred to as “the bottom surface.” The coil component  1  is disposed such that the second principal surface  10   b  faces a circuit board  102 , and therefore, the second principal surface  10   b  may be herein referred to as a “mounting surface.” The top-bottom direction of the coil component  1  refers to the top-bottom direction (the direction along the axis T) in  FIG. 1 . 
     The external electrode  21  is provided on the first end surface  10   c  of the magnetic base body  10 . The external electrode  22  is provided on the second end surface  10   d  of the magnetic base body  10 . As shown, these external electrodes may extend to the bottom surface of the magnetic base body  10 . The shapes and positions of the external electrodes are not limited to the illustrated example. For example, both of the external electrodes  21 ,  22  may be provided on the bottom surface  10   b  of the magnetic base body  10 . The external electrodes  21  and  22  are separated from each other in the length direction. 
     The magnetic base body  10  will be further described in detail with reference to  FIG. 3A . The schematic cross section of the magnetic base body  10  is shown in  FIG. 3A .  FIG. 3A  schematically shows a scanning electron microscope (SEM) image of the region A of the cross section of the magnetic base body  10  taken by SEM with a magnification ratio of 2000. As the scanning electron microscope, S-4300 available from Hitachi High Technologies Corporation can be used. The region A may be an arbitrary region in the magnetic base body  10 . The distributions of a plurality of first metal magnetic particles  31  and a plurality of second metal magnetic particles  32  on the cross section of the magnetic base body  10  are examined with a scanning electron microscope suitably with a magnification ratio of 1000 to 3000, for example. When the cross section of the magnetic base body  10  is observed, the magnification ratio of the scanning electron microscope can be adjusted between 1000 to 3000 as appropriate. 
     As shown in the drawing, the magnetic base body  10  includes the plurality of the first metal magnetic particles  31 , the plurality of second metal magnetic particles  32 , and a binder  33 . Each of the first metal magnetic particles  31  is covered with a coating film  41 . The binder  33  binds together the plurality of first metal magnetic particles  31  and the plurality of second metal magnetic particles  32 . In other words, the magnetic base body  10  includes the binder  33  and the plurality of first metal magnetic particles  31  and the plurality of second metal magnetic particles  32  bound to each other by the binder  33 . 
     In one embodiment, the first metal magnetic particles  31  and the second metal magnetic particles  32  are fine particles made of soft magnetic materials. The first metal magnetic particles  31  are made of a first magnetic material, and the second metal magnetic particles  32  are made of a second magnetic material. The first and second magnetic materials may be a crystalline or amorphous metal or alloy containing at least one element selected from the group consisting of iron (Fe), nickel (Ni) and cobalt (Co). The first and second metal magnetic materials may further contain at least one element selected from the group consisting of silicon (Si), chromium (Cr) and aluminum (Al). The first and second metal magnetic particles  31  and  32  may be pure iron particles containing Fe and unavoidable impurities, or particles of an Fe-based amorphous alloy containing iron (Fe). The Fe-based amorphous alloy includes, for example, Fe—Si alloy, Fe—Si—Al alloy, Fe—Si—Cr—B alloy, Fe—Si—B—C alloy, and Fe—Si—P—B—C alloy. The first magnetic material and the second magnetic material may be the same magnetic material or different magnetic materials from each other. The second magnetic material may be a magnetic material having a magnetic permeability smaller than that of the first magnetic material. When the first magnetic material and the second magnetic material are the same magnetic material, the average particle size of the second metal magnetic particles  32  is smaller than the average particle size of the first metal magnetic particles  31 , as will be described later. Thus the magnetic permeability of the second metal magnetic particles measured in a single particle is smaller than the magnetic permeability of the first metal magnetic particles  31  measured in a single particle. The Fe content ratio in the second magnetic material may be higher than the Fe content ratio in the first magnetic material. 
     The average particle size of the plurality of first metal magnetic particles  31  is hereinafter referred to as a first average particle size and the average particle size of the plurality of second metal magnetic particles  32  is hereinafter referred to as a second average particle size. The average particle size of the first metal magnetic particles  31  here referrers to the average particle size of the first metal magnetic particles  31  that do not have the coating film  41  thereon. The second average particle size of the second metal magnetic particles  32  is smaller than the first average particle size of the first metal magnetic particles  31 . The average particle size of the metal magnetic particles (for example, the first metal magnetic particles  31  and the second metal magnetic particles  32 ) included in the magnetic base body  10  is determined in the following manner. The magnetic base body is cut along the thickness direction (the T direction) to expose the cross section. The cross section is photographed using a scanning electron microscope (SEM) with a magnification ratio of 1000 to 3000, and the photograph is used to obtain a particle size distribution. The particle size distribution is used to determine the average particle size. For example, the 50th percentile of the particle size distribution obtained based on the SEM image can be used as the average particle size of the metal magnetic particles. The first metal magnetic particles  31  in the magnetic base body  10  may have the average particle size of 10 to 50 μm, and the second metal magnetic particles  32  may have the average particle size of 1 to 5 μm. The average particle size of the second metal magnetic particles  32  may be 10% or less of the average particle size of the first metal magnetic particles  31 . The particle size distribution of the second metal magnetic particles  32  obtained based on the SEM image may exhibit two or more peaks. In other words, the second metal magnetic particles  32  may be a particle mixture obtained mixing together two types of metal magnetic particles having different average particle sizes. 
     The binder  33  is, for example, a thermosetting resin having a high insulating property. A resin material used as the material of the binder  33  has a smaller magnetic permeability than the first magnetic material. Examples of the resin material of the binder  33  include an epoxy resin, a polyimide resin, a polystyrene (PS) resin, a high-density polyethylene (HDPE) resin, a polyoxymethylene (POM) resin, a polycarbonate (PC) resin, a polyvinylidene fluoride (PVDF) resin, a phenolic resin, a polytetrafluoroethylene (PTFE) resin, or a polybenzoxazole (PB  0 ) resin. 
     As mentioned above, each of the first metal magnetic particles  31  is covered with the coating film  41 . The coating film  41  is formed of an insulating material having an excellent insulating property. The insulating material for the coating film  41  may be an organic material or an inorganic material. The material of the coating film  41  has a smaller magnetic permeability than the first magnetic material. As the organic material for the coating film  41 , the second insulating layer  42 , and the third insulating layer  43 , epoxy, phenol, silicone, polyimide, or any other thermosetting resin can be used. When silicone is used as the organic material for the coating film  41 , the first metal magnetic particles  31  are immersed in a silicone resin solution in which a silicone resin is dissolved in a petroleum-based organic solvent such as xylene, and then the organic solvent is evaporated from the resin solution to form the coating film  41  on the surfaces of the first metal magnetic particles  31 . In order to improve the uniformity of the film thickness, the silicone resin solution may be stirred, if necessary. As the inorganic material for the coating film  41 , phosphate, borate, chromate, glass (for example, SiO 2 ), and metal oxide (for example, Fe 2 O 3  or Al 2 O 3 ) can be used. The coating film  41  may be formed by a powder mixing method, an immersion method, a sol-gel method, a CVD method, a PVD method, or various other known methods. The coating film  41  may be an oxide film formed by oxidizing metal or alloy contained in the first metal magnetic particles  31 . This oxide film is formed by performing heat treatment at 400 to 800° C. for about 20 minutes to 3 hours in a low oxygen concentration atmosphere having an oxygen concentration of about 500 to 1000 ppm. 
     It is preferable that the coating film  41  be formed to cover the entire surface of the first metal magnetic particle  31  so that the first metal magnetic particles  31  do not directly contact with other metal magnetic particles, which prevents the particles from being shorted out. Note that the coating film  41  may sometime cover only a part of the surface of the first metal magnetic particle  31 , not the entire surface. In the manufacturing process of the magnetic base body  10 , a part of the coating film  41  may incidentally come off from the first metal magnetic particle  31 . In such a case, the coating film  41  will cover only a part of the surface of the first metal magnetic particle, not the entire surface. 
     A thickness of the coating film  41  may be 5 nm to 1 nm. The material for the coating film  41  may include two or more layers made of different materials. The coating film  41  can be distinguished from the first metal magnetic particles  31  and the binder  33  based on the difference in brightness in the SEM image. The coating film  41  can also be distinguished from the binder  33  by SEM-EDS mapping. 
     As shown in  FIG. 3A , the plurality of second metal magnetic particles  32  are not coated with a film such as a coating film  41 . Therefore, each of the plurality of second metal magnetic particles directly contacts with the binder  33  on its outer surface. As described above, the first metal magnetic particles  31  are in contact with the binder  33  via the coating film  41 , while the second metal magnetic particles  32  are in direct contact with the binder  33 . 
     In the embodiment shown in  FIG. 3A , the second metal magnetic particles  32  are arranged at positions that do not overlap with a virtual center line connecting the centers of two adjacent first metal magnetic particles  31 . In this case, most of the second metal magnetic particles  32  are arranged in regions called a triple point surrounded by three or more first metal magnetic particles  31 . The centers of the first metal magnetic particle  31  may refer to the geometric centers of gravity of the first metal magnetic particles  31 .  FIG. 3A  uses broken lines to identify the imaginary lines connecting the center of gravity of the first metal magnetic particle  31  that is arranged at approximately the center of the field of view and the centers of gravity of six first metal magnetic particles  31  adjacent to the first metal magnetic particle  31  at the center of the field of view. The second metal magnetic particles  32  do not overlap with any of the six virtual lines. One or more of the plurality of second metal magnetic particles  32  may be in contact with the coating films  41  covering the first metal magnetic particles  31 . 
       FIG. 3B  shows a cross section of the region A of the magnetic base body  10  according to another embodiment of the invention. In this embodiment, the second metal magnetic particles  32  are arranged at positions that overlap with the virtual center line connecting the centers of the adjacent first metal magnetic particles  31  as shown in  FIG. 3B , Thus, even if the distance between the adjacent first metal magnetic particles  31  becomes large, a region between the adjacent first metal magnetic particles  31  is filled with the second metal magnetic particles  32 , which prevents decrease in the filling factor. 
     As shown in  FIG. 4 , the second metal magnetic particles  32  may have a flattened circular shape. For example, the aspect ratio of each of the second metal magnetic particles  32  may be in the range of 1.0 to 1.4. When the size of the second metal magnetic particle is less than 1 nm, the aspect ratio of the second metal magnetic particle may be in the range of 1.0 to 1.1. The aspect ratio of the second metal magnetic particle  32  is represented by a ratio of a shortest axis d 2  of the second metal magnetic particle  32  to a longest axis d 1  (in other words, d 1 /d 2 ). The aspect ratio of the first metal magnetic particle  31  may be smaller than the aspect ratio of the second metal magnetic particle. By setting the aspect ratios of the first metal magnetic particle  31  and the second metal magnetic particle  32  to the above range, for example, distortion inside each metal magnetic particle generated through the manufacturing process of the magnetic base body  10  can be reduced. Consequently it is possible to prevent deterioration of the relative magnetic permeability. 
     In one embodiment, the plurality of first metal magnetic particles  31  may be arranged such that the distance between adjacent first metal magnetic particles  31  is 10 times or less the average particle size of the second metal magnetic particles  32 . Alternatively, in one embodiment, the distance between the adjacent first metal magnetic particles  31  is 5 times or less the average particle size of the second metal magnetic particles  32 . Alternatively, in one embodiment, the distance between the adjacent first metal magnetic particles  31  is 3 times or less the average particle size of the second metal magnetic particles  32 . For example, the distance between adjacent first metal magnetic particles  31  may be in the range of 10 nm to 15 μm. The distances between one first metal magnetic particle  31  and adjacent first metal magnetic particles  31  may be measured for each of the first metal magnetic particles  31  in the field of view in the SEM image of the cross section of the magnetic base body  10 , and the average of the measured distances can be herein defined as the distance between the adjacent first metal magnetic particles  31 . The distance between the two adjacent first metal magnetic particles  31  referrers to the closest distance between the two adjacent first metal magnetic particles. In most cases, the closest distance between the two adjacent first metal magnetic particles  31  is a distance between the two adjacent first metal magnetic particles along the virtual center line connecting the centers of the two adjacent first metal magnetic particles  31 . When the second metal magnetic particles  32  are arranged on the virtual center line connecting the centers of the adjacent first metal magnetic particles  31 , the distance between the adjacent first metal magnetic particles  31  may exceed three times the average particle size of the plurality of second metal magnetic particles  32  depending on the particle size of the second metal magnetic particles  32 . According to one embodiment of the invention, the second metal magnetic particles  32  are arranged at positions that do not overlap with the virtual center line connecting the centers of the adjacent first metal magnetic particles  31 , so that the plurality of first metal magnetic particles  31  can make the distance between the adjacent first metal magnetic particles  31  three times or less the average particle size of the second metal magnetic particles  32 . 
     In one embodiment, the filling factor of the first metal magnetic particles  31  and the second metal magnetic particles  32  together in the magnetic base body  10  is 85 vol % or more. The filling factor of the first metal magnetic particles  31  and the second metal magnetic particles  32  together in the magnetic base body  10  may be defined as a ratio of the area occupied by the metal magnetic particles (that is, a sum of the total area occupied by the plurality of first metal magnetic particles  31  and the total area of the plurality of second metal magnetic particles  32 ) to the whole area of the field of view in the SEM image of the cross section of the magnetic base body  10 . In conventional magnetic base body, the upper limit of the filling factor was about 80 vol %. Whereas in the magnetic base body  10  of the present application, the second metal magnetic particles  32  are not coated with the coating film, and the second metal magnetic particles  32  are not situated on the virtual center line connecting the centers of the adjacent first metal magnetic particles  31 . Thus the filling factor of the first metal magnetic particles  31  and the second metal magnetic particles  32  in the magnetic base body  10  can be increased, more specifically, a filling factor of 85 vol % or more can be realized. 
     An example of manufacturing method of the coil component  1  according to one embodiment of the invention will now be described. The following describes a method of manufacturing the coil component  1  using a compression molding process. The first metal magnetic particles  31  and the second metal magnetic particles  32  are first prepared. The coating film  41  is then formed on the surfaces of the first metal magnetic particles  31 . The coating film  41  is formed on the surface of each of the plurality of first metal magnetic particles  31  by, for example, a sol-gel method. Subsequently, the first metal magnetic particles  31  coated with the coating film  41  and the second metal magnetic particles  32  are mixed to obtain a mixture of particles (mixed powder). The mixture of particles can be obtained, for example, by mixing 75 wt % of the first metal magnetic particles  31  and 25 wt % of the second metal magnetic particles  32 . The mixing ratio of the first metal magnetic particles  31  and the second metal magnetic particles  32  in the mixed particles can be adequately changed. For example, the mixture of particles may be a mixture of 90 wt % of the first metal magnetic particles  31  and 10 wt % of the second metal magnetic particles  32 . As another example, the mixture of particles may be a mixture of 60 wt % of the first metal magnetic particles  31  and 40 wt % of the second metal magnetic particles  32 . The first metal magnetic particles  31  in the mixture of particles may be in the range of 60 to 90 wt %, and the second metal magnetic particles  31  in the mixture may be in the range of 10 to 40 wt %. 
     Next, the above mixture of particles, a resin material that serves as the binder  33  after curing, and a diluting solvent are mixed and kneaded to prepare a slurry. The first metal magnetic particles  31  coated with the coating film  41  and the second metal magnetic particles  32  are dispersed in the slurry. Tarpineol (TPO) can be used as the diluting solvent. The slurry is then applied on a substrate such as a PET film in the form of a sheet, and the applied slurry is dried to volatilize the diluting solvent. Through the above process, a magnetic sheet in which the first metal magnetic particles  31  coated with the coating film  41  and the second metal magnetic particles  32  are dispersed in the resin can be obtained. 
     Subsequently, the coil conductor  25  prepared in advance is placed in a molding die, the above magnetic sheet is then placed in the molding die, and a molding pressure is applied thereto at a temperature of, for example, 50 to 150° C., and then further heated from 150 to 400° C. for curing. In this way, the magnetic base body  10  including the coil conductor  25  thereinside can be obtained. The heat treatment for obtaining the magnetic base body  10  may be performed in two steps as described above or in one step. When the heat treatment is performed in one step, molding and curing are performed during the heat treatment. In the magnetic base body  10 , the resin contained in the slurry is cured and serves as the binder  33 . The magnetic base body  10  may be warm molded at a temperature of, for example, around 80° C. The molding pressure for molding is, for example, 50 to 200 MPa. The molding pressure can be appropriately adjusted to obtain a desired filling factor. The molding pressure is, for example, 100 MPa. 
     Next, a conductor paste is applied to a surface of the magnetic base body  10 , which is produced in the above-described manner, to form the external electrodes  21  and  22 . The external electrode  21  is electrically connected to one end of the coil conductor  25  inside the magnetic base body  10 , and the external electrode  22  is electrically connected to the other end of the coil conductor  25  inside the magnetic base body  10 . The coil component  1  is obtained, as described above. 
     The following describes a coil component  101  according to another embodiment of the invention with reference to  FIG. 5 . The coil component  101  is a planar coil. As shown in  FIG. 6 , the coil component  101  according to the embodiment includes a magnetic base body  110 , an insulating plate  150  provided in the magnetic base body  110 , a coil conductor  125  provided on upper and lower surfaces of the insulating plate  150  in the magnetic base  110 , an external electrode  121  provided on the magnetic base body  110 , and an external electrode  122  provided on the magnetic base body  110  at a position apart from the external electrode  121 . 
     In the embodiment, similarly to the above-described magnetic base body  10 , the magnetic base body  110  includes the plurality of first metal magnetic particles  31 , the plurality of second metal magnetic particles  32 , and the binder  33 . The insulating plate  150  is made of an insulating material and has a plate-like shape. The insulating material used for the insulating plate  150  may be magnetic. The magnetic material used for the insulating plate  150  is, for example, a composite magnetic material containing a binder and metal magnetic particles. 
     In the embodiment shown, the coil conductor  125  includes a coil conductor  125   a  formed on the upper surface of the insulating plate  150  and the coil conductor  125   b  formed on the lower surface of the insulating plate  150 . The coil conductor  125   a  and the coil conductor  125   b  are connected by a via (not shown). The coil conductor  125   a  is formed in a predetermined pattern on the upper surface of the insulating plate  150 , and the coil conductor  125   b  is formed in a predetermined pattern on the lower surface of the insulating plate  150 . An insulating film may be provided on surfaces of the coil conductors  225   a  and  225   b . The coil conductor  125  can be provided in various shapes. When seen from above, the coil conductor  125  has, for example, a spiral shape, a meander shape, a linear shape or a combined shape of these. 
     In one embodiment of the invention, the insulating plate  150  has a larger resistance than the magnetic base body  110 . Thus, even when the insulating plate  150  has a small thickness, electric insulation between the coil conductor  125   a  and the coil conductor  125   b  can be ensured. 
     A lead-out conductor  127  is provided on one end of the coil conductor  125   a , and a lead-out conductor  126  is provided on one end of the coil conductor  125   b . In this manner, the coil conductor  125  is electrically coupled to the external electrode  121  via the lead conductor  126  and is electrically coupled to the external electrode  122  via the lead conductor  127 . 
     Next, a description is given of an example of a manufacturing method of the coil component  101 . To start with, an insulating plate made of a magnetic material and shaped like a plate is prepared. Next, a photoresist is applied to the top surface and the bottom surface of the insulating plate, and then conductor patterns are transferred onto the top surface and the bottom surface of the insulating plate by exposure, and development is performed. As a result, a resist having an opening pattern for forming a coil conductor is formed on each of the top surface and the bottom surface of the insulating plate. For example, the conductor pattern formed on the top surface of the insulating plate corresponds to the coil conductor  125   a  described above, and the conductor pattern formed on the bottom surface of the insulating plate corresponds to the coil conductor  125   b  described above. A through-hole for the via is formed in the insulating plate. 
     Subsequently, plating is performed to fill each of the opening patterns with a conductive metal. Next, etching is performed to remove the resists from the insulating plate, so that the coil conductors are formed on the top surface and the bottom surface of the insulating plate. Further, the through-hole in the insulating plate is filled with a conductive metal to form the via that connects the coil conductor  125   a  and the coil conductor  125   b.    
     A magnetic base body is subsequently formed on both surfaces of the insulating plate where the coil conductors have formed thereon. This magnetic base body corresponds to the magnetic base body  110  described above. To form the magnetic base body  110 , magnetic sheets are first fabricated. The magnetic sheet can be produced in the same manner as the magnetic sheet used in the manufacturing process of the coil component  1 . Two magnetic sheets are prepared, and the above-described coil conductor is placed between the two magnetic sheets and pressure is applied to them while they are heated. In this way, a laminated body is fabricated. The method for producing the laminated body is not limited to the above. In another method of producing the laminated body, an insulating plate on which a coil conductor has been fabricated is first placed in the molding die. Subsequently the mixture of particles in which the plurality of first metal magnetic particles  31  coated with the coating film  41  and the plurality of second metal magnetic particles  32  are mixed, and the resin material serving as the binder  33  are placed in the molding die and then they are pressurized while being heated to obtain the laminated body. The laminated body may be warm molded at a temperature of 50 to 150° C., for example, around 80° C. The molding pressure for molding is, for example, 50 to 200 MPa. The molding pressure can be appropriately adjusted to obtain a desired filling factor. The molding pressure is, for example, 100 MPa. Next, the laminated body is subjected to heat treatment at a curing temperature (for example, 150 to 400° C.) of the resin. In this way, the magnetic base body  110  containing the coil conductor  125  can be obtained. External electrodes  121 ,  122  are provided on the external surface of the magnetic base body at predetermined positions. In this manner, the coil component  101  is fabricated. 
     The following describes a coil component  201  according to another embodiment of the invention with reference to  FIG. 6 . The coil component  201  relating to one embodiment of the present invention is a winding inductor. As shown, the coil component  201  includes a drum core  210 , a winding wire  220 , a first external electrode  231   a , and a second external electrode  232   a . The drum core  210  includes a winding core  211 , a flange  212   a  having a rectangular parallelepiped shape and provided on one end of the winding core  211 , and a flange  212   b  having a rectangular parallelepiped shape and provided on the other end of the winding core  211 . The winding wire  220  is wound on the winding core  211 . The winding  220  is a conductor wire made of a metal material having excellent electrical conductivity covered with an insulation coating therearound. The first external electrode  231   a  extends along the bottom surface of the flange  212   a , and the second external electrode  232   a  extends along the bottom surface of the flange  212   b.    
     Similarly to the magnetic base body  10 , the drum core  210  includes the plurality of first metal magnetic particles  31 , the plurality of second metal magnetic particles  32 , and the binder  33 . For example, the mixture of particles obtained by mixing the plurality of first metal magnetic particles  31  coated with the coating film  41  and the plurality of second metal magnetic particles  32 , and a resin material serving as the binder  33  are mixed, then the mixed material is molded and cured to produce the drum core  210 . The drum core  210  may be warm molded at a temperature of 50 to 150° C., for example, around 80° C. The molding pressure for molding is, for example, 50 to 200 MPa. The molding pressure can be appropriately adjusted to obtain a desired filling factor. The molding pressure is, for example, 100 MPa. The curing temperature may be, for example, 150 to 400° C. The coil component  201  is produced by winding the winding wire  220  around the drum core  210 , connecting one end of the winding wire  220  to the first external electrode  231   a , and connecting the other end to the second external electrode  232   a.    
     Advantageous effects of the above embodiments will be now described. In the magnetic base body  10  according to the above embodiment, the first metal magnetic particles  31  are covered with the coating film  41  that has a low magnetic permeability, so that magnetic contact between the adjacent first metal magnetic particles  31  is prevented. In this way, it is possible to prevent the magnetic flux from concentrating at the contact points between the the first metal magnetic particles  31  due to the magnetic connection therebetween. Further, the coating film  41  may be made of an insulating material having an excellent insulating property. In this case, electrical connection between the adjacent first metal magnetic particles  31  is also prevented, which prevents increase in the eddy current. 
     In the above embodiment, the second metal magnetic particles  32  are provided such that the outer surfaces thereof are in contact with the binder  33 . That is, unlike the first metal magnetic particles  31 , the second metal magnetic particles  32  are not covered with the coating film  41  or any other insulating film. In this way, the filling factor of the metal magnetic particles (sum of the first metal magnetic particles  31  and the second metal magnetic particles  32 ) in the magnetic base body  10  can be increased as compared with conventional coil components in which the insulating film is provided on the surfaces of the second metal magnetic particles  32 . 
     In the above embodiment, the second average particle size, which is the average particle size of the second metal magnetic particles  32 , is smaller than the first average particle size, which is the average particle size of the first metal magnetic particles  31 . Thus a magnetic flux generated by change in the current flowing through the coil conductor  25  is more likely to pass through the first metal magnetic particles  31  than the second metal magnetic particles  32 . Consequently, even if the second metal magnetic particles  32  are magnetically connected to each other, the degree of concentration of the magnetic flux at the contact points therebetween is less than the degree of concentration of the magnetic flux caused by the magnetic connection between the first metal magnetic particles. Therefore, even though the second metal magnetic particles  32  are not provided with an insulating coating film, the uniformity of the magnetic flux is less affected. In one embodiment, by arranging the second metal magnetic particles between the first metal magnetic particles, it is possible to more reliably prevent the first metal magnetic particles from being magnetically connected to each other. Further, by setting the distance between the two adjacent first metal magnetic particles to 10 times or less the average particle size of the plurality of second metal magnetic particles, it is possible to prevent magnetic connection between the first metal magnetic particles without decreasing the filling factor. 
     In the above embodiment, when the second metal magnetic particles  32  are situated on the virtual center line connecting the centers of the adjacent first metal magnetic particles  31 , the distance between the adjacent first metal magnetic particles  31  is increased by the size of the second metal magnetic particles  32 . The distance (shortest distance or shortest interval) between two adjacent first metal magnetic particles  31  referrers to the distance or interval between the two adjacent first metal magnetic particles  31  along the virtual center line connecting the centers of the two adjacent first metal magnetic particles  31 . Therefore, as shown in  FIG. 3A , by disposing each of the second metal magnetic particles  32  at positions not overlapping with the virtual center line connecting the centers of the two adjacent first metal magnetic particles  31 , it is possible to reduce the distance between the adjacent first metal magnetic particles  32 . Accordingly, by providing each of the second metal magnetic particles  32  at a position that does not overlap with the virtual center lines, the filling factor of the metal magnetic particles in the magnetic base body  10  can be increased. Since a large number of the second metal magnetic particles  32  is provided, some of them may be situated between the adjacent first metal magnetic particles  31 . However, as long as the proportion of the second metal magnetic particles  32  arranged on the virtual center line connecting the centers of the first metal magnetic particles  31  is low, their effect on the filling factor of the metal magnetic particles in the magnetic base body  10  can be ignored. In one embodiment of the invention, a sectional SEM image of a region of the magnetic base body  10  that includes five or more first metal magnetic particles  31  is acquired. It can be determined that the second metal magnetic particles  32  are arranged at positions that do not overlap with the virtual center lines connecting the centers of the adjacent first metal magnetic particles  31  when it is found for each of all the first metal magnetic particles  31  in the SEM image that the second metal magnetic particles  32  do not exist on the virtual center lines connecting the center of the first metal magnetic particle  31  and the centers of the adjacent first metal magnetic particles  31 . 
     In conventional magnetic base bodies, metal magnetic particles having a relatively small particle size are also coated with an insulating film, and a resin film is often used as the insulating film. In this case, the insulating films that cover small-diameter metal magnetic particles (which correspond to the “second metal magnetic particle  32 ” in the present application) serve as a primer, which makes the small-diameter metal magnetic particles be more easily bonded to large-diameter metal magnetic particles (which correspond to the “first magnetic metal magnetism  31 ” in the present application). As a result, the small-diameter metal magnetic particles are likely to situate between the large-diameter metal magnetic particles, and the distance between the large-diameter metal magnetic particles is increased. Whereas in the above embodiment, the second metal magnetic particles  32  are in direct contact with the binder  33  without being covered with the insulating film so that the second metal magnetic particles  32  more easily move inside the binder  33  before curing at the time of molding and tend to be pushed out to regions where do not overlap with the virtual center lines (for example, the region at the triple point of the first metal magnetic particles  31 ) during pressure molding. In this way, when the second metal magnetic particles  32  are not coated with an insulating material, each of the second metal magnetic particles  32  can be easily placed at a position that does not overlap with the virtual center lines connecting the centers of the adjacent first metal magnetic particles  31 . 
     In a magnetic base body that contains two or more types of metal magnetic particles having different average particle sizes from each other, metal magnetic particles having a larger average particle size have a higher magnetic permeability than metal particles having a smaller average particle size. Therefore, magnetic flux tends to pass through a path with a high proportion of metal magnetic particles having a larger average particle size. According to the above embodiment, the first metal magnetic particles  31  having a relatively large average particle size are coated with the coating film  41  made of a material with a low magnetic permeability, while the second metal magnetic particles having a relatively small average particle size are not provided with such a coating film. Therefore, as compared with the case where the coating film is provided on the second metal magnetic particles  32 , the magnetic flux generated when a current flows through the coil is more likely to pass through the path where the second metal magnetic particles  32  exist. In this way, it is possible to make the magnetic flux distribution in the magnetic base body  10  more uniform. Consequently, concentration of the magnetic flux on the first metal magnetic particles  13  can be decreased and therefore the DC bias characteristic of the coil component  1  can be improved. 
     The operation and effects described about the magnetic base body  10  also applies to other magnetic base bodies (for example, the magnetic base bodies  110  and  210 ) according to the embodiment of the invention. Further, the operation and effects described about the coil component  1  also applies to other coil components (for example, the coil components  101  and  201 ) according to the embodiment of the invention. 
     The dimensions, materials, and arrangements of the constituent elements described herein are not limited to those explicitly described for the embodiments, and these constituent elements can be modified to have any dimensions, materials, and arrangements within the scope of the present invention. Furthermore, constituent elements not explicitly described herein can also be added to the described embodiments, and it is also possible to omit some of the constituent elements described for the embodiments.