COIL COMPONENT AND METHOD OF MANUFACTURING THE SAME

Provided is a coil component having an increased adhesion strength between an electrode layer and a plating layer included in an external electrode. A coil component according to an embodiment of the present invention includes; a base body; an external electrode provided on a surface of the base body; and a conductor electrically connected to the external electrode and wound around a coil axis, wherein the external electrode includes a first electrode layer, a second electrode layer covering the first electrode layer, and a plating layer covering the second electrode layer, wherein each of the first electrode layer and the second electrode layer contains a plurality of fillers and a resin, and wherein a proportion of a volume of the plurality of fillers in the second electrode layer is larger than a proportion of a volume of the plurality of fillers in the first electrode layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims the benefit of priority from Japanese Patent Application Serial No. 2020-034486 (filed on Feb. 29, 2020) and Japanese Patent Application Serial No. 2020-184044 (filed on Nov. 4, 2020), the contents of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to a coil component and a method of manufacturing the coil component.

BACKGROUND

A conventional coil component such as an inductor typically includes a magnetic base body made of a magnetic material, a conductor provided in the magnetic base body and wound around a coil axis, and an external electrode connected to an end portion of the conductor. Such a coil component is mounted on a substrate, for example, through electric connection between the external electrode and the substrate soldered to each other, for use as a component of various electronic devices. An example of the conventional coil component is disclosed in Japanese Patent Application Publication No. 2017-120809 (“the '809 Publication”). In the coil component of the '809 Publication, the external electrode includes an electrode layer formed by heat-treating a conductive paste containing metal fillers and a resin.

When the electrode layer is formed using the conductive paste containing metal fillers and a resin, it is common to provide a plating layer on the electrode layer. In order to improve the tight adhesion between the electrode layer and the base material, it is desirable to increase the proportion of the resin contained in the electrode layer. However, when the electrode layer contains a larger proportion of resin, a structural defect tends to form in the plating layer being grown on the electrode layer. As a result, the adhesion strength between the electrode layer and the plating layer may be reduced.

SUMMARY

One object of the present invention is to provide a coil component having an increased adhesion strength between the electrode layer and the plating layer included in the external electrode. Other objects of the present invention will be made apparent through the entire description in the specification.

A coil component according to an embodiment of the present invention is a coil component comprising: a base body; an external electrode provided on a surface of the base body; and a conductor electrically connected to the external electrode and wound around a coil axis, wherein the external electrode includes a first electrode layer, a second electrode layer covering the first electrode layer, and a plating layer covering the second electrode layer, wherein each of the first electrode layer and the second electrode layer contains a plurality of fillers and a resin, and wherein a proportion of a volume of the plurality of fillers in the second electrode layer is larger than a proportion of a volume of the plurality of fillers in the first electrode layer.

In one embodiment of the present invention, a proportion of a volume of the resin in the first electrode layer may be larger than a proportion of a volume of the resin in the second electrode layer.

In one embodiment of the present invention, adhesion strength between the first electrode layer and the base body may be higher than adhesion strength between the second electrode layer and the base body.

In one embodiment of the present invention, a proportion of the resin in the first electrode layer may be 65 vol % or smaller.

In one embodiment of the present invention, the plurality of fillers may be formed of a metal material.

In one embodiment of the present invention, the plurality of fillers may include first fillers and second fillers, and an aspect ratio of the first fillers may be 2 or smaller, and an aspect ratio of the second fillers may be 3 or larger.

In one embodiment of the present invention, among the plurality of fillers contained in the second electrode layer, a proportion of the first fillers may be 40 vol % to 70 vol %, and a proportion of the second fillers may be 30 vol % to 60 vol %.

In one embodiment of the present invention, at least a part of the plurality of fillers contained in the second electrode layer may be metal-bonded to the plating layer.

In one embodiment of the present invention, the plating layer may include a first plating layer contacting with the second electrode layer and formed of Ni.

In one embodiment of the present invention, the first plating layer may cover an entire outer surface of the second electrode layer.

In one embodiment of the present invention, the resin may be a thermosetting resin.

One embodiment of the present invention relates to a circuit board comprising any one of the above electronic components. One embodiment of the present invention relates to an electronic device comprising the above circuit board.

ADVANTAGEOUS EFFECTS

The present invention is a coil component having an increased adhesion strength between the electrode layer and the plating layer included in the external electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Various embodiments of the present invention will be hereinafter described with reference to the accompanying drawings. The constituents common to more than one drawing are denoted by the same reference signs throughout the drawings. For convenience of explanation, the drawings are not necessarily drawn to scale.

A coil component1according to one embodiment of the present invention will be hereinafter outlined with reference toFIG. 1.FIG. 1is a perspective view schematically showing the coil component1. As shown inFIG. 1, the coil component1includes a base body10, a coil conductor (a conductor)25provided in the base body10, an external electrode21disposed on a surface of the base body10, and an external electrode22disposed on the surface of the base body10at a position spaced apart from the external electrode21.

In this specification, a “length” direction, a “width” direction, and a “height” direction of the coil component1correspond to the “L axis” direction, the “W axis” direction, and the “T axis” direction inFIG. 1, respectively, unless otherwise construed from the context.

The coil component1is mounted on a circuit board (not shown). The circuit board has two land portions provided thereon. The coil component1may be mounted on the circuit board by bonding the external electrodes21,22to the land portions corresponding to the external electrodes21,22, respectively. The circuit board can be installed in electronic devices such as smartphones, tablets, game consoles, and various others. The circuit board may also be installed in an electric component of an automobile, which is a sort of electronic device.

The coil component1may be applied to inductors, transformers, filters, reactors, and various other coil components having the external electrodes21,22on the surface of the base body10. The coil component1may also be applied to coupled inductors, choke coils, and various other magnetically coupled coil components. Applications of the coil component1are not limited to those explicitly described herein.

The base body10is made of an insulating material. In one embodiment, the base body10is made mainly of a magnetic material and formed in a rectangular parallelepiped shape. In the coil component1according to one embodiment of the invention, the base body10has a length (the dimension in the L axis direction) of 1.0 mm to 4.5 mm, a width (the dimension in the W axis direction) of 0.5 mm to 3.2 mm, and a height (the dimension in the T axis direction) of 0.5 mm to 5.0 mm. The dimensions of the base body10are not limited to those specified herein. The term “rectangular parallelepiped” or “rectangular parallelepiped shape” used herein is not intended to mean solely “rectangular parallelepiped” in a mathematically strict sense.

The base body10has a first principal surface10a,a second principal surface10b,a first end surface10c,a second end surface10d,a first side surface10e,and a second side surface10f.These six surfaces define the outer periphery of the base body10. The first principal surface10aand the second principal surface10bare at the opposite ends in the height direction, the first end surface10cand the second end surface10dare at the opposite ends in the length direction, and the first side surface10eand the second side surface10fare at the opposite ends in the width direction.

As shown inFIG. 1, the first principal surface10alies on the top side of the base body10, and therefore, the first principal surface10amay be herein referred to as “the top surface.” Similarly, the second principal surface10bmay be referred to as “the bottom surface.” The coil component1is disposed such that the first principal surface10afaces the circuit board, and therefore, the first principal surface10amay be herein referred to as “the mounting surface.” The top-bottom direction of the coil component1mentioned herein refers to the top-bottom direction inFIG. 1.

Next, the base body10which is magnetic will be further described with reference toFIG. 2.FIG. 2is an enlarged sectional view schematically showing, on an enlarged scale, a sectional surface of the base body10. As shown in the drawing, the base body10contains a plurality of first metal magnetic particles11, a plurality of second metal magnetic particles12, and a binder13. The binder13binds together the plurality of first metal magnetic particles11and the plurality of second metal magnetic particles12. In other words, the base body10is formed of the binder13and the plurality of first metal magnetic particles11and the plurality of second metal magnetic particles12bound to each other by the binder13. The base body10may contain a dielectric material as a magnetic material or a non-magnetic material.

The plurality of first metal magnetic particles11have a larger average particle size than the plurality of second metal magnetic particles12. That is, the average particle size of the plurality of first metal magnetic particles11(hereinafter referred to as the first average particle size) is different from the average particle size of the plurality of second metal magnetic particles12(hereinafter referred to as the second average particle size). For example, the first average particle size is 30 μm, and the second average particle size is 2 μm, but these are not limitative. In one embodiment of the present invention, the base body10may further contain a plurality of third metal magnetic particles (not shown) having an average particle size different from the first average particle size and the second average particle size (the average particle size of the third metal magnetic particles is hereinafter referred to as the third average particle size). The third average particle size may be smaller than the first average particle size and larger than the second average particle size, or it may be smaller than the second average particle size. The first metal magnetic particles11, the second metal magnetic particles12, and the third metal magnetic particles contained in the magnetic base body10may be hereinafter collectively referred to as “the metal magnetic particles” when they need not be distinguished from one another.

The first metal magnetic particles11and the second metal magnetic particles12can be formed of various soft magnetic materials. For example, a main ingredient of the first metal magnetic particles11is Fe. Specifically, the first metal magnetic particles11are particles of (1) a metal such as Fe or Ni, (2) a crystalline alloy such as an alloy containing Fe, Si, and Cr, an alloy containing Fe, Si, and Al, or an alloy containing Fe and Ni, (3) an amorphous alloy such as an alloy containing Fe, Si, Cr, B, and C or an alloy containing Fe, Si, Cr, and B, or (4) a mixture thereof. The composition of the metal magnetic particles contained in the magnetic base body10is not limited to those described above. The first metal magnetic particles11may contain, for example, 85 wt % or more Fe. This provides the magnetic base body10with an excellent magnetic permeability. The composition of the second metal magnetic particles12is either the same as or different from that of the first metal magnetic particles11. When the magnetic base body10contains the plurality of third metal magnetic particles (not shown), the composition of the third metal magnetic particles is either the same as or different from that of the first metal magnetic particles11, as with the second metal magnetic particles12.

The surfaces of the metal magnetic particles may be coated with insulating films (not shown). The insulating films are formed of, for example, a glass, a resin, or other materials having a high insulating property. For example, the insulating films are formed on the surfaces of the first metal magnetic particles11by mixing the first metal magnetic particles11with powder of a glass material in a friction mixer (not shown). The insulating films formed of the glass material are adhered to the surfaces of the first metal magnetic particles11by the compression friction action in the friction mixer. The glass material may contain ZnO and P2O5. The insulating films may be formed of various glass materials. The insulating films14may be formed of alumina powder, zirconia powder, or any other oxide powders having a high insulating property, in place of or in addition to the glass powder. The thickness of the insulating films is, for example, 100 nm or smaller.

The second metal magnetic particles12may be coated with different insulating films than the first metal magnetic particles11. The insulating films may be oxide films formed by oxidation of the second metal magnetic particles12. The thickness of the insulating films is, for example, 20 nm or smaller. The insulating films may be oxide films formed on the surfaces of the second metal magnetic particles12by heat-treating the second metal magnetic particles12in the atmosphere. The insulating films may be oxide films containing oxides of Fe and other elements contained in the second metal magnetic particles12. These insulating films may be iron phosphate films formed on the surfaces of the second metal magnetic particles12by placing the second metal magnetic particles12into phosphoric acid and stirring. The insulating films of the first metal magnetic particles11may be oxide films formed by oxidation of the first metal magnetic particles11, whereas the insulating films of the second metal magnetic particles12may be coating films formed by a method other than oxidation of the second metal magnetic particles12.

The binder13is, for example, a thermosetting resin having a high insulating property. Examples of the binder13include 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 (PBO) resin. The binder13may also be glass or other materials and may contain insulating fillers.

The conductor25is formed in a pattern. In the embodiment shown, the conductor25is wound around the coil axis Ax (seeFIG. 1). When seen from above, the conductor25has, for example, an elliptic shape, a meander shape, a linear shape or a combined shape of these. The conductor25may have any shape such as a spiral shape.

The conductor25is formed of Cu, Ag, or any other conductive materials. The entire surface of the conductor25other than an end surface25a2and an end surface25b2may be coated with an insulating film. As shown, when the conductor25is wound around the coil axis Ax for a plurality of turns, each of the turns of the conductor25may be spaced from adjacent turns. In this arrangement, the base body10mediates between the adjacent turns.

The conductor25includes a lead-out conductor25a1at one end portion thereof and a lead-out conductor25b1at the other end portion thereof. The lead-out conductor25a1has the end surface25a2at an end portion thereof, and the lead-out conductor25b1has the end surface25b2at an end portion thereof. The lead-out conductor25a1at one end portion of the conductor25is electrically connected to the external electrode21, and the lead-out conductor25b1at the other end portion of the conductor25is electrically connected to the external electrode22.

In one embodiment of the present invention, the external electrode21extends on a part of the first principal surface10a,the second principal surface10b,the second end surface10d,the first side surface10e,and the second side surface10fof the base body10. The external electrode22extends on a part of the first principal surface10a,the second principal surface10b,the first end surface10c,the first side surface10e,and the second side surface10fof the base body10. The external electrodes21,22are spaced apart from each other. Shapes and arrangements of the external electrodes21,22are not limited to those in the example shown. Both the lead-out conductor25a1and the lead-out conductor25b1lead to the first principal surface (the mounting surface)10aof the base body10, and the end surface25a2of the lead-out conductor25a1and the end surface25b2of the lead-out conductor25b1are exposed from the base body10through the first principal surface10a.That is, the end surface25a2of the lead-out conductor25a1and the end surface25b2of the lead-out conductor25b1are exposed from the base body10through the same surface. It is also possible that the end surface25a2of the lead-out conductor25a1and the end surface25b2of the lead-out conductor25b1are exposed from the base body10through different surfaces.

Next, the external electrodes of the coil component1according to one embodiment of the present invention will be hereinafter described with reference toFIGS. 3, 4, and 5.FIG. 3is an enlarged sectional view showing, on an enlarged scale, a sectional surface around the joint between one end portion of the conductor25and the external electrode21in the coil component1shown inFIG. 1.FIG. 4is a sectional view showing the external electrode21and the conductor25of the coil component1.FIG. 5is a schematic view showing an electron microscopy image of a sectional surface of the external electrode in the coil component shown inFIG. 1. The following description on the external electrode21also applies to the external electrode22unless in specific cases. Also,FIGS. 3 to 5, which show the external electrode21, also apply to the external electrode22. As shown inFIGS. 3 to 5, the external electrode21includes a metal film23, an electrode layer24, and a plating layer26. The metal film23, the electrode layer24, and the plating layer26are stacked in this order on the first principal surface10aof the base body10through which one end portion of the conductor25(that is, the end surface25a2) is exposed. The lamination direction extends in a direction perpendicular to the surface of the base body10(the first principal surface10ain the example shown) through which one end portion of the conductor25(that is, the end surface25a2) is exposed.FIGS. 3 to 5each show a sectional surface of the coil component1cut along the lamination direction.

The metal film23contacts with the first principal surface10aand one end portion of the conductor25. The metal film23is, for example, a sputtering film, and at least a part of the metal film23and at least a part of one end portion of the conductor25(the end surface25a2) are connected with each other by metallic bond. The phrase “at least a part of one end portion of the conductor25” mentioned here refers to some region of the end surface25a2. For example, the metal film23and the end portion25a1may be connected with each other by metallic bond at a peripheral portion PP of the end surface25a2(seeFIG. 3).FIG. 3shows an example in which the metal film23and the end portion25a1of the conductor25are connected with each other by metallic bond at the entirety of the end surface25a2. In the example shown inFIG. 3, the region of the end surface25a2at which the metal film23and the end portion25a1are metal-bonded to each other includes the peripheral portion PP.

The metal film23is made of, for example, a metal such as Ag, Au, Pd, Pt, Cu, Ni, Ti, and Ta or an alloy of these metals. Metals suitable for the metal film23are those less apt to oxidation or ready to be reduced after oxidation. The metal film23is preferably made of a material having a low volume resistivity. The thickness of the metal film23is not particularly limited but may be, for example, 0.5 μm to 5 μm. The ionization tendency of the main ingredient of the metals contained in the metal film23is preferably smaller than that of the metal constituting the conductor25. The phrase “the main ingredient of the metals contained in the metal film23” refers to the metal ingredient that makes up more than a half of the metal species by weight percent among the metals contained in the metal film23. When the metal film23contains one metal, this metal is the main ingredient. By way of one example, when the conductor25is made of Cu, the metal contained in the metal film23may be Ag.

The average of the aspect ratios of the metal particles contained in the metal film23is 0.8 to 1.5. An aspect ratio of a metal particle contained in the metal film23is β/α, where α is the dimension of the metal particle in the direction horizontal to the boundary interface BI, and β is the dimension of the metal particle in the direction perpendicular to the boundary interface BI. The average of the aspect ratios of the metal particles may be an average of the aspect ratios of, for example, five, ten, or other plural number of metal particles. The metal particles contained in the metal film23are metal-bonded to each other.

The electrode layer24is disposed on the metal film23and electrically connected to one end portion of the conductor25. The electrode layer24includes a plurality of layers stacked together in the lamination direction. In the embodiment shown, the electrode layer24includes a first electrode layer24A and a second electrode layer24B covering the first electrode layer24A. In the lamination direction, the second electrode layer24B is disposed on the opposite side to the metal film23with respect to the first electrode layer24A. In the embodiment shown, the first electrode layer24A covers the metal film23and a part of the principal surface10aof the base body10. The first electrode layer24A needs to cover at least a part of the metal film23. The first electrode layer24A may also cover a part of the principal surface10aof the base body10or may not cover the principal surface10aof the base body10. The second electrode layer24B covers the entire outer surface of the first electrode layer24A (that is, the surface not in contact with the metal film23and the base body10) and a part of the second principal surface10b,the second end surface10d,the first side surface10e,and the second side surface10fof the base body10. The second electrode layer24B included in the external electrode22covers the entire outer surface of the first electrode layer24A (that is, the surface not in contact with the metal film23and the base body10) and a part of the second principal surface10b,the first end surface10c,the first side surface10e,and the second side surface10fof the base body10. The first electrode layer24A has a thickness of, for example, about 10 μm to 20 μm, and the second electrode layer24B has a thickness of, for example, about 20 μm to 30 μm. The interface between the first electrode layer24A and the second electrode layer24B may be the boundary between the resin R contained in the first electrode layer24A and the resin R contained in the second electrode layer24B. The boundary between the resin R contained in the first electrode layer24A and the resin R contained in the second electrode layer24B can be confirmed by observation under an optical microscope, for example.

The electrode layer24(that is, the first electrode layer24A and the second electrode layer24B) contain a plurality of fillers F and the resin R. In the following description, the fillers contained in the first electrode layer24A are referred to as fillers FA, the fillers contained in the second electrode layer24B are referred to as fillers FB, the resin contained in the first electrode layer24A is referred to as a resin RA, and the resin contained in the second electrode layer24B is referred to as a resin RB. The proportion of the volume of the fillers FB in the second electrode layer24B is larger than the proportion of the volume of the fillers FA in the first electrode layer24A. In one or more embodiments of the present invention, the comparison between the volume of the fillers FB in the first electrode layer24A and the volume of the fillers FB in the second electrode layer24B can be made by comparing the proportion of the area of the fillers FA relative to the area of the first electrode layer24A with the proportion of the area of the fillers FB relative to the area of the second electrode layer24B in a sectional surface of the external electrode21cut along a plane extending along the lamination direction (in other words, a plane extending parallel to the lamination direction). When the proportion of the area of the fillers FB relative to the area of the second electrode layer24B is larger than the proportion of the area of the fillers FA relative to the area of the first electrode layer24A, the proportion of the volume of the fillers FB in the second electrode layer24B is larger than the proportion of the volume of the fillers FA in the first electrode layer24A. The area of the first electrode layer24A refers to the sum of the area of the fillers FA and the area of the resin RA, both contained in the first electrode layer24A. The area of the second electrode layer24B refers to the sum of the area of the fillers FB and the area of the resin RB, both contained in the second electrode layer24B. The areas of the first electrode layer24A, the second electrode layer24B, the fillers FA, the fillers FB, the resin RA, and the resin RB in a sectional surface of the external electrode21cut along a plane extending along the lamination direction (in other words, a plane extending parallel to the lamination direction) can be measured by image processing in a scanning electron microscope (SEM) photograph obtained by imaging the sectional surface with a SEM. The fillers FA contained in the first electrode layer24A include a plurality of first fillers FA1and a plurality of second fillers FA2. Likewise, the fillers FB contained in the second electrode layer24B include a plurality of first fillers FB1and a plurality of second fillers FB2. Each of the plurality of first fillers FA1, FB1has a spherical shape with an aspect ratio of 2 or smaller. Each of the plurality of second fillers FA2, FB2has a flat shape with an aspect ratio of 3 or larger. The aspect ratios mentioned herein refer to a ratio of the dimension in the short axis direction to the dimension in the long axis direction of a filler F (that is, a value obtained by dividing the maximum particle size by the minimum particle size) in the sectional surface of the electrode layer24in the thickness direction thereof. In the sectional surface of the electrode layer24in the thickness direction thereof, the average of the maximum particle sizes of the second fillers FA2, FB2is larger than the average of the maximum particle sizes of the first fillers FA1, FB1. The average of the maximum particle sizes of the first fillers FA1, FB1is, for example, 1 μm to 10 μm, and the average of the maximum particle sizes of the second fillers FA2, FB2is, for example, 0.1 μm to 10 μm. Among the plurality of fillers FA in the first electrode layer24A, the proportion of the first fillers FA1is 30 vol % to 70 vol %, and the proportion of the second fillers FA2is 30 vol % to 70 vol %. Among the plurality of fillers FB in the second electrode layer24B, the proportion of the first fillers FB1is 40 vol % to 70 vol %, and the proportion of the second fillers FB2is 30 vol % to 60 vol %.

The first fillers FA1, FB1and the second fillers FA2, FB2are formed of a metal material having a high electrical conductivity such as Ag, Cu, Au, Pd, or Ni. Alternatively, the first fillers FA1, FB1and the second fillers FA2, FB2may be formed of an alloy such as AgPd, brass, or bronze. The first fillers FA1, FB1and the second fillers FA2, FB2may be metal fillers coated with a film of a low resistance metal such as Ag. Further, the first fillers FA1, FB1and the second fillers FA2, FB2contain a same metal as an ingredient. In the embodiment shown, both the first fillers FA1, FB1and the second fillers FA2, FB2are formed of Ag. It is also possible that the first fillers FA1, FB1and the second fillers FA2, FB2contain different metals, or the first fillers FA1, FB1and the second fillers FA2, FB2are formed only of different metals. Further, it is also possible that the first fillers FA1contained in the first electrode layer24A and the first fillers FB1contained in the second electrode layer24B contain different metals, or the first fillers FA1and the first fillers FB1are formed only of different metals. Likewise, it is also possible that the second fillers FA2contained in the first electrode layer24A and the second fillers FB2contained in the second electrode layer24B contain different metals, or the second fillers FA2and the second fillers FB2are formed only of different metals. Even when the first fillers FA1, FB1and the second fillers FA2, FB2contain different metals, the first fillers FA1, FB1and the second fillers FA2, FB2are metal-bonded to each other, and the bonding portion between the first fillers FA1, FB1and the second fillers FA2, FB2are alloyed. In this case, the combination of the metal contained in the first fillers FA1, FB1and the metal contained in the second fillers FA2, FB2is preferably selected such that the bond strength is larger than that of the metallic bond between the same metals. The bond strength of an alloy made by a combination of different metals is apparent in known literatures.

The first fillers FA1, FB1in the first electrode layer24A and the second fillers FA2, FB2in the second electrode layer24B undergo heat treatment in the manufacturing process of the coil component1. This causes the first fillers FA1, FB1and the second fillers FA2, FB2to be metal-bonded to each other. The first electrode layer24A and the second electrode layer24B may include portions in which the second fillers FA2, FB2are metal-bonded to each other with the long axes thereof oriented parallel to each other. The first electrode layer24A and the second electrode layer24B may include portions in which the first fillers FA1, FB1are metal-bonded to each other. In the interface between the first electrode layer24A and the metal film23, the second fillers FA2are metal-bonded to the metal film23. This causes the first electrode layer24A to be electrically connected to the metal film23. Further, in the interface between the second electrode layer24B and the plating layer26, the second fillers FB2are metal-bonded to the plating layer26. The long axis direction of each second filler FB2is substantially parallel to the direction perpendicular to the thickness direction of the electrode layer24. This causes the second electrode layer24B to be electrically connected to the plating layer26. Likewise, the first fillers FA1in the first electrode layer24A may be metal-bonded to the metal film23. The first fillers FB1in the second electrode layer24B may be metal-bonded to the plating layer26.

The resin RA contained in the first electrode layer24A and the resin RB contained in the second electrode layer24B are, for example, thermosetting resins. The thermosetting resins may be those commonly used for bonding application or the like, and examples of such thermosetting resins include epoxy resins, acrylic resins, phenolic resins, cyanate resins, amino resins, oxetane resins, silicon-modified organic resins, polyimide resins, maleimide resins, and BT (bismaleimide triazine) resins. Each of the first electrode layer24A and the second electrode layer24B may contain two or more thermosetting resins. The resin RA contained in the first electrode layer24A and the resin RB contained in the second electrode layer24B may be either the same or different. In one or more embodiments of the present invention, the proportion of the volume of the resin RA in the first electrode layer24A is larger than the proportion of the volume of the resin RB in the second electrode layer24B. The comparison between the volume of the resin RA in the first electrode layer24A and the volume of the resin RB in the second electrode layer24B can be made by comparing the proportion of the area of the resin RA relative to the area of the first electrode layer24A with the proportion of the area of the resin RB relative to the area of the second electrode layer24B in a sectional surface of the external electrode21cut along a plane extending along the lamination direction. When the proportion of the area of the resin RA relative to the area of the first electrode layer24A is larger than the proportion of the area of the resin RB relative to the area of the second electrode layer24B, the proportion of the volume of the resin RA in the first electrode layer24A is larger than the proportion of the volume of the resin RB in the second electrode layer24B. This makes the adhesion strength between the first electrode layer24A and the base body10higher than the adhesion strength between the second electrode layer24B and the base body10. The proportion of the resin RA in the first electrode layer24A is 65 vol % or smaller. The proportion of the resin RA in the first electrode layer24A is preferably 30 to 65 vol %. Further, the proportion of the resin RA in the first electrode layer24A is more preferably smaller than 65 vol %. The proportion of the resin RB in the second electrode layer24B is 60 vol % or smaller. The proportion of the resin RB in the second electrode layer24B is preferably25to 60 vol %. Further, the proportion of the resin RB in the second electrode layer24B is more preferably smaller than 55 vol %.

The first electrode layer24A is formed from a first conductive paste that contains first metal particles to be the first fillers FA1, second metal particles to be the second fillers FA2, and an unset resin. The second electrode layer24B is formed from a second conductive paste that contains first metal particles to be the first fillers FB1, second metal particles to be the second fillers FB2, and an unset resin. The first conductive paste and the second conductive paste are different in the proportions of the first metal particles and the second metal particles contained therein. The first metal particles and the second metal particles are the first fillers FA1, FB1and the second fillers FA2, FB2, respectively, yet to be metal-bonded by the heat treatment in the manufacturing process of the coil component1. Each of the second metal particles has a flat shape, and the curvature of the outer shape of each second metal particle is smallest at the opposite end portions E in the respective long axis direction. The average of the minimum radii of curvature of the second metal particles (that is, the curvatures of the end portions of the second metal particles in the respective long axis directions) is equal to or smaller than that of the first metal particles of 0.5 μm. By way of one example, the average of the minimum radii of curvature of the second metal particles is 0.1 μm or smaller. InFIG. 5, the end portions of the second fillers FA2, FB2metal-bonded by the heat treatment are denoted as the end portions E of the second metal particles.FIG. 5schematically shows the boundaries of the end portions of the second fillers FA2, FB2that are metal-bonded. The boundaries of the end portions of the second fillers FA2, FB2can be sighted by, for example, observing a sectional surface of the electrode layer24including the second fillers FA2, FB2under an electron microscope at a magnification of 50,000. In actual observations, the boundaries of the end portions of the second fillers FA2, FB2may not be observed clearly. For example, when an end portion of a second filler FA2, FB2is bonded to a first filler FA1, FB1or another second filler FA2, FB2, the boundary may not be observed clearly. In such a case, the boundary of the second filler FA2, FB2may be set at the boundary of the crystal grains in the second filler FA2, FB2that can be observed under an electron microscope. When the boundary of the crystal grains in the second filler FA2, FB2cannot be observed clearly, the boundary of the end portion of the second filler FA2, FB2may be set at a curved line estimated by any known estimation method from the shape of a portion of the second filler FA2, FB2other than the end portion that cannot be observed clearly.

The plating layer26is disposed on the second electrode layer24B. The plating layer26covers the entire outer surface of the second electrode layer24B (that is, the surface not in contact with the first electrode layer24A and the base body10). In the embodiment shown, the plating layer26has multilayer structure including a first plating layer26A and a second plating layer26B. The first plating layer26A contacts with the second electrode layer24B, and the second plating layer26B is disposed on the first plating layer26A. The first plating layer26A has a thickness of, for example, 5 μm to 7 μm, and the second plating layer26B has a thickness of, for example, 5 μm to 10 μm. The first plating layer26A is formed of Ni for example, and the second plating layer26B is formed of Sn for example. The first plating layer26A may also be formed of a metal or an alloy besides Ni, that forms a barrier layer corrosion-resistant to heat treatment in soldering. The second plating layer26B may also be formed of a metal or an alloy besides Sn, that has a high solder wettability. The plating layer26may also be a single-layer plating layer formed of a material including Ni or Sn.

FIG. 6is a sectional view showing, on an enlarged scale, a sectional surface of the joint between the base body10and the external electrode21in the coil component1. As shown inFIG. 6, the surface of the base body10has a plurality of indentations produced by removal of the first metal magnetic particles11and/or the second metal magnetic particles12. Therefore, the interface between the base body10and the first electrode layer24A has a plurality of indentations, which produce the anchor effect to bind together the base body10and the first electrode layer24A. Likewise, each of the interface between the first electrode layer24A and the second electrode layer24B, the interface between the second electrode layer24B and the first plating layer26A, and the interface between the first plating layer26A and the second plating layer26B also has a plurality of indentations, which produce the anchor effect to bind together the first electrode layer24A and the second electrode layer24B, the second electrode layer24B and the first plating layer26A, and the first plating layer26A and the second plating layer26B. This improves the strength of the entirety of the external electrodes21,22.

Next, a description is given of a manufacturing method of the coil component1according to one embodiment of the invention. First, the conductor25formed of a metal material or the like and having a coil shape is placed into a mold, along with a mixed resin composition prepared by mixing and kneading particles including the first metal magnetic particles11and the second metal magnetic particles12with the binder13composed of a resin or the like. The work is then compression molded such that the end surface25a2of the lead-out conductor25a1and the end surface25b2of the lead-out conductor25b1of the conductor25are exposed through the surface. The coil shape of the conductor25is not particularly limited. For example, the conductor25is made of a wire wound in a spiral shape, or it may be made of a planar coil instead of the wound wire. The conductor25may have an insulating coat. The resin in the molded product is cured to obtain the base body10having the conductor25embedded therein.

Next, the surface of the magnetic base body10in which the end surface25a2of the lead-out conductor25a1and the end surface25b2of the lead-out conductor25b1of the conductor25are exposed is smoothed to remove oxides. By way of an example, the surface of the magnetic base body10may be polished with an abrasive and then subjected to plasma etching. The particle size of the abrasive should preferably be smaller than that of the first metal magnetic particles11. For example, when the average particle size of the first metal magnetic particles11is 30 μm, an abrasive having a particle size of 25 μm is selected. Any etching method, such as plasma etching, is available that can remove oxides from the surface of the magnetic base body.

Next, the metal film23is formed. One example of the method of forming the metal film23is sputter deposition, or in particular, high density sputter deposition. In high density sputter deposition, a large electric power is applied for a short period to form a dense film while preventing overheating of the sputtered film. The sample may be cooled during sputtering, such that a larger electric power can be applied to form more dense sputtered film. With the above metals used in this method, the metal film23can be formed efficiently at a high sputtering yield. The metal film formed by sputter deposition is herein referred to as a sputtered film. The metal film23may alternatively be formed by methods other than sputter deposition that are capable of metal-bonding the end surface25a2of the conductor25to the metal film23.

Next, the electrode layer24is formed on the metal film23. In forming the electrode layer24, the first electrode layer24A is formed first. The first step to form the first electrode layer24A is to prepare a first conductive paste that contains first metal particles to be the first fillers FA1and second metal particles to be the second fillers FA2. The next step is to form a layer of the first conductive paste by the printing or other method. A heat treatment or other process follows to metal-bond the first metal particles and the second metal particles contained in the layer of the first conductive paste. By way of one example, the heat treatment is performed under the conditions of 170 to 250° C. and 30 to 60 minutes. In addition, the heat treatment is performed in a low-oxygen or reducing atmosphere, depending on the substances of the first fillers FA1and the second fillers FA2. Subsequently, the second electrode layer24B is formed on the first electrode layer24A. In forming the second electrode layer24B, the printing or other method is used to form a layer of a second conductive paste that contains first metal particles to be the first fillers FB1and second metal particles to be the second fillers FB2. A heat treatment or other process follows to metal-bond the first metal particles and the second metal particles contained in the layer of the second conductive paste. The heat treatment on the second conductive paste is performed, for example, under the same conditions as the heat treatment on the first conductive paste. Through these steps, the electrode layer24is formed that is electrically connected to the end portion of the conductor25via the metal film23.

Lastly, plating is performed to form the first plating layer26A and the second plating layer26B. Through the process described above, the external electrodes21,22are formed, and thus the coil component1is manufactured. The coil component1manufactured is mounted on the circuit board by soldering the external electrodes21,22to the corresponding land portions of the circuit board.

As described above, the external electrodes21,22of the coil component1includes the first electrode layer24A, the second electrode layer24B covering the first electrode layer24A, and the plating layer26covering the second electrode layer24B, and the proportion of the volume of the fillers FB in the second electrode layer24B is larger than the proportion of the volume of the fillers FA in the first electrode layer24A. In order to improve the tight adhesion between the electrode layer24and the base material10, it is commonly desirable to increase the proportion of the resin R contained in the electrode layer24. However, when the electrode layer24contains a larger proportion of resin R, a structural defect tends to form in the plating layer26being grown on the electrode layer24, possibly reducing the adhesion strength between the electrode layer24and the plating layer26. In the coil component1according to one embodiment of the present invention, the proportion of the volume of the fillers FA in the first electrode layer24A that contacts with the base body10is relatively small (in other words, the proportion of the volume of the resin RA is relatively large), such that the adhesion strength with the base body10is ensured. On the other hand, the proportion of the volume of the fillers FB in the second electrode layer24B that contacts with the plating layer26is relatively large (in other words, the proportion of the volume of the resin RB is relatively small), such that it can be inhibited that a structural defect forms when the plating layer26is grown on the second electrode layer24B. Accordingly, the adhesion strength between the electrode layer24and the base body10can be ensured, and simultaneously, the adhesion strength between the electrode layer24and the plating layer26can be increased.

The plurality of fillers FA, FB in the electrode layer24may include the first fillers FA1, FB1having a spherical shape and the second fillers FA2, FB2having a flat shape. The aspect ratio of the first fillers FA1, FB1may be 2 or smaller, and the aspect ratio of the second fillers FA2, FB2may be3or larger. Since each second filler FA2, FB2has a flat shape, the curvature of each second filler FA2, FB2is small at the opposite end portions E thereof in the respective long axis direction and is large at the middle portion thereof in the respective long axis direction. Therefore, the amount of energy required for the metallic bond by the heat treatment is small at the opposite end portions E of each second filler in the respective long axis direction and is large at the middle portion thereof in the respective long axis direction. Accordingly, the bonding by the metallic bond is facilitated at the opposite end portions E of each second filler in the respective long axis direction and is retarded at the middle portion thereof in the respective long axis direction. This makes it possible to inhibit the first fillers FA1, FB1and the second fillers FA2, FB2from aggregating when metal-bonded by the heat treatment, while ensuring the electric connection by the opposite end portions E of each second filler FA2, FB2in the respective long axis direction. As a result, the first fillers FA1, FB1and the second fillers FA2, FB2are inhibited from being unevenly distributed in the electrode layer24, and thus the reduction of the strength of the external electrodes21,22can be inhibited.

Among the plurality of fillers FB in the second electrode layer24B, the proportion of the first fillers FB1may be 40 vol % to 70 vol %, and the proportion of the second fillers FB2may be 30 vol % to 60 vol %. Thus, in the distribution state of the fillers FB and the resin RB in the second electrode layer24B, the resin RB can fill the space between the aggregates of the fillers FB, including the first fillers FB1and the second fillers FB2, without segregation. In particular, the resin RB can be inhibited from segregation around the bonding interface between the plating layer26and the second electrode layer24B.

At least a part of the plurality of fillers F contained in the second electrode layer24B may be metal-bonded to the plating layer26. This reduces the electrical resistance between the plating layer26and the second electrode layer24B.

Another embodiment of the external electrodes21,22of the coil component1will be hereinafter described with reference toFIGS. 7A and 7B.FIGS. 7A and 7Bare sectional views each schematically showing another embodiment of the external electrodes of the coil component1. As shown inFIG. 7A, the external electrode121according to the other embodiment may be provided on only a part of the first principal surface10aof the base body10. In this case, the plating layer26covers only the outer surface of the second electrode layer24B of the external electrode121. Further, as shown inFIG. 7B, the external electrode221according to the other embodiment may be provided on only a part of the first principal surface10aand the second end surface10dof the base body10. In this case, the plating layer26covers the outer surface of the second electrode layer24B of the external electrode221and a part of the second end surface10d.

Next, a description is given of a coil component100according to another embodiment of the present invention with reference toFIG. 8.FIG. 8is a perspective view schematically showing the coil component100. As shown, similarly to the coil component1, the coil component100includes a base body, a coil conductor25provided in the base body10, an external electrode21disposed on a surface of the base body10, and an external electrode22disposed on the surface of the base body10at a position spaced from the external electrode21. The coil component100is different from the coil component1in that it includes an insulating plate50and two conductors25. The insulating plate50is provided in the base body10, and the two conductors25are provided on the top-side surface and the bottom-side surface of the insulating plate50, respectively.

As in the coil component1, the external electrodes21,22of the coil component100includes the first electrode layer24A, the second electrode layer24B covering the first electrode layer24A, and the plating layer26covering the second electrode layer24B, and the proportion of the volume of the fillers F in the second electrode layer24B is larger than the proportion of the volume of the fillers F in the first electrode layer24A. Accordingly, for the same reason as with the coil component1, the adhesion strength between the electrode layer24and the base body10can be ensured, and simultaneously, the adhesion strength between the electrode layer24and the plating layer26can be increased.

The dimensions, materials, and arrangements of the constituent elements described for the above various embodiments 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 above-described embodiments, and it is also possible to omit some of the constituent elements described for the embodiments.

For example, the external electrodes21,22of the coil component1and the coil component100may not include the metal film23. In this case, the electrode layer24and the end portion of the conductor25may be directly connected to each other.