Method of producing chip inductor

Abstract: Supporting-grooves are formed on the inside of a metallic mold (11). Both the end portions of a coil-shaped conductive wire (12), produced by forming a metallic wire into a spiral shape, are supported thereon, so that the coil-shaped conductive wire is positioned in the center portion of the metallic mold. Magnetic slurry is cast into the metallic mold, and molded by wet pressing method, whereby a molding body having the coil-shaped conductive wire embedded therein is obtained. The molding body is fired. External electrodes (13, 14) for connection to both the end-portions of the coil-shaped conductive wire are formed on both the end-faces of the fired magnetic core.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of producing a chip inductor for use in a noise filter, a transducer, or the like. 2. Description of the Related Art As high frequency filters for eliminating radiation-noise from digital equipment such as computers and so forth, chip inductors are widely used. For example, as described in Japanese Unexamined Utility Model Publication No. 6-50312, a monolithic chip inductor has been known, in which a chip element body is formed by use of laminated ceramic layers, a coil conductor provided between the ceramic layers is connected via through-holes formed in the ceramic layers, whereby a coil is formed in the chip element body, and the leading and trailing portions of the coil are connected to external electrodes, respectively. An inductor for use in a high frequency filter is required to have a large inductance and a low resistance. In general, inductance is proportional to the square of the winding number of a coil and is inversely proportional to the length of the coil. On the other hand, a monolithic inductor of the above-described type has the problems that the production process is complicated and the production cost is high, and moreover, a large inductance cannot be attained, since the winding number of the coil cannot be increased, and also, the resistance becomes high, since the coil conductor is formed as a film-shape electrode. To solve the problems, a method of molding an inductor has been proposed, as described in Japanese Unexamined Patent Application Publication No. 8-191022, in which a magnetic ceramic is extrusion-molded into a winding-core, a conductive wire is wound around the core into a coil-shape, and further, a magnetic ceramic is extrusion-molded thereon to form a sheathing body. Thereafter, the ceramics are fired, and external electrodes are made to cover and bonded to both ends of the fired magnetic core. Thus, the external electrodes are connected to both the ends of the coil-shaped conductive wire. In this case, the production method is simple as compared with the monolithic inductor, and as the coil-shaped conductive wire, a metallic wire is used. Accordingly, advantageously, both of a high inductance and a low resistance are compatible with each other. In the above-described production methods, both of parts to become the winding-core and the sheathing body are formed by extrusion molding. The density of a molding product formed by extrusion molding is not sufficiently high. Further, in some cases, the sheathing body is not voidlessly filled in the periphery of the coil and cavities are formed between the winding-core and the sheathing body. Moreover, a binder is needed to combine the ceramic particles to each other, and causes the formation of pores at firing. Therefore, it has been difficult to produce inductors with high qualities. Further, when the sheathing body is extrusion-molded, it may occur that the coil becomes eccentric with respect to the center portion of the sheathing. Accordingly, an inductor having stable magnetic properties cannot be obtained. Further, when the firing is carried out in the state that the coil is eccentric, inconveniently, warpage or cracks may be formed, due to the shrinkage of the ceramic when it is fired. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a method of producing a chip inductor which can produce an inductor having high qualities with few inconveniences such as cracks or the like which will be caused by firing-shrinkage. To achieve the above-described object, according to a first aspect of the present invention, there is provided a method of producing a chip inductor which includes the steps of: inserting a conductive wire made of a metallic wire into a metallic mold, supporting both the end portions of the conductive wire on supporting-portions formed on the inside of the metallic mold so as to position the conductive wire in the center portion of the metallic mold, casting magnetic ceramic slurry into the metallic mold, molding the ceramic slurry cast in the metallic mold by wet pressing to obtain a molding body having the conductive wire embedded therein, firing the molding body, and forming external electrodes on both the end-faces of the fired magnetic core so as to be connected to both the end portions of the conductive wire. As described above, the conductive wire is inserted into the metallic mold, the magnetic slurry is cast, and thereafter, the wet pressing is carried out. In this case, both the end-portions of the conductive wire are supported onto the supporting-portions formed on the inside of the metallic mold, so that the conductive wire is positioned in the center portion of the metallic mold. Thereby, the conductive wire can be prevented from becoming eccentric during the wet pressing. The supporting portions may be supporting-grooves formed on the inside of the metallic mold. The molding body having the conductive wire embedded therein is produced by the wet pressing method. The molding body produced by the wet pressing method has a tight ceramic tissue and a higher density as compared with one produced by extrusion molding method. Further, since the ceramic slurry is compressed, no binder is needed or an extremely small amount of a binder is necessary. Therefore, an inductor with high qualities can be obtained by firing the molding body, since the fired magnetic core has a high density and, moreover, the generation of pores is inhibited due to less amount of a binder. As described above, the conductive wire is inserted into the metallic mold, and the wet pressing is carried out. Thus, the inductor can be formed by one molding cycle. Therefore, needless to say, the production process is simple in contrast to that for the laminated inductor and also, is simplified as compared with an extrusion molding method. In the case where the coil-shaped wire is employed as the conductive wire, a high inductance can be obtained at a lower resistance as compared with the laminated inductor. When the linear conductive wire is used, the inductance is low as compared with that obtained when the coil-shaped conductive wire is used, but the direct current resistance can be further reduced. When the molding body formed by wet pressing as described above is fired, the ceramic material is firing-shrunk. In this case, though the ceramic is shrunk, the conductive wire is not shrunk, or less shrunk than the ceramic. When the coil-shaped conductive wire is employed, voids are formed inside the coil. It may occur that a flux or the like may invade into the voids from the outside, affecting the characteristics of the inductor. Further, in some cases, cracks are formed inside the coil, in addition to the formation of voids, due to the firing-shrinkage. Moreover, in the case where the molding is carried out to obtain a plurality of the molding body in one cycle, that is, a long coil-shaped conductive wire is employed, the coil-shaped conductive wire may be deflected if both the end-portions of the coil-shaped conductive wire only are supported onto the supporting portions of the metallic mold as described above. If the molding is carried out in this state, the coil-shaped conductive wire may not be straightly disposed in the core. Thus, preferably, according to a second aspect of the present invention, the winding-core made of the fired magnetic ceramic is inserted into the coil-shaped conductive wire, before the coil-shaped conductive wire is inserted into the metallic mold. That is, since the winding-core arranged inside the coil-shaped conductive wire is not shrunk, no voids inside the coil-shaped conductive wire are formed by firing , and moreover, formation of cracks, caused by firing-shrinkage, can be prevented. Further, the winding-core is inserted into the coil. Accordingly, even if the coil is long, deflection of the coil can be prevented, attributed by the winding-core. Thus, an inductor with high qualities can be obtained. The winding-core may have the same or a different composition from that of the magnetic core provided outside the coil. If the winding-core has the same composition as the magnetic core provided outside the coil, a magnetic core which is homogeneous inside and outside its coil can be obtained. If the compositions are different, e.g., the magnetic permeabilities inside and outside the coil can be made different. Thus, the characteristics of an inductor can be easily changed. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an outside perspective view of an inductor of the present invention; FIG. 2 is a cross sectional view of the inductor of FIG. 1; FIG. 3 is a plan view of an example of a metallic mold in the present invention; FIGS. 4A, 4B, 4C, 4D, and 4E are process drawings showing a method of producing an inductor according to a first embodiment of the present invention; FIGS. 5A, 5B, and 5C are process drawings showing a method of producing an inductor according to a second embodiment of the present invention; and FIG. 6 is an outside perspective view of another example of an inductor according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 1 and 2 each show a chip inductor according to a first embodiment of the present invention. The inductor 10 is provided with a magnetic core 11 having a prism-shape. The core 11 is produced by firing a magnetic ceramic such as Ni-Cu-Zn based ferrites or the like. Regarding the shape of the core 11, different shapes and sizes, e.g., a column-shape may be employed, in addition to the prism-shape. A coil-shaped conductive wire 12 made of a metallic wire of Ag, Cu, or their alloy formed in a spiral-shape is embedded inside the core 11. Both the ends of the coil-shaped conductive wire are exposed to both the end-faces of the magnetic core 11. On the exposure faces, external electrode 13 and 14 made of thick-film electrodes are formed. Accordingly, the external electrodes 13 and 14 are electrically connected to both the ends of the coil-shaped conductive wire 12, respectively. A method of producing the chip inductor 10 having the above-described structure will be concretely described in reference to FIGS. 4A to 4E. First, a metallic mold 20 as shown in FIGS. 3 and 4A is prepared. A cavity 21 is formed by the metallic mold 20 and a lower die 26 described later. Supporting-grooves 22, which are supporting portions for supporting both the ends of the coil-shaped conductive wire 12, are formed inside of the opposite end portions of the cavity so as to have a predetermined depth D from the upper end faces, respectively. The depth D is set so that the coil-shaped conductive wire 12 is positioned in the center portion of a molding body 27 when wet press molding is carried out. The above-described supporting-grooves 22 prevent the coil-shaped conductive wire 12 from becoming eccentric when ceramic slurry 23 is cast into the cavity 21 and dispose the coil-shaped conductive wire 12 in the center portion of the metallic mold 20. Further, the shape of the supporting-grooves 22 may be determined arbitrarily. Next, the coil-shaped conductive wire 12 is inserted into the cavity 21 of the metallic mold 20, as shown in FIG. 4B, and both the ends of the coil-shaped conductive wire 12 are placed on the supporting-grooves 22. The coil-shaped conductive wire 12 in this embodiment is formed by winding an Ag wire with a wire diameter  = 200 µm into a spiral shape having an inner diameter of 1.25 mm of the coil and a coil pitch of 0.4 mm. Especially, the coil-shaped conductive wire 12 may be so long to have a length equivalent to the overall length of plural inductors for molding using multi mold-cavities to obtain a plurality of the molding bodies. Next, the ceramic slurry 23 is cast into the cavity 21 as shown in FIG. 4C, and wet pressing is performed. To 1500 g of raw material comprising a Ni-Cu-Zn based ferrite, 650 g of refined water, 0.2 wt.% on a basis of the raw material of an anti-foaming agent, and 0.5 wt.% of a dispersant are added, placed into a pot mill, and mixed with PSZ balls for 17 hours. The mixture is used as the ceramic slurry 23. After the ceramic slurry 23 is cast, the upper side of the cavity 21 is covered with a filter 24 through which only water can be passed, and a porous upper die 25 is packed thereon. Then, press-forming is carried out. That is, the lower die 26 is pushed upwardly of the lower position in the metallic mold 20, so that a pressure of 100 kgf/cm 2 , for example is applied to the ceramic slurry 23 for 5 minutes to extract water contained in the ceramic slurry through the filter 24 and the water-extracting holes 25a of the upper die 25. The molding body 27 formed as described above, as seen in FIG. 4D, has a high density, since the ceramic slurry 23 is pressed, and the ceramic slurry 23 is voidlessly filled into the periphery of the coil-shaped conductive wire 12. Thereafter, the molding body 27 is removed from the metallic mold 20. The molding body 27 is dried, e.g., at 40°C for 50 hours, and fired at 910°C for 2 hours. In this case, the molding body 27 has a high density and a high filling-degree, since the molding body 27 is formed in wet-press method. In addition, in the ceramic slurry 23, since it contains no binder, formation of pores can be prevented, and a sintered body with high qualities can be obtained. Further, with the aid of the supporting-grooves 22, the coil-shaped conductive wire 12 is prevented from becoming eccentric, and is positioned in the center portion of the sintering body. Thus, an inductor having stable characteristics can be obtained. Thereafter, as shown in FIG. 4E, the unnecessary portions at both the ends of the sintering body (the portions corresponding to the supporting-grooves 22) are cut at a predetermined length to produce the magnetic core 11. Then, the external electrodes 13 and 14 are formed on both the end-faces of the core 11 where the coil-shaped conductive wire 12 is exposed, and thereby the chip inductor 10 (see FIGS. 1 and 2) is obtained. Regarding a method of forming the external electrodes 13 and 14, e.g., Ag paste, AgPd paste, or the like is coated, dried at 150°C for 15 minutes, and baked at 800°C for 10 minutes. Ni-Sn plating may be carried out, if necessary. FIGS. 5A to 5C each show a second embodiment of the present invention. In the above-described first embodiment, the coil-shaped conductive wire 12 is inserted directly into the metallic mold 20 as shown in FIG. 4B. However, when the ceramic material is shrunk at firing, cracks or voids may be formed in the ceramic portion inside the coil-shaped conductive wire 12. Further, when a long coil-shaped conductive wire 12 is inserted for molding using multi-cavities to obtain a plurality of molding bodies, the coil-shaped conductive wire 12 may be deflected. Accordingly, in the second embodiment as shown in FIG. 5A, the coil-shaped conductive wire 12 is wound around a winding-core 28, and inserted into the metallic mold 20. The coil-shaped conductive wire 12 may be closely wound around the periphery of the winding-core 28, or may be simply inserted onto the winding-core 28. As the winding-core 28, a ceramic material having the same or a different composition from that of the magnetic core 11 may be used. At least a fired magnetic ceramic is employed. In this embodiment, the axial length of the winding-core 28 is longer than the coil-shaped conductive wire 12, and only both the end-portions of the winding-core 28 are supported on the supporting-grooves 22 of the metallic mold 20. Since the coil-shaped conductive wire 12 wound around the winding-core 28 is supported onto the supporting-grooves 22 of the metallic mold 20, the coil-shaped conductive wire 12 is prevented from being deflected, due to the rigidity of the wound core 28, even if the coil-shaped conductive wire 12 is long. Further, when the ceramic slurry 23 is cast or wet-pressed as shown in FIG. 5B, the coil-shaped conductive wire 12 is prevented from rising. The molding body 27 shown in FIG. 5C is obtained by the wet pressing. When the molding body 27 is fired, formation of cracks and voids inside the coil 12 is prevented, since the winding-core 28 is not firing-shrunk. By the firing, the part made from the ceramic slurry 23 and the part composed of the winding-core 28 are integrated with each other to produce an integrally sintered body. Thereafter, the sintered body is cut at an appropriate length, similarly to the first embodiment, to produce the magnetic core 11. The external electrodes 13 and 14 are formed on the core 11 to produce a chip inductor 10. FIG. 6 shows a chip inductor according to a third embodiment of the present invention. In this embodiment, as the conductive wire, a linear conductive wire 15 is employed. The other constitution is the same as that in the first embodiment. Accordingly, the same reference numerals are appended, and the description is omitted. Regarding the inductor using the linear conductive wire 15, the inductance is low as compared with the inductor using the coil-shaped conductive wire 12, but the direct current resistance can be reduced. Accordingly, the inductor is suitable in uses where the resistance is desired to be as low as possible. A method of producing an inductor by use of the linear conductive wire 15 is the same as that shown in FIGS. 4A to 4E, and the redundant description is omitted. The structure of the supporting portions of the metallic mold 10 for supporting both the end-portions of the conductive wire or the wiring core is not limited to the supporting-grooves 22 as described in the embodiments. Any shape and size may be available, on condition that both the end-portions of the conductive wire or the winding-core can be supported with high stability. As seen in the above-description, according to the first aspect of the present invention, the molding body having the conductive wire embedded therein is obtained by wet pressing method. Accordingly, the molding body has a higher density as compared with products formed by extrusion-molding, and a binder is unnecessary, or an extremely small amount of a binder is needed. Thus, when the molding body is fired, the fired magnetic core having a high density is obtained, and no pores are formed, since the amount of the binder is small. An inductor with high qualities can be obtained. Further, both the ends of the conductive wire are supported by the supporting portions formed in the metallic mold. Accordingly, the conductive wire is prevented from becoming eccentric, and an inductor having stable characteristics can be obtained Further, according to the second aspect of the present invention, the coil-shaped conductive wire is wound on the outer periphery of the winding-core made of a fired magnetic ceramic, and the winding-core having the coil-shaped conductive wire wound therearound is set in the metallic mold, followed by molding by wet pressing. In addition to the advantages of the method of producing a monolithic inductor in accordance to the first aspect of the present invention, voids or cracks inside the coil, which will be caused by the firing-shrinkage, can be eliminated. Even if a long coil is used for molding using multi-cavities, deflection of the coil can be prevented, due to the winding-core. Thus, a production method suitable for mass production can be realized.

Claims: A method of producing a chip inductor (10) which comprises the steps of: inserting a conductive wire (12, 15) made of a metallic wire into a metallic mold (20), supporting both the end portions of the conductive wire on supporting-portions (22) formed on the inside of the metallic mold so as to position the conductive wire in the center portion of the metallic mold, casting magnetic ceramic slurry (23) into the metallic mold, molding the ceramic slurry cast in the metallic mold by wet pressing method to obtain a molding body (27) having the conductive wire embedded therein, firing the molding body, and forming external electrodes (13, 14) on both the end-faces of the fired magnetic core so as to be connected to both the end portions of the conductive wire. A method of producing a chip inductor according to Claim 1, wherein said conductive wire is a coil-shaped conductive wire (12) made of a metallic wire defining a spiral shape. A method of producing a chip inductor comprising the steps of: winding a coil-shaped conductive wire (12) made of a metallic wire defining a spiral-shape, on the periphery of a winding-core (28) made of a fired magnetic core, inserting the winding-core (28) having the coil-shaped conductive wire (12) wound therearound into a metallic mold (20), supporting both the end-portions of the winding-core having the coil-shaped conductive wire wound therearound on supporting portions (22) provided inside of the metallic mold, and disposing the coil-shaped conductive wire in the center portion of the metallic mold, casting magnetic ceramic slurry (23) into the metallic mold, molding the ceramic slurry cast in the metallic mold, by wet pressing method to obtain a molding body having the coil-shaped conductive wire embedded therein, firing the molding body, and forming external electrodes (13, 14) on both the end-faces of the fired magnetic core so as to be connected to both the end portions of the conductive wire.

Inventors: ITO YOICHIRO, KOMATSU HIROSHI, MORIMOTO TADASHI, YAMAMOTO TAKAHIRO, ITO, YOICHIRO, KOMATSU, HIROSHI, MORIMOTO, TADASHI, YAMAMOTO, TAKAHIRO, Ito, Yoichiro, (A170) Intellectual Property Dep., Komatsu, Hiroshi, (A170) Intellectual Prop. Dep., Morimoto, Tadashi, (A170) Intellectual Prop. Dep., Yamamoto, Takahiro, (A170) Intellectual Prop. Dep.
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 2000
Main classification: H01F41
IPC codes: H01F17, H01F41, H01F27