Patent Publication Number: US-10770219-B2

Title: Coil component

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a coil component, and more particularly relates to a coil component using a drum core. 
     Description of Related Art 
     A coil component using a drum core can be surface-mounted on a printed circuit board differently from a coil component using a toroidal core, and thus has been widely used for mobile electronic devices such as a smartphone. Further, because the coil component using a drum core has a low height, it also contributes to thinning of mobile electronic devices. 
     However, in recent years, further thinning of mobile electronic devices has been desired, and in order to realize this, a further lower height has been desired for the coil component using a drum core. As one method of realizing a low height of a coil component, a method in which a magnetic top plate generally bonded to a drum core is omitted can be considered. However, in this case, because leakage of the magnetic flux increases, other circuits such as an antenna may be adversely affected. Meanwhile, because a magnetic top plate made of ferrite is fragile, if the thickness thereof is reduced, its strength becomes insufficient, and thus the magnetic top plate may be broken at the time of assembly or at the time of actual use. 
     To solve the above problems, it suffices to use magnetic-powder containing resin having flexibility instead of using ferrite as a material of the magnetic top plate. The magnetic-powder containing resin can maintain certain strength even if it is thinned. Therefore, by using the magnetic-powder containing resin as the material of the magnetic top plate, leakage of the magnetic flux can be suppressed while realizing a low height. As an example of using magnetic-powder containing resin as a material of a magnetic top plate, coil components described in Japanese Patent Application Laid-open No. H9-219318 and Japanese Patent Application Laid-open No. 2004-363178 can be mentioned. 
     However, magnetic powder contained in the magnetic-powder containing resin often has conductivity, which may cause a short circuit failure between itself and a terminal electrode or a wire. Further, the magnetic-powder containing resin has a larger thermal expansion coefficient than that of the drum core, and this causes the peel-off of the resin from the drum core due to a temperature change. 
     SUMMARY 
     It is therefore an object of the present invention to provide a coil component using a top plate containing the magnetic-powder containing resin, capable of making a short circuit failure and peel-off of the top plate less likely to occur while reducing the height. 
     A coil component according to the present invention includes a drum core having a winding core part and first and second flange parts provided on both sides of the winding core part; a wire wound around the winding core part; a plurality of terminal electrodes connected with end portions of the wire, each of the terminal electrodes being provided on an associated one of the first and second flange parts; and a top plate fixed to the first and second flange parts, wherein the top plate includes: a magnetic layer comprising magnetic powder and binder resin; and a resin layer having a smaller content of the magnetic powder than that of the magnetic layer, and wherein the resin layer is positioned between the first and second flange parts and the magnetic layer. 
     According to the present invention, the resin layer is interposed between the first and second flange parts and the magnetic layer, so that it is possible to prevent occurrence of a short circuit failure between the magnetic layer and the terminal electrode or wire. Further, by using a material having a thermal expansion coefficient falling between the thermal expansion coefficient of the magnetic layer and that of the drum core, it is possible to prevent peel-off of the top plate due to a temperature change. Further, mechanical strength of the top plate can be enhanced by the resin layer. 
     In the present invention, it is preferable that the resin layer does not substantially include the magnetic powder. This makes it possible to more reliably prevent occurrence of a short circuit failure. 
     In the present invention, it is preferable that the resin layer includes non-magnetic filler. This makes it possible to adjust the thermal expansion coefficient to a desired value. 
     In the present invention, the resin layer may cover the entire surface of the magnetic layer or may be selectively provided between the first and second flange parts and the magnetic layer. According to the former, it is possible to reduce the manufacturing cost of the top plate and reliably prevent a short circuit failure. According to the latter, it is possible to more reliably prevent peel-off of the top plate due to a temperature change. 
     The coil component according to the present invention preferably further includes an adhesive for bonding the first and second flange parts and the resin layer of the top plate. With this configuration, insulation effect by the adhesive can also be expected. 
     In the present invention, it is preferable that the magnetic layer of the top plate has a lower surface facing the resin layer and an upper surface positioned on the side opposite to the lower surface and that the density of the binder resin is higher at a surface layer part on the upper surface side than at a surface layer part on the lower surface side. This improves the insulation property of the upper surface of the top plate. This prevents occurrence of a short circuit failure due to contact between the upper surface of the top plate and another electronic component, making it possible to obtain a highly reliable coil component. In addition, the density of the magnetic powder is high at the surface layer part on the lower surface side of the magnetic layer, so that a magnetic path passing through the top plate is shortened, which makes it possible to obtain high magnetic characteristics. 
     In the present invention, it is preferable that the magnetic powder is soft magnetic metal powder. Accordingly, high magnetic properties can be obtained. Particularly, it is preferable that the soft magnetic metal powder has a flat shape. Accordingly, higher magnetic properties can be obtained. 
     According to the present invention, there can be provided a coil component capable of making a short circuit failure and peel-off of the top plate less likely to occur while reducing the height. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above features and advantages of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a coil component according to a first embodiment of the present invention when an upper surface thereof is viewed from an oblique direction; 
         FIG. 2  is a plan view of the coil component according to the first embodiment of the present invention as viewed from a mounting surface; 
         FIG. 3  is a sectional view of the coil component according to the first embodiment of the present invention; 
         FIG. 4  is an explanatory schematic sectional view of the structure of a top plate; 
         FIG. 5  is an explanatory schematic diagram of the shape of a magnetic powder contained in the top plate; 
         FIG. 6  is an explanatory schematic sectional view of the structure of a magnetic layer; 
         FIG. 7A  is an electron micrograph of one surface of the magnetic layer; 
         FIG. 7B  is an electron micrograph of the other surface of the magnetic layer; 
         FIG. 8  is an explanatory schematic diagram for explaining a manufacturing method of a sheet in which the magnetic layer is coated on a base film; 
         FIG. 9  is an explanatory schematic diagram for explaining a manufacturing method of a sheet in which the resin layer is coated on the magnetic layer; 
         FIGS. 10A to 10C  are process diagrams for explaining a manufacturing method of the coil component; 
         FIG. 11  is a perspective view of a coil component according to a second embodiment of the present invention when an upper surface thereof is viewed from an oblique direction; and 
         FIG. 12  is a sectional view of the coil component according to the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Preferred embodiments of the present invention will be explained below in detail with reference to the accompanying drawings. 
     First Embodiment 
       FIGS. 1 to 3  are diagrams showing an appearance of a coil component  11  according to the first embodiment of the present invention.  FIG. 1  is a perspective view of the coil component  11  when an upper surface thereof is viewed from an oblique direction,  FIG. 2  is a plan view thereof as viewed from a mounting surface, and  FIG. 3  is a sectional view thereof. 
     As shown in  FIGS. 1 to 3 , the coil component  11  according to the first embodiment includes a drum core  20  and a top plate  30 . The drum core  20  has a winding core  21  with an x direction being an axial direction, and first and second flange portions  22  and  23  provided at opposite ends of the winding core  21  in the x direction. The drum core  20  is made of a ceramic material having high magnetic permeability such as ferrite, and has a configuration in which the winding core  21  and the flange portions  22  and  23  are integrally formed. 
     Two wires W are wound around the winding core  21 , and opposite ends of these wires W are respectively connected to terminal electrodes E 1  to E 4  provided in the flange portions  22  and  23 . In the present embodiment, the terminal electrodes E 1  and E 2  are formed in one flange portion  22 , and the terminal electrodes E 3  and E 4  are formed in the other flange portion  23 . The terminal electrodes E 1  to E 4  are formed continuously on an xy plane of the flange portions  22  and  23  constituting a mounting surface, an xy plane of the flange portions  22  and  23  located opposite to the mounting surface and constituting an upper surface, and yz planes of the flange portions  22  and constituting outer side surfaces. In the present embodiment, the wires W are respectively connected to the terminal electrodes E 1  to E 4  provided on the upper surfaces of the flange portions  22  and  23 ; however, the wires W can be connected to the terminal electrodes E 1  to E 4  provided on the mounting surfaces of the flange portions  22  and  23 . In this case, the terminal electrodes E 1  to E 4  do not need to be provided on the upper surfaces of the flange portions  22  and  23 . 
     The application of the coil component  11  according to the present embodiment is not particularly limited to any type, and can be a general-purpose coil component for inductance, or can be a coil component for a specific application, for example, for a common-mode filter, for a pulse transformer, or for a balun transformer. Therefore, the number of wires W wound around the winding core  21 , the number of windings, the winding direction, and the winding method are not particularly limited to any specific number. The size of the coil component  11  is not particularly limited to any specific size. However, the length in the x direction is about 1.6 millimeters, the width in a y direction is about 1.0 millimeter, and the height in a z direction is from about 0.55 millimeter to about 0.65 millimeter. 
     As illustrated in  FIGS. 1 and 3 , the top plate  30  is fixed to the xy plane constituting the upper surfaces of the flange parts  22  and  23  through an adhesive  40 . As illustrated in  FIG. 4 , the top plate  30  has a laminated structure of a magnetic layer  31  (upside) and a resin layer  32  (downside). That is, the resin layer  32  is positioned between the flange parts  22 ,  23  and the magnetic layer  31 . 
     The magnetic layer  31  is made of magnetic-powder containing resin obtained by mixing magnetic powder  35  in binder resin  34 , and has higher magnetic permeability than general resin. Because the top plate  30  having the magnetic layer  31  is fixed on the upper surfaces of the flange portions  22  and  23  so as to span the wiring core  21 , the drum core  20  and the magnetic layer  31  constitute a closed magnetic path. Therefore, as compared to a case where a top plate made of only resin is used, leakage of the magnetic flux decreases, and magnetic impact on other circuits, for example, on an antenna circuit can be reduced. Further, the top plate  30  is also used as an adsorption face for handling at the time of mounting on a printed circuit board. 
     As described above, the magnetic-powder containing resin constituting the magnetic layer  31  is obtained by mixing magnetic powder  35  in binder resin  34 . The binder resin  34  preferably has a cross-linked structure by urethane bond, with acrylic ester copolymer being a main chain. Meanwhile, it is preferable to use soft magnetic metal powder having a flat shape in an xy plane for the magnetic powder  35  as shown in  FIG. 5 . When soft magnetic metal powder having a flat shape is used, it is preferable to mix the soft magnetic metal powder in the binder resin  34  so that a principal plane of the soft magnetic metal powder becomes the xy plane. Accordingly, the magnetic permeability in the x direction, being a direction of the magnetic flux passing the magnetic layer  31 , can be increased, and the soft magnetic metal powder having a flat shape also functions as an electromagnetic shield. 
     The resin layer  32  plays a role of preventing direct contact between the flange parts  22 ,  23  and the magnetic layer  31  by being interposed therebetween. The magnetic powder  35  contained in the magnetic layer  31  has conductivity, and this magnetic layer  31  may be exposed from the binder resin  34 , and thus the resin layer  32  needs to be interposed. An insulating resin material similar to that of the binder resin  34  can be used as a material for the resin layer  32 . Using the resin layer  32  made of such an insulating resin material significantly improves dielectric strength between the magnetic layer  31  and the terminal electrodes E 1  to E 4  or wire W, thereby making it possible to prevent a short circuit failure from occurring therebetween. Further, the resin layer  32  functions as a cushioning material, thereby improving impact resistance. 
     Magnetic or non-magnetic filler may be added to the resin layer  32 . However, an excessively large content of the magnetic powder may deteriorate an insulating property, so that when the magnetic powder is added to the resin layer  32 , the content of the magnetic powder needs to be less than that in the magnetic layer  31 . On the other hand, the non-magnetic filler can be added for controlling physical characteristics such as a thermal expansion coefficient. When the thermal expansion coefficient of the resin layer  32  is adjusted to a value between the thermal expansion coefficient of the magnetic layer  31  and that of the drum core  20  by addition of the non-magnetic filler, it is possible to prevent peel-off of the top plate  30  due to a temperature change. Examples of the non-magnetic filler include, e.g., talc and mica. 
     As described above, in the coil component  11  according to the present embodiment, the top plate  30  fixed to the drum core  20  has a laminated structure of the magnetic layer  31  and resin layer  32 , and the resin layer  32  is interposed between the drum core  20  and the magnetic layer  31 , so that it is possible to improve dielectric strength, impact resistance, peeling strength, and the like while reducing the height. 
       FIG. 6  is an explanatory schematic sectional view of the structure of the magnetic layer  31 . 
     As shown in  FIG. 6 , in the magnetic layer  31 , distribution of the binder resin  34  and the magnetic powder  35  in the thickness direction (the z direction) is not completely uniform, and particularly, a surface layer part  31 A and a surface layer part  31 B may have different characteristics from each other. 
     Specifically, in an inner layer part  31 C of the magnetic layer  31 , the soft magnetic metal powder  35  is distributed substantially uniformly in the binder resin  34 . Meanwhile, in the surface layer part  31 B, the density of the magnetic powder  35  is lower than that in the inner layer part  31 C, and the density of the binder resin  34  is higher than that in the inner layer part  31 C. As a result, there is less magnetic powder  35  exposed on the surface  31   b , and typically, the magnetic powder  35  is hardly exposed on the surface  31   b . In this case, substantially the entire surface of the surface  31   b  is covered with the binder resin  34 . On the other hand, a configuration of the surface layer part  31 A is substantially identical to that of the inner layer part  31 C. That is, in the surface layer part  31 A, the density of the magnetic powder  35  in the binder resin  34  is substantially the same as that in the inner layer part  31 C. Therefore, the magnetic powder  35  may be exposed to some extent from the surface  31   a.    
       FIGS. 7A and 7B  are electron micrographs of the magnetic layer  31  actually manufactured, wherein  FIG. 7A  is a photograph of the surface  31   a , and  FIG. 7B  is a photograph of the surface  31   b . In these photographs, a dark portion is the binder resin  34 , and a white portion is the magnetic powder  35 . 
     As shown in  FIG. 7A , it is understood that, in the surface layer part  31 A on the side of the surface  31   a , the density of the magnetic powder  35  is high and the density of the binder resin  34  is low, and thus when the magnetic layer  31  is shot by an electron microscope, a large amount of the magnetic powder  35  is shot white. It is also understood that a large amount of the magnetic powder  35  is exposed on the surface  31   a . On the other hand, as shown in  FIG. 7B , it is understood that in the surface layer part  31 B on the side of the surface  31   b , the density of the magnetic powder  35  is low and the density of the binder resin  34  is high, and thus when the magnetic layer  31  is shot by the electron microscope, it is shot dark across the board. Particularly, there is hardly any magnetic powder  35  exposed on the surface  31   b.    
     In this manner, the magnetic layer  31  has such a feature that the density of the binder resin  34  is higher in the surface layer part  31 B than in the surface layer part  31 A. A manufacturing process of the top plate  30  described later can cause such a difference to be generated between the surface layer parts  31 A and  31 B. 
     By utilizing such a difference in surface property, it is possible to impart more favorable characteristics to the coil component  11 . For example, when the surface  31   a  is disposed at the lower layer side (resin layer  32  side) and the surface  31   b  at the upper layer side, an insulating property on the upper surface side of the top plate  30  is improved, making it possible to prevent occurrence of a short circuit failure due to contact between the upper surface of the top plate  30  and another electronic component. In addition, the density of the binder resin  34  is low at the surface layer part  31 A on the lower surface side of the magnetic layer  31 , that is, more magnetic powder  35  exists, so that a magnetic path passing through the top plate  30  is shortened, which makes it possible to obtain high magnetic characteristics. 
     Conversely, when the surface  31   b  is disposed at the lower layer side (resin layer  32  side) and the surface  31   a  at the upper layer side, dielectric strength between the magnetic layer  31  and terminal electrodes E 1  to E 4  or wire W is increased. Further, impact resistance and peeling strength are also improved. 
     While it is not particularly limited thereto, the thickness of the top plate  30  in the z direction is preferably equal to or less than 100 micrometers, more preferably equal to or less than 75 micrometers, and particularly preferably about 60 micrometers. If the thickness of the top plate  30  is equal to or less than 100 micrometers, the height of the entire coil component  11  in the z direction can be set low. When the thickness of the top plate is reduced to 100 micrometers or less while using ferrite, the top plate may be broken due to insufficient strength. However, if the top plate  30  in which the magnetic powder  35  is mixed in the binder resin  34  is used, even if the thickness is reduced to 100 micrometers or less, there will be no breakage. While the lower limit of the thickness of the top plate  30  is not particularly limited to any size, it is preferable that the lower limit is equal to or higher than 30 micrometers. This is because if the thickness of the top plate  30  is reduced to less than 30 micrometers, the strength is not sufficient, and it is difficult to ensure sufficient magnetic properties. To suppress leakage of the magnetic flux sufficiently, it is preferable that the magnetic permeability of the magnetic layer  31  included in the top plate  30  is equal to or higher than 30. 
     Predetermined flexibility, heat resistance, and strength are required for the binder resin to be used for the magnetic layer  31 . The reason the flexibility and the strength are required is that even if the thickness of the top plate  30  is reduced, for example, to 100 micrometers or less, there will be no breakage. The reason the heat resistance is required is that there will be no deformation at the time of reflow. Therefore, a material having high strength but low flexibility, and a material having high flexibility but low heat resistance is not appropriate. Because the reflow temperature is about 260° C., at least binder resin that is not deformed at that temperature needs to be used. 
     Taking these points into consideration, in the present embodiment, binder resin having a cross-linked structure by urethane bond, with acrylic ester copolymer being amain chain is used. Regarding the composition, although not particularly limited thereto, the acrylic ester copolymer preferably has at least a copolymer structure of ethyl acrylate and a copolymer structure of butyl acrylate. This is for adding the flexibility by the copolymer structure of butyl acrylate, while ensuring high strength by the copolymer structure of ethyl acrylate. It is also preferable that the acrylic ester copolymer further has a copolymer structure of acrylonitrile. This is because the heat resistance and strength are increased by containing the copolymer structure of acrylonitrile. 
     The top plate  30  can be manufactured according to the following method. First, a binder solution in which a solute containing ethyl acrylate, butyl acrylate, and acrylonitrile having a hydroxyl group or a carboxyl group as a functional group is a main monomer is dissolved in an organic solvent such as methyl ethyl ketone is prepared. A mixed solution is prepared by mixing magnetic powder and a curing agent in the binder solution. As the curing agent, it is preferable to use isocyanate. As the isocyanate, for example, it is preferable to use, for example, aromatic isocyanate or isocyanate containing a triazine ring in the structure, and more preferably, containing a plurality of isocyanate groups in one molecule. Accordingly, the hydroxyl group or the carboxyl group contained as the functional group in the acrylic ester copolymer reacts with isocyanate to form the cross-linked structure. Further, a filler other than the magnetic powder, for example, talc or mica can be further mixed therein. 
     As shown in  FIG. 8 , the mixed solution is applied onto a base film F, the base film F is then wound around a roll while heating the base film F to dry the solvent in the mixed solution and cure the binder resin. The magnetic powder can be oriented in a predetermined direction by applying magnetic field at the time of applying the mixed solution onto the base film F. Accordingly, a sheet S 1  in which the magnetic layer  31  made of the magnetic-powder containing resin is applied onto the surface of the base film F is obtained. As the base film F, a PET film can be used. It is preferable that a content ratio of the magnetic powder in the cured magnetic-powder containing resin is from 50% to 90% by weight. If the content ratio of the magnetic powder is less than 50% by weight, sufficient magnetic permeability cannot be obtained, and if the content ratio thereof exceeds 90% by weight, the possibility that the magnetic powder falls off from a cut surface of the top plate  30  increases. 
     When the magnetic layer  31  made of the magnetic-powder containing resin is applied onto the surface of the base film F, characteristics of the magnetic layer  31  are slightly different in the surface layer part on the base film F side and in the surface layer part on an exposed side opposite thereto. It is considered this is due to the surface tension of the binder resin, which is uncured. The density of the magnetic powder  35  becomes low in the surface layer part  31 B on the base film F side and becomes high in the surface layer part  31 A on the exposed side. 
     Then, the magnetic layer  31  is peeled off from the base film F, and then, as illustrated in  FIG. 9 , a resin material constituting the resin layer  32  is applied onto the surface  31   a  positioned on the side opposite to the base film F, followed by rolling up while thermal curing is performed. As a result, a sheet S 2  in which the resin layer  32  is applied onto the surface of the magnetic layer  31  is obtained. 
     Then, as illustrated in  FIG. 10A , the sheet S 2  is stamped into a planar shape corresponding to the top plate  30  using a die. Then, an epoxy-based adhesive  40  is applied onto the stamped portion as illustrated in  FIG. 10B , and then the drum core  20  around which the wire W is wound is bonded to the stamped portion, as illustrated in  FIG. 10C . Then, the drum core  20  to which the top plate  30  is bonded is separated from the sheet body, whereby the coil component  11  of the present embodiment is accomplished. 
     By manufacturing the coil component  11  using the above method, the top plate  30  can be bonded to the drum core  20  with the surface of the magnetic powder-containing resin directed to the base film F side during the adhesive application, i.e., the surface  31   b  of the magnetic layer  31  at the side where the density of the binder resin  34  is higher, as the upper surface. The resin layer  32  may be formed without peeling off the magnetic layer  31  from the base film F. In this case, the base film F may be peeled off after completion of the process illustrated in  FIG. 10C . 
     On the other hand, when the top plate  30  can be bonded to the drum core  20  with the surface  31   a  of the magnetic layer  31  at the side where the density of the binder resin  34  is lower, as the upper surface, the base film F itself can be used as the resin layer. This can reduce the number of application processes to one, making it possible to reduce the manufacturing cost. However, in this case, as a material for the base film F, it is necessary to use not the PET resin, but a resin material having higher heat resistance. 
     Second Embodiment 
       FIGS. 11 and 12  are views each illustrating the configuration of a coil component  12  according to the second embodiment of the present invention.  FIG. 11  is a perspective view as viewed obliquely from above, and  FIG. 12  is a cross-sectional view. 
     As illustrated in  FIGS. 11 and 12 , the coil component  12  according to the second embodiment of the present invention differs from the above-described coil component  11  according to the first embodiment in that the resin layer  32  is selectively provided between the flange parts  22 ,  23  and the magnetic layer  31 . That is, the resin layer  32  is divided to two parts in a plan view. Other configurations are the same as those of the coil component  11  according to the first embodiment, so the same reference numerals are given to the same elements, and overlapping description will be omitted. 
     According to the present embodiment, the resin layer  32  is thus divided into two parts, so that even if there is a comparatively large difference between the thermal expansion coefficient of the resin layer  32  and that of the drum core  20 , the top plate  30  is unlikely to be peeled off. In the example illustrated in  FIGS. 11 and 12 , the planar sizes of the resin layer  32  and the flange parts  22 ,  23  substantially coincide with each other; however, it is sufficient that at least the resin layer  32  is interposed between a part of the flange part  22  and the magnetic layer  31  and between a part of the flange part  23  and the magnetic layer  31 . However, to ensure dielectric strength and impact resistance more reliably, it is preferable to completely cover the surfaces of the flange parts  22  and  23  in a plan view (as viewed in the z-direction) with the resin layer  32 . 
     The coil component  12  according to the present embodiment is preferably manufactured by bonding the resin layer  32  previously cut into a predetermined size to the upper surfaces of the flange parts  22  and  23  and then bonding the magnetic layer  31 . With this method, positioning of the resin layer  32  can be performed more easily than a method in which the resin layer  32  is bonded to the magnetic layer  31 . 
     It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention.