Composite magnetic material, method for manufacturing the same, and electromagnetic interference suppressing sheet

Abstract: Method of manufacturing a composite magnetizer wherein flat soft magnetic powder from which strain has been removed and magnetic coating of slurry regime consisting of a binder dissolved in a solvent are formed in a sheet by die-coating, gravure coating, or reverse coating. The binder includes a vinyl chloride resin with glass-transition temperature of not lower than 50°C.

Description: Background of the Invention: The present invention relates to an electromagnetic interference suppressor for preventing unwanted electromagnetic waves from leaking out of electronic equipments, or for providing interferences between internal circuits and influences of miss-operation caused by external electromagnetic waves. In practically, the present invention relates to an electromagnetic interference suppressor formed by a composite magnetic body (magnetizer) consisting of organic binder having soft magnetic powder dispersed therein. Recently, an importance of a measure devised to deal with electromagnetic environmental problems in electronic equipments is recognized increasingly. This is supported by a background in which many kinds of electronic equipments provide circuits with higher density than before and both active and passive elements each consisting of a semiconductor device, and the like, which radiate inductive noise are congested increasingly. These are because electronic devices such as digital electronic equipments and the like aim at responding to requirements of accelerating circuit signal processing speed, higher frequency higher performances of multifunction, down sizing and lower profiling of product configuration, in the back ground. As a result, it is often caused to occur that normal operation of equipments are disturbed. This is caused by an increase of line-to-line coupling due to electrostatic coupling and electromagnetic coupling and also by interference of radiation noise. In some cases, the noise may affect other external equipments. As countermeasures against noise caused by electronic devices with higher processing speed, more advanced function and higher density, particularly, a countermeasure against noise caused in quasi-microwave band, installation of low pass filter, shielding and the like can be taken. However, a countermeasure by components requires space for installation in the electronic device and needs to consider down sizing and low profiling in design phase. The counter measure by components is therefore not suitable for an urgent measure. In addition an inductance component contributes to real part magnetic permeability µ' and is insufficient for inductance in current quasi-raicrowave band. Further, in the event of conducting an inadvertent shielding, secondary electromagnetic coupling may be induced. In response to the problems, the above-mentioned composite magnetic body sheet for suppressing electromagnetic interference has been proposed as one of countermeasures against noise in quasi-microwave band. For example, proposals are made about composite magnetic body sheets capable of suppressing electromagnetic interference in Japanese Patent Application No. Hei 6-4864 and Japanese Patent Application No. Hei 7-183911. The above-mentioned composite magnetizer sheets correspond to quasi-microwave band and are capable of reducing radiation noise and secondary electromagnetic coupling as wave absorbers using high real part magnetic permeability µ' and imaginary part magnetic permeability µ" in a wide frequency band. Accordingly, it is possible to lighten the burden of the anti-noise measure and product development and to avoid impediments to achieve down sizing and a higher density of a circuit. Further, in the market, a component of an anti-noise measure which is easy to use similar to the above-mentioned composite magnetic body and is capable of corresponding to retrofit components is required. The component of an anti-noise measure is preferred to have frequency band capable of component correspondence, that is, low frequency (at around 10 ∼ 100 MHz) in quasi-microwave band. Further, in the circumstance that down sizing of the electronic equipments is proceeding, the composite magnetic body sheet is required to have lower profiling corresponding to the above-mentioned quasi-microwave band. The above-mentioned composite magnetic body sheet utilized for quasi-microwave band can provide less magnetic permeability as the frequency becomes lower. Accordingly, providing lower profiling in configuration is limited and effect of suppressing the electromagnetic interference reduces, as it is understood from the rise in frequency of µ". As another anti-noise measure which may be taken in small electronic equipments such as a mobile communications equipment, and the like, installing ferrite or rubber ferrite may be acceptable in idea. However, in most events, ferrite can not be installed because of the problems of a possibility to be cracked or a difficulty of placement. On the other hand, rubber ferrite which has less possibility to be cracked can not suppress noise sufficiently, even though a space for installation is reserved. Components such as coils, filters and the like are usable in the frequency of a lower side and a higher side than the frequency at around 10 ∼ 100 MHz. However, currently, no anti-noise measure which is easy to be used in the above-mentioned frequency band is found. Even if a component corresponding to such an anti-noise measure is found, change of design of a substrate, and the like requires a great deal of costs and man-hour. To solve these problems, it becomes necessary to provide a composite magnetic body sheet adoptable to the lower frequency (at around 10 ∼ 100 MHz) and capable of expressing the electromagnetic interference suppressing effect even in the lower profiling thereof. In order to provide such a composite magnetic body sheet, it becomes indispensable to enhance magnetic permeability µ' and µ" and lower the resonance frequency. In response to these requirements, for example, a proposal is made about a composite magnetic body sheet in which the suppressing effect in FM band (at around10 ∼ 100 MHz) is improved and which is adjusted to lower profiling in Japanese Patent Application No. Hei 10-168273. Improvement of the electromagnetic interference suppressing effect and lower profiling in configuration has been achieved by using the technologies disclosed in the above-mentioned Japanese Patent Application. Here, description is made about an example of methods for forming the above-mentioned composite magnetic body. As methods for forming the above-mentioned quasi-microwave band composite magnetic body sheet, there are a wet process in which a blended and agitated slurry admixture of flat soft magnetic powder, binder and solvent for dissolving the binder is formed into a film by using doctor blade, and the like, and a dry process in which an admixture of flat soft magnetic powder and binder is kneaded by kneader, and the like, and is formed by roll, and the like. Further, in the method for manufacturing a composite magnetic body disclosed in the above-mentioned Japanese Patent Application No. Hei 10-168273, flat soft magnetic powder in which stress-strain is eliminated is formed in sheets by wet process in order not to cause a strain to the flat soft magnetic powder in the forming process and pressure is added on the sheet surfaces in vertical direction by press, thus a high density is achieved. Accordingly, it is possible to enhance the electromagnetic interference suppressing effect in FM band and to correspond to lower profiling. Even though the effect of suppressing noise with frequency at around 10 ∼ 100 MHz is enhanced and profiling of the configuration becomes lower by the method disclosed in Japanese Patent Application No. Hei 10-168273, it.is possible to obtain a composite magnetic body having electromagnetic interference suppressing effect reserved therein. However, the composite magnetic body obtained by the above-mentioned method provides flame resistance insufficiently. In addition, when enogh magnetic permeability property can not be obtained sufficiently only by coating, and in the event the flat soft magnetic powder is pressed for increasing packed density to reserve high magnetic permeability, the flat soft magnetic powder gets strain in the result, and obtaining the property required is difficult. Further, the composite magnetizer sheet disclosed in Japanese Patent Application No. Hei 10-168273 with the thickness of more than 0.3 mm is poor in flexibility and provides less mechanical strength as the thickness becomes smaller in the coating process, thus handling in the post processing becomes difficult. Further more, in the above-mentioned composite magnetizer, phenomenon of secular changes caused by moisture penetration inside, in concrete, depression of density is found and coating of binder on the surface is required to improve high temperature high humidity resistance, regardless of the thickness. Moreover, according to the above-mentioned dry process, flat soft magnetic powder in which strain is removed is supplied with the strain again. It is thereby difficult to obtain desirable characteristics. EP 0 866 649 A1 discloses a composite magnetic body comprising flat soft magnetic powder, which had been subjected to an annealing process, and a heat conductive powder being dispersed in an organic binder. As the organic binder, a thermoplastic resin can be used, among which resin types polyvinyl chloride resin is mentioned. The thermoplastic resin preferably has a glass transition temperature of not less than 120°C. The composite magnetic body is obtained by a kneading and a roll moulding process. Summary of the Invention: It is therefore an object of the present invention to provide a composite magnetizer for electromagnetic interference suppressing and a method of manufacturing the composite magnetizer in which strength and electromagnetic interference suppressing effect are reserved and in which flame resistance and mechanical strength are provided sufficiently therein by filling the flat soft magnetic powder, from which strain is removed, with high density. Other objects of the present invention will become clear as the description proceeds. According to an aspect of the present invention according to claim 1, there is provided a method of manufacturing a composite magnetic body in which flat soft magnetic powder from that strain is removed and a binder are blended and kneaded to obtain an admixture, the admixture being formed in a sheet by molding means selected from the group consisting of roll molding, injection molding, and extrusion molding, characterized in that the binder includes a mixture of chlorinated polyethylene and vinyl chloride resin with glass-transition temperature of not lower than 50°C. The contents of vinyl chloride monomer in the vinyl chloride resin may be 75 ∼ 90 mol%. The mixing ratio of the chlorinated polyethylene and the vinyl chloride resin may be 4 : 1 through 1 : 4 in weight. The Flat soft magnetic powder may be oriented and arrayed in-plane direction of the composite magnetic body sheet and the packed density of the flat soft magnetic powder may be enhanced by applying roll working to the composite magnetic body sheet using a device selected from the group consisting of a reduction roll device, a calendar roll device, and a pressing device. The reduction roll device and the calendar roll device may be provided with a roll which is made of a material selected from the group consisting of a rubber with rubber hardness of not smaller than 90 and a polymeric material. A composite magnetic body or a composite magnetic body sheet may be manufactured by the method. According to another aspect of the present invention according to claim 2, there is also provided a method of manufacturing a composite magnetic body in which flat soft magnetic powder from that strain is removed and magnetic coating of a slurry consisting of a binder dissolved in a solvent are formed in a sheet by coating means selected from the group consisting of die-coating, gravure coating, and reverse coating, characterized in that the binder includes a vinyl chloride resin with glass-transition temperature of not lower than 50°C. The binder may be a mixture of chlorinated polyethylene and vinyl chloride resin with glass-transition temperature of not lower than 50°C. The contents of vinyl chloride monomer in the vinyl chloride resin may be 75 ∼ 90 mol%. The mixing ratio of the chlorinated polyethylene and the vinyl chloride resin may be 4 : 1 through 1 : 4 in weight. The flat soft magnetic powder may be oriented and arrayed in-plane direction of the composite magnetic body sheet and the packed density of the flat soft magnetic powder may be enhanced by applying roll working to the composite magnetic body sheet using a device selected from the group consisting of a reduction roll device, a calendar roll device, and a pressing device. The reduction roll device and the calendar roll device may be provided with a roll which is made of a material selected from the group consisting of a rubber with rubber hardness of not smaller than 90, and a polymeric material. A composite magnetic body or a composite magnetic body sheet may be manufactured by the method. The flat soft magnetic powder used in the present invention can be obtained in a flattened shape by applying mechanical grin ding to spherical or indefinite shaped coarse powder. In the process of grinding, strain is generated in the flat soft magnetic powder to deteriorate characteristics of the powder. However, the above-mentioned strain can be removed by conducting anneal process. It is necessary to conduct the anneal process so as to obtain the flat soft magnetic powder having desired characteristics. The conditions of the anneal process are determined, dependent on the materials of the soft magnetic powder as appropriate. As mentioned above, there are dry process and wet process for forming the admixture of flat soft magnetic powder and binder in a sheet. The wet process is more preferable for reducing the load of external force onto the flat soft magnetic powder. In the present invention, a composite magnetic body in which a flat direction of soft magnetic powder is oriented to in-plane direction of a sheet can be obtained by forming a film by die-coating, gravure coating or reverse coating. In the present invention, vinyl chloride resin with glass-transition temperature of 50°C or more is used as a binder. Therefore, the strength of binder at room temperature is increased. It becomes easier to handle the composite magnetic body in the work process. The contents of chlorine in the binder are increased 'by using vinyl chloride resin. Thereby, flame resistance is enhanced. Further, high temperature and high humidity resistance of the composite magnetic body can be improved, since polyvinyl chloride has lower water vapor permeability than polyethylene. In the present invention, the most preferable value of content of vinyl chloride monomer in vinyl chloride resin for expressing the above-mentioned effect sufficiently is 75 ∼ 90 mol% and mixing ratio of chlorinated polyethylene and vinyl chloride resin is 4 : 1 ∼ 1 : 4 in weight. It is effective to enhance the density of the composite magnetizer for obtaining the composite magnetizer with high magnetic permeability. High density can be implemented by applying roll working on the sheet after forming into a film and removing the solvent. Therefore, in the method of manufacturing the composite magnetic body sheet of the present invention, a surface of the composite magnetic body sheet is pressed vertically by rolling device utilizing press or roll. The pressing brings a has secondary effect to enhance orientation extent of the flat soft magnetic powder. During the above-mentioned process, if the press work is applied to the composite magnetic body inadvertently, strain is again generated in the flat soft magnetic powder. Therefore, a rolling device providing a roll consisting of rubber with rubber hardness of not less than 90 or consisting of polymeric material is used in the present invention. Brief Description of the Drawings: Fig. 1 is a diagram for showing the result of evaluation of relative magnetic permeability of a composite magnetic body according to a preferred embodiment of the present invention; Fig. 2 is a schematic diagram for showing a dual axis reduction roll used in a rolling process in the preferred embodiment of the present invention; Fig. 3 is a schematic diagram for showing the dual axis reduction roll used in the rolling process in which the dual axis reduction roll is press-welded and deformed; and Fig. 4 is a schematic diagram for showing a device for evaluating electromagnetic interference suppressing effect of the composite magnetic body according to the preferred embodiment of the present invention. Detailed Description of the Preferred Embodiment: Referring to Figs. 1 through 4, description will proceed to a composite magnetic body, a composite magnetic body sheet, and a method of manufacturing the composite magnetic body and the composite magnetic body sheet according to a preferred embodiment of the present invention. At first, an Fe-Al-Si alloy powder having an average diameter of 35 µm and an aspect ratio of not less than 5 is prepared as flat soft magnetic powder. The flat soft magnetic powder is subjected to the heat treatment for two hours on condition that temperature is kept at 650°C in order to remove the strain thereof. 85 weight% of the flat soft magnetic powder, 14 weight% of binder in which chlorinated polyethylene and vinyl chloride resin are blended with weight ratio of 4 : 1, 1 weight% of titanate coupling agent, and 200 weight% of toluene as solvent are weighed out, respectively. By putting all of the above-mentioned materials into a mixer to be agitated therein, the flat soft magnetic powder is subjected to a surface treatment by the coupling agent. In addition, the binder is dissolved in a solvent. A slurry for forming the composite magnetizer is thereby obtained. In this embodiment of the present invention, the binder is dissolved in the solvent to be mixed with other materials, so that the strain is not added to the flat soft magnetic powder. Incidentally, the vinyl chloride resin used in this embodiment has contents of vinyl chloride monomer of 75 ∼ 90 weight% and a glass-transition temperature of not lower than 50°C. The vinyl chloride resin contributes not only to enhancing the flame resistance of the composite magnetic body but also to improving the workability of roll forming. Next, the above-mentioned slurry is formed by die-coating into a film with a flat surface of the flat soft magnetic powder being oriented in-plane direction, so that a composite magnetic body sheet is fabricated. The composite magnetic body sheet produced in the above-mentioned operation has a thickness of about 0.08 mm. Magnetic field orientation may be conducted to orient the flat soft magnetic powder with a greater extent before the elimination of the solvent in the process. Coating by gravure coating or reverse coating is also acceptable. In addition, the composition of the above-mentioned binder is determined in consideration of the flexibility of the composite magnetic body. However, when the composite magnetic body can be more rigid, the mixing ratio of chlorinated polyethylene and vinyl chloride resin may be 1 : 4 in weight. It is possible to obtain composite magnetizer providing voluntary mechanical strength and flexibility by adjusting the ratio between 1 : 4 and 4 : 1. Accordingly, configuration of products becomes various. For example, when a very thin composite magnetic body with the thickness of not more than 0.1 mm is manufactured, blending a larger amount of vinyl chloride resin is preferable in consideration of workability in a later process. Besides, if the mixing ratio of chlorinated polyethylene and vinyl chloride resin is beyond the range as mentioned above, undesirable effects are caused to occur. Namely, when vinyl chloride resin is too much, the flexibility becomes short. On the other hand, when chlorinated polyethylene is too much, not only the mechanical strength of the composite magnetic body becomes short but also the flame resistance becomes smaller. Then, the above-mentioned composite magnetic body sheet is subjected to roll working by using the dual axis reduction roll illustrated in Fig. 2. Rolling is conducted on conditions that roll spaces are determined to be substantially 0 and that temperature is kept at 50°C. A layer 22 consisting of rubber with rubber hardness of not smaller than 90 or of polymeric material is formed around the surface of a metallic roll 23 in the roll illustrated in Fig. 2. Herein, rubber hardness thus mentioned is the value of international rubber hardness measured by rubber hardness testing method standardized in JIS, K6253. Fig. 3 is a view for showing the layer 22 consisting of rubber or polymeric material illustrated in Fig. 2 is press-welded. Since the portion is elastically deformed, a contact portion 31 of a certain extent of area is formed by the deformation as shown in Fig. 3. Accordingly, the composite magnetic body sheet rolled in the portion is rolled evenly without receiving high pressure locally. Consequently, strain is not added to the flat soft magnetic powder. Therefore it is possible to make the composite magnetic body sheet be rolled without shifting magnetic resonance frequency fr to a higher frequency side and inducing the reduction of relative magnetic permeability µ' and µ". By roll working, the density of the composite magnetic body sheet becomes 1.5 times of the density before rolling. In Fig. 1, illustrated is a result of measurement of the relative magnetic permeability characteristics of the composite magnetic body sheet. In Fig. 1, reference numerals 11 and 12 show µ' and µ" after the roll working by the roll of the above-mentioned composite magnetic body sheet, respectively. On the other hand, reference numerals 13 and 14 show µ' and µ" before the roll working by the roll of the above-mentioned composite magnetic body sheet, respectively. In addition, reference numerals 15 and 16 show µ' and µ" of the composite magnetic body sheet fabricated by a dry process, respectively. The reference numerals 15 and 16 are described for comparison. As clearly shown in Fig. 1, in the composite magnetic body sheet according to this embodiment of the present invention, enhancement of the relative magnetic permeability characteristics by the roll working is confirmed. On the other hand, in the composite magnetic body sheet fabricated by a dry process, deterioration of the magnetic permeability characteristics is confirmed, which is understood to be due to the strain added to the flat soft magnetic powder. Further, although description is herein made about a roll device with dual axis roll, calendar roll device or a pressing device is also usable as a roll device. In this case, it is necessary to form a layer consisting of a rubber with rubber hardness of not smaller than 90 or a polymeric material in a roll provided with the above-mentioned calendar roll device. It is passible to fabricate the composite magnetic body sheets thickened by piling a larger numbers of the sheets by the use of a pressing device, compared with the event of using a roll device having rolls therein. Next, referring to Fig. 4, description proceeds to an electromagnetic interference suppressing effect of the composite magnetic body sheet according to this embodiment of the present invention in comparison with that manufactured by a dry process. Fig 4. is a view for showing the outline of the device used for evaluation of the electromagnetic interference suppressing effect. The measuring device comprises an electromagnetic field wave generator 42, an electromagnetic intensity measuring instrument (spectrum analyzer) 43, a micro loop antenna 44 for radiating electromagnetic wave, and a micro loop antenna 45 for receiving electromagnetic wave, both of which have a diameter of not longer than 2 mm, respectively. Therein, the composite magnetic body sheet 41 of this embodiment is located between the two micro loop antennas. The transmittance attenuation is measured with reference to the event without the composite magnetic body sheet. In those cases, composite magnetic body sheets with the thickness of 0.3 mm and 0.5 mm are used. The result is shown in the following table. frequency 50 MHz 200MHz The present embodiment (the thickness of 0.5 mm) -17 dB -13 dB The present embodiment (the thickness of 0.3 mm) -12 dB -9 dB By a dry process (the thickness of 0.5 mm) -10 dB -8 dB As shown in the above table, an electromagnetic interference suppressing effect obtained in the composite magnetic body sheet of this embodiment having the thickness of 0.3 mm is almost equal to the value provided by the composite magnetic body sheet having the thickness of 0.5 mm fabricated by a dry process. Herein, description is made about only the composite magnetic body fabricated by the so-called wet process. However, flame resistance and mechanical strength can be achieved sufficiently in the composite magnetic body fabricated by a dry process, similarly to those of the composite magnetic body fabricated by the wet process. Even the relative magnetic permeability characteristics are improved by applying the rolling method of the present invention. However, the relative magnetic permeability characteristics before and after the rolling are inferior in values to those of the composite magnetic body fabricated by the wet process. As described above in detail, according to the present invention, a composite magnetic body in which flat soft magnetic powder having removed strain is highly filled can be obtained without adding the strain to the flat soft magnetic powder again in the manufacturing process. Accordingly, lower profiling of the composite magnetic body can be achieved. Further, the electromagnetic interference suppressing effect in the frequency band around 10 MHz ∼ 100 MHz of the composite magnetic body can be improved. Moreover, the composite magnetic body obtained according to the present invention has flame resistance and a high mechanical strength. Therefore, the composite magnetic body according to the present invention requires no reinforcing agent for coating the surface thereof. Even a composite magnetic body having a small thickness can be handled easily.

Claims: A method of manufacturing a composite magnetic body in which flat soft magnetic powder, from that strain is removed, and a binder are blended and kneaded to obtain an admixture, said admixture being formed in a sheet by molding means selected from the group consisting of roll molding, injection molding, and extrusion molding, characterized in that said binder is a mixture of chlorinated polyethylene and vinyl chloride resin with glass-transition temperature of not lower than 50°C. A method of manufacturing a composite magnetic body in which flat soft magnetic powder from that strain is removed, and magnetic coating of slurry regime consisting of a binder dissolved in a solvent are formed in a sheet by coating means selected from the group consisting of die-coating, gravure coating, and reverse coating, characterized in that said binder is a mixture of chlorinated polyethylene and vinyl chloride resin with glass-transition temperature of not lower than 50°C. A method of manufacturing a composite magnetic body as claimed in claim 1 or 2, wherein the contents of vinyl chloride monomer in said vinyl chloride resin are 75 - 90 mol%. A method of manufacturing a composite magnetic body as claimed in any one of claims 1 to 3, wherein the mixin g ratio of said chlorinated polyethylene and said vinyl chloride resin is 4:1 through 1:4 in weight. A method of manufacturing a composite magnetic body as claimed in any one of claims 1 to 4, wherein said flat soft magnetic powder is oriented and arrayed in-plane direction of the composite magnetic body sheet and packed density of said flat soft magnetic powder is enhanced by applying roll working to said composite magnetic body sheet using a device selected from the group consisting of a reduction roll device, a calender roll device, and a pressing device. A method of manufacturing a composite magnetic body as claimed in claim 5, wherein said reduction roll device and said calender roll device are provided with a roll which is made of a material selected from the group consisting of a rubber with rubber hardness of not smaller than 90, as measured according to JIS K6253, and a polymeric material.

Inventors: ONO NORIHIKO, YOSHIDA SHIGEYOSHI, ONO, NORIHIKO, YOSHIDA, SHIGEYOSHI, Ono, Norihiko, Tokin Corporation, YOSHIDA, Shigeyoshi, Tokin Corporation
Applicants: NEC TOKIN CORP, NEC TOKIN CORPORATION
Application year: 2000
Main classification: H05K9
IPC codes: H01F1, H05K9, H01F41, H01F13