Patent Description:
<CIT> relates to an electromagnetic shielding sheet. <CIT> relates to magnetic shield sheet provided with an insulating magnetic layer. There has conventionally been an increase in devices to generate low-frequency electromagnetic waves (in particular, frequencies equal to or lower than <NUM>). These devices include switching power supplies. The low-frequency electromagnetic waves (noise) generated from these devices influence, for example, a CMOS (Complementary MOS) in a digital camera, causing a problem that noise appears in captured images. Therefore, the need to shield low-frequency electromagnetic waves is increasing. Here, in order to shield the low-frequency electromagnetic waves, it is necessary to use a magnetic shield material with a significant magnetic field shield effect among electromagnetic wave shield materials.

The magnetic field shield effect using a magnetic shield material described above is generally determined by the relative permeability and the thickness of a high relative permeability material such as permalloy used as the magnetic shield material. However, high relative permeability materials such as permalloy used as conventional magnetic shield materials are expensive because they are required to be heat treated at the time of manufacturing and they contain Ni. Thus, as disclosed in Patent Document <NUM>, there is a magnetic shield material enhanced in the magnetic field shield effect relatively inexpensively by stacking a magnetic layer containing a (soft) magnetic material and an electrically conductive layer containing an electrically conductive material (material with a low electrical resistivity).

However, such conventional magnetic shield material formed by simply stacking a magnetic layer containing a magnetic material and an electrically conductive layer containing an electrically conductive material as described in the above Patent Document <NUM> cannot always have a significant magnetic field shield effect in the frequency band of the electromagnetic wave to be shielded (electromagnetic wave generated from a device around the magnetic shield material).

An object of the present invention is to solve the problems described above and to provide a magnetic shield material which can have a good magnetic field shield effect in a frequency band of the electromagnetic wave to be shielded.

In order to solve the above problem, a magnetic shield material of the present invention is defined in claim <NUM>.

In this magnetic shield material, that the thickness of the electrically conductive layer is a thickness to maximize magnetic field shield effect of the magnetic shield material in the frequency band of electromagnetic wave to be shielded.

In this magnetic shield material, it is preferred that the electrically conductive material is aluminum.

In this magnetic shield material, the electrically conductive layer can be a sheet metal including a metal foil.

In this magnetic shield material, it is preferred that the magnetic material is a soft magnetic material.

In this magnetic shield material, it is preferred that the magnetic material is an amorphous metal.

In this magnetic shield material, it is preferred that the magnetic layer is a sheet metal including a metal foil.

According to the present invention, the electrically conductive layer is designed to have a thickness corresponding to the frequency band of electromagnetic wave to be shielded. Here, in a magnetic shield material, like the magnetic shield material of the present invention, which comprises the magnetic layer containing a magnetic material and the electrically conductive layer containing an electrically conductive material, the thickness of the electrically conductive layer to maximize the magnetic field shield effect differs depending on the frequency band of electromagnetic wave to be shielded. Thus, as described above, by designing the electrically conductive layer to have a thickness corresponding to the frequency band of electromagnetic wave to be shielded (by changing the thickness of the electrically conductive layer depending on the frequency band of electromagnetic wave to be shielded), it becomes possible to obtain good magnetic field shield effect in the frequency band of electromagnetic wave to be shielded.

Hereinafter, a magnetic shield material according to an exemplary embodiment of the present invention will be described with reference to the drawings.

<FIG> is a cross-sectional view of a magnetic shield material according to the present embodiment. As shown in <FIG>, the magnetic shield material <NUM> comprises a magnetic layer <NUM> containing a magnetic material, and an electrically conductive layer <NUM> containing an electrically conductive material. More specifically, the magnetic shield material <NUM> is formed by stacking the magnetic layer <NUM> containing the magnetic material and the electrically conductive layer <NUM> containing the electrically conductive material. In this magnetic shield material <NUM>, the electrically conductive layer <NUM> has a thickness corresponding to a frequency band of an electromagnetic wave to be shielded (electromagnetic wave generated from a device around the magnetic shield material). According to the present embodiment, the thickness of the electrically conductive layer <NUM> is set to be one which maximizes the magnetic field shield effect of the magnetic shield material <NUM> in the frequency band of the electromagnetic wave to be shielded.

Materials which can be used for the magnetic layer <NUM> described above are: a sheet metal of a high relative permeability metal such as permalloy, silicon steel, iron, stainless steel, sendust or the like which is formed into a sheet shape; a metal foil of an amorphous metal; a ferrite material sintered into a sheet form; and a sheet of each of the magnetic materials described above (the high relative permeability metal such as permalloy, silicon steel, iron and stainless steel, the amorphous metal, and the ferrite material) made by powdering and compounding (mixing) each into resin, rubber or the like. Note that each magnetic material used for the magnetic layer <NUM> is basically a soft magnetic material. This is because, generally, the soft magnetic material has properties of low coercive force and high relative permeability. Note that in the measurement of magnetic field shield effect described later, a metal foil of a nanocrystalline soft magnetic material produced by crystallizing an amorphous alloy was used for the magnetic layer <NUM>.

Further, materials which can be used for the electrically conductive layer <NUM> described above are: a sheet metal of an electrically conductive metal (with low electrical resistivity) such as copper, gold, silver, nickel, aluminum or the like which is formed into a sheet shape; a deposition of each of the electrically conductive metals described above which is made by depositing each on a film, a cloth or the like by a method such as plating, sputtering, vapor deposition or the like so that the (deposition) surface can be used for electrical conduction (current can flow on the surface); and a sheet of each of the electrically conductive metals described above made by powdering and compounding (mixing) each into resin, rubber or the like. Note that in the measurement of magnetic field shield effect described later, an aluminum foil was used for the electrically conductive layer <NUM>.

Next, referring to <FIG>, an example of a method of using the magnetic shield material <NUM> of the present embodiment will be described. In the example of <FIG>, a magnetic foil <NUM> which is a metal foil of a Fe-based nanocrystalline soft magnetic material is used for the magnetic layer of the magnetic shield material <NUM>, while an aluminum foil <NUM> which is a metal foil of aluminum is used for the electrically conductive layer. In this example, the magnetic shield material <NUM> comprises a layer of PET (polyethylene terephthalate) film <NUM> and layers of double-sided adhesive tapes <NUM>, <NUM> in addition to the layers of magnetic foil <NUM> and aluminum foil <NUM> described above. The lower surface of the PET film <NUM> is adhered to the upper surface of the magnetic foil <NUM> by an acrylic resin-based adhesive. The reason for providing the layer of PET film <NUM> on the uppermost layer of magnetic shield material <NUM> is to protect the surface of the magnetic shield material <NUM>, and to increase heat resistance of the magnetic shield material <NUM>. Further, the upper and lower surfaces of the double-sided adhesive tape <NUM> are attached to the lower surface of the magnetic foil <NUM> and the upper surface of the aluminum foil <NUM>, respectively, while the upper and lower surfaces of the double-sided adhesive tape <NUM> are respectively attached to the lower surface of the aluminum foil <NUM> and the upper surface of the electronic component <NUM> which acts as an electromagnetic wave (noise) generation source. The electronic component <NUM> includes, of course, a circuit such as an IC as well as a switching power supply.

Next, referring to <FIG>, measurement results of the magnetic field shield effect of the magnetic shield material <NUM> with a structure shown in <FIG> above will be described. In this measurement, the magnetic field shield effect of the magnetic shield material <NUM> was measured by KEC method. The KEC method is a measurement method developed by KEC (Kansai Electronic Industry Development Center). According to the measurement system of the KEC method, attenuation of the intensity of magnetic field or electric field of near field in the presence of a shield material as seen from the intensity of magnetic field or electric field of near field (region near magnetic wave generation source) in the absence of the shield material is measured as shield effect in decibels. Here, the shield effect (SE) is obtained by the following equation (<NUM>). <MAT> (where E<NUM> : magnetic field intensity or electric field intensity of near field in the absence of shield material, E<NUM> : magnetic field intensity or electric field intensity of near field in the presence of shield material).

Curves A, B and C in <FIG> are curves showing the measurement results of magnetic field shield effect in the case where the thicknesses of the aluminum foil <NUM> in the magnetic shield material <NUM> shown in <FIG> are <NUM>, <NUM> and <NUM>, respectively. Further, curve D in <FIG> is a curve showing the measurement result of magnetic field shield effect in the case where the aluminum foil <NUM> is removed from the magnetic shield material <NUM> shown in <FIG>. The magnetic foil <NUM> of the magnetic shield material <NUM> used for these measurements had a thickness of <NUM> and a relative permeability (µ/µ<NUM>) of about <NUM>,<NUM>. Note that among the respective layers forming the magnetic shield material <NUM> shown in <FIG>, those that relate to the magnetic field shield performance are only the magnetic foil <NUM> and the aluminum foil <NUM>. Therefore, also in the case where the magnetic shield material <NUM> is formed by only the magnetic foil <NUM> and the aluminum foil <NUM>, and where the thicknesses of the aluminum foil <NUM> are set at <NUM>, <NUM> and <NUM>, it is possible to obtain measurement results of magnetic field shield effect similar to curves A, B and C described above. Further, for a similar reason, the curve of measurement result of magnetic field shield effect of only the magnetic foil <NUM> is essentially the same as curve D described above.

In the above measurements, a metal foil of a Fe-based nanocrystalline soft magnetic material was used for the magnetic foil <NUM> of the magnetic shield material <NUM>. As shown in Table <NUM> below, the composition (weight ratio) of the Fe-based nanocrystalline soft magnetic material was <NUM> wt% iron (Fe), <NUM> wt% silicon (Si), <NUM> wt% niobium (Nb), <NUM> wt% boron (B) and <NUM> wt% copper (Cu).

The measurement system of the KEC method described above can measure magnetic field (or electric field) shield effect of a shield material on electromagnetic waves of various frequencies by changing the frequency from a signal generator. When using the measurement system of the KEC method and observing differences in measured values of magnetic field shield effect of each magnetic shield material <NUM> in the case of stacking aluminum foils <NUM> with different thicknesses on the magnetic foil <NUM>, while changing the frequency band of electromagnetic wave to be shielded, the inventor of the present application has found that as shown by curves A, B and C in <FIG>, the frequency band (of electromagnetic wave) which makes it possible to obtain a maximum magnetic field shield effect differs (shifts) depending on the thickness of the aluminum foil <NUM> (electrically conductive layer). More specifically, it has been found that if other conditions (in particular, the thickness and relative permeability of the magnetic foil <NUM> in the magnetic shield material <NUM>) are the same, then as shown by curves A, B and C in <FIG>, the frequency (of electromagnetic wave) which enables (the magnetic shield material <NUM>) to have a maximum magnetic field shield effect shifts to a lower frequency as the thickness of the aluminum foil <NUM> increases.

Further, based on the finding described above, the inventor of the present application has arrived at a technical concept to maximize the magnetic field shield effect of the magnetic shield material <NUM> in each frequency band of electromagnetic wave by changing the thickness of the aluminum foil <NUM> depending on the frequency band of electromagnetic wave to be shielded. Note that in the following description, the frequency (of electromagnetic wave) which enables the magnetic shield material <NUM> using the aluminum foil <NUM> with a specific thickness to have a maximum magnetic field shield effect will be referred to as "peak value frequency" (of the magnetic shield material <NUM> using the aluminum foil <NUM> with the specific thickness).

Note that it is difficult for the KEC method described above to measure the shield effect on electromagnetic wave with a frequency of <NUM> or lower, and therefore, the thicknesses of the respective aluminum foils <NUM> in the magnetic shield material <NUM> in the measurements shown in <FIG> did not include a thickness (for example, <NUM>) which is considered to cause the peak value frequency described above to be <NUM> or lower.

<FIG> is a graph showing change in the peak value frequency described above where the thickness of the aluminum foil <NUM> in the magnetic shield material <NUM> shown in <FIG> is changed. In other words, <FIG> is a graph showing a correspondence relationship (combination) between the thickness of the aluminum foil <NUM> and the peak value frequency of the magnetic shield material <NUM> using the aluminum foil <NUM> with such thickness. In <FIG>, like curves A, B and C shown in <FIG>, only the peak value frequencies where the thicknesses of the aluminum foil <NUM> are <NUM>, <NUM> and <NUM> are plotted. However, curve E of approximate line shown in <FIG> is actually obtained by using a lot of measured data for combinations of the thicknesses of the aluminum foil <NUM> and the peak value frequencies.

The thickness of the aluminum foil <NUM> (electrically conductive layer <NUM>) when the intermediate value (average value) of the frequency band of electromagnetic wave to be shielded is equal to the peak value frequency described above is obtained from curve E of approximate line described above, and then the thickness of the aluminum foil <NUM> in the magnetic shield material <NUM> is set as the thickness thus obtained from curve E described above. This enables the aluminum foil <NUM> to have a thickness corresponding to the frequency band of electromagnetic wave to be shielded.

Next, referring to <FIG> and <FIG>, the influence of difference in the placement of the magnetic shield material <NUM> relative to the electromagnetic wave generation source on the magnetic field shield effect will be described. In <FIG>, the magnetic shield material <NUM> shown in <FIG> is turned upside down, in which the double-sided adhesive tape <NUM>, the aluminum foil <NUM>, the double-sided adhesive tape <NUM>, the magnetic foil <NUM> and the PET film <NUM> are stacked in order from top to bottom. Curve F shown by the solid line in <FIG> is a curve of measurement result of the magnetic field shield effect when, as shown in <FIG>, the aluminum foil <NUM> (electrically conductive layer) is placed on the electronic component <NUM>, and further thereon the magnetic foil <NUM> (magnetic layer) is placed. Further, curve G shown by the dashed line in <FIG> is a curve of measurement result of the magnetic field shield effect when, as shown in <FIG>, the magnetic foil <NUM> is placed on the electronic component <NUM>, and further thereon the aluminum foil <NUM> is placed. In these measurements, the aluminum foil <NUM> with a thickness of <NUM> was used. As understood from the curves in <FIG>, it is possible for the magnetic shield material <NUM> to have a more significant magnetic field shield effect in the case where the electrically conductive layer (aluminum foil <NUM>) is placed on the electromagnetic wave generation source (electronic component <NUM>), and further thereon the magnetic layer (magnetic foil <NUM>) is placed as shown in <FIG>, than in the case where the magnetic layer (magnetic foil <NUM>) is placed on the electromagnetic wave generation source (electronic component <NUM>), and further thereon the electrically conductive layer (aluminum foil <NUM>) is placed as shown in <FIG>.

As descried above, according to the magnetic shield material <NUM> of the present embodiment, the aluminum foil <NUM> (electrically conductive layer <NUM>) is designed to have a thickness corresponding to the frequency band of electromagnetic wave to be shielded. Here, in the magnetic shield material <NUM>, like the magnetic shield material <NUM> of the present embodiment, which comprises (has stacked) the magnetic layer <NUM> containing a magnetic material and the electrically conductive layer <NUM> containing an electrically conductive material, the thickness of the electrically conductive layer <NUM> to maximize the magnetic field shield effect differs depending on the frequency band of electromagnetic wave to be shielded. Thus, as described above, by designing the electrically conductive layer <NUM> (aluminum foil <NUM>) to have a thickness corresponding to the frequency band of electromagnetic wave to be shielded (by changing the thickness of the electrically conductive layer <NUM> (aluminum foil <NUM>) depending on the frequency band of electromagnetic wave to be shielded), it becomes possible to obtain good magnetic field shield effect in the frequency band of electromagnetic wave to be shielded.

Further, according to the magnetic shield material <NUM> of the present embodiment, the thickness of the electrically conductive layer <NUM> (aluminum foil <NUM>) is designed to have a thickness to maximize the magnetic field shield effect of the magnetic shield material <NUM> in the frequency band of electromagnetic wave to be shielded. This makes it possible to maximize the magnetic field shield effect of the magnetic shield material <NUM> in the frequency band of electromagnetic wave to be shielded.

Further, in the example of using the magnetic shield material <NUM> of the present embodiment as shown in <FIG>, the aluminum foil <NUM> with high electrical conductivity is used for the electrically conductive layer <NUM>, and therefore, the electromagnetic wave shield capability of the magnetic shield material <NUM> can be increased.

Further, in the magnetic shield material <NUM> of the present embodiment, a soft magnetic material (including Fe-based nanocrystalline soft magnetic material) with low coercive force and high relative permeability is used as the magnetic material of the magnetic layer <NUM>, and therefore, the magnetic field shield effect of the magnetic shield material <NUM> can be increased.

It is to be noted that the embodiment described above with various modifications are possible within the scope of the present invention as claimed. Next, modified examples of the present invention will be described.

The above embodiment has shown an example where the thickness of the electrically conductive layer <NUM> (aluminum foil <NUM>) is a thickness to maximize the magnetic field shield effect of the magnetic shield material <NUM> in a frequency band of electromagnetic wave to be shielded. However, the thickness of the electrically conductive layer is not limited to this, and it is sufficient if it is a thickness corresponding to the frequency band of electromagnetic wave to be shielded (thickness to increase the magnetic field shield effect of the magnetic shield material <NUM> in the frequency band of electromagnetic wave to be shielded), such embodiment not being covered by the claimed invention.

Claim 1:
A magnetic shield material (<NUM>) comprising:
a magnetic layer (<NUM>, <NUM>) containing a magnetic material; and
an electrically conductive layer (<NUM>, <NUM>) containing an electrically conductive material,
wherein the electrically conductive layer (<NUM>, <NUM>) has a thickness corresponding to a frequency band of electromagnetic wave to be shielded, wherein the thickness of the electrically conductive layer (<NUM>, <NUM>) is changed depending on the frequency band of electromagnetic wave to be shielded,
the thickness of the electrically conductive layer (<NUM>, <NUM>) is a thickness to maximize magnetic field shield effect of the magnetic shield material (<NUM>) in the frequency band of electromagnetic wave to be shielded, and
wherein the magnetic shield material (<NUM>) is placed on a generation source of the electromagnetic wave to be shielded.