Electromagnetic wave shielding sheet

Abstract: The present invention provides a thin and flexible electromagnetic wave shielding material having excellent electromagnetic wave shielding properties and mainly usable in electronic equipment. The electromagnetic wave shielding sheet according to the present invention is composed of a porous sheet containing one or plural kinds of metallic fibers, or a sintered porous sheet obtained by sintering the above porous sheet. The sheet has a thickness of 10 to 100 µm, a porosity of 70 to 95% and Gurley stiffness of 5 to 50 mg. The sheet can be made by, for example, using stainless steel fiber as the metallic fiber in accordance with a wet paper making process.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to an electromagnetic wave shielding sheet, and particularly to an electromagnetic wave shielding sheet suitable for use in electronic parts, circuit boards, flexible printed circuit boards (FPCs) and the like. 2. Description of the Related Art: With the high density assembly, high integration, and lightening and miniaturization of electronic equipment, mutual damage by electromagnetic waves from electronic parts and/or electronic equipment has become a problem, and so various preventive measures have been taken. In order to shield an object from electromagnetic waves, there is generally used a method of shielding the object with a metallic material. Examples of the metallic material used in this purpose include metal plates, wire gauzes, metallic leaves or foils (hereinafter referred to as leaf merely), platings, deposited metals and metal-containing paints. They are properly used as necessary for the end application intended. Electromagnetic wave shielding properties generally becomes better as the volume resistivity of a shielding material is lower, and its thickness is greater. However, since its specification is limited by size, price, performance, processability and the like according to its application, suitable selection is made for use. However, technologies make rapid progress at present, and particularly, development of high-function equipment by high density assembly and high integration is made in the field of electronic equipment to facilitate the provision of light-weight and small products, so that individual electronic parts are also required to have a high function. Accordingly, electromagnetic wave shielding materials used in the electronic parts are also required to be made thinner, for example, changed from metal plates to wire gauzes or metallic leaves. However, existing materials are made do for these materials under the circumstances from the viewpoint of a problem of processability. For example, in a flexible printed circuit board which is an electronic circuit board, an electromagnetic wave shielding material is laminated on one side or both sides of the circuit board to shield it from electromagnetic noise generated from other electronic parts. Since there is a limit to built-in volume, the electromagnetic wave shield is required, for example, not to impair the flexibility of the board and electromagnetic wave shielding performance. However, only the metallic leaves and wire gauzes can be used in the existing circumstances, and so the flexibility of the shielding materials remains unsatisfactory under the circumstances. By the way, examples of the metallic leaf include stainless steel leaf, aluminum leaf, copper leaf and gold leaf. A sheet-like leaf 50 µm or smaller in thickness may be produced. However, it is desired that such a metallic leaf be replaced by other materials because the weight of a part coated therewith becomes heavy due to its great specific gravity, it is poor in processability, and its cost is comparatively high. On the other hand, the wire gauze is porous compared with the metallic leaf, and so the exposed surface area of a sheet composed of the wire gauze is wider than the metallic leaf. Therefore, such a wire gauze has the advantage of enabling it to lighten the whole weight without impairing the electromagnetic wave shielding performance. However, a monofilament thinner than 50 µm in diameter cannot be obtained, and so it is difficult to obtain a wire gauze having a thickness thinner than 100 µm even in plain weave processing. Therefore, such a wire gauze involves problems of thickness and flexibility. SUMMARY OF THE INVENTION The present invention has been made in view of the foregoing circumstances in the prior art. It is thus an object of the present invention to provide a thin and flexible electromagnetic wave shielding material having excellent shielding properties and mainly usable in electronic equipment. According to the present invention, there is thus provided an electromagnetic wave shielding sheet composed of a porous sheet containing one or plural kinds of metallic fibers, wherein the sheet has a thickness of 10 to 100 µm, a porosity of 70 to 95% and Gurley stiffness of 5 to 50 mg. In the electromagnetic wave shielding sheet according to the present invention, the porous sheet may be formed of a sintered body of the metallic fibers. In the electromagnetic wave shielding sheet according to the present invention, the surfaces of the metallic fibers may be coated with a metallic material having a resistivity lower than the metallic fibers making up the porous sheet. In the electromagnetic wave shielding sheet according to the present invention, the metallic fibers come into contact with each other or mutually fusion-bonded by sintering, so that it is thin and flexible and has excellent electromagnetic wave shielding properties. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 schematically illustrates the construction of an apparatus used in the measurement of a shielding effect on electromagnetic waves. Fig. 2 diagrammatically illustrates the relationship between frequency and shielding effect as to electric waves and magnetic waves. In the Fig. 2, A means electric waves and B means magnetic waves. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS A material making us the porous sheet according to the present invention may be either magnetic or nonmagnetic so far as it is a metallic material. However, it is preferably a metal having high magnetism permeability for the purpose of shielding in a high frequency region. Accordingly, examples of metallic fibers useful in the practice of the present invention include stainless steel fiber, nickel fiber, copper fiber, aluminum fiber, silver fiber, gold fiber and titanium fiber which can be processed into an extremely small-gage wire. However, any metal may be used without any limitation on its kind so far as it is a metal which is electroconductive and capable of processing into a small-gage wire. When these metallic fibers are each formed into a sheet, its flexibility becomes better as its fiber diameter is smaller, so that the measured value of Gurley stiffness can be more reduced. However, processing becomes harder as the diameter of fiber is smaller, which is disadvantageous from the viewpoint of cost. Therefore, the fiber diameter is selected from the range of generally from 5 to 50 µm, preferably 20 µm or smaller as necessary for the end application intended. Besides, no particular limitation is imposed on the length of the metallic fiber. When the sheet is made in accordance with a wet paper making process, however, it is most preferred that the length should fall within a range of from 2 to 6 mm from the viewpoint of formation. Since the shielding against electromagnetic waves with a metallic material is mainly by virtue of reflection, it is considered that a shielding effect per unit volume becomes greater as an exposed surface is wider, and its heat radiating ability is also improved. By the way, the exposed surface area of a porous sheet becomes wider as the number of pores in the sheet is greater, individual pore sizes are smaller, and the diameter and length of fibers making up the sheet are smaller. It is thus preferred to form a sheet by suitably selecting and adjusting these conditions. The production of the porous sheet is preferably conducted in accordance with a wet paper making process. However, the sheet may also be obtained by a method for producing braid or woven fabric. A specific description will be given as to the case by the wet paper making process. One or plural kinds of metallic fibers cut into short fibers are disaggregated and dispersed in water, and a proper amount of a binder and optional auxiliaries are added thereto to mix them. The mixture is subjected to a dehydration treatment on a wire, a pressing step and a drying step to form a sheet. The sheet is heated under vacuum or in a non-oxidizing atmosphere to pyrolytically decompose the binder and the like, thereby removing them. For example, polyvinyl alcohol fiber, polyethylene fiber, polypropylene fiber, cellulose pulp or the like may be used as the binder, and a dispersing agent, surfactant, antifoaming agent and the like, which are in common use in the wet paper making process, may be used as the auxiliaries. Argon gas, nitrogen gas, hydrogen gas or the like may be used in the non-oxidizing atmosphere. The metallic fibers in the porous sheet may also be mutually sintered for purposes of electromagnetic wave shielding properties and improvement in quality. The sintering may preferably be conducting by heating the porous sheet at a temperature near the melting point of the metallic fibers under vacuum or in a non-oxidizing atmosphere, for example, at 1,120°C for 1 to 2 hours in the case of stainless steel fiber. In the present invention, the surfaces of the metallic fibers of the porous sheet may be coated with a metal having a resistivity lower than the metallic fibers. After that manner, the desired electrical conductivity and other characteristics or properties may be attained with a less amount of the metal, thereby achieving an excellent shielding effect on electromagnetic waves. The coating of the metallic fiber surface can be conducted by any known method such as electroplating, electroless plating, sputtering, vapor deposition or plasma spray coating process. Alternatively, the sintering may be conducted while causing fiber of another low-resistive metal having a melting point lower than the metallic fiber to coexist, thereby coating the surface of the metallic fiber with the low-resistive metal at the same time as the sintering. The porous sheet obtained by any of the above processes must have a thickness of 10 to 100 µm, a porosity of 70 to 95% and Gurley stiffness of 5 to 50 mg. The reasons why the thickness is limited to the range of from 10 to 100 µm are as follows. Namely, since parts of electronic equipment are shifted to those of high function and high integration, and so the volume occupied by each part tends to gradually become smaller, the porous sheet must accord with such a part. Further, when the porous sheet is used for a flexible printed circuit board or the like, it must have good flexibility. For these reasons, the above parameters are required to be preset within the above respective ranges. More specifically, if the thickness is greater than the upper limit of the above range, the occupied volume of the part naturally increases, and the Gurley stiffness of the resulting sheet is increased. If the thickness is smaller than 10 µm on the other hand, the strength of the resulting sheet is lowered, and so its durability becomes poor, and its shielding effect on electromagnetic waves is also lowered. If the porosity are lower than 70%, the density of the resulting sheet becomes high, and so its flexibility is lowered, and its Gurley stiffness is increased. If the porosity are higher than 95% on the other hand, the number of fibers making up the resulting sheet becomes fewer, and so the strength of the sheet and its electromagnetic wave shielding performance are lowered. If the Gurley stiffness is lower than 5 mg, the resulting sheet becomes too flexible and is deteriorated in handling property upon its processing. If the Gurley stiffness is higher than 50 mg on the other hand, the flexibility of the resulting sheet is deteriorated. In the present invention, the porosity and Gurley stiffness are values determined by the following respective measuring methods. (Porosity) The porosity value indicates the degree of porosity in a porous sheet and is found in accordance with the following equation: Porosity (%) = {1 - (Apparent density of sheet/True density of sheet)} x 100 In the equation, the apparent density of the sheet is a value calculated out from the basis weight and thickness of the sheet. The true density of the sheet means a density of the fiber itself (including a coating metal if any). (Measurement of Gurley stiffness) The measurement of the stiffness is conducted by using a Gurley stiffness tester (manufactured by Kumagai Riki Kogyo K.K.) and applying an appropriate load to a specimen 2 in. wide and 1.5 in. long at an apprropriate loading position (L1, L2 and L4 indicate positions 1 in., 2 in. and 4 in. distant from a pivot in the center of a pendulum, respectively). A value of the scale thus obtained is substituted for the following equation to calculate out the stiffness (mg) when a standard specimen (1 in. wide and 3.5 in. long; applying a load of 5 g at a loading position of 1 in.) is used. Gurley stiffness (mg) = Rg x W1x1+W2x2+W4x4 5 x L 2 W x 19.8 wherein Rg: scale reading; W: width of specimen (in.); L: length of specimen (in.); and W1, W2 and W4: weight (g) of weights used at the respective loading positions L1, L2 and L4. The electromagnetic wave shielding sheets according to the present invention are thin in thickness and light, have excellent flexibility and exhibit a good shielding effect on electromagnetic waves, and are hence synthetically excellent as electromagnetic wave shielding materials. Accordingly, the electromagnetic wave shielding sheets according to the present invention are extremely useful for shielding of lightened and miniaturized electronic equipment from electromagnetic waves. EXAMPLES Example 1: A sheet was made from a slurry composed of 95 parts by weight of stainless steel fiber (material: SUS316L; trade name: SUSMIC; product of Tokyo Rope Mfg. Co., Ltd.) having a fiber diameter of 8 µm and a fiber length of 4 mm, and 5 parts by weight of polyvinyl alcohol fiber (trade name: Fibribond VPB105-1; product of Kuraray Co., Ltd.; the dissolving temperature in water: 70°C) in accordance with a wet paper making process, and the resultant wet sheet was subjected to dehydrating press and dried by heating to obtain a dry sheet having a basis weight of 62 g/m 2 . The sheet thus obtained was hot press-bonded under conditions of a linear pressure of 300 kg/cm and a take-off speed of 5 m/min by means of heated rolls having a surface temperature of 160°C to obtain a hot press-bonded stainless steel fiber sheet. The shielding effect of the thus-obtained sheet on electromagnetic waves was determined and evaluated. This sheet was thin in thickness because it had been subjected to the hot press bonding, and so the sheet was flexible, and moreover had a high shielding effect on electromagnetic waves because its interfiber distance was shortened by the hot press bonding. Example 2: A sheet obtained in accordance with the wet paper making process in the same manner as in Example 1 was subjected to a sintering treatment at a heat treatment temperature of 1,120°C and a take-off speed of 15 cm/min using a continuous sintering furnace (a brazing furnace equipped with a mesh belt) in a hydrogen atmosphere without conducting the hot press bonding, thereby obtaining a sintered stainless steel fiber sheet having a basis weight of 45 g/m 2 . The shielding effect of the thus-obtained sheet on electromagnetic waves was determined and evaluated in the same manner as in Example 1. This sheet was free from the stiffness attributable to the binder by virtue of the degreasing effect and bonding between the metallic fibers by the sintering. Thus the sheet was flexible. In addition, the sheet had a high shielding effect on electromagnetic waves because the metallic fibers were directly bonded to each other. Example 3: A sintered stainless steel fiber sheet having a basis weight of 80 g/m 2 was obtained in the same manner as the production process of the sintered stainless steel fiber sheet in Example 2 except for the basis weight. The shielding effect of the thus-obtained sheet on electromagnetic waves was determined and evaluated in the same manner as in Example 1. This sheet was increased in stiffness value by the increment of the basis weight compared with the sheet in Example 2, and so its flexibility was comparatively poor. However, the metallic fibers were directly bonded to each other by virtue of the bonding between the metallic fibers by the sintering and the increase in metallic fiber content. Thus the sheet had a high shielding effect on electromagnetic waves. Example 4: A sheet was made from a slurry composed of 60 parts by weight of stainless steel fiber (material: SUS316L; trade name: SUSMIC; product of Tokyo Rope Mfg. Co., Ltd.) having a fiber diameter of 8 µm and a fiber length of 4 mm, 20 parts by weight of copper fiber (trade name: Coplon, product of Esco Co.) having a fiber diameter of 30 µm and a fiber length of 4 mm and 20 parts by weight of polyvinyl alcohol fiber (trade name: Fibribond VPB105-1; product of Kuraray Co., Ltd.; the dissolving temperature in water: 70°C) in accordance with a wet paper making process, and the resultant wet sheet was subjected to dehydrating press and dried by heating to obtain a dry sheet having a basis weight of 75 g/m 2 . The sheet thus obtained was hot press-bonded under conditions of a linear pressure of 300 kg/cm and a take-off speed of 5 m/min by means of heated rolls having a surface temperature of 160°C. The thus-obtained hot press-bonded metallic fiber sheet was subjected to a sintering treatment at a heat treatment temperature of 1,120°C and a take-off speed of 15 cm/min using a continuous sintering furnace (a brazing furnace equipped with a mesh belt) in a hydrogen atmosphere, thereby obtaining a sintered metallic fiber sheet having a basis weight of 60 g/m 2 , in which the surface of the stainless steel fiber was coated with copper by fusion bonding. The shielding effect of the thus-obtained sheet on electromagnetic waves was determined and evaluated in the same manner as in Example 1. This sheet was thin in thickness because it had been subjected to the hot press bonding, and was free from the stiffness attributable to the binder and thickness by virtue of the degreasing effect and bonding between the metallic fibers by the sintering. Thus the sheet was flexible. In addition, the sheet had a high shielding effect on electromagnetic waves because the metallic fibers were directly bonded to each other, and the stainless steel fiber was coated with copper having a resistivity lower than the stainless steel. Comparative Example 1: A sintered stainless steel fiber sheet having a basis weight of 32 g/m 2 was obtained in the same manner as the production process of the sintered stainless steel fiber sheet in Example 2 except for the basis weight. The shielding effect of the thus-obtained sheet on electromagnetic waves was determined and evaluated in the same manner as in Example 1. This sheet was increased in stiffness value compared with the sheet in Example 1 because no hot press bonding was conducted, and so its flexibility was poor. Further, its interfiber distance was long due to high porosity, and so its shielding effect on electromagnetic waves was poor. Comparative Example 2: An aluminum leaf (thickness: 27 µm) was used as a comparative electromagnetic wave shielding material to determine and evaluate the shielding effect of the leaf on electromagnetic waves in the same manner as in Example 1. This aluminum leaf had a high shielding effect on the electromagnetic waves. However, its stiffness depended on the nature and thickness of the metal itself, and so it was difficult to achieve high flexibility. Comparative Example 3: A 200-mesh wire gauze made of stainless steel was used as a comparative electromagnetic wave shielding material to determine and evaluate the shielding effect of the wire gauze on electromagnetic waves in the same manner as in Example 1. This wire gauze achieved a shielding effect on the electromagnetic waves due to its high fiber density, but was lacking in flexibility. By the way, the use of a wire gauze woven so as to make its fiber density low resulted in the achievement of a poor shielding effect on electromagnetic waves due to its large mesh size. The physical properties and evaluation results of the electromagnetic wave shielding materials in the above-described Examples and Comparative Examples are shown collectively in the following Table 2. In Table 2, the thickness and density, and the basis weight were measured in accordance with JIS P 8118 and JIS P 8124, respectively. The evaluation of Gurley stiffness and shielding effect on electromagnetic waves was conducted in the following measuring method. (Measurement of Gurley stiffness) The Gurley stiffness was measured according to the above described method. The loading position in each sheet of Examples 1 - 4 and Comparative Examples 1 - 3 and the load thereof are shown in Table 1. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3 Loading position position L1 L2 L2 L2 L2 L2 L4 Load (g) 5(W1) 5(W2) 5(W2) 5(W2) 5(W2) 25 (W2) 25 (W4) (Measurement of shielding properties) The measurement of shielding properties for evaluating the shielding effect on electromagnetic waves was conducted by means of TR17301 and TR4172 manufactured by ADVANTEST CORP. illustrated in Fig. 1. In Fig. 1, reference numeral 10 indicates a shield box (TR17301) in which a transmitting antenna 12 and receiving antenna 13 are arranged. A specimen 11 15-cm square is set in a metallic holder between both antennas. Reference numeral 14 indicates a spectrum analyzer (TR4172). The measurement of electric wave and magnetic wave shielding properties was conducted by using attached rod antenna (for electric waves) and loop antenna (for magnetic waves) to determine respective shielding properties in a frequency region of from 10 to 1,000 MHz. The shielding properties illustrated in Table 2 are values obtained from a chart recorded in the spectrum analyzer, which indicate values of the shielding effects at the frequency of 500 MHz. For example, in the case of the sheet in Example 2, the values of shielding effects on an electric wave and a magnetic wave at a frequency of 500 MHz were 51 dB and 40 dB, respectively, as apparent from the graph illustrated in Fig. 2 in which shielding effects (ordinate axis) on both electric and magnetic waves versus frequency (abscissa axis) were recorded. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3 Thickness (µm) 28 62 100 25 100 27 116 Basis weight (g/m 2 ) 62 45 80 60 32 68 255 Apparent density (g/cm 3 ) 2.2 0.7 0.8 2.4 0.3 2.5 2.2 Porosity (%) 72 91 90 71 96 0 72 Gurley stiffness (mg) 12 10 41 17 45 72 324 Shielding property Elec. wave (dB) 50 51 58 61 18 69 54 Magn. wave (dB) 37 40 47 57 2 70 53 Shielding effect on electro-magnetic waves ○ ○ ○ ○ × ○ ○ Overall evaluation ○ ○ ○ ○ × × × In the above table, the evaluation standard is as follows: Shielding effect on electromagnetic waves: ○: acceptable level; and ×: poor. Overall evaluation: ○: acceptable level; and ×: impracticable.

Claims: An electromagnetic wave shielding sheet composed of a porous sheet containing one or plural kinds of metallic fibers, wherein the sheet has a thickness of 10 to 100 µm, a porosity of 70 to 95% and Gurley stiffness of 5 to 50 mg. The electromagnetic wave shielding sheet according to Claim 1, wherein the porous sheet is a sintered porous sheet formed of a sintered body of the metallic fibers. The electromagnetic wave shielding sheet according to Claim 1, wherein the surfaces of the metallic fibers are coated with a metallic material having a resistivity lower than the metallic fibers making up the porous sheet.

Inventors: MIZUMOTO YASUHARU, NAKADA TOMOYUKI, SUZUKI TOSHIAKI, MIZUMOTO, YASUHARU, NAKADA, TOMOYUKI, SUZUKI, TOSHIAKI, Mizumoto, Yasuharu, c/o Pulp &amp; Paper Division, Nakada, Tomoyuki, c/o Pulp &amp; Paper Division, Suzuki, Toshiaki, c/o Pulp &amp; Paper Division
Applicants: TOMOEGAWA PAPER CO LTD, TOMOEGAWA PAPER CO., LTD.
Application year: 1999
Main classification: H05K9
IPC codes: H05K9, B22F5, B22F3