JP2008300800A - Surface treated metal plate and housing for electronic equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface treated metal plate which reduces a leak of an electromagnetic wave from a joint portion of an electronic equipment housing and has superior corrosive resistance, and the housing for electronic equipment. <P>SOLUTION: Disclosed is the surface treated metal plate which has a coating of 0.7 to 10 μm in average thickness containing organic resin at least on a portion of a metal or a plated metal surface and whose transfer impedance is ≤2×10<SP>-4</SP>Ω at a frequency of 10 MHz and ≤10<SP>-3</SP>Ω at a frequency of 100 MHz; and the housing for the electronic equipment using it as a joint part. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a metal plate that can be suitably used for a housing of an electronic device and has excellent electromagnetic wave shielding properties at a joint portion, and an electronic device housing manufactured using at least a part of the metal plate. In particular, the present invention relates to a metal plate and an electronic device casing that can effectively suppress malfunction of the electronic device due to radiation noise.

  Electromagnetic waves have been used for broadcasting, radar, ship communication, microwave ovens, etc. for some time, but in recent years, their use has expanded dramatically due to remarkable developments in information and communication technology. Among them, the use of the GHz band that enables transmission of large-capacity information has increased rapidly, and cellular phones (1.5 GHz), ETC (Electronic Toll Collection System) (5.8 GHz), satellite broadcasting (12 GHz), wireless LAN (Local) (Area Network) (2.45 to 60.0 GHz), on-vehicle rear-end collision prevention radar (76 GHz), and the like. On the other hand, in addition to conventional cable wiring, in general homes, the arrival of a ubiquitous society in which personal computers, televisions, and various information appliances are networked by wireless communication using microwaves and millimeter waves and are always connected to computers.

  In this way, a large number of electromagnetic wave generation sources surround us, and coupled with the reduction in size, speed, and thinning of communication devices, the radiation of unwanted electromagnetic waves and the resulting risk of malfunction are considered to have increased significantly. It is done.

  There is an electromagnetic wave shielding technique using a metal material as means for suppressing unnecessary electromagnetic wave emission (Emission) and making it difficult to malfunction even when receiving unnecessary electromagnetic wave radiation (Immunity). Non-patent document 1 describes that a metal material is suitable as an electromagnetic shielding material. The electromagnetic wave shield described in the present invention is an “electromagnetic shield” referred to in Non-Patent Document 1, and should be distinguished from an “electrostatic shield” and a “magnetic shield”. That is, an effect of preventing electromagnetic waves having a frequency of about 1 MHz or more from leaking through the material. In this sense, the metal material has an electromagnetic wave shielding effect that is significantly superior to, for example, plastic. By surrounding the generation source of the unnecessary electromagnetic wave with the metal plate, the emission is suppressed, and by surrounding the electronic circuit with the metal plate, it becomes an immunity means for protecting the circuit from unnecessary radiation from the outside. Therefore, if an electronic device casing having no gaps or joints can be created with a metal plate, a good electromagnetic wave shield can be obtained, and electromagnetic wave leakage hardly causes a problem.

  However, the electronic device casing has some joints by screwing, spot welding, helix folding, or the like. In addition, the surface of the metal plate may be coated with a film containing an organic resin for the purpose of imparting corrosion resistance and fingerprint resistance. In such a case, electromagnetic waves may leak from the joint, and the electromagnetic shielding properties of the housing are determined by the amount of leakage from the joint. Therefore, even for a metal plate, a technique for improving electromagnetic shielding properties is required.

  The prior art which aimed at the electromagnetic wave shielding property improvement of a metal plate is illustrated. In Patent Document 1, a surface-treated steel sheet in which an organic and / or inorganic film not containing chromium is formed on the surface of a zinc-based or aluminum-based plated steel sheet, the centerline average roughness is large, that is, plating with unevenness. Disclosed is a method of providing a surface-treated steel sheet having excellent electromagnetic wave shielding properties and corrosion resistance by forming a film on the steel sheet to make the distribution of the film thickness non-uniform. It is stated that the conductivity of the film is determined at the part where the film thickness of the convex part is thin, so that the above configuration is excellent in electromagnetic shielding properties and that good corrosion resistance can be obtained even if the film is non-uniform. ing.

  The prior art of the electromagnetic wave shield other than a metal plate is illustrated. Patent Document 2 discloses an electromagnetic wave shielding film and an electromagnetic wave shielding paint made of flaky conductive powder and a binder resin. It is stated that the conductive powder is preferably silver, copper, nickel, cobalt or silicon steel having an aspect ratio of 10 to 250, and the film thickness of the electromagnetic wave shielding film should be 10 to 100 μm.

  Patent Document 3 discloses an electromagnetic wave shielding material that includes an absorbing material that absorbs electromagnetic waves, and in which an electromagnetic wave reflecting material is disposed in the gap. It is stated that the electromagnetic wave absorbing material is composed of graphite or carbon black, a cross-linked polymer, a linear polymer and an alkane-based linear low molecule, and the electromagnetic wave reflecting material is a metal powder such as Ni.

  Patent Documents 4 and 5 disclose electromagnetic wave shielding materials in which coiled carbon fibers are enclosed in microcapsules and dispersed in a matrix. It is stated that various coiled carbon fibers such as C, SiC and TiC can be used as the coiled carbon fiber, and the fiber diameter is preferably 0.05 to 5 μm.

  Patent Document 6 discloses an electromagnetic wave shielding paint in which nickel fine powder and aluminum fine powder are dispersed in a modified silicone resin. It is stated that the electromagnetic shielding effect SE is recognized by the Advantest method (near field).

  Patent Document 7 discloses a magnetic shield sheet in which a conductive layer made of an iron-based metal sheet or iron-based metal powder and a binder is provided with an insulating magnetic material layer. It is stated that the magnetic shield effect in the KEC method or the Advantest method is 15 dB or more at 0.5 MHz to 10 MHz.

  Patent Document 8 discloses an electromagnetic wave shielding resin composition in which coated fine particles obtained by performing ferrite plating on the surface of carbonyl iron fine particles, which are soft magnetic powders, are dispersed in a resin. It is described that it is excellent in electromagnetic wave shielding characteristics at a high frequency exceeding 1 GHz.

  Patent Document 9 discloses an electromagnetic material for coating ferrite on a core member made of plate-like aluminum. It is said that a dispersion liquid dispersed in a liquid is applied to a member to be shielded against electromagnetic waves to coat a film having electromagnetic shielding properties.

JP 2004-156081 A JP 2002-232184 A JP 63-114635 A JP 2000-31688 A JP 2000-124658 A JP 2004-168986 A JP 2005-142551 A Japanese Patent Laid-Open No. 2005-158956 JP 2006-49335 A Yasutaka Shimizu “Latest Absorption and Blocking of Electromagnetic Waves” p205-223 Nikkei Technical Library Co., Ltd. (1999) Toshio Kudo, EMCJ 89-96 (1990) p. 51-54 Toshio Kudo, Mitsubishi Cable Industrial Time Report, No. 79 (1990) p. 21-27

  However, all of these conventional techniques have problems. Patent document 1 is the technical content which improves electromagnetic wave shielding property by providing an unevenness | corrugation to the metal plate surface. However, according to studies by the inventors, this method is not necessarily effective for suppressing leakage of electric field waves from 30 MHz to 1 GHz, which is required by the international standard (CISPR). In Patent Document 1, DC surface resistance is measured as a method for verifying the electromagnetic wave shielding effect, but it is not theoretically appropriate as an index for evaluating high-frequency shielding properties that are alternating current. In the first place, in these technical ideas, there is a concern that a plated steel plate with unevenness is coated with an organic film having a non-uniform thickness, and a portion with a thin film thickness of the convex portion becomes a starting point of corrosion.

  Each of the electromagnetic wave shielding films of Patent Documents 2 and 3 contains a conductive material such as metal powder or carbon black in a binder resin. Considering the case where this is applied to a metal plate, there is a concern that the corrosion resistance becomes poor because the original antirust function of the coating film by covering the metal plate with an insulating coating is impaired.

  Since Patent Documents 4 and 5 use a coiled carbon fiber, there is a concern that the cost is industrially high.

  Patent Document 6 has been confirmed to be effective in the Advantest method (near field), and these methods are not necessarily effective for suppressing leakage of electric field waves from 30 MHz to 1 GHz required by the international standard (CISPR). .

  Patent Document 7 confirms the magnetic shield effect at 0.5 to 10 MHz, and these methods are not necessarily effective for suppressing leakage of electric field waves from 30 MHz to 1 GHz required by the international standard (CISPR). Absent.

  Patent Document 8 discloses that it is an effective shielding means against electromagnetic waves of 1 GHz, and these methods are not necessarily for suppressing leakage of electric field waves from 30 MHz to 1 GHz required by the international standard (CISPR). It is not valid.

  Accordingly, the present invention has been made in view of such problems, and an object thereof is to reduce the leakage of electromagnetic waves from the joint portion of the electronic device casing and to have excellent corrosion resistance, and the casing for the electronic device. Is to provide.

  In order to solve the above-mentioned problems, the present inventors diligently studied the electromagnetic wave shielding characteristics against electromagnetic wave leakage from the housing joint.

  As a result, if the transfer impedance of the housing joint is low, the electromagnetic shielding characteristics are excellent, and the transfer impedance of the joint can be reduced by offsetting the impedance due to the dielectric in the film and the impedance due to the magnetic material. The headline and the present invention were completed.

The gist of the present invention is the following (1) to (6).
(1) At least a part of the metal or plated metal surface has a film having an organic resin-containing average thickness of 0.7 μm or more and 10 μm or less, a transfer impedance at a frequency of 10 MHz is 2 × 10 ⁻⁴ Ω or less, a frequency A surface-treated metal plate, wherein a transfer impedance at 100 MHz is 10 ⁻³ Ω or less.
(2) The surface-treated metal plate according to (1), wherein the film contains particles obtained by coating a dielectric with a magnetic metal.
(3) A particle having an average thickness of 0.7 μm or more and 10 μm or less containing an organic resin on at least a part of a metal or plated metal surface, and particles in which a dielectric is coated with a magnetic metal in the coating A surface-treated metal plate characterized by containing.
(4) The surface-treated metal plate according to (2) or (3), wherein the particles are contained in the film in an amount of 1% by volume to 30% by volume.
(5) The surface-treated metal plate according to any one of (1) to (4), wherein a center line average roughness according to JIS-B-0601 of the metal plate is 2.0 μm or less.
(6) A housing for electronic equipment using the surface-treated metal plate according to any one of (1) to (5) at least as a joint.

  According to the present invention, it is possible to provide a surface-treated metal plate having excellent electromagnetic shielding properties and corrosion resistance at the joint, and by using this for an electronic device casing, of course, radiation noise of 30 MHz to 1 GHz required by the international standard (CISPR). In addition, it is possible to effectively shield against radiation noise up to 10 GHz, which is expected to be generated in the future as the operating frequency increases, and to suppress malfunction of the electronic device. Accordingly, it is possible to provide an electronic device casing that is excellent in productivity and economy by eliminating or simplifying conventional electromagnetic shielding measures for joints, that is, heavy use of gaskets, spot welding, screwing, and the like. it can.

  Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

  The present inventors pay attention to the fact that the transfer impedance has theoretical consistency with respect to the electromagnetic wave shielding property (Non-patent Document 2), and reduces the electromagnetic wave shielding property by reducing the transmission impedance at the joint portion of the electronic device casing. Inspired to improve.

  That is, a metal or plated metal plate covered with a film containing an organic resin is used for the joint. As the coating, a coating containing particles in which a dielectric is coated with a magnetic metal is used.

  In the electrical equivalent circuit of the above configuration, the organic resin and particles at the joint form a parallel circuit, and if the impedance of the particles is sufficiently small relative to the organic resin, the impedance of this equivalent circuit is dominated by the impedance of the particles. It becomes the target.

  In terms of the impedance of particles, the equivalent circuit can be expressed as a term in which a term based on the capacitance of the dielectric and a term based on the inductance of the magnetic metal are connected in series.

  With the above configuration, the impedance of the dielectric having the opposite phase and the impedance of the magnetic metal cancel each other, and the transfer impedance is minimized within the measurement frequency.

  Based on the above, the contents of the present invention will be described.

  (1) relates to the thickness of the film on the surface of the metal or the plated metal and the transfer impedance of the metal and the film in order to realize the above-described electromagnetic shielding properties.

  The metal plate applicable in the present invention is not particularly limited as long as it has a shape, dimensions, strength, and workability suitable for a housing of an electronic device or a member in the housing, and steel or aluminum. Examples thereof include metal plates and alloy plates such as magnesium, copper, zinc, nickel, and titanium, and plated metal plates obtained by coating these metal plates with different metals. The metal plate constituting the housing usually has a plate thickness of 3 mm or less. When a metal plate is used as a structural member of the casing, the lower limit value of the plate thickness is usually 0.1 mm.

  The organic resin used in the present invention is not particularly limited, and any of water-soluble organic resins, emulsion-type organic resins, and solvent-based organic resins can be used. For example, olefin resins, acrylic resins, ionomer resins, epoxy resins, urethane resins, polyester resins, phenol resins, vinyl acetate resins, fluorine resins such as polytetrafluoroethylene and polyvinylidene fluoride, or polystyrene And polyethersulfone, polyphenylene ether, polyphenyl sulfide, polyamideimide, cycloolefin polymer, liquid crystal polymer and the like. These may be used alone, or two or more types may be used in combination, or a copolymer may be used (for example, ethylene- (meth) acrylic acid copolymer, (meth) acrylic acid- (meth) acrylic). Acid ester copolymer, etc.), modified with each other (for example, epoxy-modified urethane resin, acrylic-modified ionomer resin, etc.), or modified with another organic substance (for example, amine-modified epoxy resin, etc.) Also good.

  The average thickness of the film containing the organic resin is determined by observing the cross section of the specimen with a scanning electron microscope (SEM) or an optical microscope at an appropriate magnification. Collect a minimum of 10 samples from a sufficiently distant position of the metal plate, and after embedding and polishing, measure the film thickness by cross-sectional observation at 3 to 5 locations that are not unique to each sample, and obtain a total of 30-50 measurements The average value is the film thickness.

  The average thickness of the film is 0.7 μm or more and 10 μm or less. If it is less than 0.7 μm, the corrosion resistance is insufficient, and if it exceeds 10 μm, the electromagnetic wave shielding property is insufficient even with the technique of the present invention. The coating containing the organic resin may be formed on all the front and back surfaces of the metal plate used for the housing, or may be formed only on the portion used as the joint. A film containing an organic resin is essential for the part used as the joint.

The transfer impedance is measured using a Mitsubishi Electric Cable ZTR39D as a measurement jig. A detailed description of this jig is found in Non-Patent Document 3. The test material is punched into a disk shape having an inner diameter of 11 mm and an outer diameter of 63 mm, and is mounted on a jig. A sectional view of the jig after mounting is shown in FIG. ZTR39D originally has a structure in which a disk-shaped sample is sandwiched between the upper and lower outer conductors and the inner conductor at the center. In this study, the transfer impedance of the film containing organic resin is measured more accurately. For this purpose, a ring-shaped conductor (made of gold-plated copper) having an inner diameter of 20 mm and an outer diameter of 60 mm is prepared and placed on the upper surface of the test material, thereby widening the contact area between the test material and the conductor. Further, a Teflon (registered trademark) plate having a thickness of 3 mm is arranged on the back surface of the test material to prevent conduction from the back surface. In order to improve the reproducibility of the measurement, the test material is squeezed only by the weight of the upper outer conductor without screwing the upper and lower outer conductors. At this time, the average surface pressure on the surface of the test material was 0.06 MPa. The jig was connected to a spectrum analyzer (manufactured by Advantest, R3361A) with a coaxial cable, the frequency of the input side voltage was swept from 1 MHz to 1000 MHz, and the power on the output side was measured. Based on the output power P1 measured with the jig set without mounting the test material, the transfer impedance Ztr at each frequency is calculated from the power P2 when the test material is present using the following equation (I). calculate.
Z _tr = 2 × 50 × (P2 / P1) (I)

The transfer impedance is preferably 2 × 10 ⁻⁴ Ω or less at a frequency of 10 MHz, and 10 ⁻³ Ω or less at a frequency of 100 MHz. By using this for the electronic equipment casing, radiation noise of 30 MHz to 1 GHz required by the international standard (CISPR) can be easily suppressed. The lower the transfer impedance, the more advantageous. However, in the measurement method shown in the description of the present invention, it is difficult for the metal material showing a complete electromagnetic wave shield to be 10 ⁻¹⁰ Ω or less, which is the lower limit of the transfer impedance.

  Said (2) relates to the organic resin and particle | grains which a metal plate, a membrane | film | coat, and a membrane | film | coat for implement | achieving said transfer impedance characteristic. The transfer impedance within the measurement frequency is reduced by canceling out the impedance of the dielectric constituting the particles and the impedance of the magnetic metal.

The type of dielectric is not particularly limited as long as it can be coated with a magnetic metal. For example, ferroelectric barium titanate, lead titanate, potassium niobate, lithium niobate, lead niobate, strontium barium niobate, lithium tantalate, sodium potassium tartrate (Rochelle salt), potassium dihydrogen phosphate, Examples thereof include glycine trisulfide. Some of these constituent elements are substituted with an element (shifter) having an effect of shifting the Curie temperature, for example, Ba ²⁺ of barium titanate is substituted with Sr ²⁺ , Ca ²⁺ , Ti ⁴⁺ is Sn ⁴⁺ , Also included are those in which the Curie temperature is shifted to near room temperature by substitution with Zr ⁴⁺ or the like, and those in which a depressor such as CaTiO ₃ or MgTiO ₃ is added.

A paraelectric material may be used as the high dielectric constant particles. For example, magnesium titanate, calcium titanate, titanium oxide (particularly rutile type), strontium titanate, forsterite (2MgO · SiO ₂ ), steatite (MgO · SiO ₂ ) and the like can be exemplified. Alternatively, alumina, quartz, mica, resin, or the like having a dielectric constant lower than that of the above example can be used as the dielectric in the present invention.

Examples of the magnetic metal of the present invention include iron, nickel, cobalt, and alloys based on them, such as permalloy (eg, Ni-20% Fe), fine met (eg, Fe _72.5 Cu ₁ Nb ₄ Si _{13). .5} B _9), sendust (e.g. can be used Fe-3% Si) or the like: Fe-9.5% Si-5.5 % Al), electrical steel (eg.

  As means for coating the dielectric with these magnetic materials, electroless plating such as chemical reduction plating and displacement plating, vapor deposition such as CVD (Chemical Vapor Deposition) and PVD (Physical Vapor Deposition), and further, It may be selected from known methods such as a combination or a method in which a nonmagnetic or magnetic metal is coated by the above-described method and then a magnetic metal is coated by electrolytic plating. The adhesion thickness of the magnetic metal is preferably 0.005 μm or more as the average adhesion thickness. If it is thinner than that, it will be difficult for the magnetic metal to cover the entire surface of the dielectric particles, and it will be thinner than the skin depth at the frequency of the noise electromagnetic wave that you want to suppress, so the effect of coating the magnetic metal will be reduced. To do. A thicker adhesion is advantageous for noise suppression, but as the thickness increases, the overall size of the particles with the dielectric coated with magnetic metal increases, making it difficult to disperse the particles on the surface of the metal plate. It becomes. Since the average thickness of the coating on the surface of the metal plate in the present invention is 10 μm or less, the adhesion thickness of the magnetic metal is preferably 7 μm or less. If the thickness of the magnetic metal exceeds 7 μm, the diameter of the dielectric coated with the magnetic metal will exceed 14 μm, and it protrudes greatly from the film on the surface of the metal plate. This is because it greatly affects. If it is desired to obtain a smooth coating surface with less dispersion in the coating and less protruding particles, the magnetic metal deposition thickness is preferably 3 μm or less.

  The size of the dielectric coated with magnetic metal is preferably 0.01 μm or more and 15 μm or less in terms of average particle size. It is difficult to stably and economically obtain dielectric particles coated with a magnetic metal having a particle size of less than 0.01 μm as an industrial raw material. When the particle size exceeds 15 μm, it is difficult to disperse in the coating of the present invention. It is. The upper limit of the particle size in consideration of the coatability is preferably 1 μm when a water-based resin paint is used, and 10 μm when a solvent-based paint is used. The upper limit of the particle diameter in consideration of corrosion resistance is preferably equal to or less than the film thickness.

Said (3) relates to the organic resin and particle | grains which a metal plate, a film | membrane, and a film | membrane contain for implement | achieving the electromagnetic wave shielding characteristic in this invention. When the frequency at which the electromagnetic wave leakage from the junction of the electronic device casing is desired to be shielded is specified, even if the transfer impedance at a frequency of 10 MHz exceeds 2 × 10 ⁻⁴ Ω, or the transfer impedance at 100 MHz exceeds 10 ⁻³ Ω The impedance of the dielectric composing the particles and the impedance of the magnetic metal may be offset at the frequency to be shielded.

  About the metal plate in this surface treatment metal plate, a film | membrane, and the organic resin and particle | grains which a film | membrane contains, each structure mentioned above should just be selected suitably according to the characteristic calculated | required.

  Said (4) prescribes | regulates the range of the preferable volume content rate of the particle | grains in a film. If it is less than 1% by volume, the effect of improving the electromagnetic wave shielding properties is insufficient, and if it exceeds 30% by volume, it may be difficult to form a film. The optimum volume content varies depending on the combination of resin and particles used, and needs to be determined experimentally, but generally the following guidelines should be followed. When a water-soluble resin or a solvent-based resin is used as the organic resin, particles can be contained up to a high volume content. This is because the resin is indeterminate and can be formed as a binder that fills the gaps between the particles. When an emulsion type resin is used, the upper limit of the volume content is determined by the size of the emulsion diameter and the particle diameter. For example, in the case where the diameters of the emulsion and the particles are about the same, if the particle content exceeds 20% by volume, entrainment of bubbles is likely to occur. The smaller the emulsion diameter is, the higher the volume content of the particles.

  The volume content of the particles in the film is determined by observing the cross section of the specimen with a scanning electron microscope (SEM) or an optical microscope at an appropriate magnification. At least 10 samples are collected from a sufficiently distant position of the metal plate, and after embedding / polishing, cross-sectional observation is performed on 3 to 5 places where each sample is not unique. For particle identification, elemental analysis is preferably performed using an energy dispersive X-ray analyzer (EDX) or an electron beam microanalyzer (EPMA). The distribution area ratio in each analysis cross section of the particle is obtained, and this is defined as the volume content in the cross section. The average value of the total 30-50 measurements obtained is taken as the volume content of the particles in the film.

  In the film containing an organic resin, there may be constituents other than the above-mentioned particles. For example, organic-inorganic composites such as oxide additives such as silica, alumina, titania and zirconia, organic additives such as waxes and surfactants made of polyethylene and fluororesin, silane coupling agents, titanate coupling agents, etc. Examples include additives, color pigments, corrosion inhibitors, and the like.

  Said (5) prescribes | regulates the centerline average roughness of a metal plate to 2 micrometers or less. More preferably, it is 1.5 μm or less. The center line average roughness is measured as a cut-off value of 0.8 mm in accordance with JIS-B-0601. If the center line average roughness exceeds 2 μm, it should be avoided in a portion where the thickness of the film is small, for example, a portion of less than 0.7 μm, since the corrosion resistance is lowered. The surface roughness is usually about a dull steel plate, and it is not necessary to give a texture. According to the configuration of the present invention, even if the metal plate is smooth as bright, that is, the center line average roughness is less than 0.1 μm, sufficient electromagnetic shielding properties of the joint portion are exhibited.

  Said (6) is the housing | casing for electronic devices which uses the surface treatment metal plate of said (1)-(5) for at least a junction part. Examples of the electronic device casing in which the metal plate of the present invention can be applied to at least the joint include, for example, a desktop personal computer (PC), a digital home appliance such as a digital television, a copying machine, a car navigation system, a car AV, and an engine room. Car electronics equipment such as electronic equipment for automobiles and casings for in-vehicle radars. Moreover, you may use the metal plate of this invention for the junction part of housing | casing for mobile products, such as a notebook PC and a mobile telephone.

  When the metal plate of the present invention is used for the joint portion of the housing, it is preferably applied to the joint portion by screwing, spot welding, helix folding, or the like. However, there is no problem even if the joint is not applied to a portion where the metal plate is melted and joined without a gap as in seam welding.

  Hereinafter, the present invention will be described in a non-limiting manner using examples.

Example 1
(1) Tested metal plates The following three types of metal plates were used.
Steel plate: mild steel plate with a thickness of 0.8 mm SUS (stainless steel plate): SUS304 with a thickness of 1.2 mm
Al (aluminum plate): JIS 3004 with a thickness of 0.6 mm

(2) Plating layer About the metal plate which provided the plating layer further on the said metal plate, it selected from the following two types.
Zn-EG (electrogalvanizing): A metal plate was electrogalvanized using a plating bath in which sulfuric acid was added to a zinc sulfate aqueous solution.
Zn-HD (hot dip galvanization): A hot dip galvanization was performed by immersing a metal plate in a hot dip zinc bath containing 0.20% Al.

(3) Roughness measurement A stylus-type roughness meter (manufactured by Mitutoyo, Surf Test SV-3100 S4) was used to measure the surface roughness of the metal plate and the plated metal plate. The centerline average roughness was determined in accordance with JIS-B-0601 with a cut-off value of 0.8 mm.

(4) Organic resin The organic resin was selected from a total of four types of emulsion and solvent.
U: Emulsion urethane resin (Dainippon Ink, Hydran HW)
A: Emulsion acrylic resin (Mitsui Chemicals, Almatex)
M: Solvent-based melamine resin (Nippon Paint, Olga Select 100)
PE: Solvent-based polyester resin (Nippon Paint, UNIPON 400)

(5) Particles The particles shown in Table 1 were used.
Chemical reduction plating was performed by dipping in an aqueous solution having a bath composition of 26 g / L of nickel sulfate, 90 g / L of ethylenediamine ammonium citrate, and 11 g / L of sodium hypophosphite, and having a pH of 6 to 7 and a temperature of 60 ° C.
Cobalt, Ni-20% Fe, and copper were coated with Ni by chemical reduction plating and then coated with a predetermined metal by electrolytic plating.

(6) Other additives Further, 10 mass% of the following colloidal silica was added as a rust preventive pigment to all levels.
Colloidal silica (Nissan Chemical, Snowtex series)
The type of colloidal silica was selected according to the type of resin. An average particle size of 20 nm was selected.

(7) Application and drying Organic resin, particles and other additives were mixed, applied to a metal plate with a roll coater, and dried in a direct-fired drying furnace. The drying conditions (temperature, time) were adjusted appropriately according to the type of resin and the film thickness.

(8) Measurement of long diameter, volume content and average thickness of particles The cross section of the coating was observed with a scanning electron microscope to measure the long diameter, volume content and average thickness of the particles. A total of 30 measurement results were averaged for 10 samples, 3 each.

(9) Measurement transfer impedance of the transfer impedance, using Mitsubishi Cable Industries Ltd. ZTR39D measured by the aforementioned conditions, the pre-formula (I), the calculated transfer impedance Z _tr at each frequency.
The measurement was performed 5 times per sample, and the average of 3 data excluding the highest value and the lowest value was obtained.

(10) Evaluation of electromagnetic wave shielding property An aluminum plate having a thickness of 3 mm was welded to prepare a casing having a side of 550 mm, and an opening of 137 mm × 137 mm was provided only on the upper surface. This was installed in a radio semi-anechoic chamber, and a Schaffner comb generator was fixed inside the housing as an electromagnetic wave reference transmission source, and then a pulse wave was transmitted at a frequency of 10 MHz to 1000 MHz at intervals of 10 MHz. A soft gasket having a width of 5 mm (SGK5-1 manufactured by Morimiya Electric Co., Ltd.) was placed around the opening. A test metal plate of 150 mm × 150 mm was placed thereon. By placing a receiving antenna at a point 3 m away from the case in the horizontal direction and connecting it to a spectrum analyzer, the signal strength of the leaked electromagnetic wave from the case is measured, and the electric field strength of 1 μV / m is set to 0 dB (reference value). ) As dB. The measurement was performed three times, and the obtained results were averaged and compared with a VCCI standard value (standard of information technology apparatus class B: 40 dB or less for 30 MHz to 230 MHz, 47 dB or less for 230 MHz to 1000 MHz).
○: Compliant with VCCI standard value at 30 to 1000 MHz ×: Recognized measurement value not conforming to VCCI standard value at 30 to 1000 MHz

(11) Evaluation of corrosion resistance The test material was cut into 150 mm (L) x 70 mm (W), and a salt spray test prescribed in JIS-Z-2371 was performed for 72 hours. The corrosion area ratio was evaluated as follows.
Score 3: Corrosion area rate of less than 5% Score 2: Corrosion area rate of 5% or more and less than 10% Score 1: Corrosion area rate of 10% or more

  Examples of the present invention are shown in Table 2. Of the examples of the present invention, no. Nos. 3, 19, 20, and 25 are comparative examples in which no particles are used. 36, 37, 38, 39.

  In all of the inventive examples, the electromagnetic shielding properties are improved with respect to the comparative example without impairing the corrosion resistance. Further, as shown in Table 2, within the preferred range of the present invention, further excellent electromagnetic shielding properties can be obtained.

(Example 2)
The metal plates of Example 3 and Comparative Example 36 in Table 2 were used for a desktop PC casing. Measure radiation noise with a frequency of 30 MHz to 1000 MHz at a point 3 m away in a radio semi-anechoic room, and a VCCI standard value (information technology equipment class B standard: 40 MHz or less for 30 MHz to 230 MHz, 47 dB or less for 230 MHz to 1000 MHz) and Compared. As a result, Example No. No. 3 satisfies the standard, while No. 3 From 36, radiation noise exceeding the standard value was detected.

  As mentioned above, although preferred embodiment of this invention was described referring an accompanying drawing, it cannot be overemphasized that this invention is not limited to this example. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the claims, and these are naturally within the technical scope of the present invention. Understood.

It is a figure which represents typically the cross section of an example of a transfer impedance measuring jig.

Claims (6)

At least a part of the metal or plated metal surface has a film having an organic resin-containing average thickness of 0.7 μm or more and 10 μm or less,
A surface-treated metal plate having a transfer impedance of 2 × 10 ⁻⁴ Ω or less at a frequency of 10 MHz and a transfer impedance of 10 ⁻³ Ω or less at a frequency of 100 MHz.   2. The surface-treated metal plate according to claim 1, wherein the film contains particles obtained by coating a dielectric with a magnetic metal. At least a part of the metal or plated metal surface has a film having an organic resin-containing average thickness of 0.7 μm or more and 10 μm or less,
A surface-treated metal plate, wherein the coating contains particles obtained by coating a dielectric with a magnetic metal.   4. The surface-treated metal sheet according to claim 2, wherein the particles are contained in the film in an amount of 1% by volume to 30% by volume.   The surface-treated metal plate according to any one of claims 1 to 4, wherein a center line average roughness of the metal plate according to JIS-B-0601 is 2.0 µm or less.   A housing for electronic equipment using the surface-treated metal plate according to any one of claims 1 to 5 at least as a joint portion.

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