The present invention relates to an electromagnetic interference suppression sheet provided with a pressure-sensitive adhesive layer on one side of an electromagnetic interference suppression layer, and specifically relates to an electromagnetic interference suppression sheet having a structured surface formed on the side opposite the side in contact with the electromagnetic interference suppression layer, i.e. the side that bonds with its adherend.
In recent years, digital electronic devices have come to employ electromagnetic interference suppression sheets that suppress unwanted electromagnetic wave interference and transmission noise passing through wirings. An example of an electromagnetic interference suppression sheet structure is described in Japanese Unexamined Patent Publication HEI No. 07-212079, as an “electromagnetic wave interference suppressor that has an electromagnetic shielding effect, comprising a conductive support, and an insulating soft magnetic material layer provided on at least one surface of the conductive support, wherein the insulating soft magnetic material layer contains soft magnetic powder, dielectric powder and an organic binder”.
An electromagnetic interference suppression sheet is generally placed on an integrated circuit (LSI) that radiates unwanted electromagnetic waves in an electronic device or on a bus line extending from the LSI, or on wiring such as that of an FPC through which transmission noise is passing that may act as a radiant source for unwanted electromagnetic waves. One example of such a method is described in Japanese Patent Publication No. 3528427, as “a countermeasure for EMI whereby at least part of the bus line of an information processing device is coated with a composite magnetic sheet containing soft magnetic powder and an organic binder to prevent its revolution, wherein the soft magnetic powder consists of a metal magnetic body having an aspect ratio of greater than 5 and an oxide film on the surface, and the surface resistance of the composite magnetic body is at least 103Ω”, and an embodiment is also described wherein “the composite magnetic body also has a pressure-sensitive adhesive layer composed mainly of rubber, dextrin or polyvinyl alcohol”.
When a conventional electromagnetic interference suppression sheet such as described above is used for attachment to an adherend, air tends to become trapped between the adherend and the electromagnetic interference suppression sheet, making it necessary to remove the air after attachment or to carry out the attachment using an appropriate attachment working jig to prevent entrapment of air. The present inventors have discovered that, when air becomes entrapped, the distance between the electromagnetic interference suppression layer and adherend increases at those sections, thus reducing the electromagnetic interference suppression effect, and since the air entrapment is generally localized, the distance becomes non-uniform across the attachment surface and results in variation in the electromagnetic interference suppression effect for the attachment surface as a whole.
The present invention therefore provides an electromagnetic interference suppression sheet comprising an electromagnetic interference suppression layer that comprises a soft magnetic powder and an organic binder, and a pressure-sensitive adhesive layer having a structured surface with the side opposite the structured surface being laminated in contact with the electromagnetic interference suppression layer, characterized in that continuous grooves reaching to the outer perimeter of the pressure-sensitive adhesive layer are formed in the structured surface.
According to the invention, paths for air to travel outward are established between the adherend and electromagnetic interference suppression sheet when the electromagnetic interference suppression sheet is attached to its adherend, thus preventing local entrapment of air between the adherend and electromagnetic interference suppression sheet. It is therefore possible to maintain the electromagnetic interference suppression layer at a constant distance from the adherend surface without forming local air bubbles over the entire attachment surface, and consequently obtain a uniform electromagnetic interference suppression effect across the entire attachment surface.
The preceding description should not be construed as disclosing all of the embodiments of the invention nor all of the advantages of the invention.
Representative embodiments of the invention will now be explained for illustration with reference to the accompanying drawings, with the implicit understanding that the invention is not limited to these embodiments.
The soft magnetic powder in the electromagnetic interference suppression layer is preferably a material with a high relative magnetic permeability in the high-frequency range in order to increase the electromagnetic interference suppression effect, and it is preferably a material with a relative magnetic permeability of at least 1000 and more preferably at least 10,000 when measured as a bulk material. As examples of soft magnetic powders there may be mentioned carbonyl iron, iron-aluminum-silicon alloy (sendust) and iron-nickel alloy (permalloy). The soft magnetic powder may have any desired form, but in order to further increase the electromagnetic interference suppression effect it is preferably in a flaky or acicular form, and the soft magnetic powder preferably has a large aspect ratio (for example, 5:1 or greater). The soft magnetic powder may have a coercive force of less than 100 A/m as measured for the bulk magnetic body, and less 10 A/m is also suitable for use.
The electromagnetic interference suppression layer may further comprise a dielectric powder as an optional component. The dielectric powder mixes with the magnetic powder, preventing contact between the particles of the magnetic powder. Preferably, the dielectric powder has a large dielectric constant in the high-frequency range and the frequency characteristic of the dielectric constant is relatively flat. As examples of dielectric powders there may be mentioned barium titanate-based ceramics, zirconate titanate-based ceramics and lead perovskite-based ceramics.
The organic binder in the electromagnetic interference suppression layer disperses in the electromagnetic interference suppression layer the soft magnetic powder and the optional dielectric powder component in an electrically insulated state, serving to bind the powders and ensure mechanical strength for the electromagnetic interference suppression layer. As organic binders there may be mentioned thermoplastic resins including polyolefins such as polyethylene or polypropylene, polystyrene, chlorinated polyethylene, polyester, polyvinyl chloride, polyvinyl butyral, polyurethane, cellulose, nitrile-butadiene, styrene-butadiene and the like, or copolymers of the above, and thermosetting resins such as epoxy resins, phenol resins, polyamides, polyimides and the like, among which polyolefins, polystyrene and chlorinated polyethylene are most commonly used.
A larger thickness of the electromagnetic interference suppression layer is more advantageous for increasing the electromagnetic interference suppression effect or electromagnetic wave absorbing effect. However, for use in smaller electronic devices such as cellular phones, digital cameras, digital video recorders, portable audio devices and the like, the electromagnetic interference suppression sheet must be as thin as possible while still exhibiting sufficient function, from the viewpoint of packaging of other electronic parts. Particularly in the case of a flexible adherend such as a high-frequency signal cable made of FPC, the electromagnetic interference suppression sheet is preferably flexible enough not to impair the flexibility of the adherend. From this viewpoint, the thickness of the electromagnetic interference suppression layer is preferably no greater than about 1 mm, and more preferably between about 0.025 mm and 0.3 mm.
Instead of lowering the thickness of the electromagnetic interference suppression sheet, using a flexible material to form the electromagnetic interference suppression sheet is also advantageous for use of the electromagnetic interference suppression sheet for a flexible adherend such as an FPC. Thus, the breaking strength of the electromagnetic interference suppression layer of the electromagnetic interference suppression sheet is preferably no greater than 14 MPa and more preferably no greater than 7 MPa as measured according to JIS K6251. An excessively low breaking strength of the electromagnetic interference suppression layer may hamper production and prevents its application to adherends, and therefore the breaking strength is preferably at least 1 MPa and more preferably at least 3 MPa.
The pressure-sensitive adhesive layer has a structured surface on the side contacting the adherend, i.e. the exposed side opposite the side in contact with the electromagnetic interference suppression layer. Continuous grooves are formed in the structured surface, defined by a plurality of structures on the surface. The grooves are continuous open channels or thin recesses, having depth in the direction of thickness of the adhesive layer from the exposed side. The groove depths may be constant or variable, and different for multiple grooves. The grooves reach the outer perimeter of the pressure-sensitive adhesive layer, or connect with other grooves reaching to the outer perimeter of the pressure-sensitive adhesive layer. The channels created by the grooves, connecting the air between the adherend and electromagnetic interference suppression sheet with the outside, i.e. the channels formed between the adherend and electromagnetic interference suppression sheet for direction of air outward, thus prevent local entrapment of air between the electromagnetic interference suppression sheet and adherend when the electromagnetic interference suppression sheet is attached to the adherend. It is therefore possible to maintain the electromagnetic interference suppression layer at a constant distance from the adherend surface without forming local air bubbles across the entire attachment surface, and consequently obtain an essentially uniform electromagnetic interference suppression effect across the entire attachment surface.
The pressure-sensitive adhesive layer may contain a pressure-sensitive adhesive, selected as appropriate for the type of adherend on which it is to be attached. Pressure-sensitive adhesives are generally classified as polyacrylates, tackifying rubber, tackifying synthetic rubber, ethylene-vinyl acetate, silicone and the like. Suitable acrylic adhesives are disclosed, for example, in U.S. Pat. Nos. 3,239,478, 3,935,338, 5,169,727, RE 24906, 4,952,650 and 4,181,752. Preferred pressure-sensitive adhesives are reaction products of at least an alkyl acrylate and at least one reinforcing comonomer. Suitable alkyl acrylates include those with homopolymer glass transition temperatures of no higher than about −10° C., such as n-butyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, isononyl acrylate and octadecyl acrylate. Suitable reinforcing monomers include those with homopolymer glass transition temperatures of about −10° C. such as acrylic acid, itaconic acid, isobornyl acrylate, N,N-dimethylacrylamide, N-vinylcaprolactam and N-vinylpyrrolidone.
The thickness of the pressure-sensitive adhesive layer may be varied by several factors including the distance between the electromagnetic interference suppression layer and adherend, the thickness of the electromagnetic interference suppression layer, the composition of the pressure-sensitive adhesive, the form of the structures arranged for formation of the structured surface, and the type of adherend. The thickness of the pressure-sensitive adhesive layer will generally be greater than the heights of the structures of the structured surface. The thickness of the pressure-sensitive adhesive layer referred to here is the distance in the direction normal to the attachment surface, measured from the highest point of the structures of the structured surface to the interface with the other layer or material adjacent thereto, such as the electromagnetic interference suppression layer or reinforcing material.
As seen in
As mentioned above, a smaller thickness is preferred for the pressure-sensitive adhesive layer in order to increase the electromagnetic interference suppression effect. On the other hand, the adhesive force with the adherend is reduced as the pressure-sensitive adhesive layer thickness decreases, and significant adhesive force reduction can occur with adherends having particularly high surface roughnesses. In consideration of these counter factors, therefore, the thickness of the pressure-sensitive adhesive layer is preferably between about 0.02 mm and about 1.0 mm, and more preferably between about 0.02 mm and about 0.05 mm.
The shapes of the grooves formed in the structured surface of the pressure-sensitive adhesive layer may vary in width depending on the processing method, but they preferably have V-shaped, U-shaped, rectangular or trapezoidal cross-sections as seen from the traverse direction.
Grooves are usually created by embossing or forming of multiple structures in the pressure-sensitive adhesive layer. The structures may be irregularly distributed or configured in a regular pattern. The individual structures at least partially define sections of the grooves of the pressure-sensitive adhesive layer. Combining multiple structures can form continuous grooves in the structured surface of the pressure-sensitive adhesive layer.
The shapes of the structures formed in the pressure-sensitive adhesive layer to produce the structured surface may be any desired shapes. Structure shapes may be selected from among semispheres, prisms (angular prisms, rectangular prisms, cylindrical prisms and prisms with similar polygonal features), pyramids and ellipsoids, although without any limitation to these. Combinations of different structure shapes may also be used. Preferred shapes are those selected from among semispheres, prisms and pyramids. The individual structures will normally have heights of at least about 3 μm and less than the total thickness of the pressure-sensitive adhesive layer, and preferably between about 3 μm and 50 μm.
The configuration of the grooves may be any one that does not result in local entrapment of air throughout any part of the surface of the electromagnetic interference suppression sheet. This includes regular patterns and irregular patterns. As examples of groove patterns formed by groups of structures configured in regular patterns there may be mentioned perpendicular lattice, oblique lattice and hexagonal lattice patterns. The grooves may also be in a combination of multiple patterns, for example, a combination of concentric circles centered around a given point with a radial pattern that extends outward from the center point. The grooves configured in this manner either reach to the outer perimeter of the pressure-sensitive adhesive layer, or are directly or indirectly connected with other grooves that reach to the outer perimeter. This allows air bubbles to be eliminated from the electromagnetic interference suppression sheet through the grooves, regardless of where they are located.
When the group of structures is configured in a regular pattern, the average value for the pitch of the structures, i.e. the distance between corresponding points of adjacent structures, is preferably smaller to allow easier elimination of air bubbles, and the value will generally be no greater than about 0.4 mm and preferably no greater than about 0.3 mm.
The grooves have a prescribed volume per unit area of the structured surface of the pressure-sensitive adhesive layer. A larger volume of the grooves will facilitate flow of air outward from the interface between the pressure-sensitive adhesive layer and adherend. The volume of the grooves per unit area is therefore preferably at least about 1×103 μm3 per area of a 500 μm-diameter circle on the two-dimensional plane of the pressure-sensitive adhesive layer, and more preferably it is between about 1×103 μm3 and about 1×107 μm3 per area of a 500 μm-diameter circle. On the other hand, in order to ensure sufficient adhesive force of the pressure-sensitive adhesive layer for the adherend, it is essential to guarantee a prescribed contact area depending on the type of pressure-sensitive adhesive and the material and surface condition of the adherend. From this viewpoint, the initial wet out value of the pressure-sensitive adhesive layer is preferably at least about 85%.
An initial wet out test is carried out in the following manner. The test method is used to evaluate the wettability of the pressure-sensitive adhesive layer with a structured surface for a smooth transparent adherend. The device used for this method comprises a stereomicroscope (Olympus Model SZH-ZB), a microscope-mounted video camera (Cohu Model 4815), a coaxial vertical illuminator (Olympus Model TL2) and computer (Hewlett-Packard Vectra QS/20) equipped with a video digitizing board (Imaging Technologies PCVISION plus), and the video digitizing board allows the computer to photograph and digitize images. The images may then be stored and analyzed using a commercial software package (Jandel JAVA™). Light is supplied from the coaxial vertical illuminator through a lens (optical axis) to illuminate the object. The light passes through a circular polarizer mounted at the edge of a flat objective lens in the microscope. The actual procedure is carried out as follows. (1) Adhesive tape is passed once through a 2 kg roller and attached to the surface of glass (or another optically transparent flat surface). (2) The attached tape is positioned so that the interface between the adhesive and the glass can be seen through the glass with a stereomicroscope. (3) A sample is adjusted so that the glass is perpendicular to the optical axis. (4) The circular polarizer is adjusted for optimum light intensity and contrast. (5) Image analyzing software is used to capture and digitize an image. (6) The grey value window of acceptance of the software is set so as to accept only the gray values (i.e. brightness level) corresponding to the wetted regions. (7) The total wetted area 48 hours after attachment of the electromagnetic interference suppression sheet is analyzed as a percentage of the total image area.
The electromagnetic interference suppression sheet may also include a reinforcing material such as a film or nonwoven fabric. The reinforcing material is usually placed between the electromagnetic interference suppression layer and pressure-sensitive adhesive layer, but it may be situated adjacent to the electromagnetic interference suppression layer as the outermost layer of the electromagnetic interference suppression sheet.
An adhesive layer may be positioned between the reinforcing material and electromagnetic interference suppression layer if necessary for attachment of the reinforcing material to the electromagnetic interference suppression layer.
The electromagnetic interference suppression sheet may also contain a release liner with a second structured surface complementary to the structured surface of the pressure-sensitive adhesive layer. The release liner is situated with the second structured surface in contact with the structured surface of the pressure-sensitive adhesive layer.
The second structured surface of the release liner has a topography complementary to the structured surface of the pressure-sensitive adhesive layer, and for example, when V-shaped grooves are formed in sections of the structured surface, it has sharp protrusions at the sections corresponding to the second structured surface.
The release liner may be produced from any of a variety of materials. As suitable materials there may be mentioned paper or plastic materials such as thermoplastic films, examples of which include polyethylene, polypropylene, polyester, cellulose acetate, polyvinyl chloride, polyvinylidene fluoride and the like, as well as paper coated or laminated with such plastic materials, and other materials.
Embossable coated paper or thermoplastic films may be used directly, but are preferably used after silicone treatment or treatment by other methods to improve the release property. The thickness of the release liner may vary widely according to the effect required. The thickness of the release liner will generally be between about 30 μm and about 300 μm.
The second structured surface of the release liner may be formed by a method known in the prior art. As examples of such methods there may be mentioned embossing, and other mechanical processing or etching treatment methods that are commonly employed. Embossing may be accomplished by, for example, pressing an embossing roll known as a “master tool” onto one side of the release liner.
The electromagnetic interference suppression sheet can be produced by a combination of different methods known in the prior art. An example thereof will now be explained, as a process for producing an electromagnetic interference suppression sheet having a structured surface formed on the pressure-sensitive adhesive layer using a release liner with a second structured surface. A lattice of trapezoidal grooves is formed in the structured surface of the pressure-sensitive adhesive layer. The electromagnetic interference suppression sheet also includes a reinforcing material.
(1) A release liner is provided with a structured surface having a plurality of trapezoidal protrusions in a lattice configuration, i.e. a second structured surface. As an example of a release liner suitable for use there may be mentioned a polycoat liner obtained by coating both sides of a paper base material with a polyethylene coating, although a polyester base material may be used instead of a paper base material. One of the polyethylene coatings of the release liner is release treated by coating with a silicone solution. A plurality of trapezoidal protrusions are then formed in the released treated polyethylene coating by embossing using a master tool. The surface pattern of the emboss roll is thus transferred to the polyethylene coating to obtain a release liner with a second structured surface, having a plurality of trapezoidal protrusions arranged in a lattice configuration.
(2) The second structured surface of the release liner is then coated with a pressure-sensitive adhesive to a sufficient thickness to cover the entire second structured surface, thus forming a pressure-sensitive adhesive layer. For example, the polyethylene coating of a release liner having a plurality of trapezoidal protrusions formed therein is coated with an appropriately selected pressure-sensitive adhesive to a prescribed thickness, and then dried and cured. Coating of the pressure-sensitive adhesive may be accomplished by using a coating method known in the prior art, such as one using a bar coater.
(3) A reinforcing material is laminated on the pressure-sensitive adhesive layer. The reinforcing material used may be, for example, a PET plastic film with a thickness of about 6 μm. Lamination of the reinforcing material may be accomplished using a lamination method known in the prior art, such as one using a pressure bonding roller.
(4) An adhesive layer is placed on the reinforcing material. The adhesive layer may employ the same pressure-sensitive adhesive used in step (2), for example.
(5) The method for laminating the reinforcing material is used to laminate an electromagnetic interference suppression layer on the adhesive layer to complete the electromagnetic interference suppression sheet.
The steps described above may be carried out in a different order if necessary. Other treatment steps may also be added if necessary.
The electromagnetic interference suppression sheet of the invention exhibits a particularly excellent effect when attached onto integrated circuits (LSIs) that are configured at relatively high density and that operate at high frequency in electronic devices such as cellular phones, digital video recorders, digital cameras and portable audio devices, or the bus lines extending from such LSIs, or onto wirings of FPCs and the like. It may also be attached to the enclosure walls of such electronic devices, to absorb electromagnetic waves propagated by reflection within the enclosures.
When an electromagnetic interference suppression sheet according to the invention is used, it is possible to prevent local entrapment of air between the electromagnetic interference suppression sheet and its adherend during attachment. As a result, when attachment of the electromagnetic interference suppression sheet is carried out manually, for example, it is possible to attach the electromagnetic interference suppression sheet to adherends by a simple operation that does not require a special operating jig for attachment, and thus essentially avoid the reattachment procedures that are often necessary for conventional electromagnetic interference suppression sheets. Moreover, since entrapment of air is prevented, it is possible to maintain the electromagnetic interference suppression layer at a constant distance from the adherend surface over the entire attachment surface, and thus stably obtain a uniform electromagnetic interference suppression effect across the entire attachment surface.
Furthermore, because the frequency of required reattachment procedures is greatly reduced, even an electromagnetic interference suppression sheet consisting of only an electromagnetic interference suppression layer and pressure-sensitive adhesive layer without a reinforcing material, which has somewhat lower strength, is satisfactory for practical use. If a reinforcing material is used, it is possible to reduce the thickness of the electromagnetic interference suppression layer and/or reinforcing material necessary to guarantee the required level of strength for the electromagnetic interference suppression sheet. Since an electromagnetic interference suppression sheet according to the invention can therefore be formed with a smaller thickness than a conventional electromagnetic interference suppression sheet, the electromagnetic interference suppression sheet of the invention is particularly advantageous for small-sized electronic devices that have severe restrictions on the positioning and occupying space of electromagnetic interference suppression sheets. Also, because the electromagnetic interference suppression sheet can be formed with a small thickness, a more flexible electromagnetic interference suppression sheet can be produced and as a result, the electromagnetic interference suppression sheet has greater conformability with respect to complex surface shapes and three-dimensional forms.
In addition, as mentioned above, it is possible to stably obtain a uniform electromagnetic interference suppression effect across the entire attachment area, thus eliminating the need for designs with excess thickness of the electromagnetic interference suppression layer to counter in-plane variations in the electromagnetic interference suppression effect, and this contributes to a smaller electromagnetic interference suppression sheet thickness and effective utilization of thin electromagnetic interference suppression layers.
Representative examples will now be described in detail, and it will be appreciated by those skilled in the art that modifications and variations of the embodiments described below may be implemented within the scope of the claims of the present application.
Evaluation sample: An electromagnetic interference suppression sheet of the invention (sample A) was prepared with an electromagnetic interference suppression layer used in AB5010 (product of Sumitomo 3M, 0.1 mm thickness), over which was placed a 4 μm-thick pressure-sensitive adhesive layer, a 6 μm-thick PET film and a 30 μm-thick pressure-sensitive adhesive layer having a structured surface with lattice-like trapezoidal grooves at a groove depth of 10 μm, a groove pitch of 0.2 mm, a groove width of about 15 μm on the side bonding with the adherend, and a groove volume of about 1.2×106 μm3 per unit circle with a diameter of 500 μm, to fabricate the structure shown in
Evaluation method: A microstrip line with a characteristic impedance of 50Ω attached to a dielectric material base (TF-3B, product of Keycom, Tokyo) was used to simulate wiring for attachment of the electromagnetic interference suppression sheet. The measurement was carried out after removing the plastic film formed on the microstrip line. After positioning the electromagnetic interference suppression sheet sample A or B cut to a 40 min square on the line and attaching the four sides to the board, the surface was rubbed with a finger to remove as much of the entrapped air bubbles as possible from between the line and sample, after which the power loss was measured as described above. The power loss was calculated by the formula: Ploss=1−|S21|2−|S11|2 (where S11 is the reflection loss and S21 is the transmission loss), with a higher power loss signifying a greater electromagnetic interference suppression effect.
Condition of entrapped air bubbles: With sample A of the invention, it was possible to easily and completely remove air bubbles entrapped during attachment by rubbing the surface of the electromagnetic interference suppression sheet with a finger. With sample B, however, air bubbles entrapped during attachment merely shifted slightly in position when rubbed with a finger, and removal was totally impossible near the center of the attachment surface.
Power loss measurement results: When several samples A of the invention were prepared and attached to microstrip lines as adherends, uniform attachment could be consistently performed over the entire attachment surface without inclusion of air bubbles. Measurement of the power loss resulted in stable consistent values for the four evaluations, as shown by (1)-(4) in the graph in
Number | Date | Country | Kind |
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2008-017867 | Jan 2008 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US09/32072 | 1/27/2009 | WO | 00 | 7/14/2010 |