Electromagnetic wave shielding film

Abstract

An electromagnetic wave shielding film comprising a shield layer and a protective layer layered on the shield layer, wherein the protective layer includes a first protective layer that is layered on the shield layer, a second protective layer that is layered on the first protective layer, and a third protective layer that is layered on the second protective layer, and the first protective layer and the third protective layer each have a higher elastic modulus than the second protective layer.

Description

Electromagnetic wave shielding film CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from Japanese Patent Application No. 2024-56465, the entire disclosure of which is incorporated herein by reference.

[0002] The present invention relates to an electromagnetic wave shielding film.

[0003] Conventionally, as an EMC countermeasure for electronic devices, a shield that blocks electromagnetic waves that may cause noise has been formed. To form such a shield, an electromagnetic wave shielding film has been used. An electromagnetic wave shielding film typically includes a shielding layer and a conductive adhesive layer laminated on the shielding layer.

[0004] Specific applications of electromagnetic wave shielding films include flexible substrates used for transmitting high-frequency signals in electronic devices. Flexible substrates typically include a base film made of polyimide resin, a circuit pattern formed on the base film, and a coverlay that protects the circuit pattern. The coverlay has an opening that reaches a ground circuit included in the circuit pattern. When the shielding layer of the electromagnetic wave shielding film is attached to the coverlay via a conductive adhesive layer, it is electrically connected to the ground circuit via the conductive adhesive filled in the opening.

[0005] Patent Document 1 describes a further improved electromagnetic wave shielding film in which an insulating protective layer is laminated on a metal layer serving as a shielding layer. Advantages of providing such a protective layer include, for example, being able to suppress deterioration of the shielding layer and the conductive adhesive (particularly oxidative deterioration of the metal) and being able to prevent the shielding layer from shorting out with surrounding circuits.

[0006] International Publication No. 2014 / 010524

[0007] Meanwhile, flexible substrates are required to have bending resistance against repeated bending when used in electronic devices such as foldable smartphones, and electromagnetic wave shielding films for such applications are required to have bending resistance that allows them to maintain electrical connection with the ground circuit.

[0008] In view of the above circumstances, an object of the present invention is to provide an electromagnetic wave shielding film that has excellent bending resistance.

[0009] The MIT test and the seam folding test are known as methods for evaluating the flex resistance of electromagnetic wave shielding films. The present inventors focused on the modulus of elasticity of the protective layer of the electromagnetic wave shielding film in order to obtain good results in these tests. They found that increasing the modulus of elasticity of the protective layer results in good results in the MIT test, but not insufficient results in the seam folding test. They also found that decreasing the modulus of elasticity of the protective layer results in good results in the seam folding test, but not insufficient results in the MIT test. Further investigations revealed that by providing a protective layer with a multilayer structure and varying the modulus of elasticity of each layer, it is possible to obtain results at least at a certain level in both the MIT test and the seam folding test, thereby completing the present invention.

[0010] That is, the electromagnetic wave shielding film according to the present invention is as follows: (1) An electromagnetic wave shielding film comprising a shielding layer and a protective layer laminated on the shielding layer, wherein the protective layer includes a first protective layer laminated on the shielding layer, a second protective layer laminated on the first protective layer, and a third protective layer laminated on the second protective layer, and wherein the first protective layer and the third protective layer each have a higher elastic modulus than the second protective layer.

[0011] (2) The electromagnetic wave shielding film according to (1), wherein the first protective layer has a modulus of elasticity of E1 (GPa), the second protective layer has a modulus of elasticity of E2 (GPa), the first protective layer has a thickness of T1 (μm), and the second protective layer has a thickness of T2 (μm), and satisfies −10≦(E2×T2)−(E1×T1)<0.

[0012] (3) The electromagnetic wave shielding film according to (1) or (2) above, wherein the second protective layer has a higher elongation at break than the first protective layer.

[0013] (4) The electromagnetic wave shielding film according to (1) or (2) above, wherein, when the breaking elongation percentages of the first protective layer and the second protective layer are B1 and B2, respectively, 0.25≦B1 / B2≦1 is satisfied.

[0014] (5) The electromagnetic wave shielding film according to any one of (1) to (4) above, wherein the second protective layer has a breaking elongation of 50% or more.

[0015] (6) The electromagnetic wave shielding film according to any one of (1) to (5) above, wherein the total thickness of the first protective layer and the second protective layer is 1 μm or more and 5 μm or less.

[0016] According to the present invention, an electromagnetic wave shielding film having excellent bending resistance can be provided.

[0017] 1 is a schematic cross-sectional view of an electromagnetic wave shielding film according to an embodiment.

[0018] With reference to the drawings, an electromagnetic wave shielding film according to one embodiment of the present invention will be described, taking as an example a flexible printed circuit (FPC) to which the film is applied. An electromagnetic wave shielding film is adhered to a flexible printed circuit to prevent unwanted electromagnetic waves from radiating from the board and adversely affecting peripheral devices, and to prevent electromagnetic waves from entering the board from peripheral devices and causing noise in the circuit. In other words, the flexible board is an object whose entry and exit of electromagnetic waves are restricted by the electromagnetic wave shielding film. The flexible board has an attachment surface to which the electromagnetic wave shielding film is adhered.

[0019] The flexible substrate according to this embodiment includes a base film made of polyimide resin, a circuit pattern made of metal such as copper formed on the base film, and a coverlay made of polyimide resin covering the circuit pattern. The surface of the coverlay opposite the surface facing the circuit pattern is the adherend surface. The circuit pattern includes a ground circuit electrically connected to a grounded component to provide a short-circuit path for unwanted potential to the component. The coverlay has an opening penetrating the thickness direction at a position corresponding to the ground circuit. That is, in the flexible substrate, a portion of the ground circuit is exposed on the adherend surface. The electromagnetic wave shielding film 1 according to this embodiment is then adhered to the adherend surface so that its shielding layer and the ground circuit are electrically connected (hereinafter referred to as conductive).

[0020] As shown in FIG. 1 , the electromagnetic wave shielding film 1 of this embodiment includes an adhesive layer 11 for enabling adhesion to the coverlay, a shielding layer 12 laminated on the adhesive layer 11, and a protective layer 13 laminated on the shielding layer 12.

[0021] The adhesive layer 11 contains a conductive filler and a binder resin. That is, the adhesive layer 11 is a conductive adhesive layer. When the electromagnetic wave shielding film 1 is adhered to the flexible substrate, the adhesive layer 11 fills the interior of the opening and is laminated on the coverlay. This establishes electrical continuity between the shielding layer 12 and the ground circuit via the adhesive layer 11.

[0022] Examples of the conductive filler include copper particles, silver particles, nickel particles, silver-coated copper particles, gold-coated copper particles, silver-coated nickel particles, gold-coated nickel particles, silver-coated alloy particles, carbon fillers, and metal-coated resin particles.

[0023] The binder resin may be a curable resin or a thermoplastic resin. Examples of the curable resin include phenolic resins, epoxy resins, urethane resins, melamine resins, and alkyd resins. Examples of the thermoplastic resin include polyester resins, polyolefin resins, polystyrene resins, vinyl acetate resins, polyimide resins, polyamide resins, and acrylic resins. The curable resin has reaction curing properties. The curing reaction of the curable resin may be accelerated by, for example, thermal energy or by energy rays such as ultraviolet rays or electron beams. That is, the curable resin may be a thermosetting resin or a photocurable resin.

[0024] The shield layer 12 of this embodiment is a metal layer made of metal. The metal layer may contain unavoidable impurities that do not affect the shielding properties. The metal layer may be composed of a metal foil formed by rolling, or may be a deposited film formed by a deposition method such as electroplating, vacuum deposition, sputtering, or CVD. Examples of materials for forming the shield layer 12 include copper, nickel, silver, tin, gold, palladium, aluminum, chromium, titanium, zinc, and alloys of two or more of these.

[0025] The thickness of the shielding layer 12 is preferably 0.1 μm or more and 10 μm or less. When the shielding layer 12 is formed of a deposited coating such as an electrolytic plating coating, at such a thickness, grain boundaries are less likely to form and fractures are less likely to occur in the MIT test or the seam folding test. The thickness of the shielding layer 12 can be determined by observing a magnified cross section of the electromagnetic wave shielding film 1 using a SEM or the like and averaging the measured values ​​at 10 arbitrarily selected locations.

[0026] The protective layer 13 of this embodiment is formed from a resin composition exhibiting insulating properties, and includes a first protective layer 131 laminated on the shield layer 12, a second protective layer 132 laminated on the first protective layer 131, and a third protective layer 133 laminated on the second protective layer 132. The surface of the shield layer 12 on which the first protective layer 131 is laminated, and the surfaces of the first protective layer 131 and the second protective layer 132 may be subjected to a chemical treatment using a primer, a coupling agent, or the like, a mechanical roughening treatment such as blasting, or an electrical surface modification treatment such as corona discharge.

[0027] The first protective layer 131 and the third protective layer 133 each have a higher elastic modulus than the second protective layer 132. This allows the electromagnetic shielding film 1 to achieve good results in both the MIT test and the seam bending test. Specifically, when the electromagnetic shielding film 1 is bent left and right in the MIT test, the first protective layer 131 and the third protective layer 133, which have high elastic moduli, are thought to enable a large bending radius. This is thought to prevent the shielding layer 12 from being bent at an acute angle, thereby increasing the number of times the film can be bent before breaking in the MIT test. Furthermore, when the electromagnetic shielding film 1 is bent 180° in the seam bending test, the second protective layer 132, which has a relatively low elastic modulus, is compressed in the thickness direction between the first protective layer 131 and the third protective layer 133, which is thought to prevent a large tension from being applied in the bending direction to the bent portion. This is thought to reduce the degree of curvature of the shielding layer 12, thereby increasing the number of times the film can be bent before breaking in the seam bending test. In measuring the elastic modulus, a 20 μm thick coating film is first formed using the resin composition for forming each protective layer, and the coating film is pressed at 170°C and 2 MPa for 180 seconds, and then heated at 150°C for 1 hour to prepare a sample. Next, a Type 1b test piece as described in JIS K7127 is cut out from the prepared sample. The elastic modulus is then calculated from the slope of the regression line of the stress / strain curve obtained using the test piece by the measurement method as described in JIS 7161-1.

[0028] The elastic modulus of the first protective layer 131 is preferably 0.7 GPa or more, and more preferably 0.8 GPa or more. The elastic modulus of the first protective layer 131 may be 5 GPa or less, or may be 3 GPa or less.

[0029] The elastic modulus of the second protective layer 132 is preferably 0.5 GPa or less, more preferably 0.2 GPa or less, and even more preferably 0.1 GPa or less. The elastic modulus of the second protective layer 132 may be 0.005 GPa or more, 0.01 GPa or more, or 0.02 GPa or more.

[0030] The elastic modulus of the third protective layer 133 is preferably equal to or greater than the elastic modulus of the first protective layer 131, and more preferably higher than the elastic modulus of the first protective layer 131. For example, the elastic modulus of the third protective layer 133 may be 0.7 GPa or greater, 0.8 GPa or greater, preferably 1 GPa or greater, more preferably 1.5 GPa or greater, and even more preferably 2 GPa or greater. The elastic modulus of the third protective layer 133 may be 5 GPa or less, or 4 GPa or less.

[0031] Next, the second protective layer 132 preferably has a higher breaking elongation than each of the first protective layer 131 and the third protective layer 133. This enables the electromagnetic wave shielding film 1 to obtain good results in a seam folding test. Furthermore, the first protective layer 131 preferably has a higher breaking elongation than the third protective layer 133. The breaking elongation refers to the elongation at which the test piece breaks in the above-mentioned measurement of elastic modulus.

[0032] The breaking elongation of the second protective layer 132 is preferably 50% or more, and more preferably 60% or more. The breaking elongation of the second protective layer 132 may be 200% or less, or may be 175% or less.

[0033] The breaking elongation of the first protective layer 131 is preferably 20% or more, and more preferably 40% or more. The breaking elongation of the first protective layer 131 may be 100% or less, or may be 90% or less.

[0034] When the elongation at break of the first protective layer 131 and the second protective layer 132 are B1 and B2, respectively, they preferably satisfy 0.25≦B1 / B2≦1, and more preferably satisfy 0.33≦B1 / B2≦1. This enables the electromagnetic wave shielding film 1 to obtain good results in both the MIT test and the seam folding test.

[0035] The breaking elongation of the third protective layer 133 is preferably 10% or less, and may be 0.5% or more.

[0036] The thickness of the first protective layer 131 is preferably equal to or greater than the thickness of the second protective layer 132. The thickness of the third protective layer 133 may be equal to or less than either the thickness of the first protective layer 131 or the thickness of the second protective layer 132. The total thickness of the first protective layer 131 and the second protective layer 132 is preferably equal to or greater than 1 μm and equal to or less than 10 μm, and more preferably equal to or greater than 2 μm and equal to or less than 6 μm. The total thickness of the first protective layer 131, the second protective layer 132, and the third protective layer 133 is preferably equal to or greater than 2 μm and equal to or less than 12 μm, and more preferably equal to or greater than 2 μm and equal to or less than 8 μm.

[0037] The thickness of the first protective layer 131 is, for example, 0.5 μm or more and 4 μm or less. The thickness of the second protective layer 132 is, for example, 0.5 μm or more and 4 μm or less. The thickness of the third protective layer 133 is, for example, 0.5 μm or more and 2 μm or less. The thickness of each protective layer can be determined by observing a cross section of the electromagnetic wave shielding film 1 using an SEM or the like and averaging the measured values ​​at 10 arbitrarily selected locations.

[0038] Next, when the elastic modulus, elongation at break, and thickness of the first protective layer 131 are E1, B1, and T1, respectively, and the elastic modulus, elongation at break, and thickness of the second protective layer 132 are E2, B2, and T2, respectively, the electromagnetic wave shielding film 1 preferably satisfies the following condition: It is preferable that the electromagnetic wave shielding film 1 satisfies (E2×T2)−(E1×T1)<0, and more preferably satisfies −10≦(E2×T2)−(E1×T1)≦−1. This enables the electromagnetic wave shielding film 1 to obtain good results in the MIT test.

[0039] The protective layer 13 as described above can be formed from a resin composition. The resin composition may contain a thermoplastic resin, a curable resin that is cured by heat or light, a polymerization initiator, a curing agent, etc.

[0040] Examples of the thermoplastic resin include polyester resin, polyethylene resin, polypropylene resin, polystyrene resin, vinyl acetate resin, polyamide resin, polyimide resin, and acrylic resin. Examples of the thermosetting resin include epoxy resin, urethane resin, acrylic resin, melamine resin, phenol resin, and polyester resin (unsaturated polyester resin).

[0041] The elastic modulus and breaking elongation of each protective layer can be changed by changing the types and blending ratios of the thermoplastic resin and the curable resin, and by changing the types and blending ratios of the curing agents. For example, a resin that exhibits a relatively high elastic modulus and a relatively low breaking elongation is preferred for forming the first protective layer 131 and the third protective layer 133. On the other hand, a resin that exhibits a relatively high breaking elongation is preferred for forming the second protective layer 132.

[0042] The first protective layer 131 and the third protective layer 133 may contain a filler, such as resin beads, inorganic fibers, or inorganic particles.

[0043] As a method for manufacturing the electromagnetic wave shielding film 1, for example, a method can be given in which the third protective layer 133, the second protective layer 132, and the first protective layer 131 are formed in that order on a carrier film having a release-treated surface, the shielding layer 12 is formed on the surface of the first protective layer 131 by electrolytic plating, and then the conductive adhesive layer 11 is formed.

[0044] In forming the protective layer 13, when the first protective layer 131, the second protective layer 132, and the third protective layer 133 are each composed of a curable resin composition, each layer may be cured sequentially after it is formed. Alternatively, two adjacent layers in the stacking direction, such as the first protective layer 131 and the second protective layer 132, or the second protective layer 132 and the third protective layer 133, or all layers may be left in a curable state without allowing the curing reaction to proceed completely, and then these layers may be laminated and cured all at once. This method allows the protective layer 13 to exhibit high interlayer adhesion.

[0045] The protective layer 13 can be formed by forming a coating film on the shielding layer 12 using a coating liquid containing the above components and, if necessary, a solvent, and curing the coating film by heating or light irradiation. Examples of methods for forming the coating film include comma coating, gravure coating, and slot die coating.

[0046] Although one embodiment has been shown above as an example, the electromagnetic wave shielding film according to the present invention is not limited to the configuration of the above embodiment. Furthermore, the electromagnetic wave shielding film according to the present invention is not limited by the above-mentioned effects. The electromagnetic wave shielding film according to the present invention can be modified in various ways without departing from the gist of the present invention.

[0047] The present invention will be further explained below with reference to examples, but the present invention is not limited to these examples.

[0048] Example 1 (Preparation of Coating Liquids for Forming Each Layer) A coating liquid for the conductive adhesive layer was prepared by dispersing copper powder and an epoxy resin in a solvent. Next, a first coating liquid for the first protective layer was prepared by dispersing an epoxy resin, a polyamide resin, and acrylic resin beads in a solvent. A second coating liquid for the second protective layer was prepared by dispersing an epoxy resin and a polyamide resin in a solvent. A third coating liquid for the third protective layer was prepared by dispersing an acrylic polyol and an XDI isocyanate in a solvent. These coating liquids were then used to measure the modulus of elasticity and elongation at break of each protective layer using the above-described measurement methods. (Preparation of Electromagnetic Wave Shielding Film) First, a coating film was formed on the surface of a release film using the third coating liquid to form a third protective layer. Next, a coating film was formed on the surface of the third protective layer using the second coating liquid to form a second protective layer. Next, a coating film was formed on the surface of the second protective layer using the first coating liquid to form a first protective layer, thereby obtaining a protective layer consisting of the first protective layer, the second protective layer, and the third protective layer. Next, a copper thin film was formed on the surface of the protective layer by sputtering, and then a copper layer serving as a shielding layer with a thickness of 2.0 μm was formed by electrolytic plating. Finally, a conductive adhesive layer was formed on the surface of the copper layer using the adhesive layer coating liquid, thereby obtaining an electromagnetic wave shielding film with a release film. The thickness of each layer of the produced electromagnetic wave shielding film was measured using the above-mentioned measurement method.

[0049] Example 2 An electromagnetic wave shielding film was produced in the same manner as in Example 1, except that the resin in the first coating liquid was changed to a modified epoxy resin to form the first protective layer.

[0050] Example 3 An electromagnetic wave shielding film was produced in the same manner as in Example 2, except that the thickness of the first protective layer was set to 4.0 μm.

[0051] Example 4 An electromagnetic wave shielding film was produced in the same manner as in Example 2, except that the thickness of the first protective layer was set to 3.0 μm.

[0052] Example 5 An electromagnetic wave shielding film was produced in the same manner as in Example 4, except that the resin in the second coating liquid was changed to acrylic polyol and a curing agent was added to form the second protective layer.

[0053] Example 6 An electromagnetic wave shielding film was produced in the same manner as in Example 5, except that the curing agent in the second coating liquid in Example 5 was changed to another curing agent to form the second protective layer.

[0054] Example 7 An electromagnetic wave shielding film was produced in the same manner as in Example 6, except that the third coating liquid in Example 6 was changed to another acrylic polyol to form the third protective layer.

[0055] Example 8 An electromagnetic wave shielding film was produced in the same manner as in Example 7, except that the thickness of the first protective layer was set to 1.0 μm.

[0056] [Example 9] An electromagnetic wave shielding film was produced in the same manner as in Example 8, except that the resin in the second coating liquid was changed to an epoxy-based resin and a polyamide-based resin (the same as the second coating liquid in Example 6) to form a second protective layer.

[0057] Example 10 An electromagnetic wave shielding film was produced in the same manner as in Example 4, except that the resin in the third coating liquid was changed to a polyester-based resin and a melamine-based resin to form the third protective layer.

[0058] Comparative Examples 1 to 4 As shown in Table 1, test samples were prepared in which the magnitude relationships between the modulus of elasticity and elongation at break of the first and second protective layers were reversed compared to those of the examples.

[0059] Comparative Examples 5 to 8 As shown in Table 3, the protective layer was formed so as not to include the first protective layer, and samples for each test were prepared.

[0060] [MIT Test Method] Each electromagnetic wave shielding film was placed on a 50 μm thick film made of polyimide resin and bonded by thermocompression bonding (170°C, 3 MPa, 30 minutes) using a press. A test specimen was obtained by cutting into a size of 130 mm x 15 mm (length x width), and the release film was peeled off. The test specimen was subjected to an MIT folding fatigue test using an MIT folding fatigue tester (manufactured by Yasuda Seiki Seisakusho Co., Ltd., No. 307 MIT type folding fatigue tester) under the following test conditions based on the test method described in JIS P8511. (Test Conditions) Bending clamp tip R: 0.38 mm Bending angle: ±135° Bending speed: 175 cpm Load: 4.9 N (500 gf) Detection method: The resistance value of the electromagnetic wave shielding film was measured using a built-in electrical connection device.

[0061] [Method for Seam Folding Test] (Preparation of Test Samples) A ​​test printed wiring board was prepared, which included a 25 μm-thick polyimide resin base film, a circuit pattern (three copper foils, 12 μm thick, 8 mm line width) formed on the film, and a 37.5 μm-thick polyimide resin coverlay laminated via an insulating adhesive layer. Each electromagnetic wave shielding film was then placed on the coverlay of the test printed wiring board and bonded by thermocompression bonding (170°C, 3 MPa, 30 minutes) using a press to obtain a test sample. (Test Procedure) (1) A jig was prepared by sandwiching and fixing two 0.4 mm-thick glass epoxy plates between a pair of 2 mm-thick bakelites. The jig was positioned so that both bakelites extended horizontally. (2) The test sample was folded so that the electromagnetic wave shielding film was on the outside (the test printed wiring board was on the inside) and held between the pair of bakelites. (3) A 1 kg standard weight is placed on the jig via a 2 mm thick bakelite plate and held in place for 10 seconds. (4) The bakelite plate is removed along with the standard weight, and the test sample is left to stand for 1 minute. (5) Steps (3) and (4) are repeated.

[0062] [Evaluation criteria] (MIT test) ○: Number of folds to reach 10Ω is 2000 or more △: Number of folds to reach 10Ω is 1000 or more but less than 2000 ×: Number of folds to reach 10Ω is less than 1000 (Season bending test) ○: 10 or more △: 1 or more but less than 10 ×: 0 (Overall evaluation) ○: Both evaluation results are ○, or one is ○ and the other is △ ×: Both evaluation results are △, or at least one is ×

[0063]

[0064]

[0065]

[0066] 1: Electromagnetic wave shielding film, 11: Adhesive layer, 12: Shielding layer, 13: Protective layer, 131: First protective layer, 132: Second protective layer, 133: Third protective layer

Claims

1. An electromagnetic wave shielding film comprising a shielding layer and a protective layer laminated on the shielding layer, wherein the protective layer includes a first protective layer laminated on the shielding layer, a second protective layer laminated on the first protective layer, and a third protective layer laminated on the second protective layer, and wherein the first protective layer and the third protective layer each have a higher elastic modulus than the second protective layer.

2. The electromagnetic wave shielding film according to claim 1, wherein the elastic modulus of the first protective layer is E1 (GPa), the elastic modulus of the second protective layer is E2 (GPa), the thickness of the first protective layer is T1 (μm), and the thickness of the second protective layer is T2 (μm) satisfies -10≦(E2×T2)−(E1×T1)<0.

3. The electromagnetic wave shielding film according to claim 1, wherein the second protective layer has a higher breaking elongation than the first protective layer.

4. The electromagnetic wave shielding film according to claim 1, wherein, when the breaking elongation percentages of the first protective layer and the second protective layer are B1 and B2, respectively, the relationship 0.25≦B1 / B2≦1 is satisfied.

5. The electromagnetic wave shielding film according to claim 3 or 4, wherein the second protective layer has a breaking elongation of 50% or more.

6. An electromagnetic wave shielding film according to any one of claims 1 to 4, wherein the total thickness of the first protective layer and the second protective layer is 1 μm or more and 5 μm or less.

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