Electromagnetic wave shielding film

Abstract

An electromagnetic wave shielding film includes a shielding layer and a protective layer laminated to the shielding layer, the protective layer including a first protective layer laminated to the shielding layer, a second protective layer laminated to the first protective layer, and a third protective layer laminated to the second protective layer, the first protective layer and the third protective layer each having a higher elastic modulus than the second protective layer.

Description

[0001] Cross-reference of related applications

[0002] This application claims priority to Japanese Patent Application No. 2024-56465, which is incorporated herein by reference. Technical Field

[0003] This invention relates to electromagnetic wave shielding films. Background Technology

[0004] Traditionally, as an EMC countermeasure in electronic devices, shielding devices were constructed to shield electromagnetic waves that could become noise. Electromagnetic wave shielding films are used to form these shielding devices. Electromagnetic wave shielding films typically consist of a shielding layer and a conductive adhesive layer laminated on top of the shielding layer.

[0005] A specific application of electromagnetic wave shielding films is in flexible substrates used for high-frequency signal transmission in electronic devices. A flexible substrate typically comprises a base film made of polyimide resin, a circuit pattern formed on the base film, and a cover layer protecting the circuit pattern. Furthermore, the cover layer has openings that reach the grounding circuit included in the circuit pattern. When the shielding layer of the electromagnetic wave shielding film is attached to the cover layer via a conductive adhesive layer, it is electrically connected to the grounding circuit via the conductive adhesive filling the openings.

[0006] Patent Document 1 describes an electromagnetic wave shielding film that achieves further improvements, in which an insulating protective layer is laminated on a metal layer serving as the shielding layer. Advantages of this protective layer include, for example, the ability to suppress the deterioration of the shielding layer and the conductive adhesive (especially oxidation of the metal) and the ability to prevent short circuits between the shielding layer and surrounding circuits.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: International Publication No. 2014 / 010524 Summary of the Invention

[0010] The problem the invention aims to solve

[0011] However, flexible substrates used in electronic devices such as foldable screen phones require resistance to repeated bending. Furthermore, the electromagnetic wave shielding film for this application needs to maintain its electrical connection to the grounding circuit.

[0012] In view of the above, the objective of the present invention is to provide an electromagnetic wave shielding film with excellent bending resistance.

[0013] Solution for solving the problem

[0014] As methods for evaluating the bending resistance of electromagnetic wave shielding films, the MIT test and folding test are known. In order to obtain good results in these tests, the inventors focused on the elastic modulus of the protective layer of the electromagnetic wave shielding film, and found that increasing the elastic modulus of the protective layer yields good results in the MIT test, but not sufficient results in the folding test. Furthermore, they found that decreasing the elastic modulus of the protective layer yields good results in the folding test, but not sufficient results in the MIT test. Further research revealed that by making the protective layer a multi-layered structure and varying the elastic modulus of each layer, certain or better results were obtained in both the MIT test and the folding test, thus completing the present invention.

[0015] That is, the electromagnetic wave shielding film of the present invention is as follows. (1)

[0017] An electromagnetic wave shielding film, comprising:

[0018] A shielding layer and a protective layer stacked on top of the shielding layer.

[0019] The protective layer includes a first protective layer stacked on the shielding layer, a second protective layer stacked on the first protective layer, and a third protective layer stacked on the second protective layer.

[0020] The first protective layer and the third protective layer each have a higher elastic modulus than the second protective layer. (2)

[0022] According to the electromagnetic wave shielding film described in (1) above, when the elastic modulus of the first protective layer is set to E1 (GPa), the elastic modulus of the second protective layer is set to E2 (GPa), the thickness of the first protective layer is set to T1 (µm), and the thickness of the second protective layer is set to T2 (µm), -10≤(E2×T2)-(E1×T1)<0 is satisfied. (3)

[0024] According to the electromagnetic wave shielding film described in (1) or (2) above, the second protective layer has a higher elongation at break than the first protective layer. (4)

[0026] According to the electromagnetic wave shielding film described in (1) or (2) above, when the elongation at break of the first protective layer and the second protective layer is set to B1 and B2 respectively, 0.25≤B1 / B2≤1 is satisfied. (5)

[0028] According to any one of (1) to (4) above, the electromagnetic wave shielding film has a breaking elongation of 50% or more for the second protective layer. (6)

[0030] According to any one of (1) to (5) above, the electromagnetic wave shielding film has a total thickness of 1µm or more and 5µm or less.

[0031] According to the present invention, an electromagnetic wave shielding film with excellent bending resistance can be provided. Attached Figure Description

[0032] Figure 1 This is a schematic cross-sectional view of an electromagnetic wave shielding film according to one embodiment. Detailed Implementation

[0033] Referring to the accompanying drawings, an electromagnetic wave shielding film according to one embodiment of the present invention will be described using a flexible printed circuit board (FPC) as an example. To prevent unwanted electromagnetic waves from radiating from the substrate and adversely affecting peripheral devices, and to prevent electromagnetic waves from entering the substrate from peripheral devices and generating noise in the circuit, an electromagnetic wave shielding film is adhered to the flexible substrate. In other words, the flexible substrate is an object to which electromagnetic waves are restricted from entering and exiting by the electromagnetic wave shielding film. The flexible substrate has an adhesive surface for adhering the electromagnetic wave shielding film.

[0034] The flexible substrate of 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 cover layer made of polyimide resin covering the circuit pattern. The surface of the cover layer, that is, the side opposite to the side facing the circuit pattern, is the adhesive surface. The circuit pattern includes a grounding circuit, which is electrically connected to a grounded component, forming a short-circuit path for unnecessary potential to reach that component. The cover layer has an opening extending through the thickness direction at a position corresponding to the grounding circuit. That is, in the flexible substrate, a portion of the grounding circuit is exposed on the adhesive surface. Furthermore, the electromagnetic wave shielding film 1 of this embodiment is used in such a way that its shielding layer is electrically connected (hereinafter referred to as conductive) to the adhesive surface.

[0035] like Figure 1 As shown, the electromagnetic wave shielding film 1 of this embodiment includes an adhesive layer 11 for bonding with the cover layer, a shielding layer 12 stacked on the adhesive layer 11, and a protective layer 13 stacked on the shielding layer 12.

[0036] The adhesive layer 11 comprises conductive filler and adhesive resin. That is, the adhesive layer 11 is a conductive adhesive layer. When the electromagnetic wave shielding film 1 is bonded to the flexible substrate, the adhesive layer 11 fills the interior of the opening and is stacked on the cover layer. Thus, the shielding layer 12 is connected to the grounding circuit via the adhesive layer 11.

[0037] Examples of conductive fillers 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.

[0038] The adhesive resin can be either a curable resin or a thermoplastic resin. Examples of curable resins include phenolic resins, epoxy resins, urethane resins, melamine resins, and alkyd resins. Examples of thermoplastic resins include polyester resins, polyolefin resins, polystyrene resins, vinyl acetate resins, polyimide resins, polyamide resins, and acrylic resins. The curable resin exhibits reactive curing properties. The curing reaction of the curable resin can be promoted, for example, by heat energy or by energy rays such as ultraviolet light or electron beams. That is, the curable resin can be either a thermosetting resin or a photocurable resin.

[0039] The shielding layer 12 in this embodiment is a metal layer made of metal. This metal layer can contain unavoidable impurities that do not affect shielding performance. The metal layer can be made of a metal foil formed by rolling, or it can be a deposited film formed by deposition methods such as electroplating, vacuum evaporation, sputtering, or CVD. Examples of materials used to form the shielding layer 12 include copper, nickel, silver, tin, gold, palladium, aluminum, chromium, titanium, zinc, or alloys composed of two or more of these.

[0040] The thickness of the shielding layer 12 is preferably between 0.1µm and 10µm. When the shielding layer 12 is formed by deposition coating such as electroplating, grain boundaries are difficult to form at the aforementioned thickness, and fractures are unlikely to occur during MIT tests and folding tests. It should be noted that the thickness of the shielding layer 12 can be determined by averaging the measured values ​​at 10 randomly selected locations through magnified observation of the cross-section of the electromagnetic wave shielding film 1 using SEM or similar methods.

[0041] The protective layer 13 in this embodiment is formed of a resin composition exhibiting insulating properties, and includes a first protective layer 131 laminated on the shielding 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 shielding layer 12 on which the first protective layer 131 is laminated, as well as the surfaces of the first protective layer 131 and the second protective layer 132, may also be subjected to chemical treatment using a primer, coupling agent, etc., mechanical roughening treatment using sandblasting, etc., or electrical surface modification treatment using corona discharge, etc.

[0042] Furthermore, the first protective layer 131 and the third protective layer 133 each have a higher elastic modulus than the second protective layer 132. Therefore, the electromagnetic wave shielding film 1 can achieve good results in both the MIT test and the folding test. Specifically, it is believed that when the electromagnetic wave shielding film 1 is bent to the left or right in the MIT test, the first protective layer 131 and the third protective layer 133, which have high elastic moduli, can increase the bending radius. Therefore, it is believed that the shielding layer 12 can be prevented from bending at an acute angle, increasing the number of bends until the wire breaks in the MIT test. In addition, it is believed that when the electromagnetic wave shielding film 1 is bent to 180° in the folding test, the second protective layer 132, with its lower elastic modulus, is compressed in the thickness direction between the first protective layer 131 and the third protective layer 133, thus suppressing the application of large tension in the bending direction of the bent portion. Therefore, it is believed that the degree of bending of the shielding layer 12 can be made gentler, increasing the number of bends until the wire breaks in the folding test. It should be noted that in the determination of the elastic modulus, firstly, a coating film with a thickness of 20 µm is formed using the resin composition used to form each protective layer. This 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 specimen. Next, a test piece of type 1b as described in JIS K7127 is cut from the prepared specimen. Then, using the test piece, the elastic modulus is calculated from the slope of the regression line of the stress / strain curve obtained by the determination method described in JIS 7161-1.

[0043] The elastic modulus of the first protective layer 131 is preferably 0.7 GPa or more, more preferably 0.8 GPa or more. The elastic modulus of the first protective layer 131 may be 5 GPa or less, or 3 GPa or less.

[0044] 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.

[0045] The elastic modulus of the third protective layer 133 is preferably higher than or equal to that of the first protective layer 131, and more preferably higher than that of the first protective layer 131. For example, the elastic modulus of the third protective layer 133 may be 0.7 GPa or higher, 0.8 GPa or higher, preferably 1 GPa or higher, more preferably 1.5 GPa or higher, and even more preferably 2 GPa or higher. The elastic modulus of the third protective layer 133 may be 5 GPa or lower, or 4 GPa or lower.

[0046] Next, the second protective layer 132 preferably has a higher elongation at break than both the first protective layer 131 and the third protective layer 133. As a result, the electromagnetic wave shielding film 1 achieves good results in the folding test. Furthermore, the first protective layer 131 preferably has a higher elongation at break than the third protective layer 133. It should be noted that elongation at break refers to the elongation at break of the test piece during the above-mentioned elastic modulus determination.

[0047] The elongation at break of the second protective layer 132 is preferably 50% or more, more preferably 60% or more. The elongation at break of the second protective layer 132 may be 200% or less, or 175% or less.

[0048] The elongation at break of the first protective layer 131 is preferably 20% or more, more preferably 40% or more. The elongation at break of the first protective layer 131 may be less than 100% or less, or less than 90%.

[0049] When the elongation at break of the first protective layer 131 and the second protective layer 132 is set as B1 and B2 respectively, it is preferable to satisfy 0.25≤B1 / B2≤1, and more preferably to satisfy 0.33≤B1 / B2≤1. As a result, the electromagnetic wave shielding film 1 obtains good results in both the MIT test and the folding test.

[0050] The elongation at break of the third protective layer 133 is preferably below 10%. The elongation at break of the third protective layer 133 can be above 0.5%.

[0051] The thickness of the first protective layer 131 is preferably greater than or equal to the thickness of the second protective layer 132. The thickness of the third protective layer 133 can be less than or equal to the thickness of either the first protective layer 131 or the second protective layer 132. The combined thickness of the first protective layer 131 and the second protective layer 132 is preferably 1µm or more and 10µm or less, more preferably 2µm or more and 6µm or less. The combined thickness of the first protective layer 131, the second protective layer 132, and the third protective layer 133 is preferably 2µm or more and 12µm or less, more preferably 2µm or more and 8µm or less.

[0052] The thickness of the first protective layer 131 is, for example, 0.5µm or more and 4µm or less. The thickness of the second protective layer 132 is, for example, 0.5µm or more and 4µm or less. The thickness of the third protective layer 133 is, for example, 0.5µm or more and 2µm or less. It should be noted that the thickness of each protective layer can be determined by averaging the measured values ​​at 10 randomly selected locations by observing the cross-section of the electromagnetic wave shielding film 1 using SEM or similar methods.

[0053] Next, when the elastic modulus, elongation at break, and thickness of the first protective layer 131 are set to E1, B1, and T1, respectively, and the elastic modulus, elongation at break, and thickness of the second protective layer 132 are set to E2, B2, and T2, respectively, the electromagnetic wave shielding film 1 preferably satisfies the following condition: Specifically, the electromagnetic wave shielding film 1 preferably satisfies (E2×T2)-(E1×T1)<0, and more preferably satisfies -10≤(E2×T2)-(E1×T1)≤-1. Therefore, the electromagnetic wave shielding film 1 obtains good results in the MIT test.

[0054] The protective layer 13 described above can be formed from a resin composition. The resin composition may include a thermoplastic resin, a heat- or light-curable resin, a polymerization initiator, a curing agent, etc.

[0055] Examples of thermoplastic resins include polyester resins, polyethylene resins, polypropylene resins, polystyrene resins, vinyl acetate resins, polyamide resins, polyimide resins, and acrylic resins. Examples of thermosetting resins include epoxy resins, urethane resins, acrylic resins, melamine resins, phenolic resins, and polyester resins (unsaturated polyester resins).

[0056] By changing the types and mixing ratios of the thermoplastic resin and the curable resin, and further by changing the type and mixing ratio of the curing agent, the elastic modulus and elongation at break of each protective layer can be altered. For example, a resin exhibiting a higher elastic modulus and a lower elongation at break is preferably used to form the first protective layer 131 and the third protective layer 133. On the other hand, a resin exhibiting a higher elongation at break is preferably used to form the second protective layer 132.

[0057] The first protective layer 131 and the third protective layer 133 may contain fillers. Examples of fillers include resin beads, inorganic fibers, and inorganic particles.

[0058] As a method for manufacturing the electromagnetic wave shielding film 1, for example, a method can be described as follows: a third protective layer 133, a second protective layer 132, and a first protective layer 131 are sequentially formed on a carrier film having a surface after a demolding treatment; a shielding layer 12 is formed on the surface of the first protective layer 131 by electroplating; and a conductive adhesive layer 11 is formed thereon.

[0059] In the formation of protective layer 13, when the first protective layer 131, the second protective layer 132, and the third protective layer 133 are respectively composed of curable resin compositions, each layer can be cured sequentially during its formation. Alternatively, two or all layers adjacent in the lamination direction can be in a state where the curing reaction is not fully completed and they are already cured, with the first protective layer 131 and the second protective layer 132, and the second protective layer 132 and the third protective layer 133 respectively, and then cured together after being laminated. According to this method, protective layer 13 can exhibit high interlayer adhesion.

[0060] In the formation of the protective layer 13, a coating film can be formed on the shielding layer 12 using a coating liquid containing a solvent as needed in the above-mentioned components, and the coating film can be cured by heating or light irradiation. Examples of methods for forming the coating film include comma coating, gravure coating, or slot coating.

[0061] It should be noted that the above description illustrates one embodiment, but the electromagnetic wave shielding film of the present invention is not limited to the configuration of the above embodiment. Furthermore, the electromagnetic wave shielding film of the present invention is not limited to the aforementioned effects. Various modifications can be made to the electromagnetic wave shielding film of the present invention without departing from the spirit of the invention.

[0062] Example

[0063] The present invention will be further described below with reference to embodiments, but the present invention is not limited to these embodiments.

[0064] [Example 1]

[0065] (Preparation of the coating solution used to form each layer)

[0066] As a coating liquid for the conductive adhesive layer, copper powder and epoxy resin are dispersed in a solvent to prepare the adhesive layer coating liquid. Next, as a coating liquid for the first protective layer, epoxy resin, polyamide resin, and acrylic resin beads are dispersed in a solvent to prepare the first coating liquid. Furthermore, as a coating liquid for the second protective layer, epoxy resin and polyamide resin are dispersed in a solvent to prepare the second coating liquid. Furthermore, as a coating liquid for the third protective layer, acrylic polyol and XDI isocyanate are dispersed in a solvent to prepare the third coating liquid. Next, using these coating liquids, the elastic modulus and elongation at break of each protective layer are measured using the above-described measurement method.

[0067] (Fabrication of electromagnetic wave shielding film)

[0068] First, a third protective layer is formed on the surface of the release film using a third coating solution. Next, a second protective layer is formed on the surface of the third protective layer using a second coating solution. Then, a first protective layer is formed on the surface of the second protective layer using a first coating solution, resulting in a protective layer composed of the first, second, and third protective layers. Next, a copper thin film is formed on the surface of the protective layer by sputtering, followed by electroplating to form a copper layer with a thickness of 2.0 µm as a shielding layer. Finally, a conductive adhesive layer is formed on the surface of the copper layer using an adhesive layer coating solution, resulting in an electromagnetic wave shielding film with a release film. The thickness of each layer of the fabricated electromagnetic wave shielding film is measured using the above method.

[0069] [Example 2]

[0070] The first protective layer is formed by changing the resin of the first coating liquid to a modified epoxy resin. Otherwise, an electromagnetic wave shielding film is made in the same manner as in Example 1.

[0071] [Example 3]

[0072] The thickness of the first protective layer is 4.0µm. Otherwise, the electromagnetic wave shielding film is made in the same manner as in Example 2.

[0073] [Example 4]

[0074] The thickness of the first protective layer is 3.0µm. Otherwise, the electromagnetic wave shielding film is made in the same manner as in Example 2.

[0075] [Example 5]

[0076] The resin in the second coating liquid was changed to an acrylic polyol and a curing agent was added to form a second protective layer. Otherwise, an electromagnetic wave shielding film was made in the same manner as in Example 4.

[0077] [Example 6]

[0078] The curing agent of the second coating liquid in Example 5 was changed to another curing agent to form a second protective layer. Otherwise, an electromagnetic wave shielding film was made in the same manner as in Example 5.

[0079] [Example 7]

[0080] The third coating liquid in Example 6 was changed to other acrylic polyols to form a third protective layer. Otherwise, an electromagnetic wave shielding film was made in the same manner as in Example 6.

[0081] [Example 8]

[0082] The thickness of the first protective layer is 1.0µm. Otherwise, the electromagnetic wave shielding film is made in the same manner as in Example 7.

[0083] [Example 9]

[0084] The resin in the second coating liquid was changed to an epoxy resin and a polyamide resin (the same as the second coating liquid in Example 6) to form a second protective layer. Otherwise, an electromagnetic wave shielding film was made in the same manner as in Example 8.

[0085] [Example 10]

[0086] The third protective layer is formed by changing the resin of the third coating liquid to polyester resin and melamine resin. Otherwise, an electromagnetic wave shielding film is made in the same manner as in Example 4.

[0087] [Comparative Examples 1-4]

[0088] As shown in Table 1, specimens were prepared for each test in which the relationship between the elastic modulus and elongation at break of the first protective layer and the second protective layer was reversed compared to the example.

[0089] [Comparative Examples 5-8]

[0090] As shown in Table 3, the protective layer was formed without the first protective layer to prepare the test specimens for each test.

[0091] [MIT's experimental methods]

[0092] Electromagnetic wave shielding films were applied to a 50µm thick polyimide resin film and bonded using a press machine via hot pressing (170°C, 3MPa, 30 minutes). The films were cut to a size of 130mm x 15mm and the release film was peeled off to prepare test pieces. For the test pieces, MIT flexural fatigue testing was performed using an MIT flexural fatigue testing machine (manufactured by Yasuda Seiki Co., Ltd., No. 307 MIT type flexural fatigue testing machine) under the test conditions described in JIS P8511.

[0093] (Experimental conditions)

[0094] Bending fixture front end radius (R): 0.38mm

[0095] Bending angle: ±135°

[0096] Bending speed: 175 cpm

[0097] Load: 4.9N (500gf)

[0098] Test method: Measure the resistance value of the electromagnetic wave shielding film in the built-in power supply device.

[0099] [Methods of the folding test]

[0100] (Preparation of test samples)

[0101] As a test printed circuit board, a printed circuit board was prepared comprising a base film of polyimide resin with a thickness of 25µm, a circuit pattern formed on the film (3 copper foils, 12µm thick, 8mm line width), and a cover layer of polyimide resin (37.5µm thick) laminated with an insulating adhesive layer. Then, electromagnetic wave shielding films were placed on the cover layer of the test printed circuit board, and bonded using a press by hot pressing (170°C, 3MPa, 30 minutes) to obtain the test sample.

[0102] (Experimental Operation)

[0103] (1) A fixture is made by fixing two glass epoxy resin boards with a thickness of 0.4 mm between a pair of phenolic boards with a thickness of 2 mm. The fixture is arranged so that both sides of the pair of phenolic boards extend in the horizontal direction.

[0104] (2) The test sample is held between a pair of phenolic boards with the electromagnetic wave shielding film on the outside (the printed circuit board for the test is on the inside).

[0105] (3) Place a 1kg standard weight on the fixture with a 2mm thick phenolic board between it and hold for 10 seconds.

[0106] (4) Remove the phenolic plate along with the standard weights and place the test sample for 1 minute.

[0107] (5) Repeat the operations in (3) to (4).

[0108] [Evaluation Criteria]

[0109] (MIT Experiment)

[0110] ○: The number of bends required to reach 10Ω is over 2000.

[0111] △: The number of bends required to reach 10Ω is more than 1000 but less than 2000.

[0112] ×: The number of bends required to reach 10Ω is less than 1000.

[0113] (Folding test)

[0114] ○: 10 or more

[0115] △: 1 or more and less than 10

[0116] ×:0

[0117] (Overall Evaluation)

[0118] ○: Both parties' evaluation results are ○, or one party is ○ and the other party is △.

[0119] ×: The evaluation result of both is △, or at least one of them is ×.

[0120] [Table 1]

[0121]

[0122] [Table 2]

[0123]

[0124] [Table 3]

[0125]

[0126] Explanation of reference numerals in the attached figures

[0127] 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 stacked on top of the shielding layer. The protective layer includes a first protective layer stacked on the shielding layer, a second protective layer stacked on the first protective layer, and a third protective layer stacked on the second protective layer. 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, When the elastic modulus of the first protective layer is set to E1 (GPa), the elastic modulus of the second protective layer is set to E2 (GPa), the thickness of the first protective layer is set to T1 (µm), and the thickness of the second protective layer is set to T2 (µm), -10 ≤ (E2×T2) - (E1×T1) < 0 is satisfied.

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

4. The electromagnetic wave shielding film according to claim 1, wherein, When the elongation at break of the first protective layer and the second protective layer is set to B1 and B2 respectively, 0.25≤B1 / B2≤1 is satisfied.

5. The electromagnetic wave shielding film according to claim 3 or 4, wherein, The elongation at break of the second protective layer is more than 50%.

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

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