Electromagnetic wave shield film

Abstract

An electromagnetic wave shield film comprising a shield layer and a conductive adhesive layer laminated on the shield. The shield layer is a metal layer having a thickness of 6 μm or more. The conductive adhesive layer contains 30-55 mass% inclusive of conductive particles.

Description

Electromagnetic wave shielding film CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from Japanese Patent Application No. 2024-56466, the disclosure of which is incorporated herein by reference.

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

[0003] Conventionally, as an EMC measure for electronic devices, a shield is formed to block electromagnetic waves that may become noise. To form such a shield, an electromagnetic wave shielding film is used.

[0004] A known electromagnetic wave shielding film includes a shielding layer and a conductive adhesive layer laminated on the shielding layer. The conductive adhesive layer typically contains conductive particles such as metal particles and a binder resin.

[0005] Furthermore, Patent Document 1 describes an electromagnetic wave shielding film having a metal layer with a thickness of 0.5 μm or more as a shielding layer, and shows that the use of such a metal layer can effectively block electromagnetic waves.

[0006] Specific applications of electromagnetic wave shielding films include forming a shield against noise in printed wiring boards. Known printed wiring boards include those that include a base film made of polyimide resin, a circuit pattern formed on the base film, and an insulating film (e.g., a coverlay in a flexible substrate) that protects the circuit pattern. Also known printed wiring boards include those that include a ground circuit. The insulating film in this type of printed wiring board has an opening that reaches the ground circuit included in the circuit pattern. The shielding layer of the electromagnetic wave shielding film is electrically connected to the ground circuit via a conductive adhesive that bonds the insulating film and the shielding layer and fills the opening.

[0007] One method for adhering an electromagnetic wave shielding film to a printed wiring board is to overlap the conductive adhesive layer of the electromagnetic wave shielding film on the insulating film of the printed wiring board and then press-bond the electromagnetic wave shielding film to the insulating film while heating it (also described in Patent Document 1). At this time, the conductive adhesive layer flows, and the conductive particles fill the inside of openings formed in the insulating film together with the binder resin.

[0008] Furthermore, in some cases, elements are mounted on the printed wiring board to which the electromagnetic wave shielding film is attached in a reflow process, and in a typical reflow process, the printed wiring board is heated to 200° C. or higher.

[0009] International Publication No. 2013 / 077108

[0010] As described above, a thick metal layer is advantageous for improving shielding properties. However, the inventors discovered that using a thick metal layer increases the resistance between the shielding layer and the ground circuit, which are electrically connected via a conductive adhesive, and this tendency becomes particularly pronounced after reflow. One possible reason for this tendency is that, compared to a thin metal layer, a thick metal layer is less likely to deform in response to the conductive adhesive flowing toward the opening when the electromagnetic shielding film is pressure-bonded to the insulating film, thereby increasing the distance from the ground circuit. In other words, a step is formed between the surface of the insulating film and the exposed portion of the ground circuit exposed through the opening. While a thin metal layer is likely to deform toward the exposed portion in response to the conductive adhesive, a thick metal layer is less likely to deform and thus becomes separated from the exposed portion. This increase in resistance due to the reduced conformability is particularly pronounced in small-diameter openings. The increased resistance may then degrade the shielding properties of the printed wiring board.

[0011] Further investigations by the inventors have revealed that increasing the content of conductive particles in the conductive adhesive layer in order to reduce the resistance between the shielding layer and the ground circuit via the conductive adhesive increases the insertion loss in the printed wiring board.

[0012] In view of the above circumstances, an object of the present invention is to provide an electromagnetic wave shielding film that has excellent shielding properties and can reduce insertion loss.

[0013] 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 conductive adhesive layer laminated on the shielding layer, wherein the shielding layer is a metal layer having a thickness of 6 μm or more, and the conductive adhesive layer contains 30% by mass or more and 55% by mass or less of conductive particles.

[0014] (2) The electromagnetic wave shielding film according to (1) above, wherein the conductive adhesive layer contains 40% by mass or more of the conductive particles.

[0015] (3) The electromagnetic wave shielding film according to (1) or (2) above, wherein the thickness of the metal layer is 8 μm or more and 25 μm or less.

[0016] (4) The electromagnetic wave shielding film according to any one of (1) to (3) above, wherein the metal layer is made of copper foil.

[0017] (5) The electromagnetic wave shielding film according to any one of (1) to (4) above, wherein the conductive particles are dendritic copper powder.

[0018] According to the present invention, it is possible to provide an electromagnetic wave shielding film that has excellent shielding properties and can reduce insertion loss.

[0019] 1 is a schematic cross-sectional view of an electromagnetic wave shielding film according to one embodiment of the present invention, a schematic cross-sectional view of the electromagnetic wave shielding film of FIG. 1 when it is adhered to a printed wiring board, and a schematic cross-sectional view of an electromagnetic wave shielding film according to another embodiment of the present invention.

[0020] 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 case where the object to which the film is applied is a printed wiring board. An electromagnetic wave shielding film is adhered to a printed wiring board to prevent unwanted electromagnetic waves from emitting 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 printed wiring board is an object whose entry and exit of electromagnetic waves are restricted by the electromagnetic wave shielding film. The object has an attachment surface to which the electromagnetic wave shielding film is attached.

[0021] As shown in Figures 1 and 2, the electromagnetic wave shielding film 1 of this embodiment includes an adhesive layer 11 for enabling adhesion to a printed wiring board 2, and a shielding layer 12 laminated on the adhesive layer 11.

[0022] As shown in FIG. 2 , the printed wiring board 2 according to this embodiment includes a base film 21 made of polyimide resin, a circuit pattern 22 made of metal such as copper formed on the base film 21, and an insulating film 23 made of resin such as polyimide resin that covers the circuit pattern 22. The surface of the insulating film 23 is an adhesion surface 231 on which the electromagnetic wave shielding film 1 is attached. The circuit pattern 22 includes a ground circuit 221 that is electrically connected to a grounded component and provides a short-circuit path for unnecessary potential to the component. The insulating film 23 has an opening 232 that penetrates the insulating film 23 in the thickness direction at a position corresponding to the ground circuit 221. That is, before the electromagnetic wave shielding film 1 is bonded to the printed wiring board 2, a portion of the ground circuit 221 is exposed through the opening 232. When the electromagnetic wave shielding film 1 is bonded to the printed wiring board 2, a component derived from the adhesive layer 11 is filled inside the opening 232. The diameter (circle-equivalent diameter) of the opening 232 is, for example, 0.5 mm to 2 mm, and may be 0.8 mm to 1.8 mm. The opening 232 normally forms a filling space for the cylindrical adhesive layer 11 inside.

[0023] The printed wiring board 2 may be a flexible board. In this case, the insulating film 23 is usually a coverlay made of polyimide resin. The printed wiring board 2 is a shielded printed wiring board 100 to which an electromagnetic wave shielding film 1 is attached.

[0024] 1 , the electromagnetic wave shielding film 1 includes a conductive adhesive layer 11 that allows adhesion to the insulating film 23 and electrical connection (hereinafter, "conduction") between a ground circuit 221 and a shielding layer 12, and a shielding layer 12 laminated on the conductive adhesive layer 11. The conductive adhesive layer 11 contains conductive particles and a thermosetting binder resin. The shielding layer 12 is composed of a metal layer with a thickness of 6 μm or more.

[0025] When adhering the electromagnetic wave shielding film 1 to the printed wiring board 2, the electromagnetic wave shielding film 1 is superimposed on the adhesion surface 231 of the insulating film 23, and is pressure-bonded to the adhesion surface 231 while being heated at a first temperature (temporary bonding temperature) at which the binder resin can flow. At this time, the conductive adhesive layer 11 is filled into the inside of the openings 232 formed in the insulating film 23 by the binder resin that flows when heated. Thereafter, the electromagnetic wave shielding film 1 is heated to a second temperature that is higher than the first temperature and at which the binder resin can be thermally cured, and is pressure-bonded to the printed wiring board 2.

[0026] It is important that the conductive adhesive layer 11 contains the conductive particles in an amount of 30% by mass or more and 55% by mass or less relative to the total mass of the conductive adhesive layer 11. The content of the conductive particles in the conductive adhesive layer 11 is preferably 40% by mass or more and 50% by mass or less. This provides the shielded printed wiring board 100 with sufficient shielding properties and enables the insertion loss in the shielded printed wiring board 100 to be suppressed.

[0027] The reason for this effect is presumably that, in an electromagnetic wave shielding film having a thin shielding layer with a thickness of less than 6 μm, the portion covering the opening 232 recesses toward the opening 232 during the temporary fastening described above. This brings the ground circuit 221 and the shielding layer closer together, thereby enabling electrical continuity even when the conductive adhesive layer contains a small amount of conductive particles. On the other hand, in an electromagnetic wave shielding film 1 having a thick shielding layer 12 as in the present embodiment, recessing toward the opening 232 during temporary fastening is less likely to occur, meaning that the ground circuit 221 and the shielding layer 12 are less likely to come closer together, resulting in insufficient electrical continuity. In contrast, in the electromagnetic wave shielding film 1 of the present embodiment, the conductive adhesive layer 11 contains 30% by mass or more of conductive particles, which increases the number of contact points between the conductive particles between the ground circuit 221 and the shielding layer 12 and thus contributes to achieving sufficient electrical continuity.

[0028] Furthermore, an increase in the content of conductive particles in the conductive adhesive layer 11 is accompanied by a relative decrease in the content of the binder resin, which is thought to contribute to suppressing expansion of the conductive adhesive layer 11 during the reflow process. As a result, the shielded printed wiring board 100 after the reflow process can suppress an increase in the resistance value between the ground circuit 221 and the shield layer 12, which are electrically connected via the conductive adhesive layer 11, and ultimately exhibits the desired shielding properties.

[0029] Further, it is also possible to reduce the thickness of the insulating film 23 or increase the diameter of the opening 232. However, reducing the thickness of the insulating film 23 may increase insertion loss due to changes in capacitance and characteristic impedance. Furthermore, increasing the size of the opening 232 may expose circuit patterns other than the ground circuit 221, which should be covered by the insulating film 23. This may make it difficult to provide high-density wiring with narrow pitches on the printed wiring board 2. From this perspective, the electromagnetic wave shielding film 1 of this embodiment can be particularly effective when applied to a printed wiring board 2 having a thickness from the ground circuit 221 to the surface (adhesion surface 231) of the insulating film 23 of 15 μm or more, preferably 20 μm or more. The thickness here is, for example, 100 μm or less. The electromagnetic wave shielding film 1 of this embodiment can be particularly effective when applied to a printed wiring board 2 having an L / S (line and space) of 100 μm / 100 μm or less, preferably 75 μm / 75 μm or less.

[0030] Examples of the conductive particles include metal particles such as copper particles, silver particles, nickel particles, silver-coated copper particles, gold-coated copper particles, silver-coated nickel particles, gold-coated nickel particles, and silver-coated alloy particles; carbon fillers; and metal-coated resin particles.

[0031] The conductive particles may have a spherical, plate-like, needle-like, dendritic or other shape, with the dendritic shape being preferred.

[0032] The particle size of the conductive particles is preferably 2 μm or more and 20 μm or less, and more preferably 5 μm or more and 18 μm or less. The average particle size of the conductive particles can be determined from a volume-based particle size distribution measured by a laser diffraction / scattering particle size distribution measurement method.

[0033] 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.

[0034] The content of the binder resin is preferably 70% by mass or less, and more preferably 60% by mass or less, relative to the total mass of the conductive adhesive layer 11. This makes it possible to suppress expansion of the conductive adhesive layer 11 during the reflow process. The lower limit of the content of the binder resin is not particularly limited, but may be 30% by mass or more, or 40% by mass or more.

[0035] The conductive adhesive layer 11 may contain an additive such as a flame retardant. The content of the additive is, for example, 10 mass % or less with respect to the total mass of the conductive adhesive layer 11.

[0036] The thickness of the conductive adhesive layer 11 is preferably 2 μm or more and 20 μm or less, and more preferably 8 μm or more and 15 μm or less. The thickness of the conductive adhesive layer 11 means the thickness after pressing onto the printed wiring board 2 (the object), and can be determined by observing the cross section of the electromagnetic wave shielding film 1 under magnification using an SEM or the like and averaging the measured values ​​at 10 arbitrarily selected points.

[0037] The shield layer 12 of this embodiment is a metal layer made of metal. The metal layer may contain unavoidable impurities to the extent that they do not affect the shielding properties. The metal layer may be made 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. The shield layer 12 is preferably made of rolled copper foil.

[0038] The thickness of the shielding layer 12 is preferably 8 μm or more, more preferably 10 μm or more, and may be 12 μm or more. The upper limit of the thickness of the shielding layer 12 is not particularly limited, but may be 25 μm or less, 20 μm or less, or 18 μm or less. The thickness of the shielding layer 12 refers to the thickness before being pressed onto the printed wiring board 2 (the target), and can be determined by performing enlarged observation of the 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.

[0039] Next, the performance and physical properties of the electromagnetic wave shielding film 1 of this embodiment will be described.

[0040] The electromagnetic wave shielding film 1 of this embodiment can have an electric field shielding effect of more than 100 dB at 1 GHz, as measured by the KEC method. Furthermore, the electromagnetic wave shielding film 1 can have a magnetic field shielding effect of more than 80 dB, and possibly even more than 90 dB, at 1 GHz, as measured by the KEC method. The electric field shielding effect based on the KEC method can be measured using an electromagnetic wave shielding effect measuring device developed by the KEC Kansai Electronics Industry Development Center, a general incorporated association, on a 15 cm square test piece cut out from the electromagnetic wave shielding film 1. Furthermore, the measurement can be performed under conditions of a temperature of 25°C and a relative humidity of 30 to 50%.

[0041] A printed wiring board (shielded printed wiring board 100) to which the electromagnetic wave shielding film 1 of this embodiment is applied can have an insertion loss of less than −10 dB at 10 GHz. A network analyzer can be used to measure the insertion loss. Specifically, the evaluation printed wiring board is fabricated as follows: First, a 12 μm-thick copper plating layer is deposited on the surface of a base film made of polyimide resin with a thickness of 25 μm, a length of 110 mm, and a width of 30 mm. Two signal circuits (50 μm wide, with a space between the circuits of 65 μm) and two ground circuits parallel to the signal circuits are formed outside of the signal circuits by etching, forming an evaluation circuit pattern in which each circuit extends linearly in the length direction of the base film (transversely across the base film). Next, the evaluation circuit pattern is covered with a coverlay made of polyimide with a thickness of 37 μm, thereby obtaining an evaluation printed wiring board. At this time, the evaluation circuit pattern is exposed from both ends of the coverlay. Next, the evaluation shielded printed wiring board is fabricated as follows. First, a 100 mm long electromagnetic wave shielding film is laminated onto a printed wiring board for evaluation, and the resulting board is heated and pressed using a press at 170°C and 3.0 MPa for 30 minutes to obtain a shielded printed wiring board for evaluation. The exposed circuit pattern for evaluation is then connected to a network analyzer (KEYSIGHT, E5080B) via a probe (CASCADE Microtech, Z20-XD-GSSG), and a signal in the range of 10 MHz to 40 GHz is sent to the signal circuit to measure the insertion loss of the shielded printed wiring board for evaluation.

[0042] In a printed wiring board (shielded printed wiring board 100) to which the electromagnetic wave shielding film 1 of this embodiment is applied, after one reflow process, the resistance value between the ground circuit 221 and the shielding layer 12, which are electrically connected via the conductive adhesive layer 11, is 500 mΩ or less, and can also be 200 mΩ or less, or even 100 mΩ or less. Note that the above-mentioned shielded printed wiring board for evaluation can be used to measure the resistance value here.

[0043] 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.

[0044] For example, as shown in FIG. 3 , the electromagnetic wave shielding film according to the present invention may include an adhesive layer 11, a shielding layer 12 laminated on the adhesive layer 11, and a protective layer 13 laminated on the side of the shielding layer 12 opposite the adhesive layer 11. The protective layer 13 is made of an insulating resin and can suppress deterioration of the metals contained in the adhesive layer 11 and the shielding layer 12. The protective layer 13 can also prevent the shielding layer 12 from shorting to surrounding circuits. The protective layer 13 may be a single layer, or may include multiple layers including a first protective layer formed from a first resin composition and a second protective layer laminated on the first protective layer and formed from a second resin composition different from the first resin composition. The thickness of the protective layer 13 is preferably 6 μm or less, and may be 5 μm or less. It is believed that the protective layer 13, like the shielding layer 12 (composed of a thick metal layer), may have an effect of preventing electrical continuity between the shielding layer 12 and the ground circuit 221, but the adhesive layer 11 of the present invention is believed to be able to reduce the effect of such a protective layer 13.

[0045] The present invention will be further explained below with reference to examples, but the present invention is not limited to these examples.

[0046] Example 1 A protective layer (6 μm) was laminated on a transfer film (thickness 50 μm) using an epoxy resin. Next, a rolled copper foil (18 μm) was laminated on the surface of the protective layer as a shielding layer. Next, an adhesive containing 30 mass% dendritic copper particles (average particle size 13 μm) as conductive particles and 70 mass% epoxy resin as a binder resin was applied to the surface of the shielding layer using a coater to form a coating film, which was then dried. A conductive adhesive layer was then laminated, thereby obtaining an electromagnetic wave shielding film.

[0047] As shown in Table 1, the thickness of the shielding layer and the content of copper particles in the conductive adhesive layer were changed compared to Example 1, and electromagnetic wave shielding films were produced as Examples 2 to 6 and Comparative Examples 1 to 5. Furthermore, the copper particles used in Example 1 were changed to plate-like ones, and an electromagnetic wave shielding film was produced as Reference Example 1.

[0048] [Evaluation 1: Electric and magnetic field shielding properties] The electric field shielding effect and magnetic field shielding effect were measured according to the above-mentioned KEC method and evaluated based on the following evaluation criteria. The results are shown in Table 1. (Evaluation criteria for electric field shielding effect) ○: Electric field shielding effect at 1 GHz exceeds 100 dB ×: Electric field shielding effect at 1 GHz is 100 dB or less (Evaluation criteria for magnetic field shielding effect) ○: Magnetic field shielding effect at 1 GHz is 80 dB or more ×: Magnetic field shielding effect at 1 GHz is less than 80 dB

[0049] [Evaluation 2: Insertion Loss] The insertion loss was measured according to the above method and evaluated based on the following evaluation criteria. The results are shown in Table 1. (Evaluation criteria for insertion loss) ○: Insertion loss at 10 GHz is −10 dB or more ×: Insertion loss at 10 GHz is less than −10 dB

[0050] [Evaluation 3: Connection Resistance] A reflow process was carried out at 260°C, and the connection resistance between the ground circuit and the shielding layer after reflow was measured and evaluated based on the following evaluation criteria. Note that a rating of ○ indicates sufficient conductivity between the ground circuit and the shielding layer, and it is considered that the desired shielding properties can be exhibited even after reflow. The results are as shown in Table 1. (Evaluation criteria for connection resistance) ○: Resistance after reflow is 500 mΩ or less ×: Resistance after reflow is more than 500 mΩ

[0051]

[0052] 1: Electromagnetic wave shielding film, 11: Adhesive layer, 12: Shielding layer, 13: Protective layer, 2: Printed wiring board, 21: Base film, 22: Circuit pattern, 221: Ground circuit, 23: Insulating film, 231: Adhesion surface, 232: Opening, 100: Shielded printed wiring board

Claims

1. An electromagnetic wave shielding film comprising a shielding layer and a conductive adhesive layer laminated on the shielding layer, wherein the shielding layer is a metal layer having a thickness of 6 μm or more, and the conductive adhesive layer contains 30% by mass or more and 55% by mass or less of conductive particles.

2. The electromagnetic wave shielding film according to claim 1, wherein the conductive adhesive layer contains 40% by mass or more of the conductive particles.

3. The electromagnetic wave shielding film according to claim 1, wherein the thickness of the metal layer is 8 μm or more and 25 μm or less.

4. The electromagnetic wave shielding film according to any one of claims 1 to 3, wherein the metal layer is made of copper foil.

5. The electromagnetic wave shielding film according to any one of claims 1 to 3, wherein the conductive particles are dendritic copper powder.

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