Electromagnetic wave shielding film

Abstract

An electromagnetic wave shielding film comprises: a shielding layer and a conductive adhesive layer laminated thereon, wherein the shielding layer is a metal layer with a thickness of 6µm or more, and the conductive adhesive layer contains 30% by mass and 55% by mass of conductive particles.

Description

[0001] Cross-reference of related applications

[0002] This application claims priority to Japanese Patent Application No. 2024-56466, which is incorporated herein by reference. Technical Field

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

[0004] In the past, as an EMC countermeasure in electronic devices, shielding devices were formed to shield electromagnetic waves that could become noise. Electromagnetic wave shielding films were used to form such shielding devices.

[0005] As an electromagnetic wave shielding film, it is known to have a shielding layer and a conductive adhesive layer laminated thereon. The conductive adhesive layer typically contains conductive particles such as metal particles and an adhesive resin.

[0006] Furthermore, Patent Document 1 describes an electromagnetic wave shielding film having a metal layer with a thickness of 0.5µm or more as a shielding layer. Patent Document 1 demonstrates that by using this metal layer, electromagnetic waves can be effectively shielded.

[0007] One specific application of electromagnetic wave shielding films is the formation of noise shielding in printed circuit boards (PCBs). PCBs are known to include a base film made of polyimide resin, a circuit pattern formed on the base film, and an insulating film (e.g., a cover layer in a flexible substrate) protecting the circuit pattern. Additionally, PCBs are known to include a grounding circuit. The insulating film in such a PCB has an opening leading to the grounding circuit included in the circuit pattern. Furthermore, the shielding layer of the electromagnetic wave shielding film bonds the insulating film to the shielding layer and is electrically connected to the grounding circuit via a conductive adhesive filling the opening.

[0008] One method for bonding an electromagnetic wave shielding film to a printed circuit board is to overlap a conductive adhesive layer of the electromagnetic wave shielding film with an insulating film of the printed circuit board, and then press the electromagnetic wave shielding film onto the insulating film while heating it (also described in Patent Document 1). In this process, the conductive adhesive layer flows, filling conductive particles along with the adhesive resin inside the openings formed in the insulating film.

[0009] Furthermore, printed circuit boards with attached electromagnetic shielding films are sometimes used for component mounting during the reflow soldering process. In a typical reflow soldering process, the printed circuit board is heated to over 200°C.

[0010] Existing technical documents

[0011] Patent documents

[0012] Patent Document 1: International Publication No. 2013 / 077108 Summary of the Invention

[0013] The problem the invention aims to solve

[0014] As described above, a thick metal layer is advantageous in improving shielding performance. However, the inventors have observed that when a thick metal layer is used, the resistance between the shielding layer and the grounding circuit, which are electrically connected via a conductive adhesive, increases, especially after reflow soldering. The main reason for this tendency is that a thick metal layer, compared to a thin metal layer, is less likely to deform following the conductive adhesive flowing towards the opening when the electromagnetic wave shielding film is pressed onto the insulating film, thus increasing the distance between the thick metal layer and the grounding circuit. In other words, it is believed that the exposed portion of the grounding circuit from the opening forms a step with the surface of the insulating film. With a thin metal layer, it is easy to deform towards the exposed portion in a close manner following the conductive adhesive; on the other hand, a thick metal layer is less likely to undergo this deformation and become separated from the exposed portion. It is believed that the increase in resistance caused by this reduced followability is particularly significant in small-diameter openings. Moreover, there is a concern that the increased resistance may reduce the shielding performance of the printed circuit board.

[0015] Furthermore, the inventors conducted research and obtained the following insights: In order to reduce the resistance between the shielding layer and the grounding circuit via the conductive adhesive, increasing the content of conductive particles in the conductive adhesive layer increases the insertion loss in the printed circuit board.

[0016] In view of the above, the objective of the present invention is to provide an electromagnetic wave shielding film with excellent shielding performance and reduced insertion loss.

[0017] Solution for solving the problem

[0018] The electromagnetic wave shielding film of the present invention is described below. (1)

[0020] An electromagnetic wave shielding film, comprising:

[0021] A shielding layer and a conductive adhesive layer laminated thereon.

[0022] The shielding layer is a metal layer with a thickness of 6µm or more.

[0023] The conductive adhesive layer contains 30% by mass and less than 55% by mass of conductive particles. (2)

[0025] According to the electromagnetic wave shielding film described in (1) above, the conductive adhesive layer contains more than 40% by mass of the conductive particles. (3)

[0027] According to the electromagnetic wave shielding film described in (1) or (2) above, the thickness of the metal layer is more than 8µm and less than 25µm. (4)

[0029] The electromagnetic wave shielding film according to any one of (1) to (3) above, wherein the metal layer is composed of copper foil. (5)

[0031] The electromagnetic wave shielding film according to any one of (1) to (4) above, wherein the conductive particles are dendritic copper powder.

[0032] According to the present invention, an electromagnetic wave shielding film with excellent shielding performance and reduced insertion loss can be provided. Attached Figure Description

[0033] Figure 1 This is a schematic cross-sectional view of an electromagnetic wave shielding film according to one embodiment.

[0034] Figure 2 yes Figure 1 A simplified cross-sectional view of an electromagnetic wave shielding film bonded to a printed circuit board.

[0035] Figure 3 This is a schematic cross-sectional view of an electromagnetic wave shielding film according to another embodiment. Detailed Implementation

[0036] Referring to the accompanying drawings, an electromagnetic wave shielding film according to one embodiment of the present invention will be described using a printed circuit board as an example. To prevent unwanted electromagnetic waves from radiating from the board and causing adverse effects on peripheral devices, and to prevent electromagnetic waves from entering the board from peripheral devices and generating noise in the circuit, an electromagnetic wave shielding film is adhered to the printed circuit board. That is, the printed circuit board is an object whose electromagnetic waves are restricted from entering and exiting by the electromagnetic wave shielding film. This object has a surface on which the electromagnetic wave shielding film is adhered.

[0037] like Figure 1 and Figure 2 As shown, the electromagnetic wave shielding film 1 of this embodiment includes an adhesive layer 11 for bonding to a printed circuit board 2 and a shielding layer 12 stacked on the adhesive layer 11.

[0038] like Figure 2As shown, the printed circuit board 2 of 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 covering the circuit pattern 22. The surface of the insulating film 23 is the adhesive surface 231 to the electromagnetic wave shielding film 1. The circuit pattern 22 includes a grounding circuit 221, which is electrically connected to a grounded component, forming a short-circuit path for an unnecessary potential to reach that component. The insulating film 23 has an opening 232 extending through the thickness direction at a position corresponding to the grounding circuit 221. That is, in the printed circuit board 2 before the electromagnetic wave shielding film 1 is bonded, a portion of the grounding circuit 221 is exposed from the opening 232. When the electromagnetic wave shielding film 1 is bonded to the printed circuit board 2, the inside of the opening 232 is filled with a component derived from the adhesive layer 11. The diameter (equivalent circle diameter) of the opening 232 is, for example, 0.5 mm or more and 2 mm or less, or 0.8 mm or more and 1.8 mm or less. The opening 232 typically forms a cylindrical space for filling the adhesive layer 11 inside it.

[0039] The printed circuit board 2 can also be a flexible substrate. In this case, the insulating film 23 is typically a cover layer made of polyimide resin. The printed circuit board 2 is a shielded printed circuit board 100 with an electromagnetic wave shielding film 1 attached thereto.

[0040] like Figure 1 As shown, the electromagnetic wave shielding film 1 includes: a conductive adhesive layer 11 capable of being bonded to the insulating film 23 and enabling electrical connection (hereinafter, conduction) between the grounding circuit 221 and the shielding layer 12; and a shielding layer 12 laminated on the conductive adhesive layer 11. The conductive adhesive layer 11 comprises conductive particles and a thermosetting adhesive resin. The shielding layer 12 is composed of a metal layer with a thickness of 6 µm or more.

[0041] When the electromagnetic wave shielding film 1 is bonded to the printed circuit board 2, it is heated and pressed onto the adhesive surface 231 at a first temperature (temporary fixed temperature) where the adhesive resin can flow, while overlapping with the adhesive surface 231 of the insulating film 23. At this time, the conductive adhesive layer 11 is filled into the inside of the opening 232 formed in the insulating film 23 by the adhesive resin flowing due to the heating. Then, the electromagnetic wave shielding film 1 is heated to a second temperature higher than the first temperature, that is, a second temperature at which the adhesive resin can be thermally cured, and pressed onto the printed circuit board 2.

[0042] Importantly, the conductive adhesive layer 11 contains at least 30% by mass and less than 55% by mass of the conductive particles relative to its total mass. The content of the conductive particles in the conductive adhesive layer 11 is preferably at least 40% by mass and less than 50% by mass. This provides sufficient shielding to the shielded printed circuit board 100 and suppresses insertion loss in the shielded printed circuit board 100.

[0043] The reason for presuming this effect is that, firstly, when the electromagnetic wave shielding film with a thin shielding layer of less than 6µm is temporarily fixed, the portion covering the opening 232 is recessed into the opening 232. Therefore, it is believed that the grounding circuit 221 is close to this shielding layer, and conduction is possible even when the content of conductive particles in the conductive adhesive layer is low. On the other hand, it is believed that, as in this embodiment, the electromagnetic wave shielding film 1 with a thick shielding layer 12 is difficult to recess into the opening 232 during temporary fixing; that is, it is difficult for the grounding circuit 221 to be close to the shielding layer 12, and sufficient conduction cannot be achieved. In contrast, the electromagnetic wave shielding film 1 of this embodiment, by including at least 30% by mass of conductive particles in the conductive adhesive layer 11, increases the number of contact points between the conductive particles between the grounding circuit 221 and the shielding layer 12, thereby achieving sufficient conduction.

[0044] Furthermore, the increase in the content of conductive particles in the conductive adhesive layer 11 is accompanied by a relative decrease in the content of the adhesive resin, which is believed to also help suppress the expansion of the conductive adhesive layer 11 during the reflow soldering process. As a result, the shielded printed circuit board 100 after the reflow soldering process can suppress the increase in the resistance value of the grounding circuit 221 and the shielding layer 12 that are conducted through the conductive adhesive layer 11, thereby achieving the desired shielding performance.

[0045] Furthermore, reducing the thickness of the insulating film 23 or increasing the opening diameter of the opening 232 could be considered. However, if the thickness of the insulating film 23 is reduced, the insertion loss may increase along with changes in capacitance and characteristic impedance. Moreover, if the opening 232 is increased, circuit patterns other than the grounding circuit 221, which should be covered by the insulating film 23, may be exposed, raising concerns about the difficulty of setting high-density wiring with narrow spacing on the printed circuit board 2. From this perspective, it is particularly effective for the electromagnetic wave shielding film 1 of this embodiment to be applied to the surface (adhesive surface 231) of the insulating film 2 from the grounding circuit 221 to the printed circuit board 2 with a thickness of 15µm or more, preferably 20µm or more. The thickness here is, for example, 100µm or less. In addition, it is particularly effective for the electromagnetic wave shielding film 1 of this embodiment to be applied to a printed circuit board 2 with an L / S (linewidth / line spacing) of 100µm / 100µm or less, preferably 75µm / 75µm or less.

[0046] Examples of conductive particles 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, and other metal particles; carbon fillers; and metal-coated resin particles.

[0047] The conductive particles can be spherical, plate-shaped, needle-shaped, dendritic, etc., with dendritic shape being preferred.

[0048] The conductive particles preferably have a particle size of 2µm or more and 20µm or less, more preferably 5µm or more and 18µm or less. It should be noted that the average particle size of the conductive particles can be determined from the particle size distribution of a volume reference measured by laser diffraction scattering particle size distribution determination method.

[0049] The adhesive resin can be 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 a thermosetting resin or a photocurable resin.

[0050] The content of the adhesive resin relative to the total mass of the conductive adhesive layer 11 is preferably 70% by mass or less, more preferably 60% by mass or less. This helps to suppress the expansion of the conductive adhesive layer 11 during the reflow soldering process. The lower limit of the adhesive resin content is not particularly limited and can be 30% by mass or more, or 40% by mass or more.

[0051] The conductive adhesive layer 11 may contain additives such as flame retardants. The content of the additives relative to the total mass of the conductive adhesive layer 11 is, for example, 10% by mass or less.

[0052] The thickness of the conductive adhesive layer 11 is preferably 2µm or more and 20µm or less, more preferably 8µm or more and 15µm or less. It should be noted that the thickness of the conductive adhesive layer 11 refers to the thickness after pressure is applied to the printed circuit board 2 (the object), and can be obtained by averaging the measured values ​​at arbitrarily selected locations by magnifying the cross-section of the electromagnetic wave shielding film 1 using SEM or the like.

[0053] 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 the 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. The shielding layer 12 is preferably made of rolled copper foil.

[0054] The thickness of the shielding layer 12 is preferably 8µm or more, more preferably 10µm or more, and may also be 12µm or more. There is no particular upper limit to the thickness of the shielding layer 12; it may be 25µm or less, 20µm or less, or 18µm or less. It should be noted that the thickness of the shielding layer 12 refers to the thickness of the printed circuit board 2 (the object) before pressure is applied. This thickness 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 an SEM or similar method.

[0055] Next, the performance and physical properties of the electromagnetic wave shielding film 1 of this embodiment will be described.

[0056] The electromagnetic wave shielding film 1 of this embodiment exhibits an electric field shielding effect exceeding 100 dB at 1 GHz, as measured by the KEC method. Furthermore, the magnetic field shielding effect of the electromagnetic wave shielding film 1 at 1 GHz, as measured by the KEC method, exceeds 80 dB, and can even exceed 90 dB. 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 Promotion Center, using a 15 cm square test piece cut from the electromagnetic wave shielding film 1. The measurement can be performed at a temperature of 25°C and a relative humidity of 30-50%.

[0057] The printed circuit board (shielded printed circuit board 100) using the electromagnetic wave shielding film 1 of this embodiment can achieve an insertion loss of less than -10 dB at 10 GHz. The insertion loss can be measured using a network analyzer. Specifically, the evaluation printed circuit board is fabricated as follows: First, a 12 µm thick copper plating layer is deposited on the surface of a polyimide resin base film 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 line spacing of 65 µm) and two ground circuits parallel to the signal circuits on their outer sides are etched to form an evaluation circuit pattern where each circuit extends linearly (through the base film) along the length of the base film. Next, the evaluation circuit pattern is covered with a 37 µm thick polyimide capping layer to obtain the evaluation printed circuit board. At this time, the evaluation circuit pattern is exposed from both ends of the capping layer. Next, the evaluation shielded printed circuit board is fabricated as follows. First, a 100mm long electromagnetic wave shielding film was laminated onto an evaluation printed circuit board (PCB). The laminate was then heated and pressed at 170°C and 3.0MPa for 30 minutes using a press to obtain a shielded PCB for evaluation. Next, the exposed evaluation circuit pattern was connected to a network analyzer (KEYSIGHT E5080B) via probes (CASCADE Microtech, Z20-XD-GSSG). Signals in the range of 10MHz to 40GHz were transmitted to the signal circuit, and the insertion loss of the shielded PCB for evaluation was measured.

[0058] After one reflow soldering process, the resistance value between the grounding circuit 221, which is connected via the conductive adhesive layer 11, and the shielding layer 12 of the printed circuit board (shielded printed circuit board 100) using the electromagnetic wave shielding film 1 of this embodiment is 500 mΩ or less, or 200 mΩ or less, or even 100 mΩ or less. It should be noted that the resistance value can be measured using the aforementioned evaluation shielded printed circuit board.

[0059] 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.

[0060] For example, such as Figure 3As shown, the electromagnetic wave shielding film of 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 shielding layer 12 on the side opposite to the adhesive layer 11. The protective layer 13 is made of an insulating resin, which can suppress the deterioration of the metal contained in the adhesive layer 11 and the shielding layer 12. In addition, the protective layer 13 can prevent the shielding layer 12 from short-circuiting with surrounding circuits. The protective layer 13 may be a single layer, or it may include multiple layers having a first protective layer formed of a first resin composition and a second protective layer laminated on the first protective layer and formed of a second resin composition different from the first resin composition. The thickness of the protective layer 13 is preferably 6µm or less, and may also be 5µm or less. It is believed that the protective layer 13, like the shielding layer 12 (which is composed of a metal layer of thickness), can affect the conduction of the shielding layer 12 and the grounding circuit 221 in a manner that hinders the conduction of the grounding circuit 221, but it is believed that the adhesive layer 11 according to the present invention can also reduce the effect of such a protective layer 13.

[0061] Example

[0062] The present invention will be further described below with reference to embodiments, but the present invention is not limited to these embodiments.

[0063] [Example 1]

[0064] A protective layer (6µm) is laminated onto a transfer film (50µm thick) using epoxy resin. Next, a rolled copper foil (18µm) as a shielding layer is laminated onto the surface of the protective layer. Then, an adhesive comprising 30% by mass of dendritic copper particles (average particle size 13µm) as conductive particles and 70% by mass of epoxy resin as an adhesive resin is coated onto the surface of the shielding layer using a coating machine to form a coated film and allow it to dry. A conductive adhesive layer is then laminated to obtain an electromagnetic wave shielding film.

[0065] As shown in Table 1, for Example 1, the thickness of the shielding layer and the content of copper particles in the conductive adhesive layer were changed to produce electromagnetic wave shielding films as Examples 2-6 and Comparative Examples 1-5. Additionally, for Example 1, the copper particles were changed to plate-shaped to produce an electromagnetic wave shielding film as Reference Example 1.

[0066] [Evaluation 1: Shielding against electric and magnetic fields]

[0067] The shielding effects of electric and magnetic fields were measured using the KEC method described above and evaluated based on the following criteria. The results are shown in Table 1.

[0068] (Evaluation criteria for electric field shielding effectiveness)

[0069] ○: The electric field shielding effect at 1 GHz exceeds 100 dB.

[0070] ×: The electric field shielding effect at 1GHz is below 100dB.

[0071] (Evaluation criteria for magnetic field shielding effectiveness)

[0072] ○: The magnetic field shielding effect at 1GHz is above 80dB.

[0073] ×: Magnetic field shielding effect at 1GHz is less than 80dB.

[0074] [Evaluation 2: Insertion Loss]

[0075] Insertion loss was measured using the method described above and evaluated based on the following criteria. The results are shown in Table 1.

[0076] (Evaluation criteria for insertion loss)

[0077] ○: Insertion loss at 10GHz is above -10dB.

[0078] ×: Insertion loss at 10GHz is less than -10dB

[0079] [Evaluation 3: Connection Resistance]

[0080] A reflow soldering process was performed at 260°C. The connection resistance between the grounding circuit and the shielding layer after reflow soldering was measured, and the evaluation was conducted based on the following evaluation criteria. It should be noted that if the evaluation value is ○, it is considered that the grounding circuit and the shielding layer have sufficient conductivity and can still provide the desired shielding performance even after reflow soldering. The results are shown in Table 1.

[0081] (Evaluation criteria for connection resistance)

[0082] ○: The resistance value after reflow soldering is below 500mΩ.

[0083] ×: The resistance value after reflow soldering exceeds 500mΩ.

[0084] [Table 1]

[0085]

[0086] Explanation of reference numerals in the attached figures

[0087] 1: Electromagnetic wave shielding film, 11: Adhesive layer, 12: Shielding layer, 13: Protective layer, 2: Printed circuit board, 21: Base film, 22: Circuit pattern, 221: Grounding circuit, 23: Insulating film, 231: Adhesive surface, 232: Opening, 100: Printed circuit board with shielding.

Claims

1. An electromagnetic wave shielding film, comprising: A shielding layer and a conductive adhesive layer laminated thereon. The shielding layer is a metal layer with a thickness of 6µm or more. The conductive adhesive layer contains 30% by mass and less than 55% by mass of conductive particles.

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

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

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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