Composite film

Abstract

To provide a composite film having low connection electric resistance.SOLUTION: A composite film 1 includes: a copper foil 10; and a conductive layer 20 that is comprised of a conductive adhesive agent containing a conductive adhesive agent containing a conductive filler and the conductive filler, and is laminated on at least one front surface 10a of two front surfaces provided by the copper foil 10. Then, a number average particle diameter of the conductive filler is 0.12 μm or more and 7 μm or less. Also, out of two front surfaces of the copper foil 10, a development area ratio Sdr of the front surface 10a on which the conductive layer 20 is laminated is 0.01% or more and 40% or less, and a skewness Ssk is -1.0 or more and 1.0 or less, and a contact resistance 2 mΩ or more and 30 mΩ or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a composite film.

Background Art

[0002] In recent years, for electronic devices such as smartphones, medical devices, and automobiles, higher integration and higher performance have been progressing. Along with this, the importance of EMI countermeasures for suppressing electromagnetic interference (EMI) of electronic devices has been increasing. Therefore, in order to prevent problems such as malfunction of the electronic device due to electromagnetic waves from inside and outside the housing of the electronic device and deterioration of communication data quality, members such as noise filters and electromagnetic wave absorption sheets are used in the electronic device. Among such members, a composite film called an EMI countermeasure film is used in many electronic devices. The EMI countermeasure film is a composite film having a structure in which an electromagnetic wave shielding layer made of metal or the like and a conductive layer made of a conductive adhesive or a conductive adhesive are laminated. As the electromagnetic wave shielding layer, copper foil may be used from the viewpoints of high conductivity, shielding property, and industrial productivity.

[0003] And the EMI countermeasure film is very often used as a shielding film in flexible printed circuits (FPC) or chip on film technology (COF), or as a conductive tape for strengthening grounding inside the housing of an electronic device. When attaching the EMI countermeasure film to an electronic device, usually, the electromagnetic wave shielding layer is attached to a metal part inside the electronic device via the conductive layer. And when the electromagnetic wave shielding layer of the EMI countermeasure film is attached to the metal part inside the electronic device, the lower the connection electrical resistance from the electromagnetic wave shielding layer to the metal part inside the electronic device, the better the performance of suppressing electromagnetic interference (hereinafter, the performance of suppressing electromagnetic interference possessed by the EMI countermeasure film may also be referred to as "EMI resistance"). This EMI resistance can be evaluated by the above connection electrical resistance, but may also be evaluated simulatedly by wiring resistance.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] As described above, in recent years, the importance of EMI countermeasures has increased, so a composite film with a lower connection electrical resistance is required. An object of the present invention is to provide a composite film with a low connection electrical resistance.

Means for Solving the Problems

[0006] To solve the above problems, the inventors of the present invention considered that in order to reduce the connection electrical resistance (wiring resistance) of the composite film, it is effective to use a copper foil with a low contact resistance as the copper foil of the electromagnetic shielding layer, and focused on the surface shape of the copper foil. However, it was found that even when an EMI countermeasure film was produced using a copper foil with a clean and smooth surface (low surface roughness), which theoretically has the lowest contact resistance, the wiring resistance of the EMI countermeasure film did not become as low as predicted. On the other hand, experimental results were obtained that in some cases, the wiring resistance of the EMI countermeasure film was lower when using a copper foil with a relatively large surface roughness than when using a smooth copper foil with a small surface roughness, and the inventors of the present invention focused on this.

[0007] In the "Method for Measuring Surface Contact Electrical Resistance of JCBA T323:2011, Technical Standard of Japan Copper and Brass Association", it is defined that the contact resistance between the object to be measured and the gold (Au) probe is measured. When measuring the contact resistance of copper foil, since it becomes the contact resistance between metal rigid bodies, voids corresponding to the surface shapes of each other are generated at the contact interface, and thus the microscopic true contact area becomes smaller accordingly. Therefore, it is understood that when the surface roughness of the copper foil is large and the true contact area with the Au probe is small, the measured value of the contact resistance becomes large, and when the surface roughness of the copper foil is small and the true contact area with the Au probe is large, the measured value of the contact resistance tends to be low.

[0008] On the other hand, in the case of a composite film used for EMI countermeasure films, what contacts the copper foil is a conductive adhesive or a conductive bonding agent. When bonded to the copper foil, the conductive adhesive or the conductive bonding agent follows the surface shape of the copper foil. Therefore, basically, voids are unlikely to occur at the contact interface between the copper foil and the conductive adhesive or the conductive bonding agent.

[0009] From this, when the composite film is used as an EMI countermeasure film, it is considered that using a copper foil with a relatively large surface roughness rather than a copper foil with the smallest surface roughness, that is, a copper foil having a completely smooth surface, results in a smaller contact resistance of the copper foil. That is, even if the surface roughness of the surface of the copper foil increases, since the conductive layer follows the shape, voids are unlikely to occur at the contact interface between the copper foil and the conductive layer. As the surface roughness of the surface of the copper foil increases, the true contact area increases, so it is considered that the contact resistance of the copper foil becomes smaller.

[0010] When the inventors verified the above idea, it became clear that the EMI countermeasure film using a copper foil with a relatively large surface roughness has a lower wiring resistance than the EMI countermeasure film using a copper foil having a completely smooth surface. In addition, as a result of intensive research by the present inventors based on these findings, it has been revealed that in order to obtain an EMI countermeasure film with lower wiring resistance, the relationship between the number average particle diameter of the conductive filler contained in the conductive layer and the surface roughness of the copper foil is important. That is, the present inventors have found that not only does the conductive layer follow the surface shape of the copper foil, but the distribution of the conductive filler also follows the surface shape of the copper foil, so that it is necessary to increase the contact points between the conductive filler and the surface of the copper foil. For this purpose, it has been found that the relationship between the number average particle diameter of the conductive filler and the surface roughness of the copper foil is important.

[0011] Specifically, it has been found that by controlling the number average particle diameter of the conductive filler, the developed area ratio Sdr of the surface of the copper foil, the skewness Ssk of the surface of the copper foil, and the contact resistance of the copper foil within a predetermined range, the wiring resistance of the EMI countermeasure film can be significantly reduced compared to simply using a smooth copper foil with low contact resistance.

[0012] That is, a composite film according to one aspect of the present invention is a composite film having a copper foil, a conductive layer formed of a conductive adhesive containing a conductive filler or a conductive adhesive containing a conductive filler and laminated on at least one of the two surfaces of the copper foil. And, a composite film according to one aspect of the present invention is characterized in that the number average particle diameter of the conductive filler is 0.12 μm or more and 7 μm or less, the developed area ratio Sdr of the surface on which the conductive layer is laminated among the two surfaces of the copper foil is 0.01% or more and 40% or less, the skewness Ssk is -1.0 or more and 1.0 or less, and the contact resistance is 2 mΩ or more and 30 mΩ or less.

Effect of the Invention

[0013] The composite film according to the present invention has low connection electrical resistance.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0015] An embodiment of the present invention will be described. Note that the embodiments described below show examples of the present invention. Also, various changes or improvements can be made to the present embodiment, and forms with such changes or improvements can also be included in the present invention.

[0016] As shown in FIG. 1, the composite film 1 according to the present embodiment includes a copper foil 10 forming an electromagnetic wave shielding layer, and a conductive adhesive containing a conductive filler or a conductive adhesive containing a conductive filler, and the copper foil 10 is laminated on at least one of the two surfaces 10a of the copper foil 10. The composite film 1 in FIG. 1 is an example in which the conductive layer 20 is laminated only on one surface 10a of the two surfaces of the copper foil 10.

[0017] The number average particle diameter of the conductive filler contained in the conductive layer 20 is 0.12 μm or more and 7 μm or less. Also, the developed area ratio Sdr of the surface 10a on which the conductive layer 20 is laminated among the two surfaces of the copper foil 10 is 0.01% or more and 40% or less, and similarly, the skewness Ssk of the surface 10a is -1.0 or more and 1.0 or less. Further, the contact resistance of the surface 10a on which the conductive layer 20 is laminated among the two surfaces of the copper foil 10 is 2 mΩ or more and 30 mΩ or less.

[0018] Due to such a configuration, the composite film 1 according to the present embodiment has a low connection electrical resistance (wiring resistance). Therefore, since the composite film 1 according to the present embodiment has high shielding performance and excellent performance in suppressing electromagnetic interference (EMI resistance), it can be suitably used as an EMI countermeasure film.

[0019] Therefore, the composite film 1 according to this embodiment can be used as an EMI countermeasure film for various electronic devices and exhibits excellent EMI resistance. The composite film 1 according to this embodiment can be suitably used for electronic devices such as smartphones, medical devices, and automobiles, and can be particularly suitably used for mobile electronic devices such as smartphones.

[0020] In addition, the composite film 1 according to this embodiment may further include at least one of the release film 30, the insulating layer 40, and the carrier film 50. That is, a modified example of the composite film 1 according to this embodiment may have a structure in which a release film 30 is further laminated on the conductive layer 20 as shown in FIG. 2. Further, a modified example of the composite film 1 according to this embodiment may have a structure in which an insulating layer 40 for electrically protecting the copper foil 10 is further laminated on the surface 10b opposite to the surface 10a on which the conductive layer 20 is laminated among the two surfaces of the copper foil 10 as shown in FIG. 2. Furthermore, a modified example of the composite film 1 according to this embodiment may have a structure in which a carrier film 50 as a support is further laminated on the insulating layer 40 as shown in FIG. 2.

[0021] FIG. 2 shows an example of the composite film 1 including all of the release film 30, the insulating layer 40, and the carrier film 50, but the modified example of the composite film 1 according to this embodiment is not limited thereto. However, when the conductive layer 20 is laminated on both of the two surfaces 10a and 10b of the copper foil 10, the composite film 1 does not include the insulating layer 40 and the carrier film 50.

[0022] Hereinafter, the composite film 1 according to this embodiment will be described in more detail. (1) Regarding the copper foil As the copper foil 10 of the composite film 1 according to this embodiment, either an electrolytic copper foil or a rolled copper foil can be used, and a copper foil formed by a known method such as vapor deposition or sputtering can also be used. Further, a copper foil whose surface shape and thickness are adjusted by a known method such as etching or electrolytic polishing may be used as the copper foil 10 of the composite film 1 according to this embodiment.

[0023] As an example of the etching solution used for etching, CZ-8100 and CB-5602AY manufactured by Meck Co., Ltd. can be mentioned. The etching conditions such as the temperature of the etching solution, the etching time, and the stirring conditions of the etching solution can be appropriately adjusted. A copper foil with roughening plating can also be used, but since the developed surface area ratio Sdr of the surface tends to become excessively large, it is preferable to take measures such as reducing the current density and electrolysis time during roughening plating. When using a plating method, plating with copper (Cu) is preferable from the viewpoints of conductivity and contact resistance.

[0024] The thickness of the copper foil 10 of the composite film 1 according to this embodiment is preferably 2 μm or more and 40 μm or less. Further, for the purpose of preventing discoloration during storage and preventing discoloration due to the thermal history during the manufacture and use of the composite film, a copper foil subjected to rust prevention treatment after adjusting the surface shape and thickness may be used as the copper foil 10 of the composite film 1 according to this embodiment. Examples of the rust prevention treatment include plating treatment containing at least one metal among nickel (Ni), zinc (Zn), chromium (Cr), molybdenum (Mo), cobalt (Co), silicon (Si), and tungsten (W), and organic treatment using an organic compound such as benzotriazole.

[0025] The developed surface area ratio Sdr of the surface 10a on which the conductive layer 20 is laminated among the two surfaces 10a and 10b of the copper foil 10 of the composite film 1 according to this embodiment needs to be 0.01% or more and 40% or less. However, in order to make the connection electrical resistance of the composite film 1 lower, it is preferably 0.01% or more and 10% or less, and more preferably 0.05% or more and 10% or less.

[0026] Also, the skew Ssk of the surface 10a on which the conductive layer 20 is laminated among the two surfaces 10a and 10b of the copper foil 10 needs to be -1.0 or more and 1.0 or less. However, in order to make the connection electrical resistance of the composite film 1 lower, it is preferably -0.7 or more and 1.0 or less, and more preferably -0.5 or more and 0.8 or less.

[0027] Furthermore, the contact resistance of the surface 10a on which the conductive layer 20 is laminated among the two surfaces 10a and 10b of the copper foil 10 needs to be 2 mΩ or more and 30 mΩ or less. However, in order to make the connection electrical resistance of the composite film 1 lower, it is preferably 2 mΩ or more and 25 mΩ or less, and more preferably 2 mΩ or more and 20 mΩ or less. Furthermore, in order to make the connection electrical resistance of the composite film 1 lower, the kurtosis Sku of the surface 10a on which the conductive layer 20 is laminated among the two surfaces 10a and 10b of the copper foil 10 is preferably 3.0 or more and 3.6 or less.

[0028] (2) Regarding the conductive layer The conductive layer 20 of the composite film 1 according to the present embodiment is composed of a conductive adhesive containing a conductive filler or a conductive adhesive containing a conductive filler. The conductive adhesive and the conductive adhesive contain a conductive filler that imparts conductivity to the conductive layer 20 and a resin.

[0029] The conductive adhesive may have adhesiveness at normal temperature. The resin contained in the conductive adhesive may be a thermosetting resin or a thermoplastic resin. The conductive adhesive containing a thermosetting resin may be in any state of uncured, B-stage, or cured in the composite film 1.

[0030] Examples of resins contained in the conductive adhesive and the conductive adhesive include epoxy, phenolic, amino, alkyd, urethane, synthetic rubber, acrylate, acrylic, silicone, imide, isocyanate, vinyl chloride, vinyl acetate, styrene, and hydrocarbon resins and adhesives. Among these resins, epoxy resin is preferred because of its excellent heat resistance.

[0031] Examples of the conductive filler include metal particles and carbon particles. Examples of the metal type include silver (Ag), platinum (Pt), gold, copper, nickel, palladium (Pd), aluminum (Al), solder, or alloys thereof. Examples of the carbon particles include graphite particles, fired carbon particles, and plated fired carbon particles. Among these particles, copper particles and graphite particles are preferred from the viewpoints of moldability and industrial productivity when producing the conductive layer 20.

[0032] The number average particle diameter of the conductive filler needs to be 0.12 μm or more and 7 μm or less. If the number average particle diameter of the conductive filler is 0.12 μm or more, the number of contact points between the conductive filler and the copper foil 10 and the surface area of the conductive filler can be increased, so that the conductivity of the conductive layer 20 becomes excellent. On the other hand, if the number average particle diameter of the conductive filler is 7 μm or less, the conductive adhesive and the conductive adhesive can flow with respect to the shape and step of the ground (GND) opening of the printed wiring board or the metal housing, and the conductive layer 20 can easily follow, so that the target location can be sufficiently filled with the conductive layer 20.

[0033] Furthermore, in the cross-section of the conductive layer 20 that appears when the conductive layer 20 is cut in a plane perpendicular to the surface of the copper foil 10, the ratio of the cross-sectional area of the conductive filler to the cross-sectional area of the conductive layer 20 (hereinafter, may also be referred to as "cross-sectional area ratio of the conductive filler") is preferably 30% or more and 85% or less, and more preferably 45% or more and 80% or less.

[0034] If the cross-sectional area ratio of the conductive filler is 30% or more, the number of contact points between the conductive filler and the copper foil 10 and the surface area of the conductive filler can be increased, so that the conductivity of the conductive layer 20 becomes excellent. On the other hand, if the cross-sectional area ratio of the conductive filler is 85% or less, the flexibility of the composite film 1 becomes excellent.

[0035] The thickness of the conductive layer 20 is preferably 5 μm or more and 50 μm or less. If the thickness of the conductive layer 20 is 5 μm or more, the conductive layer 20 can easily follow the shape and step of the ground (GND) opening of the printed wiring board or the metal housing of the electronic device, so that the target location can be sufficiently filled with the conductive layer 20. On the other hand, if the thickness of the conductive layer 20 is 50 μm or less, the flexibility of the composite film 1 becomes excellent, so that the composite film 1 can be mounted in a thinner space.

[0036] Note that the conductive layer 20 may have isotropic conductivity or anisotropic conductivity. Also, the conductive filler may be particulate, but may also be needle-shaped or fibrous. When the conductive filler is needle-shaped or fibrous, the number average particle diameter of the conductive filler is the number average diameter calculated based on the diameter of the needle-shaped or fibrous conductive filler.

[0037] Furthermore, in order to impart flexibility to the conductive layer 20, additives such as a rubber component (carboxy-modified nitrile rubber, acrylic rubber, etc.), a tackifier, and a curing agent (isocyanate compound, etc.) may be added to the conductive adhesive or the conductive adhesive. Also, in order to improve the flame retardancy, moldability, and mechanical properties of the conductive layer 20, additives that improve those properties may be added to the conductive adhesive or the conductive adhesive.

[0038] (3) Regarding the release film The release film 30 is obtained by treating the surface of a base material with a release agent and is for protecting the conductive layer 20. That is, by covering the surface of the two surfaces of the conductive layer 20 that is opposite to the surface facing the copper foil 10 with the release film 30, a decrease in the adhesiveness and stickiness of the conductive layer 20 is prevented. Note that the release film 30 is laminated on the conductive layer 20 such that the surface treated with the release agent of the release film 30 faces the conductive layer 20.

[0039] Examples of the material of the base material of the release film 30 include resin and paper. Examples of the type of resin include polyethylene isophthalate, polybutylene terephthalate, polyethylene naphthalate, polyolefin, polyacetate, polycarbonate, polyphenylene sulfide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, polystyrene, polyvinylidene chloride, synthetic rubber, liquid crystal polymer, and the like. Further, an example of the release film 30 using paper as the base material is release paper.

[0040] (4) Regarding the insulating layer The insulating layer 40 is for protecting the copper foil 10 when the composite film 1 is mounted on an electronic device or the like. That is, by covering the copper foil 10 with the insulating layer 40, electrical contact with the copper foil 10 is prevented.

[0041] The insulating layer 40 can be formed, for example, by applying a paint containing a thermosetting resin and a curing agent to the surface 10b of the two surfaces of the copper foil 10 that is opposite to the surface 10a on which the conductive layer 20 is laminated, and semi-curing or curing. Further, the insulating layer 40 can also be formed, for example, by applying a paint containing a thermoplastic resin to the surface 10b of the two surfaces of the copper foil 10 that is opposite to the surface 10a on which the conductive layer 20 is laminated. Furthermore, the insulating layer 40 can also be formed, for example, by melt-molding a resin composition containing a thermoplastic resin in a film shape on the surface 10b of the two surfaces of the copper foil 10 that is opposite to the surface 10a on which the conductive layer 20 is laminated. In some cases, the composite film 1 may be mounted on an electronic device or the like by soldering or the like. Therefore, the insulating layer 40 preferably has heat resistance. From this point of view, the insulating layer 40 is preferably formed using a paint containing a thermosetting resin and a curing agent.

[0042] Examples of the thermosetting resin used for forming the insulating layer 40 include amide resins, epoxy resins, phenol resins, amino resins, alkyd resins, urethane resins, synthetic rubbers, ultraviolet curable acrylate resins, and the like. Among these thermosetting resins, amide resins and epoxy resins are preferred in terms of excellent heat resistance.

[0043] Examples of the thermoplastic resin used for forming the insulating layer 40 include aromatic polyether ketone, polyimide, polyamideimide, polyamide, polysulfone, polyethersulfone, polyphenylene sulfone, polyphenylene sulfide, polyphenylene sulfide sulfone, polyphenylene sulfide ketone, and the like. Known fillers may be blended in the insulating layer 40 for the purpose of imparting weather resistance, concealment, flame retardancy, designability, etc. or for coloring.

[0044] (5) Regarding the carrier film The carrier film 50 is a support for reinforcing and protecting the composite film 1. Further, the carrier film 50 is laminated on the surface of the insulating layer 40 opposite to the surface facing the copper foil 10 among the two surfaces of the insulating layer 40. Examples of the structure of the carrier film 50 include a laminated structure of a base material and an adhesive layer. As the base material of the carrier film 50, the same materials as those exemplified as the base material of the release film 30 can be used. Examples of the material for forming the adhesive layer include acrylic adhesives, urethane adhesives, rubber adhesives, and the like.

[0045] 〔Examples〕 Examples and comparative examples are shown below to more specifically explain the present invention. (A) Copper foil As the copper foils, which are raw materials for manufacturing the composite films of Examples 1 to 16 and Comparative Examples 1 to 5, copper foils with a thickness of 6 μm having the surface roughness (developed surface area ratio Sdr, skewness Ssk, kurtosis Sku) shown in Table 1 were prepared. These copper foils are commercially available electrolytic copper foils or copper foils obtained by etching the commercially available electrolytic copper foils to adjust the surface roughness. CB-5601AY manufactured by Meck Co., Ltd. was used as the etching solution for the etching treatment, and the surface roughness was adjusted by adjusting the temperature of the etching solution, the etching time, the stirring conditions of the etching solution, etc.

[0046] The surface roughness of the copper foils was measured using a confocal laser microscope VK-X1050 and VK-X1000 manufactured by Keyence Corporation. The numerical values shown in Table 1 are the average values of the measured values at any three points for both the developed surface area ratio Sdr, skewness Ssk, and kurtosis Sku of the surface. Note that the objective lens magnification of the confocal laser microscope is 100 times, the scan mode is laser confocal, the measurement size is 2048×1536, the measurement quality is High Precision, and the pitch is 0.08 μm.

[0047] Also, the developed surface area ratio Sdr, skewness Ssk, and kurtosis Sku of the surface were calculated by performing the following reference plane correction, smoothing image processing, and filter processing on the measurement data using the attached analysis software, and then performing calculations in an arbitrary field of view of 100 μm×100 μm. Reference plane correction: entire surface Smoothing: 3×3, Gaussian Filter processing: L filter 0.025 mm

[0048] Furthermore, for the surface of these copper foils on which the conductive layer is laminated among the two surfaces, before laminating the conductive layer, the contact resistance was measured by the method defined in "Measurement Method of Surface Contact Electrical Resistance of JCBA T323:2011, Technical Standards of Japan Copper Foil Association". For the measurement of the contact resistance, an electrical contact simulator CRS-113-AU manufactured by Yamazaki Precision Machinery Laboratory Co., Ltd. was used, and an Au probe with a diameter of 1 mm was used as the probe. The measurement conditions were a contact load of 0.1 N, an AC frequency of 287 Hz, and a measurement current of 1 mA. The results are shown in Table 1. Note that the numerical values shown in Table 1 are the average values of the measured values at arbitrary 10 points.

[0049]

Table 1

[0050] (B) Conductive layer A conductive layer was laminated on one of the two surfaces of the copper foil to produce the composite films of Examples 1 to 16 and Comparative Examples 1 to 5. This conductive layer was formed by coating a conductive adhesive containing an acrylic resin and a conductive filler on the copper foil in a film shape and then heating at 80 °C for 3 minutes to volatilize the solvent.

[0051] The acrylic resin is an acrylic adhesive Fine Tack CT-5030 manufactured by DIC Corporation, and the conductive filler is copper powder. The number average particle diameter of the copper powder is as shown in Table 1. The content of the conductive filler in the conductive adhesive was adjusted so that the cross-sectional area ratio of the conductive filler in the conductive layer would be the numerical values described in Table 1. Also, the thickness of the film-shaped conductive adhesive coated on the copper foil was adjusted so that the thickness of the conductive layer of the composite film would be 30 μm.

[0052] (C) Composite film For the produced composite films of Examples 1 to 16 and Comparative Examples 1 to 5, the number average particle diameter and the cross-sectional area ratio of the conductive filler were calculated. The results are shown in Table 1. A method for calculating the cross-sectional area ratio of the conductive filler will be described below. First, an arbitrary square region in the cross-section of the conductive layer was observed with a scanning electron microscope, and image data of the above-mentioned square region was obtained. The cross-section of the conductive layer is the cross-section of the conductive layer that appears when the conductive layer is cut with a plane orthogonal to the surface of the copper foil (that is, a plane parallel to the thickness direction of the conductive layer). Also, one side of the square region to be observed was set to 25 μm.

[0053] Next, the image data was processed by image editing software to create a binary image divided into the particle part of the conductive filler and the other part. Then, from the obtained binary image, the area of the cross-section of the conductive layer (that is, the area of the above-mentioned square region) and the area of the cross-section of the conductive filler were obtained, and the cross-sectional area ratio of the conductive filler was calculated by dividing the area of the cross-section of the conductive filler by the area of the above-mentioned square region. The above procedure was performed at three arbitrary locations, and the average value of these cross-sectional area ratios was calculated. The results are shown in Table 1.

[0054] A method for calculating the number average particle diameter of the conductive filler will be described below. For the particles of the conductive filler existing in the above-mentioned square region, the minimum diameter and the maximum diameter were measured, and the average value of the minimum diameter and the maximum diameter was taken as the particle diameter of the particle. Then, the particle diameters were measured for all the particles of the conductive filler existing in the above-mentioned square region, and the arithmetic average value was taken as the number average particle diameter of the conductive filler. The results are shown in Table 1.

[0055] For the composite films of Examples 1 to 16 and Comparative Examples 1 to 5, the wiring resistance was measured. A method for measuring the wiring resistance will be described below. A printed wiring board was prepared in which two sample attachment electrodes made of copper were exposed side by side. These sample attachment electrodes were square with a side length of 4 mm, and the center-to-center distance between the two sample attachment electrodes was set to 12 mm.

[0056] Resistance measurement electrodes were exposed near each of the two electrodes for attaching samples. Specifically, resistance measurement electrodes were formed on the side of the vicinity of the electrode for attaching samples that is different from the side where the adjacent electrode for attaching samples is located. The electrode for attaching samples and the resistance measurement electrode formed in its vicinity are electrically connected through the inner layer circuit of the printed wiring board and form a pair, but the two electrodes for attaching samples are not electrically connected to each other. Note that both the two electrodes for attaching samples and the two resistance measurement electrodes are subjected to electroless nickel plating and gold plating, which are common in printed wiring board applications, to minimize the effects of oxidation on the electrode surface.

[0057] First, for the two pairs of electrodes for attaching samples and resistance measurement electrodes, with the composite film not attached to the electrode for attaching samples, the electrical resistance between the electrode for attaching samples and the resistance measurement electrode was measured by the four-terminal method, respectively. Then, the sum of the two obtained electrical resistances was recorded as the measured substrate resistance.

[0058] Next, the composite film of the example or comparative example was attached so as to span between the two electrodes for attaching samples, and after being crimped using a crimping roller APR-97 for peel test manufactured by IMADA Co., Ltd., it was left standing for 30 minutes. Then, the electrical resistance between the two resistance measurement electrodes was measured by the four-terminal method, and the value obtained by subtracting the measured substrate resistance from this measured value was taken as the wiring resistance of the composite film. The measurement of the wiring resistance was performed 5 times, and the average value was taken as the wiring resistance of each of the composite films of Examples 1 to 16 and Comparative Examples 1 to 5. All of the above electrical resistance measurements were performed in an environment of a temperature of 25°C and a humidity of 40 to 50%RH.

[0059] The measurement results of the wiring resistance are shown in Table 1. Note that the measurement results of the wiring resistance shown in Table 1 are the results evaluated based on the following evaluation criteria. AA (Pass): Wiring resistance is less than 10 mΩ A (Pass): Wiring resistance is 10 mΩ or more and less than 30 mΩ B (Pass): Wiring resistance is 30 mΩ or more and less than 100 mΩ C (Fail): Wiring resistance is 100 mΩ or more and less than 500 mΩ D (Nonconforming): Wiring resistance is 500 mΩ or more.

[0060] As can be seen from Table 1, the composite films of Examples 1 to 16 had lower wiring resistance than the composite films of Comparative Examples 1 to 5. Therefore, it can be said that the composite films of Examples 1 to 16 are excellent in the performance of suppressing electromagnetic interference (EMI resistance) compared to the composite films of Comparative Examples 1 to 5. Therefore, the composite films of Examples 1 to 16 can be suitably used as EMI countermeasure films.

Explanation of Signs

[0061] 1 ··· Composite film 10 ··· Copper foil 10a ··· Surface on which the conductive layer is laminated 10b ··· Surface opposite to the surface on which the conductive layer is laminated 20 ··· Conductive layer 30 ··· Release film 40 ··· Insulating layer 50 ··· Carrier film

Claims

1. A composite film comprising a copper foil, and a conductive layer formed of a conductive adhesive containing a conductive filler or a conductive adhesive containing a conductive filler and laminated on at least one of two surfaces of the copper foil, wherein the thickness of the copper foil is 2 μm or more and 40 μm or less, the thickness of the conductive layer is 5 μm or more and 50 μm or less, the number average particle diameter of the conductive filler is 0.12 μm or more and 7 μm or less, when the conductive layer is cut in a plane perpendicular to the surface of the copper foil, in the cross-section of the conductive layer thus revealed, the ratio of the cross-sectional area of the conductive filler to the cross-sectional area of the conductive layer is 30% or more and 85% or less, of the two surfaces of the copper foil, the developed area ratio Sdr of the surface on which the conductive layer is laminated is 0.01% or more and 6.6% or less, the skewness Ssk is -1.0 or more and 1.0 or less, the kurtosis Sku is 3.0 or more and 3.6 or less, and the contact resistance is 2 mΩ or more and 30 mΩ or less, wherein the contact resistance is measured under the measurement conditions of a contact load of 0.1 N, an AC frequency of 287 Hz, and a measurement current of 1 mA by the method defined in the "Method for Measuring Surface Contact Electrical Resistance of Technical Standard JCBA T323:2011 of the Japan Copper Foil Association".

2. The composite film according to claim 1, wherein a release film is further laminated on the conductive layer.

3. The composite film according to claim 1, wherein an insulating layer for electrically protecting the copper foil is further laminated on the surface of the copper foil opposite to the surface on which the conductive layer is laminated.

4. The composite film according to claim 3, wherein a carrier film as a support is further laminated on the insulating layer.

5. The composite film according to claim 1, which is used as an EMI countermeasure film.

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