JP7704710B2 - Composite Film - Google Patents

Description

The present invention relates to a composite film.

In recent years, electronic devices such as smartphones, medical devices, and automobiles have become highly integrated and perform better, and at the same time, the importance of EMI (Electromagnetic Interference) countermeasures to suppress the electromagnetic interference of electronic devices has increased. Therefore, in order to prevent problems such as malfunction of the electronic devices and deterioration of communication data quality due to electromagnetic waves from inside and outside the housing of the electronic devices, components such as noise filters and electromagnetic wave absorbing sheets are used in the electronic devices.
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 a metal or the like and a conductive layer made of a conductive adhesive or conductive adhesive are laminated. As the electromagnetic wave shielding layer, copper foil is sometimes used from the viewpoints of high conductivity, shielding properties, and industrial productivity.

EMI control films are widely used as shielding films in flexible printed circuits (FPCs) and chip-on-film (COF) technology, or as conductive tapes for strengthening grounding within the housings of electronic devices.
When an EMI countermeasure film is attached to an electronic device, the electromagnetic wave shielding layer is usually attached to a metal part in the electronic device via a conductive layer. When the electromagnetic wave shielding layer of the EMI countermeasure film is attached to the metal part in the electronic device, the lower the connection electric resistance from the electromagnetic wave shielding layer to the metal part in the electronic device, the better the performance of suppressing electromagnetic interference (hereinafter, the performance of the EMI countermeasure film to suppress electromagnetic interference may be referred to as "EMI resistance"). This EMI resistance can be evaluated by the connection electric resistance, but it may also be evaluated in a simulated manner by wiring resistance.

Patent No. 6872449

As described above, since EMI countermeasures have become increasingly important in recent years, there is a demand for composite films with lower connection electrical resistance.
An object of the present invention is to provide a composite film having low connection electrical resistance.

In order to solve the above problems, the inventors considered that using copper foil with low contact resistance as the copper foil that is the electromagnetic wave shielding layer would be effective in reducing the connection electrical resistance (wiring resistance) of the composite film, and focused on the surface shape of the copper foil. However, it was found that even if an EMI control film is produced using copper foil with a clean and smooth surface (low surface roughness), which in principle has the lowest contact resistance, the wiring resistance of the EMI control film is not as low as expected. On the other hand, experimental results were obtained showing that in some cases the wiring resistance of the EMI control film is lower when copper foil with a relatively large surface roughness is used rather than smooth copper foil with a small surface roughness, and the inventors focused on this point.

The "Japan Copper and Brass Association Technical Standard JCBA T323:2011: Method for Measuring Surface Contact Electrical Resistance" stipulates that the contact resistance between the object to be measured and a gold (Au) probe is to be measured. When measuring the contact resistance of copper foil, the contact resistance is between two rigid metal bodies, so naturally, gaps occur at the contact interface according to the surface shapes of the two bodies, and the microscopic real contact area is accordingly smaller. Therefore, it is understood that if the copper foil has a large surface roughness and a small real contact area with the Au probe, the measured contact resistance will tend to be large, and if the copper foil has a small surface roughness and a large real contact area with the Au probe, the measured contact resistance will tend to be low.

On the other hand, in the case of composite films used as EMI protection films, it is the conductive adhesive or conductive glue that comes into contact with the copper foil, and when the film is attached to the copper foil, the conductive adhesive or conductive glue conforms to the surface shape of the copper foil, so that gaps are generally unlikely to form at the contact interface between the copper foil and the conductive adhesive or conductive glue.

For this reason, when a composite film is used as an EMI control film, it is believed that the contact resistance of the copper foil will be smaller if a copper foil with a relatively large surface roughness is used rather than a copper foil with the smallest surface roughness, i.e., a copper foil with a completely smooth surface. In other words, even if the surface roughness of the copper foil surface becomes large, the conductive layer follows the shape of the copper foil, so gaps are unlikely to occur at the contact interface between the copper foil and the conductive layer, and the true contact area becomes larger due to the larger surface roughness of the copper foil surface, so the contact resistance of the copper foil is believed to be smaller.

When the inventors verified the above idea, it became clear that an EMI protection film using copper foil with a relatively large surface roughness has a lower wiring resistance than an EMI protection film using copper foil with a completely smooth surface.
Based on these findings, the present inventors have conducted intensive research and have found that in order to obtain an EMI countermeasure film with lower wiring resistance, the relationship between the number average particle size 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 it is necessary to increase the contact points between the conductive filler and the surface of the copper foil by not only making the conductive layer conform to the surface shape of the copper foil, but also making the distribution of the conductive filler conform to the surface shape of the copper foil, and that for this purpose, the relationship between the number average particle size of the conductive filler and the surface roughness of the copper foil is important.

Specifically, it was found that by controlling the number average particle size of the conductive filler, the developed area ratio Sdr of the copper foil surface, the skewness Ssk of the copper foil surface, and the contact resistance of the copper foil within a specified range, the wiring resistance of the EMI control film can be significantly reduced compared to simply using a copper foil that is smooth and has low contact resistance.

That is, the composite film according to one embodiment of the present invention is a composite film having a copper foil and a conductive layer made of a conductive pressure-sensitive 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. The composite film according to one embodiment of the present invention has a number average particle diameter of the conductive filler of 0.12 μm or more and 7 μm or less, a developed area ratio Sdr of the surface on which the conductive layer is laminated out of the two surfaces of the copper foil of 0.01% or more and 40% or less, a skewness Ssk of -1.0 or more and 1.0 or less, and a contact resistance of 2 mΩ or more and 30 mΩ or less.

The composite film of the present invention has low connection electrical resistance.

FIG. 1 is a cross-sectional view illustrating a composite film according to one embodiment of the present invention. FIG. 2 is a cross-sectional view illustrating a modified example of the composite film of FIG. 1 .

One embodiment of the present invention will be described. Note that the embodiment described below is merely one example of the present invention. In addition, various modifications and improvements can be made to this embodiment, and such modifications and improvements can also be included in the present invention.

As shown in FIG. 1, the composite film 1 according to this embodiment has a copper foil 10 forming an electromagnetic wave shielding layer, and a conductive layer 20 made of a conductive adhesive containing a conductive filler or a conductive adhesive containing a conductive filler and laminated on at least one surface 10a of the two surfaces 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.

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. Furthermore, the developed area ratio Sdr of the surface 10a on which the conductive layer 20 is laminated, of the two surfaces of the copper foil 10, is 0.01% or more and 40% or less, and the skewness Ssk of the surface 10a is -1.0 or more and 1.0 or less. Furthermore, the contact resistance of the surface 10a on which the conductive layer 20 is laminated, of the two surfaces of the copper foil 10, is 2 mΩ or more and 30 mΩ or less.

Due to this configuration, the composite film 1 according to this embodiment has low connection electrical resistance (wiring resistance). Therefore, the composite film 1 according to this embodiment has high shielding properties and excellent performance in suppressing electromagnetic interference (EMI resistance), and can be suitably used as an EMI countermeasure film.

Therefore, the composite film 1 according to this embodiment can be used as an EMI protection 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 is particularly suitable for use in mobile electronic devices such as smartphones.

The composite film 1 according to the present 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 the present embodiment may have a structure in which the release film 30 is further laminated on the conductive layer 20, as shown in FIG. 2. In addition, a modified example of the composite film 1 according to the present embodiment may have a structure in which an insulating layer 40 that electrically protects the copper foil 10 is further laminated on the surface 10b opposite to the surface 10a on which the conductive layer 20 is laminated, as shown in FIG. 2. Furthermore, a modified example of the composite film 1 according to the present embodiment may have a structure in which the carrier film 50, which is a support, is further laminated on the insulating layer 40, as shown in FIG. 2.

Figure 2 shows an example of a composite film 1 that includes a release film 30, an insulating layer 40, and a carrier film 50, but the modified example of the composite film 1 according to this embodiment is not limited to this. However, if a conductive layer 20 is laminated on both of the two surfaces 10a, 10b of the copper foil 10, the composite film 1 does not include an insulating layer 40 and a carrier film 50.

The composite film 1 according to this embodiment will be described in further detail below.
(1) 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. Furthermore, a copper foil whose surface shape and thickness have been 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.

Examples of the etching solution used for etching include CZ-8100 and CB-5602AY manufactured by MEC Co., Ltd. Etching conditions such as the temperature of the etching solution, etching time, and stirring conditions of the etching solution can be appropriately adjusted.
Copper foil subjected to roughening plating can also be used, but since the surface development area ratio Sdr tends to become excessively large, it is preferable to take measures such as reducing the current density and electrolysis time during roughening plating. When a plating method is used, plating with copper (Cu) is preferable from the viewpoint of electrical conductivity and contact resistance.

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.
Furthermore, for the purpose of preventing discoloration during storage or discoloration due to thermal history during the manufacture and use of the composite film, a copper foil that has been 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 rust prevention treatments include plating treatments containing at least one metal selected from nickel (Ni), zinc (Zn), chromium (Cr), molybdenum (Mo), cobalt (Co), silicon (Si), and tungsten (W), and organic treatments using an organic compound such as benzotriazole.

Of the two surfaces 10a, 10b of the copper foil 10 of the composite film 1 in this embodiment, the developed area ratio Sdr of the surface 10a on which the conductive layer 20 is laminated must be 0.01% or more and 40% or less. In order to further reduce the connection electrical resistance of the composite film 1, however, it is preferably 0.01% or more and 10% or less, and more preferably 0.05% or more and 10% or less.

In addition, the skewness Ssk of the surface 10a, on which the conductive layer 20 is laminated, of the two surfaces 10a, 10b of the copper foil 10, must be -1.0 or more and 1.0 or less. In order to further reduce the connection electrical resistance of the composite film 1, it is preferably -0.7 or more and 1.0 or less, and more preferably -0.5 or more and 0.8 or less.

Furthermore, the contact resistance of the surface 10a, on which the conductive layer 20 is laminated, of the two surfaces 10a, 10b of the copper foil 10 must be 2 mΩ or more and 30 mΩ or less. In order to further reduce the connection electrical resistance of the composite film 1, however, 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 lower the connection electrical resistance of the composite film 1, it is preferable that the kurtosis Sku of the surface 10a, on which the conductive layer 20 is laminated, of the two surfaces 10a, 10b of the copper foil 10 is 3.0 or more and 3.6 or less.

(2) Conductive Layer The conductive layer 20 of the composite film 1 according to this embodiment is made of a conductive pressure-sensitive adhesive containing a conductive filler or a conductive adhesive containing a conductive filler. The conductive pressure-sensitive adhesive and the conductive adhesive contain a conductive filler and a resin that impart conductivity to the conductive layer 20.

The conductive adhesive may have adhesiveness at room 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 of the following states in the composite film 1: uncured, B-stage, or cured.

Examples of resins contained in conductive adhesives and conductive adhesives include epoxy-based, phenol-based, amino-based, alkyd-based, urethane-based, synthetic rubber-based, acrylate-based, acrylic-based, silicone-based, imide-based, isocyanate-based, vinyl chloride-based, vinyl acetate-based, styrene-based, and hydrocarbon-based resins and adhesives. Among these resins, epoxy resins are preferred because of their excellent heat resistance.

Examples of conductive fillers include metal particles and carbon particles. Examples of metal types include silver (Ag), platinum (Pt), gold, copper, nickel, palladium (Pd), aluminum (Al), solder, and alloys thereof. Examples of carbon particles include graphite particles, baked carbon particles, and plated baked 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.

The number average particle diameter of the conductive filler must 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, resulting in excellent conductivity of the conductive layer 20. On the other hand, if the number average particle diameter of the conductive filler is 7 μm or less, the conductive adhesive or conductive adhesive flows and the conductive layer 20 easily follows the shape and steps of the ground (GND) opening of the printed wiring board or metal housing, so that the target area can be sufficiently filled with the conductive layer 20.

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 sometimes referred to as the "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.

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, resulting in excellent conductivity of the conductive layer 20. 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 will be excellent.

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 conform to the shape and steps of the ground (GND) opening of the printed wiring board or the metal housing of an electronic device, so that the target area 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 is excellent, so that the composite film 1 can be mounted in a thinner space.

The conductive layer 20 may have isotropic conductivity or anisotropic conductivity.
The conductive filler may be particulate, needle-like, or fibrous. When the conductive filler is needle-like or fibrous, the number average particle size of the conductive filler is the number average diameter calculated based on the diameter of the needle-like or fibrous conductive filler.

Furthermore, in order to impart flexibility to the conductive layer 20, additives such as rubber components (carboxy-modified nitrile rubber, acrylic rubber, etc.), tackifiers, and curing agents (isocyanate compounds, etc.) may be added to the conductive adhesive or conductive pressure-sensitive adhesive. Also, in order to improve the flame retardancy, moldability, and mechanical properties of the conductive layer 20, additives that enhance these properties may be added to the conductive adhesive or conductive pressure-sensitive adhesive.

(3) Regarding the release film The release film 30 is a substrate surface treated with a release agent, and serves to protect the conductive layer 20. That is, by covering the surface opposite to the surface facing the copper foil 10, of the two surfaces of the conductive layer 20, with the release film 30, a decrease in the tackiness and adhesiveness of the conductive layer 20 is prevented. The release film 30 is laminated on the conductive layer 20 so that the surface of the release film 30 treated with the release agent faces the conductive layer 20.

Examples of the material of the base material of the release film 30 include resin and paper. Examples of types 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, etc. An example of a release film 30 using paper as the base material is release paper.

(4) Regarding the Insulating Layer The insulating layer 40 is intended to protect the copper foil 10 when the composite film 1 is mounted on an electronic device, etc. That is, by covering the copper foil 10 with the insulating layer 40, electrical contact with the copper foil 10 is prevented.

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 opposite the surface 10a on which the conductive layer 20 is laminated, among the two surfaces of the copper foil 10, and semi-curing or curing the paint. The insulating layer 40 can also be formed, for example, by applying a paint containing a thermoplastic resin to the surface 10b opposite the surface 10a on which the conductive layer 20 is laminated, among the two surfaces of the copper foil 10. Furthermore, the insulating layer 40 can also be formed, for example, by melt-molding a resin composition containing a thermoplastic resin into a film on the surface 10b opposite the surface 10a on which the conductive layer 20 is laminated, among the two surfaces of the copper foil 10.
Since the composite film 1 may be mounted on electronic devices, etc., by soldering or the like, it is preferable that the insulating layer 40 has heat resistance. From this point of view, it is preferable that the insulating layer 40 is formed using a paint containing a thermosetting resin and a curing agent.

Examples of thermosetting resins used to form the insulating layer 40 include amide resins, epoxy resins, phenolic resins, amino resins, alkyd resins, urethane resins, synthetic rubbers, and ultraviolet-cured acrylate resins. Among these thermosetting resins, amide resins and epoxy resins are preferred because of their excellent heat resistance.

Examples of thermoplastic resins used to form the insulating layer 40 include aromatic polyether ketone, polyimide, polyamide imide, polyamide, polysulfone, polyether sulfone, polyphenylene sulfone, polyphenylene sulfide, polyphenylene sulfide sulfone, polyphenylene sulfide ketone, and the like.
The insulating layer 40 may contain a known filler for the purpose of imparting weather resistance, concealment properties, flame retardancy, design properties, and coloring.

(5) Carrier Film The carrier film 50 is a support that reinforces and protects the composite film 1. The carrier film 50 is laminated on one of the two surfaces of the insulating layer 40 opposite to the surface facing the copper foil 10. An example of the structure of the carrier film 50 is a laminated structure of a substrate and an adhesive layer. The substrate of the carrier film 50 may be the same as the substrate of the release film 30. Examples of materials that form the adhesive layer include an acrylic adhesive, a urethane adhesive, and a rubber adhesive.

[Example]
The present invention will be described in more detail below with reference to examples and comparative examples.
(A) Copper foil As the raw material for producing the composite films of Examples 1 to 16 and Comparative Examples 1 to 5, copper foils having a thickness of 6 μm and having the surface roughness (surface developed area ratio Sdr, skewness Ssk, kurtosis Sku) shown in Table 1 were prepared. These copper foils were commercially available electrolytic copper foils, or copper foils in which the surface roughness of the commercially available electrolytic copper foils was adjusted by etching. CB-5601AY manufactured by MEC 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.

The surface roughness of the copper foil was measured using confocal laser microscopes VK-X1050 and VK-X1000 manufactured by Keyence Corp. The values shown in Table 1 are the average values of the measurements taken at any three points for the surface developed area ratio Sdr, skewness Ssk, and kurtosis Sku.
The objective lens magnification of the confocal laser microscope was 100 times, the scan mode was laser confocal, the measurement size was 2048×1536, the measurement quality was high precision, and the pitch was 0.08 μm.

In addition, the surface developed area ratio Sdr, skewness Ssk, and kurtosis Sku were calculated by performing the reference plane correction, smoothing image processing, and filter processing described below on the measurement data using the attached analysis software, and then performing calculations in an arbitrary field of view of 100 μm x 100 μm.
Reference surface correction: Full surface Smoothing: 3x3, Gaussian Filter processing: L filter 0.025mm

Furthermore, the contact resistance of the surface on which the conductive layer was to be laminated, out of the two surfaces of these copper foils, was measured before the conductive layer was laminated, using the method stipulated in the "Method for measuring surface contact electrical resistance, JCBA T323:2011, technical standard of the Japan Copper and Brass Association." The contact resistance was measured using an electric contact simulator CRS-113-AU manufactured by Yamazaki Seiki Kenkyusho Co., Ltd., and an Au probe with a diameter of 1 mm. 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. The values shown in Table 1 are the average values of measurements taken at 10 random points.

(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 onto the copper foil in the form of a film, and then heating at 80° C. for 3 minutes to volatilize the solvent.

The acrylic resin is an acrylic adhesive Finetac 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 was the value shown in Table 1. In addition, the thickness of the film-like conductive adhesive coated on the copper foil was adjusted so that the thickness of the conductive layer of the composite film was 30 μm.

(C) Composite Film The number average particle size and cross-sectional area ratio of the conductive filler were calculated for the composite films produced in Examples 1 to 16 and Comparative Examples 1 to 5. The results are shown in Table 1.
The method for calculating the cross-sectional area ratio of the conductive filler is described below. First, an arbitrary square-shaped region of the cross section of the conductive layer was observed with a scanning electron microscope to obtain image data of the square-shaped region. The cross section of the conductive layer is the cross section of the conductive layer that appears when the conductive layer is cut along a plane perpendicular to the surface of the copper foil (i.e., a plane parallel to the thickness direction of the conductive layer). In addition, one side of the square-shaped region to be observed was 25 μm.

Next, the image data was processed using image editing software to create a binary image that was divided into the conductive filler particle portion and the other portion. From the resulting binary image, the cross-sectional area of the conductive layer (i.e., the area of the square region described above) and the cross-sectional area of the conductive filler were obtained, and the cross-sectional area ratio of the conductive filler was calculated by dividing the cross-sectional area of the conductive filler by the area of the square region described above. The above procedure was performed for three arbitrary locations, and the average cross-sectional area ratio was calculated. The results are shown in Table 1.

The method for calculating the number average particle diameter of the conductive filler is described below. The minimum and maximum diameters of the conductive filler particles present in the above square region were measured, and the average of the minimum and maximum diameters was taken as the particle diameter of the particle. The particle diameters of all the conductive filler particles present in the above square region were then measured, and the arithmetic average value was taken as the number average particle diameter of the conductive filler. The results are shown in Table 1.

The wiring resistance was measured for the composite films of Examples 1 to 16 and Comparative Examples 1 to 5. The method for measuring the wiring resistance is described below. A printed wiring board was prepared on which two sample attachment electrodes made of copper were exposed side by side. These sample attachment electrodes were square with sides of 4 mm, and the center-to-center distance between the two sample attachment electrodes was 12 mm.

A resistance measurement electrode was exposed near each of the two sample attachment electrodes. More specifically, a resistance measurement electrode was formed on a side of the sample attachment electrode that differs from the side where the adjacent sample attachment electrode is located. The sample attachment electrode and the resistance measurement electrode formed in its vicinity are electrically connected to each other through the inner layer circuit of the printed wiring board to form a pair, but the two sample attachment electrodes are not electrically connected to each other. The two sample attachment electrodes and the two resistance measurement electrodes are both electroless nickel plated and gold plated, which are common in printed wiring board applications, to minimize the effects of oxidation of the electrode surface.

First, for two pairs of electrodes for attaching samples and electrodes for measuring resistance, the electrical resistance between the electrodes for attaching samples and the electrodes for measuring resistance was measured using the four-terminal method without the composite film attached to the electrodes for attaching samples. The sum of the two electrical resistances obtained was then recorded as the measured substrate resistance.

Next, the composite film of the Example or Comparative Example was attached so as to straddle the two sample attachment electrodes, and was pressed using a peel test pressure roller APR-97 manufactured by Imada Co., Ltd., and then left to stand for 30 minutes. The electrical resistance between the two resistance measurement electrodes was measured using the four-terminal method, and the value obtained by subtracting the measured substrate resistance from this measurement was regarded as the wiring resistance of the composite film. The wiring resistance was measured five times, and the average value was regarded 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 with a temperature of 25°C and a humidity of 40 to 50% RH.

The measurement results of the wiring resistance are shown in Table 1. The measurement results of the wiring resistance shown in Table 1 are 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 (fail): Wiring resistance is 500 mΩ or more

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 have superior electromagnetic interference suppression performance (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.

REFERENCE SIGNS LIST 1 Composite film 10 Copper foil 10a Surface on which conductive layer is laminated 10b Surface opposite to surface on which conductive layer is laminated 20 Conductive layer 30 Release film 40 Insulating layer 50 Carrier film

Claims (5)

A composite film having a copper foil and a conductive layer made of a conductive pressure-sensitive 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,
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 size of the conductive filler is 0.12 μm or more and 7 μm or less ,
In a cross section of the conductive layer that appears when the conductive layer is cut along a plane perpendicular to a surface of the copper foil, a ratio of a cross-sectional area of the conductive filler to a cross-sectional area of the conductive layer is 30% or more and 85% or less ;
the copper foil has two surfaces, the surface on which the conductive layer is laminated has a developed area ratio Sdr of 0.01% or more and 6.6 % or less, a skewness Ssk of -1.0 or more and 1.0 or less, a kurtosis Sku of 3.0 or more and 3.6 or less, and a contact resistance of 2 mΩ or more and 30 mΩ or less,
The contact resistance was 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 according to the method defined in the "Method for measuring surface contact electrical resistance of the Japan Copper and Brass Association Technical Standard JCBA T323:2011." Composite film. The composite film according to claim 1 , further comprising a release film laminated on the conductive layer. 2. The composite film according to claim 1 , further comprising an insulating layer for electrically protecting the copper foil, which is laminated on the surface opposite to the surface on which the conductive layer is laminated, of the two surfaces of the copper foil. 4. The composite film according to claim 3 , further comprising a carrier film as a support laminated on the insulating layer. The composite film according to claim 1 , which is used as an EMI countermeasure film.

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