Electromagnetic wave shielding film and shielded printed wiring board

Abstract

This invention provides an electromagnetic wave shielding film with a conductive adhesive layer exhibiting high peel strength and connection reliability, and capable of suppressing cracking of the insulating layer deposited on the conductive adhesive layer. The electromagnetic wave shielding film of this invention is characterized by comprising: a conductive adhesive layer containing conductive particles and an adhesive resin composition; and an insulating layer deposited on the conductive adhesive layer; wherein the conductive particles are composed of high-melting-point conductive particles and low-melting-point conductive particles, the high-melting-point conductive particles comprising high-melting-point flake particles and high-melting-point spherical particles, and the content of the high-melting-point flake particles is 60-80 wt% relative to the total content of the high-melting-point flake particles, the high-melting-point spherical particles, and the low-melting-point conductive particles.

Description

Technical Field

[0001] Invention Field This invention relates to electromagnetic wave shielding film and shielded printed wiring board. Prior Technology

[0002] Background Technology In the rapidly developing miniaturized and high-performance mobile devices such as cell phones, and electronic instruments such as video cameras and laptops, flexible printed circuit boards (PCBs) are often used to integrate circuits into complex mechanisms. Furthermore, their excellent flexibility is also utilized in the connection between movable parts, such as printheads, and control units. Since electromagnetic shielding is essential in these electronic instruments, PCBs with pre-implanted electromagnetic shielding (hereinafter also referred to as "shielded PCBs") are increasingly being used within these devices.

[0003] Mobile devices require multiple functions (such as camera or GPS), and to achieve this, the density of printed circuit boards is increased. In recent years, in order to meet the high performance requirements of mobile devices with communication frequencies of around 10GHz for 5G communication standards, high-level electromagnetic wave shielding films are required. When configuring printed circuit boards in a high-density configuration, the size of the mobile device itself is limited, so the method of thinning the thickness of the shielded printed circuit board is adopted. Furthermore, when it is desired to reduce the thickness of the shielded printed circuit board, the method of thinning the electromagnetic wave shielding film can be considered.

[0004] However, when the electromagnetic wave shielding film is thin, when it is heat-pressed onto a printed wiring board with height differences, the conductive adhesive layer of the electromagnetic wave shielding film will be stretched at the height difference, resulting in an increase in resistance or even damage to the electromagnetic wave shielding film itself.

[0005] Patent Document 1 discloses an electromagnetic wave shielding sheet, which, as an electromagnetic wave shielding film (electromagnetic wave shielding sheet) that can solve this problem, has a conductive layer and an insulating layer. The conductive layer contains small sheet-like conductive microparticles and a binder resin. The average aspect ratio of the small sheet-like conductive microparticles in the cross-section of the aforementioned conductive layer is 7 to 15. When the cross-sectional area of ​​the aforementioned conductive layer before heating and pressing is 100, the area occupied by the components other than the conductive microparticles is 55 to 80. The difference in the area occupied by the components other than the conductive microparticles before and after heating and pressing the aforementioned electromagnetic wave shielding sheet at 150°C, 2MPa, and 30 minutes is 5 to 25.

[0006] Previous technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2016-115725 Summary of the Invention

[0007] Invention Summary The problem the invention aims to solve In a shielded printed circuit board with an electromagnetic wave shielding film as described in Patent Document 1, small, flaky conductive microparticles may sometimes be located at the interface between the conductive layer and the insulating layer, as well as at the interface between the conductive layer and the printed circuit board. This reduces the bonding surface between the conductive layer and the insulating layer, or the contact surface between the conductive layer and the printed circuit board, resulting in a decrease in the peel strength of the conductive layer. Therefore, during use, there may be problems such as the insulation layer of the electromagnetic wave shielding film peeling off, or the electromagnetic wave shielding film itself peeling off from the printed circuit board. Furthermore, if exposed to high temperature and high humidity environments for extended periods, the small conductive microparticles may separate due to factors such as the thermal expansion coefficient of the binder resin or moisture content, leading to an increase in the resistance of the conductive layer. In other words, the reliability of the connection under high temperature and high humidity conditions is not sufficient. Furthermore, to improve connection reliability, the use of low-melting-point metal particles to bond small, sheet-like conductive microparticles together is also considered. However, if the amount of low-melting-point metal particles in the conductive layer is large, the conductive layer will expand or contract when exposed to high temperature and humidity environments for a long time, potentially causing cracks in the insulating layer deposited on the conductive layer. Regarding the reasons for the expansion or contraction of the conductive layer as described above, we believe that the heat and moisture in high temperature and humidity environments cause ionization in the low-melting-point metal, which diffuses between the small, sheet-like conductive microparticles. Combined with the thermal expansion of the binder resin, this leads to localized expansion or contraction of the conductive layer.

[0008] This invention is made to solve the above-mentioned problems. The purpose of this invention is to provide an electromagnetic wave shielding film with high peel strength and connection reliability of the conductive adhesive layer, and to suppress cracking of the insulating layer deposited on the conductive adhesive layer.

[0009] The means to solve the problem The electromagnetic wave shielding film of the present invention is characterized by comprising: a conductive adhesive layer comprising conductive particles and an adhesive resin composition; and an insulating layer deposited on the conductive adhesive layer; wherein the conductive particles are composed of high-melting-point conductive particles and low-melting-point conductive particles, the high-melting-point conductive particles comprising high-melting-point flake particles and high-melting-point spherical particles, and the content of the high-melting-point flake particles is 60-80 wt% relative to the total content of the high-melting-point flake particles, the high-melting-point spherical particles and the low-melting-point conductive particles.

[0010] In the electromagnetic wave shielding film of the present invention, the conductive particles are composed of high melting point conductive particles and low melting point conductive particles. High-melting-point conductive particles include high-melting-point small flake particles and high-melting-point spherical particles. Because the high-melting-point flake particles are sufficiently flexible, they can adapt to repeated bending of the electromagnetic wave shielding film, preventing them from easily shifting position. As a result, sufficient contact between the conductive particles is maintained, and an increase in resistance is prevented. Furthermore, if high-melting-point spherical particles are included, these particles will be embedded between the high-melting-point flake particles along the thickness direction of the conductive adhesive layer, and numerous adhesive resin components will exist between the high-melting-point flake particles. Therefore, the mechanical strength of the conductive adhesive layer is increased, and the peel strength is improved. Also, the high-melting-point spherical particles, being embedded between the high-melting-point flake particles, allow for electrical connections between them. Therefore, the shielding properties of the conductive adhesive layer are enhanced.

[0011] Additionally, in this specification, the term "high melting point small flake particles" refers to high melting point conductive particles with an aspect ratio of 18 or higher in the cross-section of the conductive adhesive layer after the electromagnetic wave shielding film has been heated and pressurized at 150°C, 2MPa, and 30min. Furthermore, in this specification, the term "high melting point spherical particles" refers to high melting point conductive particles with an aspect ratio of less than 18 in the cross-section of the conductive adhesive layer after the electromagnetic wave shielding film has been heated and pressurized at 150°C, 2MPa, and 30min. Furthermore, in this specification, the term "aspect ratio of high-melting-point conductive particles in the cross-section of the conductive adhesive layer" refers to the average aspect ratio of high-melting-point conductive particles obtained from the SEM image of the cross-section after the electromagnetic wave shielding film is heated and pressurized at 150°C, 2MPa, and 30min, and then the electromagnetic wave shielding film is cut. Specifically, a scanning electron microscope (JSM-6510LA, manufactured by Japan Electronics Corporation) is used to take images at a magnification of 3000x. The image data is then processed using image processing software (SEM Control User Interface Ver3.10) to measure the major and minor axes of 100 high-melting-point conductive particles in each image. The major axis ÷ minor axis of each high-melting-point conductive particle is calculated, and after excluding the upper and lower limits by 15%, the average value is defined as the aspect ratio.

[0012] When the electromagnetic wave shielding film of the present invention is placed on a printed wiring board, the electromagnetic wave shielding film will be subjected to heat pressing. At this point, the low-melting-point conductive particles will melt, and high-melting-point flake particles, high-melting-point spherical particles, and high-melting-point flake particles and high-melting-point spherical particles can be connected by metallic bonds. Therefore, it can better maintain the contact between conductive particles and prevent the resistance value from rising. As a result, the connection reliability of the electromagnetic wave shielding film can be improved. Furthermore, on a printed wiring board equipped with the electromagnetic wave shielding film of the present invention, electronic components are mounted using solder reflow. During the reflow of this solder, the conductive adhesive layer of the electromagnetic wave shielding film will harden, and the low melting point conductive particles will remelt, which can more strongly connect the high melting point small flake particles to each other, connect the high melting point spherical particles to each other, and connect the high melting point small flake particles and the high melting point spherical particles. The result is an increase in the overall strength of the conductive adhesive layer, as well as an increase in peel strength.

[0013] Furthermore, in the electromagnetic wave shielding film of the present invention, the content of the high-melting-point small flake particles is 60 to 80 wt% relative to the total content of the high-melting-point small flake particles, the high-melting-point spherical particles, and the low-melting-point conductive particles. If the ratio of conductive particles is within this range, the effects of including high-melting-point small flake particles and high-melting-point spherical particles, as well as the effects of including low-melting-point conductive particles, can be appropriately achieved. Furthermore, the relatively reduced content of low-melting-point conductive particles means a decrease in the amount of low-melting-point metals that can be ionized by heat and moisture. Therefore, ionized low-melting-point metals are less likely to disperse among the high-melting-point flake particles, resulting in a conductive adhesive layer that is less prone to localized expansion or contraction, thus preventing cracks in the insulating layer. Moreover, as mentioned above, high-melting-point spherical particles can penetrate between the high-melting-point flake particles. Therefore, we believe that even when the low-melting-point metals contained in the low-melting-point conductive particles are ionized, the high-melting-point spherical particles can further prevent cracks in the insulating layer by making the conductive adhesive layer less prone to localized expansion or contraction.

[0014] In the electromagnetic wave shielding film of the present invention, a metal layer may also be provided between the above-mentioned insulating layer and the above-mentioned conductive adhesive layer. If the electromagnetic wave shielding film has a metal layer, the electromagnetic wave shielding effect will be improved.

[0015] In the electromagnetic wave shielding film of the present invention, the weight ratio of the high melting point spherical particles to the low melting point conductive particles is preferably [high melting point spherical particles] / [low melting point conductive particles] = 0.5~29. The aforementioned weight ratio greater than 29 means that the weight proportion of low-melting-point conductive particles is relatively small, and the aforementioned weight ratio less than 0.5 means that the weight proportion of low-melting-point conductive particles is relatively large. That is, by setting the aforementioned weight ratio to 0.5~29, the weight proportion of low-melting-point conductive particles will not be too small or too large, and the effect of connecting high-melting-point small flake particles, connecting high-melting-point spherical particles, and connecting high-melting-point small flake particles and high-melting-point spherical particles can be easily achieved by using low-melting-point conductive particles to connect high-melting-point flake particles to each other, connect high-melting-point spherical particles to each other, and connect high-melting-point small flake particles and high-melting-point spherical particles through metallic bonding. Furthermore, the conductivity of the conductive adhesive layer is not easily reduced, and sufficient shielding can be easily obtained.

[0016] In the electromagnetic wave shielding film of the present invention, the conductive adhesive layer preferably further contains a flux, and the content of the flux in the conductive adhesive layer is preferably less than 4 wt%. If the conductive adhesive layer contains flux in the above ratio, it will become easier to connect high-melting-point flake particles to each other, high-melting-point spherical particles to each other, and high-melting-point flake particles to high-melting-point spherical particles when the low-melting-point conductive particles melt.

[0017] In the electromagnetic wave shielding film of the present invention, the melting point of the aforementioned low-melting-point conductive particles is preferably 120~190℃. If the melting point of the low-melting-point conductive particles is within the above range, they can be appropriately melted when the electromagnetic wave shielding film is hot-pressed onto the printed wiring board, and can more appropriately connect high-melting-point small flake particles to each other, connect high-melting-point spherical particles to each other, and connect high-melting-point small flake particles and high-melting-point spherical particles.

[0018] In the electromagnetic wave shielding film of the present invention, the melting point of the aforementioned high-melting-point conductive particles is preferably 300~1500℃. If the melting point of the high-melting-point conductive particles is within the above range, the high-melting-point conductive particles will not melt due to heat when the electromagnetic wave shielding film is hot-pressed onto the printed wiring board, and will not easily deform. Therefore, it can appropriately utilize the effects of containing high-melting-point small flake particles and high-melting-point spherical particles.

[0019] The shielded printed wiring board of the present invention is characterized by comprising: A printed wiring board comprising a base film, printed circuits disposed on the base film, and a cover layer disposed to cover the printed circuits; and An electromagnetic wave shielding film comprising a conductive adhesive layer and an insulating layer deposited on the conductive adhesive layer, wherein the conductive adhesive layer comprises conductive particles and an adhesive resin composition; Furthermore, the electromagnetic wave shielding film is disposed on the printed wiring board in such a way that the conductive adhesive layer and the cover layer are in contact; the conductive particles are composed of high melting point conductive particles and low melting point conductive particles, the high melting point conductive particles include high melting point flake particles and high melting point spherical particles, and the content of the high melting point flake particles is 60~80wt% relative to the total content of the high melting point flake particles, the high melting point spherical particles and the low melting point conductive particles. Furthermore, the shielded wiring board of the present invention may also have a metal layer between the above-mentioned insulating layer and the above-mentioned conductive adhesive layer.

[0020] The shielded printed circuit board of the present invention includes the electromagnetic wave shielding film of the present invention described above. Therefore, the shielded printed circuit board of the present invention can achieve the effects of the present invention described above.

[0021] In the shielded printed wiring board of the present invention, the printed circuit preferably includes a grounding circuit, and an opening is preferably formed in the cover layer to expose the grounding circuit, and the conductive adhesive layer preferably fills the opening and contacts the grounding circuit. With this structure, the conductive adhesive layer and the grounding circuit will form an electrical connection. Therefore, a good grounding effect can be obtained. Furthermore, since the conductive adhesive layer has the structure described above, even with such an opening, the conductive adhesive layer can conform to the shape of the opening and fill it. Therefore, it is not easy for gaps to form in the opening.

[0022] Invention Effects According to the present invention, an electromagnetic wave shielding film is provided, which has high peel strength and connection reliability of conductive adhesive layer, and can suppress cracking of insulating layer deposited on conductive adhesive layer. Simple Explanation of the Diagram

[0023] Figure 1 is a cross-sectional view, which schematically shows an example of an electromagnetic wave shielding film of the first embodiment of the present invention. Figure 2 is a cross-sectional view, which schematically shows the printed circuit board preparation steps of the method for manufacturing a shielded printed circuit board, and the electromagnetic wave shielding film of the first embodiment of the present invention is used in the method for manufacturing the shielded printed circuit board. Figure 3 is a cross-sectional view, which schematically shows the electromagnetic wave shielding film bonding step in the manufacturing method of the shielded printed wiring board, and the electromagnetic wave shielding film of the first embodiment of the present invention is used in the manufacturing method of the shielded printed wiring board. Figure 4 is a cross-sectional view, which schematically shows the heating and pressurizing steps of the manufacturing method of the shielded printed wiring board, and the electromagnetic wave shielding film of the first embodiment of the present invention is used in the manufacturing method of the shielded printed wiring board. Figure 5 is a cross-sectional view, which schematically shows an example of an electromagnetic wave shielding film of the second embodiment of the present invention. Figure 6 is a cross-sectional SEM image of the electromagnetic wave shielding film of Example 1. Figure 7A is a side sectional view, which schematically shows the method for measuring the connection resistance value. Figure 7B is a side sectional view, which schematically shows the method for measuring the connection resistance value. Figure 8 is a schematic diagram showing the structure of the system used in the KEC method. Implementation

[0024] Forms used to implement inventions The electromagnetic wave shielding film and shielded printed wiring board of the present invention will be described in detail below. However, the present invention is not limited to the following embodiments, and can be appropriately modified and applied without changing the essence of the present invention.

[0025] (First Implementation) Figure 1 is a cross-sectional view, which schematically shows an example of an electromagnetic wave shielding film of the first embodiment of the present invention. As shown in Figure 1, the electromagnetic wave shielding film 10 is composed of the following: a conductive adhesive layer 20, which includes conductive particles 21 and an adhesive resin composition 22; an insulating layer 30, which is deposited on the conductive adhesive layer 20; and a metal layer 40, which is disposed between the conductive adhesive layer 20 and the insulating layer 30. Furthermore, the conductive particles 21 are composed of high-melting-point conductive particles 21a and low-melting-point conductive particles 21b.

[0026] The high-melting-point conductive particles 21a include high-melting-point small flake particles 21a 1 and high-melting-point spherical particles 21a 2.

[0027] Because the high-melting-point flake particles 21a 1 are sufficiently flexible, they can also bend when the electromagnetic wave shielding film 10 is repeatedly bent, and their positions are less likely to deviate. As a result, the conductive particles 21 can be kept in sufficient contact with each other, and the resistance value can be prevented from increasing. Furthermore, if high-melting-point spherical particles 21a 2 are included, these particles will be sandwiched between the high-melting-point flake particles 21a 1 in the thickness direction of the conductive adhesive layer 20, and numerous adhesive resin components 22 will exist between the high-melting-point flake particles 21a 1. Therefore, the mechanical strength of the conductive adhesive layer 20 is increased, and the peel strength is improved. Also, since the high-melting-point spherical particles 21a 2 are embedded between the high-melting-point flake particles 21a 1, the high-melting-point flake particles 21a 1 can be electrically connected to each other through the high-melting-point spherical particles 21a 2. Therefore, the shielding properties of the conductive adhesive layer 20 are improved.

[0028] In the electromagnetic wave shielding film 10, the average particle size of the high melting point small flake particles 21a 1 should preferably be 0.5~30μm, and more preferably 1~10μm. If the average particle size of the high-melting-point flake particles 21a 1 is within this range, the high-melting-point flake particles 21a 1 will form a suitable size and strength. Therefore, the conductivity and flexural strength of the conductive adhesive layer 20 are improved. Consequently, the conductive adhesive layer 20 can be made thinner. That is, the conductive adhesive layer 20 can be thinned while maintaining the conductivity and flexural strength of the conductive adhesive layer 20.

[0029] Additionally, in this specification, "particle size of high melting point small flake particles", "particle size of high melting point spherical particles" and "particle size of low melting point conductive particles" refer to values ​​measured using the laser diffraction scattering particle size distribution measurement method of the Microtrac MT3300EXII.

[0030] Regarding the electromagnetic wave shielding film 10, after heating and pressurizing the electromagnetic wave shielding film 10 at 150°C, 2MPa, and 30min, the average aspect ratio of the high-melting-point small flake particles 21a 1 in the cross-section of its conductive adhesive layer 20 should preferably be 18~150, more preferably 20~100, and even more preferably 20~50. If the average aspect ratio of the high-melting-point flake particles 21a 1 is 18 or higher, the high-melting-point flake particles 21a 1 will possess sufficient flexibility. Therefore, when the electromagnetic wave shielding film 10 is repeatedly bent, the high-melting-point flake particles 21a 1 can also conform to the bending, and their position is less likely to deviate. Moreover, the high-melting-point flake particles 21a 1 become less prone to damage. As a result, the resistance value can be prevented from increasing. Furthermore, the adhesion of the electromagnetic wave shielding film will also be improved. If the average aspect ratio of the high-melting-point small flake particles 21a 1 is below 150, from the point of view of the number of conductive particles, it is easy to show conduction in the thickness direction, and the shielding will become good, which is an excellent feature.

[0031] In the electromagnetic wave shielding film 10, the average particle size of the high melting point spherical particles 21a 2 should preferably be 1~10μm, and more preferably 4~7μm. If the average particle size of high-melting-point spherical particles is less than 1 μm, these particles are less likely to act as steric hindrances, and the small, high-melting-point flake particles are more likely to be exposed on the surface of the conductive adhesive layer. As a result, the peel strength of the conductive adhesive layer is more likely to decrease. If the average particle size of the high-melting-point spherical particles is greater than 10 μm, the conductivity of the conductive adhesive layer will decrease, and the shielding performance will easily be reduced.

[0032] In the electromagnetic wave shielding film 10, the melting point of the high-melting-point conductive particles 21a should preferably be 300~1500℃. If the melting point of the high-melting-point conductive particles 21a is within the above range, the high-melting-point conductive particles 21a will not melt due to heat when the electromagnetic wave shielding film 10 is hot-pressed onto the printed wiring board, and will not easily deform. Therefore, the effects of including high-melting-point small flake particles 21a 1 and high-melting-point spherical particles 21a 2 can be appropriately utilized.

[0033] High-melting-point conductive particles 21a should preferably be composed of metals such as silver, copper, nickel, aluminum, and silver-plated copper, which are made by plating copper with silver. These metals have high conductivity, thus improving the conductivity of the conductive adhesive layer 20. In addition, in the electromagnetic wave shielding film 10, the high-melting-point small flake particles 21a 1 and the high-melting-point spherical particles 21a 2 can be made of the same material or different materials.

[0034] Methods for manufacturing high-melting-point spherical particles 21a 2 include atomization, which involves spraying raw material particles from a nozzle to control gas pressure. The shape of high-melting-point spherical particles 21a 2 can be controlled by adjusting gas pressure, etc. In this way, high-melting-point spherical particles 21a 2 in either a regular spherical or deformed spherical shape can be produced. Furthermore, if the atomization method is used to manufacture high-melting-point spherical particles 21a 2, the aspect ratio of the high-melting-point spherical particles 21a 2 can be made close to 1.

[0035] When the electromagnetic wave shielding film 10 is placed on the printed wiring board, the electromagnetic wave shielding film 10 will be subjected to heat pressing. At this point, the low-melting-point conductive particles 21b will melt, and can be connected to each other by metal bonding, to each other by high-melting-point small flake particles 21a 1, to each other by high-melting-point spherical particles 21a 2, and to each other by high-melting-point small flake particles 21a 1 and high-melting-point spherical particles 21a 2. Therefore, the conductive particles 21 can maintain more sufficient contact with each other, and the resistance value can be prevented from rising. Thus, the connection reliability of the electromagnetic wave shielding film 10 can be improved.

[0036] Furthermore, on a printed wiring board equipped with an electromagnetic wave shielding film 10, electronic components are mounted using solder reflow. During the solder reflow process, the conductive adhesive layer 20 of the electromagnetic wave shielding film 10 hardens, and the low-melting-point conductive particles 21b remelt, which can more strongly connect the high-melting-point small flake particles 21a 1 to each other, connect the high-melting-point spherical particles 21a 2 to each other, and connect the high-melting-point small flake particles 21a 1 and the high-melting-point spherical particles 21a 2. The result is an increase in the overall strength of the conductive adhesive layer, as well as an increase in peel strength.

[0037] In the electromagnetic wave shielding film 10, the low-melting-point conductive particles 21b should preferably be spherical. Furthermore, in the electromagnetic wave shielding film 10, the average particle size of the low-melting-point conductive particles 21b should preferably be 1~10μm, and more preferably 2~6μm. If the average particle size of low-melting-point conductive particles is less than 1 μm, then the low-melting-point conductive particles become too small to handle. If the average particle size of low-melting-point conductive particles is greater than 10 μm, the specific surface area of ​​these particles is small. In other words, the number of low-melting-point conductive particles decreases, making it difficult for them to contact high-melting-point small flake particles and high-melting-point spherical particles. When the electromagnetic wave shielding film is thermo-pressed onto the printed circuit board, it becomes difficult to connect high-melting-point small flake particles to each other, high-melting-point spherical particles to each other, and high-melting-point small flake particles to high-melting-point spherical particles.

[0038] In the electromagnetic wave shielding film 10, the melting point of the low-melting-point conductive particles 21b should preferably be 120~190℃, and more preferably 130~150℃. If the melting point of the low-melting-point conductive particles 21b is within the above range, they can be appropriately melted when the electromagnetic wave shielding film 10 is hot-pressed onto the printed wiring board, and can more appropriately connect the high-melting-point small flake particles 21a 1 to each other, connect the high-melting-point spherical particles 21a 2 to each other, and connect the high-melting-point small flake particles 21a 1 and the high-melting-point spherical particles 21a 2.

[0039] The low-melting-point conductive particles 21b are preferably composed of metals such as tin, bismuth, palladium, and indium, as well as alloys of these metals. Among these, alloys of tin and bismuth are preferred. These metals are sufficiently conductive and can form alloys with high-melting-point flake particles 21a 1 and high-melting-point spherical particles 21a 2. Therefore, it is possible to more appropriately connect the high-melting-point flake particles 21a 1 to each other, connect the high-melting-point spherical particles 21a 2 to each other, and connect the high-melting-point flake particles 21a 1 and the high-melting-point spherical particles 21a 2.

[0040] In the electromagnetic wave shielding film 10, relative to the total content of high melting point small flake particles 21a 1, high melting point spherical particles 21a 2 and low melting point conductive particles 21b, the content of high melting point small flake particles 21a 1 should preferably be 60~80wt%, more preferably 65~75wt%. If the ratio of conductive particles 21 is within this range, the effects of including high-melting-point small flake particles 21a 1 and high-melting-point spherical particles 21a 2, as well as the effects of including low-melting-point conductive particles 21b, can be appropriately achieved. Furthermore, the content of low-melting-point conductive particles 21b is relatively reduced, resulting in a decrease in low-melting-point metals that can be ionized by heat and moisture. Therefore, ionized low-melting-point metals are not easily dispersed among high-melting-point flake particles 21a 1, which makes the conductive adhesive layer 20 less prone to local expansion or contraction, thus preventing cracks from forming on the insulating layer 30. Furthermore, as mentioned above, the high-melting-point spherical particles 21a 2 will enter between the high-melting-point small flake particles 21a 1. Therefore, we believe that even when the low-melting-point metal contained in the low-melting-point conductive particles 21b has been ionized, the high-melting-point spherical particles 21a 2 can make the conductive adhesive layer 20 less prone to local expansion or contraction, thereby further preventing cracks from forming on the insulating layer 30.

[0041] In the electromagnetic wave shielding film 10, the total content of high-melting-point flake particles 21a 1, high-melting-point spherical particles 21a 2, and low-melting-point conductive particles 21b in the conductive adhesive layer 20 should preferably be 70~80wt%, more preferably 65~78wt%. If the total content of high-melting-point small flake particles, high-melting-point spherical particles and low-melting-point conductive particles is less than 70 wt%, the electromagnetic wave shielding film will easily reduce its shielding performance because the conductive particles are too few and therefore cannot easily connect electrically. If the total content of high-melting-point flake particles, high-melting-point spherical particles, and low-melting-point conductive particles exceeds 80 wt%, the content of the adhesive resin composition will relatively decrease. Since the peel strength of the conductive adhesive layer depends on the content of the adhesive resin composition, the peel strength of the conductive adhesive layer is prone to decrease.

[0042] In the electromagnetic wave shielding film 10, the content of high melting point small flake particles 21a 1 in the conductive adhesive layer 20 should preferably be 45~60wt%, more preferably 50~55wt%. In the electromagnetic wave shielding film 10, the content of high melting point spherical particles 21a 2 in the conductive adhesive layer 20 should preferably be 0.75~29.25wt%, more preferably 7~20wt%. In the electromagnetic wave shielding film 10, the content of low-melting-point conductive particles 21b in the conductive adhesive layer 20 should preferably be 0.75~29.25wt%, more preferably 3~15wt%.

[0043] In the electromagnetic wave shielding film 10, the weight ratio of high-melting-point spherical particles 21a to low-melting-point conductive particles 21b should preferably be [high-melting-point spherical particles] / [low-melting-point conductive particles] = 0.5~29. From the perspective of balancing the peel strength, connection reliability and shielding performance of the conductive adhesive layer, a ratio of 2~5 is preferred. The aforementioned weight ratio greater than 29 means that the weight proportion of low-melting-point conductive particles 21b is relatively small, and the aforementioned weight ratio less than 0.5 means that the weight proportion of low-melting-point conductive particles 21b is relatively large. That is, by setting the aforementioned weight ratio to 0.5~29, the weight proportion of low-melting-point conductive particles 21b will not be too small or too large, and the effect of connecting high-melting-point small sheet particles 21a 1 to each other, connecting high-melting-point spherical particles 21a 2 to each other, and connecting high-melting-point small sheet particles 21a 1 and high-melting-point spherical particles 21a 2 through metal bonding can be easily achieved using low-melting-point conductive particles 21b. Furthermore, the conductivity of the conductive adhesive layer is not easily reduced, and sufficient shielding can be easily obtained.

[0044] In the electromagnetic wave shielding film 10, the weight ratio of high-melting-point small flake particles 21a 1, high-melting-point spherical particles 21a 2, and low-melting-point conductive particles 21b should preferably be [high-melting-point small flake particles]: [high-melting-point spherical particles]: [low-melting-point conductive particles] = 6~8: 0.1~3.9: 0.1~3.9. If the proportion of conductive particles 21 is within this range, the effects of including high-melting-point small flake particles 21a 1 and high-melting-point spherical particles 21a 2, as well as the effects of including low-melting-point conductive particles 21b, can be appropriately achieved. On the other hand, especially if the weight ratio of low-melting-point conductive particles 21b is less than the above ratio, there will be fewer low-melting-point conductive particles, making it difficult to achieve the effect of connecting conductive particles to each other. Furthermore, the conductivity of low-melting-point conductive particles is often lower than that of high-melting-point conductive particles. Therefore, if the weight ratio of low-melting-point conductive particles is greater than the above ratio, the conductivity of the conductive adhesive layer is easily reduced, and the shielding effect is also reduced.

[0045] In the electromagnetic wave shielding film 10, there are no special restrictions on the material of the adhesive resin composition 22. It can be any thermoplastic resin composition such as styrene-based resin composition, vinyl acetate-based resin composition, polyester-based resin composition, polyethylene-based resin composition, polypropylene-based resin composition, amide-based resin composition, amide-based resin composition, acrylic resin composition, etc.; or thermosetting resin composition such as phenol-based resin composition, epoxy-based resin composition, carbamate-based resin composition, melamine-based resin composition, alkyd-based resin composition, etc. The adhesive resin composition may be a single material or a combination of two or more of these materials.

[0046] Regarding the electromagnetic wave shielding film 10, after heating and pressurizing the electromagnetic wave shielding film 10 at 150°C, 2MPa, and 30min, the ratio of the area of ​​the adhesive resin composition 22 to the total area of ​​the cross-section of its conductive adhesive layer 20 should be 60-95%. If the area ratio is less than 60%, the ratio of conductive particles 21 increases relatively, the conductive particles 21 become denser, and the flexibility of the conductive adhesive layer 20 decreases. The result is reduced compliance with elevation differences. If the area ratio is greater than 95%, the contact points between conductive particles 21 decrease, and the conductivity decreases. The result is a reduction in shielding effectiveness.

[0047] Additionally, in this specification, the term "ratio of the area of ​​the adhesive resin composition to the total area of ​​the cross-section" refers to the ratio of the area of ​​the adhesive resin composition derived from the cross-sectional SEM image after the electromagnetic wave shielding film has been cut. The specific calculation method is as follows. The cross-section of the conductive adhesive layer was observed using a scanning electron microscope (SEM). When observing the cross-section from a vertical direction using SEM, a contrast difference is generated between the adhesive resin composition and the conductive particles, allowing the shape of the conductive particles to be identified. For the cross-sectional SEM image after the electromagnetic wave shielding film was cut, the image analysis software "GIMP2.10.6" was used to binarize the adhesive resin composition and the conductive particle composition into black and white. Then, by calculating the number of black and white pixels, the ratio of the area of ​​the adhesive resin composition is calculated from the ratio of the number of pixels.

[0048] Regarding the electromagnetic wave shielding film 10, after heating and pressurizing the electromagnetic wave shielding film 10 at 150°C, 2MPa, and 30min, the distance between the high-melting-point small flake particles 21a1 in the cross-section of its conductive adhesive layer 20 should preferably be 1.5μm or more, more preferably 2.0μm or more, and even more preferably 4μm or more. Furthermore, it should preferably be 9μm or less, more preferably 8μm or less, and even more preferably 6μm or less. If the distance between high-melting-point small flake particles 21a 1 is greater than 1.5 μm, the peel strength will be improved. If the distance between high-melting-point small flake particles 21a 1 is less than 9μm, the shielding performance is improved.

[0049] In addition, the distance between high-melting-point small flake particles can be determined using the following methods. The cross-section of the conductive adhesive layer was observed using a scanning electron microscope (SEM). Next, for each image, 10 groups of adjacent high-melting-point flake particles were selected. Then, the distance between each group of adjacent high-melting-point flake particles in the thickness direction was measured. This value was averaged to determine the distance between each group of adjacent high-melting-point flake particles.

[0050] In the electromagnetic wave shielding film 10, the thickness of the conductive adhesive layer 20 is preferably 5~20μm, and more preferably 8~15μm. If the thickness of the conductive adhesive layer is less than 5 μm, the amount of conductive particles must be increased to ensure high shielding performance, which makes it impossible to maintain flexibility and peel strength. If the thickness of the conductive adhesive layer is greater than 20 μm, it will be easier to design for high shielding, but electromagnetic wave shielding films cannot be made thinner.

[0051] In the electromagnetic wave shielding film 10, the conductive adhesive layer 20 preferably contains a flux. Furthermore, the flux content in the conductive adhesive layer 20 should preferably be less than 4 wt%, and more preferably less than 2 wt%. If the conductive adhesive layer 20 contains flux in the above ratio, it will become easier to connect the high melting point flake particles 21a 1 to each other, connect the high melting point spherical particles 21a 2 to each other, and connect the high melting point flake particles 21a 1 and the high melting point spherical particles 21a 2 when the low melting point conductive particles 21b melt. Furthermore, if the flux content of the conductive adhesive layer is 4 wt% or more, the overall strength of the conductive adhesive layer will decrease, and the adhesion of the conductive adhesive layer will easily decrease. There are no special restrictions on fluxes; well-known substances such as polycarboxylic acids, lactic acid, citric acid, oleic acid, stearic acid, glutamic acid, benzoic acid, glycerol, and rosin can be used.

[0052] The conductive adhesive layer 20 may further include flame retardants, flame retardant additives, curing accelerators, tackifiers, antioxidants, pigments, dyes, plasticizers, ultraviolet absorbers, defoamers, leveling agents, fillers, viscosity modifiers, etc.

[0053] As shown in Figure 1, the electromagnetic wave shielding film 10 has an insulating layer 30. Therefore, processability is improved. Furthermore, the conductive adhesive layer 20 can be insulated from the outside.

[0054] The insulating layer 30 of the electromagnetic wave shielding film 10 is not particularly limited as long as it has sufficient insulation and can protect the conductive adhesive layer 20 and the metal layer 40. For example, it is advisable to make it from thermoplastic resin composition, thermosetting resin composition, active energy line curing composition, etc. There are no special limitations on the above-mentioned thermoplastic resin compositions, and examples include: styrene-based resin compositions, vinyl acetate-based resin compositions, polyester-based resin compositions, polyethylene-based resin compositions, polypropylene-based resin compositions, amide-based resin compositions, acrylic resin compositions, etc.

[0055] There are no special limitations on the above-mentioned thermosetting resin compositions, and examples include: phenolic resin compositions, epoxy resin compositions, carbamate resin compositions, melamine resin compositions, alkyd resin compositions, etc.

[0056] There are no particular limitations on the above-mentioned active energy line hardening components, such as polymeric compounds having at least two (meth)acryloxy groups in the molecule.

[0057] The insulating layer 30 may be made of a single material or of two or more materials.

[0058] The insulation layer 30 may also include, as needed, curing accelerators, tackifiers, antioxidants, pigments, dyes, plasticizers, ultraviolet absorbers, defoamers, leveling agents, fillers, flame retardants, viscosity modifiers, anti-caking agents, etc.

[0059] There are no special limitations on the thickness of the insulating layer 30. It can be set appropriately as needed, preferably 1~15μm, and more preferably 3~10μm.

[0060] As shown in Figure 1, the electromagnetic wave shielding film 10 has a metal layer 40. Therefore, the electromagnetic wave shielding effect is improved.

[0061] The metal layer 40 may also include a layer composed of materials such as gold, silver, copper, aluminum, nickel, tin, palladium, chromium, titanium, and zinc, and it is preferred to include a copper layer. From the perspective of conductivity and economy, copper is a suitable material for metal layer 40. Alternatively, the metal layer 40 may also comprise a layer made of an alloy of the aforementioned metals.

[0062] The thickness of the metal layer 40 should preferably be 0.01~10μm. If the thickness of the metal layer is less than 0.01 μm, it is not easy to obtain a sufficient shielding effect. If the thickness of the metal layer is greater than 10 μm, it will not be easily bent.

[0063] In the electromagnetic wave shielding film 10, an anchoring coating layer may also be formed between the insulating layer 30 and the metal layer 40. Materials for anchoring coatings include: carbamate resins, acrylic resins, core-shell composite resins with carbamate resin as the shell and acrylic resin as the core, epoxy resins, amide resins, amide resins, melamine resins, phenolic resins, urea-formaldehyde resins, end-capped isocyanates obtained by reacting end-capping agents such as phenol with polyisocyanates, polyvinyl alcohol, polyvinylpyrrolidone, etc.

[0064] Next, a method for manufacturing a shielded printed wiring board using the electromagnetic wave shielding film of the first embodiment of the present invention will be described. Figure 2 is a cross-sectional view, which schematically shows the printed circuit board preparation steps of the method for manufacturing a shielded printed circuit board, and the electromagnetic wave shielding film of the first embodiment of the present invention is used in the method for manufacturing the shielded printed circuit board. Figure 3 is a cross-sectional view, which schematically shows the electromagnetic wave shielding film bonding step in the manufacturing method of the shielded printed wiring board, and the electromagnetic wave shielding film of the first embodiment of the present invention is used in the manufacturing method of the shielded printed wiring board. Figure 4 is a cross-sectional view, which schematically shows the heating and pressurizing steps of the manufacturing method of the shielded printed wiring board, and the electromagnetic wave shielding film of the first embodiment of the present invention is used in the manufacturing method of the shielded printed wiring board.

[0065] (1) Printed Wiring Board Preparation Steps First, as shown in Figure 2, a printed wiring board 50 is prepared. The printed wiring board 50 includes a base film 51, a printed circuit 52 disposed on the base film 51, and a cover layer 53 disposed to cover the printed circuit 52.

[0066] Additionally, in the printed wiring board 50, the printed circuit 52 includes a grounding circuit 52a, and an opening 53a is formed in the cover layer 53 to expose the grounding circuit 52a.

[0067] There are no special restrictions on the materials of the base film 51 and the cover layer 53; they should preferably be made of engineering plastics. Examples of such engineering plastics include: polyethylene terephthalate, polypropylene, cross-linked polyethylene, polyester, polybenzimidazole, polyimide, polyimide-polyamide, polyether-polyimide, polyphenylene sulfide, and other resins. Furthermore, among these engineering plastics, polyphenylene sulfide film is preferred when flame retardancy is required, while polyimide film is preferred when heat resistance is required. Additionally, the thickness of the base film 51 should preferably be 10~40 μm. Furthermore, the thickness of the cover layer 53 should preferably be 10~30 μm.

[0068] There are no special restrictions on the printed circuit 52; it can be formed by etching conductive materials, etc. Conductive materials include copper, nickel, silver, and gold.

[0069] (2) Electromagnetic wave shielding film bonding steps Next, as shown in Figure 3, an electromagnetic wave shielding film 10 is prepared and disposed on the printed wiring board 50 in such a way that the conductive adhesive layer 20 is connected to the cover layer 53.

[0070] (3) Heating and pressurizing steps Next, as shown in Figure 4, heating and pressurization are applied to make the electromagnetic wave shielding film 10 adhere to the printed wiring board 50. There are no special restrictions on the heating and pressurization conditions, as long as the low-melting-point conductive particles can be melted. For example, it is advisable to use 150~200℃, 2~5MPa, and 1~10min.

[0071] Through the heating and pressurizing process, the conductive adhesive layer 20 will fill the opening 53a.

[0072] Furthermore, through the heating and pressurization process, the low-melting-point conductive particles 21b will melt, thereby connecting the high-melting-point small flake particles 21a 1 to each other, connecting the high-melting-point spherical particles 21a 2 to each other, and connecting the high-melting-point small flake particles 21a 1 and the high-melting-point spherical particles 21a 2.

[0073] Through the above steps, a shielded printed wiring board 60 can be obtained. Additionally, the shielded printed wiring board 60 is also an example of the shielded printed wiring board of the present invention.

[0074] That is, as shown in FIG4, the shielded printed wiring board 60 includes: a printed wiring board 50 having a base film 51, a printed circuit 52 disposed on the base film 51 and a cover layer 53 disposed to cover the printed circuit 52; and an electromagnetic wave shielding film 10 having a conductive adhesive layer 20, the conductive adhesive layer 20 comprising conductive particles 21 and an adhesive resin composition 22.

[0075] The electromagnetic wave shielding film 10 of the shielded printed wiring board 60 is disposed on the printed wiring board 50 in such a way that the conductive adhesive layer 20 is in contact with the cover layer 53. The conductive particles 21 are composed of high-melting-point conductive particles 21a and low-melting-point conductive particles 21b. The high-melting-point conductive particles 21a include high-melting-point small flake particles 21a 1 and high-melting-point spherical particles 21a 2, while the low-melting-point conductive particles 21b include low-melting-point conductive particles 21b.

[0076] In the shielded printed wiring board 60, relative to the total content of high melting point small flake particles 21a 1, high melting point spherical particles 21a 2 and low melting point conductive particles 21b, the content of high melting point small flake particles 21a 1 should preferably be 60~80wt%, more preferably 65~75wt%.

[0077] In the shielded printed wiring board 60, the printed circuit 52 includes a grounding circuit 52a. An opening 53a is formed in the cover layer 53 to expose the grounding circuit 52a, and the conductive adhesive layer 20 fills the opening 53a and contacts the grounding circuit 52a. With this structure, the conductive adhesive layer 20 and the grounding circuit 52a will form an electrical connection. Therefore, a good grounding effect can be obtained. Furthermore, since the conductive adhesive layer 20 has the structure described above, even with this opening 53a, the conductive adhesive layer 20 can conform to the shape of the opening 53a and fill it. Therefore, it is not easy for gaps to form in the opening 53a.

[0078] The shielded printed circuit board 60 includes an electromagnetic wave shielding film 10. Therefore, the shielded printed circuit board 60 can achieve the effects of the electromagnetic wave shielding film 10 described above.

[0079] In addition, in the shielded printed wiring board 60, an opening 53a is formed in the cover layer 53 of the printed wiring board 50 to expose the grounding circuit 52a. However, the shielded printed wiring board of the present invention may not have an opening formed in the cover layer of the printed wiring board.

[0080] (Second Implementation) Next, the electromagnetic wave shielding film of the second embodiment of the present invention will be described. Figure 5 is a cross-sectional view, which schematically shows an example of an electromagnetic wave shielding film of the second embodiment of the present invention. The electromagnetic wave shielding film 110 shown in Figure 5 has the same structure as the electromagnetic wave shielding film 10 of the first embodiment of the present invention, except that it does not have the metal layer 40. In the structure of the electromagnetic wave shielding film of the second embodiment, the conductive adhesive layer 20 serves to shield electromagnetic waves. That is, the electromagnetic wave shielding film 110 is composed of: a conductive adhesive layer 20, which includes conductive particles 21 and an adhesive resin composition 22; and an insulating layer 30, which is deposited on the conductive adhesive layer 20. The conductive particles 21 are composed of high-melting-point conductive particles 21a and low-melting-point conductive particles 21b. The high-melting-point conductive particles 21a include high-melting-point small flake particles 21a 1 and high-melting-point spherical particles 21a 2.

[0081] The ideal structure of the conductive adhesive layer 20, high-melting-point flake particles 21a 1, high-melting-point spherical particles 21a 2, low-melting-point conductive particles 21b, adhesive resin composition 22, and insulating layer 30 in the electromagnetic wave shielding film 110 is the same as the ideal structure of the conductive adhesive layer 20, high-melting-point flake particles 21a 1, high-melting-point spherical particles 21a 2, low-melting-point conductive particles 21b, adhesive resin composition 22, and insulating layer 30 in the electromagnetic wave shielding film 10.

[0082] Even with this structure, the electromagnetic wave shielding film 110 is the same as the electromagnetic wave shielding film 10 of the first embodiment described above. The conductive adhesive layer 20 has high peel strength and connection reliability, and can suppress cracks in the insulating layer 30.

[0083] Example The following describes embodiments of the invention in more detail, but the invention is not limited to these embodiments.

[0084] (Example 1) Epoxy resin was coated onto the transfer film, and an electric oven was used to heat it at 100°C for 2 minutes to produce an insulating layer with a thickness of 5μm.

[0085] Next, the high-melting-point flake particles (silver-coated copper powder), high-melting-point spherical particles (copper powder), low-melting-point conductive particles (tin-bismuth powder (tin: 72 wt%, bismuth: 28 wt%)), flux (rosin), and adhesive resin composition (cresol phenolic varnish type epoxy resin: DIC Corporation's "EPICLON N-655-EXP")) listed in Table 1 were prepared and mixed in the proportions listed in Table 1 to produce a conductive resin composition. Furthermore, the average particle size of the high-melting-point flake particles, high-melting-point spherical particles, and low-melting-point conductive particles was determined using a Microtrac MT3300EXII laser diffraction particle size distribution measurement method.

[0086] Next, a conductive resin composition is coated on the insulating layer to form a conductive adhesive layer with a thickness of 15 μm, thus obtaining the electromagnetic wave shielding film of Example 1.

[0087] (Examples 2-10) and (Comparative Examples 1-8) Except that the types, mixing ratios and contents of high-melting-point small flake particles, high-melting-point spherical particles and low-melting-point conductive particles are set as shown in Tables 1 and 2, the electromagnetic wave shielding films of Examples 2 to 10 and Comparative Examples 1 to 8 were prepared in the same manner as in Example 1.

[0088] [Table 1]

[0089] [Table 2]

[0090] (Heating and pressurization test) Prepare a 25μm thick polyimide resin board, and configure the electromagnetic wave shielding film of each embodiment and each comparative example by bonding a conductive adhesive layer to the polyimide resin board. Next, the electromagnetic wave shielding films of each embodiment and each comparative example were heated and pressurized at 150°C, 2MPa, and 30min to bond them to the polyimide resin board.

[0091] The electromagnetic wave shielding film was cut after the heating and pressurization test, and SEM images were taken. The aspect ratio of each conductive particle was measured using image processing software (SEM Control User Interface Ver3.10). The results are shown in Tables 1 and 2.

[0092] The cross-sectional SEM image of the electromagnetic wave shielding film of Example 1 is shown as a representative example. Figure 6 is a cross-sectional SEM image of the electromagnetic wave shielding film of Example 1.

[0093] (Peel strength test) In addition, under the same heating and pressurization conditions as described above, the electromagnetic wave shielding films of each embodiment and each comparative example were bonded to a polyimide resin plate, and then the peel strength when the electromagnetic wave shielding film was peeled off from the polyimide resin plate was measured. Furthermore, under the same heating and pressurizing conditions as described above, the electromagnetic wave shielding films of each embodiment and each comparative example were bonded to a polyimide resin plate. Then, after passing the film through a reflow oven twice at a TOP temperature of 260°C, the peel strength when peeling the electromagnetic wave shielding film from the polyimide resin plate was measured. The results of the peel strength test are shown in Tables 1 and 2. Additionally, according to the Unified Specification for Test Methods of Flexible Printed Wiring Boards (JPCA-DG04), the peel strength of the cover layer (above 340 N / m) is considered sufficient if the peel strength is above 0.34 N / cm.

[0094] (Connection resistance value measurement test) Using the electromagnetic wave shielding films of each embodiment and each comparative example, the resistance values ​​of the electromagnetic wave shielding films of each embodiment and each comparative example were measured using the following method. The measurement results are shown in Tables 1 and 2. Figures 7A and 7B are side cross-sectional views, which schematically show the method for measuring the connection resistance value. As shown in Figure 7A, a printed circuit board 50' for measuring resistance is prepared. It consists of two unconnected printed circuits 52 formed on a base film 51, and a cover layer 53 covering the base film 51 and the printed circuits 52. In addition, an opening 53a with a diameter of 0.5 mm is formed in the cover layer 53, which exposes a portion of each printed circuit 52.

[0095] Next, as shown in FIG7B, the electromagnetic wave shielding film 110 is bonded to the printed wiring board 50' for measuring connection resistance value in such a manner that the conductive adhesive layer 20 of the electromagnetic wave shielding film 110 in each embodiment and comparative example contacts the cover layer 53, and is then pressed using a press at a temperature of 170°C, a time of 240 seconds, and a pressure of 3 MPa. In this process, the conductive adhesive layer 20 enters the opening 53a, contacts the printed circuit 52, and the printed circuits 52 can be electrically connected to each other through the conductive adhesive layer 20. Then, resistor 70 is used to measure the resistance value between printed circuits 52 (resistance value before high temperature and high humidity load). Then, the printed wiring board 50' with the electromagnetic wave shielding film 110 attached was placed in an environment with a temperature of 85°C, a humidity of 85%, and a time of 1000 hours. The resistance value between the printed circuits 52 (resistance value after high temperature and high humidity load) was measured using resistor 70 in the same way. The results are shown in Tables 1 and 2.

[0096] (Appearance observation after high temperature and high humidity load) After measuring the resistance value under high temperature and high humidity load, visual inspection is used to confirm whether the insulation layer of each electromagnetic wave shielding film has cracks. The evaluation criteria are as follows. The results are shown in Tables 1 and 2. ○: No cracks were found in the insulation layer. ×: Cracks appear in the insulation layer.

[0097] (Evaluation of shielding capabilities) The shielding performance of the electromagnetic wave shielding films in each embodiment and comparative example was evaluated using the KEC method, which uses an electromagnetic wave shielding effect measuring device developed by the KEC Kansai Electronics Industry Promotion Center. Figure 8 is a schematic diagram showing the structure of the system used in the KEC method. The system used in the KEC method consists of an electromagnetic wave shielding effect measuring device 80, a spectrum analyzer 91, an attenuator 92 for 10dB attenuation, an attenuator 93 for 3dB attenuation, and a preamplifier 94.

[0098] As shown in Figure 8, in the electromagnetic wave shielding effect measuring device 80, two measuring fixtures 83 are arranged facing each other. The electromagnetic wave shielding films (denoted by symbol 110 in Figure 8) of each embodiment and comparative example are clamped between the measuring fixtures 83. The measuring fixtures 83 adopt the size distribution of TEM cells (Transverse Electromagnetic Cells) and are formed in a structure that is symmetrically divided in a plane perpendicular to its transmission axis direction. However, in order to prevent short circuits from being formed due to the insertion of the electromagnetic wave shielding film 110, the flat plate-shaped center conductor 84 is arranged with a gap between it and each measuring fixture 83.

[0099] The KEC method first inputs the signal output from the spectrum analyzer 91 to the measuring fixture 83 on the transmitting side via attenuator 92. Then, the signal received by the measuring fixture 83 on the receiving side and amplified by the attenuator 93 is amplified by the preamplifier 94, and the signal level is measured by the spectrum analyzer 91. In addition, the spectrum analyzer 91 outputs the attenuation amount when the electromagnetic wave shielding film 110 is provided in the electromagnetic wave shielding effect measuring device 80, based on the state where the electromagnetic wave shielding effect measuring device 80 does not have the electromagnetic wave shielding film 110 provided.

[0100] Using this apparatus, under conditions of 25°C and 30-50% relative humidity, the electromagnetic wave shielding films of each embodiment and each comparative example were cut into 15cm squares, and their shielding performance was measured at 200MHz. The measurement results are shown in Tables 1 and 2.

[0101] As shown in Tables 1 and 2, it is clear that the electromagnetic wave shielding film of the present invention has excellent peel strength, connection reliability and shielding performance, and can also suppress the formation of cracks in the insulation layer.

[0102] 10,110: Electromagnetic wave shielding film 20: Conductive adhesive layer 21: Conductive particles 21a: High melting point conductive particles 21a 1: High melting point small flake particles 21a 2: High melting point spherical particles 21b: Low-melting-point conductive particles 22: Adhesive resin composition 30: Insulation layer 40: Metal layer 50: Printed Wiring Board 50': Printed wiring board for measuring resistance value 51: Basement membrane 52: Printed Circuits 52a: Grounding circuit 53: Covering layer 53a: Opening 60: Shielded Printed Wiring Board 70: Resistor 80: Electromagnetic wave shielding effect measuring device 83: Measuring fixture 84: Central Conductor 91: Spectrum Analyzer 92, 93: Attenuators 94: Preamplifier

Claims

1. An electromagnetic wave shielding film, characterized by comprising: a conductive adhesive layer comprising conductive particles and an adhesive resin composition; and an insulating layer deposited on the conductive adhesive layer; wherein the conductive particles are composed of high-melting-point conductive particles and low-melting-point conductive particles, the high-melting-point conductive particles comprising high-melting-point flake particles and high-melting-point spherical particles, and the content of the high-melting-point flake particles is 60-80 wt% relative to the total content of the high-melting-point flake particles, the high-melting-point spherical particles and the low-melting-point conductive particles.

2. The electromagnetic wave shielding film of claim 1, wherein a metal layer is provided between the aforementioned insulating layer and the aforementioned conductive adhesive layer.

3. The electromagnetic wave shielding film as requested in item 1 or 2, wherein the weight ratio of the aforementioned high-melting-point spherical particles to the aforementioned low-melting-point conductive particles is [high-melting-point spherical particles] / [low-melting-point conductive particles] = 0.5~29.

4. The electromagnetic wave shielding film of claim 1 or 2, wherein the aforementioned conductive adhesive layer further comprises a flux, and the content of the aforementioned flux in the aforementioned conductive adhesive layer is less than 4 wt%.

5. The electromagnetic wave shielding film as requested in item 1 or 2, wherein the melting point of the aforementioned low-melting-point conductive particles is 120~190°C.

6. The electromagnetic wave shielding film as requested in item 1 or 2, wherein the melting point of the aforementioned high-melting-point conductive particles is 300~1500°C.

7. A shielded printed circuit board, characterized in that it comprises: a printed circuit board having a base film, a printed circuit disposed on the base film, and a cover layer disposed to cover the printed circuit; and an electromagnetic wave shielding film having a conductive adhesive layer and an insulating layer deposited on the conductive adhesive layer, wherein the conductive adhesive layer comprises conductive particles and an adhesive resin composition; and wherein the electromagnetic wave shielding film is disposed on the printed circuit board such that the conductive adhesive layer is in contact with the cover layer; the conductive particles are composed of high-melting-point conductive particles and low-melting-point conductive particles, wherein the high-melting-point conductive particles include high-melting-point flake particles and high-melting-point spherical particles, and the content of the high-melting-point flake particles is 60-80 wt% relative to the total content of the high-melting-point flake particles, the high-melting-point spherical particles, and the low-melting-point conductive particles.

8. The shielded printed circuit board of claim 7, wherein a metal layer is provided between the aforementioned insulating layer and the aforementioned conductive adhesive layer.

9. The shielded printed circuit board of claim 7 or 8, wherein the printed circuit includes a grounding circuit, an opening is formed in the cover layer to expose the grounding circuit, and the conductive adhesive layer fills the opening and contacts the grounding circuit.

Images