Resin sheets for flexible flat cables and flexible flat cables

Abstract

A resin sheet for a flexible flat cable according to the present disclosure is layered between a plurality of conductors arranged in parallel and a shield layer layered on the outer surface side of the plane in which the plurality of conductors are arranged in parallel, and has one or more insulating layers. The resin sheet for a flexible flat cable comprises a base insulating layer that has a tensile elastic modulus of 40-450 MPa, a dielectric loss tangent of at most 0.0014, and a relative permittivity of at most 2.3 at 25°C and 10 GHz, and that contains an olefin-based thermoplastic elastomer as the main component thereof. The average thickness of the base insulating layer is 20-450 μm.

Description

Technical Field

[0001] The present disclosure relates to a resin sheet for a flexible flat cable and a flexible flat cable. This application claims priority based on Japanese Application No. 2021-170555 filed on October 18, 2021, and incorporates by reference all the descriptions set forth in the Japanese application.

Background Art

[0002] A multi-core flat flexible flat cable is used as an electric wire for internal wiring of electronic devices. This flexible flat cable is manufactured by sandwiching a plurality of strip-shaped conductors in parallel between two insulating resin sheets and integrating them by a pressure heating process such as a thermal lamination process.

[0003] Particularly, in digital devices and the like, a flexible flat cable is used to transmit digital signals. When transmitting digital signals, it is preferable to block external electromagnetic noise. Therefore, a flexible flat cable having a conductive shield layer laminated on the outer surface of the resin sheet is often used. Also, in order to accurately transmit high-frequency signals, it is necessary to improve the dielectric characteristics.

[0004] As a conventional flexible flat cable, it has been proposed to increase the characteristic impedance between the conductor and the shield layer by interposing a low dielectric constant layer mainly composed of polyolefin between an insulating resin sheet mainly composed of polyester and the shield layer (see Japanese Patent Application Laid-Open No. 2008-047505).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

[0006] The resin sheet for flexible flat cables of the present disclosure is laminated between a plurality of parallel conductors and a shield layer laminated on the outer surface side of the parallel plane of the plurality of conductors, and has one or more insulating layers, wherein the relative permittivity at 25°C and 10GHz is 2.3 or less, the dielectric loss tangent is 0.0014 or less, the tensile modulus is 40 MPa or more and 450 MPa or less, and comprises a base insulating layer mainly composed of an olefin-based thermoplastic elastomer, with an average thickness of 20 μm or more and 450 μm or less. [Brief explanation of the drawing]

[0007] [Figure 1] Figure 1 is a schematic cross-sectional view showing a resin sheet for a flexible flat cable according to one embodiment of the present disclosure. [Figure 2] Figure 2 is a schematic cross-sectional view of a flexible flat cable in a plane perpendicular to the longitudinal direction according to one embodiment of the present disclosure. [Figure 3] Figure 3 is a schematic exploded cross-sectional view of a flexible flat cable in a plane perpendicular to the longitudinal direction according to one embodiment of the present disclosure. [Figure 4] Figure 4 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of a flexible flat cable according to another embodiment of the present disclosure. [Figure 5] Figure 5 is a cross-sectional view of the flexible flat cable shown in Figure 2, along line AA. [Figure 6] Figure 6 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of a flexible flat cable according to another embodiment of the present disclosure. [Modes for carrying out the invention]

[0008] [Issues this disclosure aims to address] Since such flexible flat cables require flame retardancy, the resin sheet for flexible flat cables must contain a flame retardant. Therefore, in the configuration of the resin sheet for flexible flat cables disclosed in the prior art, the resin surrounding the conductor contains a flame retardant, making it difficult to sufficiently lower the dielectric constant. This can easily lead to dielectric loss when transmitting high-frequency signals. Furthermore, the inclusion of a flame retardant tends to reduce flexibility. Even without a flame retardant, if a polyolefin with good dielectric properties, such as polyethylene or polypropylene, is used in the low dielectric constant layer, the flexible flat cable becomes stiff and difficult to bend.

[0009] This disclosure is made based on the circumstances described above and aims to provide a resin sheet for flexible flat cables that has good dielectric properties as well as excellent flexibility and dimensional stability.

[0010] [Effects of this disclosure] According to this disclosure, it is possible to provide a resin sheet for flexible flat cables that has good dielectric properties as well as excellent flexibility and dimensional stability.

[0011] [Description of Embodiments in this Disclosure] First, the embodiments of this disclosure will be listed and described.

[0012] The resin sheet for flexible flat cables of the present disclosure is laminated between a plurality of parallel conductors and a shield layer laminated on the outer surface side of the parallel plane of the plurality of conductors, and has one or more insulating layers, wherein the relative permittivity at 25°C and 10GHz is 2.3 or less, the dielectric loss tangent is 0.0014 or less, the tensile modulus is 40 MPa or more and 450 MPa or less, and comprises a base insulating layer mainly composed of an olefin-based thermoplastic elastomer, with an average thickness of 20 μm or more and 450 μm or less.

[0013] The resin sheet for the flexible flat cable comprises a base insulating layer mainly composed of olefin-based thermoplastic elastomer (TPO), and the average thickness of the base insulating layer is 20 μm to 450 μm, thereby ensuring flexibility while reducing the relative permittivity and dielectric loss tangent. Furthermore, the resin sheet for the flexible flat cable has a relative permittivity of 2.3 or less, a dielectric loss tangent of 0.0014 or less, and a tensile modulus of elasticity of 40 MPa to 450 MPa at 25°C and 10 GHz, thus providing the flexible flat cable resin sheet with good dielectric properties as well as excellent flexibility and dimensional stability.

[0014] The aforementioned "relative permittivity" and "dielectric loss tangent" are values ​​measured under conditions of 25°C and a frequency of 10 GHz using the cavity resonator method in accordance with JIS-C-2138 (2007). "Parallel surface" refers to a surface among multiple conductors that is parallel to the direction in which these conductors are arranged. "Main component" refers to a component with a content of 50% by mass or more, preferably 90% by mass or more. "Average thickness" refers to the average value of the thickness at any 10 points. "Tensile modulus" is the complex modulus representing the relationship between tensile stress and strain. The tensile modulus was measured using a tensile testing machine in accordance with JIS-K-7161-1:2014 "Plastics - Determination of tensile properties - Part 1: General rules".

[0015] It is preferable that the olefin-based thermoplastic elastomer is a reactor olefin-based thermoplastic elastomer having a polypropylene block (hereinafter also referred to as reactor TPO). By the olefin-based thermoplastic elastomer being a reactor TPO having a polypropylene block, the relative permittivity and dielectric loss tangent can be further reduced, and the dielectric properties of the resin sheet for the flexible flat cable can be further improved.

[0016] A shield layer side insulating layer laminated on the surface of the base insulating layer on the shield layer side may be provided, and the tensile elastic modulus of the shield layer side insulating layer may be 400 MPa or more. In the resin sheet for flexible flat cable, by having the tensile elastic modulus of the shield layer side insulating layer as the outermost layer being 400 MPa or more, the handling property during manufacturing and transportation can be improved.

[0017] A conductor side insulating layer laminated on the surface of the base insulating layer on the conductor side may be provided, and the average thickness of the conductor side insulating layer may be 3 μm or more and 20 μm or less. By having the average thickness of the conductor side insulating layer within the above range, the adhesive force with the conductor and the transmission characteristics can be improved.

[0018] The flexible flat cable of the present disclosure includes a plurality of conductors arranged in parallel, a shield layer laminated on the outer surface side of the parallel surface of the plurality of conductors, and the resin sheet for flexible flat cable laminated between the parallel surface of the plurality of conductors and the shield layer, and the resin sheet for flexible flat cable is in contact with the surfaces of the plurality of conductors.

[0019] The flexible flat cable of the present disclosure has good dielectric characteristics and includes the resin sheet for flexible flat cable of the present disclosure that is excellent in flexibility and dimensional stability. Therefore, it has excellent dielectric characteristics, is flexible, and has excellent bending performance.

[0020] [Details of Embodiments of the Present Disclosure] Hereinafter, each embodiment of the resin sheet for flexible flat cable and the flexible flat cable according to the present disclosure will be described in detail while referring to the drawings. Note that each embodiment of the resin sheet for flexible flat cable and the flexible flat cable according to the present disclosure is not limited to the dimensions shown in the drawings.

[0021] [Resin Sheet for Flexible Flat Cable]< The resin sheet 5 for a flexible flat cable shown in Fig. 1 includes a shield layer side insulating layer 2 disposed on the shield layer side of the flexible flat cable, a base insulating layer 3 laminated on the inner surface side of the shield layer side insulating layer 2, and a conductor side insulating layer 4 laminated on the inner surface side of this base insulating layer 3 and disposed on the conductor side of the flexible flat cable.

[0022] Here, the "outer surface side" and the "inner surface side" mean that when provided in a flexible flat cable, the side closer to a plurality of conductors is the "inner surface side", and the opposite side is the "outer surface side".

[0023] The resin sheet 5 for a flexible flat cable is a layer for ensuring the withstand voltage property and dielectric characteristics of the flexible flat cable. The resin sheet 5 for a flexible flat cable electrically insulates between a plurality of flat conductors 10 and, when used in a high-frequency region, functions as a capacitor that forms an electrostatic coupling intervening between the flat conductors 10 and between the shield layer 12.

[0024] The resin sheet 5 for a flexible flat cable is also called a dielectric, and the dielectric tangent (tanδ) of the resin material constituting the resin sheet 5 for a flexible flat cable is a parameter that affects the transmission characteristics of the flexible flat cable. The upper limit of the dielectric tangent of the resin sheet 5 for a flexible flat cable is 0.0014 from the viewpoint of improving dielectric characteristics and reducing dielectric loss (insertion loss), and it may be 0.0010.

[0025] The upper limit of the relative dielectric constant of the resin sheet 5 for a flexible flat cable is 2.3 from the viewpoint of improving dielectric characteristics and reducing dielectric loss (insertion loss), and it may be 2.2.

[0026] The lower limit of the tensile modulus of elasticity in the resin sheet 5 for flexible flat cables is 40 MPa, or it may be 100 MPa. On the other hand, the upper limit of the tensile modulus of elasticity is 450 MPa, or it may be 400 MPa. By setting the tensile modulus of elasticity to be between 40 MPa and 450 MPa, the strength and dimensional stability of the resin sheet 5 for flexible flat cables can be improved in a balanced manner, and the flexibility can also be improved.

[0027] The resin sheet 5 for flexible flat cables does not contain flame retardants. When the resin sheet 5 for flexible flat cables is made of a resin material that does not contain flame retardants, the dielectric loss tangent becomes smaller, which in turn reduces dielectric loss, especially in high-frequency signals.

[0028] [Shielding layer side insulating layer] The shield layer-side insulating layer 2 contains a resin. The main component of the shield layer-side insulating layer 2 is, for example, polyolefin. Examples of polyolefins include homopolymers of olefins such as ethylene, propylene, butene, and hexene, copolymers of these monomers with each other, or copolymers of these monomers with non-olefin monomers. Specific examples of polyolefins include ethylene-based resins such as low-density polyethylene, linear polyethylene (ethylene-α-olefin copolymer), and high-density polyethylene; propylene-based resins such as polypropylene and ethylene-propylene copolymer; poly(4-methylpentene-1), poly(butene-1), ethylene-vinyl acetate copolymer; and acid-modified polyolefin resins obtained by maleic anhydride modification (treatment) of these. In particular, as the main component of the shield layer-side insulating layer 2, acid-modified polyolefin is preferred to improve the adhesion between the adhesive layer 13 and the base insulating layer 3, and acid-modified polypropylene is more preferred among these. The entire resin may consist only of the main component.

[0029] The lower limit of the tensile modulus of the insulating layer 2 on the shielding layer side is preferably 400 MPa, and may also be 420 MPa. On the other hand, the upper limit of the tensile modulus of the insulating layer 2 on the shielding layer side is preferably 2000 MPa, for example. By setting the tensile modulus to be between 400 MPa and 2000 MPa, the handling of the resin sheet 5 for flexible flat cables during manufacturing and transportation can be improved.

[0030] The lower limit of the average thickness of the insulating layer 2 on the shielding layer side may be 3 μm or 5 μm. On the other hand, the upper limit of the average thickness of the insulating layer 2 on the shielding layer side may be 20 μm or 15 μm. If the average thickness of the insulating layer 2 on the shielding layer side is less than 3 μm, it may not be easy to form a uniform layer, and the adhesion strength with the adhesive layer 13 may decrease. If the average thickness of the insulating layer 2 on the shielding layer side exceeds 20 μm, the transmission characteristics of the resin sheet 5 for the flexible flat cable may decrease.

[0031] [Base insulating layer] The base insulating layer 3 is mainly composed of an olefin-based thermoplastic elastomer. The olefin-based thermoplastic elastomer has a polyolefin as the hard segment and a rubber component such as olefin-based rubber as the soft segment. Examples of polyolefins include polypropylene (PP) and polyethylene (PE). Examples of olefin-based rubbers include ethylene-propylene rubber (EPM) and ethylene-propylene-diene ternary copolymer (EPDM). Examples of olefin-based thermoplastic elastomers include a blend type of polyolefin and olefin-based rubber component, a dynamic crosslinking type in which crosslinked rubber particles are finely dispersed in the polyolefin by vulcanizing the olefin-based rubber component when mixing the polyolefin and olefin-based rubber component, and reactor TPO, which is a polymerization type of polyolefin and olefin-based rubber. Among these, reactor TPO having a polypropylene block is preferred as the olefin-based thermoplastic elastomer. By using reactor TPO having a polypropylene block as the olefin-based thermoplastic elastomer, the relative permittivity and dielectric loss tangent can be further reduced, and the dielectric properties of the flexible flat cable resin sheet 5 can be further improved.

[0032] The melting point of the olefin-based thermoplastic elastomer may be between 100°C and 180°C. Having a melting point between 100°C and 180°C allows for use in high-temperature environments. The aforementioned "melting point" is a value measured in accordance with JIS-K7121 (1987).

[0033] The lower limit of the relative permittivity of the olefin-based thermoplastic elastomer is not particularly limited, but may be, for example, 2.0. The upper limit of the relative permittivity of the olefin-based thermoplastic elastomer may be 3.0. By having a relative permittivity of 2.0 to 3.0 of the olefin-based thermoplastic elastomer, dielectric loss can be reduced while maintaining strength.

[0034] The lower limit of the dielectric loss tangent of the olefin-based thermoplastic elastomer is not particularly limited, but may be, for example, 0.0001. The upper limit of the dielectric loss tangent of the olefin-based thermoplastic elastomer may be 0.001. By having a dielectric loss tangent of 0.0001 or more and 0.001 or less of the olefin-based thermoplastic elastomer, dielectric loss can be reduced while maintaining strength.

[0035] The lower limit of the average thickness of the base insulation layer 3 is 20 μm, but it may also be 30 μm. On the other hand, the upper limit of the average thickness of the base insulation layer 3 is 450 μm, but it may also be 350 μm, 300 μm, 280 μm, or 250 μm. If the average thickness of the base insulation layer 3 is less than 20 μm, the handling properties of the resin sheet 5 for flexible flat cables may be reduced. If the average thickness of the base insulation layer 3 exceeds 450 μm, the flexibility of the resin sheet 5 for flexible flat cables may be insufficient.

[0036] [Insulating layer on the conductor side] The conductor-side insulating layer 4 is mainly composed of resin. By including the conductor-side insulating layer 4 in the resin sheet 5 for the flexible flat cable, the adhesion to the conductor can be improved. As the main component resin of the conductor-side insulating layer 4, an olefin-based thermoplastic elastomer similar to that of the base insulating layer 3 described above can be used from the viewpoint of adhesion to the conductor, low dielectric constant, and cost.

[0037] The lower limit of the average thickness of the conductor-side insulating layer 4 may be 3 μm or 5 μm. On the other hand, the upper limit of the average thickness of the conductor-side insulating layer 4 may be 20 μm or 15 μm. If the average thickness of the conductor-side insulating layer 4 is less than 3 μm, the adhesion strength to the conductor may decrease. If the average thickness of the conductor-side insulating layer 4 exceeds 20 μm, the transmission characteristics of the flexible flat cable resin sheet 5 may decrease.

[0038] [Manufacturing method for resin sheets for flexible flat cables] The method for manufacturing the resin sheet 5 for flexible flat cables comprises the steps of preparing resin compositions for forming the shield layer side insulating layer 2, the base insulating layer 3, and the conductor side insulating layer 4, respectively, and molding sheets constituting the shield layer side insulating layer 2, the base insulating layer 3, and the conductor side insulating layer 4 using each resin composition.

[0039] (Resin composition preparation process) The resin compositions for forming the shield layer-side insulating layer 2, the base insulating layer 3, and the conductor-side insulating layer 4 can be prepared by kneading a composition containing resin and other optional components such as antioxidants, pigments, processing aids, and blocking inhibitors in a kneader. Examples of kneaders include open roll kneaders, kneaders, and twin-screw mixed extruders.

[0040] (Sheet forming process) The shield layer-side insulating layer 2, the base insulating layer 3, and the conductor-side insulating layer 4 can be formed by melt extrusion methods such as the T-die method or the inflation method. The shield layer-side insulating layer 2, the base insulating layer 3, and the conductor-side insulating layer 4 may be formed as separate, independent sheets, or they may be formed as a single three-layer sheet by co-extrusion.

[0041] (Thermocompression bonding process) A resin sheet 5 for flexible flat cables can be formed by integrating three sheets or one three-layer sheet formed in this manner by heat-pressing. Heat-pressing can be performed using, for example, a heating laminator equipped with heating rollers, a heating press, etc. The heating temperature is, for example, around 80°C to 200°C. In addition, the shield layer-side insulating layer 2 and the conductor-side insulating layer 4 may be formed by applying a solution to the base insulating layer 3 and drying it.

[0042] <Flexible Flat Cable> Figure 2 is a cross-sectional view (sectional plane) of the flexible flat cable according to this embodiment, perpendicular to the longitudinal direction. Figure 3 is a schematic exploded cross-sectional view of the flexible flat cable according to this embodiment, perpendicular to the longitudinal direction. The flexible flat cable according to this embodiment is a cable used for electrically connecting equipment or for internal wiring of equipment.

[0043] The flexible flat cable 100 shown in Figures 2 and 3 comprises a plurality of parallel rectangular conductors 10, a pair of flexible flat cable resin sheets 5, a pair of shielding layers 12 that are in contact with the outer surfaces of the pair of flexible flat cable resin sheets 5 via an adhesive layer 13, and a pair of covering sheets 40 that cover the outer surfaces of the pair of shielding layers 12.

[0044] The average thickness of the flexible flat cable 100 can be, for example, 100 μm or more and 900 μm or less.

[0045] [conductor] Multiple strip-shaped rectangular conductors 10 have a stripe-like pattern arranged parallel to each other. Multiple rectangular conductors 10 are made of a conductive metal such as copper, tin-plated soft copper, or nickel-plated soft copper. Multiple rectangular conductors 10 may also be formed from foil-shaped conductive metal. In cross-section, these rectangular conductors 10 are formed in a substantially flat rectangular shape. In this embodiment, the flexible flat cable 100 is composed of four rectangular conductors 10, but the number of rectangular conductors 10 is arbitrary. Furthermore, although the flexible flat cable 100 of this embodiment is equipped with multiple rectangular conductors 10, the cross-sectional shape of the conductors is not particularly limited.

[0046] The lower limit of the average thickness of the multiple rectangular conductors 10 may be 15 μm or 25 μm. On the other hand, the upper limit of the average thickness of the multiple rectangular conductors 10 may be 150 μm or 100 μm. If the average thickness of the multiple rectangular conductors 10 is less than 15 μm, the mechanical strength of the multiple rectangular conductors 10 may be insufficient and they may break. If the average thickness of the multiple rectangular conductors 10 exceeds 150 μm, the flexible flat cable 100 may become unnecessarily thick or its flexibility may be insufficient.

[0047] [Resin sheet for flexible flat cables] As shown in Figures 2 and 3, the resin sheet 5 for flexible flat cables is laminated between a plurality of parallel conductors 10 and a shield layer 12 laminated on the outer surface of the parallel plane of these conductors 10. In other words, a pair of resin sheets 5 for flexible flat cables are laminated between a plurality of parallel conductors 10 and a shield layer 12 laminated on the outer surface of both sides of the parallel plane of the plurality of rectangular conductors 10. The resin sheet 5 for flexible flat cables is a layer that ensures the pressure resistance and high-frequency characteristics of the flexible flat cable 100. Because the flexible flat cable 100 is equipped with a resin sheet 5 for flexible flat cables that has good dielectric properties as well as excellent flexibility and dimensional stability, it has excellent dielectric properties as well as flexibility and excellent bending performance. The structure of the resin sheet 5 for flexible flat cables is as described above, and redundant explanations are omitted.

[0048] In this embodiment, a pair of flexible flat cable resin sheets 5 are laminated on both sides of a plurality of rectangular conductors 10 so that the conductor-side insulating layer 4 shown in Figure 1 abuts against the plurality of rectangular conductors 10, and then heat-pressed together. Through this heat-pressing, the conductor-side insulating layers 4 of the two flexible flat cable resin sheets 5 are filled between the rectangular conductors 10, welded together and integrated. "Filled between the rectangular conductors 10" means that the insulating layer of the flexible flat cable resin sheet 5 exists in the space between the patterns of the rectangular conductors 10. As a result, the flexible flat cable resin sheet 5 and the surfaces of the plurality of rectangular conductors 10 are in contact. In other words, the plurality of rectangular conductors 10 are covered by the pair of flexible flat cable resin sheets 5. Furthermore, the pair of flexible flat cable resin sheets 5 may be identical, or the material and thickness of each layer may differ from one another.

[0049] [Shield layer] The pair of shield layers 12 are layers that have a shielding function for noise reduction and ensuring high-frequency characteristics of the flexible flat cable 100, and are formed from metal foil such as copper foil or aluminum foil. Between each flexible flat cable resin sheet 5 and each shield layer 12, an adhesive layer 13 is provided for bonding the flexible flat cable resin sheet 5 and the shield layer 12. As the adhesive layer 13, olefin-based adhesives such as ethylene-vinyl acetate copolymer (EVA), ethylene-ethyl acrylate copolymer (EEA), maleic acid-modified polyethylene, and maleic acid-modified polypropylene can be used.

[0050] A pair of shield layers 12 are laminated on the surface of an adhesive layer 13 located on the outer side of the parallel plane of the rectangular conductors 10. In this embodiment, each of the pair of shield layers 12 is laminated on the flexible flat cable resin sheet 5 via the adhesive layer 13 such that both ends in the parallel direction of the plurality of rectangular conductors 10 (hereinafter also referred to as the conductor parallel direction) substantially coincide with both ends of the flexible flat cable resin sheet 5 in the conductor parallel direction. The shield layers may be laminated to cover the entire circumference of the flexible flat cable resin sheet. Figure 4 is a schematic cross-sectional view of a flexible flat cable with a modified shield layer. As shown in Figure 4, in the flexible flat cable 150, a pair of shield layers 22 are laminated via an adhesive layer 23 to cover the entire circumference of the flexible flat cable resin sheet 5. In this way, by providing the shield layers 22 in the flexible flat cable 150, the noise immunity and high-frequency characteristics of the flexible flat cable 150 can be maintained well.

[0051] [Resin sheet] As shown in Figure 2, the pair of covering sheets 40 consist of a base layer 42, a flame-retardant insulating layer 44, and an anchor coat layer 46. The base layer 42 is a layer for ensuring the pressure resistance of the flexible flat cable 100 and is made of, for example, polyethylene terephthalate. The flame-retardant insulating layer 44 is a layer for bonding the resin sheet 5 or shield layer 12 for the flexible flat cable 100 to the base layer 42 while ensuring the flame retardancy, pressure resistance, and degradation resistance of the flexible flat cable 100, and is made of, for example, a thermoplastic resin material. For example, a thermoplastic polyester resin containing a phosphorus-based flame retardant or a nitrogen-based flame retardant can be used as the flame-retardant insulating layer 44. Between the base layer 42 and the flame-retardant insulating layer 44, an anchor coat layer 46 is provided for bonding the base layer 42 and the flame-retardant insulating layer 44. Any material can be used as the anchor coat layer 46, but for example, a urethane-based anchor coat material can be used, which is made by mixing polyurethane, the main component, with an isocyanate-based curing agent.

[0052] The pair of covering sheets 40 cover the outer surface of the resin sheet 5 for the flexible flat cable and the shield layer 12, as well as the portion of the resin sheet 5 where the shield layer 12 is not attached. In addition, the width dimension of each covering sheet 40 along the parallel direction of the conductors is wider than the width dimension of the resin sheet 5 for the flexible flat cable and the shield layer 12. That is, both ends of the covering sheet 40 in the parallel direction of the conductors (hereinafter also referred to as both ends) extend outward beyond both ends of the resin sheet 5 for the flexible flat cable and the shield layer 12. The entire surface of both ends of the resin sheet 5 for the flexible flat cable and the shield layer 12 is covered by this extended pair of covering sheets 40. Furthermore, both ends of the base material layer 42 of the pair of covering sheets 40 are bonded to each other via a flame-retardant insulating layer 44 and an anchor coat layer 46. In this way, because the pair of covering sheets 40 are bonded to each other at both ends in the parallel direction of the conductors, it is possible to prevent the ends of the covering sheets 40 from peeling off.

[0053] Figure 5 is a longitudinal cross-sectional view of the AA line of the flexible flat cable 100. As shown in Figure 5, a pair of covering sheets 40 are bonded to the outer surface of a pair of shielding layers 12. In the flexible flat cable 100, the rectangular conductors 10 are exposed at both ends in the longitudinal direction (not shown) and are directly inserted into and connected to a connecting member (not shown).

[0054] [Manufacturing method for flexible flat cables] As an example of a manufacturing method for a flexible flat cable, in the manufacturing method of the flexible flat cable 100 shown in Figure 2, it is preferable to pre-bond the resin sheet 5 for flexible flat cables and the shield layer 12 via an adhesive layer 13. The adhesive layer 13 and the shield layer 12 can be bonded to the resin sheet 5 for flexible flat cables by heat compression. First, the resin sheets 5 for flexible flat cables, with the shield layer 12 bonded via the adhesive layer 13, are placed on both sides of the parallel surfaces of the rectangular conductors 10. Next, a pair of laminate rollers are used to press the pair of resin sheets 5 for flexible flat cables, with the shield layer 12 bonded to them, sandwiching the rectangular conductors 10 which are arranged in parallel at a predetermined interval. Then, the resin sheets 5 for flexible flat cables are bonded to each other by heat compression. By heat compression, the resin sheets 5 for flexible flat cables are filled between the multiple rectangular conductors 10, and the front and back sides of the resin sheets 5 for flexible flat cables are welded to each other. This integrates the resin sheet 5 for the flexible flat cable on the front side and the resin sheet 5 for the flexible flat cable on the back side with multiple rectangular conductors 10. The heating temperature in the heat-sealing process is, for example, around 80°C to 200°C.

[0055] Next, the covering sheet 40 is placed between a pair of laminating rollers that are pressing against each other, with a predetermined gap between them, on both the outer sides of the upper and lower shield layers 12. Then, the pair of covering sheets 40 are pressed together by the pair of laminating rollers, sandwiching the shield layers 12 between them, and the covering sheets 40 and the shield layers 12 are bonded together to produce a flexible flat cable 100.

[0056] As described above, the flexible flat cable 100 comprises a plurality of parallel rectangular conductors 10, a pair of flexible flat cable resin sheets 5 laminated on both sides of the parallel surfaces of the plurality of rectangular conductors 10, a pair of shielding layers 12 in contact with the outer surfaces of the pair of flexible flat cable resin sheets 5 via an adhesive layer 13, and a pair of covering sheets 40 covering the outer surfaces of the pair of shielding layers 12. The relative permittivity of the pair of flexible flat cable resin sheets 5 at 25°C and 10GHz is 2.3 or less, the dielectric loss tangent is 0.0014 or less, and the tensile modulus is 40 MPa or more and 450 MPa or less. Because the flexible flat cable 100 has good dielectric properties and is equipped with flexible flat cable resin sheets 5 that are excellent in flexibility and dimensional stability, it has excellent dielectric properties and is flexible and has excellent bending performance.

[0057] [Other embodiments] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of this disclosure is not limited to the configurations of the embodiments described herein, but is indicated by the claims, and all modifications within the meaning and scope equivalent to the claims are intended to be included.

[0058] In the flexible flat cable of the above embodiment, a rectangular conductor with a substantially flattened cross-section was used as the conductor, but the cross-sectional shape of the conductor is not particularly limited, and a round conductor with a circular cross-section may also be used. For example, the flexible flat cable 200 shown in Figure 6 comprises a plurality of round conductors 20 arranged in parallel, a pair of resin sheets 5 for flexible flat cables, a pair of shielding layers 12 that are in contact with the outer surface of the pair of resin sheets 5 for flexible flat cables via an adhesive layer 13, and a pair of covering sheets 40 that cover the outer surface of the pair of shielding layers 12.

[0059] The resin sheet for the flexible flat cable in the above embodiment had three layers: a shield layer-side insulating layer, a base insulating layer, and a conductor-side insulating layer. However, it may also have a configuration without a shield layer-side insulating layer or a conductor-side insulating layer. Furthermore, the resin sheet for the flexible flat cable may have a single-layer structure consisting only of a base insulating layer.

[0060] The resin sheet for the flexible flat cable described above can also be formed by dissolving the resin compositions constituting the shield layer-side insulating layer, the base insulating layer, and the conductor-side insulating layer in a solvent, applying them sequentially to the inner surface of the adhesive layer, and drying them. [Examples]

[0061] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0062] <Resin sheets for flexible flat cables No. 1 to No. 15> Resin sheets for flexible flat cables, numbered No. 1 to No. 15, were formed using the following procedure.

[0063] First, the materials listed in Table 1 were used to form the shield layer-side insulating layer, base insulating layer, and conductor-side insulating layer of the flexible flat cable resin sheets No. 1 to No. 15. The melting point, relative permittivity, and dielectric loss tangent of each material are shown in Table 1. For reactor TPO(1) to (3), reactor TPO containing polypropylene blocks was used. For TPO other than reactor TPO, a dynamic crosslinking type was used. Also, PP(1) is random polypropylene, and PP(2) is homopolypropylene.

[0064] In each of the aforementioned resin components, 0.1 parts by mass of an antioxidant and 0.1 parts by mass of a copper damage inhibitor were added to 100 parts by mass of each resin component. As the antioxidant, 3,9-bis[2-{3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy}-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[5,5]undecane, which has a semi-hindered phenol structure, was used. As the copper damage inhibitor, decamethylenedicarboxylic acid disalithyroylhydrazide was used.

[0065] The shield layer-side insulating layer, base insulating layer, and conductor-side insulating layer were simultaneously formed by co-extrusion using a multilayer T-die, while adjusting the extrusion amount of each material to achieve the average thickness shown in Table 1. Then, by producing three-layer sheets No. 1 to No. 4, No. 8, and No. 11 to No. 15, or two-layer sheets No. 5 to No. 7, No. 9, and No. 10, which were formed by integrating these layers, resin sheets No. 1 to No. 15 for flexible flat cables were obtained.

[0066] [Evaluation of resin sheets for flexible flat cables] The tensile modulus, relative permittivity, and dielectric loss tangent of the obtained resin sheets for flexible flat cables, No. 1 to No. 15, were evaluated. The evaluation method is shown below. The results for each evaluation are shown in Table 1.

[0067] (Tensile modulus of elasticity) The tensile modulus was measured using a tensile testing machine in accordance with JIS-K-7161-1:2014, "Plastics - Determination of tensile properties - Part 1: General rules".

[0068] (Relative permittivity and dielectric loss tangent) The relative permittivity and dielectric loss tangent at 25°C and 10GHz were measured using a cavity resonator manufactured by AET, Inc., and the cavity resonator path method.

[0069] (Flexibility) Based on the measured tensile modulus, the resin sheets for flexible flat cables were evaluated on a three-point scale: A, B, and C. The criteria for evaluating flexibility were as follows: an A or B rating was considered acceptable. A: The tensile modulus is less than 300 MPa. B: The tensile modulus is between 300 MPa and 450 MPa. C: The tensile modulus is greater than 450 MPa.

[0070] (Heat deformation resistance) The residual thermal deformation was measured by thermomechanical analysis (TMA) in accordance with JIS-K7197 (1991). A test indenter with a diameter of 0.5 mmΦ and a load of 10 g was used for the measurement. The evaluation criteria for thermal deformation resistance were as follows. An evaluation of A or B was considered acceptable. A: The thermal deformation retention rate at 100°C is 60% or more. B: The thermal deformation retention rate at 100°C is 40% or more but less than 60%. C: The thermal deformation retention rate at 100℃ is less than 40%.

[0071] [Table 1]

[0072] <Flexible Flat Cables No. 16 to No. 30> Flexible flat cables No. 16 to No. 30 were manufactured using the following procedure. As shown in Table 2, an insulating layer was formed using one of the resin sheets No. 1 to No. 15 for flexible flat cables, and flexible flat cables No. 16 to No. 30 were manufactured.

[0073] Twenty flat rectangular conductors, 35 μm thick and 0.3 mm wide, were used as the conductors. The resin sheets for flexible flat cables, as shown in Table 2, were placed on both sides of the 20 flat rectangular conductors, spaced apart and in contact with each conductor, and then heat-pressed. A heat roller with a temperature of 140°C to 160°C was used for heat pressing. This heat pressing softened the conductor-side insulating layers of the resin sheets for flexible flat cables on both sides, filling the gaps between the 20 flat rectangular conductors and joining them together.

[0074] Next, the adhesive layer and the shielding layer were laminated onto the flexible flat cable resin sheet by heat-pressing at 120°C. For the shielding layer, soft aluminum with an average thickness of 10 μm was used. For the adhesive layer, EVA with an average thickness of 5 μm was used.

[0075] Flexible flat cables No. 16 to No. 30 were obtained by covering the material with a three-layer coating sheet consisting of a base layer, a flame-retardant insulating layer, and an anchor coat layer, and then bonding the adhesive layer and shield layer together. A polyethylene terephthalate film with an average thickness of 12 μm was used as the base layer. A resin layer with an average thickness of 30 μm was laminated as the flame-retardant insulating layer, by adding aluminum phosphinate and melamine cyanurate to copolymerized polyester resin. A resin layer with an average thickness of 3 μm was laminated as the anchor coat layer, by adding an isocyanate-based curing agent to polyurethane.

[0076] [Evaluation of Flexible Flat Cables] The conductor adhesion strength and bending performance of the obtained flexible flat cables No. 16 to No. 30 were evaluated. The evaluation method is shown below. The results for each evaluation are shown in Table 2.

[0077] (Conductor adhesion due to the insulating layer on the conductor side) The conductor adhesion strength by the conductor-side insulating layer was measured using the following procedure. An opening was made in either the front or back side of the resin sheet for flexible flat cables to expose the conductor, and flexible flat cables No. 16 to No. 30 were manufactured using the method described above. The conductor adhesion strength was then measured by a 180° peel test, in which the conductor at the opening was peeled off in a 180° direction. The 180° peel test was performed in accordance with JIS-K6854-2 (1999). The conductor adhesion strength values ​​(N / cm) in the 180° peel test listed in Table 2 are the values ​​obtained by dividing the test value by the width of the test piece. A flexible flat cable with a conductor adhesion strength value of 0 is a type of flexible flat cable that prioritizes ease of peeling of the insulating layer and does not have adhesion between the conductor and the insulating layer.

[0078] (Bending performance) After folding a flexible flat cable in half, the radius of curvature of the bend was measured. Based on the radius of curvature, the bending performance of the flexible flat cable was evaluated on a three-level scale: A, B, and C. The evaluation criteria for the bending performance were as follows: An evaluation of A or B was considered a passing grade. A: The radius of curvature is less than 2.5 mm. B: The radius of curvature is 2.5 mm or more and less than 3.5 mm. C: The radius of curvature is 3.5 mm or larger.

[0079] (Pitch accuracy of rectangular conductors) The accuracy of the pitch (distance between each rectangular conductor) of the 20 rectangular conductors placed between the resin sheets for the flexible flat cable after heat-sealing was evaluated. Based on the product standard of ±0.05 mm, the pitch accuracy of the rectangular conductors was evaluated in two stages, A and B. An evaluation of A was considered acceptable. A: The pitch accuracy of the rectangular conductor is less than ±0.05 mm. B: The pitch accuracy of the rectangular conductor is ±0.05 mm or more.

[0080] [Table 2]

[0081] As shown in Table 1, the resin sheets for flexible flat cables No. 1 to No. 6, No. 9, No. 10 and No. 12 to No. 15, which have a base insulating layer mainly composed of an olefin-based thermoplastic elastomer, an average thickness of the base insulating layer of 20 μm to 450 μm, a relative permittivity of 2.3 or less, a dielectric loss tangent of 0.0014 or less, and a tensile modulus of 40 MPa to 450 MPa at 25°C and 10 GHz, exhibited good dielectric properties, flexibility, and dimensional stability based on heat deformation resistance. On the other hand, the resin sheets for flexible flat cables No. 7 and No. 8, which do not contain an olefin-based thermoplastic elastomer in the base insulating layer and have a tensile modulus of over 450 MPa, exhibited good dielectric properties but poor flexibility. Furthermore, No. 11, with a tensile modulus of less than 40 MPa, exhibited poor dimensional stability based on heat deformation resistance.

[0082] As shown in Table 2, flexible flat cables No. 16-21, No. 24, No. 25, and No. 27-30, equipped with resin sheets for flexible flat cables No. 1-6, No. 9, No. 10, and No. 12-15, exhibited good bending performance and dimensional stability based on the pitch accuracy of the rectangular conductors. On the other hand, flexible flat cables No. 22 and No. 23, equipped with resin sheets for flexible flat cables No. 7 and No. 8, had poor bending performance. Flexible flat cable No. 26, equipped with resin sheet for flexible flat cables No. 11, had poor dimensional stability based on the pitch accuracy of the rectangular conductors. Furthermore, flexible flat cables No. 16-19 and No. 27-30, which are equipped with resin sheets for flexible flat cables No. 1-4 and No. 12-15, and which have a base insulating layer mainly composed of olefin-based thermoplastic elastomer and a conductor-side insulating layer, also exhibited good conductor adhesion.

[0083] The results above demonstrate that the flexible flat cable of this disclosure, comprising the resin sheet for flexible flat cables of this disclosure, has good dielectric properties and dimensional stability, as well as flexibility and excellent bending performance. [Explanation of symbols]

[0084] 2. Shielding layer side insulating layer 3. Base insulating layer 4. Conductor-side insulating layer 5. Resin sheet for flexible flat cables 10 Rectangular conductors 12, 22 Shield layer 13, 23 Adhesive layer 20 Round conductors 40 Covering sheet 42 Base material layer 44 Flame-retardant insulating layer 46 Anchor coat layer 100, 150, 200 Flexible Flat Cables

Claims

1. A resin sheet for a flexible flat cable having one or more insulating layers, which is laminated between a plurality of parallel conductors and a shield layer laminated on the outer surface side of the parallel plane of the plurality of conductors, The relative permittivity at 25°C and 10 GHz is 2.3 or less, and the dielectric loss tangent is 0.0014 or less. The tensile modulus is 40 MPa or more and 450 MPa or less. It comprises a base insulating layer mainly composed of olefin-based thermoplastic elastomer, A resin sheet for flexible flat cables, wherein the average thickness of the base insulating layer is 20 μm or more and 450 μm or less.

2. The resin sheet for a flexible flat cable according to claim 1, wherein the olefin-based thermoplastic elastomer is a reactor olefin-based thermoplastic elastomer having a polypropylene block.

3. The base insulating layer further comprises a shield layer-side insulating layer laminated on the surface of the shield layer-side of the base insulating layer, The resin sheet for flexible flat cables according to claim 1 or claim 2, wherein the tensile modulus of the insulating layer on the shield layer side is 400 MPa or more.

4. The base insulating layer further comprises a conductor-side insulating layer laminated on the conductor-side surface of the base insulating layer, The resin sheet for a flexible flat cable according to claim 1 or claim 2, wherein the average thickness of the conductor-side insulating layer is 3 μm or more and 20 μm or less.

5. Multiple conductors arranged in parallel, A shield layer is laminated on the outer surface side of the parallel plane of the plurality of conductors, The flexible flat cable comprises a resin sheet for flexible flat cables according to claim 1 or 2, which is laminated between the parallel surfaces of the plurality of conductors and the shield layer, A flexible flat cable in which the resin sheet for the flexible flat cable and the surfaces of the plurality of conductors are in contact.

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