Flexible flat cable

Abstract

To provide a flexible flat cable enabling compatibility of noise resistance and flexibility.SOLUTION: A flexible flat cable 1 comprises: a plurality of wires 2 each having a conductor 21 and an insulating layer 22 provided around the conductor, the plurality of wires being arranged in parallel with each other; a pair of base films 3 holding the plurality of wires inside and exhibiting electrical insulation; and a metallic shield layer 4 interposed between the pair of base films and between the plurality of wires and each base film. An outer surface of each wire is entirely joined to and covered with the shield layer within the pair of base films.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a flexible flat cable. [Background technology]

[0002] Flexible flat cables (FFCs) typically consist of multiple thin, elongated, rectangular metal conductors attached to a resin film such as polyethylene terephthalate (PET), with a resin film fused to cover the conductors. Because of this structure, FFCs offer advantages over coaxial transmission lines and twisted-pair cables: they are thin and flexible. FFCs are often used for connecting boards within devices and in narrow spaces. However, FFCs are known to be structurally more vulnerable to external noise than coaxial cables and twisted-pair cables. Coaxial cables have an outer conductor, which prevents noise from entering the metal conductor, while twisted-pair cables have an inherent noise-canceling function. In contrast, FFCs are structurally vulnerable to noise.

[0003] Therefore, in order to add noise resistance to the FFC, Patent Document 1 discloses a shielded flat cable in which a metal shielding layer is provided on the outside of the FFC. Specifically, the shielded flat cable in Patent Document 1 is a shielded flat cable that includes multiple flat conductors arranged in parallel, a pair of insulating layers bonded to both parallel surfaces of the multiple flat conductors, and a shielding layer covering at least one of the insulating layers, and has a cable termination formed at an end in the longitudinal direction of one of the insulating layers where the flat conductors are exposed. Furthermore, a ground conductor is provided on the outer periphery of one of the insulating layers on the cable termination side, and the ground conductor is electrically connected to a predetermined flat conductor and the shielding layer.

[0004] As mentioned above, coaxial cables have long been used as transmission lines with shielding effects, but they have the problem of being thick and lacking in flexibility. The reason for the lack of flexibility in coaxial cables is the thick dielectric layer. Patent Document 2 discloses a shielded cable that includes at least a center conductor, an insulating layer surrounding the center conductor, and a shield surrounding the insulating layer, the insulating layer being made of ABS resin. Patent Document 2 further discloses that the shield is made of a metal-plated layer with a thickness of 4 μm or less. By forming the insulating layer from ABS resin and thin-film-plated the outer conductor, the cable can be made thin and yet bend-resistant. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 7040265 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-221991 Summary of the Invention [Problem to be solved by the invention]

[0006] Covering a conductor with a shielding layer, as in Patent Documents 1 and 2, is effective against external noise. However, the structure described in Patent Document 1 cannot prevent interference between adjacent conductors. To suppress such interference, a pseudo-coaxial line structure can be created by providing a ground potential line (shielded wire) around the conductor and, together with the shielding layer, surrounding the conductor at ground potential. However, in such a structure, adjacent conductors and shielded wires must be short-circuited at regular intervals (less than 1 / 4 wavelength of the highest frequency) to achieve the same ground potential. Otherwise, they may become electrically floating (floating) and become a new source of noise. To avoid this problem, multiple contact structures are required to connect the conductor and shielded wire. However, the use of metal foil and conductive adhesive layers for this contact increases costs and increases the risk of unstable contact.

[0007] In addition, in Patent Document 2, ABS resin is used as the insulator and the thickness of the outer conductor is made thin to ensure flexibility. However, if the plating layer is made thin to ensure flexibility, there is a possibility that the shielding performance will decrease.

[0008] The present invention has been made in view of the problems inherent in the prior art, and an object of the present invention is to provide a flexible flat cable that is both noise-resistant and flexible. [Means for solving the problem]

[0009] A flexible flat cable according to one aspect of the present invention comprises a plurality of electric wires arranged in parallel with each other, each having a conductor and an insulating layer disposed around the conductor; a pair of electrically insulating base films that sandwich the plurality of electric wires therein; and a metallic shielding layer interposed between the pair of base films and between the plurality of electric wires and each base film, wherein, within the pair of base films, the entire outer surface of each electric wire is joined to and covered by the shielding layer. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a flexible flat cable that is both noise-resistant and flexible. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a plan view illustrating an example of a flexible flat cable according to an embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view schematically showing a cross section taken along line II-II in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0012] The flexible flat cable according to the present embodiment will be described in detail below with reference to the drawings. Note that the dimensional proportions in the drawings are exaggerated for the sake of convenience and may differ from the actual proportions.

[0013] As shown in FIGS. 1 and 2, the flexible flat cable 1 of this embodiment includes a plurality of electric wires 2, a pair of base films 3 that sandwich the electric wires 2 therebetween, and a metallic shielding layer 4.

[0014] The flexible flat cable 1 includes a plurality of electric wires 2, which are arranged in parallel to one another along the X-axis direction. Each electric wire 2 includes a conductor 21 and an insulating layer 22 provided around the conductor 21. The conductor 21 of the electric wire 2 may be a single wire formed of a single element wire, as shown in FIG. 2, or a twisted wire formed by twisting together a plurality of element wires. Furthermore, the insulating layer 22 of the electric wire 2 may have a single-layer structure or a multi-layer structure formed of a plurality of layers, as long as electrical insulation can be ensured.

[0015] The material of the conductor 21 is not particularly limited as long as it is a conductive metal, and at least one selected from the group consisting of copper, copper alloy, aluminum, and aluminum alloy can be used. The surface of the conductor 21 may be plated. Examples of such plating include gold plating, silver plating, tin plating, and nickel plating.

[0016] The material constituting the insulating layer 22 is not particularly limited as long as it can ensure electrical insulation. Examples of resin materials that can be used to constitute the insulating layer 22 include vinyl chloride, heat-resistant vinyl chloride, cross-linked vinyl chloride, polyethylene, cross-linked polyethylene, foamed polyethylene, cross-linked foamed polyethylene, chlorinated polyethylene, polypropylene, polyamide, polyvinylidene fluoride, ethylene-tetrafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene, perfluoroalkoxyalkane, natural rubber, chloroprene rubber, butyl rubber, ethylene-propylene rubber, chlorosulfonated polyethylene rubber, and silicone rubber. These materials may be used alone or in combination of two or more.

[0017] The base film 3 is composed of a first film 31 and a second film 32, and the first film 31 and the second film 32 are each plate-shaped. The first film 31 and the second film 32 are stacked along the Z-axis direction, thereby sandwiching and holding multiple electric wires 2 inside. The thickness of the base film 3 is not particularly limited, but in order to sandwich and hold multiple electric wires 2 inside, it is preferable that the total thickness of the first film 31 and the second film 32 be larger than the diameter of the electric wires 2.

[0018] The material constituting the base film 3 is not particularly limited as long as it is a resin and has flexibility. The material constituting the base film 3 can be at least one selected from the group consisting of polyethylene terephthalate resin (PET), polybutylene terephthalate resin (PBT), fluororesin, polyvinyl chloride resin (PVC), polyphenylene sulfide resin (PPS), polyethylene resin (PE), and polypropylene resin (PP).

[0019] The first film 31 has recesses 312 formed on an inner surface 311 facing the plurality of electric wires 2, in which portions of the electric wires 2 are accommodated. The first film 31 also has the plurality of recesses 312 formed in parallel along the X-axis direction. Each recess 312 is formed from one end to the other end of the first film 31 along the Y-axis direction. Each recess 312 has a substantially semicircular cross section that follows the shape of the outer surface of the electric wires 2.

[0020] Similar to the first film 31, the second film 32 also has recesses 322 formed on an inner surface 321 facing the plurality of electric wires 2, in which recesses 322 are formed to accommodate portions of the electric wires 2. The second film 32 also has a plurality of recesses 322 formed in parallel along the X-axis direction. Each recess 322 is formed from one end to the other end of the second film 32 along the Y-axis direction. Each recess 322 has a substantially semicircular cross section that follows the shape of the outer surface of the electric wires 2.

[0021] When the first film 31 and the second film 32 are stacked along the Z-axis direction, a substantially cylindrical internal space is formed by the recess 312 of the first film 31 and the recess 322 of the second film 32, which face each other. A part of the electric wire 2 fits into the recess 312 of the first film 31, and another part of the electric wire 2 fits into the recess 322 of the second film 32, so that the electric wire 2 is accommodated and fixed in the substantially cylindrical internal space. Note that in the base film 3, a plurality of internal spaces are formed by the recess 312 of the first film 31 and the recess 322 of the second film 32, and one electric wire 2 is accommodated in each internal space.

[0022] In order to maintain impedance matching while having a planar structure, the flexible flat cable 1 is provided with a shielding layer 4 interposed between a pair of base films 3 and between the multiple electric wires 2 and each base film 3. The shielding layer 4 is made up of a first shield 41 and a second shield 42, each of which is thin-film shaped.

[0023] The shielding layer 4 can be a metal foil or a metal plating layer. The metal plating layer can be formed by electroless plating alone, or by a combination of electroless plating and electroplating. The material constituting the shielding layer 4 is not particularly limited as long as it is a metal and can prevent noise from entering the conductor 21. The material constituting the shielding layer 4 can be at least one selected from the group consisting of copper, aluminum, nickel, iron, gold, and silver.

[0024] The first shield 41 is bonded to the inner surface 311 of the first film 31, and the second shield 42 is bonded to the inner surface 321 of the second film 32. The first shield 41 is also bonded to the recess 312 of the first film 31, and the second shield 42 is also bonded to the recess 322 of the second film 32.

[0025] By stacking the first shield 41 and the second shield 42 in this manner along the Z-axis direction, the outer surface of each electric wire 2 is joined to the first shield 41 and the second shield 42. The entire outer surface of each electric wire 2 is covered with the first shield 41 and the second shield 42. At this time, the first shield 41 is interposed between the recess 312 of the first film 31 and the outer surface of the electric wire 2, and the second shield 42 is interposed between the recess 322 of the second film 32 and the outer surface of the electric wire 2.

[0026] 2, the first shield 41 and the second shield 42 are joined to each other between adjacent electric wires 2. That is, as shown in FIG. 2, the plurality of electric wires 2 are arranged in parallel along the X-axis direction, and the first shield 41 and the second shield 42 are joined to each other between adjacent electric wires 2. The first shield 41 and the second shield 42 are also joined to each other in the X-axis direction between the side surface 313 of the first film 31 and the side surface 323 of the second film 32 and the electric wires 2 adjacent to the side surface 313 and the side surface 323.

[0027] The first film 31 of the base film 3 and the first shield 41 of the shielding layer 4 may be bonded to each other with an adhesive. Similarly, the second film 32 of the base film 3 and the second shield 42 of the shielding layer 4 may be bonded to each other with an adhesive. The adhesive is not particularly limited, and various resin-based adhesives such as epoxy resin, polyimide, polyester, phenolic resin, polyurethane, acrylic resin, melamine resin, polyamideimide, and polyolefin may be used. The first shield 41 and the second shield 42 of the shielding layer 4 may also be bonded to each other with an adhesive, for example, a conductive adhesive.

[0028] As described above, the flexible flat cable 1 of this embodiment has a structure in which the outer surface of each electric wire 2 is joined to the shielding layer 4 inside the pair of base films 3, and the entire outer surface of each electric wire 2 is further covered with the shielding layer 4. The shielding layer 4 serves as a ground potential that serves as a reference for signals transmitted through the electric wires 2.

[0029] Unlike the structure of Patent Document 1, in which a shielding film is attached to the exterior, the flexible flat cable 1 of this embodiment shields each electric wire 2 with a shielding layer 4, thereby reducing crosstalk. Even if the flexible flat cable 1 is divided into individual electric wires 2, the outer surface of each electric wire 2 is covered with a shielding layer 4, so the shielding performance can be maintained. This allows for use in connecting some electric wires 2 to other boards, similar to flexible printed circuit boards (FPCs). Furthermore, if additional noise resistance is required, a metal film shield can be added to the outside of the pair of base films 3. Furthermore, unlike a structure in which coaxial cables are simply arranged, the flexible flat cable 1 has a shielding layer 4 that is not a braided wire, which suppresses degradation of transmission characteristics due to a periodic structure.

[0030] Conventional coaxial cables have a structure in which an inner conductor is surrounded by a tubular dielectric (insulator), which is further surrounded by a tubular outer conductor (shield), and the outer conductor may be metal-plated. Dielectrics are used that emphasize dielectric properties, but depending on the material, plating adhesion may be poor, so the outer surface of the dielectric may be surface-treated (roughened). For example, in the shielded cable disclosed in Patent Document 2, the surface of the insulator layer is roughened to improve surface wettability and increase the adhesion of the metal-plated layer (shield) to the insulator layer.

[0031] It is generally known that in signal transmission, current flows on the surface of the conductor (skin effect), and the equation that expresses the depth d of the surface where this current flows is as follows:

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[0032] From Equation 1, we can see that the higher the frequency of the current, the shallower the depth d of the surface through which the current flows. Therefore, for high-frequency signals, the surface roughness of the conductor cannot be ignored, resulting in degradation of transmission characteristics due to scattering loss. Furthermore, if the surface roughness of the conductor is rough, the signal transmission path becomes longer, resulting in greater propagation loss per unit length. In the shielded cable of Patent Document 2, the insulator layer is surface-treated before the metal plating layer is formed, typically with a surface roughness of several micrometers to several tens of micrometers. Due to the effects of this surface treatment, propagation loss occurs in the shield.

[0033] In contrast, in the flexible flat cable 1 of this embodiment, current collects on the surface of the shielding layer 4 formed on the inner surface of the base film 3. Furthermore, since there is no need to perform surface treatment on the inner surfaces 311, 321 of the base film 3, the surface of the shielding layer 4 is smooth. For example, if the shielding layer 4 is made of a copper-plated layer and the copper-plated layer is formed by electroless plating, the surface roughness of the copper-plated layer will be approximately 0.2 to 0.4 μm, and the surface of the shielding layer 4 will be smooth. Therefore, the shielding layer 4 will have less propagation loss due to surface roughness, and will have good transmission characteristics at high frequencies.

[0034] Next, a method for manufacturing the flexible flat cable 1 of this embodiment will be described.

[0035] First, a plurality of electric wires 2 are prepared, each having an insulating layer 22 coated around a conductor 21. The method for forming the insulating layer 22 around the conductor 21 is not particularly limited, and the insulating layer 22 may be formed by extrusion molding, or may be formed by applying a resin that constitutes the insulating layer 22, as in the case of an enameled wire.

[0036] Next, the first shield 41 is formed on the inner surface 311 of the first film 31 in which the multiple recesses 312 have been formed. The method for forming the first shield 41 is not particularly limited. For example, if the first shield 41 is made of metal foil, the metal foil may be attached to the inner surface 311 of the first film 31. At this time, the metal foil may be attached to the inner surface 311 of the first film 31 using the above-mentioned adhesive. Furthermore, if the first shield 41 is made of a metal plating layer, the metal plating layer may be formed on the inner surface 311 of the first film 31 by electroless plating or a combination of electroless plating and electroplating.

[0037] Furthermore, in the same manner as described above, the second shield 42 is formed on the inner surface 321 of the second film 32 in which the plurality of recesses 322 are formed.

[0038] Next, the electric wires 2 are placed one by one in each recess 322 of the second film 32. At this time, an adhesive may be applied between the second shield 42 and the electric wires 2 in the recess 322. Thereafter, the first film 31 is laminated on the second film 32 so that the upper parts of the electric wires 2 fit into the recesses 312 of the first film 31. At this time, an adhesive may be applied between the first film 31 and the electric wires 2 in the recess 312. An adhesive may also be applied between the first shield 41 and the second shield 42. In this way, a plurality of electric wires 2 are fixed between the first film 31 provided with the first shield 41 and the second film 32 provided with the second shield 42, thereby obtaining a flexible flat cable 1.

[0039] As described above, the flexible flat cable 1 according to this embodiment has a conductor 21 and an insulating layer 22 provided around the conductor 21, and is equipped with a plurality of electric wires 2 arranged in parallel to one another. The flexible flat cable 1 further includes a pair of electrically insulating base films 3 that sandwich the plurality of electric wires 2 therein, and a metallic shielding layer 4 interposed between the pair of base films 3 and between the plurality of electric wires 2 and each base film 3. Within the pair of base films 3, the entire outer surface of each electric wire 2 is joined to and covered with the shielding layer 4.

[0040] In the flexible flat cable 1, the entire outer surface of each electric wire 2 is covered with a shielding layer 4, which also serves as a ground potential layer. This improves the shielding performance of the flexible flat cable 1, preventing noise from entering the conductor 21 and reducing crosstalk between adjacent electric wires 2. Furthermore, since the flexible flat cable 1 has a structure in which multiple electric wires 2 and the shielding layer 4 are sandwiched between a pair of base films 3, this layered structure can be maintained even when bent, thereby increasing flexibility. Furthermore, since the electric wires 2 have a coaxial structure, the characteristic impedance can be determined by the thickness of the insulating layer 22.

[0041] In the flexible flat cable 1 of this embodiment, each base film 3 may have a plurality of recesses 312, 322 formed therein, each recess 312, 322 accommodating a part of each electric wire 2 and a part of the shielding layer 4 therein.

[0042] With this configuration, the electric wires 2 and the shielding layer 4 are fixed in the internal space formed by the recesses 312, 322, so that the laminated structure can be stabilized even when bent. Furthermore, the flexible flat cable 1 can be formed by a simple method of laminating a plurality of electric wires 2, the base film 3 having the recesses 312, 322 formed therein, and the shielding layer 4, so that the manufacturing cost can be reduced.

[0043] In the flexible flat cable 1 of this embodiment, the shielding layer 4 may have a first shield 41 laminated on one of the pair of base films 3 and a second shield 42 laminated on the other of the pair of base films 3. The first shield 41 and the second shield 42 may be joined to each other between adjacent electric wires 2.

[0044] With this configuration, the entire outer surface of each electric wire 2 can be covered with the shielding layer 4, thereby further improving the shielding performance.

[0045] In the flexible flat cable 1 of this embodiment, the base film 3 may contain a resin, and the shielding layer 4 may contain a metal thin film or metal plating.

[0046] With this configuration, the base film 3 and the shielding layer 4 become flexible, and therefore the flexibility of the flexible flat cable 1 as a whole can be improved.

[0047] In this embodiment, the recess 312 of the first film 31 and the recess 322 of the second film 32 are not essential components as long as the multiple electric wires 2 and the shielding layer 4 can be fixed inside the pair of base films 3. In this embodiment, the recess 312 of the first film 31 and the recess 322 of the second film 32 have a substantially semicircular cross section, and form a substantially cylindrical internal space. However, this embodiment is not limited to this structure, and the internal space may also be substantially rectangular.

[0048] 2, the end 411 of the first shield 41 and the end 421 of the second shield 42 are exposed to the outside. However, the end 411 of the first shield 41 and the end 421 of the second shield 42 may be located inside the side surface 313 of the first film 31 and the side surface 323 of the second film 32, and the side surfaces 313 and 323 may be joined to each other. In other words, the end 411 of the first shield 41 and the end 421 of the second shield 42 may be blocked by the side surface 313 of the first film 31 and the side surface 323 of the second film 32.

[0049] Although the present embodiment has been described above, the present embodiment is not limited to this, and various modifications are possible within the scope of the gist of the present embodiment. [Explanation of symbols]

[0050] 1 flexible flat cable 2 electric wire 3 Base film 4 Shielding Layer 21 Conductor 22 Insulating layer 41 First Shield 42 Second Shield 312,322 recess

Claims

1. a plurality of electric wires each having a conductor and an insulating layer provided around the conductor, the electric wires being arranged in parallel with one another; a pair of electrically insulating base films that sandwich the plurality of electric wires therebetween; a metallic shield layer interposed between the pair of base films and between the plurality of electric wires and each base film; Equipped with A flexible flat cable in which, inside the pair of base films, the entire outer surface of each electric wire is joined to the shielding layer and is covered with the shielding layer.

2. 2. The flexible flat cable according to claim 1, wherein each base film has a plurality of recesses formed therein for accommodating a portion of each electric wire and a portion of the shielding layer.

3. the shield layer includes a first shield laminated on one of the pair of base films and a second shield laminated on the other of the pair of base films, The flexible flat cable according to claim 1 , wherein the first shield and the second shield are joined to each other between adjacent ones of the electric wires.

4. 3. The flexible flat cable according to claim 1, wherein the base film includes a resin, and the shielding layer includes a metal thin film or a metal plating.

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