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  Flex-rigid wiring boardRelated Patent Categories: Coating Processes, Electrical Product Produced, Integrated Circuit, Printed Circuit, Or Circuit Board, Multilayer, Coating Hole WallThe Patent Description & Claims data below is from USPTO Patent Application 20080107802. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS REFERENCE OF RELATED APPLICATION [0001] This application is a divisional of U.S. application Ser. No. 10/558,289, filed Nov. 28, 2005, which is a national stage of PCT/JP05/09819, filed May 23, 2005, and further is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2004-173414, filed Jun. 11, 2004. FIELD OF THE INVENTION [0002] The present invention generally relates to a wiring board including a flexible substrate and rigid substrate, and more particularly to a wiring board characterized in the connection between a flexible substrate and rigid substrate stack-joined to each other in a rigid portion, among others, of the wiring board. BACKGROUND ART [0003] The recent portable electronic devices such as a foldable mobile phone and the like use a flex-rigid wiring board. Normally in such a wiring board, rigid or non-flexible portions and a flexible portion are joined to each other with a flexible substrate laid between them, and wiring pattern layers on the surfaces of the flexible and rigid substrates stacked together are electrically connected to each other via a plated through-hole conductive layer in the rigid portion as shown in FIG. 12 (as disclosed in the Japanese Patent Application Laid Open No. 90756 of 1993, for example). [0004] Also, there has been proposed a flex-rigid circuit board in which a multilayer rigid substrate has formed at either end thereof a cut in which an end electrode of a flexible substrate is fitted, the end portion of the flexible substrate is laid between the outermost sides of the rigid substrate and the electrode of the rigid substrate and that of the flexible substrate are electrically connected to each other (as disclosed in the Japanese Patent Application Laid Open No. 170076 of 1995, for example). [0005] However, the wiring board in which a flexible substrate and rigid substrate are electrically connected to each other through a plated through-hole with the above-mentioned conventional wiring-board technology is not advantageous in that when it is used with a frequency of higher than 1 GHz, signal propagation is delayed and high-speed signal transmission is not stable. Especially when the wiring board is used with a frequency of higher than 5 GHz, the signal propagation is more seriously delayed and instability of the high-speed signal transmission is greater. [0006] Also, the wiring board produced with the conventional wiring-board technology was found, as a result of the reliability test made in the cooling/heating cycle test, to have the electrical connection thereof deteriorated. That is, for the electrical connection through the plated through-hole in the flexible and rigid substrates, the conductive layer in the through-hole is formed by plating. Therefore, the conductive layer varies in thickness so that some of connecting terminals are not connected, namely, so-called "open" terminals are resulted. [0007] Further, since the conventional flex-rigid wiring board is formed either by fixing a flexible substrate to an outermost rigid substrate with an anisotropic adhesive or by laying a flexible substrate between both outermost rigid substrates, so it cannot be designed thinner. DISCLOSURE OF THE INVENTION [0008] The present invention has an object to overcome the above-mentioned drawbacks of the related art by providing a flex-rigid wiring thinner than the conventional one and that is excellent in electrical properties and reliability of the conductive layer penetrating flexible and rigid substrates, can transmit high-frequency electrical signals with a less delay and has a connecting structure more advantageous for assuring a stable signal transmission. [0009] The inventors of the present invention had devoted themselves to attaining the above object. The inventors found it possible to transmit GHz-level electrical signals with an improved stability by electrically connecting a rigid substrate and flexible substrate not through the plated through-hole as in the conventional technology but via an anisotropic conductive adhesive layer interposed between conductive layers of substrate portions including interconnecting electrode pads, and thus worked out the present invention. [0010] (1) The above object can be attained by providing a flex-rigid wiring board in which a rigid substrate formed from a rigid base material and a flexible substrate formed from a flexible base material are stack-joined and electrically connected to each other, wherein: the flexible substrate including a conductive layer having interconnecting electrode pads formed on at least one surface thereof, and the rigid substrate including a conductive layer having interconnecting electrode pads formed on at least one surface thereof in a position opposite to the interconnecting electrode pads on the rigid substrate, are electrically connected to each other with an anisotropic conductive adhesive layer interposed between conductive layers of substrate portions each including at least the interconnecting electrode pad. [0011] In the above flex-rigid wiring board according to the present invention, either the flexible or rigid substrate has provided on the surface thereof a coverlay having formed therein an opening through which the interconnecting electrode pads are exposed and that is filled with the anisotropic conductive adhesive. [0012] According to the present invention, a plurality of the interconnecting electrode pads exposed through the respective openings formed in the coverlay on the flexible substrate may be provided in such openings adjacent to each other and spaced 20 to 500 .mu.m from each other. [0013] Further according to the present invention, the flexible substrate may have formed therein a via-hole formed from a through-hole extending from the substrate top to bottom and filled with a conductive substance and the interconnecting electrode pads be provided just above the via-hole. [0014] Note that in the above flex-rigid wiring board structure according to the present invention, the wording "flexible substrate" or "rigid substrate" refers to both "single-layer" and "multilayer" substrate. [0015] In the flex-rigid wiring board (1) according to the present invention, since the flexible and rigid substrates are joined integrally to each other with the anisotropic conductive adhesive layer interposed between the conductive layers of the substrate portions including the interconnecting electrode pads disposed at least opposite to each other, the interconnecting electrode pads on the flexible substrate can electrically be connected to that of the rigid substrate to provide an excellent electrical connection and improved connection stability, whereby it is possible to transmit a high-frequency signal with a less delay and a stability that can be attained quickly. [0016] More specifically, when a flexible substrate and rigid substrate are electrically connected to each other through a plated through-hole as with the conventional technology, electrical signals will diffuse inside the plated through-hole and their transmission be delayed. However, in the flex-rigid wiring board according to the present invention in which a flexible substrate and rigid substrate are electrically connected through an anisotropic conductive adhesive layer, electrical signals can be transmitted without being diffused, so that a high-frequency signal can be transmitted with a less delay and a quickly attained stability. [0017] Also, by forming a plurality of interconnecting electrode pads on the flexible substrate and setting the distance between adjacent openings through which these interconnecting electrode pads are exposed, that is, the distance from the edge of one of the adjacent openings to that of the other opening (spacing distance), to 20 to 500 .mu.m, the local electrical connection between the flexible and rigid substrates via the anisotropic conductive adhesive can positively be made and the flex-rigid wiring board be designed smaller. [0018] Further, by forming a through-hole extending from the substrate top to bottom of the flexible substrate and filling the through-hole with the conductive substance to provide a via-hole and providing interconnecting electrode pads just above the via-hole, it is possible to assure a coincidence between the position of an interlayer connector at the flexible substrate and that of an interlayer connector at the rigid substrate. Also, by forming a stacked structure in which the interlayer connectors are stack-joined and electrically connected to each other, it is possible to provide a flex-rigid wiring board in which the wiring length is shorter and that is thus suitable for mounting thereon electronic parts that need a large power for their operation. [0019] (2) Also the above object can be attained by providing a flex-rigid wiring board comprising: a flexible substrate formed from a flexible base material, having a plurality of first interconnecting pads located at opposite ends thereof and a plurality of first conductor layers connected to the first interconnecting pads, respectively; Continue reading... Full patent description for Flex-rigid wiring board Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Flex-rigid wiring board patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. 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