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  Double glass cloth, and prepreg and substrate for printed wiring board using the glass clothUSPTO Application #: 20070190879Title: Double glass cloth, and prepreg and substrate for printed wiring board using the glass cloth Abstract: A double glass cloth for a printed wiring board, characterized in that it is a glass cloth composed of warps and wefts and has a double structure comprising a face side structure and a back side structure, wherein said face side structure and said back side structure are bound with a woven structure into one piece. (end of abstract) Agent: Finnegan, Henderson, Farabow, Garrett & Dunner LLP - Washington, DC, US Inventors: Yoshinori Gondoh, Makoto Someya, Yasuyuki Kimura USPTO Applicaton #: 20070190879 - Class: 442181000 (USPTO) Related Patent Categories: Fabric (woven, Knitted, Or Nonwoven Textile Or Cloth, Etc.), Woven Fabric (i.e., Woven Strand Or Strip Material) The Patent Description & Claims data below is from USPTO Patent Application 20070190879. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The present invention relates to a glass cloth used for a printed wiring board. BACKGROUND ART [0002] A double sided printed wiring board is normally manufactured following the steps below. First, in a prepreg manufacturing step, a base material such as a glass cloth is impregnated with varnish made of thermo-setting resin such as epoxy resin and a solvent, heated and dried to thereby create a prepreg. Second, in a stacking step, a single prepreg or a plurality of prepregs are stacked, copper foil is pasted to both sides of the laminated sheet obtained, and heated, pressed and cured to thereby create a coppered laminated sheet. Third, in a circuit pattern formation step, a circuit pattern made of copper foil is created on both sides of the coppered laminated sheet through photolithography and etching or plating. Finally, in a through hole processing step, through holes are formed using a drill or laser and electrical connections of both sides are secured through a publicly known step such as electroless copper plating. It is generally known that due to the heat and pressure in the above described stacking step or in the circuit pattern formation step, part of the copper foil is etched out and the dimension of the coppered laminated sheet is thereby changed. [0003] Furthermore, it is possible to manufacture a multilayer printed wiring board through a sequential molding method using the above described double sided printed wiring board as a core substrate, by overlaying the above described single prepreg or plurality of prepregs on the surface layer, pasting the copper foil on both sides of the laminated sheet obtained, heating and pressing, and hardening and adhering the laminated sheet. [0004] In order to realize higher-performance, smaller and lighter digital devices in recent years, printed wiring boards used for such devices are also required to be smaller, slimmer and denser. A possible technique therefor may be to increase the number of layers of a build-up multilayer printed wiring board using the above described sequential molding method, but a batch molding method is often used in recent years whereby double sided printed wiring boards are stacked together with an interconnecting body interposed in between, heated and pressed, and cured and adhered all at once. For the prepreg which becomes an interconnecting body (hereinafter referred to as "interconnecting prepreg" when the prepreg used to manufacture a core substrate and the prepreg which becomes the interconnecting body need to be distinguished), it is possible to form a through hole (interstitial via hole or inner via hole, abbreviated as an "IVH") right below a part land or between arbitrary layers by filling the IVH with a conductor such as conductive paste. For this reason, there are proposals of a multilayer printed wiring board having a total layer IVH structure capable of realizing a size reduction of the substrate and high density mounting. [0005] These methods of manufacturing a multilayer printed wiring board try to further achieve the aforementioned size reduction, thickness reduction and higher density by reducing the thickness of a core substrate and interconnecting prepreg which make up each layer. However, as the thickness of a material making up each layer decreases, the ease of handling of the substrate during transportation or the like deteriorates, and therefore it is necessary to improve rigidity of the interconnecting prepreg and core substrate used. [0006] Furthermore, in realizing a high-density printed wiring board, further dimensional stability of the substrate and reduction of variation in particular are required to improve yield as with the substrate and prepreg necessary for thickness reduction. With the increase of via connections between layers due to a multilayer structure, it is also necessary to reduce the coefficient of thermal expansion in the thickness direction (Z direction) to improve connection reliability in the thickness direction. [0007] There is a proposal to use a three-dimensional-structured woven glass (hereinafter referred to as "three-dimensional glass") as a base material capable of reducing the coefficient of thermal expansion in the thickness direction for a printed wiring board (see Patent Document 1 and Patent Document 2). Patent Document 1 defines the above described three-dimensional structured woven fabric as a cloth made up of fiber in the longitudinal direction, fiber in the lateral direction and fiber in the thickness direction, and more specifically describes a structure in which layers of fibers arrayed in parallel in the longitudinal direction and layers of fibers arrayed in parallel in the lateral direction are alternately stacked one atop another and fibers in the thickness direction are passed so as to weave gaps in the vertical direction which exist between the fibers in the longitudinal direction and fibers in the lateral direction. Patent Document 2 does not describe any specific three-dimensional glass structure. According to the description of Patent Document 1, the coefficient of thermal expansion in the Z direction of the printed wiring board using the three-dimensional glass is estimated to be approximately 1/3 of a conventional example using plain weave glass. Though not described in Patent Document 1, the rigidity of a printed wiring board using the three-dimensional glass is expected to be higher than that of the conventional example. [0008] However, weaving the above described three-dimensional glass requires a special loom applicable to three directions; fibers in the longitudinal direction (warps), fibers in the lateral direction (wefts) and fibers in the thickness direction and there is a large problem that the three-dimensional glass cannot be woven using a normal loom only applicable to two directions of warps and wefts. [0009] Furthermore, the three-dimensional glass also has a problem that the distribution of the amount of glass in the thickness direction drastically varies from one place to another. When through holes or IVHs are formed in a printed wiring board, differences in the distribution of the amount of glass in the thickness direction cause variations in the shape of holes. For a two-dimensional-structured woven fabric such as a plain weave glass cloth, fiber-opening process is known to have an effect in reducing the above described variations. However, in the case of the above described three-dimensional glass, since fibers in the thickness direction are arrayed and constrained on the same line in the vertical direction, even if fiber-opening process is applied, it is difficult to eliminate the variations. Because of these problems, the above described three-dimensional glass is not used as a base material for the printed wiring board. [0010] Thus, in the case of a core substrate, it is a general practice to use a technique of creating the substrate by stacking a plurality of prepregs using an available thinnest glass cloth, applying hot/pressure molding thereto to achieve high rigidity. For example, when a core substrate of 100 .mu.m in thickness is created, if the glass content is the same, a core substrate made up of two prepregs of 50 .mu.m in thickness has higher rigidity than a core substrate made up of a single prepreg of 100 .mu.m in thickness. Likewise, a core substrate created of three prepregs of 33 .mu.m in thickness is more preferable. [0011] However, a prepreg using an extremely thin glass cloth has poor yield of operability in the manufacturing step and causes a cost increase with an increase in the number of glass cloths used, which is not desirable. Furthermore, while the core substrate consisting of the above described plurality of stacked prepregs improves rigidity, it has no effect on the reduction of the coefficient of thermal expansion in the Z direction. [0012] Furthermore, in the case of the interconnecting prepreg, a method of manufacturing a prepreg by continuously impregnating a roll of glass cloth with varnish is currently in the mainstream. This involves a problem that it is difficult to stack a plurality of intermediate prepregs. Thus, as a technique capable of increasing rigidity without stacking prepregs, a manufacturing method of impregnating a plurality of glass cloths overlaid one another with varnish to obtain a single prepreg is proposed (see Patent Document 3). [0013] However, according to this technique, when a plurality of extremely thin glass cloths each having a thickness of 50 .mu.m or less are used, loose and wrinkles or the like are produced in the glass cloths and it is difficult to obtain uniform prepregs. Even if the line tension during manufacturing of prepregs is drastically increased and loose and wrinkles can be avoided, there is still a problem that large residual stress impairs stability in dimensional variations in the stacking step. Furthermore, a core substrate made up of a single prepreg converted from a plurality of glass cloths has improved rigidity but has no effect on a reduction of the coefficient of thermal expansion in the Z direction. [0014] On the other hand, there is a proposal of a printed wiring board using a glass cloth of weft backed satin weave and warp backed satin weave, which is a one-sided combination structure intended to reduce warpage and torsion of the printed wiring board (see Patent Document 4). The "one-sided combination structure" is a weave using two or more types of yarn for any one of warp and weft, and the weft backed weave is a combination structure created using one type of warp and two types of weft. In such a woven structure, the face weave is woven with warps and face side wefts and the back weave is woven with warps and back side wefts. The warp backed weave is similar to the weft backed weave with only the longitudinal/lateral relationship is inverted. [0015] The glass cloth described in Patent Document 4 provides satin weave with fewer crosses between warps and wefts to reduce crimp of woven fabric and also provides a one-sided combination structure to cancel differences between the face and back sides of satin weave. With the use of the woven fabric of one-sided combination structure, it is possible to obtain a glass cloth having a thickness of approximately 1.5 times that of a normal single structure with no difference between the face and back sides, but this does not contribute to improvement of rigidity of prepregs. Moreover, satin weave has a weak binding force between yarns, is liable to bowed filling and not suitable for a thin glass cloth. [0016] Patent Document 1: JP-B-7-36465 [0017] Patent Document 2: JP-A-7-202362 [0018] Patent Document 3: JP-A-9-151027 [0019] Patent Document 4: JP-A-2001-55642 DISCLOSURE OF THE INVENTION [0019] Problem to be Solved by the Invention [0020] It is an object of the present invention to provide a glass cloth which can be woven without using any special loom, capable of improving rigidity necessary for reducing the thickness of a core substrate and prepreg used for a double sided printed wiring board, improving productivity and reducing variations in the amount of dimensional change and coefficient of thermal expansion in the thickness direction, a prepreg using the glass cloth and a substrate for a printed wiring board using the prepreg. Means for Solving the Problem [0021] The present inventors have made every effort to solve the above described problems focusing attention to the woven structure of a glass cloth made up of warps and wefts and consequently have come to complete the present invention by discovering that using a double glass cloth having a double structure with two glass cloths stacked one atop the other and the two bound into a woven structure as a substrate makes it possible to stably obtain a prepreg having rigidity equivalent to that of two prepregs stacked and molded using one glass cloth and also reduce variations in dimensional stability and the coefficient of thermal expansion in the thickness direction. [0022] That is, the first aspect of the present invention is a double glass cloth for a printed wiring board, characterized in that it is composed of warps and wefts and has a double structure comprising a face side structure and a back side structure, wherein said face side structure and said back side structure are bound with a woven structure into one piece. [0023] The double glass cloth is preferably made up of the face side structure that consists of face side warps which only weave the face side structure, face side wefts which only weave the face side structure and common yarns which weave both the face side structure and the back side structure, and the back side structure that consists of back side warps which only weave the back side structure, back side wefts which only weave the back side structure and the common yarns which weave both the face side structure and the back side structure. Furthermore, the double glass cloth is more preferably made up of the face side structure and back side structure each having a plain weave. Moreover, the double glass cloth is most preferably made up of the face side structure and back side structure bound together at a rate of at least one location per unit structure. Continue reading... Full patent description for Double glass cloth, and prepreg and substrate for printed wiring board using the glass cloth Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Double glass cloth, and prepreg and substrate for printed wiring board using the glass cloth patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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