Adhesive bonding and miscellaneous chemical manufacture – Surface bonding means and/or assembly means therefor – Longitudinally progressive helical winding means
Reexamination Certificate
1999-05-27
2001-07-24
Yao, Sam Chuan (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Surface bonding means and/or assembly means therefor
Longitudinally progressive helical winding means
C156S174000, C156S181000, C156S428000, C156S431000
Reexamination Certificate
active
06263937
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method and apparatus for making a resin-impregnated fiber substrate suitable for use in a variety of fabrication processes and particularly suitable for use in reinforced plastic composites and printed circuit board applications. More particularly, the invention pertains to a method and an apparatus for forming a resin-impregnated web that minimizes surface imperfections and maintains a structurally favorable perpendicular arrangement of the yarns in the composite substrate.
BACKGROUND OF THE INVENTION
The current world market for resin-impregnated fiber substrates exceeds approximately 800 million yards per year. Resin-impregnated fiber substrates for composite structural applications account for 100 plus million yards (e.g., woven fiberglass, woven aramid sold under the trademark KEVLAR™, carbon, and other fabrics) while basic composites and electronic circuit boards account for 700 plus million yards (e.g., woven fiberglass fabrics).
A variety of conventional methods are commonly used to produce flat, bi-directional fiber substrates for use in other fabrication processes and structures such as reinforced plastic composites. In these conventional methods, a first set of yarns is positioned perpendicular to a second set of yarns after which the yarns are fixed in a resin to produce a substrate. The substrates are produced in the form of moldable resin-impregnated products. The moldable products can be cut into workpieces and used in the production of composite structures for the aerospace, automobile, and electronics industries. As referenced above, a common application is the use of the composite structures in the production of printed circuit boards typically used in the computer industry.
Conventional production methods for manufacturing resin-impregnated substrates often result in products that are less than suitable for particular applications. For example, advances in integrated circuit (IC) technology require a large number of integrated circuits to be placed on an individual printed circuit board. These integrated circuits have to be interconnected on the printed circuit board comprised of multiple layers (e.g., substrates with conductor traces such as copper). The surfaces of resin-impregnated substrates used in printed circuit boards often contain surface imperfections that adversely affect the conductivity of the circuit board. Minor flaws or “pimples” on the surface of the substrate are transferred to the conductive layer or layers (e.g., copper) subsequently applied to the base substrate during molding of the circuit board. Stated differently, the surface imperfections on the underlying substrate are transferred to the layers applied thereon, thus resulting in nonuniform conductive layers. Accordingly, conductive layers riddled with the surface imperfections exhibit reduced conductivity in the resulting circuits and hence degrades the overall performance of the printed circuit boards.
In order to compensate for the variations in the conductivity of the individual metal layers created by the imperfections, individual circuits having an increased cross-section are used on the printed circuit board. The increased size of the circuits limits the total number circuits that can be placed on the circuit board. Thus, the imperfections on the surface of the substrates forming the circuit board become a limiting factor in the production and performance of the printed circuit boards.
Substrates that are produced by weaving techniques further degrade the conductivity and hence limit the number of circuits on the circuit board. The weaving technique results in a weave pattern on the surface of the substrate. As conductive layers are molded on the underlying substrate, the weave pattern is transferred to the conductive layer, thus, further affecting the performance of the circuit board and further limiting the number of circuits that can be placed on the board.
One conventional method of producing resin-impregnated substrates includes the steps of weaving a fabric, applying a finish to the fabric, and then impregnating the finished fabric with a thermosetting or thermoplastic resin. The step of weaving a fabric to be impregnated includes beaming or warping yarns (i.e., winding warp yarns onto a beam in preparation for weaving or warp knitting) for the machine direction (i.e., direction in which the fabric is being produced in the machine) onto section beams, warp sizing the yarns while transferring the warp yarns from a section beam to a loom beam, placing the loom beam onto a loom, and finally applying the cross-direction (i.e., direction perpendicular to the direction in which the fabric is being produced by the machine) yarns to the machine direction yarns by either weaving the cross-direction yarns into the machine direction yarns or by warp-knitting. The terms “warp knitting” and “weft knitting” are used according to common industry standards wherein warp knitting denotes a type of knitting in which yarns generally run lengthwise in the fabric. Weft knitting is understood to describe a type of knitting whereby one continuous thread runs crosswise in the fabric.
The step of applying a finish to the fabric to be impregnated includes the preliminary step of cleaning the fabric to remove any chemicals previously applied to the yarns during the weaving or warp-knitting process. Next, the fabric is treated with additional chemicals to ensure compatibility between the woven yarns and the resin to be applied.
The impregnating step involves saturating the finished fabric with resin. The amount of resin applied to the fabric is controlled or metered to obtain the desired weight of the fabric. Conventional methods for metering the amount of resin applied includes scrapers as disclosed in U.S. Pat. No. 3,068,133 to W. H. Cilker et al.
In addition, the impregnating step requires the fabric to be heat treated to remove any residual solvents from the fabric remaining from the previous chemical treatment. The heat treatment is also necessary to partially cure the resin coating the fabric. This first conventional method of producing a resin-impregnated fiber substrate requires a variety of preliminary and subsequent processing steps other than the basic steps of weaving, applying a finish, and impregnating. Thus, the first conventional method is logistically inefficient and time consuming.
A second conventional method for producing a flat, bi-directional substrate involves a process commonly referred to as “filament winding” whereby a first set of yarns is saturated with resin and then wound at a desired angle around a flat rotating mold or mandrel to form a first set of yarns. Next, the flat mold is rotated ninety degrees from its original position and then a second set of yarns is wound around the first set of yarns, thereby forming a web of a predetermined thickness and desired pattern (e.g., checker board). Nevertheless, this process requires the manufacturer to produce multiple batches of resin for each individual substrate section. Accordingly, the second conventional method is time-consuming considering the multiple steps required to produce a single substrate section (e.g., the steps of saturating, winding, and rotating). Furthermore, the substrate is limited to the size of rotating mold.
Specifically, during the first conventional method, bobbin yarns for the warp are wound onto section beams. While the yarns on the section beams are combined to make a loom beam, warp sizes are applied to the yarns for weaving. On the loom, the sized warp yarns from the loom beam are interlaced (i.e., woven) with the weft yarns at a maximum rate of 0.42 yards per minute for a weft yarn count of forty (40) yarns per inch. The woven greige (i.e., unfinished) fabric is then heat cleaned and a finish is applied to the heat cleaned fabric which is then dried. Next, the finished fabric is shipped to a prepregger where the fabric is subsequently impregnated with resin. Prior to and following each manufacturing step referenced above, the fabric is either
ARE Industries, Inc.
Summa and Allan, P.A.
Yao Sam Chuan
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