Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
2000-03-20
2002-09-10
Lam, Cathy (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C442S205000, C174S255000, C428S901000
Reexamination Certificate
active
06447886
ABSTRACT:
TECHNICAL FIELD
The present invention is generally directed to printed circuit boards and, more particularly, to printed circuit boards exhibiting improved structural properties through the provision of a base material or substrate that is formed from an integrated, three-dimensional woven fiber structure.
BACKGROUND ART
Printed circuit boards, or PCBs, are typically provided in the form of copper-clad laminates consisting of three principal components: a base or reinforcing material; a resin system or matrix; and copper foil. Commonly employed base materials include paper, glass matte, woven glass cloth, quartz, and aramid material. In the typical process for manufacturing the laminated PCB, the base material is impregnated or coated with a resin. The resin is then polymerized in a treater or coater to a state suitable for storage and final pressing. The base material is treated by passing it through a dip pan containing the resin, and subsequently passing the impregnated base material through a set of metering rollers, such as squeeze rollers, and in turn through a drying oven to cure or semi-cure the resin. The oven is of the air-circulating or infrared type, in which most of the volatile compounds such as solvents residing in the resin are driven off. The resulting product is often referred to as a prepreg. Rigorous process control is exercised during treating in order to monitor the ratio of resin to base material, the final thickness of the prepreg, and the degree of resin polymerization. Once the prepreg has been prepared, the copper foil is applied to one or two sides of the prepreg, typically by the process of electrodeposition.
Important criteria in the production of printed circuit boards include prevention of delamination, punchability and machinability (especially drillability), uniformity of dielectric strength, tensile strength and modulus, surface flatness, dimensional stability, and measling and blistering (often caused by entrapped moisture resulting from poor drill hole quality). Dimensional stability in the lengthwise and crosswise dimensions x, y is a function of the laminate reinforcement (e.g., glass or paper), while vertical or thickness expansion in the z-direction is generally a function of the resin system or matrix. Some prior approaches to improving these properties have been concerned with improving the process of manufacturing the PCB as generally described above, while other approaches have focused on improving the structure of the base material itself.
The base material for PCBs has in the past been constructed from a multi-layer composite laminate. Laminates most widely used include materials designated FR-2, CEM-1, CEM-3, FR-4, FR-5, and GI. FR-2 laminates comprise multiple plies of paper that have been impregnated with a flame-retardant phenolic resin. FR-3 laminates comprise multiple plies of paper that have been impregnated with an epoxy-resin binder. CEM-1 is a composite having a paper core impregnated with epoxy resin. Its two planar surfaces are covered by woven glass cloth impregnated with the same type of resin. CEM-3 is a composite having an epoxy resin-impregnated non-woven fiberglass core with epoxy resin-impregnated woven glass cloth surface sheets. FR-4 laminates, perhaps the most widely used material in the PCB industry, include multiple plies of epoxy resin-impregnated woven glass cloth. FR-5 (military-type GH) laminates include multiple plies of woven glass cloth impregnated with mostly polyfunctional epoxy resin. GI laminates include multiple plies of woven glass cloth impregnated with a polyimide resin.
Woven laminates typically consist of several layers of two-dimensional plain weave fabric that have been impregnated with a resin system. One example of a layer of two-dimensional plain weave is illustrated in cross-section in FIG.
1
. The fabric is produced in a “one-up, one-down” weaving process, wherein one set of fibers
1
disposed in the 0° (x-, warp or lengthwise) direction is interlaced with another set of fibers
2
disposed in the 90° (y-, weft, fill or crosswise) direction. Because of the interlaced configuration, all of the fibers contained in this type of fabric are necessarily crimped. It is known by those skilled in the art that the crimped structure significantly reduces the mechanical properties of the resulting fabric, such as the tensile strength and modulus. In addition, it is known that such fabric has an undesirably low dimensional stability since the crimped fibers are prone to stretching. Moreover, multi-layered composites formed from layers of two-dimensional fabric weaves are prone to delamination. Still further, in the manufacture of PCBs, the waved cross-sectional profile of crimped yarns creates a significant risk of deflection of the drill bit during drilling operations. Accordingly, it has become apparent in the industry that a better performing fabric is needed in the manufacture of the base material of a PCB. This need is especially significant in view of the fact that the density and complexity of the architecture of the modern PCB is increasing.
Thus far, most approaches for producing an improved base material to adequately satisfy the requirements of modern PCB manufacture have focused on improving the structure of two-dimensional fabrics. One such approach has been to reduce the degree or extent of crimping and thereby improve the surface roughness, waviness, and evenness or flatness of the fabric. This has purportedly been accomplished by interlacing the crosswise yarns
2
of a textile fabric in its lengthwise direction with a leno interwoven binding comprising glass yarns
3
, as illustrated in
FIG. 2. A
PCB is then produced from the resulting two-dimensional fabric base layer by conventional means, i.e., the fabric is treated with resin and a copper layer is placed on the surface of the top-most layer of the impregnated fabric. An example of this approach is disclosed in U.S. Pat. No. 5,807,793 to Scari et al. It can be seen, however, from
FIG. 2
that all crosswise yarns
2
nevertheless remain interlaced with all lengthwise yarns
1
and, by necessity, all lengthwise yarns
1
remain interlaced with all crosswise yarns
2
. Moreover, the pairs of leno interwoven yarns
3
are by definition interlaced with crosswise yarns
2
. The resulting fabric is thus still characterized by crimped yarns and, in connection with the manufacture of PCBs, is subject to all of the deleterious effects attending crimped, two-dimensional designs.
Another approach is proposed in U.S. Pat. No. 5,269,863 to Middelman, which discloses a process wherein two-dimensional fiber laminates are provided in an entirely non-woven format. Referring to
FIG. 3
, double layers of crosswise threads
2
are laid in parallel relation and stretched under tension onto single and/or double layers of lengthwise threads
1
, also laid in parallel relation and stretched under tension. All layers are thus formed without any interweaving or binding among the threads
1
and
2
. The threads
1
and
2
utilized in this process are of an untwisted type such as E-glass filaments. Subsequently, the laminate is fed to a metering unit which impregnates the laminate with an epoxy resin and then to an infrared preheater to initiate curing. The impregnated laminate is then fed to a double belt press. As the laminate enters the double belt press, upper and lower copper foils
4
A,
4
B are respectively unwound from rollers onto upper and lower surfaces of the laminate. The composite laminate of filament layers and copper foils is passed through the double belt press under elevated pressure and temperature, and its continuous length is apportioned into discrete PCBs by a cutting device. It should be apparent that while the base material thus produced contains essentially crimp-free fibers, structural integrity is nonetheless compromised by the fact that the fibers are non-woven and hence bound only by the metered resin.
A critical deficiency in prior art base materials such as those just described and illustrated in
FIGS. 1-3
, as wel
Gu Pu
Lienhart R. Bradley
Mohamed Mansour H.
3-Tex, Inc.
Jenkins & Wilson, P.A.
Lam Cathy
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