Textiles: weaving – Fabrics – Materials
Reexamination Certificate
1999-05-13
2001-12-04
Falik, Andy (Department: 3765)
Textiles: weaving
Fabrics
Materials
C442S002000, C442S043000, C442S060000, C428S901000, C428S219000
Reexamination Certificate
active
06325110
ABSTRACT:
The present invention relates to a woven fabric reinforcement to be used in laminated composite structures, in particular for dielectric laminates and printed circuit boards. The specific fabric geometry and yarn disposition contribute to a balanced fabric structure that greatly reduces the movements of the resulting laminated composite structure during the successive mechanical and thermal processes for the manufacture of printed circuit boards.
BACKGROUND OF THE INVENTION
Reinforcement fabrics are successfully used today in the production of advanced dielectric composites for the electrical and electronic industries. In particular, for the production of laminates, fiberglass fabric dominates the market in the reinforcement of all types of thermosetting and thermoplastic resins.
The laminates have incorporated on one or both sides a copper foil at after several processes of photography, etching, drilling, finishing becomes a well known printed circuit board In a multi-layer board, additional layers of fabric reinforced resin and additional layers of copper foil, are laminated together and undergo additional processes of photography, etching, drilling and finishing to become rather complex multi-layer printed circuit boards.
Printed circuits boards, and the laminates used for their production, are required to have superior dimensional stability, the lowest possible bow and twist and very limited and predictable movement on the X & Y axes during the successive mechanical and thermal process steps.
Currently produced reinforcement fabrics are normally of plain weave construction, and are characterized by a large number of crossovers between warp yarns and weft yams e.g. about 500 or more which was determined during the 1950's as a consequence of the technology capabilities available then, and which has remained mostly unchanged.
The yarn used) particularly fiberglass yarn, has an average number of twist per meter ranging from 12 up to 40 turns per meter. The direction of twist commonly used is Z-twist. In Z-twist the filaments assume an ascending left to right configuration, as in the central portion of the letter Z (see FIG.
1
). In S-twist the filaments assume an ascending right to left configuraton, as in the central portion of the letter S (see FIG.
2
).
The performance challenge comes from recent technology advances in the production of printed circuit boards (and in particular high-layer count multi-layer boards), such as build-up process, micro-vias, and laser drilling, which require higher dimensional stability of the laminate and a more evenly distributed reinforcement inside the resi matrix.
U.S. Pat. No. 5,662,990 discloses that the use of untwisted yarn in the fabric reinforcement greatly improves the performance of the resulting laminate and printed circuit board. However, even better results are further obtained by changing the fabric geometry as described herein, rather than just relying only on better characteristics of single yarns.
SUMMARY OF THE INVENTION
It has now been discovered that the geometry of the reinforcement fabric is principally responsible for the dimensional stability behavior of the resulting laminated composite structure. The outstanding effects of the present invention have been remarkably obtained also for the production of thin laminates where fine fabrics with thickness ranging from 0.035 mm. up to 0.13 mm. are used, and assembled as ural in a limited number of plies. The research that was conducted brought a distinction in regards to reinforcement fabrics: above 190 grams per square meter optimal results were obtained with unidirectional fabrics, as in the commonly owned U.S. Pat. No. 5,752,550, while below 190 grams per square meter the use of unidirectional fabric gave sub-optimal results. Moreover, three primary characteristics were found to contribute to the dimensional stability behavior in an orthogonal reinforcement fabric of weight up to 190 grams per square meter in particular of plain weave construction:
1. Fabric construction geometry is extremely important: because of the fact that all finishing and impregnation production steps use continuous processes that unwind, pull, and rewind the fabric in the warp direction, it has been discovered that it is necessary to have in the warp direction at least 55% and up to about 65% of the total number of warp and weft yarns in order to maintain a satisfactory stability of the reinforcement fabric. This results in a better behavior of the dielectric laminates and printed circuit boards.
2. The number of crossovers between wrap yarns and weft yarns must be kept within a limited range per unit area, regardless of fabric weight. Up to a weight per unit area of 190 grams per square meter, the results indicate that the optimal range is between 200 and 315 crossovers per square centimeter. Outside this range, the resulting laminated composite structure shows erratic and unpredictable behavior
3. The third most influential factor in the dimensional stability behavior of laminated composite structures is the torsion present in each warp and weft yarn. It has been discovered that by utilizing about 50% (±10%) of the number of warp or weft yarns with counterclockwise torsion (Z-twist) and the remaining about 50% (±10%) of yarns with clockwise torsion (S-twist) of the same magnitude, the reinforcement fabric has an absolute neutral behavior it the resulting laminated composite structure. Torsion levels must be kept as low as possible for optimal results, but the neutral behavior has been discovered also at higher torsion levels, so that good results have been obtained in the full range between 0.4 to 40 turns per meter. This can be easily explained if we think of each yarn as a group of filaments (several 100's) having an elicoidal form. Thus, at each thermal stress the yarn behaves like a spring, where heating at high temperatures produces an elongation and cooling produces a contaction. Now, if each yarn has the same torsion direction, these thermal stresses will produce a twisting of the whole laminated composite structure. However, because the elongation due to heating and the contraction due to cooling are not linear, the outcome is a geometric deformation of the resulting laminated composite structure, which causes misregistration of the laminate in the subsequent process steps needed to form the final printed circuit board. A similar behavior can be described for mechanical stresses or combined mechanical and thermal stresses.
In a preferred embodiment of the present invention the torsion of each counterclockwise twisted yarn is neutralized by the torsion of the adjacent clockwise twisted yarn. In fact, in a preferred embodiment of the invention, each next warp yarn has opposite torsion of equal magnitude compared to the preceding warp yarn, and each next weft yarn has opposite torsion of equal magnitude compared to the preceding weft yarn. This yarn disposition acts in a similar way as a counterbalancing shaft in an automobile engine, which is designed to produce a counter-effect to the vibrations created by the engine main shaft (although the dynamic nature of that principle is altogether different from the quasi-static nature of the present invention).
Such fabric geometry and yarn disposition result in an optimally neutral behavior of the woven fabric reinforcement and the resulting laminated composite structure.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
REFERENCES:
patent: 889827 (1908-06-01), Teufel
patent: 1100060 (1914-06-01), Earnshaw
patent: 1677260 (1928-07-01), Whitman
patent: 2049743 (1936-08-01), Mack
patent: 2215938 (1940
Scari Diego
Scari Marco
Birch & Stewart Kolasch & Birch, LLP
Falik Andy
Gividi Italia S.p.A.
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