Title of Invention  A WOVEN REINFORCEMENT FABRIC FOR PRINTED CIRCUIT BOARDS 

Abstract  A woven reinforcement fabric for printed circuit boards of orthogonal construction with a weight per unit area of up to 190 grams per square meter, having warp and weft yarns, characterized by comprising a number of warp yarns which is greater than 55% of the total number of warp and weft yarns per unit area, the number of crossovers between warp yarns and weft yarns is between 200 and 315 crossovers per square centimeter; said fabric having warp and/or weft yam disposition such that between 40% and 60% of the number of warp and/or weft yarns have clockwise torsion of between 0.4 and 40 turns per meter, and between 40% and 60% of the number of warp and/or weft yarns have counterclockwise torsion of the same magnitude. 
Full Text  The present invention relates to a woven reinforcement fabric for printed circuit boards, It is also used in laminated composite structures, such as for dielectric laminates. 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 hecomes a well known printed circuit board In a multilayer 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 multilayer 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 yams 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 Ztwist the filaments assume an ascending left to right configuration, as in the central portion of the letter Z (see FIG. 1). In Stwist 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 highlayer count multilayer boards), such as buildup process, microvias, and laser drilling, which require higher dimensional stability of the laminate and a more evenly distributed reinforcement inside the resin 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. A woven fabric used in printed boards, of orthogonal construction, with a weight per unit area below 190 g/m2, is disclosed in EP399219. 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 usual 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 suboptimal 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 alt 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 yams 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 yams with counterclockwise torsion (Ztwist) and the remaining about 50% (±10%) of yarns with clockwise torsion (Stwist) 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 yam 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 contraction. 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 die 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 yam 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 yam, and each next weft yam has opposite torsion of equal magnitude compared to the preceding weft yarn. This yam disposition acts in a similar way as a counterbalancing shaft in an automobile engine, which is designed to produce a countereffect to the vibrations created by the engine main shaft (although the dynamic nature of that principle is altogether different from the quasistatic nature of the present invention). Such fabric geometry and yam disposition result in an optimally neutral behavior of the woven fabric reinforcement and the resulting laminated composite structure. BRIEF DESCRIPTION OF THE DRAWINGS The subject of the present invention will be described in the detailed description that follows also in conjunction with the drawings in which: FIG. 1 shows die definition of counterclockwise or Ztwist; FIG. 2 shows the definition of clockwise or Stwist; FIG. 3 shows the profile of the prior art reinforcement fabric; FIG. 4 shows the profile of a reinforcement fabric according to the present invention; FIG. 5 shows the XY movement of a laminate produced with 2116 fabric (state of the art); FIG. 6 shows the XY movement of a laminate produced with a reinforcement fabric according to the present invention. DETAILED DESCRIPTION OF THE INVENTION The process needed to produce reinforcement fabric, in particular glass fabric, follows the following steps: 1. Warping Operation Several hundred yams are gathered together to produce socalled primary beams. This is used because normally the numbers of warp yams needed to produce a fabric is go large that a single machine able to process all the warp yarns at once time would be inefficient in terms of quality and productivity. 2. Slashing Operation A certain number of primary beams are assembled together to form the warp yams of the fabric, by winding all the yams on a socalled loom beam. At the same time a size is applied to each yam to strengthen it and to increase its performance in the high speed looms used today. At present, the yam used in both warp and weft directions of a reinforcement fabric, in particular glass fabric, has always a counterclockwise torsion called Ztwist. For this reason, primary beams are normally produced without limitations as to their number, since they are all constructed in the same way. For example, it a particular fabric style has 3000 warp yams, its construction can be performed in many ways, including the following: A. 4 primary beams of 750 warp ends cash B. 5 primary beams of 600 warp ends each C. 6 primary beams of 500 warp ends each According to the preferred embodiment of the present invention the number of primary beams must always be even, in order to evenly distribute Ztwist and Stwist yams. As an example, items A. and C. above would have 50% of the number of primary beams made with counterclockwise torsion (Ztwist) and 50% of the number of primary beams made with clockwise torsion (Stwist), respectively. By alternating the Ztwist and Stwist primary beams one by one in the slashing machine creel, and having care to follow the same procedure in the threadin of the machine reeds, the resulting fabric will have oven warp yams with Ztwist and odd warp yarns with Stwist. It is possible to alternate them differently, as long as about 50% of the number of warp yams are with Ztwist and the remaining about 50% of the number of warp yams are with Stwist, and vice versa. 3. Weaving Operation The weaving operation is performed on weaving machines called looms. The operation consists in intertwining warp yarns and weft yams as to obtain the desired fabric construction. Yarns that run vertically along the material path (at 0°) form the warp, and those arranged crosswise (at 90°) form the weft. The intertwining arrangement between warp yams and weft yams is known as the weave. The reinforcement fabrics envisaged in the present invention are of different weaves such as satin, crowfoot, twill, basket, and other orthogonal weave types. A preferred embodiment of the present invention utilizes plain weave as the weave type. The types of looms used presently are equipped for inserting the weft into the fabric with a double weft feeder in order to decrease weft insertion speed. The present high speed of airjet or rapier looms requires this double insertion equipment which is fed by two independent yarn bobbins to drastically lower weft tension and thus avoiding overstress of the yarn. In order to have a balanced weft, as per the present invention, it is simple to use one yam bobbin with Ztwist and the other yarn bobbin with Stwist. Because the double insertion equipment draws alternatively a weft yam from the first bobbin and the next weft yam from the second yam bobbin, we can easily obtain the described balanced weft and resulting fabric. 4. Finishing Operation The reinforcement fabric obtained with the above described process can be treated in the finishing operation using conventional technology, which comprises a desizing operation, thermal or chemical, and the treatment with bonding agents in a finishing line. Reinforcement fabric constructions, in particular glass reinforcement fabrics, used by the laminate producers are characterized by different weights, and particularly in the range of from 25 grams per square meter up to 190 grams per square meter; they are characterized by having a high number of crossovers between warp and weft yarns, e.g. 500 or more. Crossovers are the points in the fabric structure where warp yams and weft yams cross each other. In a plain weave fabric, each crossover where a warp yam runs on top of a weft yarn (this warp yarn is called a raiser yam) is adjacent to a crossover where a warp yarn runs below a weft yam (this warp yarn is called a sinker yam). This is repeated in both the warp direction and the weft direction in a uniform way, so that if a plot of raiser yams (marked with a black square) and sinker yams (marked with a white square) is produced, the resulting image wilt have a checker board pattern. The undulated path that warp and weft yams have to follow has been found to be critical in the behavior of the resulting laminated composite structure and printed circuit board, where a higher number of crossovers determines additional stresses in the subsequent thermal and mechanical process steps, therefore determining and nonrepeatable behavior. In FIGS. 3 and 4 two different fabrics having same weight and similar thickness but different number of crossovers are depicted. It is evident that the reduced number of wasp yams and weft yams per unit area is compensated by higher yam count (weight of yam) to obtain the same weight of the fabric per unit area. A well known reinforcement fabric of about 100 grams per square meter weight is codenamed 2116 (see IPCEG140 specifications by The Institute for Interconnecting and Packaging Electronic Circuits). In this case fabric construction is as follows: 23.6 yarns per cm. in warp 22.8 yams per cm. in weft From the above data we can resume that the number of warp yams is 50.86% of the total number of warp and weft yams, diat the total number of crossovers is 538 crossovers per square centimeter, and that no requirement is specified for the number of twists needed, nor for their direction. A reinforcement fabric of the same weight per square area according to the present invention has the following characteristics; 20 yams per cm. in warp 12.6 yams per cm. in weft From the above data we can resume that the number of warp yams is 61.35% of the total number of warp and weft yams, that the total number of crossovers is 252 crossovers per square centimeter, and that the requirements for the number of twists needed and their disposition is according to the description given previously i.e. essentially equal torsion. When we compare test results of dimensional stability of the resulting laminated composite structure, the data are given in the Table and FIGS. 5 and 6 show the different movements of the laminates produced with fabric style 2116 (FIG. 5) and with a reinforcement fabric of the same weight per square area produced according to the present invention (FIG. 6), respectively. ax = Standard deviation of differential movement in the xdirection in ppm (parts per million); Ay = Average differential movement in the ydirection in ppm (parts per million); ax = Standard deviation of differential movement in the ydirection in ppm (parts per million). As can be obvious to someone expert in the art the new construction and yarn disposition of the fabric produced according to the present invention gives dimensional stability results that are simply unattainable by plain weave fabrics produced according to the present state of the art. WE CLAIM : 1. A woven reinforcement fabric for printed circuit boards of orthogonal construction with a weight per unit area of up to 190 grams per square meter, having warp and weft yarns, characterized by comprising a number of warp yarns which is greater than 55% of the total number of warp and weft yarns per unit area, the number of crossovers between warp yarns and weft yarns is between 200 and 315 crossovers per square centimeter; said fabric having warp and/or weft yam disposition such that between 40% and 60% of the number of warp and/or weft yarns have clockwise torsion of between 0.4 and 40 turns per meter, and between 40% and 60% of the number of warp and/or weft yarns have counterclockwise torsion of the same magnitude. 2. The woven reinforcement fabric as claimed in claim 1, having warp and weft yam disposition such that between 40% and 60% of the number of both warp and weft yarns have clockwise torsion of between 0.4 and 40 turns per meter, and between 40% and 60% of the number of both warp and weft yams have counterclockwise torsion of the same magnitude. 3. The woven reinforcement fabric as claimed in claim 1, wherein between 40% and 60% of the number of warp yarns have clockwise torsion of between 0.4 and 40 turns per meter and between 40% and 60% of the number of warp yams have counlterclockwise torsion of the same magnitude. 4. The woven reinforcement fabric as claimed in claim 1, wherein between 40% and 60% of the number of weft yams have clockwise torsion of between 0.4 and 40 turns per meter and between 40% and 60% of the number of weft yams have counterclockwise torsion of the same magnitude. 5. A laminated composite structure for printed circuit boards comprising the fabric as claimed in claims 1 to 4. 

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Patent Number  211536  

Indian Patent Application Number  IN/PCT/2001/1289/CHE  
PG Journal Number  17/2008  
Publication Date  25Apr2008  
Grant Date  05Nov2007  
Date of Filing  17Sep2001  
Name of Patentee  ISOLA FABRICS S. r. l  
Applicant Address  VIA MATTEOTTI, 120, BRUGHERIO,  
Inventors:


PCT International Classification Number  D03D 1/00  
PCT International Application Number  PCT/EP00/02680  
PCT International Filing date  20000327  
PCT Conventions:
