Abstract:
An electrical substrate material is presented comprising a thermosetting matrix of polybutadiene or polyisoprene in an amount of 25 to 50 volume %; a woven glass fabric in an amount of 10 to 40 volume %; a particulate, preferably ceramic filler, in an amount of from 5 to 60 volume %; and a flame retardant. Importantly, the composition comprises only a single resin, i.e., polybutadiene or polyisoprene. A peroxide cure initiator and/or crosslinking agent may optionally be added. The presence of a very high surface area particulate filler, preferably fumed silica, is also preferred, in that its presence results in a prepreg which has very little tackiness and can therefore be easily handled by operators. This low tackiness feature allows for the use of conventional automated layup processing, including foil cladding, using one or more known roll laminators. While the prepreg of this invention is tack-free enough to be handled relatively easily by hand, it is also tacky enough to be tacked to itself using a roll laminator (e.g., nip roller) at room temperature. The composition of this invention is particularly well suited for making electrical circuit substrates for microwave and digital circuits, typically in the form of the thermosetting composition being laminated onto one or both opposed surfaces to a metal conductive foil such as copper.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 322,890, filed on Oct. 13, 1994, now U.S. Pat. No. 5,571,609. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to a method of making thermosetting composites and the resulting product which preferably comprises electrical circuit laminate materials. More particularly, this invention relates to an electrical circuit laminate comprising (1) a thermosetting resin of polybutadiene or polyisoprene; (2) a woven fibrous web impregnated with the resin; and (3) inorganic particulate filler such as silica, titania and the like. In accordance with an important feature of this invention, the filler loadings for both the woven web (fabric) and particulate filler are selected such that individual layers are relatively tack free thereby allowing ease of handling for lamination without the need for B-staging. The material of this invention allows for relatively low lamination temperatures. 
     Commonly assigned U.S. Pat. No. 5,223,568 (which is fully incorporated herein by reference) describes a thermosetting composition which is particularly useful for making electrical substrate materials. In general, U.S. Pat. No. 5,223,568 describes a composition formed from the steps of: 
     (a) providing a moldable thermosetting composition that includes (1) polybutadiene or polyisoprene resin which is a liquid at room temperature and which has a molecular weight less than 5,000 and (2) a solid butadiene or isoprene-containing polymer capable of cross-linking with the polybutadiene or polyisoprene resin; 
     (b) forming the composition into a shape; and 
     (c) curing the composition to produce the electrical substrate material including subjecting the composition to a high temperature cure condition at a temperature greater than about 250° C. and less than the decomposition temperature of the composition. This composition thus comprises a two component system, the first component being the polybutadiene or polyisoprene resin and the second component being the solid butadiene or isoprene-containing polymer, all of which are subjected to the high temperature curing cycle (e.g., greater than 250° C.). 
     In preferred embodiments, the solid polymer is a thermoplastic elastomer block copolymer. 
     U.S. Pat. No. 5,223,568 also describes a composition with a dielectric filler (i.e., a material having a dielectric constant greater than about 1.2 at microwave frequencies) homogeneously dispersed throughout the composition to the extent that when the composition is cured the properties of the cured article, e.g., dielectric constant and coefficient of thermal expansion, do not vary more than about 5% throughout the article. 
     In preferred embodiments, the composition of U.S. Pat. No. 5,223,568 further includes a crosslinking agent capable of co-curing (i.e., forming covalent bonds) with the polybutadiene or polyisoprene resin thermoplastic elastomer, or both. Examples of preferred crosslinking agents include triallylcyanurate, diallylphthlate, divinyl benzene, a multifunctional acrylate, or combinations of these agents. 
     When the electrical substrate material disclosed in U.S. Pat. No. 5,223,568 includes a dielectric filler, the volume % of the filler (based upon the combined volume of resin, thermoplastic elastomer, crosslinking agent, if any, and filler) is between 5 and 80%, inclusive. Examples of preferred fillers include titanium dioxide (rutile and anatase), barium titanate, strontium titanate, silica (particles and hollow spheres); corundum, wollastonite, polytetrafluoroethylene, aramide fibers (e.g., Kevlar), fiberglass, Ba 2  Ti 9  O 20 , glass spheres, quartz, boron nitride, aluminum nitride, silicon carbide, beryllia, or magnesia. They may be used alone or in combination. 
     The method disclosed in U.S. Pat. No. 5,223,568 provides a wide variety of shaped articles having favorable isotropic thermal and dielectric properties. These properties can be tailored to match or complement those of ceramic materials, including gallium arsenide, alumina, and silica. Thus, the cured articles can replace ceramic materials in many electronic and microwave applications, for example, as specialized substrates for high speed digital and microwave circuits. Examples of microwave circuits include microstrip circuits, microstrip antennas, and stripline circuits. The cured products are also useful as rod antennas and chip carriers. 
     While well suited for its intended purposes, the circuit laminate materials described in U.S. Pat. No. 5,223,568 do suffer from several drawbacks. For example, these prior art materials require a high temperature cure (e.g., lamination) of greater than about 250° C. (482° F.); and this requirement is problematic for several reasons. First, conventional circuit fabrication equipment often exhibit temperature limits of 360° F., which is below that of the 482° F. requirement. Still another drawback is that flammability ratings required by Underwriters Laboratory UL 94-VO lead to the need for inclusion of a flame retardant additive such as a bromine-containing fire retardant. Unfortunately, typical bromine-containing fire retardant additives cannot withstand the high temperature cure conditions of greater than 250° C., undergoing decomposition and/or chemical degradation at these temperatures. 
     In addition to the foregoing need for lowering the cure (lamination) temperature of the circuit laminates, there is also a problem with the inherent tackiness associated with the polybutadiene or polyisoprene laminate prepregs. Because of the tacky nature of the prepreg, it is not practically feasible to use a continuous, automated layup process for laminating circuit materials. Roll lamination equipment is commonly available, however, and the ability to provide a polybutadiene or polyisoprene based prepreg of the type described in U.S. Pat. No. 5,223,568 which is also usable in an automatic layup process would greatly reduce the manufacturing cost and processing time, leading to a significant increase in the commercial success of the resultant circuit material products. 
     In addition to U.S. Pat. No. 5,223,568, there are other prior art patents and literature of some interest which respect to the use of filled circuit laminates in general, and polybutadiene based circuit laminates in particular. For example, in an article entitled &#34;A New Flame Retardant 1,2-Polybutadiene Laminate&#34; by N. Sawatari et al., IEEE Transactions on Electrical Insulation, Vol. EI-18, No. 2, April 1983, the limitations on presently-used polybutadiene based laminates is discussed, including the fact that 1,2-polybutadiene (PBD) is difficult to control in the semi-cured condition (the B-stage), difficult to make non-tacky, highly flammable, and exhibits low copper bond. The article thereafter describes a composition which addresses these issues. The composition described uses a high percentage of very high molecular weight PBD to eliminate tackiness and B-staging. Also, a low molecular weight, modified PBD resin is used as a minor component to aid in copper bond and flow during lamination. There is no mention of using filler of any type. 
     U.S. Pat. No. 5,264,065 to Kohm describes a base material for printed wiring boards where inert filler is used to control Z-axis CTE in fiberglass-reinforced thermoset resins. The Kohm patent discloses a range of 45-65 weight % fiberglass reinforcement and a range of 30 to 100 parts filler per 100 parts of the polymer. There is no disclosure in Kohm of a polybutadiene or like material for the resin system. 
     U.S. Pat. No. 4,997,702 to Gazit et al. discloses a circuit laminate having an epoxy resin system which also includes inorganic fillers or fibers in the range of 20-70 weight % of the total composite. The fibers include both glass and polymeric fibers and the fillers include clay or mineral (e.g. silica) particulate fillers. 
     U.S. Pat. No. 4,241,132 to Pratt et al. discloses an insulating board comprising a polymeric matrix such as polybutadiene and a filler consisting of polymeric filler, for example fibrous polyproylene. In all cases, the dielectric constant or dissipation factor of the resin matrix is matched to the fibrous reinforcement in order to obtain an isotropic composite. 
     European Patent No. 0 202 488 A2 discloses a polybutadiene-based laminate wherein a high molecular weight, bromine-containing prepolymer is used to reduce tack and flammability of a 1,2-polybutadiene resin. Similarly, in Japanese Patent No. 04,258,658, a high molecular weight compound is added to a tacky PBD resin to control tack. The compound utilized is a halogen-containing bismaleimide which provides flammability resistance, as well as good copper bonding and heat resistance. There is no mention of the use of fillers and the resulting laminate will have a relatively high dissipation factor. 
     An article entitled &#34;1,2-Polybutadienes-High Performance Resins for the Electrical Industry&#34;, by R. E. Drake, ANTEC &#39;84 pp. 730-733 (1984), generally discloses conventional polybutadiene resins for use in laminates and specifically discloses the use of reactive monomers to co-cure with the PBD. 
     U.K. Patent Application No. 2 172 892 A generally discloses laminates composed of styrene-containing and thermoplastic copolymers with unsaturated double bonds and polybutadiene. 
     Notwithstanding the foregoing examples of laminate composites, there continues to be a need for improved polybutadiene laminates having a combination of electrical, chemical, mechanical and thermal properties which are not presently available including flame retardance, improved CTE, ability to tailor dielectric constant, improved dissipation factor, low cost and low tackiness. 
     SUMMARY OF THE INVENTION 
     The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by the electrical substrate material of the present invention. In accordance with one embodiment of the present invention, an electrical substrate material is provided which comprises a thermosetting matrix which includes a polybutadiene or polyisoprene resin and an unsaturated butadiene or isoprene containing polymer in an amount of 25 to 50 volume percent; a woven glass fabric in an amount of 10 to 40 volume %; a particulate, preferably ceramic, filler in an amount of from 5 to 60 volume %; a flame retardant; and a peroxide cure initiator. A preferred composition has 18% woven glass, 41% particulate filler and 30% thermosetting matrix. 
     The foregoing component ratios, and particularly the relatively high range of particulate filler, is an important feature of this invention in that this filled composite material leads to a prepreg which has very little tackiness and can therefore be easily handled by operators. This low tackiness feature allows for the use of conventional automated layup processing, including foil cladding, using one or more known roll laminators. While the prepreg of this invention is tack-free enough to be handled relatively easily by hand, it is also tacky enough to be tacked to itself using a roll laminator (e.g., nip roller) at room temperature. In addition, another important feature of this invention is the low amount of glass fabric filler relative to the higher range of particulate filler which leads to improved (lower) CTE in the Z axis or thickness direction, improved electrical performance (e.g., dissipation factor), lower cost and the ability to tailor dielectric constant through appropriate selection of particulate fillers. 
     Still another important feature of this invention is that the cure temperature for lamination is significantly lower than required by U.S. Pat. No. 5,223,568 with lamination temperatures typically in the range of 330° to 425° F. The low temperature cure conditions allow for the use of bromine containing fire retardant additives as well as permitting the use of conventional circuit fabrication (e.g., lamination) equipment. The relatively low temperature cure is achieved using an appropriate amount of organic peroxide such as dicumyl peroxide and t-butyl perbenzoate peroxide. 
     In accordance with another embodiment of the present invention, an electrical substrate material is provided which comprises a thermosetting matrix which includes a single resin, polybutadiene or polyisoprene resin, in an amount of 25 to 50 volume %; a woven glass fabric in an amount of 10 to 40 volume %; and a particulate filler in an amount of from 5 to 60 volume %. A preferred composition has 18% woven glass, 41% particulate filler and 30% thermosetting matrix. Preferably, the composition further comprises a high surface area filler, preferably fumed silica, to reduce tack; a flame retardant; and a peroxide cure initiator. 
     The electrical substrate material of this invention includes a plurality of woven webs (such as E-glass webs) embedded in a mixture of the polybutadiene or polyisoprene based resin system and inorganic filler (e.g., silica) laminated between one or two sheets of conductive foils (e.g., copper) to produce a circuit board material which is especially well suited for microwave applications. Of course, if very thin (e.g., less than 3 mil thickness) cross-sections are desired, then only a single saturated web may be used for the dielectric. 
     The above-discussed and other features and advantages of the present invention will be appreciated and understood by those of ordinary skill in the art from the following detailed description and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings, wherein like elements are numbered alike in the several FIGURES: 
     FIG. 1 is a schematic representation of a processing line for the continuous manufacture of the prepreg in accordance with the present invention; 
     FIG. 2 is a cross-sectional elevation view of a circuit substrate material in accordance with the present invention; and 
     FIGS. 3-5 are test data depicting the improved (lower) tackiness of the B-staged circuit substrate in accordance with the polybutadiene or polyisoprene/unsaturated polymer embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention comprises a thermosetting matrix in the amount of 25 to 50 volume %, a woven glass fabric in the amount of 10 to 40 volume %, and inorganic particulate filler in the amount of 5 to 60 volume %. In addition, the present invention includes one or more organic peroxide initiators in an amount of 1.5 to 6 phr and a bromine-containing fire retardant additive in an amount effective to provide fire retardance. 
     The resin system, fillers, cross-linking agents, woven web, fire retardant additives, peroxide initiators, processing conditions and representative constructions together with Examples will now be discussed in detail. 
     Resin System 
     In one embodiment of the present invention, the resin system used in the electrical substrate material of this invention is a thermosetting composition generally comprising (1) a polybutadiene or polyisoprene resin (or mixture thereof); and (2) an unsaturated butadiene or isoprene-containing polymer capable of participating in cross-linking with the polybutadiene or polyisoprene resin during cure. The polybutadiene or polyisoprene resins may be liquid or solid at room temperature. Liquid resins may have a molecular weight greater than 5,000 but preferably have a molecular weight of less than 5,000 (most preferably between 1,000 or 3,000). The preferably liquid (at room temperature) resin portion maintains the viscosity of the composition at a manageable level during processing. It also crosslinks during cure. Polybutadiene and polyisoprene resins having at least 90% 1,2 addition by weight are preferred because they exhibit the greatest crosslink density upon cure owing to the large number of pendent vinyl groups available for crosslinking. High crosslink densities are desirable because the electrical circuit substrates exhibit superior high temperature properties. A preferred resin is B3000 resin, a low molecular weight polybutadiene liquid resin having greater than 90 weight % 1,2 addition. B3000 resin is commercially available from Nippon Soda, Ltd. 
     The unsaturated polymer preferably comprises a thermoplastic elastomer and more preferably includes a linear or graft-type block copolymer that preferably has a polybutadiene or polyisoprene block with at least 50% by weight, 1,2 addition and a thermoplastic block that preferably is styrene or α-methyl styrene. The high proportion of 1,2-addition in the polyisoprene or polybutadiene block leads to high crosslink densities after the curing step, as is the case with the polybutadiene or polyisoprene resin described above. A preferred copolymer is a styrene-butadiene-styrene triblock copolymer, e.g., Kraton DX1300 (commercially available from Shell Chemical Corp.). 
     The thermoplastic elastomer may also contain a second block copolymer similar to the first except that the polybutadiene or polyisoprene block is hydrogenated, thereby forming a polyethylene block (in the case of polybutadiene) or an ethylene-propylene copolymer (in the case of polyisoprene). When used in conjunction with the first copolymer, materials with greater &#34;toughness&#34; can be produced. Where it is desired to use this second block copolymer, a preferred material is Kraton GX1855 (commercially available from Shell Chemical Corp.) which is believed to be a mixture of styrene-high 1,2 butadiene-styrene block copolymer and styrene-(ethylene-propylene)-styrene block copolymer. Particularly preferred compositions are those in which the resin is polybutadiene, the first block copolymer is styrene-butadiene-styrene triblock copolymer (m=n=1), and the second block is styrene-(ethylene-propylene)-styrene triblock copolymer (m=n=1), the ethylene-propylene block being the hydrogenated form of an isoprene block. 
     Thus, in a preferred embodiment, the unsaturated polymer comprises a solid thermoplastic elastomer block copolymer having the formula X m  (Y--X) n  (linear copolymer or ##STR1## (graft polymer) where Y is a polybutadiene or polyisoprene block, X is a thermoplastic block, and m and n represent the average block numbers in the copolymer, m being 0 or 1 and n being at least 1. The composition may further include a second thermoplastic elastomer block copolymer having the formula W p  --(Z--W) q  (linear copolymer) or ##STR2## (graft copolymer) where Z is a polyethylene or ethylene-propylene copolymer block, W is a thermoplastic block, and p and q represent the average block numbers in the copolymer, p being 0 and 1 and q being at least 1. 
     Preferably, the polybutadiene or polyisoprene resin and the polybutadiene or polyisoprene block of the first block copolymer making up the thermoplastic elastomer have at least 90% by weight 1,2 addition. The volume to volume ratio of the polybutadiene or polyisoprene to the thermoplastic elastomer preferably is between 1:9 and 9:1, inclusive. 
     Other free radical curable polymers which can co-cure with butadiene polymers may be added (such that 1,2-butadiene polymers are still the major polymeric ingredient) for specific property or processing modifications. Such possible modification purposes include toughness, adherability to copper foil and copper plating, and prepreg handling characteristics. These co-curable polymers include random and block copolymers of primarily 1,3-addition butadiene or isoprene with styrene, alpha-methyl styrene, acrylate or methacrylate, or acrylonitrile monomers; homopolymers or copolymers of ethylene, such as polyethylene, ethylene-propylene copolymer and ethylene-propylene-diene terpolymers, ethylene-ethylene oxide copolymers; natural rubber; norbornene polymers such as polydicyclopentadiene; hydrogenated diene polymers such as hydrogenated styrene-isoprene-styrene copolymers and butadiene-acrylonitrile copolymers; and others. Levels of these co-curable polymers should be less than 50% of the total polymeric component. 
     In still another preferred embodiment of the present invention, the resin system comprises only a single resin, i.e., either polybutadiene or polyisoprene alone, without a second polymeric component. As with the two-component resin systems, the polybutadiene or polyisoprene resins may be liquid or solid at room temperature. Liquid resins may have a molecular weight greater than 5,000 but preferably have a molecular weight of less than 5,000 (most preferably between 1,000 or 3,000). The preferably liquid (at room temperature) resin portion maintains the viscosity of the composition at a manageable level during processing. 
     Polybutadiene and polyisoprene resins having at least 90% 1,2 addition by weight are preferred because they exhibit the greatest crosslink density upon cure owing to the large number of pendent vinyl groups available for crosslinking. High crosslink densities are desirable because the electrical circuit substrates exhibit superior high temperature properties. A preferred resin is B3000 resin, a low molecular weight polybutadiene liquid resin having greater than 90 weight % 1,2 addition. B3000 resin is commercially available from Nippon Soda, Ltd. 
     Particulate Filler Material 
     The volume % of the filler (based upon the combined volume of the resin system, woven fabric and particulate filler) is between 5 and 60%, inclusive and preferably between 30% and 50%. Examples of preferred fillers include titanium dioxide (rutile and anatase), barium titanate, strontium titanate, silica (particles and hollow spheres) including fused amorphous silica and fumed silica; corundum, wollastonite, aramide fibers (e.g., Kevlar), fiberglass, Ba 2  Ti 9  O 20 , glass spheres, quartz, boron nitride, aluminum nitride, silicon carbide, beryllia, alumina or magnesia. They may be used alone or in combination. 
     In an important and preferred feature of this invention, the particulate filler is present in an amount which is (1) greater than the amount (in volume %) of thermosetting composition preferably the ratio of filler to thermosetting composition is 45:55) and (2) greater than the amount (in volume %) of the woven fabric. 
     Particularly preferred fillers are rutile titanium dioxide and amorphous silica because these fillers have a high and low dielectric constant, respectively, thereby permitting a broad range of dielectric constants combined with a low dissipation factor to be achieved in the final cured product by adjusting the respective amounts of the two fillers in the composition. To improve adhesion between the fillers and resin, coupling agents, e.g., silanes, are preferably used. 
     As will be discussed hereinafter, the material described herein is preferably used as a dielectric substrate in a circuit laminate wherein a layer of metal is laminated thereto. Preferably, the filler material and quantity thereof is selected so as to provide the substrate with a coefficient of thermal expansion which is equal or substantially equal to the coefficient of thermal expansion of the metal layer. 
     With respect to the single resin embodiment of the present invention, a very high surface area particulate filler such as fumed silica may be additionally used to prevent tackiness and stickiness in the prepreg. The preferred fumed silica is available from Degussa under the trade name AEROSIL 200, and has a surface area of around 200 m 2  /g, with a typical primary particle size of 12 nm. 
     The amount of fumed silica used may be in the range from about 0.2 volume % to about 5 volume percent, and preferably in the range from about 0.5 volume % to about 1.5 volume %. Thus, the loading level of such silica may be as low as 1% by weight and still yield a prepreg with very similar tack characteristics to those found in the two-component system. Apparently, such very high surface area silica is much more effective at reducing tack than the standard sized amorphous silica (10 μm median), as the addition of only about 1% fumed silica made a large reduction in tack to a prepreg already containing 66% amorphous silica. 
     Woven Web 
     The fiber reinforcement comprises woven, thermally stable webs of a suitable fiber, preferably glass (E, S, and D glass) or high temperature polyester fibers (e.g., KODEL from Eastman Kodak). The web is present in an amount of about 10 to 40 volume %, and preferably about 15 to 25 volume % with respect to the entire laminate. Such thermally stable fiber reinforcement provide the laminate with a means of controlling shrinkage upon cure within the plane of the laminate. In addition, the use of the woven web reinforcement renders a dielectric substrate with a relatively high mechanical strength. 
     Preferred examples of the woven fiberglass web used in the present invention are set forth in the following Table: 
     
         ______________________________________FIBERGLASS WOVEN WEBSManufacturer    Style   Thickness (in.)______________________________________Fiber Glast     519-A   0.0015Clark-Schwebel  112     0.0032Clark-Schwebel  1080    0.0025Burlington      106     0.0015Burlington      7628    0.0068______________________________________ 
    
     Fire Retardant Additive 
     Preferably, the present invention includes a flame retardant such as a bromine-containing flame retardant in an amount of 20 to 60 volume % or 2 to 6 PHR. Examples of suitable brominated flame retardants include Saytex BT 93W (ethylene bistetrabromophthalimide), Saytex 120 (tetradecabromodiphenoxy benzene) or Saytex 102 (decabromo diphenoxy oxide). 
     Curing Agent 
     A curing agent is added to the composition to accelerate the curing reaction. When the composition is heated, the curing agent decomposes to form free radicals, which then initiate crosslinking of the polymeric chains. Preferred curing agents are free radical cure initiators such as organic peroxides, e.g., dicumyl peroxide, t-butylperbenzoate and t-butylperoxy hexyne-3, all of which are commercially available. The peroxide curing agent is provided in an amount of between 1.5 and 6 phr. 
     Processing 
     In general, the two-component thermosetting compositions are processed as follows. First, the polybutadiene or polyisoprene resin, thermoplastic elastomer, particulate fillers, curing agents, flame retardants, and coupling agent (if any) are thoroughly mixed to form a slurry in conventional mixing equipment. The mixing temperature is regulated to avoid substantial decomposition of the curing agent (and thus premature cure). Mixing continues until the particulate filler is uniformly dispersed throughout the resin. The particulate filler may be pretreated with coupling agents (preferably silanes) in a separate step for more efficient use of the agents. 
     Next, conventional prepreg manufacturing methods can be employed. Typically the web is impregnated with the slurry, metered to the correct thickness, and then the solvent is removed (evaporated) to form a prepreg. 
     One processing system useful in making a prepreg in accordance with the present invention is shown in FIG. 1 and includes (from left to right) a roll 10 of woven glass web 11 which unwinds through a first accumulator 12 to a series of drive rolls 14. The web 11 then passes into the coating area 16 where the web is passed through a saturation tank 20 (which contains the mixture of polymer, particulate filler, solvent and other components) and then through a pair of metering rolls 22. The web 11 thereafter travels the length of a drying tower 18 for a selected period of time until the solvent is evaporated from the web whereupon the web passes through drive rolls 28, a second accumulator 30 and finally web 11 (which is now a prepreg) is wound onto roll 32. 
     The lamination process entails a stack-up of one or more prepreg layers between one or two sheets of conductive foil (copper). This stack-up is then densified and cured via lamination or a combination of lamination and oven baking. 
     The stack-up is cured in a conventional peroxide cure step; typical cure temperatures are between 330° and 425° F. (165° to 218° C.). In accordance with an important feature of this invention, unlike U.S. Pat. No. 5,223,568, no additional high temperature cure step is needed to increase crosslink density. It will be appreciated that the &#39;568 patent requires a high temperature cure where the temperature is greater than about 250° C. but less than the decomposition temperature of the resin (typically about 400° C.). 
     Referring now to FIG. 2, a cross-sectional view of electrical substrate material in accordance with the present invention is shown generally at 40. Electrical substrate 40 has been laminated in accordance with one of the processes described above wherein a woven web 42 is impregnated with a resin system/filler composition 44 and laminated between two copper foils 46, 46&#39; to produce a circuit board laminate. As discussed above with reference to the processing conditions, the resin system 44 may either be cast onto woven web 42 using known prepreg manufacturing equipment or web 42 may be saturated by resin system 44 by sandwiching web 42 between a pair of bond plys formed from resin system 44 and laminating the stack up together with the copper cladding 46, 46&#39;. While FIG. 2 depicts a single layer of woven web 42, it will be appreciated that typically a plurality of layers of saturated web 12 will be used in forming circuit laminates in accordance with the present invention. However, a single layer as shown in FIG. 2 is desirable where very thin cross-sections (less than 3 mils) are required. 
     With respect to the single-resin embodiment according to the present invention, processing and laminate production was similar to the methods described above. Thus, the components were mixed with solvent (xylene), saturated onto 1080 fiberglass, and dried at room temperature to make prepregs. These prepregs were then stacked (6 layers), placed between sheets of 1-oz. ED copper foil and laminated according to the above procedures. Initially, typical lamination conditions were used, i.e., heating at 350 of for 1.5 hours at 350 psi (Samples 6 and 7). 
     However, the properties of samples laminated at 350 psi indicated that the samples were not being fully densified: the specific gravity, dielectric constant, and copper adhesion were all low. For this reason, samples were also laminated at 989 psi (Example 8). The resulting laminates displayed considerably better copper adhesion, as well as more typical specific gravity and dielectric strength values. The low dielectric strength and dissipation factor values were still not optimal, probably due to the tendency of the fumed silica to adsorb moisture onto its surface. (The manufacturer&#39;s literature in fact states that up to 1.5% by weight of the silica could be water.) Either a pre-drying or surface treatment step using silane or other hydrophobic coating may be employed to resolve these problems. Any silane or other hydrophobic coating would need to be compatible with the resin system of the present invention. 
     EXAMPLES 
     The following non-limiting examples further describe the electrical substrate material of the present invention. 
     A. Tackiness 
     In an effort to quantify the tackiness of the prepreg of this invention, a peel test was developed and applied to a number of samples. 
     Test Description 
     Attempting to simulate a 90° peel test, an Instron (Model 1125) was used to peel apart 2 layers of prepreg. The individual layers of prepreg were 1&#34; wide and 12&#34; long and were stuck together by rolling a 10 lb roller over them in the lengthwise direction. A 50 lb tensile load cell was used in conjunction with a 15 ipm crosshead speed to measure the peel force or tack. For verification of the data, the tests were duplicated using a 10 lb cell and 12 ipm. 
     Test Samples 
     Four prepreg formulations were created for this test as shown in the table below. Using only 106 style fiberglass, prepregs were made in which the particulate filler, resin and rubber contents varied. 
     
         ______________________________________   Prepreg Component, % by volume             Resin   Rubber  WovenSample No.     Filler  (PBD)   (SBS)   Glass Other______________________________________2075-62-1 25      32      20      18    52075-62-2 34      26      16      18    62075-62-3 25      41      11      18    52075-62-5 34      34      9       18    5______________________________________ 
    
     Results 
     The measured peel values range from 0 to 0.35 pli. FIG. 3 plots the tack versus SBS (rubber) content for the two filler loadings used. From this plot, the resulting tack is more sensitive to rubber content at the lower filler loading. Alternatively, FIG. 4 plots the tack versus filler content. In this case, the resulting tack is more sensitive to filler content at the lower rubber loading. FIGS. 3 and 4 lead to the conclusion that the combination of filler and rubber volumes effect tack. If one looks at the resulting resin contents versus tack (FIG. 5), there is a good correlation between resin volume present and tack. Put simply, it appears that an important factor for tackiness is the amount of resin present and to reduce tack it does not matter whether the resin is displaced by filler or rubber so long as the resin is displaced. 
     B. Two-Component Polybutadiene/Polyisoprene Example Formulations 
     The following five examples show representative electrical, thermal and mechanical data for the two-component laminates of the present invention. In accordance with an important feature of this invention, the dissipation factor of the resultant laminate is less than or equal to 0.007, which renders this material well suited for use in microwave applications. 
     Example 1 
     This is a preferred embodiment. The laminate consisted of five layers of prepreg for a total dielectric thickness of 0.023&#34;. Note the high filler and low rubber contents. 
     Example 2 
     This sample contained a particulate filler loading lower than the preferred embodiment but had a higher rubber content. The laminate was made with five layers of 1080 woven glass resulting in a dielectric thickness of 0.021&#34;. 
     Example 3 
     This sample contained titania (TiO 2 ) particulate filler compared to the silica (SiO 2 ) found in the other examples. The laminate contained 10 layers of 1080 woven glass resulting in a dielectric thickness of 0.028&#34;. 
     Example 4 
     This laminate had the rubber replaced in the formulation with more resin. The laminate&#39;s construction utilized four layers of 1080 woven glass and resulted in a dielectric thickness of 0.016&#34;. 
     Example 5 
     This sample contained more rubber than resin. The resulting laminate was 0.020&#34; thick and utilized five layers of 1080 woven glass. 
     
         ______________________________________Component, Source        Sample No.(% by total weight)        1       2       3     4     5______________________________________Formulations, Examples 1-5B3000 Resin, Nippon        14.6    13.5    12    21.2  4.4SodaD1300 (Kraton) Rubber,        3.8     9.3     3     0     13.1ShellFB-35 Fused Silica,        0       44.7    0     0     51.3DenkaMinsil 5 Fused Silica,        51      0       0     45.7  0MincoTicon HG Titania, TAM        0       0       53.2  0     0Woven Glass, 25      26      27    26    25Clark-SchwebelSilane       0.5     0.4     0.6   0.4   0.5Brominated Flame        4.5     4.6     3.7   5.3   4.4RetardantCatalyst (peroxide)        0.6     1.5     0.5   1.4   1.3Properties, Examples 1-5Dielectric Constant*        3.47    3.35    9.54  3.22  3.42Dissipation Factor**        0.0035  0.0042  0.0059                              0.0053                                    0.0035Specific Gravity (g/cc)        1.83    1.75    2.47  1.76  1.83Copper Bond (pli)        4.4     5       2.1   3.2   3.7Water Absorption (% wt)        N/A     0.09    N/A   N/A   0.67______________________________________*Dielectric constant (Dk) values are the averages of the measured Dk&#39;sfrom a 1-10 Ghz frequency sweep.**Dissipation Factor (Df) values are the lowest recorded value of agiven1-10 Ghz frequency sweep.Specific gravity test method:           ASTM D792-91Copper bond:    IPC-TM-650                     2.4.8Water absorption           IPC-TM-650                     2.6.2.1 (with 48 hr exp.) 
    
     In general, the preferred formulations of the present invention minimize fiber content and maximize particulate filler content. This high particulate filler content (preferred ratio of resin to filler is 45:55) leads to improved (lower) CTE in the Z-axis direction, improved electrical performance, lower cost and other features and advantages. While the prior art discussed above generally discloses filled polybutadiene laminates and laminates based on other resin systems which employ fiber and particulate fillers, no prior art to which Applicants are aware utilize the combination of: 
     1. thermosetting matrix comprised of polybutadiene or polyisoprene resin and an unsaturated butadiene or isoprene containing polymer in an amount of 25 to 50 volume %; 
     2. woven fabric reinforcement in an amount from 10 to 40 volume %; and 
     3. particulate filler in an amount of 5 to 60 volume %. 
     In general, the present invention utilizes a higher particulate filler level and a lower woven fabric level. Preferably, the use in the thermosetting composition of a high molecular weight copolymer permits the use of these higher levels of particulate fillers, all of which leads to improved Theological control, reduced tack and reduced shrinkage. More particularly, the combination of an extremely high loading level of particulate filler, woven glass and an unsaturated polymer while still being processable using standard industry equipment represents an important advance in the circuit materials field. In addition, the unexpected result that the particulate filler can be used to eliminate tackiness of the liquid polybutadiene resin represents still another important feature of this invention. 
     C. Single Resin Polybutadiene/Polyisoprene Formulations 
     The following examples show representative electrical, thermal, and mechanical data on the preferred single resin embodiment of the laminate of the present invention, wherein only a single resin system comprising a polybutadiene or a polyisoprene resin is used. The entire SBS rubber portion of the formulation is replaced on a weight basis with additional liquid resin. 
     Example 6 
     The final laminate properties of this example, where the unsaturated rubber component is replaced by liquid resin on a weight basis, are very similar to those of the two-component system. The key differences are a loss of flexural strength and in increase in flexural modulus, as well as in increase in xylene absorption. Since the unsaturated rubber component can act as a toughening agent, the changes in flexural properties are expected. However, direct replacement of the unsaturated rubber component with additional liquid resin resulted in very tacky, sticky prepregs. This level of tack can pose handling problems in some applications. 
     Example 7 
     In this Example, very high surface area fumed silica has been added to the composition, thereby eliminating tack and stickiness in the prepreg. Lamination was performed at 350 psi. Except for dissipation factor and dielectric strength, the resulting materials have very similar properties to those observed in the above-described two-component system. Additional treatment of the fumed silica may improve these properties. 
     Example 8 
     In this Example, lamination of a single resin system containing fumed silica was laminated at 989 psi. 
     Example 9 
     Example 9 is a representative two-component resin system according to the invention described above, comprising a polybutadiene or polyisoprene resin and an unsaturated polymer, included for comparison purposes. 
     
         ______________________________________Component, Source        Sample No.(% by total weight)          6       7        8     9______________________________________Formulations, Examples 6-9B3000 Resin, Nippon          16.9    16.7     16.7  12.6SodaCE44I Amorphous Silica,          49.5    49       49    46.2CE MineralsAerosil 200 Fumed Silica,          --      0.7      0.7   --DegussaA174 Silane, OSI          0.5     0.5      0.5   0.4Luperox 500R Catalyst,          0.7     0.7      0.7   0.6Elf AtochemBT-93W Flame   7.4     7.4      7.4   7.0Retardant, Albemarle850BD (Vector) --      --       --    3.2Rubber, DexcoWoven G1ass,   25      25       25    30Clark-SchwebelProperties, Examples 6-19Lamination Pressure (psi)          350     350      989   350Thickness (mils)          23.4    23       24    20Copper Bond (pli)          5.0     3.0      4.8   5.4% Bow (after etch on          7.2     7.1      5.6   6.0one side)% Twist (after etch on          6.3     6.3      6.9   5.0one side)Dielectric Strength          783     688      631   800+(V/mil)Specific Gravity (g/cc)          1.89    1.79     1.86  1.86MD Flexural Strength          27.2    28.3     31.3  33.5(Kpsi)CMD Flexural Modulus          23.8    21.6     26.3  28.0(Kpsi)Dielectric Constant*          3.40    3.25     3.46  3.45Dissipation Factor**          0.0041  0.0045   0.0055                                 0.0040Xylene Absorption          2.55    1.98     1.87  1.60(% wt)______________________________________*Dielectric constant (Dk) values are the averages of the measured Dk&#39;sfrom a 1-10 Ghz frequency sweep.**Dissipation Factor (Df) values are the lowest recorded value of agiven1-10 Ghz frequency sweep.Specific gravity test method:           ASTM D792-91Copper bond:    IPC-TM-650                     2.4.8Xylene absorption           IPC-TM-650                     2.6.2.1 (with 48 hr                     exposure) 
    
     The above Examples 6-9 demonstrate that a single resin composition according to the present invention can be used to form circuit board laminates with desirable properties. With the addition of a small amount of a high surface area particulate, preferably fumed silica, the composition is readily workable for manual and automated handling, and thus economical to prepare. 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.