WO1997048752A1

WO1997048752A1 - Composite structures and prepreg therefor

Info

Publication number: WO1997048752A1
Application number: PCT/US1997/008943
Authority: WO
Inventors: John P. Everett; Douglas E. Turek
Original assignee: The Dow Chemical Company
Priority date: 1996-06-17
Filing date: 1997-05-23
Publication date: 1997-12-24
Also published as: AU3286697A

Abstract

A prepreg useful for the production of composites and copper foil-laminated or similar printed circuit boards having a high heat resistance and high dimension stability, comprises a syndiotactic vinylaromatic polymer fibrous base material and a thermosetting resin composition.

Description

COMPOSITE STRUCTURES AND PREPREG THEREFOR

The present invention relates to a thermosetting resin-impregnated prepreg. More particularly, the present invention relates to a thermosetting resin composition- impregnated prepreg that is valuable for the production of composite structures such as printed circuit boards (PCB's) having excellent heat resistance and dimensional stability and in which the electncal resistance remains very large even in an electric field under high-temperature and high-humidity conditions

Recently, PCB's compπsing aramid fiber based prepregs having an excellent heat resistance and dimensional stability have been developed for applications in which a high heat resistance and dimensional stability are required. These PCB's are prepared by laminating a copper foil and at least one prepreg formed by impregnating a woven fabπc, paper-like sheet omonwoven fabric composed of polymeric fibers with a varnish composition compnsing a rigid thermosetting resin such as an epoxy resin, vinyl ester resin, or the like.

For example, US-A-5,436,301 discloses a printed circuit board formed by impregnating a fabric composed mainly of aramid fibers with a resin composition compπsing an epoxy resin and a curing agent. Additional PCB's based on mixtures of glass, aramid and other polymeπc fibrous base materials and resins such as vinyl ester resins are disclosed in: JP-A-6/255027 (9/13/94), JP-A-3/113642 (5/14/91 ), JP- A-1/196337 (8/8/89), and JP-A-62/274688 (11/28/87). In US-A-5,165,990, a stampable sheet useful for making of printed circuit boards was disclosed by depositing a mixture of syndiotactic polystyrene (including fibers thereof), glass fiber reinforcement and optionally a resinous component and heating the resulting mixture under pressure. According to the reference, the SPS resin formed the matπx of the resulting composite.

High heat resistance fibers are desired for use in preparing prepregs in order to provide necessary dimensional stability in the composite or PCB, for example, to resist soldenng or other high temperature operations. Materials having improved dielectπc constant also are desired in order to obtain thinner PCB's or PCB's where conductive lines are more closely spaced. Improved wet strength of the prepreg and faster resin impregnation and adhesion are also desired. Resins adapted for use in the large volume manufacture of fibers and nonwovens are also desired. Finally, low density, solvent resistant fibers that are compatible with plating and etching solutions and that have low hydroscopicity are further desired A prepreg and composite comprising a fibrous material able to attain all of the forgoing objectives is highly desired.

Summary of the Invention According to the present invention there is provided a prepreg comprising a fibrous base material comprising syndiotactic vinylaromatic polymeric fibers and a resin composition comprising a thermosetting resin impregnated in the base material.

Also included within the present invention is a resulting cured composite, comprising the foregoing prepreg composition wherein the thermosetting resin composition has been cured.

Finally, there is provided a printed circuit board comprising the foregoing composite and one or more conductive metal layers contained thereon.

Because the syndiotactic vinylaromatic fibers used in the present invention possess high crystallinity and the molecular orientation thereof is maximized, the resulting prepregs and composites made therefrom possess good physical properties. The dimensional stability as reflected in high stiffness and low coefficient of thermal expansion (CTE), chemical resistance and temperature resistance of the resulting prepreg and composites are all improved. In addition because the dielectric constant of syndiotactic vinylaromatic polymer fibers is much lower than those of the conventional aramid fibers, and the density thereof is low, the prepregs, composites and pπnted circuit boards of the present invention possess desirable final physical properties and are amendable to use in existing high volume manufacturing processes.

Detailed Descπption of the Invention Syndiotactic vinylaromatic polymers especially include homopolymers and copolymers of vinyl aromatic monomers prepared by coordination polymerization thereof under conditions to provide a high degree of syndiotacticity. Most highly preferred are those polymers containing greater than 75 percent syndiotacticity at a racemic tnad, preferably greater than 95 percent syndiotacticity at a racemic tπad. Highly preferred polymers include syndiotactic polystyrene and syndiotactic copolymers of styrene with Ci.* ring alkyl- or bromo- substituted styrenes, especially p-vinyltoluene and πng brominated or dibrominated styrenes. The latter brominated styrenes are particularly useful in the preparation of syndiotactic vinylaromatic polymers that are inherently resistant to ignition. Such polymers are known in the art having been previously disclosed in, for example, US-A-4, 680,353; US-A-4,959,435; US-A-4,950,724; and US-A-4,774,301.

The syndiotactic fibers used in the base material of the present invention desirably are highly crystalline and ordered fibers produced, for example, by shaping a syndiotactic vinyl aromatic resin into fibers by melt-spinning, spun-bonding, solution fiber spinning, flash spinning, melt-blowing or other conventional techniques used in the manufacture of fibers and nonwovens. The syndiotactic vinyl aromatic polymer fibers may be in the form of short fibers, pulpy fibrils or a mixture thereof. The pulpy fibril can be formed for example by gπnd-pulveπzing the fibers.

Preferred fibers (especially for the wet laid or dry laid process) are those from 0.1 to 200 mm in length, more preferably from 1 to 25 mm in length, having a thickness from 0.001 to 1.0 mm, more preferably from 0.005 to 0.1 mm. Further preferred are fibers having 0.02 percent or less equilibrium moisture content. If the equilibrium moisture content of the fiber base mateπal used in the present invention is high, when the resulting PCB is placed in an electπc field under high-temperature and high-humidity conditions, metal electrodes on the surface of the substrate may be ionized due to reaction with such water and a hydrate may be formed. As a result, a metal oxide may be deposited on the positive electrode and reduced metal may be deposited on the negative electrode, and thus a migration of the metal values occurs.

The syndiotactic vinylaromatic fibers are formed into a base material which may be a woven fabπc, a knitted fabric, a nonwoven fabric or a paper-like sheet. Wet laid or dry laid and spun bonded fabrics are preferred due to the fibers' increased oπentation and crystallinity compared to fabrics based on less-oriented fibers. Such fabrics may be calendered to improve porosity and impregnability of the thermosetting resin composition. A wetting agent may be included with the fibers to improve the dispersabihty thereof in a wet-laid process. Alternatively, the aromatic syndiotactic vinylaromatic fibers may be dispersed in the epoxy resin composition, and the PCB extruded or cast into a desired, optionally laminated or complex shaped article.

The fibrous base mateπal usable for the present invention may include other fibrous materials in addition to the syndiotactic vinyl aromatic polymer fibers. Preferred fibrous base mateπals compnse 2.5 to 100% by weight, preferably 5 to 100% by weight, of the aromatic syndiotactic vinylaromatic fibers and 97.5 to 0% by weight, preferably 95 to 0% by weight, of at least one other type of fiber. The other fibers can be contained in the base material, so long as a realization of the intended object or functional effect of the present invention is not hindered. For example, the other fibers can be selected from glass fibers, carbon fibers, polyether ketone fibers, polyether ether ketone fibers, polyester imide fibers, polyimide fibers, wholly aromatic polyester fibers, polyphenylene sulfide fibers, aromatic polyamide fibers, aramid fibers, and ceramic fibers. Such fibers can be mixed with the syndiotactic vinylaromatic fibers by comingling continuous fiber bundles or mixing short-cut staple fibers in the wet laid process for forming of the base material. In a preferred embodiment the fabncs may be prepared by co-continuously spinning fibers or using wet or dry laid techniques with mixtures of glass or aramid fibers with syndiotactic vinylaromatic resin fibers. Additional ingredients to improve adhesion between fibers and/or the thermoset matπx such as coupling agents, for example maleated polymers, including maleic anhydride modified polyphenylene oxide, or maleic anhydride modified syndiotactic vinylaromatic polymers, or binders to improve the wet strength of the base fabric may be used. Additives, such as flame retardants including brominated polystyrene, brominated syndiotactic vinylaromatic polymers, antimony tπoxide, and polytetrafluoroethylene may be added to the fibrous base material as well as to the thermosetting resin composition. Suitable thermosetting resins used to form the prepreg of the present invention include those resins known and used previously to form printed circuit board prepregs. Especially preferred resins include the well known epoxy resins, polyimide resins, cyanate-ester resins, bismaleimides-tnazine resins, unsaturated polyester resins, and vinylester resins. Blends of the foregoing resins with materials containing reactive functionality such as styrene/ maleic anhydπde copolymers may also be used. Examples of suitable epoxy resins include:

(A) Diglycidyl ether compounds compπsing the reaction product of bisphenol A or halogenated bisphenol A compounds with epichlorohydπn (for example, D.E.R.™ 592 A80 resin available from The Dow Chemical Company). (B) Polyglycidylether compounds comprising the reaction product of polyhydnc alcohol compounds which are the reaction product of bisphenol A or halogenated bisphenol A compounds with an alkylene oxide in the presence of an acid or alkali catalyst, with epichlorohydπn (for example, EP-4000 supplied by Asahi- Denka). (C) Phenol-novolak epoxy compounds (for example, Dow Epoxy Novolac D.E.N ™ 438)

(D) o-cresoi-novolak epoxy compounds (for example, Epikote 180S65 supplied by Yuka-Shell Epoxy). The cuπng agent contained in the epoxy resin composition is not particularly cπtical, but in general, at least one member selected from dicyandiamide compounds, aromatic polyamines, anhydπdes and phenolic resins can be used.

A curing promoter can be further incorporated in the epoxy resin composition usable for the present invention. Imidazoles, imidazolines and tertiary phosphines such as tnphenylphosphine are preferably used.

Suitable flame-retardant epoxy resin compositions usable for the prepreg of the invention include a mixture of a brominated bisphenol A epoxy resin which is a copolymeπzation product of tetrabromobisphenol A and bisphenol A with epichlorohydπn, with a heat-resistant o-cresol-novolak type epoxy resin and dicyandiamide as the cuπng agent.

Preferred epoxy resins having improved heat resistance and good metal laminate peel resistance are resins comprising the reaction products of:

(I) glycidyl ether compounds which are reaction products of epichlorohydπn with polycondensation products of bisphenol A or halogenated bisphenol A and formaldehyde

(II) a bisphenol A- or bisphenol F-type glycidyl ether compound, and

(III) bisphenol A, bisphenol F or tetrabromobisphenol A.

The thermosetting resins can be combined with various diluents to form a varnish or resin composition for ease of application to the base mateπal. For example, at least one member selected from acetone, methylethylketone, toluene, xylene, methylisobutylketone, ethyl acetate, ethylene glycol monomethyl ether, N,N'- dimethylformamide, N,N-dιmethylacetamιde, methanol and ethanol can be used. The resin composition is impregnated into the base mateπal in any suitable manner such as by spraying, dipping, soaking, injecting, or otherwise coating the resin composition thereon. The prepreg may be shaped or further modified by drilling or shaping and additional components, such as connectors, terminals or electronic components added prior to final curing.

Preferably, in the prepreg of the present invention, the weight ratio of the fibrous base mateπal to the resin composition is from 20/80 to 90/10, preferably from 30/70 to 75/25, more preferably from 35/65 to 65/35. If this weight ratio is outside the above-mentioned range, the obtained prepreg has an unsatisfactory dimensional stability, and insulating property, and the heat resistance of the prepreg may not be satisfactory for subsequent soldering operations. The thermosetting resin composition may further comprise one or more additional ingredients such as a cuπng promoter, a lubricant, a flame retardant, a stabilizer, a release agent, an inorganic or organic filler, fine particles of a fluorine- containing polymer, a pigment, or a dye.

The prepreg of the present invention can be prepared by impregnating the aromatic syndiotactic vinylaromatic fiber base material with the thermosetting resin composition, drying, and optionally partially curing the resulting prepreg, if necessary, by customary procedures. The cured product results by initiating or completing crosslinkmg of the thermosetting resin, generally by heating the prepreg, optionally after further forming or shaping the prepreg into the desired shape. Alternatively the mixture compπsing the fibers and thermosetting resin can be extruded into the desired shape and cured to form the desired cured prepreg

The pπnted circuit board of the present invention can be prepared by combining a predetermined number of prepregs, adding a copper foil on the surface of the prepreg or prepreg laminate, and integrally curing the laminate by heating under compression. A metal circuit layer can also be added by vapor deposition or by subsequent lamination of the metal layer to the surface of the cured prepreg and etching or machining the desired circuit design thereon.

Examples

The present invention will now be descπbed in detail with reference to the following examples. Unless stated to the contrary, parts and percentages are based on weight. The skilled artisan will recognize that the invention may be practiced in the absence of any material not specifically recited. Example 1

A calendered non-woven fabric (10 cm x 10 cm) of syndiotactic polystyrene (SPS) fibers having a basis weight of 95 g/m² was prepared. Into this fabric a resin solution was impregnated. The resin solution consisted of a blend of the following components: 125 parts bisphenol A epoxy resin solution (D.E.R.™ 592 A80 epoxy resin, available from The Dow Chemical Co.), 34 parts of dicyandiamide curing agent solution (both solutions being 10 percent weight in methyl glycol solvent) and 0.8 parts of a 2-methyl imidazole solution (10 percent weight in methanol). The resin impregnated paper was dried at 170°C for four minutes to produce a tack-free prepreg. Four sheets of the prepreg were pressed together (175°C for two hours at 1.4 bar) producing a consolidated and thermoset laminate structure of 0.799 mm thickness.

Differential Scanning Caloπmetry (DSC) analysis determined that the laminate had two glass transition temperatures, at 104.5°C and 168.5°C respectively, corresponding to semi-crystalline SPS thermoplastic and thermoset epoxy respectively. Measurement of dielectric constant using a Hewlett-Packard HP 4291 A network analyzer coupled to a HP 16453A test cell from 1 MHz to 1.8GHz remained at a constant 3.0 value. Dimensional stability measurement analysis using a Mettler TMA 30 gave a Z-axis length change of 1.61 percent across a temperature range of 30 to 200°C.

Example 2

A vinyl ester resin, 100 parts of DERAKANE™ 41 1 -45 (available from The Dow Chemical Co.), was mixed with two parts of benzoyl peroxide catalyst and 0.2 parts silane anti^'-foaming additive (Byk-A-150, available from Byk Corporation). The resin blend was then impregnated in a non-woven mat produced from SPS fibers with a basis weight of 100g/m² using a roller followed by successive build-up of mat and resin until six layers had been combined. The laminate was left to gel, exotherm and cool before a two hour post-bake at 90°C was carried out. The laminate had a thickness of 1.07 mm. The Tg of the laminate was measured and a single inflection point at 106.2°C recorded. Six 2 cm² square samples were cut from the laminate, three of which were soaked in distilled water and three in methylene chloride, both for 24 hours. The water and solvent pick-up after immersion were 0.18 and 0.76 percent respectively. Example 3

A wet-laid nonwoven fabric having a density of 50 g/rrf is prepared from a 50/50 (weight) blend of undrawn melt spun syndiotactic polystyrene fibers (approximately 13 mm long and 30 urn in diameter) and p-aramid fibers (approximately 10 mm long and 10 urn in diameter, TWARON brand aramid available from Akzo Inc). The tissue-like fabric is subsequently pressed 10 minutes at 180°C and 450 kN) to form a thin, paper-like material.

A resin varnish comprising a 68/32/0.10 weight percent blend of epoxy resin (blended from 40 parts D.E.R.™ 592 A80 epoxy resin, available from The Dow Chemical Co., 5 parts tetrabromo bisphenol-A, available from Great Lakes Co., 13 parts Quatrex™ 6410 epox asm also available from The Dow Chemical Co.), styrene maleic anhydride copolymer (SMA-3000 from Elf-Atochem, Mn approximately 3,000 and Mw approximately 10,000) and catalyst (boric acid and 2-ethyl-4-methyl imidazole, V1 weight ratio, each used as a 10 percent by weight solution in methanol) in a solvent mixture of methyl ethyl ketone and 1-methoxy-2-hydroxypropane is prepared. 15cm² sheets of the reinforcement fabric are immersed in the resin varnish. The resin impregnated fabric sheets are then dried and B-stage cured in a large hot-air circulating oven for 3 minutes at 160 °C. 10 cm² squares are cut from the centers of the resulting prepregs. The 10cm² prepreg squares are then put into a Brucker minipress and laminated at a temperature of 190°C for 2 hours with a constant pressure of 25 kg/cm². On removal from the press the excess resin flow is trimmed from the product providing a fabricated laminate. A similar laminate is prepared from a spun-bond fabric of only syndiotactic polystyrene fibers (SPS-pure). z-Direction dimensional stability (z-direction coefficient of thermal expansion, z-CTE), are measured using a Mettler TMA-40 instrument, under IPC method No. 2.4.24, Revision B. x,y-Directιon dimensional stability is measured using a DuPont 2100 CTE apparatus. Dielectric constant (Dk) and dissipation factor (Df) are measured within an accuracy of ± 5 % for Dk and 15 percent for Df at 23 °C. Resin content in the laminate is calculated by subtracting fabric weight from the weight of the final cured laminate. Differential Scanning Calonmetry (DSC) analysis shows a well defined transition around 180°C corresponding to the epoxy matrix, and a less defined transition at 100°C corresponding to the presence of the SPS fibers. Results are contained in Table 1.

Claims

What is Claimed is:

1. A prepreg compπsing a fibrous base material comprising syndiotactic vinylaromatic polymenc fibers and a resin composition comprising a thermosetting resin impregnated in the base material.

2. A prepreg according to claim 1 wherein the thermosetting resin comprises an epoxy resin, polyimide resin, cyanate-ester resin, bismaleimide-tπazine resin, uπstaurated polyester resin, or a vinylester resin.

3. A prepreg according to claim 2 wherein the epoxy comprises:

(A) a diglycidyl ether compound comprising the reaction product of bisphenol A or halogenated bisphenol A compounds with epichlorohydnn,

(B) a polyglycidylether compound comprising the reaction product of polyhydnc alcohol compounds which are the reaction product of bisphenol A or halogenated bisphenol A compounds with an alkylene oxide in the presence of an acid or alkali catalyst, with epichlorohydnn, (C) a phenol-novolak epoxy compound, or

(D) a o-cresol-novolak epoxy compound.

4. A prepreg according to Claim 1 wherein the fibrous base mateπal comprises a mixture of a syndiotactic vinyl aromatic polymeric fiber and a glass fiber, carbon fiber, polyether ketone fiber, polyether ether ketone fiber, polyester imide fiber, polyimide fiber, wholly aromatic polyester fiber, polyphenylene sulfide fiber, aromatic polyamide fiber, aramid fiber or ceramic fiber.

5. A composite formed by layering one or more prepregs of any one of claims 1 -4.

6. A cured composite according to claim 5.

7. A printed circuit board comprising a composite according to claim 6.