WO1997038564A1

WO1997038564A1 - Composite dielectric material

Info

Publication number: WO1997038564A1
Application number: PCT/US1997/007061
Authority: WO
Original assignee: Arlon, Inc.
Priority date: 1996-04-09
Filing date: 1997-04-07
Publication date: 1997-10-16

Abstract

Composite dielectric materials are provided having exceptional thermal stability of dielectric constant and low matrix coefficient of thermal expansion. These materials comprise fabrics (12), either woven or nonwoven, impregnated with filled (16) thermosetting resins (14). Also provided is a method of manufacture of these dielectric materials. The dielectric materials are useful in single layer and multilayer printed circuit boards.

Description

COMPOSITE DIELECTRIC MATERIAL

BACKGROUND OF THE INVENTION

The present invention relates to low dielectric constant composite materials useful in single layer and multilayer printed circuit boards.

A known dielectric material finding wide use in printed circuit boards is a laminated composite of fiberglas fabric impregnated with a thermosetting epoxy resin, referred to by the National Electronic Manufacturers Association (NEMA) classification as FR-4. This composite is produced by impregnating fiberglas fabric with a thermosetting epoxy resin. The resin in the impregnated fabric is partially cured to form a dry, flexible sheet in which the resin is in an intermediate cure state, termed a "Bⁿ- stage or "pre-preg" sheet. One or more of these sheets of pre-preg may be stacked together to a desired thickness and laminated together by further curing under heat and pressure to form a laminated composite in which the resin reaches a fully-cured, ^MC^M stage state.

During the lamination process, the B-stage resin of the pre- preg sheet is converted to fully-cured c-stage resin and the sheets of pre-preg may be bonded to one or two sheets of copper foil during the lamination process so that the laminated composite consists of dielectric material clad on one or both sides with copper foil. This composite material is referred to as FR-4 copper clad laminate and this composite may be further fabricated into single and double sided printed circuit boards. Where very high circuit densities are required, multilayer printed circuit boards have been provided. Thin dielectric FR-4 copper clad laminate is fabricated into single or double sided circuit patterns, called innerlayers, and one or more of these innerlayers are interleaved with one or more sheets of B-stage pre- preg and laminated together under heat and pressure to form a homogeneous, void free multilayer structure. This lamination process converts the B-stage resin of the pre-preg into C-stage resin, bonding the innerlayers together and providing electrical insulation between the circuit layers. The multilayered structure is then further processed into a nultilayer printed circuit board.

Dielectric materials having lower dielectric constants and dissipation factors than those of conventional materials are needed because of increasing signal speeds and operating frequencies of electronic systems. Lower dielectric constant dielectric materials both decrease capacitive coupling and increase the speed of the electronic signal, with the result that electronic systems now process data at greater and greater speeds. FR-4 laminate has a relatively high dielectric constant, approximately 5.0 at 1 megahertz, resulting from the high dielectric constant contribution of the fiberglas (6.11), averaged with the lower dielectric constant of the resin (3.4). To achieve a lower dielectric constant material, laminated composites comprised of fiberglas fabric impregnated with fluorocarbon resins have been developed. These laminated composites can have a dielectric constant of as low as approximately 2.5 at 1 megahertz. However, fluorocarbons are not thermosetting resins and are difficult to fabricate into multilayer printed circuit boards.

In copending application USSN 08/495,324, in the name of one of us, composite dielectrics are disclosed in which a high strength fabric is coated with a fluoropolymer containing a particulate filler. And U.S. Patent 4,680,220 discloses a dielectric composite material having a fabric base in which at least some of the fibers of the fabric are fluorocarbon fibers which have been treated to render them wettable by an uncured thermoset resin. On impregnation with a thermosetting resin, this composite has a dielectric constant of less than 3.5.

Particulate ceramic fillers for various polymers, useful in electrical applications, are also known, including quartz, silica, mica, talc (magnesium silicate) , feldspar, clays, boron nitride, glass beads and quartz or glass microballoons. Particulate ceramic fillers and synthetic mineral powders are disclosed in copending application S.N. 495,324, for fluorocarbon polymers.

Other dielectric materials have been developed which employ fabrics other than fiberglas in combination with thermosetting resins. Laminated composites of polyaramid fibers and epoxy resins are known which have a dielectric constant of about 3.8, which is considerably higher than the fluorocarbon composites. Molded, filled hard shaped articles are disclosed in U.S. Patent 5,223,568 including thermosetting compositions of both a polybutadiene or polyisoprene resin which is a liquid at room temperature and which has a molecular weight less than 5,000 and a large number of pendent vinyl groups, and a solid butadiene- or isoprene-containing polymer (e.g., a thermoplastic elastomer). Examples of fillers include titanium dioxide (rutile and anatase) , barium titanate, strontium titanate, silica (particles and hollow spheres) ; corundum, wollastonite, polytetrafluoroethylene, aramide fibers (e.g., Kevlar) , fiberglas, Ba₂Ti₉O₂₀, glass spheres, quartz, boron nitride, aluminum nitride, silicon carbide, beryllia, or magnesia.

SUMMARY OF THE INVENTION

A composite dielectric material is provided comprising a fabric substrate impregnated with a filled polymeric composition containing about 60% to about 40% by weight of polymer and about 40% to about 60% by weight of a particulate inorganic filler dispersed within the polymer. The polymer is a thermosetting polymer selected from the class consisting of epoxy, imide, a cyanate ester, cyanate ester/bismaleimide, bismaleimide-triazine- epoxy blends and butadiene polymers, the preferred polymer being butadiene styrene, divinyl benzene graft terpolymer. The preferred inorganic particulate material is a powder of aluminum magnesium silicate and the fabric substrate is preferably fiberglas or a polyaramid fabric. In forming printed circuitry components, a layer of electrically conducting material, i.e., copper foil, is adhered to at least one side of a sheet of the composite dielectric material of the invention.

Dielectric materials of the invention generally have dielectric constants less than about 10.0, and may have dielectric constants less than about 3.5 and have dissipation factors less than about 0.005 - 0.006. A further processing advantage is provided in that the composite dielectric material is a thermosetting polymer which, when cured to its B-stage state, is substantially tack-free at room temperature.

Also provided are multilayer electrical circuits constructed using the dielectric material of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

Fig. 1 is a schematic perspective view, partially in cross- section and partially cut-away, of the composite article according to the invention, shown adhesively bonding two electrically conductive metal foils.

Fig. 2 is a schematic flow diagram of a preferred production process for making sheets of the composite according to the invention.

Fig. 3 is a schematic perspective illustration of the various components to be assembled and then placed in a press to effect bonding of component sheets or foils using the composite adhesive of the invention.

Fig. 4 is a graph of the dielectric constant of one embodiment of the composite material of the invention, a preferred embodiment thereof, over the temperature range of -10*C to 140*C.

Fig. 5 illustrates the dielectric constant of a composite material known in the art over the. same temperature range of Fig. 4.

DETAILED DESCRIPTION OF THE

INVENTION AND PREFERRED EMBODIMENTS

WITH REFERENCE TO THE DRAWINGS

Composite dielectric materials are provided having exceptional thermal stability of dielectric contant and low matrix coefficient of thermal expansion. These materials comprise fabrics, either woven or nonwoven, impregnated with filled thermosetting resins. Also provided is a method of manufacture of these dielectric materials. The dielectric materials are useful in single layer and multilayer printed circuit boards.

The invention is best described in detail with reference to the accompanying drawings wherein Fig. 1 shows the basic composite material according to the invention. That composite includes a substrate of a strength-reinforcing fabric 12 which has been impregnated as shown with a composition which penetrates the interstices of the fabric 12 and forms a continuous matrix surrounding the fabric and penetrating its voids. The matrix generally comprises about 40% to 60% by weight of a thermosetting polymer 14 and about 60% to 40% by weight of an inorganic particulate filler material 16 dispersed within the thermosetting polymer. In the coated fabric composite, the weight of the woven glass reinforcement generally accounts for about 20% to 60% by weight of the total system. The composite material of the invention may be used to construct printed circuit board innerlayers 10 depicted in Fig. 1 wherein electrically conductive layers 18, such as copper foils are adhered to one or both sides of the impregnated fabric, as shown. A polybutadiene-styrene divinyl benzene terpolymer is the preferred resin system, see, e.g., "VHSIC low dielectric constant printed wiring boards contract F33615-84-C-1415" (Air Force Wright Aeronautical Laboratory Final Government/Industry briefing) . Other resins may be employed such as polyi ides, FR-4 epoxy, cyanate esters, fluorocarbons and other printed wiring board substrate resins known for this application.

The strength reinforcing substrate 12 shown in Fig. 1 is preferably glass, quartz or S2-glass fabric. A high strength polymeric fabric or nonwoven sheet such as a polyaramid fabric may also be used. Various paper-like substrates may be employed for some applications. The preferred glass fabric is "style 1080" (JP Stevens Co. or Clark Schwebel Co.). Other similar reinforcing fabrics may be employed, so long as they meet the strength and temperature requirements of the application.

The preferred filler powder 16 is either a synthetic aluminum magnesium silicate or fused silica powder, with aluminum magnesium silicate being the most preferred particulate filler for this application. Conductors 18 of copper foil and other conductive layers may be employed so long as they can be adhered to the basic composite building block of the invention, the impregnated fabric.

A process for making the composite of the invention is depicted schematically in Fig. 2. As shown in Fig. 2, the ingredients used in preparing the filled polymer system are introduced (40) into mixing vessel 48. These ingredients include the resin/solvent, the catalyst and the particulate filler to be employed. The ingredients are mixed under moderate shear by mixer 46 to disperse the filler particles uniformly within the resin and this mixture 52 is then passed through filter 54 to remove unwanted impurities. The filtered mixture 56 is pumped into process vessel 58, kept uniform by optional agitator 68 with the excess being recirculated back into mixing vessel 48.

Glass fabric is a preferred reinforcing substrate. When glass is employed as the reinforcing substrate, fiberglas fabric (style 1080, 106) from roll 60 is fed over guide roll 62 and into and through the coating composition 56 contained in process vessel 58. Upon exit from vessel 58, metering rolls (not shown) may optionally be employed to control the coating thickness applied to the glass fabric, which effectively controls the resin content ultimately applied to the substrate. The coated fabric exits the coating section to oven 72 which has multiple heating zones 72(a)- (c) , wherein the mixture is partially dried of carrier solvents, leaving the resin, filler, catalyst and glass fabric as substantially a tack-free B-stage prepreg. The temperature upon exit from oven 72 is preferably kept below 350'F, to prevent decomposition of the catalyst.

The coated, partially dried fabric is passed over guide rolls 74 as shown and into and through second oven 76 in which additional solvent evaporation occurs. Within oven 76, the residual volatiles content is preferably reduced to 1.0% or lower. The preferred temperature in oven 76 is 350*F or below, also to prevent decomposition of the catalyst.

The dried coated fiberglas fabric passes around guide roll 78 as shown and is collected as a continuous coated roll 80. In the manufacture of printed circuit boards, the coated roll 80 is cut into desired sheet sizes and copper foil is adhered to the desired number of layers of the reinforcod composite substrate to build a multilayer circuit having filled polymer/glass dielectric insulation layers. This composite construction 86 is passed into a hydraulic press 82 containing heated pressure platens 84, indicated schematically. The platens, in general, are heated with steam, and cooled with water. The composite laminates 86 are described in detail below. In press 82, the required number of plies of coated stock, together with associated copper foil or other conductive plies, are consolidated under heat and pressure.

The material is plied into a book (86 of Fig. 3) , and held in the press, under a vacuum exceeding 28 inches Mercury for a minimum of

30 minutes. The material held under a pressure of 200 to 300 psi is heated gradually from room temperature at a heating rate of 4'F

(2'C) per minute to a cure tempera ure of 435^*F, for a time of 2 to

4 hours to achieve cure. While this process is suitable for making double sided clad laminates, somewhat higher or lower temperatures and or pressures may be employed for making multilayer circuit boards. The optimum processing conditions in multilayer pressing appear to be a heating rate of 4'F per minute, with approximately 30 minute dwell time at 200'F, then 4'F per minute rise to 300^*F, with a 30 minute dwell, and then heat to a final cross-link cure C-stage cure at 425*F for 2 hours, at a pressure of 200 psi. As indicated, somewhat lower or higher temperatures, pressures and heating rates may be more efficient for other applications and other circuit geometries.

Fig. 3 illustrates schematically a typical construction of a multilayer composite 86 according to the invention. Backing plates 88 are used to provide uniform pressure distribution against the press platens 84 (not shown) ; padding layers 90, of glass reinforced silicon rubber, kraft paper or other compliant material, sandwiched with aluminum foil for ease of release from press and plate surfaces, are used to produce a uniform pressure distribution and uniform heat distribution between the platens and the multilayer composite. Face plates (or caul plates) 92 are used to produce a smooth surface in the laminate-to-copper foil cladding. Copper foil 96 is plied (drum side out, treatment side toward prepreg layers 98, smooth side out) facing caul plate 92. The composite laminate 86 is not limited to one laminate between platens 84 as shown, but may be constructed of several similar laminates sandwiched between platens 84 (not depicted) by stacking multiple laminate buildups such as conductors 96, prepreg plies 98, conductors 96 constituting a laminate buildup, between caul plates 92 separating each individual laminate buildup. The core of the laminate 98 is composed of one or more composites of the coated and impregnated reinforcing fabric containing the filler according to the invention. Several such composite laminates can be placed between the press platens 84 to form book 86 between each set of platens 84.

Fig. 4 illustrates the variation in dielectric constant of a dielectric composite of the invention over the range of temperature of -10*C to 140*C and Fig. 5 illustrates the variation in dielectric constant over this temperature range for an unfilled, cyanate ester glass composite laminate known in the art. The composite of the invention has a lower dielectric constant, which decreases with increase in temperature, compared to known composites which tend to increase with temperature (Fig. 5) . This represents a significant improvement in performance for thermosetting composite constructions resulting from lowering of the magnitude of the dielectric constant and allows faster circuits to be designed utilizing the composite of this invention.

The following examples are intended to be illustrative of the invention but will not limit the scope of the claims in any way.

EXAMPLE 1

The basic ingredients used to prepare the coating composition of the invention were introduced into mixing vessel 48 as follows: 150 pounds of Ricon XBK 250 series (Ricon Resins, Inc., Grand Junction, CO) styrene-polybutadiene divinyl benzene terpolymer, contained in 150 pounds of toluene, was prefiltered through a 50 mesh filter 54 into vessel 48. To this was added a solution of dissolved catalyst (3402 grams of dicumyl peroxide, pre-dissolved in 15 pounds of toluene) . 150 pounds of synthetic magnesium aluminum silicate (INCOR Corporation, HC-11, 400 mesh) was then added slowly while stirring to maintain consistency and uniformity of the mixture. An additional 90 pounds of toluene was added to keep the mixture uniform in the 50-60% solids range and to coat the glass with a 50-60% solids mixture of filler and resin.

This blended mixture, identified as 40 in Fig. 1, was filtered to remove all impurities through a 300 mesh filter screen 54 and the filtered mixture was then introduced into coating vessel 58, where it was continuously recirculated with the mixture in mixing vessel 48 to maintain filler loading and consistency.

E-glass fabric 64 (Style 1080, 38 inch wide fabric, JPS finish 9827) from roll 60 was guided over roll 62 to and through process vessel 58 wherein the coating composition 52 impregnated the fabric and formed a coating thereon before passing through metering rolls (not shown) which control the gap spacing, thus controlling resin and filler loading application. Metering roll gap settings of 23 mils (open) provided the application of resin content yielding a final composition containing 20% glass reinforcement fabric, 40% polymer and 40% filler, all by weight. The composite then passed to and through oven zones 72 and 76 at temperatures of 300 degrees F, where substantially all volatiles were removed, without causing the peroxide to decompose or initiate partial or complete reaction of the terpolymer system. This was performed at a line speed of 20 feet per minute to achieve proper drying of the material, yet not initiate advancement through peroxide decomposition. Somewhat higher or lower process temperatures, or higher or lower line speeds are necessary to make prepregs which are not sufficiently over-staged, yet sufficiently volatile free (less than 1% remaining toluene volatile content.)

The impregnated, coated, dried fabric was taken up on roll 80, upon which it was packaged for storage, and could be accessed as needed to make the composite circuit boards according to the invention.

Material from a roll 80 was sheeted into 38 by 50 inch sheets. The coated glass fabric was plied into 6 plies 98 (Fig. 3) and copper foil 96 (Fukuda electrodeposition) was plied (drum side toward the resin) on each outer surface of the sandwich. This construction provided a 30 mil copper clad laminate following cure of the resin. To provide laminates for multilayer circuit boards. the resin was cured as follows: 6 plies of 1080 stock, each 5 mil thick, coated to an applied weight of 20% glass, 80% of a 50/50 resin/filler mix were plied together with associated copper foil, drum side out. The composite was then consolidated under heat and pressure. The material was plied in books (86) , held within the press under vacuum for 30 minutes at room temperature. The material was then held under a pressure of 200 to 300 psi, starting at room temperature, and gradually heated from room temperature at a heating rate of 4*F (2*C) per minute to a temperature of 435'F, for a time of 2 to 4 hours to achieve cure.

E AMPLE ? Using identical preparation techniques, but on proportionally smaller scale, mixtures were prepared as follows:

Descrip¬ 0% filler 30% filler 50% filler 60% filler tion

Ricon 256 300 grams 300 grams 300 grams 300 grams 50% in toluene

Dicumyl 7.5 grams 7.5 grams 7.5 grams 7.5 grams peroxide

HC11 64.3 grams 150 grams 225 grams filler

toluene 25 grams 90 grams 120 grams Properties of the composite dielectrics of these examples are shown in the following table:

* Tg by TMA, IPC-TM-650, method 2.4.24C (units in *C)

CTE by TMA, IPC-TM-650, method 2.4.24C (units in ppm'C^"1)

Peel Strength, measured by IPC-TM-650, method 2.4.8C (units in pounds per inch of width)

2-sided Copper clad Solderfloat blister resistance, minutes at

500*F per IPC-TM-650, method 2.4.13.1

Dielectric Constant at 1 MHz per IPC-TM-650 method 2.5.5.3

(unitless number)

Dissipation Factor at 1 MHz per IPC-TM-650 method 2.5.5.3

(unitless number) EXAMPLE 3

A composite laminate according to the invention comprising the 0.030 inch thick, 6-ply laminate of Example 1 was tested for the following physical properties, with the results indicated below:

Test; Value Test MethPfl

Specific Gravity 1.54 ASTM D-792(A)

DK 1MHz 3.278 IPC-TM-650-2.5.5.3 DF 1MHz 0.00249 IPC-TM-650-2.5.5.3

DK 10GHz 3.2478 IPC-TM-650-2.5.5.5 DF 10GHz 0.00267 IPC-TM-650-2.5.5.5

Flexural Strength psi(Mpa) MD 186,000 (1282) ASTM D-790 CD 135,000 (930)

Flexural Modulus psi(Mpa) MD 380,000 (665) ASTM D-790

CD 330,000 (583)

Tensile Strength psi(Mpa) MD 27,163 (187) ASTM D-638

CD 20,925 (140)

Tensile Modulus psi(N/m²) MD 230,474 (1588) CD 105,634 (728)

Thermal Conductivity 100'C W/M/'K 0.446 ASTM E-1225

Arc Resistance seconds 65 ASTM D- 95

Bond Strength Lbs/in. Cond A 4.8/4.8 IPC-TM-650.2.4.8 After thermal stress 5.6/5.6 EXAMPLE 4 Sheet material prepared according to Example 1, but without the use of filler, was compared to the filled sheeting of Example 1 according to this invention. The unfilled sheeting was assembled in four plies, pressed between platens at 200 psi for 15 minutes at 70*F. These unfilled prepregs stuck together (blocked) under these conditions and attempts to separate them resulted in cohesive failure (tearing) of the resin. Prepregs of Example 1, containing filler, were pressed in an identical manner and, following pressing, the plies easily separated without sticking or blocking. This accelerated test indicates that the material of the invention will not block at room temperature and is substantially tack-free.

While the invention has been disclosed herein in connection with certain embodiments, examples and detailed descriptions, it will be clear to one skilled in the art that modifications or variations of such details can be made without deviating from the gist of this invention, and such modifications or variations are considered to be within the scope of the claims herein below.

Claims

CLAIMS What is claimed is:

1. A composite dielectric material comprising:

(a) a fabric substrate, said substrate impregnated with

(b) a filled polymeric composition, wherein said filled polymeric composition comprises about 60% to about 40% by weight of a polymer and about 40% to about 60% by weight of a particulate filler dispersed within said polymer.

2. The material of claim 1 wherein said filled composition comprises about 50% of said polymer and about 50% of said particulate filler.

3. The material of claim 1 wherein said polymer is a thermosetting polymer.

4. The material of claim 3 wherein said polymer is one selected from the class consisting of epoxy, imide, cyanate ester, cyanate ester/bismaleimide, bismaleimide-triazine-epoxy blends and butadiene polymers.

5. The material of claim 3 wherein said polymer is butadiene styrene, divinyl benzene graft terpoly er.

6. The material of claim 1 wherein said particulate material is a powder of aluminum magnesium silicate.

7. The material of claim 1 wherein said fabric substrate is fiberglas.

8. The material of claim 1 wherein said fabric substrate is a polyaramid fabric.

9. The material of claim 1 having a layer of electrically conducting material adhered to at least one side thereof.

10. The material of claim 9 wherein said conducting material is copper.

11. The material of claim 1 having a dielectric constant less than about 10.0.

12. The composite dielectric material of claim 1 wherein said polymer is a thermosetting polymer cured to its B-stage state, which material is substantially tack-free at room temperature.

13. A multilayer electrical circuit composite comprising at least two conductive circuits laminated together by a composite adhesive dielectric material comprising:

(a) a fabric substrate, said substrate being impregnated with

14. The multilayer composite of claim 13 wherein said filled composition comprises about 50% of said polymer and about 50% of said particulate filler.

15. The multilayer composite of claim 13 wherein said polymer is a thermosetting polymer.

16. The multilayer composite of claim 15 wherein said polymer is one selected from the class consisting of epoxy, imide, cyanate ester, cyanate ester/bismaleimide, bismaleimide-triazine-epoxy blends and butadiene polymers.

17. The multilayer composite of claim 15 wherein said polymer is butadiene styrene, divinyl benzene graft terpolymer.

18. The multilayer composite of claim 13 wherein said particulate material is a powder of aluminum magnesium silicate.

19. The multilayer composite of claim 13 wherein said fabric substrate is fiberglas.

20. The multilayer composite of claim 13 wherein said fabric substrate is a polyaramid fabric.

21. The multilayer composite of claim 13 having a layer of electrically conducting material adhered to at least one side thereof.

22. The multilayer composite of claim 21 wherein said conducting material is copper.

23. The multilayer composite of claim 13 having a dielectric constant less than about 10.0.

24. The multilayer composite of claim 13 wherein said polymer is cured to its B-stage state, which material is substantially tack-free at room temperature.