WO1993017860A1

WO1993017860A1 - Cyanate ester microwave circuit material

Info

Publication number: WO1993017860A1
Application number: PCT/US1993/002277
Authority: WO
Inventors: Samuel Gazit; Robert L. Bush; Jeffrey B. Otto
Original assignee: Rogers Corporation
Priority date: 1992-03-13
Filing date: 1993-03-12
Publication date: 1993-09-16

Abstract

A new and improved laminated microwave circuit material (10) comprises a non-woven glass web (5 to 20 vol. %) impregnated with a cyanate ester resin (35 to 68 vol. %) and filled with a low dielectric constant (e.g., silica), high dielectric constant (e.g., titania) filler or other suitable ceramic particulate filler (25-55 vol. %). Intermediate values of dielectric constant may be achieved by filler mixtures of silica, titania and other suitable fillers. Preferably, the filler is coated with a material which renders the filler hydrophobic such as silane, titanate or zirconate coatings. This microwave circuit material (10) of the present invention is preferably formulated to be flame retardant and has many features and advantages relative to prior art and microwave materials including prior art cyanate ester/woven glass materials. For example, the combination of resin, non-woven glass web and filler provides a low dissipation factor (Df) of less than 0.008 thereby permitting the material of the present invention to be used in rellatively demanding consumer and commercial applications (and some less demanding military applications).

Description

CYANATE ESTER MICROWAVE CIRCUIT MATERIAL

Background of the Invention:

This invention relates generally to microwave circuit materials. More particularly, this invention relates to a new and improved microwave circuit

material comprising a non-woven fiberglass web

impregnated with a cyanate ester matrix and filled with a particulate filler material. This cyanate ester based microwave circuit material exhibits a low

dissipation factor of less than 0.008, has a dielectric constant (K') which can be manipulated and tailored in the range between 3 and at least 10, and has a

relatively low cost. Preferably, this circuit material is also formulated to be flame retardant.

Microwave circuit materials are well known and are used in a large number of applications. Certain military and other demanding commercial applications require the use of relatively expensive fluoropolymer based microwave circuit materials. However, there is a developing need for lower cost microwave materials for less demanding commercial and consumer uses such as, for example, antennas for wireless communication in the home or office; or in connection with cellular

communications.

Presently available lower cost microwave materials include epoxy based FR-4 materials, polyimide based materials, cyanate ester/woven glass materials and bismaleimide triazine based materials. In microwave applications, the electrical property known as

dissipation factor (Df) is important, with lower values of Df providing improved performance. Unfortunately, epoxy, polyimide and cyanate ester/woven glass

microwave circuit materials exhibit less than desirable Df values .of generally 0.024, 0.021 and 0.009,

respectively. While bismaleimide triazine based materials (BT resins from Mitsubishi Gas Chemical

Company) exhibit Df values in the range of

0.0015-0.0140, the material versions which have a low Df are problematic from the standpoint of having a glass transition temperature under 200°C. Another drawback of all the above woven glass reinforced circuit materials, is the fact that their Z-axis coefficient of thermal expansion is much higher (50-70 ppm/°C) than the desirable 16 ppm/°C value of

copper.

Summary of the Invention:

The above-discussed and other drawbacks and deficiencies of the prior art are overcome or

alleviated by the microwave circuit material of the present invention. In accordance with the present invention, a new and improved laminated microwave circuit material comprises a non-woven glass web (5 to 20 vol. %) impregnated with a cyanate ester resin (35 to 68 vol. %) and filled with a low dielectric constant (e.g., silica) or high dielectric constant (e.g., titania, alumina or other suitable materials)

particulate filler (25-55 vol. %) or mixtures of high and low dielectric constant fillers (e.g., silica, titania, alumina or other suitable materials) for intermediate dielectric constant levels. Preferably, the filler is coated with a material which renders the filler hydrophobic such as silane, titanate or

zirconate coatings. This circuit material is

preferably formulated to be flame retardant.

The microwave circuit material of the present invention has many features and advantages relative to prior art microwave materials including prior art cyanate ester/woven glass materials. For example, the combination of resin, non-woven glass web and filler provides a low dissipation factor (Df) of less than 0.008 thereby permitting the material of the present invention to be used in relatively demanding consumer and commercial applications (and some less demanding military applications).

The use of a non-woven glass web (5 to 20 vol. %) is a critical feature of this invention as it permits the final laminate to obtain a wide range of cyanate ester resin (35 to 68 vol. %) and a relatively large amount of particulate filler (25 to 55 vol. %). In contrast, prior art woven glass webs are not suitable or useful with the present invention as woven glass will not permit the relatively large amounts of filler associated with the present invention. In addition, nonwoven webs provide significant improvements to woven webs in that woven webs tend to propagate fractures along the woven glass layers while nonwoven webs, due to the random nature of the fiber's direction, do not exhibit such tendency for fracture propagation. The reduction in fracture propagation using the nonwoven web of this invention leads to improved (e.g., faster and more reliable) drilling (for feature formation) since the tendency of the woven glass to fracture requires slower drilling to avoid cracks and fractures between the woven glass layers.

The ability to include a relatively wide volume range of cyanate ester resin and large amounts of particulate filler to the laminated microwave circuit material of this invention is important for several reasons. First, while the cyanate ester is a

relatively expensive resin, the use of this resin in a range of 35 to 68 vol. % leads to relatively low Df of less than 0.008. This low Df is important,

particularly when compared to higher Df associated with prior art epoxy, polyimide and cyanate ester/woven web microwave circuit materials. Second, the use of particulate filler in the range of 25 to 55 vol. % is important in that (a) the large amount of filler provides the ability to specifically tailor the final dielectric constant of the laminate in the range of 3 to at least 10 or higher; (b) the large amount of filler lowers the coefficient of thermal expansion (CTE) of the substrate in the z direction to improve the plated throughole reliability; (c) the large content of the relatively inexpensive filler lowers the required amount of cyanate ester thereby lowering the overall cost of the final laminate; and (d) the

particulate filler acts to arrest any crack or fracture propagation.

The above-discussed and other features and

advantages of the present invention will be appreciated and understood by those skilled 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: FIGURE 1 is a cross-sectional elevation view of a microwave circuit laminate in accordance with the present invention; and

FIGURE 2 is a cross-sectional elevation view of a multilayer circuit laminate in accordance with the present invention.

Description of the Preferred Embodiment:

Referring to FIGURE 1, the present invention comprises a microwave circuit material 10 composed of a substrate 12 laminated on one or both outer planar surfaces to a foil of conductive material 14, 14'

(generally copper). Substrate 12 comprises a non-woven glass web impregnated with a cyanate ester resin and filled with particulate filler material. Preferably, substrate 12 also includes flame retarding compounds, a catalyst system for curing the cyanate ester resin, and a hydrophobic coating on the particulate filler.

The cyanate ester resin is present in an amount of 35 to 68 vol. % with respect to the volume of substrate 12. The cyanate ester resin used in this invention is from the family of aryl dicyanate monomers and their prepolymers containing the ring-forming cyanate

(O-C≡N) functional group, and more particularly is an ester of bisphenols and cyanic acid which

cyclotrimerize to substituted triazine rings upon heating. Curing of the cyanate ester resin forms a thermoset plastic comprising three dimensional networks of oxygen-linked triazine rings and bisphenol units (termed polycyanurates). In accordance with the present invention, three particularly advantageous and preferred cyanate ester resins are Quatrex 7187 resin available from Dow Plastics, REX 366 resin and

derivatives (such as REX 379) available from

Rhone-Poulene, Inc. (now Ciba-Geigy, Inc.) and the AroCy B resin family (such as B-50) also available from Ciba-Geigy Corp.

The fibrous carrier used in this invention must be a non-woven web (as opposed to a woven web or fabric) in order to absorb (or carry) large amounts of resin and particulate filler. In addition, the non-woven web must be made of primarily glass fibers (as opposed to polymeric fibers) to maintain a sufficiently low Df (less than 0.008), to be thermally stable during solder reflow operations and to also maintain a low materials cost. The non-woven glass web is present in an amount of 5-20 vol. % with respect to the volume of the substrate 12. The non-woven glass fiber is preferably E-glass or D-glass and a particularly preferred and advantageous non-woven glass fiber web is Viledon Style T 1786 or T 1792 available from Freudenberg Nonwovens, Viledon Industrial Products Div., Chelmsford,

Massachusetts.

The particulate filler is present in an amount of 25-55 vol. % and is selected to manipulate the K' of the microwave circuit material 10 to be either in the range of 3 to at least 10 or higher. The particulate filler used in the present invention must be

electrically non-conductive. A suitable low K' filler preferably comprises fused silica (Si0„) particles. In general, the particle size cutoff should be less than 60 microns and more preferably less than 45 microns. By "cut-off size", it is meant the largest detectable equivalent spherical diameter (e.g., overall average diameter) of each particle. The shape of the particles may vary and is not critical to the

functioning of the microwave circuit material of this invention. Two particularly preferred and advantageous silica filler particles are Denka spherical type FB-35 available from Performance Materials Division of Denki Kagaku Kogyo Kabushiki Kaisha, Tokyo, Japan and GP-7I available from Harbison-Walker Refractories, Calhoun, Georgia. A high K' filler preferably comprises titania (TiO₂), alumina, barium nanotitanate or barium

tetratitanate. The high K' fillers similarly have a particle size cut-off of less than 60 microns,

preferably less than 45 microns. Particularly

preferred and advantageous high K' fillers include TiOnia VC available from SCM Chemicals.

In addition to manipulating the K' and the Df of the circuit laminate, the particulate filler also lowers the coefficient of thermal expansion (CTE) of the substrate 12 in the z direction. The CTE of the filler and the composition of the dielectric is

selected such that the filler will lower the CTE in the Z direction of substrate 12 close to 16 pρm/°C.

Typical CTE values of the circuit material of the present invention are 30-35 ρpm/°C. These low CTE fillers provide improved plated through hole

reliability.

Preferably, the filler particles are coated with a material which renders the filler hydrophobic and therefore lessens the water absorption of the circuit material 10. Suitable hydrophobic coatings are silane, titanate and zirconate coatings. Examples of specific silanes, titanates and zirconates useful in the present invention are described in coassigned U.S. Patent Nos . 4,849,284 and 5,024,871 as well as U.S. Application S.N. 279,474 filed December 2, 1988 (now U.S.

Patent

), all of which are incorporated herein by reference. These coating materials also provide improvements to circuit fabrication processes such as plating and etching. The hydrophobic material is initially coated onto the particulate filler in an amount of about 1 to 2 weight % with respect to the filler particles. The curing of the cyanate ester resin requires a catalyst. Preferably, this catalyst is cobalt

acetylacetonate or manganese acetylacetonate, either of which is combined with nonyl phenol co-catalyst. An effective amount of catalyst used to catalize curing of the cyanate ester is in the range of 50 to 150 ppm of the active metal ion with respect to the cyanate ester resin. Preferably, 100 ppm of catalyst is used. The preferred co-catalyst amount is 2% relative to the reactive cyanate ester resin components.

Aromatic brominated compounds are added to the formulation to render the dielectric material flame retardant. Such compounds are decabromo-di phenyl oxide (Saytex 102), tetradecabromo-diphenoxy benzene (Saytex 120), and ethylene bis-tetrabromo phthalimide (Saytex BT-93) all from Ethyl Corporation. The brominated compounds are present in an amount of 2-4 vol. %. Preferably, the flame retardant is used in an amount effective to obtain a flammability rating or class of 94V-0.

Substrate 12 has a thickness of about 0.005 to 0.060 inch (preferably 0.02 to 0.03 inch) while conductive sheets 14, 14' may range in thickness between 18 micron and 70 micron. Substrate 12 is a rigid material with a Tg in the range of 180 to

260°C. This high Tg is advantageous in that the CTE of a glassy material is about 5 times smaller than the CTE in the rubbery state; and is comparatively higher than the Tg of typical epoxy based microwave circuit materials (about 130ºC).

The microwave circuit material of the present invention is preferably made from the following lamination process: PROCESS

The cyanate ester in solution (typically 85% solids in MEK or methylene chloride) is first mixed with the catalyst system at room temperature, to thoroughly dissolve the catalyst and the co-catalyst in the resin solution. The particulate filler or mixture of fillers and the flame retarding compounds are then added to the resin, and thoroughly mixed to form a homogenous suspension. The particulate filled cyanate ester resin is then combined with the non-woven web by using one of several processes including: extrusion, casting or calendering. The saturated web is then passed through an oven at about 170°C to completely remove the solvent, and to B-stage the cyanate ester resin. Curing of the dielectric between two sheets of copper is carried out in a flat bed lamination press, at 170-180°C for about one hour. Post curing is invoked at 225°C for 1-3 hours.

The following non-limiting examples for the present invention are set forth below in Examples 1-17:

The circuit composite laminate described above is also well suited for use in the manufacture of

multilayer circuits. In the case of multilayer

circuits, one or more discrete layers of bond plys (B-staged sheets) are stacked between sheets of the above-described cyanate ester based microwave circuit laminate. Lamination conditions depend upon the multilayer circuit features (e.g., througholes, vias, circuit lines, pads, etc.) and the type of conductive material (e.g., copper) and thickness (e.g., one ounce, two ounce, etc.).

Preferably, the bonding ply is a cyanate ester pre-preg (or B-stage) composite of the type described herein optionally differing only in a lower particulate filler concentration (as low as 10%) and also a lower catalyst concentration (as low as 10 ppm). Lower filler concentrations provide improved flow leading to improved bonding; while lower catalyst concentrations are designed to control the resin polymerization during initial manufacturing of the bond ply (B-stage

prepreg). Thus, the composition of a bonding ply in accordance with the present invention will be in the same ranges as described above with the exception that the filler range is 10-55% and the catalyst

concentration is 10-150 ppm.

Manufacture of the bonding ply of this invention is essentially similar to the manufacture of the circuit laminate of this invention wherein a continuous

lamination process is utilized. The manufacturing process for the bonding ply uses a carrier sheet which serves as a release material. This carrier sheet may be a silicone coated paper, a polymeric film or any other suitable carrier. Examples of suitable polymeric films are polyimide film (e.g., Kapton film from

DuPont) or polyester film (e.g., Mylar film from

DuPont). Turning now to FIGURE 2, a multilayer microwave circuit board in accordance with the present invention is shown generally at 16. Multilayer board 16

comprises a plurality of layers of substrate material 18, 20 and 22, all of which are comprised of an

electrical substrate material in accordance with the present invention. Each substrate layer 18, 20 and 22 has a conductive pattern 24, 26, 28, and 30,

respectively thereon. Note that a substrate layer having a circuit pattern thereon defines a circuit substrate. Vias 32 and 34 interconnect selected circuit patterns in a known manner.

In accordance with the present invention, separate sheets 36 and 38 of substrate material having a

composition in accordance with the prepreg formulation of the present invention are used as an adhesive or bond ply to laminate individual circuit substrates together. In a preferred method of forming such a laminate, a stack-up of circuit substrates alternated with one or more layers of the bond ply is made. This stack-up is then cured at a sufficient temperature and pressure whereby the entire multilayer assembly is bonded together to form a homogeneous construction with consistent electrical and mechanical properties

throughout.

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.

What is claimed is:

Claims

CLAIM 1. A microwave circuit material consisting essentially of:

(1) a substrate sheet, said substrate having a dissipation factor of less than 0.008 and including;

(a) a non-woven fibrous glass carrier in an amount of 5-20 vol. % with respect to said substrate;

(b) cyanate ester resin impregnating said non-woven glass web, said cyanate ester resin being present in an amount of 35-68 vol. % with respect to said substrate;

(c) non-electrically conductive particulate filler in an amount of from 25-55 vol. % with respect to said substrate; and

(2) a layer of conductive material on at least a portion of a surface of said substrate sheet.

CLAIM 2. The material of claim 1 wherein said

substrate includes:

at least one aromatic brominated compound in an amount effective to render said substrate flame

retardant.

CLAIM 3. The material of claim 2 wherein:

said aromatic brominated compound is present in the range of about 2 to 4 volume %.

CLAIM 4. The material of claim 2 wherein:

said aromatic brominated compound is selected from the group consisting of decabromo-di phenyl oxide, tetracabromo diphenoxy benzene and ethylene

bis-tetrabromo phthalimide.

CLAIM 5. The material of claim 1 including:

a coating on said particulate filler, said coating being selected from the group consisting of silane, titanate and zirconate materials.

CLAIM 6. The material of claim 1 wherein:

said particulate filler has an equivalent spherical diameter of less than 60 microns.

CLAIM 7. The material of claim 1 wherein:

said particulate filler is selected from the group consisting of silica, titania, alumina, barium

nanotitanate and barium tetratitanate.

CLAIM 8. The material of claim 1 wherein:

said substrate has a dielectric constant of at least 3.

CLAIM 9. The material of claim 1 wherein:

said particulate filler is present in an amount of 35 to 55 vol. %.

CLAIM 10. The material of claim 1 wherein:

said substrate has a thickness of between 0.005" and 0.060".

CLAIM 11. The material of claim 1 wherein:

said substrate has a glass transition temperature in the range of 180°C to 280°C.

CLAIM 12. The material of claim 1 including:

a catalyst system comprising cobalt acetylacetonate or maganese acetylacetonate combined with nonyl phenol co-catalyst, said catalyst system having a metal ion component in an amount effective to catalyze said cyanate ester resin.

CLAIM 13. The material of claim 12 wherein:

said effective amount of metal ion component is in the range of 50 to 150 ppm relative to the cyanate ester resin.

CLAIM 14. The material of claim 1 wherein:

said non-woven glass web is selected from the group consisting of E-glass and D-glass.

CLAIM 15. The material of claim 1 wherein:

said substrate exhibits a coefficient of thermal expansion in the range of 30-35 ppm/^°C.

CLAIM 16. The material of claim 1 wherein at least a portion of said cyanate ester resin comprises:

aryl dicyanate monomers and their prepolymers containing the ring-forming cyanate (O-C≡N)

functional group.

CLAIM 17. The material of claim 16 wherein said cyanate ester resin further comprises:

an ester of bisphenols and cyanic acid which cyclotrimerize to substituted triazine rings upon heating.

CLAIM 18. A microwave circuit material free of woven glass web, comprising:

(1) a substrate sheet, said substrate including;

CLAIM 19. A prepreg bonding ply for use in a microwave circuit consisting essentially of:

a substrate, said substrate having a dissipation factor of less than 0.008 and including;

(b) cyanate ester resin impregnating said non-woven glass web, said cyanate ester resin being present in an amount of 35-68 vol. % with respect to said substrate; and

(c) non-electrically conductive particulate filler in an amount of from 10-55 vol. % with respect to said substrate.

CLAIM 20. A prepreg bonding ply free of woven glass web for use in a microwave circuit comprising:

a substrate, said substrate including;

(c) electrically non-conductive particulate filler in an amount of from 10-55 vol. % with respect to said substrate.

CLAIM 21. In a multilayer circuit including at least a first circuit layer and a second circuit layer, the improvement comprising:

an adhesive layer sandwiched between the first and second circuit layers, the adhesive layer having a dissipation factor of less than 0.008 and comprising:

a substrate, said substrate including; (a) a non-woven fibrous glass carrier in an amount of 5-20 vol. % with respect to said substrate;

CLAIM 22. In a multilayer circuit including at least first circuit layer and a second circuit layer, the improvement comprising:

a substrate, said substrate being free of woven glass web and including;