WO1992022422A1

WO1992022422A1 - Shape retaining flexible electrical circuit

Info

Publication number: WO1992022422A1
Application number: PCT/US1992/005094
Authority: WO
Inventors: Samuel Gazit; Gary W. Adams
Original assignee: Rogers Corporation
Priority date: 1991-06-19
Filing date: 1992-06-17
Publication date: 1992-12-23

Abstract

A printed circuit board composed of a low Tg epoxy filled with a low CTE filler in the range of 45 to 65 % by volume and laminated to electrically conductive sheets is presented. The printed circuit board of this invention does not include any woven or nonwoven fabrics. The printed circuit board is flexible in the sense that it can be bent to any desired multiplanar shape and will retain that shape after installation as required by electronic interconnection systems.

Description

SHAPE RETAINING FLEXIBLE ELECTRICAL CIRCUIT

Background of the Invention:

This invention relates to the field of printed circuit boards. More particularly, this invention relates to printed circuit boards comprising an epoxy substrate reinforced with a particulate material and/or short fibers which can be processed in a manner similar to rigid printed circuit board or hardboards but thereafter are bendable to retain multiplanar shapes. The process for manufacturing a rigid printed circuit board or hardboard is well known in the art. The hardboard is produced in a panel form with the particular circuitry being etched, plated, screened or stamped thereon. Rigid printed circuit board of this type must necessarily only be used for single-plane hardboard applications since any bending would result in cracking and/or breaking.

In order to connect single-plane hardboards to other hardboards within the electronic device, expensive multiboard interconnections must be utilized. These interconnectors add both to parts costs and labor costs as well as increasing the complexity of a given installation. Multiplane circuitry can be achieved by:

1. Combinations of two or more rigid board segments interconnected by flexible jumper cables. 2. The mother-daughter board arrangement, using edge card connectors.

3. Building a flexible circuit which is then selectively stiffened in sections which are designed for component mounting. 4. Molding circuitry to form three dimensional circuit shapes.

The above-discussed well known problems of building multiplanar circuits with both conventional rigid and flexible circuit boards and molded circuits have been overcome and alleviated by a novel bendable, shape retaining circuit board material described in U.S. Patent Application Serial No. 778,603 filed September 20, 1985, which is assigned to the assignee hereof, all of the contents of which are incorporated herein by reference. This material is commercially available from Rogers Corporation, Rogers, Connecticut under the trademark BEND/flex. The circuit board material of prior USSN 778,603 is made utilizing conventional hardboard processes. The circuit board is produced in sheet form, and can be converted into a printed circuit by using conventional hardboard processing techniques including component mounting. Thereafter, the unique properties of the prior material allow the printed circuit board to be formed into a predetermined three dimensional shape and thereafter mounted into electronic equipment. The formed printed circuit board will not crack and has sufficient stiffness to retain its shape after installation. The manufacturing process of the circuit material of USSN 778,603 includes forming a nonwoven web substrate of long (1-3 inches) polyester and glass fibers, impregnating and saturating the web with an epoxy solution, and thereafter drying the web to drive off any solvent. The dry, tacky web is then laminated on one or both sides with sheets of copper to form a sheet of printed circuit board material. As with hardboard material, the sheet can be etched, punched, drilled or blanked out to form any desired circuit configuration and finally, the circuit with the mounted components can be formed or bent into a multiplanar configuration.

An important feature of the circuit material of USSN 778,603 is its bendability and shape retention at room temperature. This key feature is achieved by carefully selecting the epoxy resin to exhibit a glass transition temperature (T ) at or near room temperature. The T is typically in the range of 10-60°C and preferably about 40-50°C. This glass transition is broad and spreads over 20-30°C. In addition to the epoxy matrix and nonwoven fabric web of glass and polyester fibers, the commercial bendable composite also includes flame retardant fillers. Above the T up to solder temperature, the circuit material of USSN 778,603 has a high thermal expansion coefficient (CTE) . The high CTE causes any plated through holes, used to connect conductive patterns on one side to the other, to fracture upon repeated thermal cycling. The fracturing occurs due to a CTE mismatch between the dielectric material and the conductive plated through hole which is conventionally copper. Therefore, it is desirable to reduce the CTE of the dielectric material to more closely match that of copper which is approximately 16 parts per million (PPM)/°C. Also, the polyester fibers which are used to reinforce the circuit material of USSN 778,603 melt at solder temperature causing the composite to embrittle. An improved bendable and shape retaining circuit board material is disclosed in U.S. Patent 4,997,702, which is also assigned to the assignee hereof and incorporated herein by reference. In accordance with Patent 4,997,702, the circuit material of USSN 778,603 is modified to contain up to 70% by weight of low CTE fillers and/or an additional weight fraction of higher melting point fiber reinforcement to improve the overall thermal and mechanical properties of the material. As a result, a prior art problem of limited plated through hole reliability due to low CTE in the Z axis is improved by filling the circuit substrate (comprised of a nonwoven web impregnated with epoxy) with low CTE particles such as glass spheres, silica or milled microglass fibers in a preferred filling range of about 25-35% by volume. In addition, a prior art problem of limited heat resistance at solder reflow temperatures is alleviated by replacing the polyester component in the nonwoven fabric web with a more thermally stable fiber such as an aromatic polyamide, a polyacrylonitrile or a similar polymeric fiber. The resultant circuit material will then easily withstand temperatures typical to the solder reflow process without deleterious effects. As mentioned, the preferred filing range in U.S. Patent 4,997,702 is 25-35%. This range is preferred due to difficulties incorporating more low CTE particles along with the long fiber reinforcement. When the volume loading of filler particles is greater than about 35%, the long fiber reinforcement restricts the web impregnation process, segregating the filler particles across the dielectric thickness. The resultant composite material is inhomogeneous containing fiber, resin and filler rich regions. Also, the presence of the organic fibers actually increases the CTE value of the composite in the Z-axis of the laminate. The long glass fibers are less detrimental to the CTE than the organic fibers but also reduce the bendability and shape retention properties of the composite laminate.

The epoxy resin based composition disclosed in Patent 4,902,732 is an example of an epoxy composite for electronic applications. The epoxy composite disclosed in Patent 4,902,732 contains a curable epoxy resin blend and a phenolic novolac organopolysiloxane block copolymer. This epoxy composite is used to encapsulate semiconductor devices and may contain up to 90% inorganic filler such as powdered quartz. The inorganic filler is added to reduce the CTE of the composite to more closely match the CTE of the semiconductor device. The function of the organopolysiloxane block copolymer is to reduce the internal curing stress so that the composite is more resistant to crack formation. However, the composition described in Patent 4,902,732 has a high glass transition temperature and therefor would not be useful as a bendable, shape retaining circuit substrate. Additionally, no mention of the use of this composition as a printed circuit substrate is made. An epoxy resin/filler composition for a slightly different application is disclosed in Patent 4,528,305. The epoxy composite disclosed in Patent 4,528,305 consists of a high T (greater than 149°C) epoxy matrix filled with low CTE filler. The examples in Patent 4,528,305 demonstrate that increasing the levels of low CTE filler significantly decreases the CTE of the composites. The rigid composites in Patent 4,528,305 were prepared to have a CTE close to that of aluminum for use as modeling stock.

Summary of the Invention:

The present invention relates to a circuit board material comprising a substrate composed of a low T epoxy matrix filled with low CTE fillers up to 65% by volume, the substrate being laminated under heat and pressure on one or both sides to a layer of conductive material such as copper.

The circuit substrate of this invention has several advantages over conventional circuit substrates. Like the circuit materials of USSN 778,603 and U.S. Patent

4,997,702, the circuit substrate may be formulated with a glass transition temperature (T ) around room temperature and the substrate will exhibit sufficient rigidity to be fabricated into printed circuit boards in a similar manner to conventional hardboards. After fabrication, the circuit substrate of this invention can be bent to retain multiplanar shapes. However, in addition to its bendability and shape retension properties, the circuit substrate of this invention exhibits a Z-axis CTE significantly less than disclosed in either USSN 778,603 or U.S. Patent 4,997,702. Also, the circuit substrate of this invention does not include a nonwoven web fabric (or a woven fabric) as do the prior art materials of USSN 778,603 and U.S. Patent 4,997,702.

The circuit substrate of this invention may be made using several known processing techniques including extrusion and casting.

The circuit material in accordance with the present invention has many significant features and advantages. For example, the flexible/bendable epoxy system used in the present invention comprises di and polyanhydrides or diacids tailored to provide a low T in the range of 10-60°C. The use of an g epoxy/anhydride or diacid polymer system also achieves excellent chemical resistance to solutions used in circuit fabrication as well as providing an excellent and permanent bonding between the substrate and a copper foil. This epoxy polymer system also provides flame retardancy which is improved even further through the use of flame retarding additives such as antimony oxide. The use of a low CTE filler provides improved plated through hole (PTH) reliability. Moreover, appropriate surface treating of the filler (e.g., silane, titanate and/or zirconate coatings) leads to reduced water absorption which results in improved reliability of the circuit material in a humid environment. This surface treatment also provides improved resistance to solutions used in circuit fabrication.

The lack of a web (either woven or non-woven) in the laminate of the present invention is a particularly important feature as it results in the use of relatively more epoxy in the matrix therefore leading to an overall lower cost due to the relatively high costs for webs as compared to epoxy resins. The absence of a web also allows for higher loadings of the relatively inexpensive fillers which again contributes to an overall low cost for the circuit laminate of the present invention. Finally, the elimination of the web improves (e.g. increases) the flexibility of the circuit substrate.

Still another important feature of this invention is a thickness range for the substrate of between about 5-30 mils. Because this material retains a degree of flexibility at relatively large thicknesses (e.g.,

15-30 mils), the present invention provides important solutions where thick, yet flexible circuit designs are required. This feature is especially significant since prior art polyester and polyimide based substrates of comparable thickness (15-30 mils) are rigid and so could not be used where a flexible and bendable material is required.

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.

Description of the Preferred Embodiment:

In general, the present invention comprises a composite composed of a low T epoxy matrix filled with low CTE fillers. As used herein, the term

"fillers" means particulate fillers or short fibers. The composite of this invention does not include or contain any fabric, whether woven or nonwoven.

The following is a general description of the invention and the method used to prepare the examples. An epoxy/curing agent mixture is blended with the particulate filler using a paddle type planetary mixer such as a ROSS mixer. Blending the particulate filler with the epoxy curing agent mixture can also be done using a two roll mill. Filler loading of particles/fibers should be at least 45% by volume with a preferred filler loading level of particulate filler being 50-65 volume % to minimize the CTE value by approaching the maximum volume packing fraction of the filler. The maximum packing fraction (and corresponding maximum loading level by weight) varies with filler particle shape, particle size, particle size distribution and surface chemistry of the filler particles. Examples of typical low CTE mineral fillers are clays or mineral fillers such as wollastonite, bentonite, kaolin, diatomaceous earth, mica, beta-eucryptite, silica, quartz, glass microspheres or beads, hollow glass or ceramic microspheres, milled glass or mineral fibers, alumina, aluminum silicate, sodium aluminum silicate and calcium carbonate.

Silane, titanate, zirconate and other coupling agents such as 3-aminopropyl-triethoxysilane, 3-glycidyloxypropyl-trimethoxysilane, zirconium IV l,l(bis-2-propenolatomethyl) butanolato-tris (2-amino) phenylato, neopentyl(diallyl)oxy-tri(N-ethylenediamino) ethyl titanate, tetra (2,2 diallyoxymethyl)butyl-di(ditridecyl)phosphito titanate, neopentyl(diallyl)oxy-tri(dioctyl)pyro-phosphato zirconate may be added either directly to the epoxy-filler mixture during blending or may be used to pretreat the filler particles prior to mixing. Coupling agents such as those previously described may be added to: a) improve the mechanical properties of the composite such as increasing the tensile strength and elongation of the composite, b) decrease moisture absorption and c) increase dimensional stability. The epoxy prepolymers used may be any commercially available di or multifunctional epoxy, however liquid or semisolid epoxies are preferred for ease of processing. While a solvent can be added to aid in the mixing of the high level of filler particles, solvent can lead to difficulties during lamination to conductive metal sheets. Epoxies and curing agents with a flexible molecular backbone are also preferred so that the T of the composite is around room g temperature and crazing or cracking upon bending is avoided. The viscosity of the resin-filler mixture can be reduced by incorporating a monofunctional epoxy into the formulation. Addition of a monofunctional epoxy also aids in preventing embrittlement of the composite during thermal cycling. Examples of di- or multifunctional epoxies which can be used are: diglycidyl ether of bisphenol A, multifunctional glycidyl ether of phenolic novolac, aliphatic triglycidyl ether, aliphatic diglycidyl ether, polyglycol diglycidyl ether, diglycidyl ether of dibromoneopentyl glycol, diglycidyl ether of 1,4 butanediol, diglycidyl ether of neopentyl glycol, diglycidyl ether of resorcinol, and triglycidyl ether of para-aminophenol. Monofunctional epoxies such as: dibromophenyl glycidyl ether, phenyl glycidyl ether, cresyl glycidyl ether, p-tert-butyl phenyl glycidyl ether, butyl glycidyl ether and alkyl glycidyl ethers may be added to reduce the epoxy/filler/curing agent mixture viscosity. Flexible polyanhydride or diacid curing agents such as aliphatic polyamides, aliphatic amido-amines, polysebacic polyanhydride, polyadipic polyanhydride, polydodecanedioic polyanhydride or polyazelaic polyanhydride are preferred. However, other anhydride curing agents may be used if the glycidyl ether backbone is sufficiently flexible to result in a T around room temperature (10 to 60°C) for the cured composite. Typical anhydride curing agents used for electrical applications are: dodecenyl succinic, hexadhdropethalic, NADIC methyl, phthalic, succinic, tetrahydrophthalic and chlorendic anhydrides. The corresponding diacids of any of the aforementioned anhydrides (including the flexible backbone anhydrides) may also be used as curing agents.

The anhydride to epoxy ratio is important and may be varied from 0.5:1 to 1.5:1. The preferred range of anhydride to epoxy ratio is between 0.6:1 and 0.8:1. This preferred range results in optimum electrical and mechanical properties of the composite.

The ratio of monofunctional to difunctional epoxy is preferably 1:2.5 (functional group equivalence).

A flame retardant is added in an amount effective to provide flame retardance. Preferably, the flame retardant is antimony trioxide or a synergistic combination of antimony and brominated compounds.

The circuit material of this invention can be made in several ways, including extension and casting processes. By the casting process, the filled epoxy is mixed with a solvent and spread on a copper foil using a blade or a knife. The solvent is then removed by evaporation in an oven and the resin is B-staged. Another layer of copper foil is then placed on top of the filled resin layer. The sandwiched dielectric between the two copper foils is then placed in a press for curing of the epoxy resin, and to form the circuit material. The extrusion processing is discussed with reference to Examples 1-6. The following six non-limiting example formulations were made by compounding the epoxy curing agent mixture in a paddle type mixer at a slightly elevated temperature (approximately 50-60 C) . Any catalyst required was added when the mixing was nearly complete. The epoxy/curing agent/filler mixture was rolled into flat sheets using a calendar and laminated to 1 ounce/ft copper using a combination of heat and pressure. The lamination temperatures ranged were about 400°F while lamination pressures were approximately 800 psi. The lamination times were 15-30 minutes and did not appear to be critical over this range in determining the laminate properties.

Example 1

Component Parts bv Weight VQ I

Diglycidyl ether of bisphenol A 72.4

(Shell Epon 828)

Dibromo phenyl glycidyl ether 60.0

(Nippon Kaya u BR-250)

Polyazelaic Polyanhydride 63.0

(Anhydrides and Chemicals PAPA)

44 micron particle size cutoff 456.0 51.0 amorphous silica

Dimethyl amino methyl phenol 1.5

(Anhydrides and chemicals AC-10)

Antimony Trioxide 47.0

Example 2 Component Parts by Weight Vol.%

Diglycidyl ether of bisphenol A 37.1

(Shell Epon 828)

Dibromo phenyl glycidyl ether 30.7

(Nippon Kayaku BR-250)

Polyazelaic Polyanhydride 32.2

(Anhydrides and Chemicals PAPA)

44 micron particle size cutoff 308.0 64.0 amorphous silica

Milled glass fiber (10 micron diameter, 16.0

40 micron maximum length)

3-glycidyloxypropyl-trimethoxysilane 4.5

(Dow Corning Z-6040)

Dimethyl amino methyl-phenol 1.2

(Anhydrides and Chemicals AC-10)

Antimony Trioxide 26.0

Example 3

Component ^r s .by Weight Vol.%

Diglycidyl ether of bisphenol A 75.0

(Dow DER 330)

General purpose amido amine 75.0

(Hi-Tek Polymers Epi-Cure 855)

44 micron cutoff glass microspheres 278.0 49.0

Dibromo phenyl glycidyl ether (Nippon

Kayaku Br-260) 35.5

Antimony Trioxide 48.0

Example 4

Component

Diglycidyl ether of bisphenol A

(Shell Epon 828)

Alkyl glycidyl ether

(Hi-Tek Polymers Heloxy 8)

General purpose amido amine

(Hi-Tek Polymers Epi-Cure 855)

44 micron cutoff glass microspheres

Dibromo phenyl glycidyl ether (Nippon

Kayaku Br-260)

Antimony Trioxide

Example 5 Component Parts bv Weight Vol.%

Multifunctional glycidyl either of phenolic 100.0 novolac (Dow DEN 431)

Aliphatic triglycidyl ether 30.0

(Hi-Tek Polymers 5044)

Polysebacic polyanhydride 100.0

(Anhydrides and Chemicals PSPA)

Dimethyl aminomethyl phenol 1.5

(Anhydrides and Chemicals AC-10)

3-glycidyloxypropyl-trimethoxysilane 6.

44 micron particle size cutoff silica 375. 45.0

Dibromo phenyl glycidyl ether (Br 250) 73,

Antimony Trioxide 64.

Example 6

Component Parts by ei Vol.

Diglycidyl ether of bisphenol A 75.0

(Shell Epon 828)

Polyazelaic polyanhydride 37.0

(Anhydrides and Chemicals PAPA)

Dimethyl amino methyl phenol 1.0

44 micron diameter cutoff glass 227.0 48.0 microspheres

Dibromo phenyl glycidyl ether (Br 250) 49.0

Antimony Trioxide 51.0

The glass transition temperatures (T_'s) of Examples 1-6 were approximately 20-30oC as measured by thermo-mechanical analysis (TMA) . The T 's of the examples as measured by DSC were as follows:

Example T (°C)

3 16

4 17

5 12

6 16 The coefficient of thermal expansion (CTE) of the prior art circuit material (described in U.S. Patent Application 778,603) is approximately 300-400 PPM°C (parts per million per degree Celsius) while the CTE of the circuit material described in U.S. Patent 4,997,702 is typically 200-300 PPM/°C. The circuit material described in Example 1 has a CTE of approximately 90 PPM/°C while the circuit material described in Example 2 has a CTE of approximately 100 PPM/°C.

A significant feature of this invention is that the inorganic filler material is present in an amount (45 vol. % or greater) effective to lower the CTE of the substrate in the Z direction to improve plated through hole reliability of the circuit material composite. The CTE is preferably in the range of equal to or less than 100 ppm/°C.

The circuit materials of Examples 1 and 2 were shown to be bendable and shape retaining at room temperature. Samples of these examples approximately 0.015" thick laminated to 1 ounce/ft 2 copper were bent at a radius of curvature of 0.15" without cracking or crazing of the copper or substrate. The substrates were laminated to copper on one side and were bent using a simple bending tool. The samples were bent with the laminated copper both towards and away from the radius of curvature of bending. An overbending angle of approximately 40 (130 total) for a retained angle of 90° was required. Samples clad with copper on both sides were also prepared and similarly retained their bent shape.

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 circuit material composite being free of any woven or non-woven webs comprising:

(a) a substrate comprising an epoxy resin system and inorganic filler material filling said epoxy resin system; said epoxy resin system having a glass transition in the range of about 10-60 C, said glass transition having a breadth of about 20 to 30°C, said epoxy resin system comprising a multifunctional or difunctional epoxy and an anhydride, polyanhydride or diacid curing agent, said epoxy being present in an amount of 0.5:1 to 1.5:1 with respect to said curing agent; said inorganic filler material comprising at least 45% by volume of the total composite and being effective to lower the coefficient of thermal expansion of the substrate in the Z direction to improve plated through hole reliability of the circuit material composite; and (b) a layer of conductive material permanently bonded on at least a portion of a surface of said substrate, said composite being flexible in the sense that it can be bent to a desired multiplanar shape and will retain that shape.

CLAIM 2. The composite of claim 1 including: at least one coupling agent on said filler material.

CLAIM 3. The composite of claim 2 wherein: said coupling agent is selected from the group consisting of silane, titanate and zirconate coupling agents.

CLAIM 4. The composite of claim 1 wherein: said substrate has a thickness of between 5-30 mils,

CLAIM 5. The composite of claim 1 wherein: said inorganic filler material is selected from the group consisting of particulate filler and short fibers.

CLAIM 6. The composite of claim 1 wherein: said filler material is selected from the group consisting of clays or mineral fillers.

CLAIM 7. The composite of claim 1 wherein: said filler material is selected from the group consisting of wollastonite, bentonite, kaolin, diatomaceous earth, mica, beta-eucryptite, silica, quartz, glass microspheres or beads, hollow glass or ceramic microspheres, milled glass or mineral fibers, alumina, aluminum silicate, sodium aluminum silicate and calcium carbonate.

CLAIM 8. The composite of claim 1 wherein said epoxy resin system further comprises: a monofunctional epoxy present in the amount 1:2.5 ratio of monofunctional to di-functional epoxy.

CLAIM 9. The composite of claim 1 wherein: said filler material comprises 45-65% by volume of the total composite material.

CLAIM 10. The composite of claim 9 wherein: said filler material comprises 55-65% by volume of the total composite material.

CLAIM 11. The composite of claim 1 including: antimony tiroxide or a synergistic combination of antimony and brominated compounds in an amount effective to provide flame retardance.

CLAIM 12. The composite of claim 1 wherein: the coefficient of thermal expansion for the circuit material composite is in the range of equal to or less than about 100 ppm/ C.

CLAIM 13. A circuit material composite being free of any woven or non-woven webs, consisting essentially of:

(a) a substrate comprising an epoxy resin system and inorganic filler material filling said epoxy resin system; said epoxy resin system having a glass transition in the range of about 10-60°C, said glass transition having a breadth of about 20 to 30°C, said epoxy resin system comprising a multifunctional or difunctional epoxy and an anhydride, polyanhydride or diacid curing agent, said epoxy being present in an amount of 0.5:1 to 1.5:1 with respect to said curing agent; said inorganic filler material comprising at least 45% by volume of the total composite and being effective to lower the coefficient of thermal expansion of the substrate in the Z direction to improve plated through hole reliability of the circuit material composite; and (b) a layer of conductive material permanently bonded on at least a portion of a surface of said substrate, said composite being flexible in the sense that it can be bent to a desired multiplanar shape and will retain that shape.

CLAIM 14. The composite of claim 13 including: at least one coupling agent on said filler material.

CLAIM 15. The composite of claim 14 wherein: said coupling agent is selected from the group consisting of silane, titanate and zirconate coupling agents.

CLAIM 16. The composite of claim 13 wherein: said substrate has a thickness of between 5-30 mils,

CLAIM 17. The composite of claim 13 wherein: said inorganic filler material is selected from the group consisting of particulate filler and short fibers,

CLAIM 18. The composite of claim 13 wherein: said filler material is selected from the group consisting of clays or mineral fillers.

CLAIM 19. The composite of claim 13 wherein: said filler material is selected from the group consisting of wollastonite, bentonite, kaolin, diatomaceous earth, mica, beta-eucryptite, silica, quartz, glass microspheres or beads, hollow glass or ceramic microspheres, milled glass or mineral fibers, alumina, aluminum silicate, sodium aluminum silicate and calcium carbonate.

CLAIM 20. The composite of claim 13 wherein said epoxy resin system further comprises: a monofunctional epoxy present in the amount 1:2.5 ratio of monofunctional to di-functional epoxy.

CLAIM 21. The composite of claim 13 wherein: said filler material comprises 45-65% by volume of the total composite material.

CLAIM 22. The composite of claim 21 wherein: said filler material comprises 55-65% by volume of the total composite material.

CLAIM 23. The composite of claim 13 including: antimony tiroxide or a synergistic combination of antimony and brominated compounds in an amount effective to provide flame retardance.

CLAIM 24. The composite of claim 13 wherein: the coefficient of thermal expansion for the circuit material composite is in the range of equal to or less than about 100 ρpm/°C.