Cyanate Ester Resins with Low Dielectric Properties and Applications

Chapter 4 CYANATE ESTER RESINS WITH LOW DIELECTRIC PROPERTIES AND APPLICATIONS Arthur W. Snow, Leonard J. Buckley Naval Research Laboratory, Washington, DC, USA

Contents 1. Introduction 2. Historical Developments 3. Cyanate Ester Resin Chemistry 3.1. Cyanate Ester Monomers 3.2. Cyanate Ester Polymerization 4. Dielectric Properties 4.1. Measurements 4.2. Structure-Property Relationships 4.3. Comparison with Other Resins 4.4. Calculation of Composite Dielectric Constants 4.5. Cyanate Ester Resin Conditions 5. Applications 5.1. Microelectronics 5.2. Conmiunications 6. Conclusion Acknowledgment References

189 190 192 193 193 195 195 197 199 200 201 206 206 207 210 211 211

1. INTRODUCTION Early in the history of the development of cyanate ester resins, it was recognized from a consideration of polymer structure that these resins had potential for low dielectric applications. The polymerization reaction and resin structure are depicted in Figure 1. The processing characteristics, chemical versatility, and properties associated with this class of thermoset resins have qualified it for low dielectric applications that include printed circuit boards, microelectronic interconnects, antenna coatings, radomes, and sonar domes. The purpose of this chapter is to review the chemistry and structure-property relationships of cyanate ester resins that are relevant to low dielectric applications. Although the scope of this chapter focuses on low dielectric aspects of cyanate ester resins, excellent reviews have appeared for general and/or other specialized aspects [1-4]. Handbook of Low and High Dielectric Constant Materials and Their Applications y edited by H.S. Nalwa Volume 1: Materials and Processing ISBN 0-12-513907-1/$30.00 All rights of reproduction in any form reserved.

189

SNOW AND BUCKLEY

i

5=N ^\.

/

NnVo-A-O^N

N=C-0-A-0-C=N

A 0/

A \0.

! Fig. 1. General representation of the cyanate ester monomer, cyclotrimerization, polymerization, and cyanurate polymer network structure.

2. HISTORICAL DEVELOPMENTS Cyanate ester resin technology was made possible by the discovery of a very simple and efficient synthesis that is readily adaptable to industrial scale production. This synthesis involves the addition of a base (usually triethylamine) to a phenol-cyanogen halide mixture in such a way that an excess of phenoxide is rigorously avoided: ArOH + CICN + (C2H5)3N -> ArOCN + (C2H5)3NH+Cr This method was patented by Grigat and Putter [5], assigned to Bayer AG in 1963, and later published in the open literature [6, 7, 8]. It was further discovered that the cyanate functional group undergoes a nearly quantitative cyclotrimerization to a symmetrical triazine structure. This reaction occurs thermally or at lower temperatures with Lewis acid or base catalysts. When dicyanates are derived from bifunctional phenols, this leads to the cross-linked structure depicted in Figure 1. Thermosets formed by this chemistry were also patented in 1963 [9]. Cyanate resins based on resorcinol, bisphenol E, and bisphenol A were developed by Bayer AG in the 1960s, and the bisphenol A dicyanate resin was evaluated relative to the corresponding epoxy in glass fiber laminates for application as printed wiring boards in 1968 [10]. The dielectric data comparison presented in Table I shows the lowering of the dielectric constant and particularly the loss tangent resulting from substituting the cyanate resin for the epoxy in the laminate. It was discovered soon thereafter that the cyanate ester and the epoxy functional groups co-react and that a blend of the two resins produced the most cost-effective laminate [11]. The resin derived from bisphenol A dicyanate was marketed as Triazine A or TA resin and was used predominantly in circuit board applications; the low dielectric properties and high thermal stability were the attractive features [12]. Sales volume (exceeding 14 tons in 1974) and worldwide markets for this resin, particularly in the United States and Japan, increased rapidly until 1977 [13]. At that time a severe problem was experienced in a southeast Asian production facility where multilayer boards were reported to be "blowing apart" when subjected to a soldiering operation. The problem was traced to moisture absorption that caused degradative chemistry (i.e., the hydrolytic formation of carbamate and its decomposition to gaseous by-products at elevated temperatures [1]). This problem was internally evaluated by testing modifications to the TA resin, and Bayer concluded that extensive research would be necessary to overcome the difficulty [13]. Based on this issue and a consideration of sales volume relative to other

190

CYANATE ESTER RESINS WITH LOW DIELECTRIC PROPERTIES AND APPLICATIONS

Table I. Comparative Evaluation of Cyanate Ester^ and Epoxy^ Fiberglass Laminates for Printed Circuit Board Applications in 1968 [10] Frequency e'

tan^

50 Hz

Epoxy/glass

Cyanate/glass

5

4.2

IkHz

5

4.2

IMHz

5

4.4

0.03

0.005

IkHz

0.03

0.004

IMHz

0.025

0.004

50 Hz

" Bisphenol A dicyanate. ^Diglycidyl ether of bisphenol A.

products, Bayer decided to withdraw Triazine A from the market. The technology was licensed to Mitsubishi Gas Chemical in 1978 and Celanese in 1984. Mitsubishi Gas Chemical developed and marketed a BT resin based on a blend of the monomer or prepolymer of the dicyanate of bisphenol A and the bismaleimide of methylene dianiline. This resin has a higher Tg than Triazine A that results in a lower dissipation factor and a lower dielectric constant at higher temperatures (> 175 °C) along with improved mechanical properties [14,15]. Its primary application is improved performance in printed circuit boards. Celanese Speciality Resins initiated research on cyanate ester resins in 1981 and focused on six areas recognized as limitations of Triazine A: hydrolytic stability, cure catalysis, melt viscosity control, low temperature curing, toughening, and flame retardance [16]. Monomers from a variety of bisphenols were produced. Five of these monomers and their prepolymers were commercially marketed under the trade name AroCy®, and several others were available as developmental monomers [17]. This research and commercialization progressed from Celanese through a series of companies (Interez, Hi-Tek Polymers, Rhone-Poulenc) to Ciba-Geigy in 1992. Targeted applications included printed circuit boards, composites, adhesives, and radomes. More recently, Dow Chemical developed a cycloaliphatic phenylcyanate resin designated as XU71787. It is derived from low molecular weight adducts of phenols and dicyclopentadiene and is targeted for low dielectric applications, principally printed circuit boards [18, 19]. Allied Signal has developed a cyanated novolac resin designated FT resin or Frimaset FT [20, 21]. This is a nonvolatile curing phenolic resin that has an improved stability and shelf life in the uncured state along with a very high Tg and thermal stability in the cured state. The structures of commercially available monomers for the corresponding cyanate resins along with physical characteristics are presented in Table II. The demand for higher speed in electronics, resins compatible with integrated circuit technology processing, and lower loss transmission of microwaves in communication applications has placed more interest and value on low dielectric constant resins. Development of these properties is a focus of current and future research in cyanate ester resins.

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SNOW AND BUCKLEY

Table II.

Structures and Physical Characteristics of Commercialized Cyanate Ester Resins [2]

Structure of resin

Trade name

Tg-

s'

tan 5

monomer/precursor

and supplier

rc)

(1 GHz)

(1 GHz)

AroCy B Ciba-Geigy

257

2.79

0.006

AroCy F Ciba-Geigy

270

2.54

0.005

AroCy L Ciba-Geigy

259

2.85

0.006

AroCy M Ciba-Geigy

244

2.67

0.005

RTX 366 Ciba-Geigy

179

2.53

0.002

XU-71787 Dow Chemical

252

2.8

0.005

2.97

0.007

Bisphenol A dicyanate N C O - ^ ^ - H ^ ^ OCN 6F Bisphenol A dicyanate

NCO-^-+-^-OCN Bisphenol E dicyanate H3C

NCO^ H3C

CH3

H V K H ^ O C N CH3

Tetramethylbisphenol F dicyanate H3C CKi KbC CH3

henol M dicyanate

OCN Dicyclopentadienyl bisphenol dicyanate OCN

/OCN

\

0C» OCN

Primaset PT Allied Signal

>300

Cyanated novalac resin "Thermomechanical analysis measurement.

3. CYANATE ESTER RESIN CHEMISTRY The cyanate ester polymerization reaction in Figure 1 offers a very attractive and seductively simple route to low dielectric resins. The cyanate functional group of the monomer, which has a significant dipole, is converted to a symmetrical triazine linkage in the polymer where symmetry cancels any permanent dipole in the carbon-nitrogen ring. A great chemical versatility resides in the structural feature depicted by "-A-" in Figure 1. In principle any substructure can be built in, and those structures desirable from a low dielectric perspective would be associated with very low dielectric constant materials such as polyethylene or poly(tetrafluoroethylene) segments. However, in practice, issues of monomer stability, cure and processing characteristics, and compromise with other physical properties restrict options and complicate this approach. These issues will be discussed in this section's summary of cyanate ester monomer and polymerization chemistry.

192

CYANATE ESTER RESINS WITH LOW DIELECTRIC PROPERTIES AND APPLICATIONS

3.1. Cyanate Ester Monomers As indicated in Section 2, cyanate ester monomers are prepared by reacting a phenol or alcohol with a cyanogen halide in the presence of a base at low temperature as is illustrated for the synthesis of the dicyanate of bisphenol A in Figure 2. The base (usually triethyl amine) forms the alkoxide that is cyanated by the cyanogen halide to form the organic cyanate monomer. Monomer stability is derived from the nature of the organic radical to which the cyanate functional group is bonded and from the susceptibility of the cyanate functional group to chemical species that cause premature polymerization or side reactions. The three known classes of organic cyanate monomers are alkyl cyanates, aryl cyanates, and fluoroalkyl cyanates. Alkyl cyanates have an instability toward isomerization to an isocyanate and subsequent trimerization to an isocyanate. The isomerization is a consequence of the cyanate group's pseudohalogen character, where dissociation to an ion pair precedes the isomerization [22]. An exception is the dicyanate of l,4-dihydroxybicyclo[2.2.2]octane, which is stable under ambient conditions but isomerizes instead of polymerizing when a catalyst is added [23]. Aryl cyanates are stabilized against isomerization by a resonance interaction, and a wide variety of stable aromatic cyanate ester monomers are known. The single-ring aromatic cyanate ester monomers (e.g., resorcinol dicyanate and hydroquinone dicyanate) are a very reactive subgroup [2]. The reactivity of the cyanate ester functional group is strongly influenced by the electron withdrawing character of the structure to which it is bonded. The dipole associated with the cyanate group is significant (3.93 D in C6H5OCN [24]), and one cyanate group on the ring activates the reactivity of the other. Fluoroalkyl cyanates are stabilized against isomerization by the electron withdrawing effect of the fluoroalkyl group. This class of cyanate esters is the most reactive, and special care must be taken in its purification and storage [25]. Monomer susceptibility to hydrolysis is also an important factor in monomer stability. The hydrolysis of the bisphenol A dicyanate to the dicarbamate is depicted in Figure 3. Although the bisphenol A dicyanate monomer is not susceptible to this reaction under neutral conditions, substitution of trifluoromethyl groups for the methyl groups does result in a small susceptibility, and the fluoroalkyl cyanate ester monomers are more susceptible [25]. Purification is also a very important factor in monomer stability. Impurities from the synthesis reaction (unreacted phenol/alcohol, triethyl amine, triethylammonium bromide, etc.) can catalyze the polymerization reaction at unwanted times. As monomer purity approaches the 100% level, polymerization reactivity ceases [26].

3.2. Cyanate Ester Polymerization The cyclotrimerization polymerization depicted in Figure 1 is an exothermic (—106 kJ percyanate equivalent) step growth process that can be driven to 90-99% conversion with sufficiently high reaction temperatures and/or catalysts. Many classes of compounds serve as catalysts. In their original work, Grigat and Putter [6] identified protic acids, Lewis

\ = /

CHaX^

<0*'C

\ = /

CH3\=/

Fig. 2. Synthesis of the bisphenol A dicyanate monomer.

Fig. 3. Hydrolysis of the bisphenol A dicyanate monomer.

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SNOW AND BUCKLEY

NH R-0-C=N

H-O-R

R-0-C=N

R-O-C^N

R-O-H

VY""

(a)

"4.0

N

¥ . 'HNII

H

^a

b I

I R

R

i '

N^

^0-^F

.oy-Y" N ^ N

R-~ M^-^

+

0-cf

R

NH

H-O-R

b I R

(b) Fig. 4. (a) Cyanate ester polymerization mechanism catalyzed by (a) an alcohol or phenol and (b) by a transition metal in combination with an alcohol or phenol.

acids, and bases. Currently, active hydrogen compounds (e.g., alkyl phenols) are used in combination with transition metal complexes (e.g., cobalt acetylacetonate, copper naphthenate) [1]. Use of appropriately formulated catalysts will lower cure temperatures from a 230-280°C range to a 121-177 °C range [1]. The mechanism is dependent on the catalyst conditions. With only the phenol present, the first step is believed to be formation of a iminocarbonate, which sequentially combines with two cyanate groups to form the cyanurate structure and regenerate the phenol as depicted in Figure 4 [2]. The addition of a transition metal is believed to serve as a coordinating template to promote a more facile cyanurate ring formation [1]. Determination of the gel point in cyanate ester resins has been found to be dependent on conditions of cure (i.e., melt system, solution, atmosphere, etc.). Theoretical statistical and kinetic models have predicted gelation at 50% conversion [2]. Experimentally, gel point conversions in the 50-64% range have been observed, and the discrepancy may be attributed to cyanate conversion that does not contribute to network formation such as side reactions and intramolecular cyclizations [1]. The cyclotrimerization reaction must be advanced to high conversion to achieve resin properties useful in applications. Unreacted cyanate ester groups are sites for hydrolysis and further decomposition. They are also sites of polarity that detract from the low di-

194

CYANATE ESTER RESINS WITH LOW DIELECTRIC PROPERTIES AND APPLICATIONS

electric properties. Reasonably well purified cyanate ester monomers, when cured without added catalysts, require temperatures of 225-280 °C for 6-12 h to achieve conversions of 80-90%. Catalysts are required to achieve conversions of 95-99% as well as practical resin cure schedules. In many cases lower cure temperatures are required, particularly when working with polyethylene fiber reinforcements.

4. DIELECTRIC PROPERTIES When an electric field impinges on matter, there is a polarization response by the constituent electrons, nuclei, and dipoles. Energy from the electric field is either stored by the polarization or dissipated in the form of heat. This has the effect of delayed signal propagation speeds in embedded conduction lines, increased reflection of electromagnetic radiation, and power loss in the form of dissipated heat. An ideal dielectric insulator would be a material that has no interaction with an electromagnetic field. From a practical perspective, the best low dielectric material has the minimum interaction. The origins of the modes of electronic, atomic, and dipolar orientation polarization in polymeric materials have been described from a chemical structure perspective [27, 28]. Electronic polarization pertains to a very rapid electric field-induced distortion of electron density surrounding the nuclei at the molecular structure level. This response occurs at optical frequencies and is influenced by the polarizibility of the electrons associated with constituent atoms. Atomic polarization is caused by electric field-induced displacement of the positively charged nuclei from their unperturbed positions. Because the nuclei are much heavier than the electrons, the displacements and frequency are much smaller, making this effect nearly negligible compared with that from the electron. The dipolar orientation polarization results from movement of a chemical structure that possesses a permanent dipole to align with the electric field. This effect can be large and may be the dominant contributor in liquids and gases. In solids with random dipolar orientation and restricted molecular mobility, this effect is comparable to that from the electronic polarization. Absorbed moisture is a particularly important consideration because the dipole of water is quite large and its mobility is frequently high. The dielectric constant, E', and dissipation factor, e" [or loss tangent, tan5 = e"/s'], are the parameters that describe the utility of low dielectric materials to serve in applications such as printed circuit boards, microelectronic interconnects, radomes, and antenna coatings. Physically, the dielectric constant is a measure of a material to resist a flow of charge in a specified electric field relative to a vacuum. When the electric field oscillates, the current resulting from the flow of charge is composed of a charging component and a loss component. The dissipation factor is the ratio of the loss current to the charging current, and is a measure of the stored electromagnetic energy that is converted to a leakage current and ultimately dissipated as heat. Although these two parameters factor differently into various applications and vary with frequency, temperature, and moisture content, in all cases it is desirable to have them as low as possible. At present, a low dielectric material is a material with a dielectric constant less than or equal to 2.7 and loss tangent less than or equal to 0.002. 4.1. Measurements Measurement of s' and tan 5 involve measurement of the complex permittivity from which are resolved the real permittivity or dielectric constant, s\ and imaginary permittivity or dielectric loss, s": s* =

s'-js"

195

SNOW AND BUCKLEY

A direct approach toward the measurement of the complex permittivity is a capacitance measurement involving electrode/sample contacts. The dielectric constant is calculated from the equation e' = Ct/soA where s' is the dielectric constant, C is the measured capacitance, t is the sample thickness, £0 is the permittivity of free space, and A is the electrode/sample contact area. This equation is valid when the stray capacitance is totally eliminated by shielding and the addition of a guard electrode. There are corrections that can be employed and these are given in an ASTM test method [29]. This technique has several disadvantages compared to noncontact methods involving waveguide, cavity, or free space measurements. Errors associated with electrode contact area, increased capacitance, and dissipation factor due to the inductance and resistance of leads, along with a limited range of high frequency, make this technique less desirable for exact measurements. A noncontact method based on the perturbation of the resonance curve (frequency and bandwidth) of a cavity by a sample yields data at the resonance frequency of the cavity and is therefore not useful for a broadband response showing the frequency dispersion [30]. The perturbation method requires that the sample be small compared to the volume of the cavity and positioned symmetrically in a region of maximum electric field. The advantage of this technique is the relatively straightforward sample preparation where sample size and shape are limited only by the size of the cavity. A waveguide technique using either a rectangular waveguide or a coaxial transmission line allows a broadband measurement covering frequencies well into the millimeter wave region. Reflection measurements using this configuration were pioneered by von Hippel and are still commonly used today [31]. A method for determining the complex permittivity and permeability from the transmission and reflection coefficients obtained from a sample in a waveguide was described by Nicolson and Ross [32]. In this method the scattering parameters (5) are used to calculate the transmission and reflection coefficients (T and R, respectively) as shown in the equations

5ii =

{l-T^)R/{l-T^R^)

521 =

{l-R^)T/{l-T^R^)

These coefficients are then used to calculate the complex permittivity and permeability (6 and fi, respectively) as shown in the equations [(/xA) + l]i/2 T = exp[-X/xf-1)1/2] where L is the path of the sample in the waveguide. Waveguide measurements have been used effectively to characterize small quantities of new materials over broad frequency ranges [33]. Several data reduction models can be used to determine the material properties, and one should be selected that best fits the type of samples under investigation. For the data reduction to permittivity values, an iterative technique can be used that does not have any calculation anomalies at frequencies where the sample thickness is an integer multiple of half of the wavelength [34]. This model can be used most efficiently for nonmagnetic relatively low loss samples. It is not sensitive to the position of the sample within the test configuration, which makes sample manipulation quite facile. The rectangular sample must fit precisely in the short direction to minimize error. The preciseness of this fit is dependent on the wavelength (smaller wavelengths require a more perfect fit). Using a coaxial transmission line as the waveguide requires a very precise fit of the sample into the apparatus that is often difficult to achieve via machining processes. The sample shape is annular and is often difficult to produce without flaws. There are also potential problems associated with ohmic contact of the sample and the sample holder [35].

196

CYANATE ESTER RESINS WITH LOW DIELECTRIC PROPERTIES AND APPLICATIONS

Free space methods in which a sample is placed between transmitting and receiving antennas can be used to measure the intrinsic material properties [36]. A relatively large sample is needed to avoid edge diffraction effects. The accuracy increases as the frequency increases into the millimeter wave region where the wave propagation is quasi-optical. 4.2. Structure-Property Relationships Structural features that contribute to low dielectric properties are those that correlate with lower polarizability per unit volume. Such features include the magnitude and concentration of permanent and induced dipoles, molecular packing efficiency, molecular mobility (Tg), electron localization in chemical structures, etc. The data in Table II reflect some of these trends for the series of aromatic cyanate ester resins. In the cured resin the phenylene cyanurate structure (see Fig. 5) has permanent dipoles associated with the carbon-oxygen and carbon-nitrogen bonds of the cyanurate ring and induced dipoles associated with the phenylene and triazine structural units. Substituents peripheral to the triazine ring are noncoplanar as indicated by X-ray structure of a model compound [37] and conformational energy minimization calculations [ 1 ]. As the cyanurate structure is diluted with the hydrocarbon linking unit by increasing the number of rings, there is a progressive decrease in s': resorcinol dicyanate (3.2) > novolac cyanate (3.08) > bisphenol A dicyanate (2.91) > bisphenol M dicyanate (2.64). This concentration of these phenylene cyanurate structural units within the cured resin matrix (Qy) may be calculated from the resin density (p), monomer functionality ( / ) , conversion (or), and repeat unit molecular weight (M^) [38]: Ccy = c.(//3)(p/M«) Figure 6 displays a plot of this quantity (calculated approximating or ^ 1) against the corresponding cyanate resin dielectric constant. Two sets of data points are connected. The upper curve connects those data points for hydrocarbon aromatic cyanurate resins, and the lower curve connects those data points for the aliphatic fluoromethylene cyanurate resins. The hydrocarbon aromatic cyanurate resin data display a continuous decrease in dielectric constant as the cyanurate structural unit is diluted by the hydrocarbon spacer. The single-ring dicyanate (resorcinol dicyanate) resin has the highest density of cyanurate groups, followed by the compact biphenyl dicyanate, then the single-atom-connected two-ring dicyanates, then those with an incremental number of methyl pendant groups, and finally the three-ring dicyanate. Further dilution would cause a leveling off of this curve. Polystyrene with a dielectric constant of 2.5 would, in a crude manner, represent an infinite dilution of the cyanurate structure but with the phenyl groups still in the resin matrix. Polyethylene with a dielectric constant of 2.3 is representative of an aliphatic hydrocarbon resin matrix. Aromatic cyanurates falling above and below the curve are those with heteroatom substitution in the spacing group. Where the heteroatom substitution is a sulfur- and/or oxygen-containing group, dipoles are generated and the dielectric constant is enhanced.

2

0.

T

0. CH2

Phenylene cyanurate Fig. 5.

Methylene cyanurate

Phenylene cyanurate and methylene cyanurate polymer linking structures.

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SNOW AND BUCKLEY

3.6 M,C C H , M,C C H ,

NCO-

3.4 3.2 N

X

o 3.0 c

o c o a o 5 o b

2.8 2.6 2.4 NCOCH2{CF2)nCH20CN

2.2 2.0 1.0

1.5

2.0

2.5

3.0

3.5

Cyanurate Structural Units per 1000 A^ Fig. 6. Structure-property correlation of the dielectric constant (1 GHz) with the density of the cyanurate polymer linkage structures in the resin matrix. The filled circles correspond to aromatic cyanate ester resins, and those connected by the solid line are hydrocarbon aromatic cyanate ester resins. The monomer structures designate the specific resin used for each data point. The open circles correspond tofluoromethylenealiphatic cyanate ester resins. The numbers beside each data point correspond to the number of CF2 units in the fluoromethylene chain for that particular resin.

The sulfone group is a very polar substitution, and the data point corresponding to the bisphenol sulfone dicyanate is displaced significantly above the curve. The ether and thioether connecting groups are weakly polar, and their placement is much closer to the hydrocarbon curve. Fluorine substitution reduces polarizability, and this effect is reflected in a lower dielectric constant if dipoles are not generated. In Figure 6, replacing the two methyl groups of bisphenol A dicyanate with the trifluoromethyl groups of 6F bisphenol A dicyanate shows this effect. While the larger size of the trifluoromethyl group causes a greater dilution of the cyanurate structure, there is substantial depression of the dielectric constant illustrated by the distance the corresponding data point for the 6F bisphenol A dicyanate resin falls below the aromatic cyanurate line in Figure 6. The fluoromethylene cyanurate data series also displays a continuous curve. This curve is displaced to a significantly lower dielectric constant level. This reflects both the fluoromethylene character of the spacing group and the absence of the phenylene structure attached to the cyanurate group (see Fig. 5). After the fluoromethylene chain length exceeds six units, the dielectric constant appears to level off at a value of 2.3 rather than

198

CYANATE ESTER RESINS WITH LOW DIELECTRIC PROPERTIES AND APPLICATIONS

approaching the poly(tetrafluoroethylene) value of 2.05. It has been speculated that the fluoromethylene chains in the cyanurate resin matrix are more kinked than extended, and that these kinks are sources of dipoles [39]. There is also a dipole associated with the CH2CF2 junction that enhances the dielectric constant at the higher end of the fluoromethylene cyanurate curve [39]. The effect of inefficient molecular packing is a way of incorporating free volume into the resin matrix with an attendant reduction in dielectric constant. One way on a small scale to do this is to incorporate bulky substituents in the monomer structure. The ortho substitution of the methyl groups in the tetramethyl bisphenol F resin (see Table II) positions these methyl groups above and below the cyanurate ring in the cured structure and inhibits close ordered packing. This effect is reflected in a lower resin density (and a smaller Qy). The reduction in dielectric constant is significant compared with the bisphenol A dicyanate resin (see Table II). On a larger scale, the nanofoam approach may be employed. In this case a thermally labile oligomer, such as one derived from propylene oxide or caprolactone, is covalently bonded to the thermoset matrix and depolymerized at a temperature greater than that for resin cure (>260°C) to leave submicron voids in the matrix. The effective void content depresses the dielectric content but at a cost to the mechanical properties. 4.3. Comparison with Other Resins Cyanate ester resins have an attractive range of dielectric properties when considered against other candidate materials. Table III presents representative dielectric characterization of relevant thermosets and thermoplastics. For the thermosets and most of the Table III. Relative Dielectric Permittivity of Various Thermoset and Thermoplastic Engineering Resins and Polymers^ Relative dielectric permittivity, s'

Resin/polymer Nylon 6

5.5

Phenolic

4.8

Polyurethane

4.1

Polyimides

4.1-5

FR-4 epoxy

3.6

Epoxy-anhydride

3.2-3.5

Epoxy-Jeffamine

3.0

Polyester (diallylphthalate)

3.2

Poly(phenylene sulfide)

3.2

Poly(ether imide)

3.1

Polysulfone

3.0

Polycarbonate

3.0

Poly(dimethylphenylene oxide)

2.6

Polystyrene

2.5

Polyethylene

2.3

'^ References 40 and 41 and unpublished data.

199

SNOW AND BUCKLEY Table IV. Relative Dielectric Permittivity of Various Fluorinated Research Resins Resin/polymer

s'

Ref.

2.3-2.8

42-47

Perfluorocyclobutane ethers

2.4

48

Tetrafluorocyclophane

2.4

49

Fluoromethylene cyanurates

2.3-2.7

39

Fluoroacrylics

2.1-2.4

50

Fluoroallyl siloxanes

2.3-2.4

51

Poly(tetrafluoroethylene)

2.0

52

Teflon AF

1.9

52

Fluorinated polyimides

thermoplastics, the polarity and concentration of the resin linkage structure correlate with the dielectric constant. Traditional circuit board and radome resins (epoxies and polyesters) have served the purpose well because of their ease of processing along with their mechanical, thermal, and adhesive properties and cost. With the current demand for the higher performance resulting from substitution of a lower dielectric resin, cyanate esters offer a good cost-performance ratio compared with other engineering plastics. The lower polarity associated with the cyanate ester resin's cyanurate linkage offers a way to maintain these properties associated with a thermoset while depressing the dielectric to the 2.5-3.0 range. With the exception of polyolefins, a resin with a dielectric constant below 2.5 will require an appropriate substitution of fluorine. Research following this approach has been undertaken with several classes of resins as depicted in Table IV. As with the fluoromethylene cyanate esters described in the previous section, the lowering of the dielectric constant is dependent on the degree to which the more polar polymer linking structures can be diluted by the fluorocarbon substitutents. There are practical limits to this approach. With increasing fluorine substitution, a deterioration in other application-important properties is realized. These properties include mechanical (modulus, creep), thermal (Tg, thermal expansion), processing (solubility), and cost (fluorinated reagents, multiple synthesis steps).

4.4. Calculation of Composite Dielectric Constants The dielectric constant of resins loaded with separate phase reinforcements or toughening agents may be calculated from a knowledge of the volume fraction and dielectric constant of each phase. Found to be very useful for this purpose is the model [53] ^lam-

Vrsn[(2/3) + ( 4 f / 3 4 s n ) ] + ^mf

where Vrsn and e^^^ are the volume fraction and dielectric constant of the resin, and Vrnf and e'^^ are the volume fraction and dielectric of the reinforcement. Important composite reinforcements and dielectric data are listed in Table V. A composite panel can be produced with a dielectric constant intermediate between the two components as determined by volume fraction weighting factors. Dielectric constants of laminates prepared with a 5 5 70% tetramethyl bisphenol F resin (AroCy®M) volume fraction and the fillers indicated in Table V were reported to agree within 2% of the calculated value [2].

200

CYANATE ESTER RESINS WITH LOW DIELECTRIC PROPERTIES AND APPLICATIONS

Table V. Dielectric Permittivity of Composite Reinforcements [40] Reinforcement

e'

tan 5

E-glass

6.3

0.0037

S-glass

6.0

0.002

D-glass

4.6

0.0015

Quartz

3.7

0.0002

Kevlar 49

3.7

0.002

Spectra

2.2

0.0002

Poly(tetrafluoroethylene)

2.0

0.0002

Microballoons

1.3

—

4.5. Cyanate Ester Resin Conditions Permittivity measurements reflect the strength, concentration, and mobility of dipoles in resin. For a particular thermoset resin, factors that are important determinants of s' and tan 5, in addition to its general molecular structure, are the degree of conversion, the presence of guest molecules not bonded to the network (catalyst, moisture, plasticizers, etc.), and the glass transition temperature. These parameters are also dependent on the temperature and frequency of the measurement. The conversion of resin cure relates to both the percent transformation of monomer functional groups to polymer linkages and the density of molecular packing (or, inversely, free volume) in the thermoset matrix. For cyanate ester thermosets to have useful mechanical properties, a conversion of greater than or equal to 85% should be attained. In this range of 85-100% conversion, significant changes in the dielectric constant may occur. As the conversion of bisphenol A dicyanate progresses from 86 to 98%, the dielectric constant decreases from 3.05 to 2.91 (unpublished data), and as the conversion for bisphenol M dicyanate increases from 83 to 99%, the dielectric constant decreases from 2.80 to 2.66 [2]. Two factors are responsible for this phenomena. First, the cyanurate linkage is less polar than the cyanate functional group so that the conversion of the last 15% of the unreacted cyanate groups diminishes the dielectric constant. Second, in cyanate resins the specific volume passes through a minimum with conversion near the gel point and increases at higher conversions [54]. This is a consequence of an advancing glass transition temperature with conversion, and it results in a less dense molecular packing and consequently a lower dielectric constant. Catalysts are added to cyanate ester monomers and prepolymers to attain required conversions in practical reaction times. These catalysts are usually composed of an organic ligand transition metal complex dispersed in an alkyl phenol as described in Section 3.2. Typical concentrations are 0.1-g transition metal complex and 2-g nonylphenol in 100-g resin. At this concentration, the effects on dielectric constant and tan 5 are negligible [2]. The absorption of water has a major impact on dielectric properties of thermoset resins. Liquid water has a very large dielectric constant (80 at 20 °C) and is effective in converting microwave energy into heat. These effects have been measured quantitatively under conditions of 48-h boiling water immersion for the series of aromatic cyanate ester resins relative to BMI (bismaleimide-methylenedianiline) and TGMDA/DDS epoxy (tetrafunctional epoxy-diaminodiphenylsulfone) resins and are summarized in Table VI [55]. The cyanate esters are clearly more water resistant: water uptakes range from 0.8 to 2.5% and the corresponding dielectric constant increases by 10-15%, while the tan 5 loss factors increase by a

201

SNOW AND BUCKLEY

Table VI. Effect of Water Absorption" on the Permittivity of Cyanate Ester [55]

Dry

Wet

%H20

eVtan(5

e7tan(5

absorbed

2.6/0.001

2.9/0.003

0.6

2.7/0.005

3.1/0.015

1.8

2.75/0.003

3.25/0.010

1.4

^^^^0<}O^OCN

2.8/0.003

3.3/0.011

1.4

NCO-^K+^^!^-OCN

2.9/0.005

3.4/0.015

2.5

NCO-/~yS-^~yOCN

3.1/0.004

3.6/0.014

2.4

Bismaleimide/methylene dianiline

3.5/0.005

4.6/0.023

4.2

4.0/0.022

4.8/0.039

5.8

Resin monomer structure H3C CH3H3C CH3

NCO-^y-h^^OCN

H3C

CH3

Tetraglycidyl methylene dianiline/ 4,4'-diaminodiphenylsulfone "48-h boiling water immersion

factor of 2.5-4.0. An interesting feature of the water absorption rates is that they are significantly faster for the cyanate ester resins than for the BMI or TGMDA/DDS resins. It has been speculated that the cyanate ester resins have a relatively large free volume fraction, which can accommodate a rapid ingress and egress of low molecular weight compound, whereas a thermoset matrix with strong dipoles swells by clustering water around these dipoles [2]. If this clustered water is tightly bound, its dielectric response is something intermediate between free water (s' = 80) and ice (f' = 4.2). Moisture absorption can be further depressed by the incorporation of fluorine. Figure 7 depicts the water absorption-time dependence for 23 °C water immersion of the fluoromethylene cyanate ester resin (F6Cy), the 6F bisphenol A dicyanate resin (AroCy®F), and a Jeffamine-epoxy (230-MW amine-terminated oxypropylene oligomerdiglycidylether of bisphenol A). The lower water absorption of F6Cy reflects fluoroaliphatic character and higher fluorine content. Both cyanate ester resins display an equilibrium saturation well before the epoxy. The resulting changes in the dielectric constant and tan 5 are presented in Table VII. These results illustrate the importance of reducing moisture absorption to a minimum. Moisture absorption can also have a deleterious effect on the processing of cyanate ester laminates as printed circuit boards. During processing, the circuit boards are rapidly heated (T ~ 250 °C) to conduct a soldering operation. If the boards were previously stored under humid conditions, the rapid heating causes delamination or blister formation. The degradative reaction is proposed to involve hydrolysis of the cyanuric ester linkage and a subsequent degradation of the cyanuric acid to ammonia and carbon dioxide (see Fig. 8). This reaction has been modeled, and conditions for avoiding blister formation have been delineated [56].

202

CYANATE ESTER RESINS WITH LOW DIELECTRIC PROPERTIES AND APPLICATIONS

Water Absorption (23^C/immer9ion)

40

60

100

timeV2 (hr)1/2 Fig. 7. Time dependence of moisture absorption by Jeffamine-epoxy, AroCy F, and F6Cy castings immersed in water at 23 °C. Table VII. Effect of Moisture Absorption^ on the Dielectric Constant and tan 5 (1 GHz) on the Resins Depicted in Figure 7 [25] Dry

Wet

%H20 absorbed

e7tan5

Resin NCOCH2(CF2)6CH20CN

2.29/0.018

2.43

0.20

6F bisphenol A dicyanate

2.54/0.005

2.72

1.20

Jeffamine-epoxy

2.94/0.016

3.09

1.84

^23 °C water immersion at saturation.

'^•^

^ r

^ 0

HoO

/

N=/o

VOH HO-A-( >=N

^

^^

0 ^

^

N=/o

^

j ^ ^

HoOy

HoO

HO NH3

+

CO2

HoO

iT VOH

HO-A-OH

HO

Fig. 8.

Hydrolysis of aromatic cyanate ester resin to phenol, ammonia, and carbon dioxide.

The magnitude of the effect of temperature on thermoset resin dielectric properties correlates with the temperature difference between the measurement and glass transition, the measurement frequency, and the strength of dipoles in the matrix. Temperature dependent measurements (50-300 °C) of the dielectric constant and dissipation factors at 1 Hz and 1,

203

SNOW AND BUCKLEY

10, and 100 kHz on the bisphenol E dicyanate have been reported [2]. This resin has a Tg of 258 °C. Up to 210 °C both measurements are relatively constant and frequency independent. At 250 °C the 1-Hz data display the onset of a large increase, and at 300 °C the dielectric constant increases from 3.0 to 4.5, while the dissipation factor increases from 0.015 to 2.0. The higher frequency measurements (1,10, and 100 kHz) have progressively higher temperature onsets and much smaller increases. The dicyclopentadiene-phenol cyanate resin (XU71787; see Table II) exhibits a similar response over four decades of frequency (see Fig. 9) [57]. The Tg of this resin is 256 °C. Like the bisphenol E dicyanate, it displays an onset to increases in dielectric constant and loss factor at approximately 50 °C below its glass transition. Clearly the effect on the dissipation factor is larger. When stronger

0.0100 r-

V

.e-~-ioo,,^ v '

Fig. 9. XU-71787 cyanate ester dielectric constant (upper) and dissipation factor (lower) dependence on temperature and frequency. [Source: G. W. Bogan, M. E. Lyssy, G. A. Monnerat, and E. P. Woo, SAMPE J. 24, 19 (1988). Reprinted by permission from the Society for the Advancement of Material and Process Engineering.]

204

CYANATE ESTER RESINS WITH LOW DIELECTRIC PROPERTIES AND APPLICATIONS

dipoles are in the resin matrix, such as the hydroxy! group in an epoxy resin, these temperature effects are even more pronounced [57]. When the fluoromethylene cyanate resin tan 5 parameters (see Table VII) are compared with those for the aromatic cyanate resins (Table II), the order of magnitude difference probably reflects the much lower Tg of the fluoromethylene resins (Tg = 80-100 °C [39]). A relatively flat E' and tan 5 frequency dependence is desirable in microelectronic and communication applications, particular for high frequency circuits used in pagers (800 MHz), car phones (900 MHz), GPS links (1.6 GHz), satellite communications (412 GHz), and aircraft radar (8-100 GHz). The frequency dependence of E' and tan 5 for high and low Tg cyanate ester (AroCy F and F6Cy, respectively) and epoxy [metaphenylenediamine-bisphenol A diglycidyl ether (MPDA-epoxy) and Jeffamine D230bisphenol A diglycidyl ether (JA-epoxy), respectively] resins are presented in Figure 10. The low Tg resins with the larger permanent dipoles have a steeper frequency dependence. The epoxy resins have a large dipole associated with the hydroxyl and amine functionalities

4.0 3.6

MPDAepoxy

c o c o

3.2

tio

2.8

o

JA-epoxy AroCy-F

2.4 2.0

400

ik— •'•

*

4

- 4 .•

•

•

—-

4

••

A

-

4

*

^ F6Cy •

t

t

f

t

t

600

800

1000

1200

1400

1600 1800

Frequency (MHz) 0.030 MPDAepoxy

0.025 ^

0.020

c

g

0.015 1-

in

S

0.010 0.005 0.000 400

600

800

1000

1200

1400

1600 1800

Frequency (MHz) Fig. 10. Dielectric constant and loss tangent dependence on frequency for epoxy and cyanate ester resins. [MPDA-epoxy, metaphenylene diamine-diglycidylether of bisphenol A; JA-epoxy, Jeffamine D230diglycidylether of bisphenol A; AroCy F, 6F bisphenol A dicyanate; F6Cy, NCOCH2(CF2)6CH20CN; PTFE, poly(tetrafluoroethylene).]

205

SNOW AND BUCKLEY

of the polymer linkage and display a larger dielectric constant and frequency dependence than the cyanates. The MPDA-epoxy has a larger density of these linkages than the JAepoxy. The loss tangent is more sensitive to functional group mobility and consequently Tg. For the cyanate esters, AroCy F has a larger dielectric constant than F6Cy, but a lower tan ^, reflecting the higher Tg of AroCy F.

5. APPLICATIONS The importance of low dielectric materials to microelectronics and communications was recognized many years ago [31, 58], but in recent years, with emphasis on higher speed, smaller size, higher frequencies, and lower operational power requirements, it has assumed a more critical role in advancing these technologies. In microelectronics, low dielectric materials separate conducting lines and directly influence the speed of the device operation and the heat buildup within it. In communications, low dielectric materials in the form of electromagnetically transparent coatings and shells (radomes) protect sensitive antennas from weather elements, corrosion, and other airborne debris, and their reflection and absorption characteristics influence the sensitivity of reception and transmission. 5.1. Microelectronics As components of microelectronic devices, low dielectric materials serve as an interconnect material that separates conducting lines within a multichip module or between modules in a printed circuit board. Originally, the printed circuit board was developed to distribute power to various mounted components. Conventionally, it was a laminate of a flame retardant epoxy reinforced with fiber glass. The advances in microelectronics technology and semiconductor lithography have resulted in a large scale integration of circuit board functions with microelectronic components. Advanced circuit boards are now multilayer boards with more stringent demands for high-speed transmission, low heat losses, higher circuit density, minimal dimensions, and reduced power consumption [40]. Similarly, multichip modules rely on thin layers of low dielectric polymer film to insulate thin layers of metalized conductors on silicon substrates. The influence of the interconnect material on the performance of an electronic device is determined by the effects of the dielectric constant and loss tangent on the dielectric loss, signal propagation delay, and cross talk [59]. When an electrical signal propagates down a given conducting line, it generates an electromagnetic field that permeates into the surrounding interconnect material. The propagation time of the signal is determined by length of the conducting line and the velocity of the signal. A dielectric interaction between the electric field from the signal and the surrounding dielectric medium detracts from the signal's energy (dielectric loss) and slows its velocity (propagation delay). The dielectric loss and signal propagation delay are related to s' and tan 8 as dielectric loss = 23.7 • (f/c) • e'^''^ • tan5 propagation delay = s'^^^/c where / is the frequency and c is the speed of light. The dielectric loss is predominantly determined by the loss tangent, which for candidate materials may vary over two decades while the dielectric constant typically varies by a factor of 2 (see Table II). Reducing dielectric loss also reduces the buildup of heat within the component. The propagation delay is determined by the square root dependence on the interconnect dielectric constant. Thus lowering the dielectric constant will increase the signal transmission speed. Cross talk is related to the distance the electric field permeates into the interconnect material and varies with the dielectric constant. If two conducting lines are positioned too close together, cross talk will occur where the electric field generated by a signal in one line can induce a signal

206

CYANATE ESTER RESINS WITH LOW DIELECTRIC PROPERTIES AND APPLICATIONS

Table VIII. Material Properties of Printed Circuit Board Laminates Based on Quartz-Reinforced Poly(tetrafluoroethylene) [60] and Cycloaliphatic XU71787 Cyanate Ester [57] Matrices Property

PTFE/silica

XU71787/quartz

Resin content

50%

48.5%

f'

2.8

3.2

tan 5

0.0012

0.0022

CTE;c/

25 ppm/°C

ll.lppm/°C

CTEz«

40ppm/°C

52 ppm/°C

Elastic modulus

1.0 GPa

2.8 GPa

"CTE denotes coefficient of thermal expansion.

in the second line. Reducing the dielectric constant of the interconnect reduces the required distance between conducting lines, which translates into a smaller, faster device. Printed circuit boards based on poly(tetrafluoroethylene) (PTFE) filled with silica represent a current state of the art for achieving low dielectric properties [59, 60]. The typical resin-filler content is 50%. They have excellent dielectric properties {s' = 2.8 and tan 5 = 0.0012) and moisture resistance (^0.15%). Table VIII displays comparable characterization for quartz-reinforced PTFE matrix and the cycloaliphatic XU71787 cyanate ester matrix printed circuit board laminates. Other issues, such as materials cost, processing complexity, adhesion, dimensional stability, and mechanical properties, are factors that favor thermoset replacements such as cyanate ester resins. As this tradeoff gap narrows possibly by way of the bisphenol M dicyanate or fluoromethylene cyanate ester resins, thermosets will work their way into this application. For interlayer dielectrics the polyimides, particularly fluorinated polyimides, represent the state of the art [45, 61]. Dielectric constants typically run from 2.5 to 2.8. The combination of the trifluoromethyl substitution and the polyimide structure also provides for low moisture absorption (0.6-1.9% at 85% RH) and very low thermal expansion (220 ppm/°C) coefficients. When the fluorocarbon content is further increased by addition of more fluoromethylene segments, a tradeoff occurs between a substantial moisture absorption decrease (^0.1%) and a thermal expansion increase (~100 ppm/°C) [62]. The fluoromethylene cyanate ester resin has been evaluated as a thin film spin-on dielectric [63]. It displays good planarization, gap-fill, and adhesion to silicon oxide [63] in addition to low moisture absorption (0.12% ambient; 0.20% 23 °C immersion) and low dielectric constant (2.3) associated with heavily fluorinated (~55 wt%) polymers [25]. 5.2. Communications In communications and radar applications, low dielectric materials serve as antenna coatings and radome skin components. Again the dielectric constant and loss tangent are critical material parameters. Large differences between dielectric constants at the air-material interface cause reflections that result in signal loss and signal distortion. Figure 11 presents a schematic of a surface-mounted radome illustrating transmission, absorption, and reflection of radio/microwave signals. Transmission of the electromagnetic wave through the material is accompanied by a power loss due to absorption of the signal. This absorption results in heating of the material that may become severe in high power transmissions. Minimizations of the dielectric constant to reduce reflections and of the loss tangent to

207

SNOW AND BUCKLEY

Transmission

Absorption

Reflection

Fig. 11. Sketch of a radome over an antenna dish illustrating the transmission, absorption, and reflection of a radar signal.

reduce absorption are critical to this application. The reflection from a radome surface or panel is quantified by a power-reflection coefficient (|/?P), and the absorption is quantified by an absorption coefficient (K) or power-transmission coefficient {\T\^) [64]. These parameters are a functions of s' and tan 5 and also of the radome wall thickness and the transmission wavelength. For the purpose of initial materials assessment, simplified expressions that approximate the maximum values of these parameters and factor out the wall thickness/wavelength effect can be used. These are expressions

l^lmax = [(^'-l)/(^'+l)f K = (tan5)/2 1 - \T\^ = [is' -h 1)/2V6^]NTC

tan5

where A^ is the number of half wavelengths in the wall thickness. The reflection is only affected by 6\ and is very important, because back reflections inside the radome can distort the signal or possibly damage the power source. Figure 12 depicts the dependence of l^lmax o^ ^ ' - 1 ^ ^^^ ^' range of two to four, small decreases in the dielectric constant will significantly reduce the radome reflection. The absorption and transmission are linearly dependent on tan 8 and the thickness of the radome, and the transmission has a minor dependence on f'. Figure 13 depicts this \T\^ dependence on f' at tan 5 values of 0.001,0.01, and 0.03. The tan 5, which can vary over 2 orders in magnitude depending on the material and amount of absorbed moisture, is a critical factor in the performance of the radome. At values greater than 0.03, the power-transmission coefficient is too small for the material to be considered useful in a radome application [64]. Minimizing tan 5 and the wall thickness are key issues for enhancing transmission. For the ideal material, l/^l^ax would approach 0 and | r p would approach 1. The purpose of a radome is to provide protection for antennas and other radio/microwave signal receiving systems while minimally affecting the signal with regard to power loss and distortion. Moisture absorption and impact resistance are additional critical properties. Small quantities of adsorbed moisture cause large increases in the dielectric

208

CYANATE ESTER RESINS WITH LOW DIELECTRIC PROPERTIES AND APPLICATIONS

^ ^Q

c

0.8

o o

0.6

i

i

i

i

i

|

i

t

i

i

|

i

i

t

i

|

i

t

t

i

|

i

i

i

'

'

c o

0.4 I

k

0.2 0.0

10

15

25

20

Dielectric Constant Fig. 12. Dependence of the power-reflection coefficient, |/?r, on variation in the dielectric constant.

1.00 0.001

c

i

0.95 I-

o o c o *w m

0.90

c o

0.85 0.80

o 0.

0.75 10

15

Dielectric Constant Fig. 13. Dependence of the power-transmission coefficient, \T\^,on variation in the dielectric constant at tan 5 values of 0.001, 0.010, and 0.030.

constant and particularly in the loss tangent. On aircraft flying at sonic and supersonic speeds, the impacts of rain, ice, and small debris can cause severe physical damage. Although radomes are not load-bearing structures, they should be tough enough to resist impact-induced microcracking, which becomes a serious source of water ingression. The selection of materials and design for construction is made based on these factors. There are generally two types of radome designs: solid wall and sandwich radomes. Solid wall radomes are used where resistance to impacts is an important factor such as in high-speed aircraft. They are typically fabricated from fiber-reinforced thermoset composites with a thickness gaged to the amount of impact resistance required. Sandwich radomes consist of a low density and low dielectric core (either a honeycomb or rigid foam) and a thin inner and outer skin (usually a fiber-reinforced thermoset or thermoplastic composite). These radomes are mounted on low-speed aircraft, ships, and shore installations.

209

SNOW AND BUCKLEY

Table IX. Comparative Dielectric Properties of Cyanate, Bismaleimide, and Epoxy Quartz Fiber-Reinforced Laminates for Radome Evaluation [69] Cyanate

BMI

Epoxy

e' (33 GHz)

3.25

3.44

3.62

tan<5(33GHz)

0.005

0.014

0.016

Moisture absorption

1.3%

4.5%

5.5%

tan(5 (33 GHz/wet)

0.006

0.022

—

Property

The reinforcing fibers used depend on the level of electromagnetic performance and mechanical robustness required. Traditionally, quartz and various grades of glass fibers are used (see Table V). More recently, ultrahigh molecular weight polyethylene fibers (Spectra®, Allied Signal) have been utilized for this application with very exciting results [65, 66]. This material has a very unique combination of very low dielectric properties {s' — 2.2; tan 5 = 0.0002), attractive mechanical properties (similar to aramids), and moisture resistance. With this fiber, the reinforcement becomes the lower dielectric component, and improvement of resin dielectric properties will have a major impact on radome composites. Spectra® is temperature limited (melting point 130°C; service temperature <100°C), which restricts it to low-speed applications and presents a special challenge for resin cure conditions. Spectra® has been successfully prepreged with cyanate ester and is available from Bryte Technologies, Inc. Cured laminates are claimed to display s' and tan 5 of 2.50 and 0.0006, respectively [67]. Cyanate ester resins were first patented as a matrix resin for radome fabrication in 1991 [68]. Comparative studies of cyanate ester, bismaleimide, and epoxy resins in quartz fiber laminates were conducted to demonstrate the relative potential of these materials for radome applications [69]. Of these radome composites the cyanate ester laminate displayed the lowest s' and tan 5 as indicated in Table IX. Additionally, the cyanate ester composite has the flattest s' and tan 5 frequency dependence and the lowest moisture absorption.

6. CONCLUSION Cyanate ester resins made their appearance in the 1960s and in both the research and application arenas have matured in work through the 1990s. Like all materials development, the drivers are unique properties in the science arena and performance and cost in the applications arena. The uniqueness of the cyanate ester resin resides in its polymerization and structure (Fig. 1). It is a simple one-component thermoset system with a simple well-defined network structure. This has provided a solid foundation on which scientific structure-property studies have been built. The high symmetry and low polarity of the network structure are responsible for the low dielectric character of this resin system. The limits as to how low the cyanate ester resin dielectric constant can be pushed may be extrapolated from Figure 6 and were described in Section 4. For applications, there are always tradeoffs between conflicting properties and between cost and performance. In the future, performance demands for low dielectric materials will inevitably increase as more is demanded from microelectronics and communications. Based on the currently known property data base, the materials availability, and the processing simplicity, cyanate ester resins have a healthy performance to cost ratio. Future developments in low dielectric materials will probably be in the area of unique low density morphologies such as nanofoams and in the area of polyolefin or PTFE fiber-reinforced composites.

210

CYANATE ESTER RESINS WITH LOW DIELECTRIC PROPERTIES AND APPLICATIONS

Acknowledgment The authors gratefully acknowledge David A. Shimp of Ciba-Geigy and Ernst Grigat for many helpful discussions and the office of Naval Research forfinancialsupport. References 1. T. Fang and D. A. Shimp, in "Progress in Polymer Science" (O. Vogel, ed.). Vol. 20, pp. 61-118, Elsevier, Amsterdam, 1995. 2. I. Hamerton, ed., "Chemistry and Technology of Cyanate Ester Resins." Chapman and Hall, Glasgow, 1994. 3. R. B. Graver, in "High Performance Polymers: Their Origin and Development" (R. B. Seymour and G. S. Kirshenbaum, eds.), pp. 309-316. Elsevier, New York, 1986. 4. V. A. Pankratov, S. V. Vinogradova, and V. V. Korshak, Russ. Chem. Rev. 46, 278 (1977). 5. E. Grigat and R. Putter, German Patent 1,195,764 (1963). 6. E. Grigat and R. PUtter, Chem. Ben 97, 3012 (1964). 7. E. Grigat and R. Putter, Angew. Chem., Int. Ed. Engl. 6, 206 (1967). 8. E. Grigat, Angew. Chem., Int. Ed. Engl. 11, 949 (1972). 9. E. Grigat and R. Putter, German Patent 1,183,507 (1963). 10. R. Kubens, H. Schultheis, R. Wolf, and E. Grigat, Kunststoffe 58, 827 (1968). 11. R. Kubens, H. Schultheis, R. Wolf, E. Grigat, H.-D. Schminke, and R. Putter, U.S. Patent 3,562,214 (1971). 12. K. K. Weirauch, P G. Gemeinhardt, and A. L. Baren, Soc. Plast. Eng. Prepr. 22, 317 (1976). 13. J. Franke and G. J. Schexnayder, Polym. Mater. Sci. Eng. 71, 619 (1994). 14. M. Gaku, K. Suzuki, and N. Ikeguchi, "Proceedings of the 13th Electrical/Electronics Insulation Conference," 1977, p. 11. 15. S. Ayano, Chem. Econ. Eng. Rev. 10, 25 (1978). 16. D. A. Shimp, Polym. Mater. Sci. Eng. 71,623 (1994). 17. D. A. Shimp, J. R. Christenson, and S. J. Ising, "Cyanate Ester Resins—Chemistry, Properties and Applications," Technical Bulletin, Ciba-Geigy, Ardsley, NY, 1991. 18. T. G. Millard and P M. Puckett, Polym. Mater Sci. Eng. 71, 625 (1994). 19. E. P Woo and D. J. Murray, U.S. Patent 4,713,442, 1987. 20. S. Das and F DeAntonis, Polym. Mater Sci. Eng. 71, 627 (1994). 21. S. Das, D. C. Prevorsek, and B. T. DeBona, "Proceedings of the 21st International SAMPE Technical Conference," 1989, p. 972. 22. K. A. Jensen, M. Due, A. Holm, and C. Wenti-up, Acta Chem. Scand. 20, 2091 (1966). 23. J. C. Kauer and W. W. Henderson, J. Am. Chem. Soc. 86,4732 (1964). 24. D. Martin and W. M. Brause, Chem. Ber 102, 2508 (1969). 25. A. W. Snow, L. J. Buckley, and J. P Armistead, J. Poly Sci., Part A: Polym. Chem. 37, 135 (1999). 26. M. Bauer, J. Bauer, and G. Kuhn, Acta Polym. 37,715 (1986). 27. A. R. Blythe, "Electrical Properties of Polymers." Cambridge University Press, Cambridge, U.K., 1979. 28. C. C. Ku and R. Liepins, "Electrical Properties of Polymers. Chemical Principles," Chap. 2. Hanser, Munich, 1987. 29. ASTM Standard Test Method for A-C Loss Characteristics and Permittivity of Solid Electtical Insulating Materials, Publication D150, American Society for Testing and Materials, 1992. 30. A. Parkash, J. Vaid, and A. Mansingh, IEEE Trans. Microwave Theory Tech. MTT-27, 791 (1979). 31. A. von Hippel, "Dielectric Materials and Application." MIT Technology Press, Cambridge, MA, 1954. 32. A. M. Nicolson and G. Ross, IEEE Trans. Insti-um. Meas. IM-19, 377 (1970). 33. L. J. Buckley and K. E. Dudeck, Synthetic Metals 52, 353 (1992). 34. J. Baker-Jarvis, E. Vanzura, and W. Kissick, IEEE Trans. Microwave Theory Tech. 38, 1096 (1990). 35. J. Pameix, M. E. Kadiri, and G. Tourillon, Synthetic Metals 25, 299 (1988). 36. R. Cook and C. Rosenberg, J. Phys. D: Appl. Phys. 12, 1643 (1979). 37. A. S. Jessiman, D. D. MacNicol, P. R. Mallinson, and I. Vallance, J. Chem. Soc, Chem. Commun. 1619 (1990). 38. A. W. Snow, R. F. Cozzens, W H. Echols, J. P Armistead, and D. A. Shimp, "Proceedings of the 33rd International SAMPE Symposium," 1988, p. 422. 39. A. W. Snow and L. J. Buckley, Macromolecules 30, 394 (1997). 40. S. J. Mumby, / Electron. Mater. 18, 241 (1989). 41. P. R. Karmel, G. D. Colef, and R. L. Camisa, "Introduction to Electromagnetic and Microwave Engineering." Wiley, New York, 1998. 42. G. Hougham, G. Tesoro, A. Viehbeck, and J. D. Chapple-Sokol, Macromolecules 27, 5964 (1994). 43. A. E. Feiring, B. C. Auman, and E. R. Wonchoba, Macromolecules 26, 2779 (1993). 44. T Ichino, S. Sasaki, T. Matsuura, and S. Nishi, J. Polym. Sci., Part A: Polym. Chem. 28, 323 (1990). 45. B. C. Auman, Mater Res. Soc. Symp. Proc. 381, 19 (1995).

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