Materials suppliers are constantly refining their recipes in search of products that offer greater value and increased reliability for a variety of commercial, industrial, and military applications
Jack Browne
Technical Director
Microwave materials represent building blocks for high-frequency circuits and systems. Whether they are used to hold circuit traces, absorb or suppress radiofrequency interference (RFI), or form resonant structures, high-quality microwave materials must provide stable performance with time and temperature and often in harsh environments. Although the topic deserves a textbook, this brief survey only hopes to provide a quick update on recent developments among the high-frequency industry's many suppliers of microwave materials.
Power dissipation and thermal management have long been concerns for suppliers of printed-circuitboard (PCB) materials. In cellular communications base stations, for example, the RF power amplifiers are among the most expensive single components and among the most likely to fail when heat generated by the amplifier is not properly dissipated. Numerous options are available for achieving heat dissipation, including the use of large heat sinks, fan cooling, amplifier PCBs with heavy metal backplanes, and even liquid-cooled enclosures. Almost all of these options can help dissipate heat from an amplifier while also adding cost.
One approach that brings effective heat dissipation without increasing the cost involves increasing the thermal conductivity of the PCB laminate material, which has traditionally been limited. In a presentation last Fall to the International Wireless Industry Consortium (IWPC, www.iwpc.org) entitled "Expanding the Thermal Management Tools for RF Infrastructure & Power Amplifiers--Low Loss Thermally Conductive PTFE and Thermally Conductive FR-4 Laminates," Russ Hornung, technical marketing manager for Arlon (www.arlonmed.com) addressed the issue of thermal management through a new line of microwave laminate materials. His company, with a history of more than 100 years in materials development, was one of the earliest suppliers of polytetrafluoroethylene (PTFE) laminate material (DiClad 522) for electronics applications (to Collins Radio in 1949) and one of the first manufacturers to disperse ceramic powder into PTFE substrate material (the firm's Epsilam 10 product from the early 1970s). Arlon launched thermally conductive FR-4 (99ML) PCB material in 2005 and has continued to enhance the technology.
In his presentation, Hornung noted that research by TriQuint Semiconductor (www.triquint.com) on GaAs heterojunction bipolar transistor (HBT) devices has shown that a 10C increase in device temperature can result in a doubling of its failure rate. By using a laminate material with improved thermal conductivity, an amplifier's active devices can be maintained at lower operating temperatures and under higher-reliability conditions. Hornung's presentation served to review some of Arlon's advanced laminate materials, including the TC600 and TC350 laminates.
The TC600 material, named for its dielectric constant of 6.15 at 10 GHz, exhibits particularly low loss tangent of 0.0022 at 10 GHz. It can be thought of as similar to the firm's AD600 material (with the same dielectric constant), but with greatly improved thermal properties, including a coefficient of thermal expansion (CTE) of 8 PPM/C in the X and Y directions and 17 PPM/C in the Z direction. With thermal conductivity of 1.4 W/m-K in the X and Y directions and 1.1 W/m-K in the Z direction, the material offers double the thermal conductivity of available laminate materials with that same dielectric constant, and also exhibits low moisture absorption of only 0.01 percent.
In addition to the TC600 material, the lower-dielectric-constant TC350 material (an improved thermal version of the firm's AD350A material) features a dielectric constant of 3.5 at 10 GHz with low loss tangent of 0.0025 at 10 GHz. It offers thermal conductivity of almost 50 percent higher of available materials with that same dielectric constant by merit of its thermal conductivity of 0.80 W/m-K in the Z direction (compared to 0.45 W/m-K for the AD350A material). The TC350 material exhibits CTE of 8 PPM/C in the X and Y directions ad 17 PM/C in the Z direction. This level of microwave laminate thermal stability is particularly critical for circuits and components requiring excellent phase stability. Like the TC600 material, the TC350 material exhibits moisture absorption on only 0.01 percent.
In his presentation, Hornung also reported that Arlon is developing nextgeneration materials for the company's 99 Series of products. In fact, the company recently announced the release of one of these multifunction epoxy laminate and prepreg products, Arlon 91ML. The thermally conductive FR-4 material is compatible with lead-free solder processes for RoHS compatibility. It provides through-plane (Z direction) thermal conductivity of 1.0 W/m-K and in-plane (X and Y direction) thermal conductivity of 2 W/m-K in prepreg thicknesses as thin as 0.003 in. It provides excellent thermal stability at lead-free solder temperatures. Hornung's presentation also reported on the notyet- announced Arlon 92M material, which is aimed at higher-performance solutions requiring excellent thermal management. In general, increased thermal conductivity at the laminate level can provide improved reliability for the PCB's solder joints and attached components.
Also concerned with maintaining excellent dimensional stability across wide temperature ranges, the Advanced Circuit Materials Division of Rogers Corp. (www.rogerscorporation.com) recently introduced its RO4500 Series laminates. Designed to extend the capabilities of the firm's RO4000 Series laminates into antenna applications, the new materials are available with a range of dielectric constants from 3.3 to 3.5 at 10 GHz and with loss tangents ranging from 0.0020 to 0.0037 at 10 GHz. The ceramic-filled, glassreinforced hydrocarbon-based material provides good dimensional stability and excellent passive intermodulation distortion for wireless antenna applications. It is compatible with all standard PCB processes, including those developed for FR-4 PCB materials.
The RO4500 materials are available in standard panel sizes of 24 x 18 in. and 48 x 36 in. As an example, the RO4533 material has a dielectric constant of 3.3 with 0.08 uniformity across the panel. The loss factor is 0.0020 at 2.5 GHz and 0.0025 at 10 GHz. The CTE is 13 PPM/C in the X direction, 11 PPM/C in the Y direction, and 37 PPM/C in the Z direction with thermal conductivity of 0.6 W/m-K. The material exhibits dielectric strength of better than 500 V/ mil with dimensional stability of better than 0.2 mm/m.
Another supplier of PCB materials based on different combinations of PTFE, ceramic materials, and woven glass is Park Electrochemical Corp. (www.parkelectro.com). The firm supplies Nelco RF and microwave materials in many different product families, including NY9000 series PTFE/wovenglass composite and N4350-13 modified epoxy-based material.
Morgan Electroceramics (www.morganelectroceramics.com) offers a range of temperature-stabilized PCB materials with dielectric constants as low as 6.5 for amplifiers, through 20.0 for antenna circuits, and as high as 35.0 for filters. For example, the company's D20 material provides dielectric constant of 20.0 1 with temperature coefficient of 0 1 PPM/C. With thermal conductivity of 7.0 W/m-K, the material is usable past 7 GHz and features moisture absorption of less than 0.01 percent.
In developing PCB solutions for highspeed digital and multilayer circuits, Taconic Advanced Dielectric Division (www.taconic-add.com) created its fastRise 27 multilayer nonreinforced prepreg material. Based on ceramic, thermoset, and PTFE materials, it is ideal for use with the company's low-loss laminates and is designed to eliminate skew in high-speed differential transmission lines caused by fluctuations in dielectric constant. The fastRise materials exhibit a dielectric constant of 2.70 at 10 GHz and also at 40 GHz, with loss factor of 0.0014 at 10 GHz that only rises to 0.0017 at 40 GHz. The high-speed material, with thermal conductivity of 0.25 W/m-K, exhibits CTE of 59 PPM/C in the X direction, 70 PPM/C in the Y direction, and 72 PPM/C in the Z direction. The laserablatable material is ideal for a variety of applications through millimeter-wave frequencies, including in semiconductor testing, military systems, and in automotive radar systems.
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In some cases, high-frequency manufacturers develop extensive in-house materials capabilities in support of their own or custom components and highfrequency electronics assemblies, such as Anaren (www.anaren.com), Dielectric Laboratories (www.dilabs.com), and W. L. Gore & Associates (www.gore.com). (For more on Gore, don't miss the feature story celebrating the company's 50 years of PTFE, beginning on p. 84 of the February issue of Microwaves & RF or online at www.mwrf.com.) For example, Dielectric Laboratories (Cazenovia, NY), perhaps best known for its highperformance fixed single-layer capacitor (SLC) products, manufactures its own high-performance ceramic materials not only for the capacitors but for custom resonators, filters, and oscillators.
As detailed in a three-part article series last year in this magazine, Dielectric Laboratories leveraged its materials expertise in working with two other companies within the parent Dover Technologies group, Vectron International (www.vectron.com) and K & L Microwave (www.klmicrowave.com), to produce miniature, high-frequency voltage-controlled oscillators (VCOs) and switched filter banks, respectively (see Microwaves & RF, September, October, and November 2007). Dielectric Laboratories has produced single-frequency cavity resonators (SFCRs) from the lower microwave range through millimeter-wave frequencies that serve as high-quality-factor (high-Q) building blocks for filters and oscillators in automotive, commercial communications, and military applications. For example, based on the firm's CF material, a 5-GHz SFCR measures just 8,1 x 8,1 x 3.0 mm with a temperature coefficient of frequency of a mere -2.3 PPM/C from -60 to +125C. A 67-GHz SFCR built from the company's FS material measures just 1.6 x 1.6 x 1.0 mm with a temperature coefficient of frequency of -7.3 PPMC from -60 to +125C. Both resonators feature frequency tolerance within 0.1 to 1.0 percent.
Of course, some applications call for materials that suppress, rather than transfer, high-frequency signals. In response to requests for a simple solution to electromagnetic interference (EMI),
ARC Technologies (www.arc-tech.com) developed its Wave-X line of products for noise-reduction applications from 5 MHz to 40 GHz. The flexible RoHS-compliant material is well suited for conformal applications. Its low outgassing properties make it suitable for space-based applications. In order to achieve consistent EMI suppression, the size of the metal particle fillers in theWave-X materials is precisely controlled, with particles uniformly distributed across thin, flexible sheets. The sheets, which can be supplied with optional peel-and-stick backing, are available in a range of thicknesses. In addition, Wave- X materials can be injection molded for EMI absorbing enclosures, and can be extruded directly over wire and cable systems for custom solutions.
Wave-X product lines include WX Series sheets and rolls with typical EMI suppression of 10 dB or better from 100 MHz to 3 GHz. These products are available in standard thicknesses of 0.005, 0.010, 0.020, and 0.040 in. The WK Series of thin EMI sheets are only 0.002 in. thick and can be used for applications from 100 MHz to 3 GHz, and the WD Series of sheets and rolls carry a UL94-V0 flammability rating.
An absorber supplier gaining some notoriety recently was Emerson & Cuming Microwave Products, when its ECCOSORB(R) MF-117 magnetically loaded epoxy was selected for a millimeter-wave waveguide termination in the international radio astronomy Atacama Large Millimeter Array (ALMA). The material was chosen for its capabilities at millimeter-wave frequencies (84 to 116 GHz) as well as its capability to function at cryogenic (4 K) temperatures. The ALMA system, one of the largest radio astronomy antenna systems in the world, will be comprised of an array of 12-m antennas with baselines of several kilometers. The ALMA radio astronomy project is a collaboration between research scientists from Europe, Japan, and North America in cooperation with the Republic of Chile.
For most practical applications, the company offers a free problem-solving kit, the Eccosorb Solutions RF Interference Problem Solving Kit, available upon request from its web site (www.eccosorb.com). The handy kit contains samples of the firm's different ECCOSORB absorber products, including ECCOSORB BSR-1/SS6M mode absorbing magnetic elastomer for use from 27 to 60 GHz, ECCOSORB GDS/SS-6M mode absorbing magnetic elastomer for use from 6 to 35 GHz, and ECCOSORB MCS/SS-6M mode absorbing magnetic elastomer for use from 0.8 to 8.0 GHz. The kit also includes several samples of the company's ECCOSORB absorbing dielectric foam materials.
Cuming Microwave (www.cumingmw.com) offers a variety of low-loss dielectric materials including RoHScompliant C-STOCK LOW K material with dielectric constants of less than 2.0. The syntactic foam materials are suitable for antenna, waveguide, and coaxial support applications. The company's C-RAM SFC-HC is a high-power broadband pyramidal honeycomb RF absorber made from specially treated Pyramidal honeycomb with a steep pyramid design for an impedance gradient. With no forced air, the absorbers can handle up to 10 W/in.2 of RF energy with phenolic-coated material and 3 W/in.2 with the Neoprene-coated Honeycomb material. With forced air cooling, the phenolic-coated material can handle 20 W/in.2 with air velocity of 200 ft/min and 80 W/in.2 with air velocity of 600 ft/min. With forced-cir cooling, the Neoprene-coated material can handle 6 W/in.2 with air velocity of 100 ft/min and 12 W/in.2 with air velocity of 300 ft/min.
In addition, Cuming Microwave offers its own RF Problem Solver Kit that includes several of the firm's most commonly used absorber materials for applications from 100 MHz to 40 GHz. The materials are supplied with peeland- stick pressure-sensitive adhesive backing layer that allows the absorbers to be cut to form and stuck into place. A kit can be requested by visiting the web site or sending an e-mail request to [email protected].
Pacific Ceramics, Inc. (www.pceramics.com) supplies diversified lines of microwave ceramic materials, including rare earth iron garnets, calcium vanadium garnets, lithium and magnesium ferrites, and titanate dielectrics. The company supports the design and manufacture of passive components for telecommunications and military applications with full process control over forming, firing, and machining.
Because of the importance of advanced materials to military and aerospace applications, major contractors either maintain materials research facilities or invest heavily in subcontracted research efforts. BAE Systems (www.bae-systems.com), for example, pursues semiconductor device research and development at its Microwave Electronics Center (MEC) in Nashua, NH. The firm, with 6-in.-wafer molecular-beam-epitaxy (MBE) and electron-beam (e-beam) lithography capabilities, fabricates GaAs MESFET and PHEMT and InP MHEMT devices on GaAs substrates and GaN HEMT devices on silicon carbide (SiC) substrates. The MEC, which fabicates both low-noise and high-power devices, defines all device gates by means of e-beam lithography and relies on automated cassette-tocassette handling of wafers.
In addition to "conventional" semiconductor materials, BAE Systems is also involved in the development of "smart materials" based on active-frequency-selective-surface (AFSS) technology, such as what has become known as "stealthy wallpaper." Made in panels, the material can be built into rooms and turned on and off to provide frequency-selective suppression of RF signals from different devices, thus controlling access to wireless communications equipment within the room and making it possible to prevent communications from those devices to outside of the room. The innovative wallpaper can be used to ensure security with cellular telephones and wireless local area networks (WLANs) in sensitive facilities.
As mentioned earlier, this brief report can only begin to scratch the surface of the advanced materials that are critical to high-frequency circuit and system design, with key materials such as lowtemperature-cofired-ceramic (LTCC) and superconducting materials left for the topics of a future report. In the meantime, those seeking more information on suppliers of microwave materials can find guidance in the online version of the Microwaves & RF Product Data Directory, at www.mwrf.com/