Microwave Materials Move Beyond Thermal Management

March 19, 2009
To serve higher-frequency designs, emerging materials are maintaining low loss and stability while enabling engineers to coax more performance out of smaller products.

Materials are not just tasked with bonding and separating circuit traces and ground lines on circuit boards. They must also channel life-shortening heat away from components. In addition, materials must provide stable foundations for circuits over both time and temperature even as new designs call for higher power and increased thermal stability. To enable future designs, materials are growing increasingly sophisticated and promising increased thermal conductivity, lower loss, and more. Current material processing approaches are derived from solutions as diverse as nanotechnology and tunable technologies. No matter what tack they take, however, material advances are enabling new generations of components, semiconductors, and systems.

For an example of the impact that materials can have on a system, a paper from Arlon, Inc. asserts that thermally conductive substrates offer a variety of advantages over the more traditional materials used for printed-circuit-board (PCB) substrates. The paper, titled "Improvements in Microwave Laminates for Amplifier Reliability and Efficiency," is authored by Russell R. Hornung, Michael T. Smith, and John C. Frankosky. They note that engineers can ensure that heat flows away from temperaturesensitive devices and solder joints by increasing a laminate's thermal conductivity. The maximum temperatures seen at device junctions are then minimized, extending device operating lifetimes and improving design reliability. In addition, work hardening is reduced while the thermal excursion caused by the thermal coefficient of expansion is minimized.

If the right engineered ceramics are chosen, the microwave laminate will exhibit greater phase stability across temperature excursions. For example, phase-stable ceramics can dampen the temperature sensitivity of the low-loss polytetrafluoroethylene (PTFE) resin used in microwave laminates. As a result, designers can minimize uneven frequency response due to dielectric-constant drift as the operating temperature changes. In contrast, antenna designs can achieve lower-gain performance via a significant change in resonance frequency and bandwidth rolloff at specific frequencies. In higher-frequency applications in particular, the paper notes that materials that offer lower electrical loss reduce heat generation in passive components like couplers, filters, and feed networks. Material s that pull heat away from active components through either the x/y- or z-direction help improve the reliability of those components.

RF and microwave applications call for low loss and low variance in dielectric constant. Traditionally, the materials that offered these properties included PTFE and other low-loss thermoset polymers. According to Arlon's paper, non-woven fiberglass-reinforced PTFE is available with dielectric constants from roughly 2.2 to 2.33. In contrast, woven fiberglass- reinforced PTFE is available with dielectric-constant values of about 2.2 to above 3.0. By adding ceramic fillers, the dielectric constant of PTFE-based materials can reach 10.2 and higher.

PTFE was originally created at W.L. Gore & Associates. Currently, Gore provides electromagnetic- interference (EMI) shielding solutions like the GORE snapSHOT, which is a multicavity, board-level EMI shielding product for wireless communication devices. Essentially, it is a lightweight, metallized plastic shield that can be thermoformed to virtually any design. The attachment mechanism uses solder spheres as individual mechanical snap features. The shield is metallized with tin on the outside only, which leaves the inside surface insulative. The shield supports narrower ground traces, less space between components, and reduced overall thickness when compared to circuit designs employing existing shielding solutions.

The firm also offers a family of composite organic dielectric materials for chip-package substrates and PCBs. For instance, the GORE G410 Prepreg organic substrate promises to allow the production of reliable, high-performance, single-chip substrate packages using modified PCB construction techniques. At 500 MHz, the dielectric constant for a split post resonant cavity is 3.4. The loss tangent is 0.008.

Another company with a long heritage in the microwave materials space is Rogers Corp. At this month's IMAPS Device Packaging show in Scottsdale/Fountain Hills, AZ, the company spotlighted the RO2808 and ULTRALAM 3000 laminates. The RO2808 ceramic-filled PTFE material boasts a dielectric constant of 7.6. According to the company, this material offers the stability and high-frequency performance of low-temperature co-fired ceramic (LTCC) substrates. The RO2808 material flaunts a low-loss tangent of 0.002 or less and can be supplied as 1-mil-thick boards to support low-profile designs.

The firm's ULTRALAM 3000 series Liquid Crystalline Polymer (LCP) circuit material is characterized by a low and stable dielectric constant of 2.9. Its moisture absorption is less than 0.04 percent by weight. These double-clad copper laminates are available in very thin constructions of 0.001, 0.002, and 0.004 inches. Previously available in a panel form, Ultralam 3000 is now offered in a roll.

According to Rogers, it was the first company to offer resistive copper foils on high-frequency materials. Recently, the company partnered with Ohmega Technologies to develop the RT/duroid 6002PR and RT/duroid 6202PR. Although the RT/duroid 6202PR is similar to the RT/duroid 6002PR, it uses a limited amount of woven glass for increased mechanical support of thin dielectrics. The objective of this development was to significantly decrease the resistor variation. Industry-wide variation on high-frequency materials was often in excess of 20 percent. With RT/duroid 6002PR and RT/duroid 6202PR, less than 10 percent variation was achieved.

To provide a high degree of dimensional stability and registration consistency for multilayer designs, Arlon's CLTE-AT micro-dispersed ceramic PTFE composite utilizes a woven fiberglass reinforcement. The composite stands out for its loss tangent of 0.0013. CLTE-AT also uses smoother copper styles with average surface roughness (Rz) of less than 4.0 m while many competitive offerings use rougher 9-to-10-m copper. As a result, it vows to reduce insertion loss and transmission-line resistance while maintaining the peel strength required for thin traces on thin laminates. At around 200 MHz, the CLTE-AT exhibits a dielectric constant of 3.0. It offers a coefficient of thermal expansion (CTE) of just 8 ppm/C for the x- and y-axis and 20 ppm/C for the z-axis.

The conductive foam dubbed WE-LS shields RF modules. Hailing from Wurth Electronics Midcom, Inc., it is a polyolefin foam with nickel-copper (NiCu) metallization. To suppress RF frequencies, it offers resistance to 0.02 /mm2. Weighing 98 to 150 g/m2, this material promises high flexibility and durability. The 390105, for example, exhibits 87 dB insertion loss with 24.5 tensile strength.

In the thermal-conductivity arena, Master Bond, Inc. has developed a two-part adhesive system with a thermal conductivity in excess of 22 (BTUin/ft2hrF). Called EP21AN, this electrical insulator boasts a dielectric strength of more than 400 V/ mil and a volume resistivity above 103 -cm. The EP21AN adheres to a range of substrates including metals, ceramics, glass, and many plastics.

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Bonding also is the goal of NanoFoil from Reactive NanoTechnologies, Inc. or RNT. This patented product is fabricated by vapor-depositing thousands of alternating nanoscale layers of aluminum (Al) and nickel (Ni). When activated by a small pulse of local energy from electrical, optical, or thermal sources, the foil reacts to precisely deliver localized heat to temperatures of +1500C in thousandths of a second.

To improve power-amplifier efficiency in the handset, STMicroelectronics and Paratek have been cooperating to advance the next generation of Paratek's ParaScan materials technology for high-volume manufacturing. They plan to jointly develop tunable products that improve total radiated power (TRP) for mobile phones. By relying on STMicroelectronics' Integrated Passive and Active Devices (IPAD) technology, the companies plan to leverage the ParaScan materials to create high-quality, high-reliability tunable capacitors.

These tunable capacitors, in turn, will allow the implementation of dynamic impedance matching for wireless handsets. In doing so, they will improve PA efficiency and battery life while reducing the likelihood of dropped or missed calls. With RF tuning, mobile-handset antennas can operate more effectively through adaptive impedance-matching circuits. Because the antennas can be much smaller, the adoption of such technology also enables manufacturers to make mobile handsets smaller, thinner, and more iconic.

Another materials-based partnership seeks to help handheld-device manufacturers miniaturizethis time through integration. Specifically, SABIC Innovative Plastics and LPKF Laser & Electronics AG have teamed to combine LPKF's Laser Direct Structuring (LDS) technology with SABIC Innovative Plastics materials. Today's original equipment manufacturers (OEMS) use molded interconnect devices that integrate circuit tracks into the plastic parts, which are created using LDS technology. The laser sculpts an intricate, three-dimensional structure on the molded plastic housing in preparation for metallization. SABIC Innovative Plastics offers plastics with LDS-enabled LNP compounds based on polyphthalamide (PPA), polyphenylene oxide (PPO), nylon, and polycarbonate/acrylonitrile butadiene styrene (PC/ABS). Thanks to this partnership, it is now possible to integrate electronic and mechanical functionality, such as a mobile-phone antenna, in a single module (Fig. 1).

Because material advances directly translate into technology firsts, military research into materials is well supported. For example, scientists at the Naval Research Laboratory's Materials Science and Technology Division recently demonstrated the ability to control the spin population of the individual quantum shell states of self-assembled indium-arsenide (InAs) quantum dots (QDs). Using a spin-polarized bias current from an iron (Fe) thin-film contact, the scientists determined the strength of the interaction between spin-polarized electrons in the s, p, and d shells.

Semiconductor QDs are nanoscale circular disks of one semiconducting materialtypically 3 nm high by 30 nm in diameterembedded within layers of a second material. They are well suited for a variety of quantum information processing, electronic, and spintronic applications. The NRL researchers monitor the shell population and spin polarization by measuring the polarized light emitted as a function of the bias current from the Fe contact. From a detailed analysis of the electroluminescence (EL) spectra, the researchers were able to obtain the first experimental measure of the exchange energies between electrons in the s- and p-shells and between electrons in the p- and d-shells. These energies describe the degree of interaction between these quantum levels.

In the future, some are predicting that zinc oxide (ZnO) will cover many applications ranging from thin-film transistors to sensors. According to NanoMarkets, ZnO is often combined with other materials. Alternatively, it can be fabricated into new types of nanostructures that each have their own unique properties. In addition, Rogers is finishing development of a new type of high-frequency material aimed mainly at airborne antenna programs. The RT/ duroid 5880LZ material offers a dielectric constant of 1.96 and temperature coefficient of dielectric constant of +22 ppm/C with a density of 1.37 gm/cm330 percent lighter than PTFE/glass.

Going forward, material advances also will be increasingly software centric, as engineers are increasingly turning to software to simulate issues with their circuit-board materials or real-world designs. For instance, DuPont Microcircuit Materials and CAD Design Software have announced the integration of the DuPont GreenTape LTCC materials and manufacturing processes into CAD Design Software's electronicdesign- automation (EDA) design tools for Ceramic (hybrid/MCM-LTCC) circuit design (Fig. 2).

By incorporating the DuPont Green Tape 951 and 943 systems, CAD Design Software's Ceramic Design tool automates DuPont's recommended LTCC processes. The ability to select materials from CAD Design Software's Materials Library allows the user to set up custom technology parameters to begin a ceramic design or choose one from the list of preset technology files. Those files have all of the DuPont recommended minimum entity widths and spacings incorporated for efficient design and manufacturing flow. In addition, CAD Design Software has incorporated many CAM tools for the ease of processing a ceramic design.

About the Author

Nancy Friedrich | Editor-in-Chief

Nancy Friedrich began her career in technical publishing in 1998. After a stint with sister publication Electronic Design as Chief Copy Editor, Nancy worked as Managing Editor of Embedded Systems Development. She then became a Technology Editor at Wireless Systems Design, an offshoot of Microwaves & RF. Nancy has called the microwave space “home” since 2005.

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