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 10°C 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.