This post authored by John Coonrod, Technical Marketing Manager, and team originally appeared on the ROG Blog hosted by Microwave Journal.

Millimeter-wave frequencies were once few and far-between, in terms of applications and circuits using frequencies above 30 GHz. But that is about to change quickly, with Fifth Generation (5G) wireless networks and automotive radar systems both incorporating millimeter-wave frequency bands. For many circuit designers, these frequencies may represent uncharted territory and may require some thought not only about a suitable printed-circuit-board (PCB) material, but of the optimum transmission-line technology, board layouts, and connector launches. Many circuit designers face new challenges with the inevitable increase of millimeter-wave applications.

Circuit designers familiar with a particular transmission-line technology may ask: Can’t I stick with microstrip at these higher frequencies, if the PCB material delivers the performance I need? Microstrip is widely used in circuits from about 300 MHz to 30 GHz. Above 30 GHz, at millimeter-wave frequencies (30 to 300 GHz), microstrip suffers increased radiation loss and problems with spurious propagation modes. Designers working on circuits with both microwave and millimeter-wave transmission lines will often make a transition from microstrip to grounded coplanar-waveguide (GCPW) transmission lines which, when designed and fabricated properly, have little or no radiation loss and minimal spurious mode propagation.

For circuits with wideband coverage and without transitions between different transmission-line technologies, stripline is often used from lower microwave frequencies to millimeter-wave frequencies. However, forming a signal launch from a coaxial connector to stripline on a PCB has never been easy at microwave frequencies, and can become more challenging at higher, millimeter-wave frequencies with the shrinking dimensions of transmission-line structures. Ideally, the transition from the coaxial domain of a high-frequency connector to the parallel plane of a stripline PCB should be smooth, with little or no signal loss or reflections and no spurious modes. Assuming well-matched signal launches, stripline can be an excellent choice of  transmission line for millimeter-wave PCBs, although circuit fabrication is somewhat more involved than when forming microstrip or GCPW transmission lines.

Easier to Build?

Microstrip and GCPW circuits are attractive for their ease of assembly, each with a single dielectric layer with ground plane on bottom and signal conductors and components on top. Since the circuitry is exposed, components can be attached directly to the transmission lines on the signal plane. Stripline, on the other hand, surrounds its signal conductors with dielectric layers which in turn have ground planes on top and bottom. Because stripline’s signal conductors are buried in a multiple-layer circuit assembly, making connections between components and the signal conductors is never routine. Signal connections in microwave stripline PCBs are typically made by means of conductive viaholes: holes drilled through the dielectric layers and plated with conductive metal. Plated viaholes, or plated through holes (PTHs) as they are known, provide short, electrically conductive signal paths through the dielectric layers but also add their own capacitance and inductance values to the circuitry, impacting performance at higher frequencies. They become part of a circuit diagram (which must be modeled) at millimeter-wave frequencies.

Effective use of stripline transmission-line technology for millimeter-wave PCBs depends on finding the optimum plated viahole structure for low-loss, low-reflection transmission of high-frequency signals to the embedded signal plane. The transition provided by well-formed viaholes through stripline circuitry is essential not only for energy from signal-launch connectors but any electrical connections made to and from external components.

Laser technology can be an effective means of forming the small viaholes, or microvias, needed for stripline PCB interconnections at millimeter-wave frequencies. Precisely controlled laser drilling systems are designed to cut micron-sized microvias by burning through the top copper ground plane of a stripline circuit assembly, through the dielectric material beneath it, and to the signal plane lying between the two dielectric layers. Copper plating is applied and, in this way, a conductive path is formed through the hole from the top copper layer to the signal plane beneath. Such microvias can be formed with extremely small diameters and with the short lengths needed for thin dielectric materials typically used at smaller-wavelength, millimeter-wave frequencies.

By using this commercially available laser-based microvia-forming process, excellent performance can be achieved in stripline interconnections at millimeter-wave frequencies. Larger PTHs formed in stripline circuit assemblies can add unwanted capacitances and inductances at millimeter wavelengths, even in the shortest lengths.

Low-loss, low-reflection signal launches in stripline have been commonly realized in circuits for use to about 40 GHz; it can be difficult to achieve the good match and construction between connector interface and viahole for stripline circuits with coaxial launches at frequencies above 40 GHz. However, the choice of PCB material can play a role in the effectiveness of stripline circuits at millimeter-wave frequencies, based on recent experiments with RO3003™ laminates from Rogers Corp. Using these materials with standard stripline transmission-line structures, low-loss coaxial signal launches were measured to as high as 60 GHz. With several minor modifications, it should be possible to achieve practical coaxial-to-stripline signal launches out to 80 GHz using these same circuit materials.

When considering a PCB material that can support microvias for millimeter-wave circuits, stability at those higher frequencies is a key requirement. RO3003 circuit material has shown excellent mechanical and electrical stability above 30 GHz. It is mechanically stable, with the stability typically realized on other glass-reinforced materials as part of a multilayer construction. However, RO3003 laminates do not use glass reinforcement, so microvias can be laser-formed reliably and consistently without the effects from the lasered glass. RO3003 features coefficient of thermal expansion (CTE) closely matched to that of copper, so that microvias remain structurally and electrically sound even with thermal cycling. Regardless of the choice of transmission line, RO3003 circuit material, with its consistent dielectric constant (Dk) and dissipation factor (Df) over a wide range of frequencies, is a logical starting place for those higher-frequency circuits.

While there may not be one perfect transmission-line technology for millimeter-wave circuits, the choice of a starting point—the PCB material—can make a difference in the final performance possible at those higher frequencies. Microstrip and GCPW technologies support many millimeter-wave circuit applications with ease of fabrication and testing, but it has been shown that stripline is capable of excellent circuit performance at millimeter-wave frequencies when teamed with the right circuit materials. 

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I am ROGERS. Meet Bobo Hu.

On January 24, 2017, in Power Electronics, Rogers Corporation, by sharilee

Meet more of our hard-working employees. Bobo Hu has been part of our Power Electronics Solutions group for 10 years.

“I make compact 3D designed busbars that power DC-DC converters and motor drives so your car can go that extra mile.”

Bobo
Rogers Power Electronics Solutions Division provides advanced materials technologies that significantly increase efficiency, manage heat, and ensure the quality and reliability of power electronics. From insulated bipolar gate transistors (IGBT) and high voltage direct current (HVDC) systems to satellite power management and vehicle propulsion, our power electronics solutions can be found in a variety of high performance packaging, cooling, and power conversion / distribution devices.

About our Employees

Throughout our organization, our cultural behaviors describe how our employees work and are judged by our customers, business partners, investors, and each other.

Live Safely: I actively prevent injuries for everyone, everywhere, every day.

Trust: I respect people and trust them to do the right thing.

Speak Openly: I courageously seek and speak the truth.

Innovate: I create market-driven solutions that lead to customer success.

Just Decide: I make informed decisions rapidly to drive progress.

Simply Improve: I continuously simplify how I do things to achieve excellence.

Deliver Results: I align and achieve my goals to deliver our “Must-Do” results.

Together, we are changing our culture as we help change the world around us. For over 180 years, the employees of Rogers Corporation have focused on our customers, delivering world-class solutions to meet their most demanding materials challenges.

Dr. George J. Kostas believed in the development of human potential and the pursuit of innovation. He was a visionary, a patriot, and benefactor of the George J. Kostas Research Institute for Homeland Security at Northeastern University. In December, Dr. George J. Kostas passed peacefully in his sleep at the age of 97.

George_KostasDr. Kostas is special to Rogers as the Kostas Institute is home to the first Rogers’ Innovation Center, a unique academic-industry partnership focused on building closer linkages between academic research, industry know-how, and commercialization of research. The Center’s goal is to develop breakthrough innovations in advanced materials to address global challenges for clean energy, safety and security, and Internet connectivity. Rogers’ expertise closely aligns with Northeastern’s focus on use-inspired research in health, security, and sustainability.

Dr. Kostas is the son of Greek immigrants. He graduated from Northeastern University with a bachelor’s degree in chemical engineering in 1943. He com­pleted the exec­u­tive MBA pro­gram at Columbia Uni­ver­sity in 1967 and in 2007, he received an hon­orary doc­torate of sci­ence from Northeastern.

Dr. Kostas is a pioneer in synthetic rubber manufacturing. He held positions at the U.S. Synthetic Rubber program during World War II and later served as Director of Research and Development at General Tire and Rubber Corp. He founded Techno-Economic Services Co. in 1972 where his patents lead to the development of a revolutionary process to plate aluminum in an atomic form on metal substrates to render them resistant to motion.

His vision for the Kostas Institute was of fostering collaborative, use-inspired research aimed at expanding the capacity of the nation and its communities, critical systems, and infrastructure to withstand, respond to, and recover from manmade and natural catastrophes.

In speaking for the Rogers Innovation Center team, Shawn Williams, Vice President, R&D, said, “We are honored to carry on Dr. Kostas’ vision of working in a high-trust environment that brings together academia, industry, and government researchers and practitioners.”

To learn more about Dr. Kostas’ rich and interesting life, visit his obituary.

This post authored by John Coonrod, Technical Marketing Manager, and team originally appeared on the ROG Blog hosted by Microwave Journal.

Space may be the final frontier, but the orbiting satellites that provide this planet’s satellite communications (satcom) outer infrastructure may seem even more inaccessible. In what may be one of the most hostile operating environments for electronic equipment, these satellites contain circuits that absolutely must not fail. The satcom systems in those satellites demand circuit materials capable of maintaining excellent performance and high reliability even under those stressful, in-orbit conditions. Few circuit materials can handle the challenging requirements of satellite systems; the ones that do have the special characteristics that make it possible.

The TMM® family of thermoset circuit materials from Rogers Corp. is a lineup of ceramic, hydrocarbon, thermoset polymer composites with a proven track record of high reliability in satellite circuit applications. Available with dielectric constants (Dk values) from 3.27 to 12.85 in the z-axis (thickness), they exhibit a particular set of characteristics that are well suited to the challenging operating environments of orbiting satellites.

What type of material characteristics are needed for space? Low outgassing is one critical requirement for any circuit material that must survive the vacuum environment of a satellite. Outgassing is the release of gas trapped within a solid, such as a PCB material. Once released, the gas can condense on different surfaces within a satellite, potentially causing problems with some circuits and subsystems. (For more on outgassing, check out the ROG Blog from November 19, 2010: What Is Outgassing And When Does It Matter?”)

The outgassing process typically occurs very slowly, over a long period of time, and requires meticulous testing to determine a circuit material’s amount of outgassing. A testing procedure has been developed by the American National Standards Institute (ANSI) and is defined in the ANSI/ASTM E595-84 standard. NASA, which uses that test method along with its own SP-R-022A test procedure to evaluate materials for outgassing based on changes in mass under vacuum conditions, has found materials based on polytetrafluoroethylene (PTFE), such as RT/duroid® from Rogers Corp. as well as the TMM hydrocarbon composite circuit materials, to be highly resistant to outgassing.

In addition to a vacuum, circuit materials in space must deal with temperature extremes that exceed most applications. Space is often thought of as cold and dark, and satellites in the shadow of Earth and without the moderation from the atmosphere can reach quite cold temperatures. Conversely, when exposed to sunlight without that atmosphere, a satellite’s operating environment can achieve furnace-like temperatures. It is this cycling between temperature extremes during a satellite’s normal orbiting paths, whether geosynchronous or geostationary that can place tremendous temperature-based stress on a circuit material, requiring a PCB material with outstanding thermal properties for satellite applications.

One of these key thermal characteristics that can be used to gauge a circuit material’s suitability for satellite applications is how much the material’s Dk changes with temperature over the operating temperature range. Ideally, a circuit material for space will not only handle a wide temperature range, but exhibit very little change in Dk over that temperature range. This material parameter, the thermal coefficient of dielectric constant (TCDk), serves as a barometer for material stability in applications that must endure wide temperature swings, in commercial, industrial, and military systems as well as in space. For the 50-Ω impedance that is characteristic to most high-frequency circuits designed for satcom use, a change in circuit material Dk will cause a change in impedance, resulting in variations in circuit performance, such as shifts in the amplitude and phase characteristics of high-frequency transmission lines.

For in-space circuit applications, it is important to use a circuit material with the lowest possible TCDk value, to minimize performance variations due to changes in Dk with temperature. The TMM materials are formulated for an operating temperature range of -55 to +125°C to handle the temperature extremes of space and satellite environments. These materials also change very little in Dk value over that wide temperature, with Dk increasing slightly for the TMM materials with the lowest Dk values and Dk decreasing in very small amounts for TMM materials with Dk values at 6 and higher.

For example, for TMM 3 laminate with a Dk of 3.27 in the z-axis (thickness) at 10 GHz, the TCDk is a very low +37 ppm/°K. The other TMM material with a slight positive shift in Dk with temperature is TMM 4 laminate, with a Dk of 4.50 in the z-axis at 10 GHz. The Dk decreases almost insignificantly with temperature with the TMM 6 material, which has a Dk of 6.00 in the z-axis and an extremely low TCDk of -11ppm/°K. Typically, a TCDk with an absolute value of 50 ppm/°K or less is considered quite good.

The TMM family of circuit materials offers circuit designers the options of designing with a wide range of Dk values, making it possible to save space in a satellite through the circuit miniaturization depending upon the choice of material Dk. This can be accomplished by using PCB materials with higher Dk values (achieving transmission lines with the same characteristic impedance as circuits with larger dimensions on PCB materials with lower Dk values). The tradeoff for such circuit miniaturization is usually poor TCDk, although this is not the case with the higher-Dk-value TMM materials. For example, TMM 10 material, with a Dk of 9.20 in the z-axis at 10 GHz, has a low TCDk of -38 ppm/°K. For extreme miniaturization, the TMM 13i circuit material has a Dk of 12.85 in the z-axis with a still reasonable TCDk value of -70 ppm/°K.

The TMM 13i material is formulated to be highly isotropic, with a Dk value close to12.85 in all three axes. Most materials are anisotropic, with a z-axis Dk value that differs from the Dk values of the x and y axes. For most circuits, such as microstrip and stripline circuits, the z axis is the direction of interest, since the electromagnetic fields (EM) of those transmission lines are mainly through the thickness of the material. But for circuits with EM fields in the x-y plane, an isotropic material will provide more predictable performance. For designs requiring isotropic circuit materials, the TMM 10i material is an isotropic version of the standard anisotropic TMM 10 material. The price for the highly isotropic behavior in TMM 10i material is a slightly higher Dk than TMM 10 material, at 9.80 in the z-axis at 10 GHz, compared to 9.20 for TMM 10 material.

Because changing temperatures play such a strong role in the choice of circuit material for space, another key material parameter for satellite circuit designers is coefficient of thermal expansion (CTE), which gauges how a circuit material changes dimensionally with heating and cooling. Since most materials will contract with extreme cold and expand with heat to some degree, it is rare to have a material with a CTE value of 0 ppm/°K. Ideally, the value should be as low as possible or as close to the value of conductive materials, such as copper (at about 17 ppm/°C), used on the PCB so that dielectric and metal will contract and expand together for minimal stress with temperature. In all three axes, the TMM materials exhibit CTE values ranging from 15 to 26 ppm/°K–quite close to that of copper for high circuit reliability even in the wide range of temperatures in satellite environments.

Screen shot 2014-08-08 at 1.33.54 PMROG Mobile App

Download the ROG Mobile app to access Rogers’ calculators, including the popular Microwave Impedance simulation tool, literature, technical papers, and the ability to order samples of the company’s high performance printed circuit board materials.

Ask an Engineer

Do you have a design or fabrication question? Rogers Corporation’s experts are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.

 

Marc Cotnoir has announced his retirement from Rogers to occur as of the end of 2016.

Marc began his career with us 37 years ago. He has held several roles over the years including marketing management, sales management, and most recently EMS Director of Sales, North America.

Marc’s contributions to Rogers are unparalleled. He is dedicated, talented, and brings out the best in his team and associates.

We wish him the best as he embarks on the next steps of his journey. Enjoy your retirement, Marc!

cotnoir_retirement2

 
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