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

Much of the buzz on the show floor at the 2017 IMS in Honolulu was about millimeter-wave devices and circuits. At one time, frequencies above 30 GHz were considered “exotic” and only for military or scientific applications. But times have changed, and available spectrum is scarce. Millimeter-wave frequencies are now used in commercial vehicular radars, and big plans are being made for these small wavelengths in Fifth Generation (5G) wireless communications networks, in support of moving massive amounts of data quickly. More and more design engineers are faced with developing practical millimeter-wave circuits to 77 GHz and beyond. But first, they must decide upon the best transmission-line technology for those high frequencies as well as the circuit material that can support those circuits with quality, low-loss signal propagation. Drivers and cell-phone users everywhere will be counting on them!

At microwave frequencies, microstrip is by far the most popular transmission-line technology, compared to stripline and coplanar waveguide (CPW). It has a signal plane on the top copper layer and bottom ground plane. It is relatively simple and cost-effective, and allows surface mounting of components for ease of construction.

Unfortunately, as signal frequencies move into the millimeter-wave range, microstrip circuits can behave like antennas, radiating electromagnetic (EM) energy away from a desired signal propagation path and resulting in much higher radiation losses than at lower frequencies. Microstrip radiation losses are also dependent upon the thickness and dielectric constant (Dk) of the circuit substrate material. Thinner substrates suffer less radiation loss than thicker substrates. Also, circuit materials with higher Dk values have less radiation loss than circuit materials with lower Dk values.

In microstrip, the effective Dk is a combination of the Dk of the substrate material and air, since EM waves in a microstrip transmission line propagate in part through the dielectric and in part through the air above it. In contrast to microstrip, stripline is like a flattened coaxial cable. It consists of a conductor surrounded by top and bottom dielectric layers which in turn are covered by ground planes. The Dk of stripline is the same as that of the dielectric material, since air is not involved in the propagation process.

CPW circuits are fabricated with a number of variations, including as standard, grounded coplanar waveguide (GCPW), and conductor-backed coplanar waveguide. Standard CPW metallizes parallel conductors (in the form of a flat waveguide) on the top of a dielectric layer, with ground metal areas just beyond the conductors. GCPW adds a bottom ground-plane layer but requires plated-through-hole (PTH) viaholes through the dielectric substrate material to connect the top and bottom ground planes. The extra ground planes on the top copper layer helps GCPW achieve high isolation between signal lines and can be designed to minimize spurious wave propagating modes. Placement of the PTH viaholes is critical, and can impact transmission-line impedance and loss.

Like microstrip, GCPW has an effective dielectric constant as the result of EM waves propagating through the dielectric material as well as through the air around the conductors. GCPW, like microstrip, also allows surface-mounting of components for ease of fabrication, in contrast to stripline where PTH vias need to connect the components on the outer circuit layers to the inner signal layer. In terms of millimeter-wave frequencies, GCPW has lower dispersion than microstrip, with less radiation loss, and is capable of supporting higher-frequency propagation than microstrip circuits. GCPW also achieves more effective suppression of spurious propagation than microstrip, and is more amenable to practical signal-launch configurations (such as from waveguide, cables, and connectors) at millimeter-wave frequencies than microstrip.

Finding the Right Circuit Material

If GCPW is the optimum transmission line for millimeter-wave circuits, it should then be fabricated on a circuit material with optimum characteristics for millimeter-wave frequencies. Since signal power tends to decrease with increasing frequency, an optimum circuit material for millimeter-wave circuits should have low loss at those high frequencies. The insertion loss of millimeter-wave transmission lines is due mainly to the aforementioned radiation losses, conductor losses, and dielectric losses. Radiation losses tend to be design-specific, whereas conductor and dielectric losses will depend upon the choice of circuit material.

Dielectric losses are a function of the type of dielectric material, and usually well defined by a material’s dissipation factor (Df), with lower values indicating lower dielectric losses. A circuit material capable of consistent performance at millimeter-wave frequencies will also exhibit minimal variations in Dk, so that dielectric losses do not change dramatically with frequency.

In considering circuit materials for millimeter-wave circuit applications, the thermal coefficient of dielectric constant (TCDk) parameter provides reliable insight into the stability of a material’s Dk with temperature. The TCDk parameter provides an understanding of what to expect of a particular circuit material’s performance at millimeter-wave frequencies, with lower TCDk values indicating less change of Dk with temperature and less variations in frequency phase response resulting from variations in Dk with temperature.

Conductor losses can be traced to a number of variables at millimeter-wave frequencies, including the surface roughness of the copper conductors and the choice of plated finish for the conductors. Copper is an excellent conductor, but increasing surface roughness results in increasing conductor loss and greater propagation phase delays. The main area of concern for copper surface roughness is at the copper-substrate interface, with conductor loss due to the copper surface roughness increasing as a function of increasing frequency. The small wavelengths of millimeter-wave signals result in less skin depth in the circuit material as part of EM propagation, and circuit materials with greater copper surface roughness will more severely impact the insertion loss and phase response at millimeter-wave frequencies. The effect of copper surface roughness on insertion loss is also dependent upon the thickness of the circuit material, with thinner circuits affected more by copper surface roughness than thicker circuits.

At millimeter-wave frequencies, circuit materials with excessive copper surface roughness will have more impact on the conductor loss of microstrip circuits than on the conductor loss of GCPW circuits. Switching to a circuit material with smoother copper finish will bring less of an improvement in conductor-loss performance for a GCPW circuit than for a microstrip circuit, especially at millimeter-wave frequencies. In particular, tightly coupled GCPW circuits, which feature closely spaced conductors and ground areas, are less subject to the effects of copper surface roughness than loosely coupled GCPW circuits (with greater spacing between conductors and ground).

An optimum circuit material for millimeter-wave circuits should cause minimal phase angle variations, since such behavior can be critical to many millimeter-wave applications, such as 77-GHz vehicular radar systems. By minimizing variations in certain material-based attributes, such as copper thickness, Dk, conductor width, and substrate thickness, variations in phase angles can be minimized at millimeter-wave frequencies.

Additional details on finding the right combination of circuit material and transmission-line technology are available as part of the Microwave Journal webinar, “Design Considerations and Tradeoffs for Microstrip, Coplanar and Stripline Structures at Millimeter-wave Frequencies,” presented by John Coonrod. 

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

Recipes are often refined with time, in hopes of improving the results. Such is the case with RF/microwave circuit laminates, created from carefully blended mixtures of materials, with the goal of achieving the best possible results in electrical and mechanical performance. Over the years, many different formulations have been applied to create high-frequency circuit materials. The efforts have led to a variety of current circuit laminate choices for a wide range of high-frequency applications and performance requirements.

The high-frequency material perhaps most familiar to users of circuit laminates is polytetrafluoroethylene, more commonly known as PTFE. It is a synthetic thermoplastic fluoropolymer formed of carbon and fluorine. It has a high molecular weight and low coefficient of friction, the main reason it is often used to create “non-stick” surfaces. With a dielectric constant (Dk) of 2.1, PTFE has excellent dielectric properties at microwave frequencies.

PTFE has been a “building-block” material for microwave circuit laminates for some time. It is combined with other materials to modify its electrical and mechanical properties to the requirements of high-frequency circuit designers. For example, PTFE-based circuit materials are typically reinforced with woven glass for improved mechanical stability. The woven-glass reinforcement will raise the material’s Dk value and also decrease material expansion as a function of temperature, better matching the coefficient of thermal expansion (CTE) of the circuit material to that of its copper conductors. PTFE-based laminates also use ceramic fillers to achieve higher Dk values and to fine-tune other material properties, such as CTE.

At one time, the choice of circuit laminates for high-frequency, thin-film circuits came down to almost an “either/or” decision for circuit designers: fabricate it on lower-cost FR-4 circuit material or on higher-performance (and higher-cost) PTFE-based laminates (or alumina ceramic substrates, in the case of high-frequency thick-film circuits). FR-4 really refers to a family of circuit materials based on woven-glass-reinforced flame-retardant epoxy. The material is popular for its low cost and ease of circuit fabrication, but suffers degraded electrical performance at higher frequencies, typically above about 500 MHz, and many circuit designers had learned their own “cutoff frequency point” below which they could use FR-4 and above which required a PTFE-based circuit laminate.

While well-established and accepted for high-frequency circuits, PTFE is just one of a number of “ingredients” in currently available high-frequency circuit laminates, which also include thermoplastic materials such as polyphenyl ether (PPE), polyphenylene oxide (PPO) epoxy resin, and hydrocarbon-based materials with ceramic fillers. Some high-frequency and high-speed applications have encouraged the development of even more exotic circuit laminate formulations, such as liquid-crystalline-polymer (LCP) materials for flexible circuits and polyetheretherketone (PEEK) thermoplastic materials for extremely high operating temperatures (to about +200°C). In fact, for circuits at microwave frequencies, the number of circuit laminate options seems to grow with time, with newer material formulations promising improvements in the key characteristics that define circuit laminate performance for printed-circuit boards (PCBs), including Dk, dissipation factor (Df), coefficient of thermal expansion (CTE), thermal coefficient of dielectric constant (TCDk), thermal conductivity, moisture absorption, and long-term aging.

Comparing Compositions

How do these different high-frequency material compositions compare? First of all, it is important to note that not all PTFE-based circuit laminates are created equal. Early PTFE-based laminates were reinforced with woven glass to reduce the inherently high CTE of PTFE alone. Further improvements in performance were possible for PTFE-based circuit laminates by adding micro-fiber glass to the mixture in RT/duroid® 5880 circuit material from Rogers Corp. PTFE-based laminates were further improved by adding special ceramic materials as fillers, not only to modify the Dk but to alter certain properties of the material to make them easier to process when fabricating PCBs.

In the case of RT/duroid 6002 circuit board material from Rogers Corp., it is based on PTFE but without woven-glass reinforcement. By adding special ceramic filler, the Dk of the base PTFE material is raised to a value of 2.94 that is highly consistent (within ±0.04) through a sheet of RT/duroid 6002 and with low Df (0.0012) and CTE through the z-axis (thickness) closely matched to that of copper for reliable plated through holes. In fact, the process of adding ceramic filler to a base material such as PTFE allows “fine-tuning” of the material’s ultimate Dk value, so that PTFE-based circuit laminates can be formulated with many different Dk values.

Through experimentation, it was also found that ceramic filler could also be used to fine-tune the Dk values of circuit materials other than PTFE, such as the thermoset hydrocarbon resin materials that are the basis for the TMM® laminates from Rogers Corp. For example, through the addition of different amounts and types of ceramic filler, TMM laminates achieve Dk values ranging from 3 to 13. These resin-based materials are somewhat easier to process into PCBs than PTFE-based circuit laminates, although the absence of glass reinforcement does result in some other challenges for circuit fabrication. To overcome those challenges, a circuit laminate formulation based on ceramic-filled hydrocarbon resin, but with woven-glass reinforcement—RO4350B™ circuit material from Rogers Corp.—was created to provide improved CTE and temperature stability while also maintaining the ease of PCB processing associated with hydrocarbon (non-PTFE)-based circuit laminates.

More recent circuit material formulations have included thermoset hydrocarbon-based PPE and PPO circuit laminates, typically reinforced with woven glass for improved mechanical stability. As noted earlier, such materials can offer unique benefits, such as ease of circuit fabrication and improved long-term aging characteristics. However, they are also limited to lower Dk values and tend to exhibit more rapidly increasing dielectric loss (Df) with frequency than PTFE-based materials and ceramic-filled, hydrocarbon-based circuit laminates.

This sampling of different circuit material compositions hints at some of the differences among the material choices. For example, whether they are glass reinforced or not, special ceramic fillers which are used in PTFE-based circuit materials contribute to good CTE and TCDk performance levels; they also make possible a wide range of Dk values for PTFE-based circuit laminates, from about 3 to 10. Without ceramic filler, PTFE-based circuit materials achieve better loss characteristics (low Df), but with degraded CTE and TCDk compared to ceramic-filled PTFE-based materials. As a general trend, PTFE-based circuit laminates with higher Dk values will exhibit higher Df values and are more anisotropic with increased Dk.

Ceramic-filled, hydrocarbon-based circuit laminates fortified with woven glass typically have higher Df (greater loss) than PTFE-based materials, although they also offer typically better CTE, TCDk, and thermal conductivity than PTFE-based circuit laminates. PPE and PPO-based circuit laminates also have higher Df values than PTFE-based circuit materials, or about the same values as hydrocarbon-based circuit laminates when tested at about 10 GHz or less. For the special features of those PPE and PPO-based circuit materials, including excellent long-term aging characteristics, they suffer higher moisture absorption than the other types of high-frequency circuit laminates.

For high-frequency circuit designers, more choices in circuit laminate compositions are available than ever before, each with its own benefits and tradeoffs. The requirements of a particular application can usually help to speed up and simplify the choice.

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

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

Congratulations to our Advanced Connectivity Solutions team on their 100th ROG Blog post. For six years they have been providing technical advice about PCB design for RF/microwave applications. Here’s to 100 more posts!

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Predicting the future is never easy. Similarly, knowing which types of circuit materials will be needed for the next decade’s RF/microwave applications can be difficult to predict, but the past can provide invaluable guidance. With this being the 100th installment in this series, the previous six years of ROG Blogs provide a bit of a map for the high-frequency road ahead and what might be needed in terms of electrical and mechanical characteristics for what are expected to be large-volume applications in this industry, including in radar-based automotive electronics systems, Fifth Generation (5G) wireless communications systems, and Internet of Things (IoT) sensors almost everywhere.

This first six years of the ROG Blog offered guidance on the use of many different types of circuit materials from Rogers Corp., for everything from high-frequency analog circuits to high-speed digital circuits. It explored the effects of circuit material characteristics on the performance of different types of high-frequency transmission lines, including microstrip, stripline, and various types of coplanar waveguide (CPW) transmission lines. And it has examined how the choice of printed-circuit-board (PCB) material impacts the performance of many different types of components, such as low-noise amplifiers, power amplifiers, delay lines, filters, and resonators.

In the next few years, this industry is expected to face new challenges in circuit design, with the high-volume growth of automotive electronics systems, 5G wireless, IoT, even with the steady growth of existing wireless applications such as WiFi and WLAN.

For many high-frequency circuit designers, wireless communications systems such as Third Generation (3G), Fourth Generation (4G), and Long Term Evolution (LTE) have represented rapid growth areas. But even without a standard in place, excitement is growing for the coming of 5G wireless communications systems and the type of services they will provide in the years to come, including fast wireless data with almost zero latency. Transferring high-speed digital signals will be an important part of any future communications network and the ROG Blog from January 22, 2015, “Selecting PCB Materials For High-Speed Digital Circuits,” detailed how RO4003™ circuit materials provided the proper mix of characteristics for speeds to 25 GB/s and beyond.

Higher-frequency (millimeter-wave) signals are expected to play important roles in 5G next-generation communications systems, and the ROG Blog has already provided several installments on choosing materials for millimeter-wave circuits, such as “Making The Most of Millimeter-Wave Circuits” and “Matching Materials To Millimeter-Wave Circuits.” Since millimeter-wave frequency bands are planned for high-data-rate backhaul links throughout 5G networks, the need for circuit materials capable of reliable, low-loss performance at 50 through 70 GHz should continue to grow, prompting more ROG Blogs on this topic.

The expected boost in the number of wireless signals in use during the next decade should also focus circuit designers’ attention on the material characteristics needed for low-PIM performance. An earlier ROG Blog, “Perusing PCBs For Low PIM Levels,” explained the role of circuit materials in the design of PCB antennas and how circuit material characteristics should be chosen to minimize PIM. That blog presented Rogers’ RO4725JXR™ and RO4730JXR™ circuit laminates as non-PTFE, halogen-free circuit materials with the characteristics needed to minimize PIM. With the growing number of wireless signals to be generated during the coming decade, in high-volume applications such as 5G and IoT, the importance of minimizing PIM only increases and certainly should be a recurring topic of this ROG Blog series.

The first six years of the ROG Blog series provided circuit specifiers with key insights on different aspects of circuit materials, such as material parameters important for impedance matching, for dissipating heat, and for minimizing losses. ROG Blogs have explored such things as the importance of circuit laminate finish, in “Finish Makes a Difference in Broadband PCB Loss,” and the reason for using a high-dielectric-constant (Dk) circuit material, in “Harness High-Dk Circuit Materials.” The next 100 ROG Blogs hope to provide guidance on the best use of these high-frequency materials, as starting points for what appears to be many high-frequency, high-speed circuits in the decade to come.

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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.

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We are pleased to announce that Rogers Corporation has signed a definitive agreement to acquire Arlon, LLC, currently owned by Handy & Harman Ltd. (NASDAQ: HNH), for $157 million, subject to closing and post-closing adjustments. The transaction, which is subject to regulatory clearances, is expected to close in the first half of 2015. Rogers intends to finance the transaction through a combination of cash and borrowings under an existing bank credit facility.

RogersCorporation logoBruce Hoechner, President and Chief Executive Officer of Rogers said, “This transaction is truly a unique strategic fit for both Rogers and Arlon. We are energized by the opportunity to serve our customers with our complementary capabilities and technologies in circuit materials and engineered silicones and to enhance value for our shareholders. We look forward to closing this acquisition as another significant milestone in Rogers’ growth as a premier global engineered materials solutions company.”

arlon_logoArlon: A Strong, Strategic Fit

The proposed acquisition of Arlon is consistent with Rogers’ strategy as it adds complementary solutions to its Printed Circuit Materials and High Performance Foams business segments and expands Rogers’ capabilities to serve a broader range of markets and application areas.

Arlon’s circuit materials product family positions Rogers for additional growth in the rapidly expanding telecommunications infrastructure sector, as well as in the automotive, aerospace and defense sectors. Arlon produces its circuit materials in Bear, Delaware; Rancho Cucamonga, California; and Suzhou, China.

The engineered silicones product family of Arlon will further diversify the Company’s solutions and market opportunities in sealing and insulation applications. Arlon will bring new capabilities in precision-calendered silicones, silicone-coated fabrics and specialty extruded silicone tapes. Used primarily for electrical insulation, these materials serve a wide range of high reliability applications across many market segments, including aviation, rail, power generation, semiconductor, foodservice, medical and general industrial. This product family is primarily manufactured in Bear, Delaware.

Revenue and operating income for the Arlon segment of Handy & Harman Ltd. were $100.4 million and $16.7 million, respectively, for the trailing twelve months ended September 30, 2014 (compiled based on amounts reported by Handy & Harman Ltd. in Forms 10-K and 10-Q filed with the Securities and Exchange Commission).

Press Release: Rogers Corporation Signs Definitive Agreement to Acquire Arlon, LLC

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Screen shot 2014-08-08 at 1.33.54 PMNow you can access Rogers’ PCB materials resources with the ROG Mobile App. Quick and easy access to calculators, literature, technical papers. You can even request samples on your smartphone or tablet

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