Tracking Trends in RF Laminate Formulations
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.
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.
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