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. (www.rogerscorp.com) 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.
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