- Buyers Guide
Military Microwaves Supplement
Satellites in outer space are vital and increasingly important parts of modern communications, as signals are constantly being bounced off orbiting spacecraft to enable everything from telephone calls to video entertainment. Printed-circuit boards (PCBs) in space and satellite-communications (satcom) applications are often taken for granted, but they are special, since they must function reliably in extremely hostile environments. The choice of PCB material for these applications should be carefully considered, since these circuits must literally last a lifetime, or at least through the life of the satellite. Of course, satcom systems also work with Earth-based earth stations, but even the circuitry for these parts of the systems has special requirements, including minimizing distortion from passive intermodulation (PIM).
What makes satellite and space-based RF/microwave circuit applications so challenging? One of the main concerns for any PCB intended for use in space is a phenomenon known as outgassing. This refers to the release of gas trapped within a solid, such as a PCB. Since circuit laminates are formed of composite materials, including dielectric and conductive materials and a variety of fillers, they can be porous in nature. They can contain gas that has been trapped during the PCB manufacturing process or from curing the laminate/prepreg, which can be released under certain conditions, such as the high-vacuum environments of space. In fact, because outgassing is such a concern in space, NASA has developed precise procedures for evaluating PCB materials to determine each material’s outgassing characteristics. For space applications, a number of different circuit materials have been developed by Rogers Corporation with low outgassing characteristics, including some types of RT/duroid® circuit materials, which are based on polytetrafluoroethylene (PTFE) with inorganic filler materials, such as glass and ceramic materials, and temperature-stable TMM® series hydrocarbon composite circuit materials.
Another concern for electronic circuits in space is the impact of the large swings in temperature that can take place within a spacecraft or satellite and how these temperature swings can alter a PCB material’s characteristics, such as its dielectric constant (Dk). Temperature coefficient of dielectric constant (TCDk) is a useful data-sheet parameter for comparing different PCB materials and their suitability for space-based applications. This PCB parameter is essentially a measure of how much a material’s Dk changes with large swings in operating temperature (which can be quite large in space). Ideally, PCB materials targeting space-based applications should have the lowest TCDk values possible. Stable Dk is important for maintaining consistent impedance matches for active circuits, such as amplifiers, and passive circuits, such as antennas, under the changing operating conditions of space.
Since minimizing weight is an important goal for any PCBs installed in satellites and space-based systems, the density of RF/microwave circuitry for space continues to increase, with a growing use of multilayer circuit boards. Interconnections between circuit layers are typically made by means of plated through holes (PTHs) and, operating within the wide temperature extremes and temperature cycling of space, the long-term reliability of these PTHs can become a concern. The PCB parameter known as coefficient of thermal expansion (CTE) is one means of comparing materials for the expected reliability of their PTHs under broad operating-temperature ranges. The CTE of a PCB’s dielectric material is often closely matched to that of the copper conductor to minimize stress between the dielectric and conductor as temperature changes. It is the CTE of a PCB’s z-axis that offers insight into the expected reliability of the PTHs over temperature, ideally with a CTE in the circuit material’s z direction that has been engineered for minimal dimensional changes in the PCBs PTHs across those wide temperature extremes of space.
Various other circuit material characteristics must be considered when choosing a PCB material for space, to determine a material’s electrical and mechanical stability under the hostile operating conditions of space. Circuit materials in space must operate under vacuum conditions, and may also be required to handle high-voltage corona fields in space. Some space applications may need to survive high-voltage conditions, with the capability of handling high dielectric breakdown voltage. Also, the compact, dense circuits typically designed for satellites and space applications can benefit from the capability of dissipating heat within a tight space, such as the confines of a space craft. The measure of a PCB’s thermal conductivity provides a means of comparing the capabilities of different circuit materials to dissipate heat in space. For both active and passive circuits in space, losses can also be a concern, as measured by a PCB’s dissipation factor (Df), which should be as low as possible when selecting a circuit material for minimal loss.
In terms of practical PCB material choices for outer space, Rogers TMM series circuit materials, such as TMM 3 and TMM 10, and its RT/duroid materials, such as RT/duroid 6002, offer many of the qualities favorable for demanding space-based applications. For example, TMM 3 and TMM 10 are thermoset circuit materials engineered for high PTH reliability. TMM 3 has a low Dk of 3.27 in the z-axis at 10 GHz with low Df of 0.0020 in the z-axis at 10 GHz. For those seeking a PCB material with higher Dk, TMM 10 has a Dk of 9.20 in the z-axis at 10 GHz with low Df (0.0022 at 10 GHz). Both of the TMM materials feature low TCDk values and CTE characteristics that help conquer the temperature cycling of space-based applications, maintaining stable Dk values over those wide temperature ranges.
In addition, Rogers RT/duroid 5880LZ laminate is a circuit material with many characteristics well suited for space. It is very light in weight, as measured by a specific gravity of 1.3 to 1.4, with a low z-axis CTE (41.5 ppm/°C) for good PTH reliability over wide temperature ranges. It has a low Dk value (1.96 at 10 GHz in the z-axis) and low loss, as measured by a dissipation factor of 0.0019 at 10 GHz.
Of course, the requirements for PCB materials for space-based applications are among the most demanding of all RF/microwave applications. Materials that can survive in space are candidates for a host of other challenging applications, in commercial and military circuits, including in secure communications, radar, and electronic-warfare (EW) systems, especially when the expected operating conditions might be “less than ideal.” Although the Rogers materials mentioned above have properties that can make them suitable for a variety of demanding applications, it is important to remember that every application is different, therefore the designer must determine the suitability of Rogers circuit materials for each application.
Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.
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