Microwave Tech - How to choose PCB material suitable for satellite communication systems?

Space may be the last exploration territory of mankind, but the orbiting satellites that provide satellite communications (satcom) to the earth and its auxiliary infrastructure still seem so far away. For electronic equipment, space may be one of its worst working environments, and the various components of the satellite must not fail. Satellite communication systems require PCB material to maintain excellent performance and high reliability in harsh environments and in orbit operation. Few PCB material can meet the demanding and challenging requirements of satellite systems, and only those PCB material with special characteristics can be competent.

What type of PCB material can meet the working environment in space? For satellites working in a vacuum environment, the low outgassing rate of PCB material is a crucial condition. Outgassing rate is the release of gases trapped in solids, such as those in PCB material. Once the gas is released, it can condense on the surfaces of different devices in the satellite, which may cause malfunctions in circuits and systems.

Usually the deflation process is very slow, takes a long time, and requires precise detection to determine the amount of PCB material deflation. The American National Standards Institute (ANSI) developed a test method for outgassing rate and defined it in the ANSI/ASTM E595-84 standard. The National Aeronautics and Space Administration (NASA) uses this standard in conjunction with its internal SP-R-022A test method to test the quality change of material after gassing under vacuum conditions to evaluate the gas release rate. Tests found that material based on polytetrafluoroethylene (PTFE), such as Rogers’ RT/duroid and TMM hydrocarbon composite PCB material, have a high degree of resistance to outgassing.

The TMM series of thermosetting PCB material have been proven to be applicable to satellite communication systems that require high reliability. It is composed of a series of ceramics, hydrocarbons and thermosetting polymers. Its dielectric constant (Dk value) in the z-axis direction (thickness direction) ranges from 3.27 to 12.85, and its excellent characteristics are very suitable for orbiting satellites and similarly challenging working environments.

In addition to vacuum conditions, PCB material in space must be able to be applied to various extreme temperatures beyond conventional applications. The space environment is usually cold and dark. When the satellite is in the shadow of the earth, the ambient temperature will be quite low because there is no atmospheric regulation. On the contrary, when the satellite is exposed to sunlight, the operating environment of the satellite can reach the temperature of a stove. Satellites in orbit continue to cycle under such extreme temperatures. Whether in the application of geostationary satellites or geostationary satellites, it will bring great temperature shock to the circuit board material, so PCB material are required to have particularly good thermal properties.

How to measure whether PCBmaterial are suitable for satellites? One of the key characteristic indicators is: the rate of change of the dielectric constant of the PCB material with the operating temperature. Ideally, PCB material used in space can not only be suitable for a wide temperature range, but also have very small changes in dielectric constant within this temperature range. The temperature coefficient of dielectric constant (TCDk) of the PCB material can clearly reflect the stability of the material. In commercial, industrial, military systems, and space environments, PCB material must withstand large temperature fluctuations. The characteristic impedance of most high-frequency transmission lines used in satellite communications is 50Ω. Changes in the dielectric constant of PCB material will cause changes in characteristic impedance, leading to differences in circuit performance, such as changes in amplitude and phase characteristics.

In space circuit applications, it is very necessary to use PCB material with a low temperature coefficient of dielectric constant (TCDk), which can reduce the performance changes caused by the temperature change of the dielectric constant. The working temperature range of TMM material design can be from -55°C to +125°C, which can cope with the extreme temperature of satellites in the space environment. Under extreme temperature, the dielectric constant of these PCB material changes very little. For TMM material with the lowest dielectric constant value, the dielectric constant will increase slightly; for TMM material with a dielectric constant value of 6 and higher, the dielectric constant The constant will decrease slightly.

For example, for a TMM3 laminate with a dielectric constant of 3.27 in the z-axis (thickness) direction at a frequency of 10 GHz, the TCDk is very low, only +37 ppm/°K. Another TMM PCB material whose dielectric constant changes in the positive direction is TMM4 laminate, which has a dielectric constant of 4.50 on the z-axis at a frequency of 10 GHz. The decrease in the dielectric constant of TMM6 PCB material with temperature is almost negligible. Its dielectric constant in the z-axis direction is 6.00 and has an extremely low TCDk of -11 ppm/°K. Generally, PCB material with an absolute value of TCDk less than or equal to 50 ppm/°K are considered to have fairly good temperature characteristics.

The TMM series of PCB material provide circuit designers with a wide range of selectable permittivity values. Designers can realize circuit miniaturization and save space by choosing the dielectric constant value of the PCB material. This can be achieved by using a PCB material with a higher dielectric constant value (the circuit size of a circuit with a low dielectric constant value PCB material is relatively large when the transmission line has the same characteristic impedance circuit). Usually the price of such circuit miniaturization is the slightly poorer material TCDk, although this is not the case with TMM material with higher dielectric constant values. For example, TMM10 material has a z-axis dielectric constant value of 9.20 at 10 GHz, which has a TCDk value as low as -38 ppm/°K. In order to achieve extreme miniaturization, the dielectric constant of TMM13i PCB material in the z-axis is 12.85, and its TCDk value is -70 ppm/°K, which is still acceptable.

The TMM13i PCB material is highly isotropic, and its dielectric constant values in the three directional axes (X, Y, Z) are all close to 12.85. Most material are anisotropic, and the z-axis dielectric constant is different from the x- and y-axis dielectric constant values. For most circuits, such as microstrip and stripline circuits, the main concern is the dielectric constant in the z-axis direction, because most of the electromagnetic field (EM) of these transmission lines passes through this direction of the material. But for circuits with EM fields in the x-y plane, isotropic material can provide predictable performance. For circuits that need to use isotropic material, TMM10i material has better isotropic properties, and it is an upgraded version of the standard TMM10 material. The z-axis dielectric constant value of TMM10i material is slightly higher than that of TMM10 material. TMM10i has a z-axis dielectric constant of 9.80 at a frequency of 10GHz, and TMM10 material is 9.20.

Temperature changes play a decisive role in the choice of PCB material used in space, and another key parameter that circuit designers care about is the coefficient of thermal expansion (CTE) of PCB material. CTE can be used to measure the dimensional changes of PCB material when heating and cooling. Since most PCB material will expand and contract to a certain extent, material with a CTE of 0 ppm/°K are very rare. Ideally, the CTE value should be as low as possible or close to the value of conductive material, such as copper foil covering the PCB material (CTE is about 17 ppm/°C), so that the medium and the copper foil in contact with the copper foil can produce the smallest changes with temperature. Stress. The CTE value of TMM material on the three axes (X, Y, Z) ranges from 15 to 26 ppm/°K, which is quite close to copper. Therefore, even in a satellite environment with a large temperature range, its circuit still has a high Reliability.