Capacitors for Spacecraft: Withstanding a Harsh Radiation Environment

Dennis Zogbi’s terrific MarketEye column “Space-Based Passive Components: Global Market Update: 2016” prompted me to look into the effects of radiation on passives, and capacitors in particular, intended for use on spacecraft. Passives represent more than 80% of the electronic parts used on spacecraft, so radiation can be a major concern since space applications cannot afford to suffer a performance outage or loss of any function.

There are four primary causes of radiation in space: trapped electrons, trapped protons, solar protons and cosmic rays.

The Van Allen radiation belts contain charged particles trapped in the magnetic field surrounding the Earth. Trapped electrons are negatively charged particles relatively low in mass, but extremely energetic. They are typically found is very high orbits such as the geosynchronous orbits that are approximately 36,000 km above the planet. The inner Van Allen belt consists largely of highly energetic protons, with energy exceeding 30,000,000 electron volts. The peak intensity of these protons is approximately 20,000 particles per second crossing a spherical area of one square cm in all directions. Protons trapped in the magnetic field exist in high concentrations in Low Earth Orbit (LEO), which is defined as 1400km –2000km from the Earth’s surface. Solar protons are similar to trapped protons except they are ejected from the sun during a solar flare event.

Cosmic rays come from all directions and consist of approximately 85% protons, 14% alpha particles, and 1% heavy ions, together with x-ray and gamma-ray radiation. Heavy ions are massive, highly charged particles that can cause severe damage to electronic devices. In space, these particles generally have so much energy that they easily ionize atoms, freeing negatively-charged electrons. In electronic devices, this ionization process creates excess charge, which can produce both transient and lasting effects. Most effects are caused by particles with energies between 0.1 and 20 GeV.

Dielectrics used as capacitor insulators have a wide range of physical and structural characteristics which give rise to differences in their electrical behavior. Exposure to radiation is one of the factors which may influence these characteristics and could result in significant changes of capacitor performance within a circuit.

All dielectric materials become electrically conductive when exposed to ionizing radiation. From a circuit applications standpoint, the most important effect of radiation on a capacitor is the induced conductivity in the dielectric material. The amount of radiation-induced conductivity (RIC) can vary widely with dielectric material type. Depending on the nature of the material, conduction may cease very quickly (nanoseconds) after radiation or persist for a longer period.

Dimensional change of the capacitor plate spacing is the principal cause of capacitance changes during irradiation. This dimensional change is most pronounced when radiation-sensitive materials, generally organics, are used in one or more parts of the capacitor's construction. Pressure buildup from gas evolution and swelling causes physical distortion of capacitor elements and thus changes the spacing.

During radiation capacitor leakage resistance decreases and, as a result, the time constant of the circuit will also decrease. If the capacitor is in a critical timing circuit, the timing circuit may produce errors that affect system performance.

Research has shown that:

• In general both MIL and space application specifications classify conductive polymer capacitors as not radiation sensitive. Conductive polymer capacitors have been evaluated at total ionization dose (TID) test using a Cobalt 60 source up to 200k rad irradiation at dose rates of 500 rad/hr without impact to the part performance.
• Teflon (PTFE) has demonstrated a rather high susceptibility to radiation damage. Degradation of physical properties occurs when irradiated due to liberated fluorine atoms and the production of entrapped fluorocarbon gases. Tensile strength decreases and the material becomes brittle. Embrittlement becomes severe with extended irradiation [107 rads] and the polytetrafluoroethylene can crumble.
• Ceramic insulating materials show virtually no change in properties (including dielectric constant) with X-ray irradiation to doses of 107 rads and higher.
• When tantalum capacitors are irradiated with ionizing radiation, electrons and holes (positively-charged atoms) are excited into mobile states. The result can be radiation induced discharge in caps with several volts initial bias and radiation-induced voltage buildup across initially unbiased capacitors. Wet tantalum capacitors have been used in space applications for over 40 years, from the early days of the manned space flight programs.
• Electrolytics are general avoided for space applications (although there are mil spec'ed electrolytics available). Electrolytics may undergo parameter changes from either ionizing or burst radiation, depending on dosage. Aluminum electrolytic capacitors can outgas water and organic vapors..The dielectric also can be weakened, and DC leakage may rise as a result of bombardment. NASA found that aluminum capacitors with solid conductive polymer exhibits significant advantages over aluminum capacitors that use a liquid electrolyte.

Recently, AVX’s full range of space-level BME X7R dielectric MLCCs – spanning 0603 to 1812 case sizes, 2.2nF to 8.2μF capacitance values, and 16V to 100V ratings – qualified for use in aerospace designs and US military applications under S-311-P-838 for applications including I/O filtering and bulk storage in switch mode power supplies (SMPS) in space and satellite communications and launchers.

The National Aeronautics and Space Administration’s (NASA) S-311-P-838 specification defines the requirements for high reliability, ceramic dielectric, multilayer, base metal electrode, and fixed-value chip capacitors for high-reliability space applications. It also specifies the processing verification and inspections required by assemblies used in flight.

Devices supplied to this specification are subjected to ultrasonic examination, which may be performed prior to capacitor termination (in accordance with MIL-PRF-123 requirements) or after capacitor termination, at the manufacturer’s option. The voltage temperature characteristic of the dielectric is referenced to the +25 °C value, applicable over the entire temperature range of -55 °C to +125 °C, and ±15% of capacitance.

AVX’s space-level BME X7R MLCCs are said to provide enhanced resistance to mechanical stress by allowing for more board flexure than standard terminations, especially in large case sizes. Tested using MIL-SPEC standards and methods, including 100% ultrasonic examination in compliance with the S-311-P-838 specification, the NASA-approved series is rated for 16 – 100V and 2.2nF to 8.2μF with three capacitance tolerances (±5%, ±10%, and ±20%).

AVX says it is the first and only manufacturer authorized to supply BME X7R MLCCs for use in US military and aerospace applications, AVX’s space-level BME X7R MLCCs in case sizes 0603 – 1812 are also the first such capacitors approved for use in European Space Agency applications.

An upcoming event focusing on commercial passive components in space environments is the 2nd ESA Space Passive Component Days, to be held at ESA ESTEC, The Netherlands, 12 – 14 October, 2016.