Private companies are doing a lot more in space today than they were ten years ago, including ferrying supplies to the International Space Station (ISS), landing and reusing rockets and creating constellations of small communications satellites. Murray Slovick published his note on impact to passive electronic components in TTI Market Eye article.
This commercialization of the space industry is taking the form of programs driven by a government agency for private (i.e. non-government) entities; low-cost missions (such as CubeSats); or the activities of private space companies focused primarily on the commercial aspects of space exploration.
However, while passive components represent more than 70 percent of parts used in space applications, according to estimates – for example, capacitors are among the most used devices on a satellite and there can be as many as 10,000 on each spacecraft – these uses are not driving product development.
Why? Simply put, products with relatively low overall volume and high entry cost are difficult to amortize. As such, designers of spacecraft electronics are finding radiation-hardened, space-qualified components to be expensive and difficult to find.
In fact, of all the radiation-hardened electronic components manufactured, only an estimated 10 to 15 percent go into NASA programs. Of the remainder, 60 percent go into commercial initiatives and 25 percent into military projects.
Key Selection Criteria
To give one example, the radiation effects on electronic devices are a primary concern for space-level applications. Outside the protective cover of the Earth’s atmosphere, the solar system is filled with radiation. The natural space radiation environment can damage electronic devices and the effects range from a degradation in parametric performance to a complete functional failure.
Commercial off-the-shelf (COTS) devices generally tend to be more sensitive to radiation effects. Also with small satellite designs, there is less structural mass shielding the electronics. Thus COTS parts need stringent radiation-behavior assessment, which is a long, costly and uncertain process. For unmanned missions the total dose is not of great concern: often the requirements are limited (10,000 to 20,000 rads), shielding is effective and the latest silicon technologies (such as chips fabricated on SOI) are inherently rad-resistant.
Outgassing is another major concern. The vapors coming off of plastics, glues and adhesives can deposit material on optical devices, thereby degrading their performance. Outgassing of volatile silicones in low Earth orbit can cause a cloud of contaminants around the spacecraft. Contamination from outgassing, venting, leaks and thruster firing can also degrade and modify the external surfaces of the spacecraft.
Key criteria when selecting a component for use in space include destructive physical analysis results, failure histories, and qualification and screening test results. Parts used in space systems in general are subjected to 100 percent parts-level inspections and testing to provide high assurance of quality and reliability. There is also a trend to use commercial-grade (often aerospace or automotive) parts that are derated. For example, in tantalum capacitors, 50 percent or higher reduced application voltages versus rated voltage are employed to deal with reliability issues and to reduce the end cost of the electronic device.
The automotive industry can be considered the “cousin” for many aspects of space-rated products and quality/system requirements, but it should be noted that the parts in a car will spend much of their lives in an “off” state, unlike those in space systems where harsh conditions are of much higher concern when assessing the product’s lifetime.
Testing for the Challenges of Spaceflight
Electronic components face a host of challenges while in space, from extreme temperature variation to radiation.
For example, NASA’s Orion spacecraft, being designed to take humans to Mars and into deep space, may have to endure temperatures that can approach over 2,000 degrees C. On the moon, during a lunar day and night, the temperature on the surface can vary from “only” around -200 degrees C to 200 degrees C, and the radiation can be deadly to humans.
As a spacecraft travels on missions beyond the protection of Earth’s magnetic field, it will be exposed to a harsher radiation environment than it would see in low-Earth orbit with greater amounts of radiation from charged particles and solar storms that can cause disruptions to critical electronic equipment.
Temperature is of great concern with any capacitor, which is why capacitors on a circuit board should not be mounted close to heat sources. When operating temperatures of capacitors, resistors and connectors increase by 30 to 50 degrees C, there is a concomitant reduction of reliability. However, passive parts that withstand those temperatures are already used in automotive and drilling applications where high heat is one of the main constraints.
In the future, gallium nitride (GaN)-based applications may become more common. The high power capability of GaN can lead to thermal dissipation issues; an increase of temperature is envisaged for equipment linked to GaN which may withstand temperatures of 150 degrees C.
A temperature cycle test, also called a thermal shock test, exposes the device under test to fluctuating temperatures to detect failure that can be caused by repeated exposure to rapidly varying temperatures.
For use on space missions, passive components must tolerate gravitational stresses of up to 2,000 g, or up to 3,000 g in some use cases. Any test for mechanical shock must simulate this application of sudden force or abrupt change in motion. It must aim to observe the disturbance in operating characteristics or damage caused by the repetitive vibrations/shocks. The components of most concern are typically parts with thin wires (e.g. thermal sensors, relays) or mechanical position-monitoring parts (potentiometers) and the like, which can exhibit potential mechanical issues.
A constant acceleration test is a high-stress simulation test in which semiconductor devices are exposed to constant acceleration to determine its effects on the devices. This centrifuge test exposes mechanical and structural weaknesses not necessarily detected in vibration or mechanical shock testing
A general reduction in size is a trend that space-based applications share with other electronic devices. Passive parts follow this trend with the reduction in size of connectors, chip capacitors and resistors; use of SMD designs; and improvements of key parameters such as ESR for tantalum capacitors (or, a reduction in the number of capacitors used). Reduced-size silicon capacitors are expected to have the same capacitance value and potential use at high temperature.
The main capacitor technologies used in space applications include:
- Ceramic capacitors
- Tantalum (Ta), Solid Electrolyte (MnO2) capacitors
- Metalized film, plastic or foil capacitors. (The knowledge acquired over the years in polyester film technology has permitted the miniaturization of these capacitors and their adaptation to the various environmental constraints found in space applications.)
Aluminum Electrolytic Capacitors, widely used in the industrial market, are generally not used in space applications due to their propensity to throw off their electrolyte through a vent when temperature and pressure increases.
Miniaturization will continue in the years to come. Another means of miniaturization, already used in some industries, is to embed resistors in the substrate. An embedded component is defined as an active or passive device that is placed or formed on an inner layer of an organic circuit board, module or chip package such that it is buried inside the structure when completed, as opposed to being on the top or bottom surface.
The primary passive devices currently being embedded are resistors and capacitors; however there is also some effort focused on inductors.
Ceramic chip capacitors are potentially fragile and are therefore sensitive to mounting conditions, thermal mismatch with the printed circuit board and constraints applied to the boards (i.e. vibrations and shocks). Tantalum capacitors must be tested with regard to temperature stability, capacitance ratio, surge current capabilities, shock, vibration and thermal shock.
Capacitor types with silver cases should be avoided since short circuits can appear due to silver electroplating; the internal heat rises which liberates gases and can lead to catastrophic failure.
For these reasons, tantalum hermetic parts with double-sealing have to be used for this type of capacitor. If the capacitor is no longer hermetic, the predominant failure mode is a gradual loss of capacitance under operation and, ultimately, an open circuit condition resulting from the electrolyte’s vaporization into the atmosphere.
Space Qualification Standards
How are electronic components qualified for use in space? Certain certifications are used to denote the quality of the components. Some of the main certifications are:
- ARC-STD-8070.1 The Space Flight System Design and Environmental Test defines engineering design and environmental test requirements and guidelines for Class C and D space flight systems. Class C: represents an instrument or spacecraft whose loss would result in a loss or delay of some key national science objectives. Class D denotes technical risk is medium by design. Many credible mission failure mechanisms may exist.
- MIL-PRF-38535 is a United States military specification that establishes the general performance and verification requirements of single die integrated circuit electronics. The specification defines some high dose rate Radiation Hardness Assurance levels.
- AS9100 is a widely adopted and standardized quality management system for the aerospace industry. Major aerospace manufacturers and suppliers worldwide require compliance and/or registration to AS9100 as a condition of doing business with them.
- MIL-STD-883 establishes uniform methods, controls, and procedures for testing microelectronic devices suitable for use within military and aerospace electronic systems including basic environmental tests to determine resistance to conditions surrounding military and space operations.
- MIL-PRF-38534 applies to multi-chip modules and systems, such as power modules or DC-DC converters.
- JESD22 is a solid state device packaging standard for evaluating the reliability of packaged solid state devices. JESD-22 establishes the physical, electrical, mechanical, and environmental conditions under which these packaged devices are to be tested.
- Space organizations, such as NASA and ESA, may have their own general requirements as well.
Note: Due to the ongoing COVID-19 situation, the fourth edition of Space Passive Components Days has been postponed to October 2022. It will be held in Noordwijk, The Netherlands.