Electronic components in commercial and military space systems experience radiation stresses that include cosmic rays, protons, electrons and other particles. Ground-based, laboratory tests in a controlled environment at fixed dose rates do not compare to the space environment, for a number of reasons. In part, the actual environment is quite complex. To effectively simulate it, we would have to know each particle type, its energy, direction, and position at any given moment of time. An added complication is that particles in space are influenced by spacecraft and packaging shielding. In addition, solar activity and magnetic storms can greatly affect particle spectra. As a result, the total dose response of devices that exhibit enhanced low-dose-rate sensitivity (ELDRS) when placed in a space environment is uncertain.
Extensive ground-based irradiation tests have demonstrated that many bipolar devices, most notably lateral PNP transistors, exhibit ELDRS. Qualified Manufacturer List V (space level; QML-V) does not specify a radiation requirement, but suppliers like Intersil Corp. (Milpitas, CA) and Texas Instruments Inc. (TI; Dallas, TX) have begun to “up-screen” their product assurance flow to include radiation tests and other added reliability testing. For vendors to supply reliable product, they must convert more than 20 years of key observations and research, technical articles and academic studies into a good product assurance flow that can become a standard part of their test and qualification procedures.
ELDRS testing is a standard part of Intersil’s product assurance flow, say Philip Chesley, product line director for high reliability and Nick Vanvonno, radiation advisor. The company uses a three-phase approach to radiation testing. In 2009, they initiated a low-dose-rate testing program of 13 legacy parts, posting the results on the web. Currently, all newly developed parts are fully tested at high- and low-dose rates as a routine part of product development. Finally, they have introduced wafer-by-wafer low-dose-rate acceptance testing as a complement to current high-dose-rate acceptance testing.
Most older satellites have components that have not been subjected to ELDRS testing, but we do know that HDR tests alone are not sufficient for true reliability of a component in space. It's not that satellites are dropping out of orbit—at least not because of electronics failures—but continued operation in the face of extended low-dose exposure has been a problem. “[Satellites] aren't falling out of the skies, they’ve just stopped working in some cases,” says Jim Salzman, high-reliability and radiation expert at TI. Safely launching a multi-billion-dollar satellite, or more important astronauts, into space, must always be the highest priority.
Many questions remain about the best way to reliably test electronic components so that they function reliability in the harsh environment of space, especially over long deployments (20-plus years is not unusual). Cost is a big factor, as is the amount of time needed to test the devices. “We can’t test a component for 15 years, to determine if it’s going to work in a lengthy space application, therefore accelerated testing is used across the industry. However, accelerated testing has drawbacks, as in the discovery of ELDRS. As a result, the testing for ELDRS was added to the 883 Military Test Standards, method 1019 years ago, and is an integral part of TI’s space qualification flow,” says Salzman. TI has several bipolar processes that are inherently ELDRS free and immune to latch-up, such as TI’s BICOM silicon-on-insulator (SOI) complementary SiGe process.
Experts are investigating many ways to accelerate ELDRS testing, but it will take time as we traverse the learning curve in this area. Let’s face it—we’ve only been doing this space stuff for less than 50 years. We know that ELDRS testing may take months in some cases, but since low-dose radiation is what the semiconductors actually experience in space, we must continue to pursue this solution with vigor, innovation and experimentation.
What do you think about ELDRS testing? Do you think current methods are effective? Can you think of better ways to simulate extended exposure? What strategies would you suggest?
About the author
Stephen Taranovich has 40 years of experience in the electronics industry. He received his MSEE from Polytechnic University (Brooklyn, NY) and his BEEE from New York University (Bronx, NY). Steve is also chairman of the Educational Activities Committee for IEEE Long Island.
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