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Microelectromechanical Systems and Microstructures in Aerospace 351

(expressed as standard deviation). Given the dependence of MEMS reliability on
the operating conditions encountered during the life cycle, it is important that such
conditions be identified accurately at the beginning of the design process.

16.2.2 DE-RATING AND REDUNDANCY
One method to develop reliable systems is the use of redundancy. Civilian and
military project engineers design systems and electronic circuits with redundancy so
that if one system fails, the second or even third system will operate in its place. Use
of redundancy in critical electronic systems can cover for unexpected or unpredict-
able failure mechanisms during the required mission lifetime. There are different
levels of redundancy that are used on spacecraft. The geostationary operational
environmental satellites (GOES) each have two parallel systems to operate their
instruments. The Earth Observing System (EOS) can require redundancy down to
individual electronic parts.
In determining redundancy requirements, a design engineer considers past
experience, the additional costs, the additional weight, the additional space re-
quired, the particular project’s requirements, and especially the criticality of each
function. Failure modes and effects analysis (FMEA) are performed in the design
phase of a spacecraft to determine the criticality of a function. Other analyses such
as stress analyses, worst-case analyses, and trend analyses assess the reliability and
criticality of a system. Statistical analyses determine how many redundant systems
will meet the reliability requirements of the project. The space station program
specifies requirements for the criticality of particular functions. For Space Station
Manned Base (SSMB) functions for crew survival, two redundant systems are
required. For SSMB functions for station survival, a single redundant system is
required.
Another method used to develop a reliable system is to de-rate parts for their
respective applications. Although de-rating programs are not available for MEMS
devices, the same principle of operating well within a parts margin is applied. The
approach NASA takes to de-rating is to run all electrical, electronic, and electro-
mechanical (EEE) parts well within their respective safe operating areas (SOA).
The SOA of a part depends on its design and performance ability. Each part type is
derated to the guidelines found in MIL-STD-975 or in accordance with the indi-
vidual program de-rating requirements (e.g., SSP 30312, EEE Parts Derating and
7
End of Life Guidelines). In general, parts de-ratings reduce the factors that limit the
SOA of a part to increase reliability and device longevity. These include tempera-
ture, voltage, current, cycles, and power consumption. Space flight parts have
specified operating areas between 55 and 1258C. By de-rating the operating
temperature of a specific component, the failure rate may reduce by a factor of
five for active devices. Certain part types will have an extended operating life when
de-rated in terms of power consumption. In addition, de-rating minimizes the
impact of aging affects such as the drift of electrical parameters. Although the
term de-rating applies to microelectronics and not to MEMS, operating within
reduced margins is prudent and should be required on all space programs. The

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