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338 MEMS and Microstructures in Aerospace Applications

TABLE 15.2
Various Environmental Pairs — Continued
Low Pressure and Explosive Solar Radiation and Explosive
Atmosphere Atmosphere Solar Radiation and Particulate
At low pressures, an electrical This combination produces It is suspected that this
discharge is easier to develop no added effects. combination will produce high
but the explosive atmosphere temperatures.
is harder to ignite.

Solar Radiation and Solar Radiation and Shock or
Solar Radiation and Ozone Vibration Acceleration
This combination increases the Under vibration conditions, These combinations produce no
rate of oxidation of materials. solar radiation deteriorates added effects.
plastics, elastomers, oils, etc.
at a higher rate.

Shock and Vibration Vibration and Acceleration Particulate and Vibration
This combination produces no This combination produces Vibration might possibly increase
added effects. increased effects when the wearing effects of sand and
encountered with high dust.
temperatures and low
pressure in the hyper-
environmental ranges.

Each environmental factor that is present requires a determination of its impact
on the operational and reliability characteristics of the materials and parts compris-
ing the equipment being designed. Packaging techniques should be identified that
afford the necessary protection against the degrading factors.
In the environmental stress identification process that precedes selection of
environmental strength techniques, it is essential to consider stresses associated
with all life intervals of the MEMS. This includes operational and maintenance
environments as well as the preoperational environments, when stresses imposed on
the parts during manufacturing assembly, inspection, testing, shipping, and instal-
lation may have significant impact on MEMS reliability. Stresses imposed during
the preoperational phase are often overlooked; however, they may represent a
particularly harsh environment that the MEMS must withstand. Often, the environ-
ments MEMS are exposed to during shipping and installation are more severe than
those encountered during normal operating conditions. It is probable that some of
the environmental strength features that are contained in a system design pertain to
conditions that will be encountered in the preoperational phase rather than during
actual operation. Environmental stresses affect parts in different ways and must also
be taken into consideration during the design phase. Table 15.3 illustrates the
principal effects of typical environments on MEMS.

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