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70 MEMS and Microstructures in Aerospace Applications

reliability can be greatly impacted. Systems are expected to operate continuously in
orbit or in deep space for several years without performance degradation. For most
low-power applications, properly designed heat conducting paths are sufficient
to remove heat from the system. The placement of MEMS devices is therefore
of great importance. The basic rule is that high power parts should not be placed
too close to one another. This prevents heat from becoming concentrated in a
localized area and precludes the formation of damaging ‘‘hot spots.’’ However,
some special high power boards require more intensive thermal management
mechanisms such as ducting liquid cooling fluids through printed wiring assemblies
and enclosures.
Aging effects of temperature are modeled after the Arrhenius or Eyring equa-
tions, which estimate the longevity of the subsystem. Similarly, the effects of
voltage or power stress can be estimated using an inverse power model. From the
microelectronic world comes a very mature understanding of the factors, such as the
Arrhenius activation energy or the power law exponent, dependent on the part type
being evaluated, and the expected dominant failure mechanism at the modeled
stress level. However, the activation energy is based on electrochemical effects
which may not be the predominant failure mode especially in the mechanical
aspects of the MEMS device. Lack of an established reliability base remains a
precautionary note when evaluating MEMS for space applications.
Accelerated stress testing can be used to activate latent failure mechanisms. The
temperatures used for accelerated testing at the parts level are more extreme than
the temperatures used to test components and systems. The latter temperatures
exceed the worst-case predictions for the mission operating conditions to provide
additional safety margins. High-temperature testing can force failures caused by
material defects, workmanship errors, and design defects. Low-temperature testing
can stimulate failures from the combination of material embrittlement, thermal
contraction, and parametric drifts outside design limits.
Typical test levels derived from EEE parts include the following:

. High-temperature life test is a dynamic or static bias test usually performed
between 125 and 1508C.
. Temperature–humidity testing is performed at 858C and 85% RH (pack-
aged).
. Temperature–pressure testing, also known as autoclave, is performed at
1218C at 15 to 20 psi (packaged).

Often, the space environment presents extreme thermal stress on the spacecraft.
High-temperature extremes result from the exposure to direct sunlight and low
temperature extremes arise because there is no atmosphere to contain the heat
when not exposed to the sun. This cycling between temperature extremes can
aggravate thermal expansion mismatches between materials and assemblies.
Large cyclic temperature changes in temperature can cause cracking, separation,
and other reliability problems for temperature sensitive parts. Temperature cycling
is also a major cause of fatigue-related soldered joint failures.

© 2006 by Taylor & Francis Group, LLC