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72 MEMS and Microstructures in Aerospace Applications

Vibration forces can be stimulated by acoustic emissions. The acoustic envir-
onment of a spacecraft is a function of the physical configuration of the launch
vehicle, the configuration of the propulsion system and the launch acceleration
profile. The magnitude of the acoustic waves near the launch pad is increased by
reflected energy from the launch pad structures and facilities. The first stages of a
spacecraft (e.g., solid-rocket boosters) will usually provide a more demanding
environment. The smaller the total vehicle size, the more stressed the payload is
likely to be. The closer the payload is located to the launch pad, the more severe the
acoustic environment will be.
Random vibration and multivibration tests (i.e., swept sine or frequency sine
combined with random vibration) are typically performed. The use of vibration as a
screen for electronic systems continues to increase throughout the industry (includ-
ing airborne avionic, ground, military shipboard, and commercial applications).
Electronic assemblies in space applications must not degrade or fail as a result
of mechanical shocks which are approximately 50 to 30,000g for 1.0 and 0.12 sec,
respectively. To reduce effectively the negative effects of shock energy, electronic
assemblies must be designed to transmit rather than absorb the shock. The assembly
must therefore be stiff enough to achieve a rigid body response. Making individual
electronic devices as low in mass as possible ensures that there is an overall increase
in shock resistance of the entire assembly.
Commercial manufacturers of mass produced MEMS devices such as acceler-
ometers for air bag deployment have incorporated shock and drop tests to their
routing quality screens.

4.2.3 CHEMICAL EFFECTS
Chemical effects on MEMS devices are covered under three categories. These
divisions are high-humidity environments, outgassing, and flammability. Moisture
from high-humidity environments can have serious deleterious effects on the
electronic assemblies particularly MEMS devices. Moisture causes corrosion,
swelling, loss of strength, and affects other mechanical properties. To protect
against moisture effects, electronic packages are typically hermetically sealed.
However, many MEMS devices, especially those used for environmental sensors,
cannot be hermetically sealed and require additional precautions. Systems are
normally specified to operate in an environment of less than or equal to 50% RH.
(A maximum of 50% RH is specified for the Space Shuttle.) Outgassing of moisture
from sources such as wire insulation or encapsulants must be factored into the
amount of humidity expected in an enclosed environment. Exposure during mission
and launch is limited by the control of the environment. Prior to launch, the
humidity of storage and processing must be controlled. Hermetic packaging
schemes are preferred for space applications. The integrity of the package seal
and the internal environment of the parts correlate directly with their long-term
reliability. Moisture-related failure mechanisms might occur externally or internally
to the packaged part. External moisture-related failure mechanisms include lead
corrosion, galvanic effects, and dendrite growth. Internal moisture-related failure

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