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342 MEMS and Microstructures in Aerospace Applications
cracking of seams, delamination, loss of hermeticity, leakage of fill gases, separ-
ation of encapsulating materials from components and enclosure surface, leading to
the creation of voids, and distortion of support members.
A thermal shock test may be specified to determine the integrity of solder joints
since such a test creates large internal forces due to differential expansion effects.
Such a test also has been found to be instrumental in creating segregation effects in
solder alloys, leading to the formation of lead-rich zones, which are susceptible to
cracking effects.
15.5.3 SHOCK AND VIBRATION
MEMS are often subjected to environmental shock and vibration during both
normal use and testing. Such environments can cause physical damage when
deflections cause mechanical stresses that exceed the allowable working stress of
the constituent parts.
Natural frequencies of items comprising the MEMS are important parameters
that must be considered in the design process since a resonant condition can be
produced if a natural frequency is within the vibration frequency range. The reson-
ance condition will greatly amplify subsystem deflection and may increase stresses
beyond the safe limit.
The vibration environment can be particularly severe for electrical connectors,
since it may cause relative motion between connector elements. In combination
with other environmental stresses, this motion can produce fretting corrosion. This
generates wear debris and causes large variation in contact resistance. Reliability
improvement techniques for vibrational stress include the use of stiffening, control
of resonance, and reduced freedom of movement.
15.5.4 HUMIDITY
Humidity can cause degradation of MEMS as discussed previously. Reliability
improvement techniques for humidity and salt environments include use of her-
metic sealing, moisture-resistant material, dehumidifiers, protective coatings or
covers, and reduced use of dissimilar metals.
Deleterious effects may be exacerbated with high humidity; for example, crack
30
growth has a dependence on moisture that is well documented. Electrical perform-
ance may change as moisture enters gaps in a vapor form and condenses as water
droplets, causing surface tension which may induce a piezoresistive stress effect. 31
Perhaps best known is the relationship of adhesion and friction of polycrystalline
silicon MEMS. 32 This dependence is reduced, but not eliminated, when molecular
coatings are applied to the surfaces. Antistiction coatings have the ability to penetrate
into the intricate side wall and under-surface spaces in three dimensions. Thus, these
coatings extend the operating life of MEMS devices by reducing stiction. 33
15.5.5 RADIATION
Electromagnetic and nuclear radiation can disrupt performance levels and, in some
cases, cause permanent damage to exposed devices. Therefore, it is important that
© 2006 by Taylor & Francis Group, LLC