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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
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                   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




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