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Thermal Effects

High temperatures impose a severe stress on most electronic devices including MEMS, often causing mechanical failure or resulting in deterioration due to chem­ical effects. MEMS design inherently requires small sizes with high part densities. This generally requires a cooling system to provide a path of low thermal resis­tance from heat-producing elements to a heat sink. Adequate life with such thermal stresses usually demand the use of heat dissipation devices, cooling systems, thermal insulation, and heat-resistant materials.

Conversely, low temperatures experienced by MEMS can have a reliability impact. These problems usually are typically associated with mechanical system elements. They include mechanical stresses produced by differences in the coef­ficients of expansion (contraction) of metallic and nonmetallic materials, embrit-tlement of nonmetallic components, mechanical forces caused by freezing and expansion of entrapped moisture, stiffening of fluid constituents, etc. Typical exam­ples include cracking, delaminations, binding of mechanical linkages, and excessive viscosity of lubricants. Reliability improvement techniques for such low tempera­ture stresses include the use of heating devices, thermal insulation, and cold-tolerant materials.

Additional stresses are produced when MEMS are exposed to sudden changes of temperature or rapidly changing thermal cycling conditions. These conditions generate large internal mechanical stresses in structural elements, particularly when dissimilar materials are involved. Effects of thermal shock-induced stresses include cracking of seams, delamination, loss of hermeticity, leakage of fill gases, separation 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 check the integrity of solder joints since such a test creates large internal forces due to differential expansion. Repetitive stress may create segregation effects in solder alloys leading to the formation of lead-rich zones, which are susceptible to cracking effects.

Date: 2015-02-28; view: 166

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