Power supply life is affected by three kinds of stresses: thermal, mechanical, and electrical. A quality design anticipates each of these and takes necessary steps to minimize both their occurrence and their impact. Thermal stress is the most challenging and insidious stress, because it manifests itself in so many ways. By their nature, power supplies dissipate heat: a 1000 W supply operating at an admirable 90% efficiency is still producing 100 W of heat. Thermal stress takes two forms: static and dynamic. Static thermal stress means operation at elevated temperatures, which degrades components and their basic materials. Bulk capacitors may begin to dry out, or their seals may be stressed, and even resistor coatings may begin to deteriorate and break down. Interconnection and mating areas can expand and mismatch. Dynamic stress is associated with the heating and cooling cycles which occur as the supply output goes from full load to low load, or is turned on and off. Each time this happens, the structures and connections expand and contract, and micro-cracks can eventually develop due to differing coefficients of thermal expansion between materials. Mechanical stress severity depends on how and where the supply will be installed and used. This stress can cause both intermittent and hard failures, as cracks develop and circuit connections start to open and, in some cases, re-connect. Perhaps the supply is subject to vibration in normal use, or there is unexpected flexing of the circuit board, connections, or cabling. Mechanical stress can also result from an improper manufacturing process, such as a fastener which is over-torqued. Electrical stress occurs when a component is operated beyond its rated value, either through poor selection or one-time events. For example, a capacitor may be rated to 100 VDC, but sees a 150 VDC spike in operation. Or a resistor is specified to handle up to 1A current but sees a higher-current pulse due to a circuit transient or external ESD event. The result is premature aging and early failure in many cases.