Power supply life is affected by three kinds of stress: thermal, mechanical, and electrical. Of these, thermal stress is the most challenging and insidious stress because it manifests itself in so many ways. As we have seen, by their nature, power supplies dissipate heat: a 1000W supply operating at an admirable 90% efficiency is still producing over 100W of heat. However, it is not just the supply's own dissipation which causes the supply to operate at higher temperatures. Most of the power that the supply provides to the electronics eventually ends up as dissipated heat within the enclosure as well (some may be used outside the box to drive loads such as motors) thus adding to the overall thermal load and heat rise of the product. Thermal stress takes two forms: static and dynamic. Static thermal stress results from operation at elevated temperatures, which degrades components and their basic materials. Bulk capacitors may begin to dry out, or their seals may break down; similarly resistor coatings may begin to deteriorate and connector expansion can result in contact mismatch. Dynamic thermal stress is associated with 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, device structures and connections expand and contract, allowing micro-cracks to develop due to the differing coefficients of thermal expansion between materials. Such repeated cycling ultimately causes outright breaks and failures. However, it is difficult to estimate the deterioration in product reliability due to dynamic stress because the heating and cooling rates of these cycles also affect their impact.