Failure rate has three key phases: infant mortality, useful life, and wear-out. There's a higher failure rate during the "infant mortality" phase, which usually lasts around low double-digit hours. These failures are generally due to poor workmanship and shoddy components, and can be found through pre-shipment burn-in of a power supply. The second, and longest phase is "useful life", during which the supply operates properly. During this phase the failure rate is low and constant. The final phase is the "wear out" phase, where the supply fails as its components reach the end of their operating life. Common mechanisms for wear-out include fan bearings going bad, electrolytic capacitors drying out, and stress cracks developing after thousands of thermal cycles.