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26 The PCB Magazine • March 2015 What is reliability? Reliability is the prob- ability of a product performing its intended function over its specified period of usage, and under specified operating conditions, in a man- ner that meets or exceeds customer expecta- tions [1] . The catalyst for the emergence of reliability analysis goes back to 1906, when Lee de Forest invented the triode vacuum tube. The vacuum tube initiated the electronics revolution, en- abling a series of applications such as the radio, television, radar, etc. The vacuum tube is recog- nized by many as the technology that allowed the Allies to win the so-called "Wizard War" of World War II. Ironically the vacuum tube was the main cause of equipment failure and tube replacements were needed five times more of- ten than all other equipment [2] . In 1937, Waloddi Weibull invented the Weibull distribution, and in 1951 he delivered his hallmark American paper on this subject claiming that his distribution applied to a wide range of problems. He showed several exam- ples ranging from fiber strength of cotton to by Patrick Valentine OM GrOup ELECTrOnIC ChEMICaLS the fatigue life of steel [3] . In 1952 the Advisory Group on Reliability of Electronic Equipment (AGREE) was jointly established by the Depart- ment of Defense (DoD) and the American Elec- tronics Industry. In 1956 the Radio Corpora- tion of America (RCA), a major manufacturer of vacuum tubes, released a significant report, "Reliability Stress Analysis for Electronic Equip- ment" (TR-1100), which presented a number of models for estimating failure rates. Then, in 1959, the "RADC Reliability Notebook" came into existence, and in 1961, a military reli- ability prediction handbook format known as MIL-HDBK-217 was published. Numerous other reliability documents have been published ever since. Historically, reliability engineering of elec- tronics has been dominated by the belief that the life or percentage of complex hardware fail- ures that occur over time can be estimated, pre- dicted, or modeled. Ironically, there is little, if any, empirical field data from the vast majority of verified failures that shows any correlation with calculated predictions of failure rates [4] . Modern reliability tests try and simulate as- sembly and field use conditions. These methods include solder shock, air-to-air thermal cycling, liquid-to-liquid thermal cycling, interconnect Feature Reliability Testing and Statistics

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