New-Tech Europe | April 2016 | Digital edition

Figure 1: The bathtub curve, failure rate plotted against time with the three life-cycle phases: infant mortality, useful life and wear-out.

Figure 2: Curve showing the probability that a component is still operational over time.

Conversely, assessment is the most accurate way of predicting failure rate, but requires greater time and resources. This method subjects a suitable number of final units to an accelerated life test at elevated temperature, with carefully controlled and increased stress factors. One method, the HALT (highly accelerated life test) approach, tests a number of prototype units under as many conditions as possible, with cycling of temperature, input voltage, output load, and other impacting factors. HALT testing seeks to fatigue a component, PCB, subassembly, or finished product through either intense stressing for fewer cycles, or low level stressing for more cycles. A second method, HASS (highly accelerated stress screen) testing is an accelerated reliability screening technique used to reveal latent flaws not detected by environmental stress screening, burn-in, or other test methods. HASS testing uses stresses beyond initial specifications, but still within the capability of the design as determined by HALT.

characterized by an exponential factor, so only 37% of the units in a large group will last as long as the MTBF number; second, for a single supply, the probability that it will last as long as its MTBF rating is only 37%; and third, there is a 37% confidence level likelihood that it will last as long as its MTBF rating. Additionally, half the components in a group will have failed after just 0.69 of the MTBF. It should also be noted that this formula and curve can be adapted to calculate the reliability of a system: Where A is the sum total of all components failure rates ( A = 1n1 + 2n2 + … + ini) Calculating the failure rate Three methods can be used to calculate failure rates, prediction (during design), assessment (during manufacturing) and observation (during service life). Prediction uses a standard database of component failure rates and expected

life, typically MIL-HDBK-217 for military and commercial applications or Telcordia for telecom applications. The MIL approach requires use of many parameters for the different components and includes voltage and power stresses, while Telcordia requires fewer component parameters and can also take into account lab- test results, burn-in data, and field- test data. Finally, the MIL approach yields MTBF data, while Telcordia produces FIT numbers, (failures per billion hours). Using these databases and techniques means several, often incorrect, assumptions need to be made, such as the assumption that the design is perfect, the stresses are all known, everything is operated within its ratings, any single failure will cause complete failure, and the database is current and valid. But, it is the least time consuming method and by applying it consistently across different designs, it can indicate the relative reliability of topologies and design approaches, rather than absolute reliability.

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