FIT and MTTF / MTBF (3): What types of MTBF figures can be obtained from manufacturers?
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A. Basically, there are two types of MTBF figures: Theoretical and operational MTBF. When a manufacturer introduces a new product to the market, there is no operational data on how the product will perform. Typically, a theoretical MTBF figure will be calculated based on similar products (in the case of multilayer ceramic capacitors, this figure will be based on similar materials and class). This number is primarily used as a benchmark for reliability. It is calculated from the FIT number. It is also important to note that these MTBF figures are estimated based on a theoretical model and they are limited by the assumptions upon which they were based. Therefore it cannot typically account for random or unusual conditions such as a temporary quality problem during the manufacturing process.

Operational MTBF is provided when a component has been in use for a period of time. Actual failures of the component can be analyzed and calculations can be made thereafter. This figure is derived by analyzing field information and is typically lower than theoretical MTBF because it includes some human element and unforeseeable problems not accounted for in theoretical MTBF.

*JEDEC Standard JESD85, "Methods for Calculating Failure Rates in Units of FITs", JEDEC Solid State Technology Association, Arlington, VA, July 2001.
*R. A. Dovich, "Reliability Statistics", ASQ Quality Press, Wisconsin, 1990.
*Zzyzx Peripherals, Inc., "Mean Time Between Failure (MTBF) and Availability – A Primer", Technical Paper, June 13, 2001 Revision A.

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