Realistic reliability requirements for the stresses in use

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This paper discusses, with examples, dependency of the reliability estimates or requirements on the use environments, including the details of reliability changes of electronic and mechanical components as a function of environmental and operational stresses as well as of their duty cycle. It also provides a discussion of the fundamental differences between the product validation and reliability demonstration or assessment. The paper shows how the same product used in a different location of a system, e.g. vehicle, or in merely different orientation that affects its natural heat transfer, can have a very different reliability estimate. For that reason, the reliability requirements or goals need to be tailored for the product actual expected use. The reliability details such as dependency on the product use cannot be numerically tailored at all times for all locations and details of use, especially contractually. Therefore specification of reliability requirements in a form of a reliability range in average and or a minimum reliability in the harshest use profile is needed rather than as a fixed numerical value. The most excessive environmental and operational stresses are not expected be constant and always present when the product is in actual use; they are expected to be present for only a fraction of the life time, thus the knowledge of the use profile is essential to prepare analysis or design an accelerated reliability life test is specifically important. This analysis along with the reliability demonstration/growth test needs to be tailored to cover the use with the aggressive, normal, and light stresses dependent on the percent time spent in those environments, and with realistically expected stresses of the product during those times. To determine realistic and effective reliability goals or requirements, it is necessary to have in mind the manner how an item will be used and also the environments or locations of its use. Specifying a value of a required MTTF or MTBF may introduce unwanted problems, as they are average values and, just as reliability is, are highly dependent on the item's use its operational and environmental stresses and also of life or product use duration. It is more appropriate to specify minimum reliability value for the aggressive use in the harshest environment that the product might be used. This is even more important in cases where a product, besides electronics, contains mechanical components where and MTTF is just an average value calculated from the stress/strength criteria and resultant reliability.