This study discusses, with examples, the dependency of 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 and of their duty cycles. It also discusses the fundamental differences between product validation and reliability demonstration or assessment. This study shows how the same product used in a different location of a system, e.g., a vehicle, or in a different orientation that affects its natural heat transfer, can yield a very different reliability estimate. For that reason, reliability requirements or goals should be tailored for the actual expected use of the product. Reliability details such as dependency on the product use cannot be numerically tailored at all times for all locations and conditions of use, especially contractually. Therefore, specification of reliability requirements is needed in the form of a reliability range in average conditions, or a minimum reliability under the harshest use profile, rather than a fixed numerical value.
The most excessive environmental and operational stresses are not expected to be constant and ever-present when the product is in actual use. Such stresses are expected to be present for only a fraction of the lifetime of the product; therefore, knowing the use profile is essential to preparing an analysis or designing an accelerated reliability life test. This analysis and the reliability demonstration/growth test should be tailored to cover product use under aggressive, normal, and light stresses depending on the percentage of time spent in each of those environments, and with realistically expected stresses on the product during those times.
To determine realistic and effective reliability goals or requirements, it is necessary to consider the manner in which an item will be used and the environments or locations of its use. Specifying a required value of Mean Time To Failure (MTTF) or Mean Time Between Failures (MTBF) may introduce unwanted problems, as MTTF and MTBF are average values that, just as reliability is, are highly dependent on the use, operational and environmental stresses, and expected life or use duration of the product. It is more appropriate to specify a minimum reliability value for aggressive use in the harshest environment in which the product might be used. This practice is even more important in cases where a product contains not only electronics, but also mechanical components for which MTTF is just an average value calculated from stress/strength criteria and resultant reliability.
Author's Note: References 1 and 2 should be considered foundational documents for this entire study; as such, they are not notated individually within the text.