While MIL-HDBK-217 provides guidance, this is not enough to provide information on how long the product will be useful in the field. Only when the MTBF calculation is adjusted for a constant failure rate, then is it a worthwhile value. Let me explain…
There is software available with models that conform to MIL-HDBK-217 which allows for all of the components from the electrical drawing to be entered.
Figure 1. Example Input for MTBF calculation
The problem is these theoretical calculations can result in MTBF values of more than 500,000 hours (>57 years). This is of course not a good indication of how long the product will function in operation and is so high it is not worth mentioning.
This is a useful tool to find weak points in designs, but for the user of the product the information is not a good indication of failure rate. The MTBF will decrease in very high or very low temperatures, or other extreme environmental conditions. System designers need a realistic guarantee of how the product will perform in their application. As an example, airborne-targeting systems require a component to operate without failure for 20,000 hours of operation. Obviously, further methods are involved than providing the theoretical value provided by the software.
In order for MTBF to provide this information, only the flat region of the bathtub curve (See figure below from Wikipedia) where the failure rate is constant can be considered. This is a typical practice for military specifications and is also called “the useful life period.” To remove the infant mortality failures, intense and accelerated testing per the expected system conditions using industry-accepted methods (i.e. MIL-STD-810) is completed.
Experienced manufacturers are able to provide a reliable MTBF value. Unless it is tested and reported in a certain way, it is not a useful indication of failure rate.
Figure 2. Bathtub Curve (Source: Wikipedia)
MTBF is very often mis-used in the camera industry. Numbers are given without an understanding of the assumptions and meaning. While MTBF is an indication of reliability, it does not represent the expected lifetime of the product. If the wear-out-failure mechanisms are unknown or undefined and assumptions during MTBF estimates are unrealistic the MTBF value is meaningless.