The MTBF calculation standard MIL-HDBK-217 is the oldest standard of its kind, but has not been updated since its final last edition F, Notice 2 of 1995.
Considering that MTBF calculation standards are based on field experience, the latest edition of MIL-HDBK-217 is based on field data that is now 25 years old.
- You can never go wrong with MIL-HDBK-217. If in doubt, always opt for Mil-HDBK-217 (however, the results are always worse than for all other standards).
- Mostly used for MTBF calculation
- Is often used in the field of functional safety.
25 year old field data in no way reflects the quality level of today's electronic components, and one can even safely assume that today's commercial electronics are considerably more reliable than military grade quality at that time, despite the RoHS directive.
For this reason, the ANSI/VITA 51.1 so to say-standard was issued by a consortium of military and commercial aerospace companies. This consortium also included the then Relexsoftware company, whose Relex software is available today through PTC under the name Windchill Quality Solutions. ANSI/VITA 51.1 provides concrete instructions for the quality classification of the different component types. As a result, improvements of between a factor of 1 and 100 are achieved at the component level, and at the PCB level, however, only of about a factor of 3 to 5 compared to the strict application of MIL-HDBK-217 in the original sense, which is still quite pessimistic compared to other standards.
- Despite ANSI/VITA 51.1, the following issues remain problematic:
Not all component types are addressed,
- For connectors, switches, and other component types with already relatively high failure rates, it is recommended to use the corresponding MIL-HDBK-217 model in its original form. Therefore, it is possible to "ruin" the MTBF of the entire assembly with a single switch or a few connectors.
Both of the above points mean that the improvement in MTBF results is only a factor of 3 to 5, despite the application of ANSI/VITA 51.1 at the PCB level. Whereas in the past it was the ceramic capacitors that significantly determined the overall MTBF, today it is the switches and connectors.