Who are you fooling with MTBF Predictions?
All models are wrong, some are useful. ~ George E. P. Box
If you know me, you know I do not like MTBF. Trying to predict MTBF, which I consider a worthless metric, is folly.
So, why the article on predicting MTBF?
Predicting MTBF or creating an estimate is often requested by your customer or organization. You are being specifically asked for MTBF for a new product.
You have to come up with something.
The options available include the Mil Hdbk 217, various other similar standards, IEC 61709 and VITA 51.2. There may be others.
Mil Hdbk 217 is out of date
The Mil Hdbk 217F Notice 2 was published in Feb 1995. There is work to update the standard to the G revision and is making progress. It’s not out yet (as far as I can determine), and promises to incorporate more failure mechanism and physics of failure approach.
In the meantime, do not use Mil Hdbk 217 as it is sorely out of date.
IEC 61709 Physics of Failure Reliability Prediction limited scope
The IEC 61709 Rev 2.0 published in 2011 is major step in the right direction. While limited in scope and using simplistic model, it provides a means for vendors to conduct and report product testing that user may convert to their specific use conditions.
This is wonderful.
The standard still uses failure rates and modifications as the structure. You will notice familiar equations for electrolytic capacitor life and the Arrhenius equation, and a many more.
The equations and data used for fitting parameters are conservative. The idea, in part, is to bridge the 217 approach and physics of failure approach.
Like many IEC standards it doesn’t provide sufficient background and references for formulas and fitting parameters. It doesn’t discuss the many underlying assumptions, which are needed to properly apply the formulas.
VITA 51.2 Physics of Failure Reliability Predictions is better
Now we’re getting there.
Good stuff in this document. More models and it includes some of the how and why to apply, including assumptions. While not a full textbook, it make a major step forward.
Downside, not widely accepted in the contracting world. The Mil Hdbk’s and IEC standards seem to dominate. The VITA 51.2 is good work and should start to become the standard going forward.
We can hope.
Yes, it takes work In order to predict the future, the best way is to wait and measure it. Often we do not have time for this approach.
We need to make decisions today about design and assembly decisions that may impact product performance 20 years in the future. Even 5 years is difficult.
Starting with an outdated model is sure way to be wrong. Using a current physical of failure model may require some thinking, additional data and a bit of research.
It is worth it?
What decisions are you making and are they important? If so, then use the best technology available. It may take some work.
So be it.
How do you make your reliability predictions? What are you being asked to forecast? How successful are you?