Abstract

Chris and Fred discuss … MTBFs! Fred’s favorite! We (should) know that Mean Time Between Failure (MTBF) is not reliability. So what do we do if we somehow need to get reliability from nothing but the MTBF.

Key Points

Join Chris and Fred as they discuss an often and unfortunate problem of reliability engineers being asked to ‘divine’ reliability from nothing but MTBFs and Failures in Time (FITs) numbers which are essentially the inverse of the MTBF for electronic components. So what do you do?

Topics include:

• Acknowledge that if all you have are MTBFs, then you don’t have enough information. It is a reality. Even if your boss or management team don’t want to hear it.
• Components with the same MTBF are unlikely to have the same reliability at specific usage durations. It all comes down to failure mechanisms that drive what we call ‘shape’ parameters. And these shape parameters can mean that two components with the same MTBF might have an 80 % chance of failing in a warranty period … or a 1 % chance. So you can to look up the Physics of Failure (PoF) or use engineering judgment to understand the nature of failure and combine that with the MTBF to get some estimate on reliability.
• Get on the phone. Speak to suppliers and vendors (even those that have just given you MTBFs on the specification sheet), and ask what the real story might be.
• Focus on the vital few. The chassis of a vehicle is unlikely to fail as often as the transmission of a vehicle. So why should we ever focus on understanding how long it takes for the chassis to fail? If you ever abandon a vehicle in a forest, the last thing that will rust away will be the chassis. So focus on learning how the transmission … or the weakest part of your system … will fail. The VITAL FEW. Not the TRIVIAL THOUSANDS.
• Think about Bayesian analysis. This is a fancy (but simple!) way of combining lots of ‘expert assessments’ to come up with estimates of things like reliability. And when we say ‘experts,’ this could be the different members of the design and manufacturing teams. You WILL be amazed by how accurate this can be … if you do it well.

Enjoy an episode of Speaking of Reliability. Where you can join friends as they discuss reliability topics. Join us as we discuss topics ranging from design for reliability techniques to field data analysis approaches.

SOR 699 Only have MTBFsChristopher Jackson

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