Unsure how to read MTTF value on datasheet - Page 1
Forum section: electronics / beginners

author: floxia
author_rank: Contributor
timestamp: April 02, 2025, 09:25:19 am
content: Hi there!As per the title, I am unsure how the MTTF value is interpreted on the datasheet.I do not understand whether, in this specific case, it is 100 × 10,000 hours = 1 million hours or that every 100 units tested the average was 10000 hours?Datasheet:http://en.jetekcn.com/web/viewer.html?file=/Upload/1683332738.pdf&title=K78XX-500Any clarification on this would be greatly appreciated!

author: RoGeorge
author_rank: Super Contributor
timestamp: April 02, 2025, 12:30:21 pm
content: Yes, it is 1 million hours.

author: SteveThackery
author_rank: Super Contributor
timestamp: April 02, 2025, 06:28:24 pm
content: And now for something extremely unintuitive: the probability of any particular example surviving to its MTTF (or MTBF) is 35%. Not 50% as many people might imagine.Anyway, totally irrelevant to you - a million hours is 114 years.

author: floxia
author_rank: Contributor
timestamp: April 03, 2025, 04:47:11 am
content: Quote from: RoGeorge on April 02, 2025, 12:30:21 pmThanks for clarifying this!Quote from: SteveThackery on April 02, 2025, 06:28:24 pmThanks for sharing this, well let's say that the unit only survives 35% of expected lifetime is still 40 years, which is still great.

author: golden_labels
author_rank: Super Contributor
timestamp: April 03, 2025, 10:44:59 am
content: Quote from: floxia on April 03, 2025, 04:47:11 amThis is not what it says. 35% of items survives past MTTF, not the expected value being 35% of MTTF. The expected value is 114 years, and 2/3 of items fail earlier.This is modeled with theexponential distribution, with quartiles being:Q1: 0.29 · mean ≈ 33 yearsQ2: 0.69 · mean ≈ 79 yearsQ3: 1.39 · mean ≈ 158 yearsIn other words: the least “lucky” 25% fails before 33 years, the next quarter survives until 79 years, the next quarter until 158 years, and the next quarter beyond that. But also 10% doesn’t survive 12 years, and the most “unlucky” 1% dies within the first year.However, take those values with a reasonable safety margin. MTTF is a parameter in a model. It’s not reality. It only defines curve’s shape, not predict the future. It’s also calculated from a sample covering only the very begining of that curve, making the higher values pretty wobbly and uncertain. On top of that it’s specified for particular test conditions. Test conditions are not your device’s conditions. It doesn’t include being exposed to random voltage spikes, cosmic rays, unexpected moisture ingress, extreme thermal cycling, other components misbehaving and stressing it, cat playing with the device, or cockroaches making themselves a bed under the component. It’s also not a bathtub curve: aging is not taken into account. Don’t expect to see 25% of product batch actually alive at their 160th birthday. They’ll likely crumble into pieces by then.Finally: those are probabilistic values. They should never be taken out of context of probabilistics. All the values mentioned hold exactly only for infinitely large number of items. Your finite subset may as well die entirely on the first day.

author: SteveThackery
author_rank: Super Contributor
timestamp: April 03, 2025, 11:59:53 am
content: Quote from: floxia on April 03, 2025, 04:47:11 amWhat@golden_labelssaid!It is probably worth giving it a moment's thought so you are sure about what's going on.If you want to take reliability and longevity into account there are several important points to understand, some of which may seem counterintuitive:1/ MTTF and MTBF are based on thefailures occurring at randomduring the normal operational life of a product. They donottake into account the effects of ageing. The life expectancy of a product (determined by ageing mechanisms) and its reliability are two completely different things. Indeed it is not unusual to have a life expectancy much less than the MTTF.2/ Some people think that MTBF is about reliability and MTTF is about longevity. They are wrong. MTBF and MTTF are mathematically the same. The difference is as follows: MTTF refers to failures that stay failed, and MTBF refers to failures that are repaired. For example, a transistor cannot be repaired once it has failed - it must be replaced - so it is appropriate to refer to its MTTF. On the other hand a typical circuit board can be repaired and put back into service, so we talk about its MTBF.I've been always considered MTTF and MTBF to be awkward ways of expressing reliability. I think the concept of "failure rate" is easier and more intuitive. It is actually the reciprocal of the MTTF: 1/MTTF. (Same with MTBF, of course.)For example, assume we have a circuit board with a 1000 hour MTBF. Here is what that actually means:a/ Any particular example will fail randomly with an average rate over time ofone failure per 1000 hours. Note that this is anaverage over a long period.b/ Theprobabilityof any particular example failing in the next hour (or any other hour) is1 in 1000.A worked example will make the benefits of failure rates compared with MTBFs clear:Suppose you had a switching transistor, a current limiting resistor and an LED in series. If any component fails, the circuit fails - the LED won't light up. The MTBFs are:Transistor: 500 hoursResistor: 10000 hoursLED: 1000 hoursWhat is the MTBF of the whole system (the three components working together)? It isn't obvious how to work it out! But now convert the MTBFs to failure rates:The failure rates are:Transistor: 1/500 = 0.002 failures per hourResistor: 1/10000 = 0.0001 fphLED: 1/1000 = 0.001 fphNow add the failure rates:System failure rate = 0.002 + 0.0001 + 0.001 =0.0031 failures per hourSystem MTBF = 1 / 0.0031 =323 hoursThus you can see that using failure rates makes it very easy to calculate a system failure rate and MTBF, even when the system comprises all sorts of different components.Note: the MTBFs I've used are ridiculously low and not realistic, so don't quote them!(PS: this is a quote from a previous post of mine in a different thread.)