calculate MTBF

For a component? For an entire system? 217 or Bellcore/Telcordia or ...

The Wikipedia article isn't a bad place to start. If you're tasked with doing a full-up parts-stress reliability prediction analysis, good luck!

--
Rich Webb     Norfolk, VA

Get the failure rate of each part (specified in FITS, failures per billion hours), add them up, take the reciprocal, multiply by a billion.

John

Thanks

The National site is good and I found the info for the their parts.

Microchip no luck, IR no luck. So where do you normally find the information?

I suppose one also assumes perfect ESD procedures, and perfect lead free soldering.

For a lithium ion battery charger.

My brief post was a cry of despair. I ship these by the hundred. Now a BIG US company wants them and I get asked for a bucket load of information.

The two commonly used docs are MIL-HBK-217 and the Bellcore spec. We purchased the Bellcore reliability doc, about $1000 or so, and use its numbers unless we can get a specific FIT value from a vendor. That cost us about $50 a page, or $300 a page for the three pages that are useful.

Occasionally a customer asks us for an MTBF value, so we grind out the numbers. Our actual field failure rates seem to be well below the values that we calculate.

Yes.

John

You're new at this so, even if you're not doing a MIL-STD prediction, I'd strongly recommend downloading MIL-HDBK-217 from the online site (search on MIL-HDBK-217 in the Document ID field) and skimming though sections 3 and 4. The Bellcore process is similar.

John's basic equation is correct. You can use the tables in 217 to estimate the reliability of items for which the manufacturer does not provide the info.

There are provisions to consider connections (e.g., "Quantity of Hand Soldered PTHs [plated through holes]"). It's a hell of a lot of work to do a full analysis.

--
Rich Webb     Norfolk, VA

[snip] [snip]

Firefox give me "This Connection is Untrusted" for that site. ...Jim Thompson

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One reason we paid for the Bellcore TR-332 document is that their numbers are much better. I assume it's the difference between commercial and military applications.

John

Jim Thomps>Firefox give me "This Connection is Untrusted" for that site.

"Certificate Authorities" are a scam (much like IEEE) and Mozilla buys into the scam.

?sid=3D08/08/04/0058217+*-website-*-using-*-self-signed-*.*.*.*.*-*.*-*.*+=*-*-bundle-*-*-*-*-*+migrate-away-*-*.*-*+hey-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*=-*-*-*-*-*-*-*+*-pretending+*-*-little-sense+not-just-*-paying-customers+*-=*-*-*-*-*-*.*.*.*.*.*-*.and-no-certificate+*-click-four-times-*-*-*-*-*-*-*=-*+*-*-almost-useless-*-*-*-*+inc+inc+inc+looks.MORE.scary.and.LESS.secure#=

24465811

Just pretend like Mozilla doesn't know WTF they're talking about.

Their root certificate is back in the .mil domain rather than the "normal" root authorities that are built into most current browsers. There may be some reluctance on the part of browsers or it could be a statutory limitation.

--
Rich Webb     Norfolk, VA

217?

Who said there was no such thing as a true random number generating algorithm?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
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845-480-2058

email: hobbs at electrooptical dot net
http://electrooptical.net

We use Bellcore because the calculated MTBFs come out much higher.

John

It's been a long time since I did any of that stuff but IIRC the MIL calcs used 125°C for the high temp and Bellcore used 85°C. This would account for the better numbers. Art

the First paragraph in the 217 states that the analysis results cannot be used for contractual requirements. The MTBF numbers calculated are only for a guidance.

Try telling that to managment once they get that MTBF number in their hot little hands. :)

..as long as they PAY for the info and time to compile it.

Given the customer I'll go with 217. I'm trying to avoid spending a week doing this so I'm looking at other solutions.

I've seen a few websites that offer online calculation ($500) or send the BOM and results in 48 hours ($1500). I've also seen some programs that calculate MTBF.

Anyone tried these options?

Nope. They would prefer the BOM and do the calculation themselves. Since I won't give the BOM it's down to me sort it out.

When I got roped into doing one for a piece of shipboard equipment (nobody else would even admit to knowing how to spell "RM&A"), the company had a copy of Relex that I used for the crunching. Packages like that do save time, since they have the tables "built in" and one just needs to select the appropriate factors for each component. It can also make auditing a bit easier, as well as printing pretty reports.

You might also want to start with the Appx A "Parts Count" method which is less detailed but does give you a ballpark figure (but one that tends to be more conservative).

The intended use of either method is to establish an "apples to apples" comparison baseline, sort of like the EPA estimated MPG ratings. You don't expect to get exactly that MPG but you can have some reasonable expectations if one vehicle is rated 18 and another 32.

The other big use is to help out the logistics chain in estimating the initial spare parts buy. The reliability prediction is paired with a FMECA (Failure Modes, Effects, and Criticality Analysis) to help decide which, how many, and where spares need to be stocked. Once the equipment has a field history, of course, the actual failure rates and spares usage drive the numbers.

--
Rich Webb     Norfolk, VA

Given the FIT numbers, you can do a reasonable product by hand in a half hour maybe.

John