# calculate MTBF

R

#### Raveninghorde

Jan 1, 1970
0
How do you calculate MTBF?

R

#### Rich Webb

Jan 1, 1970
0
How do you 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!

R

#### Raveninghorde

Jan 1, 1970
0
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.

R

#### Raveninghorde

Jan 1, 1970
0
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!

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.

R

#### Rich Webb

Jan 1, 1970
0
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.

You're new at this so, even if you're not doing a MIL-STD prediction,
<https://assist.daps.dla.mil/quicksearch/> (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.

R

#### Rich Webb

Jan 1, 1970
0
On Thu, 21 Jan 2010 12:43:20 -0500, Rich Webb

[snip]
You're new at this so, even if you're not doing a MIL-STD prediction,
<https://assist.daps.dla.mil/quicksearch/> (search on MIL-HDBK-217 in
the Document ID field) and skimming though sections 3 and 4. The
Bellcore process is similar.
[snip]

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

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.

A

#### Artemus

Jan 1, 1970
0
John Larkin said:
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

R

#### Raveninghorde

Jan 1, 1970
0
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.

You're new at this so, even if you're not doing a MIL-STD prediction,
<https://assist.daps.dla.mil/quicksearch/> (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.

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?

R

#### Raveninghorde

Jan 1, 1970
0
..as long as they PAY for the info and time to compile it.

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.

R

#### Rich Webb

Jan 1, 1970
0
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?

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.

J

#### JosephKK

Jan 1, 1970
0
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.

Basically it is the _statistical_ point where the root sum square failurerate
of the individual components becomes 50% probability.

R

#### Rich Webb

Jan 1, 1970
0
Basically it is the _statistical_ point where the root sum square failure rate
of the individual components becomes 50% probability.

Note that for the often used exponential distribution (applicable for a
constant failure rate), we will then have the reliability at time t as
R(t) = exp(-t/MTBF). So the probability that any one component actually
survives to its MTBF is only about 37%.

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