sccook1@cox.net wrote:
> I have not been following this thread very closely,
Someone had a disk failure, and asked the question "how do they get
those huge MTBS's" or similar.
I replied they (MTBF's which seem to vary between 500,00 and 1.4 million
hours) assume you replace the device at the end of a service life, which
is typically 5 years. So it basically tells you the mean time you will
have to wait for a failure if you throw lots of usable disks away.
> but did anyone bring up the aapproach whereby devices are subjected to added
> stress; e.g., high temperature "Burn In" aka "Accelerated Life Test" as part
> of a regime of establishing reliability for a device.
That was not specifically addressed. I personally was the first person
to reply to the question and just assumed anyone understanding MTBF
would know that accelerated tests, and or extrapolation from shorter
tests would be necessary in the case of devices having a lifetime beyond
where it is practical to make measurements.
>
> In the microelectronics world, they uses MIL-STD-883. During the process of
> "Qualification" the device is subjected to a variety of tests and
> environments to demonstrate deisgn adaquacy. "1000 Hour Life Test" is a
> common part of qualification and involves an extended burn-in for 1000 hours
> at 125 deg C (typical). The hardware is tested every 250 hours or so.
That does not seem unreasonable. You know there is going to be diffusion
of the metallic conductors to the silicon, the rate of which will
increase with temperature. I am sure other failure mechanisms will often
have predicable.
> I believe there is some information in the literature that equates Life Test
> with MTBF... but I can't remember what it is.
>
> There, I feel better now...
>
> Steve (WG7K)
I am however puzzled by the comments of WB2WIK/6 stevek@jmr.com when he
states this:
> MTBF is a funny statistical tool: Per even
> MIL-HDBK-217 and Bellcore standard definitions,
> MTBF may be determined in any way that
> arithmetically accomplishes the calculaton of a mean.
> Including this one: 1 million hours MTBF
> may be described as having 1 million
> devices in operation for one hour
> each, with failure rate of one
> device or less.
> If 100% of the population all failed
> after 61 minutes, this lot would still
> have an MTBF of 1 million hours by that definition.
I can't for the life of me understand how if 100% of devices in a sample
of 1,000,000 all failed within 61 mintes can you draw the conclusion
that the MTBF is 1,000,000 hours. I would have thought that is
statistically valid data to prove the MTBF is <= 61 minutes, rather than
1,000,000 hours.
Even if 100% work after 60 minutes, the confidence that you could put on
any one device lasting 1,000,000 hours is negligsable.
I have not however read the mil spec - I only looked at the Seagate
(might be another disk manufacturer) and determined the MTBF, which
might suggest disks last on average for 50-150 years is an incorrect
interpretation.
You know the old proverb - there are lied, damm lies and then there are
statistics!
--
David Kirkby,
G8WRB
Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/
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