MTTF (Mean Time to Failure) calculation help needed - Opto-electronic device

[spamman - 2009]

All: We are designing an opto-electronic device and will be using software to import all the reliability data for all components to calculate an MTTF for our customer. Now they would like us to also calculate the "required" MTTF based on the following data:

12 year product life
Usage of 2080 hours per year
less than or equal to 1% product return rate year over year.

How can I back that into an estimated MTTF?

Thanks: Ed

[Tim Folkerts]

If you assume an exponential failure model, then the number of units that are still good = the reliability R, would be

R = exp[-lambda t] = exp[-t/MTTF]

Take the ln of both sides and do a little algebra

ln(R) = - t/MTTF

MTTF = -t/ln(R)

So of you want <= 1% failure each year, you need R >= 0.99, so

MTTF = -2080 hr/ln(0.99) = 207000 hr

• If you wanted 1% failure over the entire 12 yr (not 1% each year), then MTTF = -(2080*12)/ln(0.99)
• The the failures do not follow an exponential model, then you would need to analyze it for the specific model.

Tim

[spamman - 2009]

Tim:

Thank you for the quick reply. I believe the exponential model is correct since these are electronic devices.

I am trying to wrap my arms around the 12yr life that the customer is expecting and how that affects the calculation. If they require a maximum failure rate of 1% each year, I am not sure if it is appropriate to assume the 1% for the entire 12 year span.

I think they are looking at the 12 year mark as to where the failure rate will begin to climb again and thus need to be replaced.

Do we assume the 12 year mark is simply for the customer to manage end of life for that device or does it impact the calculation?

Thanks: Ed

[Tim Folkerts]

If you truly accept the exponential model for failures, then it doesn't matter how long you wait. If 1% fail the first year, then 1% of the remaining units will fail the second, and 1% of the units remaining after 2 years will fail the third year, etc.

[spamman - 2009]

Thanks again. We just had that debate over here as well about that. Our customer had stated 12 years but we are thinking they use another model due to the other devices in their image chain that can degrade over time. Ours fits the model used. Do you think they use another model that takes into account an overall product lifetime? I really appreciate our discussion. Thanks: Ed