MTBF & Useful Life

In my experience the following is a good way to look at this:

After initial startup and burn in of equipment the MBTF should be a good indicator of when you will begin to see a rise in the rate of failures. The rate of rise in failures may vary widely with the type of equipment it is. PLCs and such tend to operate well beyond their published MBTFs but LCD monitors and bulbs etc.. may fail fairly closely together near the MBTF. We recently experienced this in our plant with TFT touch panels. The accuracy of the MBTF depends on how well the manufacturer performed the analysis. Siemens and the other large manufacturers usually have accurate numbers, but some of the other cheaper or smaller companies MBTF’s may not be very accurate. Bottom line is get to know your manufacturers and it will very soon become apparent who’s numbers can be trusted and who’s can’t.

To be safe you could use an arbitrary multiple of the MBTF like 1.25 to give you an estimate of useful life. After this you will probably experience failures (device type and manufacturer depending) at a significantly increasing rate.

If anyone works in the Siemens QA department they may be able to add in some more exact numbers.