I have on a few occasions calculated the MTBF using the FITS of all the components on a BOM and something like the telcordia standard in some spreadsheet tool I was given. We get a number say 1 Million hours, and everyone nods approvingly because 1 Million is a very big number.
I have trouble understanding the practical use of this number though. Doing some reading I understand this is supposed to be the average time between failures, but does not mean 1 unit will run for 1 million hours. If however I had 1 Million units I think it means I should expect a failure once an hour?
Can I then extrapolate that and say if I ran my 1 million units for a year or 8760 hours I should expect 8760 failures? Would it then make sense to say that's a 0.9% failure rate?
The final thing I don't quite get is MTTF vs MTBF. I understand that MTTF is when the unit can't or won't be repaired, but I don't really understand the difference. For electronic designs at least no one has ever wanted to talk about MTTF even though most of my products are throw away if they break.