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These numbers are mainly used to calculate the probability a system will remain in working condition for a given time frame. They are often misinterpreted and misused.
These numbers are in no way a guarantee that the part will last for the stated amount of time. Obviously, the higher the MTBF, the higher chance a part will perform for a longer period of time at the given conditions. MTBF is not related to life expectancy of the operating life of the product and therefore has nothing to do with product lifetime.
For example, let us suppose a TDK capacitor has a MTBF number of 6,918 years (16.5 FIT), this does not mean that the capacitor will last for that long (obviously no one will be able to test the part for that amount of time). If we assume that there are 8,760 hours in a year (365 days x 24 hrs per day) and we want to find out what the probability the capacitor operating without failure for 10 years is, we can do the following math:
- 6,918 yrs x 8,760 hrs/yr = 60,601,680 hrs
- 10 yrs x 8,760 hrs/yr = 87,600 hrs
- 87,600 hrs/60,601,680 hrs = 0.0014455
- e-0.0014455 = 0.99856
Since probability is a percentage, this simply means that the part has a 99.9 percent chance of functioning correctly in the 10 year period.
It is obvious that 6,918 years is a ridiculously long time for a capacitor to fail. However, it is important to take into account that a capacitor does not function by itself. It is part of a circuit that performs some function for a larger system. Sometime, if a component in the circuit failed, the whole circuit will cease to operate. This will result in downtime for the system and render the product unavailable for use. To calculate the MTBF for the whole system, take the inverse of the sum of all the FIT figures. Let’s look at an example to get a clearer understanding of what a FIT number means. Let’s say that there are 70 capacitors in use for a particular circuit. All of these capacitors have a FIT number of 16.5 (same as previous example). To find the MTBF you would do the following calculations:
- MTBF = 1/(16.5 x 70) = 0.0008658008658
- 0.0008658008658 x 109 = 865,800 hours
For 70 capacitors in the circuit, the MTBF of all of them combine has dropped significantly. If we want to know the chance of the circuit still being operational after 10 years, it will result in only a 90.3% chance following the same calculation steps for the previous example. Also, notice that the MTBF is now more reasonable, 865,800 hours is roughly 98 years. Also consider that this is in terms of operating hours. So for something that runs 24/7, the average fail time would be 9.88 years. However, if it is used in a car, for example, then it would be much longer.
Assume that a car will operate 4 hours per day, 1460 hours per year; the MTBF will be 865,800/1460, or 593 years. The probability of the car not failing in 10 years would now be e-(10/593) = 0.983, or 98.3%. This is much better compared to 90.4%. Another way to used MTBF figure is in the calculation of availability of a system.