FIT means Failure In Time. It indicates, how many units fail within a certain timespan e.g. 10^9 hours.

1 FIT = 1 Failure in 10⁹ hours

As the timespan may vary, it is recommended to indicated it. (10⁹ hours is widely used)
Measuring the number of failures over time provides a failure rate : λ.

The Mean Time Between Failure (MTBF), or for components, the Mean Time To Failure (MTTF) is the distribution for a population of parts. MTTF is the reciprocal of the failure rate.

MTBF = 1 / λ

In a first step, you must collect all FIT's from every component in your circuit. This data mayst be delivered by the manufacturer, obtained by simulation or by own measurement.

It is good practice to do this with the spread-sheet of your choice. By that, you can calculate the sum of all FITs times their occurence.



And yes : Manufacturers don't like to publish their FITs. So there is some guess-work involved.

With the above, we can now calculate the Probability for Failure = F(t) and the Probability for Survivor = R(t).

R(t) = exp(-λ * t)

In our example this becomes :

R(5 years) = exp(-3328.3 * 10⁹(per hour) * 5 (years) * 365 (days/year) * 24 (hours/day)

= 0.8643 or 86.43 %.

And F(t) = 1 - R(t) becomes : F(5 years) = 0.1357 or 13.57 %

MTTF = 1 / λ

In our example this becomes :

MTTF = 1 * 10⁹ / 3328.3 = 300453.7 hours or 34.4 years.

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