FIT and MTBF
1 FIT = 1 Failure/10^9 Device Hours (114,155 years).
The Mean Time Between Failures (MTBF), represents the average time you can expect a device to operate without failing.
So for example, the conversion between FIT (1489 FIT) and MTBF(?) is as follows:
1,489 FIT or an MTBF of 77 years (114,155 years/1,489 FIT) = 77 years MTBF
(Parts per million) PPM
Since the failure rate is assumed to be a constant, it is acceptable to express Early life failure rate (ELFR) in terms of FIT.
It is often desired to express the ELFR in PPM.
However, PPM is a measure of the cumulative fraction failing per device, whereas FIT is a measure of fraction failing per device-hour.
Thus, when ELFR is expressed in PPM, the early life period must also be specified.
ELFR(in ppm per tELF) = [tELF x 10^-3 x ELFR in FIT];
ELFR(in FIT) = [1/(tELF x 10^-3) x ppm];
Where early life period (tELF) is the specified early life period as defined by the user or the supplier.
For example 1 year = 356 days 24 h/day = 8,760 hours.
Early life failure rate (ELFR):
Early life failure rate (ELFR) measurement of a product is typically performed during product qualifications or as part of ongoing product reliability monitoring activities.
These tests measure reliability performance over the products first several months in the field.
For more information, please see JESD74A at: http://www.jedec.org/sites/default/files/docs/JESD74A.pdf