FIT (failures in time)
Beginner
A way to count rare failures: one FIT means one failure in a billion hours of chips running.
Novice
Failures per billion () device-hours. A part rated at 10 FIT is expected to fail once in 100 million hours of operation, so a fleet of 100,000 such parts would see about one failure every 1,000 hours, roughly six weeks.
Expert
Assumes a constant failure rate, the flat floor of the bathtub curve. A system’s FIT is the sum of its components’ FITs. Qualification data with zero failures gives only an upper bound, at a chosen statistical confidence.
Explained in After tapeout (Extras).
See also: Bathtub curve, HTOL (high temperature operating life).