Bathtub curve
Beginner
The shape of failure rate over a product’s life: many early failures, then a long quiet period, then rising failures as parts wear out.
Novice
Failure rate plotted against age, in three parts: early failures (high and falling), useful life (low and roughly constant) and wear-out (rising).
Expert
Burn-in and screening attack the first region; design rules for electromigration, oxide breakdown and transistor aging push the third beyond the product’s lifetime. FIT numbers describe the middle.