When the failure rate is shown on the vertical axis and time on the horizontal axis, the curve of the failure rate looks like a bathtub. This is called as a bathtub curve.
The three failure stages are described below.
[Early failure stage]
This is the period where relatively early failures occur following the first operation of a semiconductor device, and is characterized by a gradual decline in the number of failures over time. This phenomenon is caused because products with latent failures cannot be completely eliminated during the sorting process. These latent failures reveal themselves in a short time after first use of the semiconductor device, due to temperature, voltage, and other stresses.
[Random failure stage]
Once the early failures have been eliminated, the failure rate stabilizes at an extremely low level, but latent failures may reveal themselves at random over a long time and thus their number does not decline to zero. Since failures occur at random, the failure rate is almost constant.
[Wear-out failure stage]
This is the period where the devices fail due to wear, fatigue, and so on, with the failure rate rising over time.