What is the "bathtub" curve?
In
the 1950’s, a group known as AGREE (Advisory Group for the Reliability of
Electronic Equipment) discovered that the failure rate of electronic equipment
had a pattern similar to the death rate of people in a closed system.
Specifically, they noted that the failure rate of electronic components
and systems follow the classical “bathtub” curve.
This curve is shown below and has three distinctive phases:
1.
An “infant mortality” early life phase characterized by a decreasing
failure rate (Phase 1). Failure
occurrence during this period is not
random in time but rather the result of substandard components with gross
defects and the lack of adequate controls in the manufacturing process. Parts fail at a high but decreasing rate.
2.
A “useful life” period where electronics have a relatively constant
failure rate caused by randomly occurring
defects and stresses (Phase 2). This
corresponds to a normal wear and tear period where failures are caused by
unexpected and sudden over stress conditions.
Most reliability analyses pertaining to electronic systems are concerned
with lowering the failure frequency (i.e., lconst shown in the Figure) during this
period.
3.
A “wear out” period where the failure rate increases due to critical
parts wearing out (Phase 3). As
they wear out, it takes less stress to cause failure and the overall system
failure rate increases, accordingly failures do
not occur randomly in time.
The
failure rate is represented by the height of the curve (lconst
shown in the figure)
and is not related to the length of the curve (i.e., longevity).
It is therefore possible to have a long or short useful life period for a
given failure rate.
Electronic
systems reliability engineering theory is usually most concerned with the height
of the failure rate curve during the useful life of a system life (i.e., the
Phase 2 portion of the curve). Many
studies have shown that the height of the curve (magnitude of system failure
rate) is directly proportional to applied stress.
In the general design of electronic systems, the stresses which are most
influential to reliability include electrical (voltage and current), thermal,
vibration, and humidity. Every
effort is made during the design process to mitigate these stresses through
steps such as device derating, good thermal design, dampening vibration and
hermetic sealing. Derating is the
practice of operating devices significantly below their electrical and thermal
ratings to reduce the probability that marginal components will fail due to
transient over stress conditions during the useful life of a system.
For
more Information:
Bazovsky,
I. Reliability Theory and Practice, Prentice Hall (1961). | 
REPERTOIRE
Bathtub Curve
