The Bathtub Curve and Product Failure Behavior
Part One - The Bathtub Curve, Infant Mortality and Burn-in by Dennis J. Wilkins
Retired Hewlett-Packard Senior Reliability Specialist, currently a ReliaSoft Reliability Field Consultant
This paper is adapted with permission from work done while at Hewlett-Packard. Reliability specialists often describe the lifetime of a population of products using a graphical representation called the bathtub curve. The bathtub curve consists of three periods: an infant mortality period with a decreasing failure rate followed by a normal life period (also known as "useful life") with a low, relatively constant failure rate and concluding with a wear-out period that exhibits an increasing failure rate. This article provides an overview of how infant mortality, normal life failures and wear-out modes combine to create the overall product failure distributions. It describes methods to reduce failures at each stage of product life and shows how burn-in, when appropriate, can significantly reduce operational failure rate by screening out infant mortality failures. The material will be presented in two parts. Part One (presented in this issue) introduces the bathtub curve and covers infant mortality and burn-in. Part Two (presented in next month's HotWire) will address the remaining two periods of the bathtub curve: normal life failures and end of life wear-out.
Figure 1: The Bathtub Curve
The bathtub curve, displayed in Figure 1 above, does not depict the failure rate of a single item, but describes the relative failure rate of an entire population of products over time. Some individual units will fail relatively early (infant mortality failures), others (we hope most) will last until wear-out, and some will fail during the relatively long period typically called normal life. Failures during infant mortality are highly undesirable and are always caused by defects and blunders: material defects, design blunders, errors in assembly,...