Extension of the Duane Plotting Technique

A Duane plot is a straight line relating the log of cumulative failure rate (or of cumulative MTBF) to the log of cumulative equipment operating hours in a series of equipment tests. The straight line provides an easy translation from the cumulative line to a line of instantaneous failure rate (or to instantaneous MTFB). This technique has also been applied to the early failure rates of operating integrated circuits (ICs); the cumulative plot has the same usefulness in data smoothing, and the same relationship to instantaneous failure rate applies. The cumulative failure rate plot produces a curved line instead of a straight line. The Duane relationship applies at any time point, however, and depends only on the slope of the line at that time. For either application, the cumulative data should not be forced to fit a straight line; such forcing causes the early reliability history to be lost, and the latest history may be improperly determined from too few points. This is not just of historical interest; it concerns whether reliability is still improving usefully, or whether a point of limited return is being approached. An additional point of concern is indicated by the example of rapidly reducing failure rates in early life of semiconductor devices. For equipments using many of these devices, the Duane plot assumption of a constant failure rate during each equipment test period is probably not true.