SH7059 Datasheet, PDF (913/1042 Pages) Renesas Technology Corp – 32-Bit RISC Microcomputer

Section 28 Reliability
28. Reliability
28.1 Reliability
A failure rate curve represents an index of the reliability of a semiconductor device. The failure rate curve traces a bathtub
shape over the course of time, as is shown in figure 28.1. The curve is divided into three periods according to the type of
failure phenomena: an initial failure period, a random failure period (functional lifetime), and a wear-out failure period.
Initial failures, which occur during the initial failure period, are caused by contamination with foreign matter and localized
chemical pollution; these can be eliminated by screening. Wear-out failures in the final period are caused by the
deterioration of materials that make up semiconductor devices during long periods of usage. Random failures, which occur
during the random failure period, are thought to occur in cases where a device with a minor failure is not removed by
screening, and so is shipped, and then fails during the customer's production process or in the field, and in cases where a
failure which should normally not have occurred until the wear-out period occurs earlier because of variations in
production. Therefore, the reliability of semiconductor device is secured by appropriate screening to reduce the presence
of initial failures and high reliability design to prevent the occurrence of wear-out failures. The reliability of a product is
confirmed by producing a large quantity of prototypes for checking of the initial failure rate and executing accelerated life
testing to identify the wear-out failure time in a realistic environment.
Initial failure
period
Functional lifetime
Wear-out failure
period
Random failure period
Time
Figure 28.1 Failure Rate Curve (Bathtub Curve)
The reliability of products is estimated on the assumption that products developed for the automotive sector are used in a
tougher environment than products for the consumer and industrial sectors. The representative failure phenomena of
semiconductor devices, such as the dielectric breakdown of oxide films and electromigration in wiring, constitute wear-out
failures. The stress factors in such failures are the voltage, current, and temperature applied to devices while they are in
use. Since the temperature range for the guaranteed operation of products for use in automobiles is conventionally −40°C
to 85°C, their reliability in terms of the above failure phenomena has to be confirmed by accelerated life testing at all
temperatures in this range. Operation at temperatures in excess of 85°C leads to failure within a short time, since high
temperatures induce failures in semiconductor devices.
Rev.3.00 Mar. 12, 2008 Page 823 of 948
REJ09B0177-0300