FAQ (Technical Support)/Precautions

8. FAILURE RATE

8.1 CONCEPT OF FAILURE RATE

  The failure rate is a widely used indicator of reliability in semiconductors. It describes the rate of failure in a given time period. The failure rate changes over time, as is shown in Figure 8-1. The symbols (a to d) in Figure 8-1 are also noted in parentheses below.

Figure 8-1. Time-related Changes in Failure Rate

• Early failure stage: During this stage, failures occur at a high rate following the initial operation of semiconductor devices. Latent causes of failure that were not detected during previous screening tend to become apparent soon after the device is initially put into operation. Since these failures are mainly due to manufacturing defects or material defects, they occur very soon and thus the failure rate declines rapidly over time. Advances in manufacturing process control technologies have helped to reduce the failure rate during the early failure stage.
  In addition, screening has enabled failures that occur during the early failure stage (a) to be experienced as part of the manufacturing process, which has also helped to lower the failure rate once the manufactured devices begin operating.



• Random failure stage: During this stage, various failures that were not screened out occur with random frequency. The failure rate during this period remains fairly constant since failures that occur during this period are sporadic (random) and are due to complex factors, such as design quality and the use environment. Methods to suppress the failure rate (c) and to increase the life of semiconductors before they reach their wear-out failure stage, include building an extra reliability margin into the semiconductor's design and establishing a safety margin in the semiconductor's use conditions.



• Wear-out failure stage: During this stage, failures occur with increasingly frequency over time and are caused by age-related wear and fatigue. Such failures are often seen in products that have outlived their useful life. In devices whose components can be replaced to extend useful life (called repairable system), analyzing the start of their wear-out failure stage as the sum of (a) and (b) and the manner in which wear-out failures occur (d) can make it possible to implement preventive maintenance whereby certain components are replaced before they fail.

8.2 CALCULATING THE FAILURE RATE

  The semiconductor failure rate (l) is determined from the reliability of a device inherent in its design and the conditions under which it is used by the customer. For the former parameter, the basic failure rate is Ea = 0.7, lb (fit) = 10. For the latter parameter, the conditions include the temperature at which the device is used, and for transistors and FETs, the applied voltage. (All other conditions are those that comply with the recommended condition specifications in data sheets and data books).

1) Calculation procedure

2) Calculation formula

The device's failure rate (l) is determined based on the device-specific basic failure rate (lb) and the use conditions. After considering the hypothetical use environment, the failure rate can be calculated via the following steps.