On July 1st, Toshiba Corporation's Semiconductor Company and Storage Products Company consolidated to form Semiconductor & Storage Products Company.This page describes reliability information of semiconductor products.

Constant Stress and Step Stress

[As of April, 2011]

There are two types of accelerated lifetime testing: constant stress and step stress. In a constant stress test, the time-dependent failure distribution of a test sample subjected to constant stress at several stress levels is observed. In a step stress test, stress is applied to a test sample gradually in stepped increments, and the step at which failure occurs is observed.
A typical constant stress test is the application of the constant stress of power or ambient temperature exceeding the maximum rating. Weibull distribution is often used to verify that the failure mode has not been changed by the test. The validity of the accelerated test is confirmed if the shape parameter m of the Weibull distribution remains unchanged by the accelerated stress.
Figure 1 shows Weibull plots when the power consumption of a silicon transistor is changed. It is evident from the figure that parameter m is constant regardless of the power consumption level.

This is [Figure 1 Weibull Distribution and Shape Parameter for Transistor Accelerated Lifetime Test].

Figure 1 Weibull Distribution and Shape Parameter for Transistor Accelerated Lifetime Test

This same result should occur in both constant tests and step tests.
Thus, a step test produces the failure data corresponding to at least one constant stress. If the failure mode of the previous step is the same, a step test can by used to determine the critical temperature for the component and to estimate its lifetime. Figure 2 shows an example.

This is [Figure 2 Failure Rate Estimation Step Stress].

Figure 2 Failure Rate Estimation Step Stress