Reliability HotWire

Reliability HotWire

Issue 134, April 2012

Reliability Basics

Using Extrapolated Intervals in ALTA Degradation Analysis

In previous versions of ALTA, the degradation analysis results (i.e., extrapolated failure times) were available as point data, which do not fully reflect the variability of the estimated parameters. In Version 8, you have the option to extrapolate the failure times as interval censored data, which allows you to capture the uncertainty that comes with having only a few degradation measurements for each test unit. This article provides an example on how to use this new feature.

Example

Consider a device that has a critical parameter (capacitance) that degrades linearly over time, where the rate of degradation varies with operating temperature. A device failure would occur when the capacitance value changes by 15% or more. The normal operating temperature of the device is 337 K. 10 units were tested under two accelerated temperatures: 353 K and 373 K. The percent change in capacitance was recorded for each unit at six inspection intervals. The degradation data were recorded in an ALTA degradation folio, as shown next.

The following figure shows the Main page of the control panel and the analysis settings.

The Degradation Model area allows you to model how the capacitance degrades over time at each accelerated temperature. The Critical Degradation value is set to 15, which means that failures would occur when the degradation value has dropped to 85% of the original capacitance. The selected degradation model (in this case, the Linear model) extrapolates the time when each test unit is expected to reach the critical degradation value under the accelerated stress condition.  

The ALTA Model area allows you to analyze the failure times that have been extrapolated in order to quantify the life of the device under normal operating conditions. In this example, the model is set to an Arrhenius-Weibull model and the normal use stress is set to 337 K. In addition, the Use extrapolated intervals check box is selected to estimate the failure times as interval censored data.

To view the results of the degradation analysis, click the button in the Degradation Results area. The "Degradation Fit Results" sheet shows the calculated degradation parameters and their standard deviations. The “Extrapolated Failure/Suspension Time” sheet shows the extrapolated failure time intervals.


Degradation Fit Results

 


Extrapolated Failure/Suspension Times

The following figure compares the use level probability plots of the degradation analysis with the extrapolated intervals and without the extrapolated intervals (with 90% two-sided confidence bounds). The width of each interval is affected by the number of measurements for each device, such that an increase in the number of measurements will result in narrower intervals.


Degradation Analysis with and without Extrapolated Intervals

Note that although Version 8 now automatically performs the accelerated life testing data analysis directly within the same folio as the degradation analysis, you still have the option to transfer the extrapolated failure times to an ALTA standard folio and continue the analysis from there, if desired. To do this, choose Degradation > Transfer Life Data > Transfer Life Data to New Folio.

Conclusion

This article uses an example to illustrate the new “Use extrapolated Interval” feature available in ALTA 8. The software uses the variance of the parameters and the 90% confidence bounds to estimate the time intervals in which each unit is expected to fail. This allows you to take into account the variability of the parameters due to sample size.