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 ArrheniusWeibull 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% twosided 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.
