Background
In many cases, the life of a product is a function of stress and some
other engineering variable, like materials, vendors or operation type.
For this type of product, ALTA 7 PRO provides the general log-linear
life-stress relationship, which allows you to analyze up to eight stress
types and specify an underlying relationship for each stress.
Experiment and Data
A sample of electronic components are subjected to a
quantitative accelerated life test in which three stress types are
applied to the units. The stress types include temperature, voltage and
a third indicator variable to describe whether the units are operated
continuously or turned on and off. The stress profile for this test is
presented in Table 1 and the time-to-failure and time-to-suspension data
are presented in Table 2. The normal use stress levels are 328K for
temperature and 10V for voltage.
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Table 1 Stress Profile Summary
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| Table 2 Failure and Suspension Data
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Analysis
The
analysis is performed in an ALTA Standard Folio for grouped
times-to-failure with suspensions data with three stress columns for
temperature, voltage and operation type and a subset ID column. The
operation type is treated as an indicator variable, with the discrete
values of 0 and 1 to represent on/off and continuous operation,
respectively. The stress profile identifier is entered in ALTA's
multi-purpose Subset ID column, which is renamed to Stress Profile.
The
general log-linear life-stress model is used and the Weibull distribution is used as the underlying life distribution for
the data set. In the Stress Tranformation window, the transformation relationship
is specified for each
stress type. Temperature follows an Arrhenius model, voltage follows
a power model and no transformation is performed on the operation
type.
Figure 1 shows the ALTA Data Folio with data entered and parameters
calculated.
Once
the parameters are estimated, a variety of plots and results can be
obtained.
The
Weibull probability plot for the data is presented in
Figure
2. This plot can be used to examine the choice of an underlying
life distribution and the assumption of a common slope (shape
parameter) at all stress levels. The linearity of the data and the
fact that the data for each stress level appears parallel reinforce
the assumptions made.
Life
vs. stress plots can be very useful in assessing the effect of each
stress on a product's failure. In this case, since the life is a
function of three stresses, three different life vs. stress plots are
available. These plots are created by holding two of the stresses
constant at the desired use level and varying the remaining stress.
Figure
3 displays the life vs. stress plot for temperature.
Figure
4 displays the life vs. stress plot for voltage.
The
effects of the two different operation types on life can be observed
in Figure 5. It can be seen that the on/off
cycling has a greater effect on the life of the product in terms of
accelerating failure than the continuous operation. In other words, a
higher reliability can be achieved by running the product
continuously.
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ALTA
7 Case Studies
Version
7
