Reliability HotWire: eMagazine for the Reliability Professional
Reliability HotWire

Issue 17, July 2002

Tool Tips

*How do I view a plot of Life vs. Stress, Acceleration Factor vs. Stress, or Standard Deviation vs. Stress for multiple stresses at the same time in ALTA 6?

This can be accomplished easily using the Side-by-Side Plots utility in ALTA 6. The Acceleration Factor vs. Stress plot will be used to illustrate the process and you can then apply the same process to generate the Life vs. Stress and Standard Deviation vs. Stress plots.

Although the Side-by-Side Plots utility can be used with data sets that have only one stress, the question deals with two or more stresses and therefore the process requires a data set that contains two or more stresses used in the analysis. Let's assume an accelerated test was conducted using temperature and voltage and the data set was analyzed using ALTA 6. The Use Level Probability plot is shown below.

Use Level Probability Plot

Open the Side-by-Side Plots utility by clicking on its icon, which is located on the panel to the right of the plot,

Side-by-Side Plots icon

or by selecting Side-by-Side Plots from the Plot Options menu. The Side-by-Side Plots window will load and automatically generate plots based on the previous selections. For this example, make sure that Vary Plots is selected and then only the AF vs. Stress option is selected. The Plot All Stresses option will become enabled (this option will only become enabled if Life vs. Stress, Std vs. Stress, or AF vs. Stress is the only plot option selected). Next, simply click Refresh Plots and the Side-by-Side Plots utility will generate the correct number of plots. The figure below shows the options that have been selected in the panel on the right side of the plot and the plots that are created. In this example, two Acceleration Factor plots were generated because two stresses were used in the analysis. The top plot varies temperature (first stress) and the bottom plot varies voltage (second stress).

AF vs. Stress Plot for All Stresses

*In ALTA 6 PRO, what do the Time Varying Stress Calculation options on the Calculations tab of the User Setup mean?

The Time Varying Stress Calculations options are available in ALTA 6 PRO but not in ALTA 6 Standard because they apply to Cumulative Damage calculations, which are only available in the professional version of ALTA. The next figure displays ALTA 6 PRO's User Setup window with these options displayed at the bottom of the Calculations page.

ALTA 6 PRO User Setup, Calculations Page

To understand the impact of these settings, consider an accelerated test where a step-stress is being applied and one of the units under test fails during a step, as shown next.

Plot Indicating a Failure Occurring During a Step

It is assumed that the transition from a stress of 120 to 180 occurs instantaneously. So at the time of failure, is the stress equal to 120 or 180 for the purposes of the analysis? If Implement Stress at Beginning of Step is selected, then 120 will be used during the estimation of the parameters. However, if Implement Stress at End of Step is selected, then 180 will be used. The option selected will determine how this scenario is handled by ALTA 6 PRO when estimating the parameters.


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