ALTA Example 1 - Voltage Step-Stress
An electronic component is subjected to a voltage stress, starting at 2 V (use stress level) and increasing to 7 V in stepwise increments. The following steps (in hours) are used to apply stress to the products under test: 0 to 250, 2 V; 250 to 350, 3 V; 350 to 370, 4 V; 370 to 380, 5 V; 380 to 390, 6 V; and 390 to 400, 7 V. These steps are shown next in a Stress vs. Time plot.
The objective of this test is to determine the B10 life and the mean life (often called "mean time to failure," or MTTF) of these components at the normal use stress level of 2 V.
In this experiment, the overall test time is 385 hours. If the test were performed at use conditions, one would expect the test duration to be approximately 1,700 hours if the test were run until all units failed.
Experiment and Data
Eleven units are available for the test. All eleven units are tested using the same stress profile. Units that fail are removed from the test and their total time on test recorded. The following failure times (in hours) are observed in the test: 280, 310, 330, 352, 360, 366, 371, 374, 378, 381 and 385.
Step 1: Using ALTA PRO, create a new project with a standard folio for ungrouped times-to-failure data, with voltage as the stress type and a use stress level of 2 V, as shown next.
Step 2: After creating the data sheet, add a new stress profile to the project by choosing Insert > Tools > ALTA Stress Profile. Rename the stress profile folio to "Voltage Step."
In a stress profile, a time-dependent stress is defined in segments. The stress applied during these segments can be a constant value (as is the case in this step example) or defined as a function of a time variable (t). The stress profile for this analysis is shown next.
Click the Validate Stress Profile icon on the control panel to confirm that the segments are continuous.
Step 3: Return to the standard folio. In the Model area of the control panel, choose Cumulative Damage > CD- Weibull, then click the Stress Transformation link and apply the logarithmic transformation, as shown next.
Step 4: Enter the times-to-failure data, then assign the "Voltage Step" stress profile to each data point by clicking a cell in the Voltage column and selecting the profile from the drop-down list that appears. Click Calculate to estimate the parameters, as shown next.
Multiple plots and reports are now available to ascertain the adequacy of fit, as well as visualize the results and compute the required values.
Step 5: The following figures show the B10 life and mean life at the 2 V use stress level, calculated with ALTA's Quick Calculation Pad (QCP). ALTA contains many other tools for obtaining results and reports for your analyses, including a use level probability plot and a Cox-Snell residuals plot.