Voltage Step-Stress

ALTA Example 1 - Voltage Step-Stress
Software Used: ALTA 7 PRO

Background
An electronic component is subjected to a voltage stress, starting at 2V (use stress level) and increasing to 7V in stepwise increments as shown in Figure 1. The following steps, in hours, are used to apply stress to the products under test: 0 to 250, 2V; 250 to 350, 3V; 350 to 370, 4V; 370 to 380, 5V; 380 to 390, 6V; and 390 to 400, 7V.

The objective of this test is to determine the B(10) life and the mean life (often called "mean time to failure, "MTTF or MTBF) of these components at the normal use stress level of 2V.

In this experiment, the overall test time is 385 hr. If the test were performed at use conditions, one would expect the test duration to be approximately 1700 hr 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 times-to-failure are observed in the test, in hours: 280, 310, 330, 352, 360, 366, 371, 374, 378, 381 and 385.

Analysis
Step 1: Using ALTA 7 PRO, the analyst begins by creating a new project with a Standard Folio for ungrouped times-to-failure data with voltage as the stress type and setting the use stress level as 2V, as shown in Figure 2.

Step 2: After creating the Data Sheet, the analyst adds a new Stress Profile to the project, defines the Stress Profile and renames it as "Voltage Step."

In a Stress Profile, you can define a time-varying stress 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 displayed in Figure 3.

Step 3: The analyst selects the cumulative damage life-stress model and the Weibull distribution, then uses the Stress Transformation window to specify Logarithmic (Power LSR) as the transformation to be applied to the Voltage stress, as shown in Figure 4.

Step 4: Figure 5 displays the ALTA Standard Folio with the analysis options selected (cumulative damage life-stress model and Weibull for the underlying life distribution) and the times-to-failure data entered. Notice that the Voltage Step Stress Profile has been assigned to each data point. Figure 5A displays the method for assigning the appropriate time-dependent stress profile to a data point in ALTA. After entering the data and selecting the appropriate analysis options, the analyst clicks Calculate to estimate the parameters. It is that simple! At this point, the analysis is complete. Multiple plots and reports are available to ascertain the adequacy of fit as well as visualize the results, and compute the required values.

Step 5: Figure 6 displays the B(10) life and mean life at the 2V use stress level, calculated with ALTA's Quick Calculation Pad (QCP). ALTA contains many other tools for obtaining results and reports for your analyses. Figure 6A and Figure 6B display a use level probability plot and a Cox-Snell Residuals plot for the analysis.