ALTA Two-Stress Example

A team of reliability engineers is instructed to perform an accelerated life test and use the accelerated test data to extrapolate a product's use level failure behavior. They are asked to estimate, with 90% confidence, the time at which the product will have an unreliability of 10% (i.e., the B10 life).

The normal operating temperature for this product is 328 K, and the normal operating voltage is 2 V. It was decided to accelerate both stresses in the test. The following table shows the data from the test.

Time Failed (Hr)	Temperature (K)	Voltage (V)
620	348	3
632	348	3
658	348	3
822	348	3
216	378	3
246	378	3
332	378	3
400	378	3
380	378	5
416	378	5
460	378	5
596	378	5

Add a new ALTA standard folio by choosing Insert > Folios > ALTA Standard Folio. Since the data set contains only exact failure times, do not select any of the check boxes in the Options for the Times-to-Failure Data Type area. Then, select Hour (Hr) from the Units drop-down list and click Next.
On the second page of the setup window, select Temperature and Voltage as the stress types. Set the use level temperature to 328 with K as the unit, and set the use level voltage to 2 with V as the unit.

Click OK to close the setup window and view your new standard folio. In the data sheet of your new folio, select the check boxes inside the Temperature and Voltage column headers. Both stresses will now be used in the calculation for the selected model.

On the Main page of the folio’s control panel, select the TNT-Weibull model (i.e., the temperature-nonthermal life-stress relationship combined with the Weibull life distribution).
Enter the above data into the data sheet, then click Calculate. The parameters of the model will be automatically computed and displayed in the Analysis Summary area, as shown next.
To obtain the two-sided 90% confidence bounds on the product's B10 life, choose ALTA > Analysis > Quick Calculation Pad or click the icon on the control panel.

In the QCP, choose to calculate the BX% Life with Two-Sided Bounds. Select Hour for the time units and then make the following inputs:
- Stress = 328; 2
- BX% Life At = 10
- Confidence Level = 0.9

Click Calculate to display the results, as shown next.

The results show, with 90% confidence, that the time at which the product will have an unreliability of 10% is between 459.7681 and 853.1253 hours.

To view a plot of the unreliability vs. time at the use stress level, go to the Plot page of the control panel and select Unreliability vs. Time from the Plot Type drop-down list. The plot is shown next.