Stress Profile Example

A reliability engineer is asked to determine, with a confidence level of 90%, whether an electronic component has a B10 life of at least 350 hours under ordinary conditions.

To simplify the analysis, the engineer will assume that the ordinary stress conditions for the component consist of a constant stress level of 2 V. However, because the ordinary stress conditions are known to involve escalating stress values, the component will be subjected to a time-dependent voltage stress during testing. The following table describes the stress values that will be applied during the test.

Starting Time (Hr)	Ending Time (Hr)	Stress Level (V)
0	250	2
250	350	3
350	370	4
370	380	5
380	390	6
390	400	7

All eleven units were tested to failure using the same stress profile. The following failure times were observed in the test.

Failure Time (Hr)	Stress Level
280	Time-dependent
310	Time-dependent
330	Time-dependent
352	Time-dependent
360	Time-dependent
366	Time-dependent
371	Time-dependent
374	Time-dependent
378	Time-dependent
381	Time-dependent
385	Time-dependent

Create a Life-Stress Data Folio and Stress Profile

1. Choose Home > Insert > Life-Stress Data. Since all the data are exact failure times, clear all 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.

2. On the second page of the setup window, select Voltage as the stress types. Set the use stress level to 2 with V as the unit. Click OK to close the setup window and create the new life-stress data folio.

3. Enter the failure times into the data sheet.

4. On the control panel of the data sheet, select CD-Weibull. Click Stress Transformation and apply the Logarithmic transformation to the stress.

5. To create a stress profile:

1. Click the icon on the control panel.

2. In the Profile window, rename the profile to "Voltage Step." Enter the segment end times and stress values into the data sheet, as shown next.
   
   

3. On the control panel, select Continue with last value. Click the Validate Stress Profile icon to validate the information in your new profile. Finally, click the Plot icon to view a stress vs. time plot of the profile.

4. Click OK to save the profile with the project and close the window.

6. To assign the stress profile to a failure time, double-click the corresponding cell in the Voltage column of the data sheet, then click the drop-down arrow and choose Voltage Step from the list. After you have filled out the data sheet, it should appear as shown next.

Analyze the Data Set

1. Click the Calculate icon on the folio's control panel. The parameters of the model will be automatically computed and displayed in the Analysis Summary area of the control panel.

2. To estimate the B10 life of the device, choose Life-Stress Data > Analysis > Quick Calculation Pad or click the icon on the control panel.

3. In the QCP, choose to calculate the BX% Life with Lower One-Sided confidence bounds. Select Hour for the time units and then make the following inputs:

• Stress = 2

• BX% Life At = 10

• Confidence Level = 0.9

4. Click Calculate to display the results. The results show that the component's estimated B10 life is 507.9438 hours. The lower one-sided 90% confidence bound is shown to be 300.4821 hours. Thus, the test's results do not show with 90% confidence that the component's B10 life is at least 350 hours.