You are the project manager for a PCB that must function in an environment from -10° to 80°C. The PCB will experience one thermal cycle per day in this range in its application. It must function for 20 years. The customer requires less than a 5% failure rate. You want to perform an accelerated test to duplicate this experience in less than 2 months. Your thermal cycle chamber can cycle through -20° to 140° in 4 hrs, or 6 cycles in one day. So, in the field, the board will experience 365.25 days/year x 20 years = 7305 days, or cycle at one cycle per day. Your chamber can provide 6 cycles/day x 30 days/month x 2 months = 360 cycles in two months.
In the book Practical Reliability Engineering, author Patrick O’Connor proposes an acceleration factor (p. 334). Using this formula, we can calculate the acceleration factor to be 23.67. Therefore, 7305 field cycles will be 309 lab cycles. See Figure 1.
Figure 1. The calculation for the number of lab cycles to be equal to the number of field cycles.
The experiment is performed in the lab for 360 cycles on 100 samples. Of the 100 samples, 3 fail at thermal cycles below 309. These samples failed at 303, 305 and 307 cycles. Using confidence intervals on portions, we see that the 95% confidence interval is 6.4%, slightly higher than the 5% we had hoped.
Figure 2. The confidence interval on portions to calculate the 95% confidence limit of projected failures
We will need to perform root cause failure analysis to understand what causes the failures and take corrected action.