The data generated at the accelerated testing conditions of HTOL (1,000 hrs at 125C or equivalent) is translated to lifetimes at the end user operating conditions (10 years at 55C), by using the Arrhenius equation with an activation energy of 0.7eV. The Chi Squared statistical distribution is used to calculate the confidence intervals (60 and 90%) on the failure rate data based on the number of units HTOL tested.

 
 
Failure Rate = X²/ (2 . N . H. At)

Where:
X² = Chi Squared Distribution, value depends on number of failures and confidence interval
N = Number of units HTOL tested
H = Duration of HTOL testing
At = Acceleration Factor from Test to Use Conditions (Ea = 0.7eV and Tuse = 55C)
e.g. 150C to 55C, At = 258
135C to 55C, At = 128
125C to 55C, At = 77
85C to 55C, At = 8


Then:
Equivalent Device Hours = N. H. At
MTTF = 1 / (Failure Rate)
FIT = (Failure Rate) x 1e9


This calculation will yield artificially high FIT rates when the sample sizes are small.