The failure rate is typically defined in FIT units. One FIT equals 1 failure per 1
billion device hours. For example, 5 failures expected out of 1 million components
operating for 1,000 hours have a failure rate of 5 FIT. The following is the failure
rate calculation method:

where:

x^{2} = Chi-squared value at a desired confidence level and (2f + 2)
degrees of freedom, where f is the number of failures.

The acceleration factor is calculated using the Arrhenius relationship:

where:

*E*
_{a} = Thermal activation energy
(0.7 eV is assumed and used in failure rate calculation except EPROM in which 0.58 eV is
used).

*A* = Acceleration factor

*k* = Boltzmann's constant, 8.617164 x 10^{-5} eV/K

*T*
_{J1} = Use junction temperature in Kelvin (K = °C + 273.16)

*T*
_{J2} = Stress junction temperature in Kelvin (K = °C + 273.16)