The failure rate equation used by the test data extrapolation method is:

Where:

Chi2(a,b) = Chi-square value at level a and b degrees of freedom

b = 2 * (Number of Failures + 1)

The value of a depends on the type of confidence bound.

For the lower bound:

a = Confidence Level / 100

For the upper bound:

a = 1 − (Confidence Level) / 100

The confidence level is specified in percentages. Hence, it is converted to a fraction or probability value by dividing it by 100.

AF = The acceleration factor, which equals:

E = The activation energy (eV).

K = Boltzmann’s constant (8.617342E-5eV/Degrees K).

T = The absolute temperature (Degrees K).

Toperating = 1 / (273.15 + temperature rise + case)

Tburn-in = 1 / (273.15 + burn-in temperature)

Chi2 is based on the confidence level. It is calculated using the Chi-square distribution. For more information, see Chi-Square Distribution.