Calculation Methodology

Thermal acceleration for semiconductor failure mechanisms is commonly determined using the Arrhenius equation:

A_ft = exp { E_a / k ( 1 / T_u - 1 / T_s )}

Where: E_a is the activation energy in ev. (0.70)
K is the Boltzmann's constant (8.617 x 10^-5)
T_u and T_s are the use and stress temperatures respectively, expressed in °K

For Volterra products, the HTOL stress test is typically run at an ambient temperature of 125°C. The junction temperature is estimated by measuring the power dissipated by the component during stress and the thermal characteristics of the package. The use (junction) temperature is assumed to be 55°C.

Furthermore the HTOL is performed at a supply voltage = 1.1 x V_cc, with a typical clock frequency of 1MHz. No voltage acceleration is assumed (A_fv = 1).

The long term failure rate (in FIT) is calculated assuming a Chi-Squared distribution at 60% confidence level, as follows:

HTOL Failure Rate [FIT] = 10 ⁹ * Chi ² ( 2 n + 2 ) / ( 2 * A_f * #Device-hours )

Where: n is the number of failures
A_f is the total acceleration = A_ft x A_fv

The Early Failure Rate (EFR) in Defective Parts per Million, based on 48 hours of HTOL stress, is calculated as follows:

Early Failure Rate [DPM] = 10 ⁶ * Chi ² ( 2 n + 2 ) / ( 2 * #Device )