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Acceleration factor and failure rate (FIT) calculator

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KEY POINTS

The acceleration factor tells you how many hours of use one test hour represents; for temperature it follows the Arrhenius equation AF = exp[Ea/k · (1/Tu − 1/Ts)]. With Ea = 0.7 eV, a 125 °C test and 55 °C use, AF is about 78, so 1,000 hours of HTOL equals about nine years of use. Even with zero failures, the upper confidence bound on the failure rate is not zero: 231 units × 1,000 hours gives about 50 FIT at 60% confidence.

Acceleration factor

Acceleration model

Activation energy Ea (eV)Test temperature Ts (°C)Use temperature Tu (°C)

Acceleration factor AF77.66×

Use time equivalent to 1,000 test hours77,660h

(in years)8.9years

Defaults are commonly cited reference values (Ea = 0.7 eV for HTOL; n = 2.66 and Ea = 0.79 eV for the Peck model; m = 2 to 2.5 for solder joint temperature cycling; n = 2 and Ea = 0.8 to 1.0 eV for copper interconnect EM). Real Ea depends on the failure mechanism and varies widely, for example about 0.3 to 0.7 eV for time-dependent oxide breakdown and 0.5 to 0.7 eV for aluminum interconnect EM.

Failure rate (FIT) and MTTF

Units tested N (units)Test time t (h)Failures r (units)Confidence level (%)

Failure rate51.08FIT

MTTF2,230years

χ² (60%, 2 degrees of freedom)1.833

Equivalent device hours N·t·AF1.794×10⁷

1 FIT is one failure per 10⁹ device hours. Even with zero failures, the upper confidence bound on the failure rate is not zero; at 60% confidence, χ² = 1.833 is used. The standard HTOL in JEDEC JESD47 (125 °C, 1,000 hours, 3 lots × 77 units = 231 units, zero failures) matches this calculator's defaults.

Acceleration factor by activation energy (temperature term only)

Arrhenius acceleration for test 125 °C / use 55 °C and 1,000 test hours.
Ea	Acceleration factor	Equivalent use time	In years
0.3 eV	6.46	6,460 h	0.7 years
0.5 eV	22.4	22,400 h	2.6 years
0.7 eV	77.7	77,700 h	8.9 years
0.9 eV	269	2.69×10⁵ h	30.7 years
1.1 eV	934	9.34×10⁵ h	106.6 years

The idea behind accelerated testing

Semiconductors usually last more than ten years, far too long to test in real time before launch. Tests therefore apply stresses stronger than use conditions (temperature, humidity, voltage, current density, temperature swing) to speed up degradation without changing the failure mechanism. The acceleration factor expresses how many hours of use one test hour represents.

Temperature acceleration follows the Arrhenius equation from reaction kinetics: failures with a higher Ea are more temperature-sensitive and have larger acceleration factors. Overestimating Ea makes lifetime estimates optimistic, so choosing Ea is the most debated part of a test plan.

FAQ

What is 1 FIT?

One failure per 10⁹ device hours. A failure rate of 50 FIT corresponds to an MTTF of about 2,300 years for a single device.

Which activation energy should I use?

It depends on the failure mechanism. 0.7 eV is a common default for HTOL, while the calculator's notes give about 0.3 to 0.7 eV for time-dependent oxide breakdown and 0.8 to 1.0 eV for copper electromigration.

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Last updated: October 23, 2026