Semiconductor glossary · Test and reliability

What Is Lifetime Extrapolation?

Lifetime extrapolation extends a model fitted to accelerated-test measurements toward the conditions of actual use. The result is an estimate outside the measured range, with assumptions that must be checked.

Extend the line beyond the measured range

The Renesas reliability handbook gives the Arrhenius lifetime relationship L＝A exp(Ea/kT). Taking its logarithm gives ln L＝ln A＋Ea/kT, a straight line in ln lifetime versus reciprocal absolute temperature.

Measure lifetime at accelerated-test temperatures, fit the line, and extend it toward the lower-temperature use condition. With 1/T on the horizontal axis, the slope is Ea/k. Ea is obtained by multiplying the slope by k; with 1/(kT) as the horizontal axis, the slope is Ea.

The diagram uses 150℃, 85℃ and 30℃ to distinguish test and use conditions. The 10³h, 10⁵h and 10⁷h labels are illustrative log-lifetime ticks, not certified lifetime predictions for a device.

A line fitted through test measurements is extended toward the use temperature. Lifetime ticks are illustrative, not certified device predictions.

A mechanism change can bend the line

The dominant failure mechanism can differ between the high-temperature test and actual use. The Arrhenius line can then change slope. Extending the high-temperature line through an unmeasured transition can predict a longer lifetime than the mechanism operating in use actually permits.

Check that the accelerated test reproduces the relevant mechanism and that the measured temperatures support the chosen model. A visually straight fitted segment does not establish what happens beyond that segment.

A mechanism change alters the slope. A straight-line extension can overestimate lifetime compared with a bent relationship.

Extrapolation distance affects uncertainty

Greater distance from measured conditions amplifies sensitivity to slope error and model assumptions. Two measured points give a slope; at least three points are needed to check whether those points support a straight line, although even three do not prove a model outside the tested range.

The handbook’s humidity-resistance example uses 30℃ for use and 85℃ for testing: a difference of 55℃, with an assumed activation energy of 1.0 eV. Convert temperatures to kelvin for reciprocal-temperature calculations; a Celsius temperature difference alone is not the acceleration factor.

Connect the estimate to application conditions

The ROHM EcoGaN paper discusses lifetime estimates from DHTOL and SALT accelerated tests and how to extend them to soft-switching applications. It considers switching-waveform simulations for an LLC resonant converter and a Totem-pole PFC operating in critical-conduction mode.

Application lifetime needs the actual stress conditions, including voltage and switching behavior. The handbook’s Eyring model illustrates how voltage and humidity can supplement the temperature dependence. Device-specific validation remains necessary.

Trade-offs

A wider test-to-use gap may save test time but requires more extrapolation and stronger confidence in the mechanism and model. Testing closer to use conditions reduces that gap but usually takes longer.

The diagram’s dashed continuation and bent actual line show why the faster test can be optimistic. More data near a suspected transition can be more valuable than simply extending a high-temperature fit farther.

Frequently asked questions

What is lifetime extrapolation?

It estimates lifetime under use conditions by extending a relationship fitted to accelerated-test results beyond the measured range.

Why is the Arrhenius relationship a straight line?

Taking the logarithm of L＝A exp(Ea/kT) gives ln L＝ln A＋Ea/kT, which is linear in reciprocal absolute temperature.

What does the slope represent?

The slope of ln L against 1/T is Ea/k. Multiply it by k to obtain Ea. If the horizontal axis is 1/(kT), the slope is Ea.

How many temperatures are needed?

Two points determine a slope; three or more help check whether the measured data support a straight line. This does not establish validity outside the measured range.

What is the main pitfall?

A change in the dominant failure mechanism between test and use conditions can bend the line. Ignoring that change can overestimate lifetime.

Can voltage or humidity be included?

Yes, where a suitable model and evidence support them. The cited handbook discusses the Eyring model for combined temperature, humidity and voltage dependence.

Is this used for real applications?

Yes. The cited ROHM paper uses SALT-derived acceleration relationships in estimating soft-switching application lifetime and discusses DHTOL and application waveforms.

Compare the concepts

An acceleration factor converts exposure between specified conditions. Lifetime extrapolation uses the fitted relationship to estimate lifetime outside the measured range. Activation energy controls the temperature-dependent slope for a particular mechanism.