MTBF/FIT すいてい員
FIT
Failure in Time is one of the units of failure rates defined as one failure per 109 device-hours, one PPM (parts per million) per 1,000 hours of operation or one failure per 1000 devices run for one million hours of operation. Expressed in mathematical form, that is FIT = 1 failure /109 device-hours.
MTTF
Mean time to failure, is the expected time to failure of a system. Non-repaired systems can fail only once. Therefore, MTTF is equivalent to the mean of its failure time distribution.
The calculation of product life
Below is the calculation process of product life under 55℃.
According to Arrhenius model:
AFT = exp((Ea/k)×(1/Tu - 1/Ts)) = tu/ts
AFT = Acceleration factor due to
tu = Time at use temperature (e.g. 55°C)
ts = Time at stress temperature (e.g. 150°C)
k = Boltzmann's Constant (8.617 x 10-5 eV/°K)
Tu = Temperature at Use (°K)
Ts = Temperature at Stress (°K)
Ea = Activation Energy (e.g. 0.7 ev)
X² = Chi Squared Distribution, value depends on number of failures and confidence interval
TDH (Total Device Hours) = Number tested × Test hours
The failure rateλ is calculated by using the formula:
λ = [X² /(2×TDH ×AFT)] ×109
MTTF (Mean Time to Failure) = 1/λ
MTBF: Mean time between failure, is the expected time between two successive failures of a system. Therefor, MTBF is a key reliability matric for a system that can be repaired or restored.
MTTF: Mean time to failure, is the expected time to failure of a system. Non-repaired systems can fail only once. Therefore, MTTF is equivalent to the mean of its failure time distribution.
MTTR: Mean to Restoration.
MTTF =∑T1/ N
MTTR =∑(T2+T3)/ N
MTBF =∑(T2+T3+T1)/ N
MTBF= MTTF+ MTTR
For non-repairable system, MTBF=MTTF
