Abstract
As a kind of electronic products, the failure rate of LED products follows the traditional bathtub curve.
Corresponding to the different regions in the failure rate
curve of the LED products, this paper proposes two
different lifetime evaluation methods. In the first method,
the lumen maintenance lifetime of the LED device is
estimated by only considering the failure of lumen
maintenance, corresponding to the aging failure stage.
The L70 lifetime, indicating the time when the lumen
maintenance of the LED product degrades to 70% of its
initial value, can be estimated through an accelerated life
test with a high temperature stress. Then the L70 lifetime
of the LED product under the normal operating condition
is calculated based on the accelerated test data by using
the degradation coefficient method. It is shown that a
suitable temperature stress in accelerated test will not
create other failure modes in the test LED lamp, and
therefore can reduce the test time to estimate the mean
replacement period of a batch of lamps. The other method
is proposed in correspondence to the accidental failure
stage of the LED product. In this case, it is assumed that
the lifetime of the LED product follows an exponential
distribution. Then the average life expectancy under the
normal operating condition, mean time to failure (MTTF)
of LED devices and not repairable lamps can be estimated
from the point estimates and confidence intervals from the
experimental data of certain reliability tests. From the
experimental results, it is proved that more accurate
reliability and lifetime expectancies of both the LED
devices and products are addressed from the point
estimates and confidence intervals calculations.