M. Sawant1 and A. Christou 1
1Materials Science Department And Department of Mechanical Engineering University of Maryland, College Park MD, US.
Abstract:
While use of LEDs in fiber optics and lighting
applications is common, their use in medical diagnostic
applications is very rare. Since the precise value of
light intensity will be used to interpret patient results,
understanding failure modes is very important. We
used the Failure Modes and Effects Criticality Analysis
(FMECA) tool to identify the critical LED failure
modes. Once the critical failure modes were identified,
the next step was the generation of time to failure
distribution using Accelerated Life Testing (ALT) and
Bayesian analysis.
ALT was performed on the LEDs by driving them in
pulse mode at higher current density J and higher
temperature T. This required the use of accelerating
agent modeling. We have used Inverse Power Law
model with J as the accelerating agent and the
Arrhenius model with T as the accelerating agent. Such
power law dependence originates directly from the
electromigration assumption of the failure mechanism,
The Bayesian modeling began by researching
published articles that can be used as prior information
for Bayesian modeling. From the published data, we
extracted the time required for the optical power output
to reach 80% of its initial value (our failure criteria).
Analysis of published data for different LED Materials
(AlGaInP, GaN, AlGaAs), the Semiconductor
Structures (DH, MQW) and the mode of testing (DC,
Pulsed) was carried out. This data was converted to
application conditions of the medical environment.
Many of the LED degradation mechanisms occur
simultaneously. The weakest link causes the actual
failure. This leads us to believe that Weibull
distribution is the most suitable distribution for time to
failure of the LEDs. We used this rationale to develop
the Bayesian likelihood function. In this study, we
report the results of our ALT and develop the Bayesian
model as an approach for analyzing LED suitability for
numerous system applications.