A test set for LEDs life model estimation

Development of a Life Model for Light Emitting Diodes Stressed by Forward Current

“…By considering the experimental couples , the parameters and of the IPM expressed by relationship (1) can be derived very simply via the linear regression technique, given that (1) becomes (2) According to the EM, the relationship between MTTF and is expressed by (3) In this case, the application of the linear regression technique to the experimental couples provides the parameters and of the EM, given that (3) turns into (4) As shown by (2) and (4), (1) and (3) provide straight lines in log-log and semi-log coordinate systems, respectively, with slopes , and if is in ordinate, and is in abscissa. For this reason, coefficient (or ) can be defined as the Stress Endurance Coefficient.…”

“…The stress is taken as constant with time In this case, life model parameters can be easily estimated from experimental times-tofailure data obtained at different levels of the single stress. 1 More precisely, the parameters [3] are computed by regressing the estimates of a reference percentile (e.g., the median, or the Mean Time To Failure (MTTF)) of the time-to-failure distributions against the test stress levels. This is a phenomenological approach, given that it is used no matter the kind of phenomenon that leads the component to failure.…”

“…In [3], a life model for LEDs subjected to constant forward (direct) current as the single applied stress was developed by employing an experimental set-up for ALT based on LED luminosity as the diagnostic property. Time to failure was correlated to the driving current instead of some other physical quantity, such as the junction temperature, because the main stress of a LED is forward current.…”

“…In the present paper, in fact a technically extended version of [3], the outcomes of further investigations based on this ALT technique are presented. The model derived in [3] is improved via an additional experimental campaign consisting mostly of accelerated life tests performed under step-varying forward current, and aiming at more realistic time-to-failure estimates.…”

“…The model derived in [3] is improved via an additional experimental campaign consisting mostly of accelerated life tests performed under step-varying forward current, and aiming at more realistic time-to-failure estimates. Indeed, most literature studies about the endurance of LEDs focused on the effect of thermal stress [4]- [6], although also packaging [7] was considered, and different failure modes and mechanisms [8] were analysed.…”