Electro-optical strength assessment of white LEDs multichip modules for automotive forward-lighting application: Failure analysis and packaging influence

Performances of Lighting Emitting Diode (LED) are now suitable for automotive high beam / low beam lighting applications. Due to high brightness light sources needs in automotive, LEDs are packaged in multichip module (e.g. 4 chips in series), to deliver up to 1000 lumens at 1A. Currently, different packaging strategies have been implemented in term of chip configuration, bonding, down conversion phosphor layer and mechanical protection to optimize performances. Beyond the performances, and from an automotive point of view, it is crucial to know the exact behavior and reliability of these optoelectronic devices in an automotive context. Current reference documents such as automotive regulations and qualification test (Automotive Electronics Council, AEC-Q101) do not take all of the automotive mission profile into consideration. In this context, a methodology is proposed to define and anticipate failure behaviors of each LEDs module through a robustness study (over-stress). A setup measurement has been developed focusing on the monitoring of electrical, optical, and thermal parameters of these modules during thermal and electrical step-stress robustness tests. A specific methodology and physical measurement issues on a multichip configuration are detailed. Indeed, these new multichip optoelectronic devices force to overall characterizations (chips in series) and drive the emergency of a new behavior to be considered, the partial failure: how could we estimate the failure of a single chip into a multichip configuration? Secondly, based on these results, we extract the strength and weakness of each LEDs module packaging strategy against thermal or electrical over-shots allowing defining a safe operating area of such devices. Failure modes, failure mechanisms and related physical phenomena are investigated considering the overall LEDs module architecture.