The influence of the phosphor layer as heat source and up-stream thermal masses on the thermal characterization by transient thermal analysis of modern wafer level high power LEDs

Abstract

In the last years Waver Level LED Packages (WLP-LEDs) were developed. They are thin film flip chips where the sapphire substrate remains attached on top of the epitaxial light emitting layer (EPI) which can be assembled directly on a printed circuit board. The thermal resistance and the thermal path of WLP-LED packages are measured by transient thermal analysis and transient finite element simulation. This study investigates the impact of the upstream thermal masses, i.e. the sapphire (SP), phosphor layer (PL) and the side coating (SC) on the transient thermal impedance curve and the cumulative structure function. It is shown that the standard approach to extract thermal properties by features (steps) within the structure function is misleading for thermal networks with upstream thermal load and distributed heat source (EPI and PL) because they influence the shape of the structure function. By transient thermal measurements and finite element (FE) simulation the transient thermal measurements are analysed to extract information about the thermal parameters and the thermal path. Starting from the analysis of the blue flip chip LED (FC-LED, no PL and no SC) the FE-model is set up. Stepwise the FE model is extended and the influence of the PL and the SC on the transient thermal measurement is investigated. A FE model is validated and calibrated which allows simulating the transient thermal curves of these modern LEDs. Using the model the impact of structural changes in the LED package on the transient thermal curves can be identified for reliability analysis.