Study of thermal resistance in light emitting diode

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Date
2005-06
Authors
Abdul Karim, Norfidathul Aizar
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Abstract
The application of optoelectronic products particularly light emitting diode' (LED) is becoming more prominent recently due to the availability of high photon energy blue, green and red LEOs. Because of this, many incandescent bulb and cold cathode fluorescent tube applications are now being replaced by the new generation of LEOs due to its better colour gamut and longer life advantages. Thermal management of LEOs is also becoming equally important to enable these LEOs to be driven at higher power without sacrificing its reliability performance due to thermal loads. Due to strong competition in the LED business, shorter design cycle is a must to ensure continuous new product introduction to the market. This can be achieved through a fast and accurate modelling prior to prototyping. In this study, thermal modelling techniques for LED packages are discussed. There are three types of LED packages studied: a through hole LED (Type-1), single chip surface mount LED (Type-2) and multi chip surface mount LED (Type-3). Each of the LED packages was modelled in detail using a commercial computational fluid dynamic (CFO) codes. To reduce simulation time, orthotropic thermal conductivity of LED chips in Type-3 was used to simplify the chip structure. To reflect the actual test set-up, thermal test board for each LED package was also modelled in a steady state natural convection system. Effect of radiatio~ heat transfer was also investigated in the simulation. The models were validated on the actual samples. Thermal resistance between junction to pin and between junction to ambient of each LED packages were measured using thermal resistance tester following EIAIJESD measurement method. Error of measurement was also obtained. The deviation of simulated model versus measured values was within 5%. Based on the validated simulation and applying key design considerations, a series of sensitivity analyses were done. Effect of lead frame's thermal conductivity and leadframe's width on junction temperature and junction to pin thermal resistance were analysed in Type-1 LED. There were three types of copper alloy analysed in the simulation and the most effective alloy for Type-1 is Olin 194. Leadframe width was varied from 0.5mm to 2.5mm and the most effective width was found to be 1.6mm. Combining both results, junction to pin thermal resistance of Type-1 LED was reduced from the original 240°CNV to 117°CNV. In Type-2 LED package, effect of die-attach material at different bond line thickness, addition of heat spreader and effect of heat spreader sizes wer~ analysed. Three types of die attach material were studied in Type-2 LED. In all cases, the reduction of thermal resistance is not significant if the bond line thickness is thin which in this studies it is at 5JLm. It was also found that by adding heat spreader, junction to pin thermal resistance was reduced from the original 352°CNV to 202°CNV. The thermal resistance is reduced further by reducing the spreader thickness from O.22Smm to 0.071 mm and by increasing the spreader area from 1.0mm2 to S.12mm2. Combination of heat spreader, reduction in spreader thickness and enlarging the spreader area had reduced the junction to pin thermal resistance of Type-2 from the original 352°Cf\N to 91°Cf\N. The prototypes of the thermally improved LEOs were characterised. The deviation of simulated model versus measured values was within 10%. With this result, it showed that the reduction in thermal resistance of the LED packages can be achieved through correct modeling techniques as shown in this study.
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Many incandescent bulb and cold cathode fluorescent tube applications , now being replaced by the new generation of LEOs due to its better colour gamut
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