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Thermal Resistance Characterizations on Chip On Board LED Packaging
Keywords: LED packaging, Chip On Board, thermal resistance
High Brightness Light-Emitting Diodes (HB-LEDs) are key devices for the solid state lighting applications. Contrarily to other lighting devices, most of the heat resulting from non radiated power has to be dissipated by conduction through the different levels of the packaging. Thermal conduction has to be addressed at all levels of packaging and especially at the chip or device level to keep the junction temperature as low as possible. The thermal performance of a LED package is characterized by the so-called “junction to case thermal resistance – Rth J-C. Today most single chip packages have a Rth J-C that is above 8 W/K. Chip On Board technology is very promising since it may enable to achieve a better miniaturisation, a design flexibility and of course a better thermal management. In this paper, we present a detailed study of the thermal resistances in such package technologies. The study was carried out by bonding LED chips directly on Metal Core Printed Circuit Board substrates with various thermal interface materials : adhesives and solders. The thermal resistance is calculated from thermo-electrical and optical measurements. Thermo-electrical measurements are used to determine the injected power and the junction temperature in various thermal conditions using a prior calibration with the diode forward voltage method. Optical measurements are used in a integrating sphere to retrieve the accurate emitting light and thus indirectly the heat dissipated. By integrating multi-chip packages, it is shown that we can derive separately the thermal resistance of the chip and the board. We show an improvement of the thermal performances by a factor of 5 using a COB technology compared to a classical surface mounting. These measurements are supported by a computed thermal analysis using the Ansys software that enable a discussion of the results.
Adrien Gasse, Research Scientist
CEA LETI Minatec

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