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The market of LEDs increases exponentially over the years as well as their power. However, at the same time the mean time to failure should remain superior to 100,000 hours. The major difficulty comes from the mechanical stress in the LED structure, which is roughly proportional to the inverse of the thermal conductivity. Consequently, it is really important to determine thermal conductivity of the materials used in LEDs. In the same way, the thermal conductivity of films is lower than the thermal conductivity of the bulk material. Unfortunately, thermal conductivity measurement is difficult on two-dimensional structures. Then an appropriate method had to be developed. The 3 omega thermal conductivity measurement method has been used extensively to measure the thermal properties of bulk and thin film dielectric materials. Tests on different materials will allow to find materials with a higher thermal conductivity. The thermal conductivity measurement will be elaborated by two different methods defined the isotropic and anisotropic properties, the "slope method" and the 2D heat conduction.

ABSTRACT: The power of LEDs increases exponentially over the years,while the mean time to failure (MTTF) should remain >100000 hours. The reliability requirement limits the junction temperature and the thermo elastic stresses, which are roughly inversely proportional tothe thermal conductivity of the heat spreaders. The 3omega method has been set up to measure dynamically the thermal conductivity of anisotropic thin layers preferably in situ. CONCLUSIONS: The 3omega method measurement set up is operational from 10 Hz to 10 kHz The measured thermal conductivity of silicium deviates 50%, so the set upneeds to be improved. The length of the heater has already been checked. The goodness of fit of the sensor/heater calibration R² = 0.99998 The goodness of fit of the 3omega slope method R² = 0.99