Thermal analysis

Despite its high efficiency, LEDs face the issue of high heat dissipation densities during operation. For instance, a LED, with an electrical power rating of 1 W and an efficiency of 90 percent, produces a surface heating power density of 100 kW/m². This corresponds to approximately twice the heating power density of a commercially available cooking plate. On the one hand, high operating temperatures have a negative effect on the lifetime of the LED (the lifetime decreases exponentially with increasing temperature); on the other hand, they cause an undesirable colour change in the LED in the long term.

With the help of infrared thermography, the surface temperature of LED modules can be determined in a contactless and spatially-resolved manner. It is possible to identify the areas of the component which are subject to a higher thermal load. Often, the regions of interest are not directly on the surface, but are covered by other materials such as encapsulation materials or domes, which are only partially transparent to infrared light. To characterize the transmittance of these materials in the infrared spectral range, Fourier transform infrared (FTIR) spectroscopy is used to determine the influence of different materials on the thermography measurement. To be able to map hotspots within a LED package, a more accurate spatial resolution is required. For these measurements, an infrared microscope can be used at the Fraunhofer Application Center, which enables lateral resolutions in the range of a few micrometres.

For the theoretical description of the heat transfer within a component, the heat capacity, the thermal conductivity, and the geometry of the materials and components involved must be known. Calorimetric and microscopic measuring methods are available at the Fraunhofer Application Center. The results of the investigations can be used to enhance the composition of the components in order to increase their efficiency and operational lifetime.

Reference

Wagner, F.; Malvisalo, T.; Nolte, P. W.; Schweizer, S.:
Analysis of Thermal Diffusivity of Metals using Lock-in Thermography
13. Quantitative InfraRed Thermography (QIRT) Conference, July 4-8, 2016, Gdańsk, Poland
DOI: 10.21611/qirt.2016.093

Nolte, P. W; Ziegeler, N.; Rimbach, A. C.; Schweizer, S.:
Suitability of Lock-in Infrared Thermography for Luminescent Glass Development
Quantitative InfraRed Thermography Journal (2019)
DOI: 10.1080/17686733.2019.1646448

Ziegeler, N.; Nolte, P. W.; Schweizer, S.:
Application of Infrared Thermography to Thermal Transient Measurements
25th International Workshop on Thermal Investigations of ICs and Systems (THERMINIC 2019), 25-27 September 2019, Lecco, Italy
DOI: 10.1109/THERMINIC.2019.8923882

Thermal analysis describes methods in which the physical and chemical properties of a substance or mixture of substances are measured as a function of temperature or time. The sample is subjected to a controlled temperature program. The versatility of dynamic differential calorimetry (DSC) offers optimal conditions for research and development, quality assurance and process optimization.

Temperature and phase properties

At the Fraunhofer Application Center Soest, dynamic differential calorimetry (DSC) is used to determine characteristic temperature points of heat transfer processes (melting, crystallization, polymorphous transition, reactions, glass transition) as well as the corresponding enthalpies. It is also possible to determine the specific heat capacity.

A DSC measuring cell consists of a furnace and an integrated sensor with designated positions for the sample and reference crucibles. The sensor areas are connected to thermocouples. This makes it possible to measure both the temperature difference between the sample and reference side (DSC signal) and the absolute temperature of the sample and reference side and thus the heat flow. The device available at the Fraunhofer Application Center Soest allows investigations on a wide variety of materials up to 1500 °C. Oxidation-sensitive materials can be examined either in vacuum up to 10^-4 mbar or in an inert gas atmosphere.