Thanks to their increasing light output, high-power LEDs are becoming an interesting option for new applications in medical products, industrial image processing and UV hardening, to name just a few. Hundreds of very densely packed power LED chips are needed to illuminate large-surface objects; this can lead to thermal problems.
On top of that, high demands are placed on unchanging optical parameters, especially in UV applications, such as wavelength, chromaticity, forward voltage and light output. Fluctuating temperatures or very inhomogeneous temperature distribution across the power module are unacceptable. A joint project sponsored by VDI/VDE-IT Bavaria faced these challenges and developed compact LED modules with a high packing density and output. The Electronic Applications Division at CeramTec GmbH supplied innovative aluminium nitride (AlN) liquid-cooled heat-sinks; the Fraunhofer IZM institute in Oberpfaffenhofen designed the thermal and fluidic cooling; the Fraunhofer IZM institute in Berlin populated the ceramic heat-sinks with LEDs using new bonding techniques; and Excelitas Technologies GmbH & Co KG (formerly PerkinElmer Elcos) in Pfaffenhofen produced the functional LED module with an optical compound and electrical and fluidic interfaces.
The significant increase in output of the new cooling modules was made possible by several development projects. For example, the usual thermal bottleneck seen in glued components was avoided altogether through the development of new chip assembly techniques using optimized sintering processes for direct LED assembly on AlN ceramic heat-sinks. The metalized Alunit ceramic creates efficient thermal coupling between the chip and the coolant. Another area of emphasis was the development of a special thermal management system with even temperature distribution over the entire module that also takes other general conditions into account such as scalability in every direction and simple handling. The CeramCool Box that resulted from this effort also allows quick adaptation of the illumination to the respective application requirements without any complex optics. An edge length of only 40mm for 1600W The compact CeramCool Box is made for homogeneous and efficient cooling of packing densities up to 100W/cm². With an edge length of just 40x40mm² and a height of only 16 mm, it has a total cooling capacity of 1600W. With an efficiency rating of 25 percent, this corresponds to 400W of optical power, or roughly 45,000 lumens with common high-power LEDs. The remaining 1200W need to be efficiently dissipated as heat, which is a challenge that already begins with the heat transfer from the component to the carrier substrate. Power densities of this magnitude challenge conventional bonding techniques for die bonding. Even highly filled Ag conductive adhesives exhibit a thermal conductivity of little more than 1W/mK, which already results in a bottleneck for efficient cooling. Add variable adhesive layer thicknesses and even the best cooling concept cannot compensate for the absolute and relative temperatures.
Figure 1: The compact CeramCool Box is made for homogeneous and efficient cooling of packing densities up to 100W/cm². Within a 16x40x40mm form factor, it provides a total cooling capacity of 1600W at 90°C. This translates into a temperature delta of 60K to the coolant.
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