We present a high power density UV LED module for a wavelength of 395 nm with an optical power density of 27.3 W/cm 2 . The module consists of 98 densely packed LED chips soldered onto an Al 2 O 3 ceramic board. Thermal and optical measurements were conducted. The module was cooled by a forced air heat sink for the characterization experiments. A room temperature liquid gallium alloy was used as thermal interface material between substrate and heat sink with a TiN barrier layer to prevent corrosion. Further a technology to replace Al 2 O 3 ceramics by aluminum as substrate ispresented. An initial characterization shows that aluminum with a dielectric layer for electrical insulation is a competitive substrate material for efficient heat removal. IntroductionIndustrial applications using ultraviolet (UV) light are extremely progressive and gain large annual growth rates. UV light is used for curing paints, lacquers, coatings, and adhesives. Water treatment using UV light offers a large ecological benefit compared to the use of chemicals. These tasks are performed so far by high power mercury gas discharge lamps. But applications using high power UV light emitting diodes (LED) are gaining increasing attention. LED technology promises easier operation, fast switching, less power consumption, and longer lifetime than gas discharge lamps. Currently LEDs in the near UV range from 400 nm down to a wavelength of approximately 365 nm with hundreds of milliwatts optical output power per chip are available. With these LEDs compact and powerful curing solutions become possible.In contrast to gas discharge lamps, LEDs emit a narrow spectrum around a peak wavelength. Therefore applications requiring only a narrow spectral band in the near UV benefit from their use due to reduced power and cooling requirements. Another possibility to save power in non-continuous processes is the ability to switch LEDs on to full power only when needed in a cycle. In these cases many gas discharge lamps can be replaced with more energy efficient, LED based solutions [1].LEDs with much shorter wavelengths, e.g. 280 nm necessary for water purification, become more and more powerful. So the experience gathered with the construction of curing systems can be reused by exchanging the LED chips.To operate a high power UV LED source reliably some conditions have to be taken into account. The lifetime and efficiency of the system is closely associated with the junction temperature of each LED chip used. Therefore the thermal properties of the packaging are crucial for the system design.

We present optical measurements of an LED module consisting of 98 UV LEDs with an emission wavelength of 395 nm soldered onto a ceramic substrate within an area of 211 mm 2 . The module is mounted to a high performance microstructured water cooler. This cooler enables a maximum optical power density of 45.9 W/cm 2 at a forward current of 1350 mA and 447.9 W electrical input power.Further we describe the development of an LED module based on an aluminum substrate with thick film printed insulator and conductor layers and embedded, meander shaped water cooling channels. Numerical and experimental studies with different channel cross-sections are shown. Finally experimental results for this kind of UV LED module with 98 LED chips are presented and compared to the ceramic based module.

We present an improved cooling for a high power density UV LED module for a wavelength of 395 nm. The module consists of 98 LED chips soldered on a thick film printed alumina substrate on an area of 2.11 cm 2 . We investigated cooling by a commercial water cooler as well as by a surface micro cooler developed by our own.Further we describe a technology to replace alumina by aluminum as substrate material. A module consisting of 25 UV LEDs was optically characterized without and with liquid encapsulation.Finally we conducted numerical studies to develop an easily producible, sufficiently powerful, and robust water cooler. Based on the results we present a water cooler design with cooling channels embedded in the aluminum substrate of an LED module, removing the interface between LED substrate and heat sink. IntroductionMany industrial applications today use UV light. It is used to cure paints, lacquers, coatings, and adhesives with low emissions and timesaving. Waste water treatment with UV light offers an important ecological advantage compared to chemical treatment. Currently these applications use high power gas discharge lamps. But high power UV LED sources are gaining increased attention. LED technology promises easier operation, faster switching, lower power consumption, variable light output, and longer lifetime than gas discharge lamps. UV LED chips for the near UV spectral range at wavelengths between 400 nm and 365 nm are currently available with several hundred milliwatt of optical output power. These LEDs enable powerful UV LED light sources.Applications needing just a small spectral band benefit from lower power and cooling demands, because in contrast to gas discharge lamps LEDs emit only in the required spectral band and no light has to be filtered out. Additional power can be saved through an efficient control in non-continuous processes due to the instantaneous light output after switching on the LEDs.As lifetime, efficiency, and reliability of LED systems is closely associated with the junction temperature of every single LED, the thermal properties of the packaging and the cooling system are crucial issues.We present substantial improvements in the cooling of an LED cluster built on a ceramic substrate using a high performance microstructured water cooler. Additionally we show a novel LED cluster built on a thick aluminum substrate with thick film printed dielectrics and silver traces. The LEDs are

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An LED module consisting of 98 UV-LEDs with an emission wavelength of 395 nm placed on a ceramic substrate of 211 mm2 is presented. The module is cooled by a forced air heat sink as well as a high performance microstructured water cooler to lower the thermal resistance. For high thermal conductance a liquid metal as the thermal interface material between substrate and heat sink is used. With the forced air heat sink a maximum irradiance of 27.3 W/cm2 at a forward current of 700 mA and 220 W electrical input power was achieved. The microstructured water cooler enabled an almost doubling of the electrical input power (430 W) while maintaining the chip's maximum temperature. For a reduction of the module's thermal resistance a thick film process for aluminum sheet metal substrates was developed. A prototype LED module with 25 UV-LED chips on an area of 54 mm2 achieved a maximum optical power density of 31.6 W/cm2 at a forward current of 900 mA using a forced air heat sink. For an improved cooling of the LED chips a chip-on-heat sink-technology with embedded water cooling channels is developed to eliminate the thermal interface between substrate and heat sink.