Index matched fluidic packaging of high power UV LED clusters on aluminum substrates for improved optical output power

Abstract

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

Very high power density LED modules on aluminum substrates with embedded water cooling

“…The maximum allowed chip temperature of 125 °C is reached at 1400 mA and an electrical input power of 465 W with a slight decrease of the optical output power. Compared to previous results [2], this is an increase by 35 W at a 6 °C higher cooling water temperature and a 12 % lower flow rate. This improvement was possible through a tighter thermal coupling of the substrate to the cooler.…”

“…If all chips would have emitted UV light, we could expect a maximum optical power density of more than 25 W/cm 2 . Previously published results [2] have shown an optical power density of up to 31.6 W/cm 2 on a forced convection air heat sink. For these former results an LED module with a thinner insulation layer of 40 µm and only 25 LED chips was used.…”

“…An even larger channel cross-section further decreases the flow resistance. The different design of the commercial cooler shows an even higher flow rate and lower pressure loss, but this cooler is also prone to clogging due to particles in the cooling water and its very narrow channels [2].…”

“…The frame is screwed on the water cooler and presses down the corners of the module to ensure a tight connection between ceramic substrate and water cooler. A thermal interface material (TIM) [2], a gallium based, room temperature liquid metal, between cooler and substrate leads to high heat transfer capabilities. Additionally, an elastic silicone sheet is placed between module and frame to prevent bending stress as well as to compensate for thermal expansion.…”

High power density LED modules with silver sintering die attach on aluminum nitride substrates

“…The used surface micro cooler was made from copper with internal micro structures to provide an excellent heat transfer to the cooling water [4]. Fig.…”