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Posted on 29 June 2019

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Using optimized sintering processes for direct LED assembly

A new combination of Alunit ceramic and innovative liquid cooling makes it possible to create extremely compact power electronics. LED arrays up to 100W/cm² and 45,000lm on an area of 40x40mm² are possible. To cool these packing densities the entire thermal path has been optimized – from the LED assembly to the ceramic heat-sink.

By Rüdiger Herrmann, Key Account Manager, Electronic Applications Division at CeramTec GmbH, Marktredwitz and Dr. Rafael Jordan, Photonics Coordinator, Fraunhofer IZM, Berlin

 

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 development partners have demonstrated their expertise in every step of the process – from the design and production to the characterization and verification:

• The Electronic Applications Division at CeramTec GmbH, Marktredwitz, supplied innovative aluminum nitride (AlN) liquid-cooled heatsinks.
• The Fraunhofer IZM institute in Oberpfaffenhofen designed the thermal and fluidic cooling.
• The Fraunhofer IZM institute in Berlin populated the ceramic heatsinks with LEDs using new bonding techniques.
• Excelitas Technologies GmbH & Co KG (formerly PerkinElmer Elcos) in Pfaffenhofen then 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 on 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 adaption 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 40 x 40 mm² and a height of only 16 mm, it has a total cooling capacity of 1600W. With an efficiency rating of 25%, 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 call into question conventional bonding techniques for die bonding. Even highly filled Ag conductive adhesives exhibit a thermal conductivity of little more than 1 W/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.

The compact CeramCool Box is made for homogeneous and efficient cooling of packing densities up to 100W/cm²

The Fraunhofer IZM institute in Berlin approached the problem with the bottleneck using new soldering and sintering techniques. The lower thermal resistance of this metallic bond created an excellent thermal coupling with the metalized Alunit substrate. They tested different combinations of LEDs, sintered metals and the ceramic substrate to ensure dependable adhesion. In addition to the electric conductors, this requires that the soldering points and sintering pastes are placed directly on and bonded permanently with the high-performance ceramic heat-sink without creating thermal barriers and without the risk of delamination (difference in thermal expansion coefficients). In this case the chip can be bonded directly on the heatsink. With production costs in mind they developed techniques for collective bonding that deliver a high degree of placement accuracy with considerably lower costs.

Alunit ceramic: high thermal conductivity, excellent dielectric strength

Ceramic heat-sinks take on a key role in efficient cooling because achieving the required temperatures is only possible when the base material exhibits high thermal conductivity with direct metalization. Alunit ceramic not only enables efficient thermal coupling with the coolant but also ensures the spreading of heat to minimize temperature differences within the module. What's more, during the research project CeramTec succeeded in achieving series extrusion of AlN ceramics with exceptional thermal conductivity. This process was the world's first of its kind at the time and enables rod-shaped bodies and tube systems made of ceramic with high thermal conductivity, mechanical stability and dielectric strength. The CeramCool Box has multiple parts and is produced using a dry pressing process followed by solid-state sintering. The shaping of the various prototype geometries takes place in the green stage using CNC machining as this method allows for the fast manufacturing of low-cost test modules.

The temperature profile of the CeramCool Box is homogeneous. Packing densities up to 100W/cm² are efficiently cooled.

If only passive cooling using air convection is possible in spite of the extensive waste heat, then uneven heating up of the LEDs in places to much higher than 100°C is unavoidable. For this reason the CeramCool Box allows for the hookup of an active water cooling system. A conventional chiller such as those used in PC technology is perfectly sufficient for heat dissipation. To ensure that the CeramCool Box is as simple to handle as possible, the designers limited the number of cooling water connections to a single inlet and outlet. Moreover, thanks to the ceramic construction, system developers can choose the coolant and even use the heat-sink in aggressive environments.

Symmetrically arranged spiral condensers with innovative multi-level flow paths ensure even cooling all the way to the exterior

What makes the CeramCool Box particularly innovative is found inside the ceramic heat-sink: Four symmetrically arranged spiral condensers with innovative flow paths ensure even cooling all the way to the exterior. The interior ceramic walls are a mere one millimeter “thick”. The enables the coolant to get closer to the heat source than any other concept with a comparably long system lifetime. The CeramCool Box uses Alunit, an AIN ceramic material with a thermal conductivity at room temperature of >170W/mK. Its high heat conduction enables this ceramic to deliver superior heat spread even with very thin walls. In conjunction with the sintering technology described above, this guarantees excellent heat transfer from the heat source to the coolant.

The scalable metalized module cools LED arrays up to 100W/cm², or 45,000lm on an area of 40x40mm².

The efficient and even temperature distribution has been proven in the thermal characterization using IR thermography and electrical junction temperature measurement. Thanks to the innovative interior design of the heat-sink, the measurements showed that the temperature only reaches 90°C with a coolant flow of 180l/h and an ambient and cooling water temperature of 30°C! For more precise measurement of the junction temperature than is possible via the wavelength shift in the UV range, a special measuring apparatus was developed that can determine the temperature via the forward voltage within an accuracy of one degree Celsius. These measurements have also confirmed the simulated results.

 

 

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