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. Researchers 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 heat-sink. With production costs in mind, the researchers developed techniques for collective bonding that deliver a high degree of placement accuracy with considerably lower costs.
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 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.
Figure 2: The temperature profile of the CeramCool Box is homogeneous. The unit is made from an aluminium nitride ceramic material called Alunit.
When only using passive cooling through air convection, the LEDs experienced uneven heating at temperatures much higher than 100°C. For this reason, the CeramCool Box was designed to allow coupling with an active water cooling system. A conventional chiller such as those used in PC technology is perfectly sufficient for heat dissipation. For handling simplicity, the designers limited the number of cooling water connections to a single inlet and outlet. The ceramic construction means the heat-sink can be used in aggressive environments and allows system developers to choose the coolant. Inside the ceramic heat-sink, four symmetrically arranged spiral condensers ensure even cooling all the way to the exterior. The interior ceramic walls are a mere one millimeter “thick”, enabling the coolant to get closer to the heat source than any other concept, with a comparably long system lifetime. The Alunit (AIN) ceramic material exhibits a thermal conductivity greater than 170W/mK at room temperature, supporting superior heat spread even when designed with very thin walls. In conjunction with the sintering technology described above, this guarantees excellent heat transfer from the heat source to the coolant.
Figure 3: Symmetrically arranged spiral condensers with innovative multi-level flow paths ensure even cooling all the way to the exterior.
Thermal characterization using IR thermography and electrical junction temperature measurements proved the efficient and even temperature distribution. Thanks to the innovative interior design of the heat-sink, measurements showed temperature peaks of only 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, a special measuring apparatus was developed that can determine the temperature via the forward voltage within an accuracy of one degree Celsius.
About the author:
. Rüdiger Herrmann
is Key Account Manager in the Electronic Applications Division at CeramTec GmbH – www.ceramtec.de
– He can be reached at R.Herrmann@ceramtec.de
. Dr. Rafael Jordan
is the Photonics Coordinator at the Fraunhofer IZM institute in Berlin - www.izm.fraunhofer.de
- He can be reached at Rafael.Jordan@izm.fraunhofer.de
This article originated from EE Times Europe magazine