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Fig. 2: conventional MBPCB materials cannot match the thermal performance of the Nanotherm substrates. circuit board production. And because the base material is aluminium, its cost is competitive. This means it may be used as a more thermally efficient replacement for conventional MBPCBs. Testing the performance of LEDs with a variety of substrate types shows how dramatic the difference in performance can be at the device level when the performance of the substrate is improved. Figure 2 shows a comparison of the thermal conductivity of Nanotherm LC (Laminated Copper) alongside a selection of commonly used thermal substrates. By improving the flow of waste heat from the LED through the substrate to a heat sink, Nanotherm reduces the heat at the LED’s junction temperature - see figure 3. For instance, a popular Cree XP-E LED driven at 1.5A operates around 20°C cooler when mounted on Nanotherm than when mounted on a common 2W/mK insulated metal substrate. Nanotherm, then, can offer improved thermal performance over conventional MBPCBs. It may also be used as a reducedcost replacement for high-performance aluminium nitride (AlN) ceramic tiles. Today, metallised AlN ceramics are the gold standard of thermal substrates, offering thermal impedance as much as a factor of 10 lower than typical insulated metal substrates. But the extremely high cost of AlN tiles has restricted their use to the most demanding applications. Here, however, Nanotherm technologies can provide an alternative. In the Nanotherm LC material described above, the thin adhesive layer between the dielectric and the copper circuit layer makes for a slight thermal bottleneck. This can be almost eliminated in higher-performance versions of Nanotherm. Nanotherm PS (for ‘Printed Silver’) uses conventional thick-film processes to form a silver circuit layer directly on the alumina dielectric. Because it uses standard equipment and processes, this material is relatively cheap to manufacture, but its electrical isolation rating limits it to low-voltage applications. Nanotherm DM (for ‘Direct Metallisation’) uses semiconductor fabrication processes to form a thin Titanium and Copper seed layer on the nano-ceramic. The circuit layer can then be formed on the copper using common PCB photo-lithographic and plating techniques. AlN ceramic tiles are expensive because they require an exotic base material; Nanotherm PS and Nanotherm DM, by contrast, have the same nano-ceramic-on-aluminium composite structure as Nanotherm LC. Applications for Nanotherm DM, such as heavy-duty power supplies, are able to benefit from comparable performance to AlN ceramic tiles, sometimes at as little as half the cost. New uses for isolated aluminium As the above description shows, new nano-ceramic techniques for coating aluminium with an electrically isolating crystal alumina layer offer an improved combination of low cost and low thermal impedance, ideal for high-temperature electronics systems. The technology is not limited, however, to flat aluminium boards: any aluminium extrusion may be coated. Today, this supports, for instance, Chip-on-Heat-Sink (CoHS) substrates, in which a pre-formed circuit layer is bonded to an aluminium/ alumina composite heat sink, with no intervening circuit board or Thermal Interface Material (TIM). Further uses for electrically isolated aluminium are already being found, for instance in brick power supply designs, and the ingenuity of electronics design engineers will no doubt find more. Now, Nanotherm is giving system designers a means to lower device operating temperatures, extend device lifetimes and increase efficiency and output across a range of demanding high-temperature applications, at a cost competitive with conventional thermal substrates. Nano-ceramic technology appears to represent the future for the thermal substrate industry, exposing the weaknesses of epoxy-based products and giving manufacturers for which the cost of AlN tiles is too high a means to enjoy hugely improved thermal performance. Fig. 3: a Cree LED runs substantially cooler when mounted on a Nanotherm MBPCB than when on commonly used conventional MBPCBs. Fig. 4: Examples of extruded aluminium heat sinks with screen printed circuit with Ag thick film paste, directly on top of a 30um thin nanoceramic layer. www.electronics-eetimes.com Electronic Engineering Times Europe December 2013 23


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