Thermal Management
which is a significant reduction from the much thicker layers used in conventional materials.
Independent testing of the dielectric layer shows that it has a thermal impedance of 0.012°
C.cm2/W. Even when the copper foil is bonded onto its surface with a glue line of 4-5 micron, this extremely low thermal impedance results in a substrate whose thermal properties still exceed the leading metal-clad PCBs available today.
isolation which allows a 10 micron nano-ceramic layer to be used, and a power electronics applications may require a higher 1500V(dc) isolation which would be provided by 25 micron layer of nano-ceramic. So, the substrate is ‘tuned’ to the requirements of the application, always providing the thinnest possible dielectric to meet electrical performance requirements and thus helping to minimise thermal impedance.
What does this means for a designer of a thermal
management PCB? It gives the designer the opportunity to reduce device temperature. This has the two-fold effect of improving device performance (a device will maintain its datasheet performance parameters) and also extend the device lifespan (we are all familiar with the equation that says reducing the device temperature by 10°C doubles the life of the
Figure 3: Comparison of thermal conductivity for thermal management materials
Testing using thermal measurement of devices has shown that when compared with leading metal-clad products, device temperature could be significantly reduced by using Nanotherm’s material. In a comparison with a typical 1W/mK material, and a Cree XP-E device driven at 1.5A, a temperature reduction of around 20°C was observed. The Nanotherm MBPCB topped the list of all substrate materials tested for its ability to transfer heat away from components.
component). This new substrate material also offers the scope to push the boundaries of design; increasing device and power density on the same PCB footprint. To the end user, a finished Nanotherm MBPCB looks, and
processes, no differently to any other thermal management substrate. The PCB is finished with the required soldermask and a surface finish suitable for assembly, that is, conventional finishes for solder assembly (Pb-free Solder, Immersion Silver, OSP, etc) or wire- bondable finishes such as ENIG and ENEPIG (for Aluminium and Gold wire bonding respectively). The material is compatible with Pb-free processing and is ROHS compliant.
Conclusion Figure 4: Graph of LED temperature rise (over ambient) for various common MCPCBs
Electrical breakdown voltage Electrical breakdown voltage (BDV) of the dielectric is also a key performance criterion. Here, Nanotherm offers an interesting approach; because the coating is grown onto the base aluminium it is possible to control the thickness of nano-ceramic layer to meet the customer’s requirement. For example, a low voltage LED lighting application may only require 500V(dc)
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As designs drive up device and power density, so the need for better thermal substrates increases. Cambridge Nanotherm is already well on its way to realising its next generation product – the Nanotherm PLUS MBPCB takes out the adhesive layer and applies the metallisation directly onto the surface of the nano-ceramic. This method significantly reduces thermal resistance between the circuit layer and the aluminium heat-spreader. At a composite thermal conductivity of c.140W/mK, the thermal performance begins to leave behind that of even the best thermal management dielectrics, even when used with copper heat- spreading layers, and begins to
approach that of aluminium nitride (AlN) metallised ceramics. The Nanotherm solution offers that level of performance at a fraction of the cost of metallised AlN and offers the best cost-benefit performance of substrates for component packaging and LED die array manufacture. Nanotherm PLUS is a fully inorganic substrate material, eliminating epoxy degradation over time and loss of adhesion strength as the product is thermally cycled and ages.
In addition, the ability of nano-ceramics to be applied to any 3d aluminium form opens up new possibilities for applying circuitry directly onto a conventional aluminium heatsink. The same bonding process used in the flat PCB product can be used to apply a circuit to an extrusion. This is already available in prototype form for client’s development purposes.
Cambridge Nanotherm |
www.camnano.com
An automotive headlight with circuit directly on CNC machined heatsink
Steven Curtis is VP Product Development at Cambridge Nanotherm Ltd
Components in Electronics March 2013 13
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