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EETE DEC 2013

LED LIGHTING Thermally conductive nano-ceramics simplify LED lighting assemblies By Steve Curtis Manufact uring a high-performance thermal substrate is a balancing act: it involves applying a dielectric layer to a metal base plate to gain sufficient electrical isolation for the circuit’s tracks and pads, while providing the least possible resistance to the passage of heat from components to the base plate. Too thick a dielectric layer, and the heat cannot escape fast enough; too thin, and the system is at risk of forming a short circuit between the tracks and the base plate. Spurred by growing demand for high-density power electronics systems and LED lighting equipment, however, the thermal substrate industry has been developing new materials and production processes that dramatically shift the balance between dielectric strength and thermal performance. The latest nano-ceramic materials are now posing a challenge to existing metal-backed PCB (MBPCB) technologies by providing a better combination of high performance and competitive cost. Epoxy: a poor thermal interface There are two contributors to the thermal impedance of a substrate: the impedance of the layers of the substrate; and the impedance at the interface between the layers. So in an MBPCB, the thermal impedance of the metal back or base plate is not the problem: the problem is the dielectric layer, and the interfaces between the circuit layer, the dielectric layer and the base plate. In the most widely used kind of thermal substrate, insulated metal substrate, the dielectric layer is a filled epoxy (epoxy mixed with ceramic particles) sandwiched between the base plate and the copper circuit layer. Filled epoxy, which has relatively poor thermal properties, supports a minimum thickness of some 30μm, and its glued interface to the base plate suffers from high thermal impedance. While an epoxy dielectric layer is applied thermo-mechanically to the base plate, nano-ceramic material can be grown directly on it. Alumina (aluminium oxide) for instance, a crystalline ceramic material which is both electrically isolating and thermally conducting, may be grown on an aluminium base plate. The purpose of nano-ceramic technology is to create a ceramic coating with properties peculiarly well suited to high-temperature electronics applications. Tiny crystals between 20nm and 40nm in size are packed densely together, providing an excellent electrically isolating layer which also provides an impermeable barrier to the materials used in circuit board formation. The process for growing the nano-ceramic layer allows for precise control of the thickness of the dielectric. This means that it can be tuned to the needs of the application: the minimum 10μm layer is sufficient for low-voltage applications such as LED lighting, offering a breakdown voltage of 500V. A 25μm layer provides a 1,500V rating. Higher voltages may be obtained by applying a thicker layer of nano-ceramic material. The junctions between crystals also have a remarkable ability to absorb the expansion and contraction of the base plate due to extreme changes in temperature, a property that has been proven in motor sport and aerospace applications. The resulting nano-ceramic material has obvious advantages over conventional insulated metal substrate. The dielectric layer is atomically bonded to the base plate, providing for an excellent thermal interface. What’s more, an adhesive layer no more than 5μm thick is sufficient to bond the copper circuit layer to the dielectric – see figure 1, drastically reducing the thermal impedance compared to the much thicker layer of epoxy in conventional products. With the copper circuit layer bonded to the alumina nanoceramic layer, the resulting ‘biscuit’ is ready for circuit formation and assembly in exactly the same way as a conventional MBPCB. With a thermal conductivity of 7.2W/mK, the dielectric layer conducts heat twice as well as even the best epoxy-based metal substrate. Nano-ceramic might be a promising material, then, but is it suitable for mass production and for use in electronics systems? In Nanotherm, a new patented process commercialised by Cambridge Nanotherm, an alumina layer grows on the surface of an aluminium base plate when it is suspended in a bath of electrolyte and subjected to complex electrical pulses. The great advantage of this process is that it is highly controllable, so that the thickness of the dielectric can be chosen at the level of single microns, while at the same time it uses as its main material aluminium, which is both abundant and cheap. At first glance, this process might seem similar to other ways of growing ceramic on metal, such as anodising, plasma electrolytic oxidation or micro-arc oxidation. In practice, however, none of these methods creates a dense enough layer to provide the electrical isolation values that the electronics industry requires. Nanotherm, by contrast, produces a material proven in demanding applications, and can be scaled up for high-volume Steve Curtis is Chief Engineering Officer at Cambridge Nanotherm - www.camnano.com Fig. 1: Lay-up of a nano-ceramic MBPCB. 22 Electronic Engineering Times Europe December 2013 www.electronics-eetimes.com


EETE DEC 2013
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