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March 2014 • The PCB Magazine 17 THERMALLY CONDUCTIVE SUBSTRATES & THERMAL MANAGEMENT continues above chart that aluminium nitride and bo- ron nitride give the best thermal conductivity with good breakdown voltage, alumina gives a higher breakdown voltage but with lower ther- mal conductivity, the filler blend needs to be carefully selected bearing in mind ultimate per- formance requirements balanced against long term reliability. Traditional PCB substrates are designed to have good electrical insulation, but due to the glass component they also tend to be good ther- mal insulators as well. The increased thermal conductivity, of con- ventional glass style prepregs is achieved by loading the resin with thermally conductive ceramic fillers. These materials can be used to build multilayer PCB structures which can then be bonded to a heat-dissipating base using ther- mally conductive prepregs. Insulated metal substrates are commercially available, and they offer cost effective perfor- mance with straightforward fabrication, good mechanical stability and a range of thermal conductivities to suit particular configurations. IMS are not a new concept, these materials have been available since the 1960s, but only recent- ly has the volume demand driven the develop- ment of new and improved versions and war- rantied volume production. Functions of the dielectric layer: • Thermal conduction • Electrical insulation • Promote adhesion—bonding to copper and aluminium • Choice of woven-glass reinforced or non-reinforced dielectric The dielectric serves a number of functions, including, thermal conduction, insulation and promoting good bonding. Generally the dielec- tric is a laminating resin, loaded with thermal conductive ceramic filler, designed to give a good bond to the copper foil, the base laminate and to the metal substrate, if it is a pure IMS. It may be glass-reinforced or unreinforced. Conventional FR-4 glass reinforced dielec- trics create a thermally resistive, as well as an electrically resistive barrier. At this stage ther- mally conductive glass reinforced dielectrics currently allow in excess of 12x more heat trans- fer than standard FR4, the use of non-reinforced dielectrics is significantly increasing this figure to, currently, in excess of 25x and still develop- ing As can be seen above, the choice between reinforced and un-reinforced dielectric is a trade-off, glass reinforced material offer a mar- ginally better thickness uniformity and high- er breakd own voltage (obviously the two are linked), and the cost is lower. The trade-off is that the ultimate thermal conductivity of the glass-reinforced material is less (because there's not as much room in the cross-section to ac- commodate the required thermally-conductive filler and glass is a thermal as well electrical in- table 2: comparison of woven-glass reinforced and non-reinforced ims.