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March 2014 • The PCB Magazine 15 THERMALLY CONDUCTIVE SUBSTRATES & THERMAL MANAGEMENT continues and under-bonnet automotive devices are built using direct bonded copper (DBC) substrates on ceramics with good thermal conductivity, particularly materials such as beryllia, silicon nitride or aluminium nitride, which are signifi- cantly better heat conductors than alumina, al- though at a significantly higher cost. Other areas seeing a significant increase in the use of IMS are power conversion systems (telecom, industrial, high voltage regulator, power supplies) and hybrid/electric motor con- trol applications. These sectors typically use a more complex constructions of IMS printed circuits. Heavy copper weights for high current low voltage applications are used, and the en- gineering flexibility offered by IMS lends itself readily to this sector. The metal substrate can be of varying thicknesses with complex engineer- ing requirements that may need milling, shap- ing, threading or even anodizing. Four Main LED Substrate Types 1. Metal cored PCBs/IMS—insulated metal substrates 2. Conventional PCB—mid- and high-T g FR-4 3. DBC—direct bonded copper 4. LTCC—low temperature co-fired ceramic (LTCC) LED manufacturers have been adopting packaging technologies from the power-elec- tronics field with the result that they can offer efficient thermal coupling from the semicon- ductor to the interconnecting substrate the de- vice is mounted upon. However, although com- mon printed circuit materials have good elec- trical insulation properties, they generally pro- vide excellent thermal insulation as well. Heat dissipation has traditionally been promoted by the incorporation of internal or external heat sinks and fans: Heavy copper inner layers with thermal via holes, or bonded aluminium heat ladders—all of which work, but potentially add significant cost in terms of materials and pro- cessability. The family of thermally conductive printed circuit laminates, known as IMS, have now be- come widely established as a preferred base ma- terial for the manufacture of circuits for HB-LED applications, and offer cost-effective perfor- mance with straightforward fabrication, good mechanical stability and a range of thermal conductivities to suit particular configurations. Whilst other materials are available, in terms of thermal conductivity, there remain signifi- cant challenges in terms of processability, cost and material robustness. Figure 2: thermal dissipation of a typical led build.