PCB007 Magazine

PCB-Dec2017

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44 The PCB Magazine • December 2017 by Gareth Parry, P.Eng AISMALIBAR Thermally conductive dielectrics used in the PCB substrate have been steadily growing in de- mand and application, with a surge following the development of LED lighting. Many thermally conductive dielectrics are similar to the standard FR-4 dielectrics used in the PCB industry: epoxy (or polyimide) rein- forced with woven glass with the addition of a conductive filler. The type, size, geometry, and concentration of the filler dictate the thermal/ mechanical properties of the dielectric. Typical conductivity values are 1.0–3.2 W/mK. In com- parison, standard FR-4 has a thermal conductiv- ity of 0.3W/mK, and copper is 398 W/mK. Why the need for a thermally conductive dielectric? The simple answer is thermal management of the system. Components mounted to the PCB or IMS need power to operate and the bi-prod- uct is thermal energy. Current running through a copper conductor will generate heat due the resistance of the copper. The greater the resis- tance, the more thermal energy that must be dissipated. As the temperature of the conductor increases, so does the resistance. In an uncon- trolled design, one can experience a runaway condition where the increase in temperature drives an increase in resistance which drives an increase in temperature and so on, eventually destroying the circuit. In most situations this is not the case, but for high-power applications, and high-speed signals, thermal management is a real concern. It is true that LEDs generally do not produce much heat, and are cool to the touch. Then why the exponential growth in demand for thermally conductive dielectrics? To put it in perspective, the energy consumed by a 100-watt incandescent bulb produces around 12% heat, 83% IR and only 5% visible light. The typical low watt LED might produce15% visible light and 85% heat [1] . But with high power LEDs (>1 watt), it is essential to remove the heat through efficient thermal management. Without good heat sinking, the internal temperature of the LED rises, and this will cause the LED charac- FEATURE

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