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16 The PCB Magazine • March 2014 THERMALLY CONDUCTIVE SUBSTRATES & THERMAL MANAGEMENT continues LEDs, like all light sources, are not 100% efficient—they generate heat. This heat causes the LED chip's temperature to rise. As the tem- perature rises, it can reach a point where the light conversion efficiency actually decreases as the input power is increased. Also, as the tem- perature rises, the long-term life of the LED de- creases: the brightness of the light permanently drops. The heat generated by an LED is quite differ- ent from the heat generated by a regular incan- descent bulb. Because the LED heat is trapped in the chip, heat must be conducted out via a path of low heat resistance. This keeps the LED at a temperature where it will function effi- ciently and have a lifetime that maps to the life of the application. It is necessary to remove the heat, not only from the LED chip, but also from the LED light platform, by conducting it into a heat sink. The heat sink could be the light fix- ture itself. The best conductor of heat is metal, so heat sinking the LED light to a large metal object delivers optimal heat conduction, which is where IMS or MCPCBs step in. Conversely, incandescent bulbs radiate most of their heat into the surrounding air, which as- sists significantly in cooling the bulb. Unlike LEDs, incandescent bulbs do not conduct sub- stantial heat into the Edison socket, which stays relatively cool. IMS materials generally consist of a thermal- ly conductive dielectric layer, heavily loaded with ceramic type fillers, sandwiched between a copper foil and an aluminium plate, the di- electric may be unreinforced, or woven-glass re- inforced. Due to its superior mechanical prop- erties, aluminium remains the most preferred backing substrate however; alternatives include copper and steel plate. The choice of filler is always a balance be- tween thermal conductivity, cost and electri- cal insulation properties. You can see from the table 1: effect of filler on thermal conductivity and breakdown voltage. (Key: ++ Better + good - Bad - - worse)