PCB007 Magazine

PCB-Mar2015

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44 The PCB Magazine • March 2015 Feature by Les Round SpIrIT CIrCuITS Thermal Management for LED Lighting Applications PCB substrates designed for thermal man- agement have been around for a number of years, traditionally servicing power-related ap- plications; however, there are now many more suppliers and substrates emerging to meet the growing demand from LED lighting products. The LED package emits light forward and any excess heat is designed to be dissipated from the base of the component, usually through a be- spoke thermal pad or through either the anode or cathode pads. Like other electronic compo- nents, the failure rate of an LED doubles with every 10°C increase in junction temperature. So based on the fact that reliability and longevity are key requirements for the successful uptake of LED lighting, good thermal management is an essential element in this growth. A wide range of available LEDs put vary- ing thermal demands onto the PCB substrate. Low-wattage (0.25W LEDs) and low-density applications are typically dealt with by using standard, single-sided FR-4 or CEM PCBs, where all the heat must be dissipated at the surface and the thermal performance is enhanced by using large copper lands (for heat spreading) and higher copper weights when required. The FR-4/CEM materials are very good thermal in- sulators and so obtain little or no benefit from a secondary heat sink and the operating tempera- ture is directly influenced by the ambient tem- perature and although this does limit the use of this technology, it still represents a significant part of the LED market. It should be noted that there are some new FR-4/CEM style laminates that have been developed with a higher thermal conductivity, which allow the LEDs to benefit from secondary heat-sinking. For mid-power (1.0W LEDs), moderate den- sity applications, where the thermal require- ments are beyond the capability of a standard, single-sided PCB, the next level of thermal per- formance comes from FR-4 PTH PCBs using thermal vias to enhance heat dissipation. The heat generated by the LED spreads across the pad and then down the plated via holes to a large copper area on the other side of the board,

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