Printed Circuit Design & Fab - July 2008 - (Page 25)

ThERMAL ManaGEMEnt directly from the filament to its surroundings by thermal radiation with some conduction due to the glass. The primary path of heat transfer in an LED device is usually from the junction to the system enclosure. The LED device manufacturer provides the package level thermal management. For the manufacturer, the biggest concern is minimizing the thermal resistance from the junction to the outside of the package. Some LEDs, typically small devices mounted on panels, have leads that form the main thermal conduction path and for these devices the thermal resistance from the junction to the leads is most critical. Package design varies by manufacturer and type of LED, but the concepts between packages are similar. In FiGurE 1, the LED chip is typically attached by a bond layer to a metal interconnect layer which is then attached to a ceramic substrate and an electrically isolated thermal pad. The entire package is designed to maximize optical output and move heat away from the back of the LED chip. Rudi Hechfellner, applications manager for Philips Lumileds Lighting, San Jose, California says that thermal management is by far the most critical aspect of LED system design. Hechfellner pointed out that even the most thermally efficient LED devices requires that a cooling system be developed around it. He said that because most traditional lighting methods radiate heat, they do not have that level of thermal issues. Many systems manufacturers have much more experience in the electrical and mechanical than in the thermal aspects of design. “What the engineering community needs is a change of their mindset and think thermal first and electrical later,” Hechfellner said. “Thermal represents 90% of today’s design challenges for LED systems manufacturers while electrical and mechanical together provide only 10%.” The nature of an LED package is such that even as LEDs increase in efficiency, the challenge of thermal management will not disappear. As light output reduces with temperature, a greater proportion of the electrical power is turned into heat, further increasing the temperature. The light output from an LED reduces as it ages, so its heat output may increase over time, accelerating the rate of degradation. A common cause of lumen depreciation in white LEDs is a yellowing of the phosphor, which may be heat or environmentally induced but does not necessarily mean that the chip is working less efficiently or that there is more heat being generated. Thermal management solutions must become more effective in removing the heat dissipated by an LED over its useful life. ! FiGurE 1. Schematic of a high-power leD package. System Level Design Considerations The design considerations are different for every LED and care must be taken to understand the metrics and performance of the LED being used in the application. The essence of LED system design is transferring the heat efficiently from the LED thermal spreader, slug or wire leads to the ambient surroundings. First of all, a secure and thermally efficient bond must be provided between the slug and the circuit board pad. The thermal connection typically runs through a small thermal via in the PCB to a large copper area on JULY 2008 another layer. Heat is typically conducted through this layer to the enclosure or an external heatsink. Understanding these system level thermal management challenges are integral right from the start, in the PCB design phase. An external heatsink may be required in situations where an exceptionally large amount of heat is dissipated within the enclosure. Copper and aluminum are commonly used materials for LED heat sinks. Optimizing the geometry of the heat sink is a critical concern in many applications, as the heatsinkto-air thermal resistance is often significant. Heat sink performance varies depending upon factors such as the material, number of fins, fin thickness, base thickness, etc. External heatsinks extend the surface area available for heat to transfer to the ambient air. The optimum design depends on the local air flow conditions that are affected by the introduction of the heatsink, increasing the design challenge. Copper offers superior thermal conductivity, while aluminum is lighter and less expensive. In some cases, PCBs made of materials that improve heat transfer through the board may be used. These boards may be made of ceramic, coated steel, aluminum or some of the newer, thermally enhanced PCB laminate materials. The most difficult LED applications are those that require an airtight enclosure to protect the LED from its environment. One way to address this challenge is to use an enclosure material having a high thermal conductivity. In other cases, more elaborate measures may be required. One example is an airto-air heat exchanger design that uses internal fans to circulate hot air over internal fins, which conducts the heat into the walls of the enclosure. External fans are then used to move cool ambient air over fins fitted to the outside of the enclosure and remove the heat. Heat transfer is then via a series of convection and conduction steps. Obviously, there are a large number of design variables that need to be considered when designing LED systems. Optimizing the thermal design is critical for a number of reasons. The DoE’s fact sheet on thermal management notes that excess heat affects both short-term and long-term LED performance. The reversible short-term effects are color shift and reduced light output. Minimizing color shift is critical for back lighting printEd circuit dESign & fAB 25

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Printed Circuit Design & Fab - July 2008

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