When it comes to highpower lighting applications in the military and harsh industrial environments, reliability and efficiency are key features to consider. To maximize both of these qualities, lightemitting diode (LED) technology is coming into play with brighter, more flexible lighting solutions. LEDs are longer-lasting, appear brighter and are more versatile and more durable than incandescent and compact fluorescent lighting (CFL).
The quality of the light and the lifetime reliability of LEDs are both temperature-dependent, so thermal management is critical for maximizing LED output. Because LEDs are semiconductors, they emit a substantial amount of waste heat in a very small area. As a result, designers must consider the most appropriate thermal management solutions that can collect the highly concentrated waste heat and then move or dissipate it.
Thermal Management Options
Designers have several options to keep LEDs cool using passive thermal management technologies, such as heat pipe-based assemblies or k-Core® Encapsulated annealed pyrolytic graphite (APG). These passive thermal management technologies help LED manufacturers protect sensitive electronic devices and ensure the high performance that gives LEDs an edge over other lighting options like incandescent and CFLs.
Some constraints, including limited height budget and the need for reduced or no electrical power consumption or noise, make heat pipe-based thermal solutions especially attractive. The selection of the proper heat pipe cooling solution is dependent upon each customer’s specifications and design constraints. For example, a natural convection cooling solution involves rejecting heat to the ambient environment by natural convection, i.e., no fans. This is typically very difficult to do when considering LEDs, except when heat pipes are used. Figure 1 illustrates an LED cooling design that uses heat pipes to contact the LEDs directly. The heat pipe transports the heat to multiple thin plate fins that are spaced properly to allow the air to circulate naturally regardless of the orientation of the LEDs. The configuration of the fin stack is variable.
In many conventional systems, LED electronic devices are mounted on copper alloy substrates using soft or hard die attach. While these copper alloy substrates are inexpensive and reliable, they possess thermal conductivity properties that are about 50% that of copper, or 200 W/m·K. More favorably, copper alloy substrates have a Coefficient of Thermal Expansion (CTE) that is closer in value to various LED semiconductor materials than more traditional thermal management materials such as pure copper and aluminum. As opposed to a heat pipe system that relies on two-phase heat transfer through the evaporation and condensation of a working fluid, copper alloy substrates rely on solid conduction heat transfer, and as a result, the distance that these copper alloy materials can transport heat with a given temperature difference is very limited.
Another disadvantage of copper alloy solid materials is mass, as these solutions may add significant mass to the overall system. Using a solid conductor like diamond can improve thermal conductivity and CTE matching, but a diamond based solution is very expensive. The optimum approach is a replacement material or system that has a high thermal conductivity and a matching CTE at a reasonable cost.
Heat Pipes and Heat Pipe Thermal Ground Planes (TGPs)
Heat pipes typically have three components: a vacuumtight, sealed containment shell or vessel; working fluid; and a capillary wick structure. The traditional heat pipe is a cylindrical tube that transfers thermal energy from a heat-generating source, such as an LED, through its collection of the heat into the evaporator section of the heat pipe, and then movement of the heat by the working fluid vapor flow to the heat pipe condenser section, where the working fluid condenses and releases the heat to a cooling medium, such as the ambient air, a circulated liquid, or a cold plate for final dissipation. Typically, the heat flux is reduced in the condenser section, facilitating heat transfer to traditionally less capable media such as air. The capillary wick structure allows the heat pipe to develop the capillary action for the returning liquid from the condenser to the evaporator.
For most electronics cooling applications, water is the most commonly used working fluid; other working fluids can be used in applications requiring operation at unusually high temperature (>300°C) or low temperatures (<0°C). Capillary wick structures have many configurations, with typical structures being internal grooves, wire or screen mesh, and sintered powder metal. Sintered powder metal offers the most advantages in terms of capillary pumping capability for long distance transport, heat flux capacity, freeze/thaw tolerance and high gravitational or acceleration environments.