As electronic technologies advance in defense and aerospace applications, heat fluxes are becoming larger and more centralized[1,2]. In order to meet the performance specifications of new technologies, thermal solutions must also advance. The push for more capable thermal technologies is evident with the U.S. Air Force Research Laboratory (AFRL) signing an agreement with private companies and contractors for the development of innovative vapor cooling technologies capable of handling high heat fluxes.
Basic aluminum spreaders are no longer a viable means of spreading the heat as increased heat fluxes result in large thermal gradients across aluminum alone. Embedding heat pipes, a passive two-phase thermal solution, into an aluminum or copper spreader could distribute the heat flux to areas where it could be easily removed without a large thermal gradient. This technology is referred to as a HiK™ plate.
HiK™ heat spreaders utilize heat pipes distributed throughout a metallic spreader to disperse heat and increase the efficacy of wide convection cooling regions. A longstanding challenge with utilizing heat pipe solutions in aerospace applications is the acceleration conditions the thermal solution must endure. Heat pipe performance varies greatly under various acceleration conditions, so a unique HiK™ plate is required to meet performance requirements under all loading conditions.
Benefits of HiK Plates
From a cost and weight standpoint, aluminum is an ideal heat spreader material. However, under certain cases the relatively low thermal conductivity of aluminum can cause component temperatures to exceed maximum allowable limits. In this case it becomes common to swap the aluminum heat spreader for a copper heat spreader.
Figure 1 and Figure 2 show temperature contour plots of a circuit-card carrier heat spreader made out of aluminum and copper respectively at the interface between the spreader and a mating heatsink. While the copper spreader is able to reduce the temperature at the heatsink interface by 16% compared to the aluminum spreader, it increases the weight of the spreader 3.25 times. Figure 3 shows the same temperature contour plot, but for an aluminum HiK™ heat spreader.
The aluminum HiK™ spreader reduces the maximum temperature by 23% compared to the pure aluminum heat spreader, but only increased the weight by 5%. In certain high-heat flux applications, a copper HiK™ heat spreader can further reduce component temperatures by reducing the conduction gradient from the component to the heat pipes.
Figure 4 shows a temperature contour plot of a copper HiK™ version of this heat spreader. For this case the temperature at the heatsink interface is essentially the same between the copper HiK and the Aluminum HiK™, but the copper HiK™ was chosen as it significantly reduced the conduction gradient from the high-power components to the heat pipes. The accompanying table summarizes the impact on weight and temperature that the different heat spreaders had relatively to an aluminum heat spreader.
Electronic devices in the aerospace industry often operate under high acceleration loads. Typically, these loads are specified for a duration of time rather than as a steady-state condition. For the heat spreader discussed earlier, the customer had a requirement that the electronic components must stay under maximum allowable temperatures when the heat spreader was experiencing 6G acceleration for a duration of one minute.
A traditional copper or aluminum heat spreader will not be thermally impacted by acceleration loads. However, because HiK plates function through the use of embedded heat pipes they must be designed to account for acceleration loading. Figure 5 shows the operating curve for the same heat pipe under 1G and 6G loading. In order for a heat pipe to properly function, the amount of power carried by the pipe must fall below this curve. If the acceleration causes the operating curve to drop too much, the heat pipe will not be able to carry all of the power. This can cause the pipes to dry out and dramatically increase the temperature of components while the heat pipe is experiencing an acceleration loading. Once the acceleration stops, the heat pipe will function at full capacity again.
A heat pipe operates by evaporating its working fluid at the heat source, then moving the vaporized fluid to a condenser where it will condense and return to the evaporator via an internal wick structure. When the acceleration of the heat pipe increases, the gravitational pressure drop the liquid must overcome increases. If the gravitational pressure drop becomes too large, the wick cannot pump the fluid back to the evaporator and the heat pipe will dry out or cease operating.
Acceleration Resistant HiK Design
In order to protect components from exceeding maximum temperatures while under acceleration loading, the HiK spreader was designed so that critical components would always have a heat pipe capable of carrying enough power during acceleration loading. This was accomplished by aligning heat pipes in perpendicular directions to each other within the heat spreader.
In order to verify the design before proceeding with production of the parts, the heat spreader was analyzed under the acceleration loads specified by the customer and then tested. Based on heat pipe theory, ACT was able to predict the performance of certain critical heat pipes under the acceleration loads and built this into the simulation. However, certain heat pipes were difficult to predict in terms of their their behavior, so the pipes were treated as not being able to function in the simulation. This was done to ensure that the design would be conservative.
After performing the analysis, a prototype spreader was built and tested to validate the analysis. Figure 6 shows the centrifuge that was used to perform the testing. During the test, heat was applied to a critical component location, and the temperature rise was monitored for a duration of one minute. Figure 7 and Figure 8 show the results of the test compared to the temperature predicted by analysis for the worst-case acceleration loads. The maximum temperatures predicted by analysis agree with the test results within 4%, and the HiK heat spreader successfully passed the customers operational acceleration requirement.
With increased centralized heat fluxes in high acceleration application electronics, innovative thermal solutions are required to maintain operable component temperatures. Unique HiK™ plate designs can be used in high acceleration applications to effectively spread heat to an area where the heat can be removed. A well-designed HiK™ plate can reduce thermal gradients over an all-aluminum design significantly without a substantial weight increase.
This article was written by Jared Tower, Product Development Engineer; Dan Fritch, Lead Engineer; and Jens Weyant, Lead Engineer; Advanced Cooling Technologies, Inc. (Lancaster, PA). For more information, visit here .
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