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Comparing the Impact of Different Heat Sink Materials on Cooling Performance

Fri, 04/30/2010 - 7:00am
Michael Haskell, Thermal Engineer, Advanced Thermal Solutions, Inc., www.Qats.com

This article addresses the impact of material choice on heat sink performance. First, we evaluate different materials using mechanical samples and a research quality wind tunnel. This testing compares a constant heat sink geometry made from copper, aluminum, and graphite foam. Next, an application-specific heat sink study is presented using CFD (computational fluid dynamics) software. In this study, a heat sink was designed in 3D CAD to cool a dual core host processor. The performance of both an aluminum and copper design was then evaluated using CFD.

Copper, Aluminum, and Graphite Foam
The reported thermal properties of engineered graphite foams have enhanced their consideration as heat sink materials. To evaluate graphite foam as a viable material for heat sinks, a series of tests were conducted to compare the thermal performance of geometrically identical heat sinks made of copper, aluminum, and graphite foam respectively. Testing was conducted in a research quality laboratory wind tunnel where the unducted air flow was consistent with typical applications.

Foam experiments by Coursey et al. [1] used solder brazing to attach a foam heat sink to a heated component. The solder method reduced the problematic interfacial resistance when using foams, due to their porous nature. Directly bonding the heat sink to a component has two potential drawbacks. First, the high temperatures common in brazing could damage the electrical component itself. The other issue concerns the complicated replacement or rework of the component. Due to the low tensile strength of foam (Table 1), there is a greater potential for heat sink damage than with aluminum or copper [2]. If the heat sink is damaged or the attached component needs to be serviced, direct bonding increases the cost of rework.

To avoid these problems, the foam heat sink can be soldered to an aluminum or copper carrier plate. This foam-and-plate assembly can then be mounted to a component in a standard fashion. The carrier plate allows sufficient pressure to be applied to the interface material, ensuring low contact resistance.

For a more direct comparison, in this study, all heat sinks were clamped directly to the test component without a carrier plate as a baseline for all three materials. Shin-Etsu X23 thermal grease was used as an interface material to fill the porous surface of the foam and reduce interfacial resistance.

Five J-type thermocouples were placed in the following locations: upstream of the heat sink to record ambient air temperatures, in the heater block, in the center of the heat sink base, at the edge of the heat sink base, and in the tip of the outermost fin.

A thin film heater was set at 10 W during all testing, and the heat source area was 25 mm x 25 mm, or one quarter of the overall sink base area, as shown in Figure 1. Cardboard and FR-4 board were both used to insulate the bottom of the heater, The estimated value (characterization parameter) of ?JB (junction-to-board) is 62.5°C/W. Throughout testing, the value of ?JB was 36, which is 92x greater than that of ?JA (junction-to-ambient).

QATS-Table1

Figure 1. Tested heat sink shape and dimensions.  

Table 2. Tested heat sink geometry.


Figure 2.  Experimental heater and measurement setup.

Results
As expected, the traditional copper and aluminum heat sinks performed similarly. Their main difference was the higher thermal conductivity of copper, which reduced spreading resistance.

During slow velocity flow conditions, the lower heat transfer rate means that convection thermal resistance makes up a large portion of the overall ?JA (thermal resistance, junction-to-ambient). As airflow speed increases, the convection resistance decreases, and the internal heat sink conduction resistance is more of a factor in the overall ?JA value. This behavior is seen in Table 3 and when comparing the different heat sink materials. The graphite heat sink’s thermal performance was only 12 percent lower than aluminum at low flow rates. However, the performance difference increased to 25-30 percent as the flow rate increased.

Due to the lack of a solder joint, the foam heat sink experienced a larger interfacial resistance when compared to the solid heat sinks. This difference can be seen when comparing ?HEATER-BASE in Table 3. To decouple the effect of interfacial resistance, ?BASE-AIR can be calculated. When ignoring interfacial resistance in this manner foam performs within 1 percent of aluminum at 1.5 m/s, and within 15 percent at 3.5 m/s. 

Figure 3.  Heat sink thermal resistance as a function of velocity.


Table 3.  Specific thermal test results.

Graphite foam-derived heat sinks show promise in specific applications, but exhibit several drawbacks in mainstream electronics cooling. Due to the frail nature of graphite foam, unique precautions must be taken during the handling and use of these heat sinks. When coupled to a copper base plate, graphite foam can perform with acceptably small thermal spreading resistances. However, the foam’s lower thermal conductivity reduces thermal performance at high flow velocities compared to a traditional copper heat sink.

The mechanical attachment needed to ensure acceptable thermal interface performance without soldering or brazing also hinders foam-based heat sinks from being explored in mainstream applications. Despite these challenges, the thermal performance-to-weight ratio of foam is very attractive and well-suited to the aerospace and military industries, where cost and ease of use come second to weight and performance.

Thermal Software Comparison of Aluminum and Copper Heat Sinks
A challenging thermal application was considered. This involved the use of a dual core host processor on a board with a limited footprint area for a heat sink of sufficient size. A heat sink with a stepped base was designed to clear onboard components. It provided sufficient surface area to dissipate heat (Figure 4).

Due to the complexity of the heat sink, machining a test sample from each material was not practical. Instead, CFD was used to predict the performance difference between the two materials and determine if the additional cost of copper was warranted. 

Figure 4. Stepped-base maxiFLOW heat sink (ATS).

Because of the sink’s stepped base and long heat conduction path, spreading resistance was a major factor in the overall thermal resistance. Due to its higher thermal conductivity (400 W/m?K) the effect of copper in place of aluminum (180 W/m?K) is shown in Table 4. The CFD software predicted a 21 percent improvement using copper in place of aluminum. More importantly, it reduced the processor case temperature below the required goal of 95°C.

The performance improvement with copper is due to the reduced spreading resistance from the processor die to the heat sink fins. This effect is shown in Figure 5, where the base temperatures of both heat sinks are obtained from the CFD analysis and plotted together. The aluminum heat sink shows a hotter center base temperature and a more pronounced drop off in temperature along the outer fins. The copper heat sink spreads the heat to all fins in a more even fashion, increasing the overall efficiency of the design. This temperature distribution can be seen in Figures 6 and 7, which were created with CFDesign software. 

Table 4. Heat sink performance at 55°C (ambient).


Figure 5. Effect of heat sink material on temperature distribution.


Figure 6. Aluminum stepped-base heat sink simulation.

Figure 7. Copper stepped-base heat sink simulation.

Conclusion
Design engineers have many materials at their disposal to meet the challenging thermal needs of modern components. Traditional materials such as aluminum and copper are joined by new technologies that bring improvements in cost, weight, or thermal conductivity. The choice between a metallic or foam heat sink can be difficult because thermal conductivity provides the only available information to predict its performance. The first method for determining material selection is a classic thermodynamics problem: what effect does conductivity have on the overall thermal resistance in my system? Only once this is answered can the benefits of cost, weight, and manufacture be addressed.

References:

1. Coursey, J., Jungho, K. and Boudreaux, P. “Performance of Graphite Foam Evaporator for Use in Thermal Management,” Journal of Electronics Packaging, June 2005.
2. Klett, J., “High Conductivity Graphite Foams,” Oak Ridge National Laboratory, 2003.

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