By Michael Haskell, Thermal Engineer
and Norman Quesnel, Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc.
(This article was featured in an issue of Qpedia Thermal e-Magazine, an online publication produced by Advanced Thermal Solutions, Inc. (ATS) dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine.)
As thermal solutions for today’s electronics grow more challenging, demand rises for novel cooling ideas or materials. As a result, the proven methods of analytical calculations, modeling and laboratory testing are sometimes bypassed for a quick “cure-all” solution. Evolutionary progress is required of the thermal industry, of course. But, despite the urgency to introduce new ideas and materials, thorough testing should be performed in determining the thermal performance of a solution before it is implemented.
This article addresses the impact of material choice on heat sink performance. First, an evaluation of different materials is made in a laboratory setting, 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 computational fluid dynamics (CFD) 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.
Laboratory Tests of Copper, Aluminum, and Graphite Foam
The stated thermal properties of engineered graphite foams have enhanced their consideration as heat sink materials. Yet, the literature is void of a true comparison of these materials with copper and aluminum. 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.
(The results for ducted and jet impingement flows, though similar to the unducted case, will be presented in a future article along with a secondary graphite foam material.)
Earlier foam experiments by Coursey et al.  used solder brazing to affix 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) a greater potential for heat sink damage occurs than with aluminum or copper . 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.
In this study, the 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 watts 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. Both cardboard and FR-4 board were used to insulate the bottom of the heater, The estimated value of Ψjb is 62.5°C/W. Throughout testing, the value of Ψjb was 36–92 times greater than that of Ψja.
As expected, the traditional copper and aluminum heat sinks performed similarly. The main difference was due to 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 Θja.
As flow speed increases, the convection resistance decreases, and the internal heat sink conduction resistance is more of a factor in the overall Θja value. This behavior is evident in the table below, and when comparing the different heat sink materials. The graphite heat sink’s thermal performance was only 12% lower than aluminum at low flow rates. However, the performance difference increased to 25-30% as the flow rate increased (Table 3).
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 ΨHEATER-BASE in Table 3. To decouple the effect of interfacial resistance ΨBASE-AIR can be calculated. When ignoring interfacial resistance in this manner foam performs within 1% of aluminum at 1.5 m/s, and within 15% at 3.5 m/s.
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 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 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.
Because of the stepped base and a long heat conduction path, spreading resistance was a major factor in the overall thermal resistance. The effect of copper in place of aluminum due to its higher thermal conductivity (400 and 180 W/m*K respectively) is shown in Table 4. The CFD software predicted a 21% 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 using CFDesign software.
Design engineers have many materials at their disposal to meet the challenging thermal needs of modern components. Classic materials such as aluminum and copper are joined by new technologies that bring improvements in cost, weight, or conductivity. The choice between a metallic, foam or plastic heat sink can be difficult because thermal conductivity provides the only available information to predict 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.
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 Graphitic Foams, Oak Ridge National Laboratory, 2003.