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Research Papers

# Thermal Analysis of Miniature Low Profile Heat Sinks With and Without Fins

[+] Author and Article Information
V. Egan

Department of Mechanical and Aeronautical Engineering, Stokes Institute, University of Limerick, Limerick, Irelandvanessa.egan@ul.ie

P. A. Walsh, E. Walsh, R. Grimes

Department of Mechanical and Aeronautical Engineering, Stokes Institute, University of Limerick, Limerick, Ireland

J. Electron. Packag 131(3), 031004 (Jun 23, 2009) (11 pages) doi:10.1115/1.3144150 History: Received April 18, 2008; Revised February 05, 2009; Published June 23, 2009

## Abstract

Reliable and efficient cooling solutions for portable electronic devices are now at the forefront of research due to consumer demand for manufacturers to downscale existing technologies. To achieve this, the power consumed has to be dissipated over smaller areas resulting in elevated heat fluxes. With regard to cooling such devices, the most popular choice is to integrate a fan driven heat sink, which for portable electronic devices must have a low profile. This paper presents an experimental investigation into such low profile cooling solutions, which incorporate one of the smallest commercially available fans in series with two different heat sink designs. The first of these is the conventionally used finned heat sink design, which was specifically optimized and custom manufactured in the current study to complement the driving fan. While the second design proposed is a novel “finless” type heat sink suitable for use in low profile applications. Together the driving fan and heat sinks combined were constrained to have a total footprint area of $465 mm2$ and a profile height of only 5 mm, making them ideal for use in portable electronics. The objective was to evaluate the performance of the proposed finless heat sink design against a conventional finned heat sink, and this was achieved by means of thermal resistance and overall heat transfer coefficient measurements. It was found that the proposed finless design proved to be the superior cooling solution when operating at low fan speeds, while at the maximum fan speed tested of 8000 rpm both provided similar performance. Particle image velocimetry measurements were used to detail the flow structures within each heat sink and highlighted methods, which could further optimize their performance. Also, these measurements along with corresponding global volume flow rate measurements were used to elucidate the enhanced heat transfer characteristics observed for the finless design. Overall, it is shown that the proposed finless type heat sink can provide superior performance compared with conventional finned designs when used in low profile applications. In addition a number of secondary benefits associated with such a design are highlighted including lower cost, lower mass, lower acoustics, and reduced fouling issues.

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## Figures

Figure 1

Labeled picture of the Micronel U16LM-9 fan used throughout experimentation with inlet orifice and outlet vent highlighted

Figure 2

Plot showing the Micronel U16LM-005GK-9 driving fan’s characteristic curve along with the predicted system curve for an optimal finned heat sink configuration as predicted by Eq. 4

Figure 3

Optimum fin spacing for maximum heat transfer density

Figure 4

Schematic of finned heat sink outlining the overall dimensions and the optimized parameters, b and tfin

Figure 5

Photograph of fan and (a) finned heat sink or (b) finless heat sink used during experimentation with scale relative to a one Euro coin. The copper top cover is partially removed to reveal heat sink interior geometry.

Figure 6

Schematic showing (a) dissipation of QTOTAL by forced and natural convection cooling and (b) corresponding thermal resistance network

Figure 7

Schematic of experimental setup used for PIV measurements

Figure 8

Relationship between thermal resistance and fan rotational speed for both finned and finless cooling solutions

Figure 9

Plot outlining the resistances to the flow of heat by forced convection (FC) and by secondary heat dissipating mechanisms (losses)

Figure 10

Relationship between the forced convective heat transfer coefficient and fan rotational speed for both finned and finless cooling solutions

Figure 11

Normalized PIV plots depicting the flow within the finless heat sink for fan speeds of (a) 8000 rpm normalized with 4 m/s, (b) 5500 rpm normalized with 2.8 m/s, and (c) 3000 rpm normalized with 1.5 m/s, respectively

Figure 12

Normalized PIV plots depicting the flow within each of the finned heat sink channels for fan speeds of (a) 8000 rpm normalized with 2.7 m/s, (b) 5500 rpm normalized with 1.5 m/s, and (c) 3000 rpm normalized with 0.6 m/s, respectively

Figure 13

Magnified image of Fig. 1 showing flow in the entrance region of the finned heat sink in channels 1–4 and for a fan speed of 8000 rpm. Velocities are normalized with respect to the maximum velocity of 2.7 m/s and channel walls are shown in black with regions left uncolored so that vectors can be discerned.

Figure 14

Measured fan characteristic curve at 8000 rpm with measured operating points for both the finned and finless heat sink highlighted. Linear system curves also plotted through these operating points and the origin.

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