Pin Fin Heatsinks Supply Compact Cooling Power

Sep 1, 2002 12:00 PM
By Barry Dagan, Cool Innovations Inc., Concord, Ontario, Canada

Pin fin heatsinks provide excellent cooling solutions for devices dissipating heavy thermal loads. Due to a thermally efficient physical structure and use of highly conductive materials, pin fin heatsinks pack substantial cooling power into small envelopes (Photo 1).

The structural efficiency stems from their round pins, the omnidirectional pin configuration, and large surface areas typical to pin fins.

An omnidirectional structure lets air enter the heatsink from every direction, and the round pins enhance air turbulence between the pins. Plus, the large surface area produces a unique structure that outperforms most other heatsink structures.

Generally manufactured of highly conductive alloys, pin fins employ pure aluminum and oxygen-free copper. Use of such materials provides a performance premium. For example, pure aluminum for forged pin fin heatsinks provides conductivity premium of 100% over AL380 — a common aluminum alloy used in the production of various heatsinks manufactured with a casting process.

When selecting heatsinks, designers often assume that cooling power and surface area are positively correlated and opt for fins with dense arrays. For optimal cooling, however, designers must account for the magnitude of incoming airflow as well as surface area.

To achieve optimal cooling, surrounding airflows must be able to flush through the heatsink's pin configuration. When the airflow flushes through a heatsink, the pins are exposed to considerable air turbulence; the boundary layers of still air are broken, which generates effective convection. When the heatsink's resistance to air is greater than the airflow, the airflow can't penetrate the pin structure and the heatsink doesn't provide efficient cooling. Thus, when selecting or designing heatsinks, designers must consider both the heatsink's resistance to air and its surface area.

Pin Density

When selecting heatsinks for environments with high airflows, those with dense pin arrays, (for example, with large surface areas) are appropriate. In such instances, incoming airflows are powerful enough to overcome the high resistance to air flow presented by the dense pin array. As a result, it exposes large surface area to incoming airflows, generating substantial cooling power.

For environments with slow-moving air and heatsinks that operate in the natural convection mode, designers can obtain optimal performance using sparsely configured pin fins. When exposed to slow-moving air, heatsinks with high pin densities present too strong of a resistance to incoming airflows and are not as effective. As a result, heatsinks with much smaller surface areas are suitable.

To further improve the heatsink selection process, designers may use CFD (Computational Fluid Dynamics) analysis tools. Using these tools, designers can customize pin densities, pin diameters, and the ratio of pin height to base thickness for optimal solutions for specific applications.

The following are the results of a thermal experiment to test the behavior of dense and sparse pin fins in two environments: high speed and low speed. The tested heatsinks have a 2-in.² footprint and are 1.1-in. high. The dense pin fin, UltraCool 2-202011R, has 117 pins, while the sparse pin fin, UltraCool 2-202011M, has 66 pins.

When placed in a wind tunnel with 200 LFM (linear feet per minute) airflow, the low-density pin fin outperformed the dense solution by 11%. When exposed to 600 LFM, the dense pin fin provided a 17% cooling premium, showing that both surface area and surrounding airflows are crucial in the heatsink selection process.

Metallurgy

Pin fins come in aluminum, copper and a copper/aluminum hybrid form. Aluminum pin fins are the most common, mainly due to cost and weight considerations. However, for certain applications, copper and copper/aluminum hybrids are appropriate due to their additional cooling power and ability to quickly spread dissipated heat along the base of the heatsink.

Copper pin fin heatsinks provide, on average, a 20% cooling premium when compared with aluminum pin fin heatsinks. However, as a result of the copper's significant conductivity premium, copper pin fins also spread heat along their base more rapidly. Consequently, copper pin fins are suitable for devices that dissipate heavy thermal loads and those that have small and focused heat sources.

Impingement Cooling

Pin fins are much more effective when used in the impingement cooling mode. In this mode, the heatsink has a fan mounted directly over it, blowing the air down into the pin structure (Photo 2). When compared with the traditional cooling from the side mode (at 500 LFM to 600 LFM) approach, impingement cooling provides a performance enhancement of up to 20%. At the same time, in impingement cooling, the cooling power spreads evenly along the heatsink, eliminating the “hot end effect” typical of cooling from the side.

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