As heat loads continue to escalate in electronic equipment, designers seek out more powerful heat sinks to cool their electronic devices. In many instances, designers are turning to pin fin heat sinks, which are considered to be one of the most effective heat sink technologies available.
Pin fin heat sinks, (see Figure 1), which contain an array of vertically oriented round pins made of copper or aluminum, deliver substantially greater performance than standard heat sinks with flat fins. The aerodynamic nature of the round pins and their omnidirectional configuration enable pin fin heat sinks to transfer heat very efficiently from the heat generating device to the ambient environment. As a result, this advanced heat sink style is used in a wide variety of applications and industries, wherever difficult cooling challenges arise.
Although standard pin fin heat sinks provide substantial levels of cooling, there are applications in which even greater cooling power is required. With these applications in mind, two innovative derivatives of the pin fin heat sink were developed. Splayed pin fins and hybrid pin fins both possess the round pins associated with the standard pin fin heat sink. But as result of their structural and metallurgical enhancements, these two new heat sink styles push heat sink performance to higher levels.
Splayed Pin Fin Heat Sinks
Splayed pin fin heat sinks possess the same structural properties as standard pin fin heat sinks, but their pins are oriented differently. Unlike standard pin fins that possess a vertical pin array, splayed pin fin heat sinks feature pins that are bent outward gradually (see Figure 2).
By bending the pins outward, the distance between the pins is increased substantially so that incoming airstreams do not have to penetrate through a tight pin array. As a result, the resistance to incoming airstreams is reduced significantly versus that of a standard pin fin heat sink with similar footprint and pin count. The impact of increased pin spacing on heat sink performance is magnified at lower air speeds because weak airstreams have less power to penetrate the array of pins.
With splayed pin construction, the additional distance between the pins does not require a reduction in the surface area of the heat sink. Contrast that with vertical pin fins, where any added distance between the pins requires the elimination of some pins, and therefore less surface area and less potential cooling power. So, at lower airspeeds, splayed pin fin heat sinks offer the advantage of reduced resistance to airflow without sacrificing any surface area. As a result, splayed pin fins are recommended for low and moderate airspeed environments and for natural convection cooling. These environments have air speeds ranging from 0 to 400 LFM.
Low airspeed environments are becoming more and more common due to the large number of devices that fill today’s pc boards. With more devices packed more densely on boards, there are more objects to block and slow the flow of air. In these environments, splayed pin fin heat sinks are up to 40% more efficient than standard pin fin heat sinks that possess the same surface area. Splayed pin fins are available in copper and aluminum variations and range in footprint from 0.5 inches x 0.5 inches to 2.0 inches x 2.0 inches.
To illustrate the enhanced cooling performance delivered by splayed pin fin heat sinks, consider two sample heat sinks—one with splayed pins and the other with standard vertical pins. The heat sinks used in the experiment both have a footprint of 2 inches x 2 inches, an overall height of 1.1 inches, and 225 pins. The surface area of each heat sink is the same.
Both heat sinks are tested under three airflow conditions—200 LFM, 100 LFM and still air (natural convection)—and their thermal resistances are measured from case to ambient. When exposed to 200 LFM of airflow, the thermal resistance of the vertical pin fin heat sink measures 0.69°C/W versus 0.62°C/W for the splayed pin fin heat sink. So the splayed pins produce a 11% reduction in thermal resistance. When the airspeed is lowered to 100 LFM, the thermal resistance of the vertical pin fin heat sink increases to 0.88 °C/W versus 0.75 °C/W for the splayed pin fin model. Again, the performance improvement is 17%.
Finally, the two heat sinks are tested in still air. In this case, the thermal resistance of the vertical pin fin unit measures 6.70 °C/W versus just 4.83 °C/W for the splayed pin fin—a 40% improvement. This result demonstrates how the splayed pin fin heat sink offers its greatest advantage at lower airspeeds.
Hybrid Pin Fins
Hybrid pin fins feature the same pin structure as standard pin fin heat sinks, but change the material used in the base. Unlike standard pin fin heat sinks that are either composed of aluminum or copper, hybrid pin fin heat sinks consist of aluminum pin fins that are reflowed onto a copper plate. Hybrid pin fins range in footprint from 1.5 inches x 3.0 inches to 8.2 inches x 8.2 inches with heights ranging from 0.3 inches to 1.3 inches (see Figure 3).
Hybrid heat sinks are designed for devices that feature small and focused heat sources. Such devices require heat sinks that are significantly larger than the devices they reside on. These heat sinks are also intended for multi-device cooling, in which a single heat sink is used to cool more than one component.
Both of these scenarios are becoming more popular as designers try to maximize the space available for cooling purposes. They do so by using application space that is not directly over the heat generating device. In each case, the heat sink must be able to spread the heat quickly along its base to operate efficiently. Otherwise the areas of the heat sink far away from the device will not able to provide any cooling
When compared to all-copper heat sinks, hybrid heat sinks provide similar spreading power, as the spreading of the heat occurs just along the base. The major advantage of hybrid heat sinks over all-copper models is their lighter weight. Copper is approx. 3.2 times the weight of aluminum. So, depending on the size of the heat sink, hybrids may be up to 50% lighter then all-copper heat sinks of the same size.
To illustrate the premium associated with hybrid pin fins, consider two heat sinks developed for multi-device cooling. Each heat sink measures 5 inches x 5 inches x 0.8 inches, but one model is all copper, while the other is a copper-aluminum hybrid. Although the all-copper heat sink weighs twice as much as the hybrid, its thermal resistance (as measured from case to ambient) is only 2% to 3% lower than the hybrid.
In the most-demanding cooling applications, designers can deploy splayed pin fin heat sinks and hybrid pin fin heat sinks to achieve the required cooling without making excessive tradeoffs in heat sink size or weight. As with vertical pin fin designs, these new variations are highly customizable. Designers can tailor heat sink footprints, pin counts, and other parameters for optimum cooling in their applications.
Barry Dagan, P. E. is Chief Technology Officer of Cool Innovations, Inc, Barry has more then 25 years of experience in thermal management design and holds multiple patents.
Barry’s contact information:
Phone: 905 760-1992
Company Website: www.coolinnovations.com
141 Cidermill Avenue, #2,
Concord, Ontario L4K 4G5, Canada