Cooling the pin-fin way

Barry Dagan
Technical Director
Cool Innovations Inc.
Concord, Ontario
Canada

EDITED BY MILES BUDIMIR

Increasing the processing power of electronic equipment often means that more transistors have to be squeezed into tighter packages. And higher circuit densities increase the amount of heat that must be dissipated. One of the most common ways to dissipate heat is to use a convection-type heat sink. However, because these heat sinks can consume 25 to 45% of the total system volume, smaller and more compact heat sinks are needed. Pin-fin heat sinks represent an emerging technology designed to solve complex thermal problems where heat loads are substantial and space is limited.

HEAT SINK BASICS
The efficiency of various heat-sink types depends mainly on three factors: surface area, structure or shape, and material. Cooling capabilities relate directly to surface area; the larger the surface area, the more heat that can be dissipated. Physical structure is another factor. Proper structure increases turbulent airflow which creates a more efficient heat sink.

The heat-sink material is also crucial. Copper, for instance, has superior cooling qualities to aluminum because the thermal conductivity of copper is much higher than that of aluminum. At room temperature, copper has a thermal conductivity of 401 W/m-K while aluminum is 235 W/m-K. Consequently, all other factors being equal, a heat sink made of copper dissipates more heat than a heat sink made of aluminum.

A technical term that can be used to compare various heat-sink technologies is volumetric efficiency, or VE. VE is the product of thermal resistance and heat-sink volume, where thermal resistance equals temperature increase per watt (°C/W) and heat-sink volume equals footprint area times height.

The lower the thermal resistance, the more effective a heat sink is, and the smaller the heat-sink's volume (but larger surface area) the more efficient it will be. Consequently, a low volumetric efficiency number means a more efficient heat sink.

THE PIN-FIN ALTERNATIVE
Among existing heat-sink technologies, pin-fin heat sinks represent an efficient cooling solution. They have a large surface area in relation to any other heat-sink volume. Also, the round pins and pin spacing let blown air create a significant amount of turbulence between the pins. This breaks up boundary layers around the pins, creating high convective thermal coefficiencies. In addition, pin-fin heat sinks are made of highly thermally conductive aluminum alloys or copper.

Pin-fin heat sinks consist of a base and an array of embedded pins. Parameters such as base-plate dimensions (both footprint and thickness), pin length, thickness and density, and material (aluminum, copper, or copper-aluminum hybrids) fit most specifications. As a result, pin fins can easily be customized to fit various applications depending on heat loads involved, available space, and airflow.

The volumetric efficiency of pin fins is higher than most other heat-sink shapes. As a result, pin-fin heat sinks are significantly smaller and lighter compared to their cooling ability. In one instance, switching to a pin fin heat sink while keeping the same 1 × 1-in. footprint dropped the temperature by 10°C. In some instances, pin fins are two to 10 times more efficient than extrusion-type heat sinks.

The amount of airflow in a system is a key factor when selecting a heat sink. As a rule of thumb, forced-convection heat sinks outperform natural-convection heat sinks, all other factors being equal, by a factor of 10 to one. However, certain heat sinks are more efficient in forced convection and vice versa. Controlling pin density ensures an optimal design for various levels of airflow. In general, a high pin density is most efficient in forced convection and a low pin density is more efficient in natural convection.

Pin-fin heat sinks are efficient in impingement cooling where a fan placed on top of the heat sink blows air directly on the surface. The pin-fin structure for impingement cooling lowers the thermal resistance. Impingement cooling also distributes the air evenly along the heat sink. This eliminates the temperature gradient and the associated hot end typical of traditional side cooling, and extends the life span of the device.

Pin-fin heat sinks do not contain bonding agents, so most plating methods are acceptable. This means that aluminum heat sinks can be anodized and copper heat sinks can be oxidized. Consequently, pin-fin heat sinks can withstand harsh industrial or commercial environments.

Here are a few examples that illustrate the efficiency of pin-fin heat sinks.

ASICs, or application specific ICs, generally dissipate from 1 to 10 W. ASICs that dissipate more than 2 W require extensive cooling. ASICs are often mounted on daughterboards, leaving a limited amount of space for cooling. The distance between boards is generally between 0.4 and 1.0 in. and the amount of airflow is limited. Heat sinks often mount on top of ASICs using doublesided thermally conductive adhesive tape.

A pin-fin heat sink with a footprint of 1.2 in. and a total height of 0.7 in. under forced convection can cool 10 W within a temperature rise of 17°C on its base. Thermal resistance is 1.67°C/W.

In natural convection, a pin-fin heat sink with a footprint of 1 in. and a total height of 0.5 in. intended for natural convection (low pin density) can cool 2 W within a temperature rise of 40°C on its base. Thermal resistance here is 20.05°C/W.

Pin-fin heat sinks are also used to cool IGBTs (insulated gate bipolar transistors) in impingement cooling. A heat sink with a 5 × 5-in. footprint and a total height of 1.3 in. can cool 625 W of dissipated power within a temperature rise of 50°C on its base, using standard fans. Thermal resistance is 0.08°C/W. Similarly, a 2.4 × 2.4-in. heat sink with a 1.1-in. total height can cool 185 W of dissipated power within a temperature rise of 50°C on its base, using a standard fan. Thermal resistance is 0.27°C/W.