Selecting Pin Fin Heat Sinks for Surface Mount Devices

TODAY'S CUTTING EDGE INTEGRATED CIRCUITS DISSIPATE SUBSTANTIALLY HEAVIER LOADS OF HEAT THAN EVER BEFORE. At the same time, the premium associated with miniaturized applications has never been greater, and space allocated for cooling purposes is on the decline. These factors have forced design engineers to seek more efficient heat sink technologies.

One of the more powerful cooling technologies that have emerged in recent years is the pin fin technology. The unique pin fin design generates substantial cooling power and is highly suitable for "hot" devices and applications that have limited space for cooling.

Pin fin heat sinks for surface mount (SM) devices are available in a variety of configurations, sizes and materials. The following step-by-step breakdown will aid in selecting the most suitable heat sink for your application.

1. COMPILE DATA REQUIREMENTS
The following is a summary of the information that should be gathered prior to selecting a heat sink:

• Power dissipated by the device
• Maximum safe junction temperature of the device
• Junction-to-case temperature
• Maximum ambient temperature of the system
• Dimensions of the device
• Material of the package (metal, ceramic or plastic)
• Available room for the heat sink (footprint and height)
• Airspeed approaching the heat sink

The required thermal resistance of the heat sink can be determined from the thermal data and ambient temperature.

2. SELECT THE MATERIAL
Pin fin heat sinks for SM devices are available in aluminum and copper variations. Copper is by nature significantly more conductive than aluminum, and alloys used for copper pin fin heat sinks (see Figure 1) are twice as conductive as ones used for aluminum pin fins. However, in terms of cooling power, copper pin fins are typically only 5 to 15 percent more powerful then identically structured aluminum pin fin

The cooling premium that is generated by copper pin fin heat sinks is offset by weight and cost issues. In terms of weight, copper pin fins are substantially heavier than identically structured aluminum pin fins, by a ratio of 3.1 to 1. In terms of price, copper pin fins are substantially more expensive. Thus, for the majority of cooling scenarios, aluminum pin fins are recommended. Copper pin fin heat sinks are recommended for extreme cooling needs, in which the additional cooling power is required and for devices that possess small and focused heat sources.

Devices that possess small and focused heat sources do not only require a heat sink that offers substantial cooling power (i.e. low thermal resistance), but also one that will be able to rapidly spread the heat along the base in order to prevent a hot spot at the junction of the chip. In such scenarios, the highly conductive copper can quickly spread heat along the base and, as a result, a copper heat sink performs both as a heat spreader and a heat sink simultaneously. A commonly used semiconductor package that requires copper heat sinks is the open die flip chip package, in which the heat sink is placed directly on top of the exposed silicon.

3. SELECT A PIN CONFIGURATION
Pin fin heat sinks are comprised of a base and an embedded array of round pins. They are available in a variety of pin configurations that feature varying pin diameters and pin densities. When selecting a heat sink, the pin configuration has to match the system's available airspeed. Airspeed environmentscan be divided into three groups: high (400-800 LFM), moderate (200-400 LFM) and low speed (0-200 LFM) environments. Figure 2 illustrates three different pin configurations for high, moderate and low air speeds.

When selecting a heat sink, one must ensure that the pin configuration does not present more resistance than the airflow can break through. Otherwise, if the approaching airspeed is not high enough, the air will stall in between the pins, and poor heat convection will result.

In high airspeed environments, a dense heat sink will provide the best possible cooling solution, as its large surface area will be efficiently flushed through the air. However, in a moderate airspeed environment, a densely configured heat sink will produce too much resistance to incoming airflows and the heat sink will be rendered inefficient. In such an environment, surface area must be given up, and a moderately spaced heat sink should be selected.

In low airspeed environments, in order to allow surrounding airflows to penetrate into the pin array, the spacing between the pins must be increased even more. Consequently, the heat sink will possess limited total surface area and thus limited cooling power. A new type of heat sink design that addresses the specific requirements of low airspeed cooling is the splayed pin fin design. Splayed pin fin heat sinks (see Figure 3) are sparsely configured to allow weak, low speed air, to penetrate into their pin arrays. However, due to their innovative design, they possess substantially more surface area than other types of low airspeed heat sinks. When compared to traditional pin fins, splayed pin fins offer a 20% to 30% performance premium in low-airspeed environments.

4. SELECT A FOOTPRINT
In most cooling scenarios, the footprint should match, or be slightly larger than, the footprint of the device it is placed on. However, for devices that dissipate heavy heat loads, a heat sink with a matching footprint may not possess sufficient surface area to provide adequate cooling power. In such scenarios, overhanging a heat sink with a larger footprint than the device may be the only viable solution. Overhanging a larger, standard, heat sink over the device is not possible-in all applications due to the existence of devices that sit beside the device being cooled. In instances where space is available in some, but not all directions, pin fins provide ideal solutions due to their highly customizable nature. Pin fins can be easily customized to meet any required square or rectangular shapes without any hard-tooling charges. As a result, if space is available in one or two directions, a rectangular heat sink can be implemented.

5. SELECT A HEIGHT
When selecting a heat sink, one must try to make the best use of any vertical space that is available for cooling purposes. Unless an off-the-shelf pin fin possesses the desired height, a standard pin fin can easily be customized to meet application-specific height requirements.

Unlike most other heat sink technologies, forged pin fin heat sinks can be easily trimmed to meet the required height, in a cost-effective fashion (see Figure 4). The forging technology enables trimming to any feasible height without any associated tooling charges. In addition, if required, pins can be trimmed to multiple heights rather than a single height.

6. SELECT AN ATTACHMENT METHOD
The three attachment methods that are most commonly used to attach heat sinks to SM devices are thermally conductive glue, thermally conductive double-sided adhesive tape and mechanical clips. The following is a short summary of important characteristics of each of the main attachment methods, that should be considered when selecting an attachment method.

Thermally conductive glue adds a relatively low source of thermal resistance, is easy to apply, and is considered to be reliable. However, it is messy, requires some skill in the application process and is difficult to remove.

Thermally conductive tape is cost effective and is easy to apply. However, it may not be suitable for high-reliability application and is not suitable for components that are smaller then 0.5" x 0.5". The use of tape requires a certain flatness of the package as well as proper cleaning of the part of the package that comes in contact with the tape.

Mechanical clips provide a low source of thermal resistance. They are considered to be the most reliable alternative and consequently are often used in high-reliability applications. At the same time, mechanical clips are relatively expensive and generally require time to assemble.

CONCLUSION
Keep in mind that for every 10°C reduction in junction temperature, a device's life expectancy doubles. By taking into account a heat sink's material, pin configuration, footprint, height and attachment method and matching those parameters to the cooling requirements of a particular application, the reliability of a system will be greatly improved.