How to Select a Heatsink

Operating temperature and thermal performance are directly related; the lower, the better – and the higher a device’s reliability. Thermal management is more critical than ever for electronic product design as semiconductor devices continue scaling smaller with higher power densities.

Heatsinks and other enhanced cooling solutions are an effective means of reducing power consumption by lowering junction temperature. By increasing the surface area in direct contact with the coolant, heatsinks allow more heat to be dissipated while lowering the device operating temperature.

Though the overall thermal budget is generally the key deciding factor of which heatsink to use, this blog will examine precisely how to choose the ideal heatsink depending on the project and application needs.

Step One: Thermal Resistance

Determining the heatsink thermal resistance is the first step when beginning selection. The maximum case temperature, ambient air temperature ()*and power dissipation () of the component can be used to find the required thermal resistance () of the heat sink: 

*Note:  typically ranges from 35 to 45°C if the external air is used and from 50 to 60°C if the component is enclosed.

To find the approximate heat sink volume, designers can utilize this equation alongside charts provided by heatsink suppliers. For an example of these charts, please click here. For forced convection applications, these charts and/or performance graphs can be valuable for identifying heatsinks, determining minimum flow velocity, and deciding if liquid-cooled cold plates are necessary.

In Example 1, the power dissipation is 100 watts, and a maximum case temperature of 95°C is used with an ambient temperature of 25°C – resulting in 0.75 °C per watt thermal resistance. In this instance, a heat sink of approximately 105 cubic inches is be required. When forced air is applied, however, 20 cubic inches would suffice.

Step Two: Choosing Heatsink Materials

Heatsinks need to perform with high thermal conductivity. As such, their materials must be carefully considered.

Though copper is the “gold” standard of thermal conductivity, it can be costly. So, aluminum has become an industry standard, balancing cost-effectiveness and thermal conductivity. Depending on its surface area and finish, aluminum’s performance can vary slightly.

Step Three: Determining Required Volume

After materials, designers should determine the required volume of the heatsink. Usually, a chart and/or graph provided by the manufacturer providing ranges of volumetric thermal resistance of a heat sink under different flow conditions make this process quite simple.

At this stage, classifying the heat sink’s airflow is also necessary. Airflow will either be natural, mixed, or high flow forced convection.

• Natural Convection: No externally induced flow. The free flow of air surrounding the heat sink is the sole means of heat transfer.
• Forced Convection: The flow of air is induced by mechanical means, often a fan or blower.

Step Four: Fins

Fins create shapes capable of dissipating large heat loads and are often strategically utilized in highly efficient extruded heatsinks. By “squeezing” or extruding the aluminum, long sections are created and cut to proper part lengths. If needed, mounting holes or other features can then be machined.

Alternately, Forged Pin heat sink geometry can provide increased surface area for heat transfer. This is achieved by manufacturing the piece under low temperature and high pressure. This ensures no impurities or air bubbles get trapped in the heatsink material, yielding very high quality.

Forged Pins are especially useful for forced air convection applications, as they allow for the ability to force air in multiple directions through these pins. Whether for high power density or small size electronic packaging with forced convection cooling, these heatsinks are an ideal fit.

Step Five: The Finish – Anodization and Mounting

The finish is another important consideration. For instance, anodization–a standard finish–offers a range of benefits.

By thickening and increasing the toughness of the naturally occurring protective oxide layer on the surface of a metal part, it improves the electrical isolation, wear, and corrosion resistance of a heatsink. This finish also improves surface emissivity or the effectiveness in emitting energy as thermal radiation.

Clips can also help to accomplish fast assembly of heat dissipaters. However, the clip must be designed specifically for a given heatsink and semiconductor package to provide proper pressure. The clip will also provide lower interface thermal resistance compared to other assembly methods.

Though a heat sink lowers the fluid-side thermal resistance, clips are worth noting as heatsinks also introduce an interface resistance across the contact formed between itself and the package case. Depending on the circumstances, this contact resistance may be significant, impeding heat flow and lowering the heatsink’s overall effectiveness.

Ohmite’s patented clip system eliminates the use of screws and holes for installation. This provides users an easier, more streamlined assembly process. The elimination of mounting hardware enables a maximum surface area per unit design and reduces cost.

Summary

In summary, when choosing the proper heatsink for an application, designers must consider several factors.

To calculate thermal resistance, several parameters are needed, including the maximum case temperature, ambient air temperature, and power dissipation.

Ohmite offers multiple configurations and sizes to accommodate various application needs and designs, focusing on extruded designs and those that maximize surface area by adding or extending fins with clip mounting or other creative designs.

Ohmite’s heat sink series can also accommodate different wattages and mounting options; from surface mount devices to multiple unit extrusions, Ohmite has a heat sink solution for popular TO series packages.

For more information, visit www.ohmite.com.