Guide to Heat Sinks

The heat transport and evacuation process is closely related to heat sink performance. Thermal resistance must be considered when selecting a heat sink; it is measuring how much heat flow is resisted.

The standard range for ambient air temperature is 25°C to 45°C or 77°F to 113°F. The average temperature range due to an enclosed application or closely located to another heat source is 50°C to 70°C or 122°F to 158°F.

The thermal resistance number obtained from the equation is used in conjunction with a chart provided by heat sink suppliers to find an approximate heat sink volume. Figure 1 illustrates one such chart from Ohmite’s Thermal Product Guide, which provides more information on thermal resistance equations and calculating your heat sink requirements.

Figure 1. A thermal resistance vs. volume chart helps determine the required heat sink volume.

To determine the necessary flow velocity, engineers can use a chart or performance graph to identify a heat sink for forced convection applications and determine if a liquid-cooled cold plate is needed. Figure 2, also from Ohmite’s Thermal Product Guide, shows how the required velocity can be calculated based on dissipation requirements and ambient temperature.

Figure 2. Air velocity can be calculated based on dissipation requirements and ambient temperature.

The most common materials used for heat sinks are aluminum alloys and copper for high thermal conductivity, corrosion resistance, and heat absorption. Copper is about 60% higher than aluminum for thermal conductivity, although it is less malleable for the extrusion process, so aluminum is considered an industry standard.

There are many manufacturing methods for heat sinks regardless of materials, including CNC machining, milling, forging, and more. The most common manufacturing method is extrusion with aluminum alloys. 

According to Gabrial International, aluminum alloy 1050 has a higher thermal conductivity value of 229 W/m K, although this material cannot withstand the manufacturing processes for heat sinks.

Engineers measure the thermal conductivity of aluminum in ambient temperatures through what is called °C/W of power dissipation. According to Gabrian International, aluminum alloys 6061 and 6063  are commonly used, with thermal conductivity values of 166 and 201 W/m K, respectively.

Ohmite’s heat sinks include Aluminum Alloy 6063-T5 or similar materials. According to the alloy supplier, Smiths Metal Centres Limited, 6061 aluminum alloy is heat treatable, corrosion-resistant, and provides medium to high strength for heavy-duty applications. 6063 aluminum alloy is also corrosion-resistant and is perfect for the extrusion process per Smiths Metal Centres Limited.

Another important step in selecting a heat sink is to deduce the required volume. This usually can be determined from a chart and/or graph provided by the manufacturer, entailing the spectrum of volumetric thermal resistance based on various flow conditions. Manufacturers provide datasheets with more specifications detailing applications, weights, BOMs, thermal resistance, max airflow, and more. One can use the graph and determine whether the heat sink will meet the thermal design requirements.

Ohmite heat sinks are designed to secure TO-126, TO-218, TO-220, TO-247, and TO-264 packages, plus provide thermal solutions for TO-252, TO-263, and TO-268 SMD devices.

The fins on a heat sink are essential to the cooling medium, so having an accurate design and placement of the fins is critical. Extruding heat sinks enable the creation of shapes capable of dissipating large heat loads via these strategically arranged fins. In the extrusion, process aluminum is heated to a point just below melting temperature and then extruded through a form and cut into the necessary part lengths. Then the part is machined for creating mounting holes and other features.

The fin is the final point of contact for the heat; the fins conduct the heat away and increase the surface area used to release heat.

Common types of fins for heat sinks also include, in addition to extruded, fins that are bonded (attached to the base with conductive thermal resin), skived, stamped (sheet metal stamped into desired shapes), forged, and CNC machined. Forged heat sinks, for example, can be made with thin round pins or thin fins and with high-density geometries.  

Flared heat sink fins reduce the flow resistance pushing more air through the fin channel.  

If the application has room, slanted fins will increase the surface area for better performance. Slanted fins are naturally longer fins, so another factor is the higher the aspect ratio (fin height compared to fin distance), the less effective the fins will be.

Modifying the fins to be thicker or shorter will amplify their efficiency. As mentioned earlier, copper is more conductive than aluminum which will also improve the fin efficiency.

Fin efficiency will change if one or more of several factors change, e.g., adding to the fin thickness, reducing the fin length, the thermal conductivity of the fin increases, the thermal conductivity of the fin material changes, or the airflow velocity changes.

In general, the more surface area a heat sink has, the better it works.  A pin-fin heat sink is a heat sink that has pins extending from its base. Another type of heat sink fin arrangement is the straight fin which runs the entire length of the heat sink.

Pin-fin heat sink performance is better than straight fins when the fluid flows axially along the pins.

Plate fin heat sinks also have fins along the whole length of the heat sink that are straight and large to optimize the surface area and cooling power. Plate fin heat sinks do not perform well when the fins are oriented perpendicular to the direction of airflow.

One of the most common surfaces finishes for heat sinks is anodization. The anodizing process is only for aluminum alloys to boost durability, hardness, corrosion resistance, and wear resistance.

Put another way, the emissivity of a surface is the percentage of the surface that emits rather than reflects.  This phenomenon is described by Planck’s law, which sets forth the relationship between temperature and electromagnetic radiation for a so-called black body. In practical terms, emissivity is the tendency of a surface to radiate away energy as its temperature increases. A material with higher emissivity will radiate away energy more efficiently.

The anodizing process involves a combination of electricity and acid that also adds to the aluminum oxide layer with three anodizing processes ranging in thickness from 0.00005" - 0.003". If an efficient heat transfer path from the device to the environment can be found or provided, the component is much less likely to overheat. Beyond optimizing the heat sink surface area and fin efficiency, the heat sink also requires a secure attachment which helps to reduce thermal resistance.

This is non-trivial because a heat sink lowers the fluid-side thermal resistance but also introduces an interface resistance across the contact formed between itself and the package case. 

Typical attachment methods include thermally conductive tape or epoxy, clips and push pins with ends that expand after installing. The component must also t remain in thermal contact with its heat sink, given a reasonable amount of shock and vibration.

Electronics scaling for power delivery and removal due to reduced dimensions, higher operating voltages, and increasing power presents a need for advanced solutions. Fortunately, heat sinks can be easily modified to fit any application. They can be configured to customer specifications for length, surface finish, mounting holes, and custom machining. A heat sink’s design flexibility allows chill plate lengths to be configured, as well as mounting hole patterns to accept a multitude of devices.

While heat sinks are reliable over time, in a normal operating environment the heat sink could become clogged with dust and other debris. The dust reduces the cooling power of the heat sink by lessening the surface area available for cooling. Compressed air can be used to blow the dust out of and away from the heat sink.

Ohmite heat sinks can be fitted with a patented clip system, eliminating the use of screws and holes for installation. Spring clips provide maximum repeatability via a constant spring force over repeated assembly/disassembly as the spring action locks electronic components in place.

Clips provide a uniform, consistent pressure to keep the sink’s mating surface against the component (often with an interface material between them). Because the clips provide consistent pressure, no torque-gauging tools are needed. To provide proper pressure, the clip must be specially designed for a particular heat sink and semiconductor package.

This system requires no tools, and you can lock in the device for a proper thermal connection with one finger. The elimination of mounting hardware enables a maximum surface area per unit design. Overall, the clipping system provides users with a more streamlined assembly process with the ability to accommodate fan sizes up to a standard 60mm x 60mm for active cooling. The goal here is to provide users with an easier, more streamlined assembly process.

In addition to fast assembly, the clip provides lower interface thermal resistance than other assembly methods. This is non-trivial because while the use of a heat sink lowers the fluid-side thermal resistance, it also introduces an interface resistance across the contact formed between itself and the package

To cite a specific example, the C Series Heat Sink System offers flexible, high-performance, and compact heatsinks with an exchange cam clip system for TO-126, TO-220, TO-247 and TO-264 devices. Read more about it here or view these products in Ohmite’s catalog. Ohmite’s experts will guide you with your heat sink selections.

Ohmite Manufacturing Company offers different heat sink series to 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. Options include mounting, anodizing, and extrusion length, with many extrusion profiles available. It offers multiple sizes and configurations to accommodate many application needs and designs. Ohmite’s product offering will have the heat sink required for your application. Ohmite also has been the leading provider of resistive products for high current, high voltage, and high energy applications for over 95 years.