Industry Developments: Extrusion Profile Heat Sinks

Extruded metal heat sinks are among the lowest cost, widest used heat spreaders in electronics thermal management. Besides their affordability, extruded heat sinks are lightweight, readily cut to size and shape, and capable of high levels of cooling.

Most extruded heat sinks are made from aluminum alloys, mainly from the 6000 alloy series, where aluminum dominates. Trace amounts of other elements are added, including magnesium and silicon. These alloys are easy to extrude and machine, are weldable, and can be hardened.

Common alloys for extruded heat sinks are the 6063 metals. These can be extruded as complex shapes, with very smooth surfaces. 6061 aluminum is also used for extrusions. Its tensile strength (up to 240 MPa) is superior to 6063 alloys (up to 186 MPa). In addition to heat sinks, these aluminum alloys are popular for architectural applications such as window and door frames. 

The surfaces of these metals can be anodized to replace their naturally occurring surface layer of aluminum oxide. Anodizing provides more heat transfer, corrosion resistance and better adhesion for paint primers. It is an electrochemical process that increases surface emissivity, corrosion and wear resistance, and electrical isolation.

The Extruding Process

Aluminum alloys are popular for extruding as heat sinks because they provide both malleability and formability. They can be easily machined and are as little as one-third the density of steel. This results in extrusions that are both strong and stable, at a reduced cost relative to other materials.

The aluminum extrusion process starts with designing and creating the die that will shape the heatsink extrusion. Once this has been done, a cylindrical billet of aluminum is heated up in a forge to high temperatures, generally between 800-925°F (427-496°C). Next, a lubricant is added to the aluminum to prevent it from sticking to any of the machinery. It is then placed on a loader and pressure is applied with a ram to push heated aluminum through the die.

During this process, nitrogen is added in order to prevent oxidation. The extruded part will pass completely through the die and out the other side. It has now been elongated in the shape of the die opening. The finished extrusion is then cooled, and if necessary, a process of straightening and hardening creates the finished product.

They can be cut to the desired lengths, drilled and machined, and undergo a final aging process before being ready for market. [4]

Finished heat sinks typically come with anodized surfaces, which can enhance their thermal performance. Alternatively, a chromate finish provides some corrosion protection, or can be used as a primer before a final paint or powder coating is applied. [5]

Shapes of Extruded Heat Sinks

Extrusions tooling heat sink profiles range from simple flat back fin structures to complex geometries for optimized cooling. They can be used for natural (passive) or forced convection (active) with an added fan or blower. Extruded profiles can also include special geometries and groove patterns for use with clip or push pin attachment systems.

6063 aluminum alloy has a thermal conductivity of 201-218 W/(mK). Higher tensile strength 6061 aluminum’s thermal conductivity ranges from 151-202 W/(mK).

Besides choosing the aluminum alloy, selecting an optimal extruded heat sink should factor in its overall dimensions and weight, the specified thermal resistance, and the extrusion shape (flat-back, flat-back with gap, hollow, double-sided, etc.). [7]

Extruded heat sinks can be designed with very thin, and thus more, fins than other sink types. They can be extruded with aspect ratios of around 8:1, which can greatly optimize heat sink performance. A heat sink’s aspect ratio is basically the comparison of its fin height to the distance between its fins.

In typical heat sinks the aspect ratio is between 3:1 and 5:1. A high aspect extruded heat sink has taller fins with a smaller distance between them for a ratio that can be 8:1 to 16:1 or greater.

Linear Cellular Alloys (LCAs) are metal honeycombs that are extruded using powder metal-oxide precursors and chemical reactions to obtain near fully dense metallic cell walls. Either ordered periodic or graded cell structures can be formed. In this work, the performance of heat sinks fabricated from stochastic cellular metals is compared to that of LCA heat sinks. Flash diffusivity experiments are performed to determine the in situ thermal properties of cell wall material. The pressure drop for unidirectional fluid flow in the honeycomb channels and the total heat transfer rate of LCA heat sinks are experimentally measured. These measurements are compared to values predicted from a finite difference code and commercial computational fluid dynamics (CFD) software.

A three-dimensional finite element model of a multichip module (MCM) has been developed by using ANSYS? finite element simulation code. The model has been used for thermal characterization of the module. In addition, optimum dimensions of an external heat sink, which maintains the specified device’s junction temperature within desired operating temperature limits, are determined as functions of air flow rate and power density of surrounding semiconductor devices. Parametric studies have been performed to study the effects of heat sink height, width and length on junction-to-ambient thermal resistance of a high power application specific integrated circuit (ASIC) device found in the MCM assembly. A set of curves are generated to select either heat sink dimensions or air speed for a given design requirements. Influence of the power output of surrounding devices on the thermal performance of the high power ASIC device is also assessed. The predicted results indicate that the ASIC device’s junction temperature as well as junction-to-ambient resistance increase as the power of the surrounding packages increases. This effect diminishes if a sufficiently large heat sink is used to cool the package.

There are different metals with different properties, some metals are used for luxury purposes such as diamond and gold, others are used for building purposes such as brass, nickel, steel, copper, and many more. Every piece of equipment, to work efficiently, requires a good building block. And while engineering important components it is extremely important to look at the qualities of the material that are going to be used and it is also important to keep a check on the factors that can affect the material, Aluminium in this case.

Aluminum is considered the best option for engineering heat sinks because it is cost-friendly, lightweight and most importantly has great thermal conductivity.

Which Metals Conduct Heat The Best?

Copper and Aluminium among other metals have the highest thermal conductivity. Before using metal in any sort of application it is very important to check the thermal conductivity of that metal. The rate of thermal conductivity helps to decide which metal should be used for a specific purpose. Aluminum is a great conductor of heat, which makes it useful for constructing heat exchangers. On the other hand, steel is a very poor conductor of heat which makes it useful for high-temperature environments. This is why Aluminum is preferred to be used in constructing a heat sink.

Thermal Conductivity

Heat transfers in three ways; radiation, convection, and conduction. Thermal Conduction is a process where two objects of different temperatures come into contact with one another and when they meet fast-moving molecules from the warmer object transfer the energy to the slow-moving molecules in the cooler object.

Aluminum heat sinks

Aluminum is considered beneficial for electrical device managers. It is a great metal to be used in the construction of critical power cooling systems. Improvement in extrusion profile technology has made it possible to engineer heat sinks which call for a blend of greater strength and lighter weight.

Aluminum in comparison with other metals such as copper has lower thermal conductivity but it is far too difficult to extrude them into the shape of a heat sink. Secondly, Aluminium is a lightweight metal, which is also another property that other metals do not possess.

Heat Sinks

Heat sinks are mainly used inside computers to cool down the CPU(Central Processing Unit), they are also used in lighting devices, LEDs, and power transistors.

Heat sinks are designed in a way to have a large surface area to maximize the contact with the fluid medium, such as air or liquid coolant to absorb heat and direct it away from the device.

Aluminum alloys are preferred to be used in constructing heat sinks. This is because Aluminium is lighter and cheaper than copper.

How does a heat sink work?

Computers heat up and if the heat is not removed from the device it can actually damage the entire system. To direct the heat away from the system it is necessary to install a heat exchanger. Heat sink directs the heat away from the computer, it does this by transferring the heat generated in the system to a fluid medium such as air or a liquid coolant, whereby it is directed away from the device. 

What is the purpose of a heat sink?

The purpose of a CPU heatsink is basically the maintenance of the computer. Without a heat sink, the system can overheat and therefore can stop working efficiently. To ensure smooth working of the device it is important to install a heat sink to direct generated heat away from the system and prevent overheating.

Why is a heat sink important?

As stated above, a skived fin heatsink is vital for extending the life of a lighting device. It absorbs unnecessary heat and directs it away from the device. Heat sinks increase the efficiency of the device by removing the excess heat which is why it is an extremely important component. Without a heat sink, computers or other related devices can expire quicker. Heat sinks keep the system cool and provide a good working environment to the other components which heat up quite quickly.

Factors Affecting Aluminum Heat Sink Quality

Quality Requirements For Ingots

The blend of alloys in an ingot must be strictly monitored and controlled, for purification purposes. To make sure that the structure and properties are not imbalanced it is important to make sure that the alloys are homogenized. The surface of the ingot must be smooth and there must not be any sand. The end of the ingot must be flat.