Resources | LpR Article | Technologies | Thermal Management | Feb 18, 2019

Materials, Manufacturing and Technologies for Designing Passive Cooling Devices

Until recently, incandescent bulbs and fluorescent lights were the dominant light sources. Today it’s the energy efficient, durable and, in the meantime, cost effective, LED. But to achieve these attributes appropriate cooling is still necessary and due to progress over the last few years the need for active cooling solutions is limited to a few high power applications as long as the material and technology is carefully chosen. John Broadbent, Managing Director at Columbia-Staver Ltd., describes the different technologies and materials and gives practical advice on how to find the right solution for an application.

A decade ago when purchasing a light bulb there was just little choice, the incandescent bulb was pretty much all one could get. Recent developments have brought new options with the introduction of LED’s. The light emitting diode (LED) bulb can operate for 20,000 to 50,000 hours, at least five times the length of any comparable bulb. In respect to energy consumption LEDs put all contenders to shame. Whilst purchase cost of these bulbs are a little bit higher, over the lifetime of the bulb dramatic savings can be made.

LED lighting can be used in a huge variety of applications that include simple domestic lighting, street lights and traffic control, building illumination, plant growth and even for curing some epoxy products. The LED has been successfully used in these applications, and many more, but in order to get the legendary longevity out of the LED technology, their critical junction temperatures must be maintained at all times and in all operating conditions. Maintaining this temperature requires careful consideration of their thermal management requirements.

A large proportion of LED applications are deployed in the built environment and in urban settings with the need to adhere to strict noise limitations. It is no surprise, therefore, that over two thirds of LED cooling solutions are based on natural convection passive designs.

Only a few specialist higher power applications may need more sophisticated cooling solutions using pumped liquid cooling or fans to increase the air speed. Therefore, the scope of this article is on natural convection cooling solutions and will describe heat sinks based on die-casting, cold forging and heat pipe assemblies. Aesthetic aspects, which can often be achieved using various surface treatments in different colors and or textures, are also considered, as the solution is often visible.

Die Casting

Die cast heat sinks are particularly well suited for the high volume, low cost LED market, where solutions are limited in the amount of output power. Whilst this manufacturing method is low cost, it does facilitate the inclusion of multiple design features. Once incorporated into the die cast mold, these features are repeated for each casting cycle. Design engineers can make use of this technology feature to design heat sinks with additional functionality.

Figure 2: Example of a die cast heat sink and its application. Die cast heat sinks are the low cost solution for mass production of standard productsFigure 2: Example of a die cast heat sink and its application. Die cast heat sinks are the low cost solution for mass production of standard products

Die-casting requires the production of an open and shut mold or die, into which molten metal is injected to form the component. Once the metal is cooled down enough, the die is opened and the component is removed. The die then closes, and the process gets repeated to produce the next component. This allows for high rate of production and can be further increased by duplicating the component within the mold thereby casting more than one component at a time. Die-casting can maintain excellent repeatability of features to high mechanical tolerances.

When selecting the casting material to be used for the heat sink, two major groups of alloys can be considered.

Best suitable alloys for die casting:
• Silicon (Si) based alloys
• Zinc (Zi) based alloys

In the first group of silicon (Si) based alloys, it is in particular, ADC12. This alloy is attractive to heat sink designers due to its excellent casting properties; its ability to fill narrow cavity features such as rips and fins. ADC12 also offers an attractive thermal conductivity of 90 – 96 W/m∙K and can be easily powder coated or otherwise treated to enhance its performance or appearance after casting.

The second group of zinc (Zi) based alloys are often referred to by their original trademark of Zamak, with Zamak 3 being the most widely used alloy in Europe. Whilst the Zinc based alloys do offer a slightly more favorable thermal conductivity, ranging from 105 – 113 W/m∙K. The density of the zinc based alloys in the range of 6600 kg/m³ in comparison to around 2700 kg/m³ for aluminum silicon alloys will increase the component weight by a factor of 2.4. The material is more aggressive, causing faster tool wear and has a harder to treat surface. These factors often put the zinc-based alloys behind the aluminum-based alloys as the prime choice for heat sink designers.

A third alloy family, which is not widely used for pure heat sink applications, but needs mentioning for reasons of completeness, is the group of magnesium-based alloys. These are predominantly used for applications, where weight and structural strength of the components are critical considerations.

Cold Forging

When the thermal performance of the heat sink demands thermal conductivity in excess of 120 W/m∙K, die cast solutions cannot be considered. Cold forging can be done with a range of materials 120 W/m∙K and above. The technology can also offer large diameter heat sinks with tall high density fins. Designs can also offer orientation independent solutions for high volume- low cost applications.

Figure 3: Cold forged aluminum heat sinks offer better thermal conductivity than die cast productsFigure 3: Cold forged aluminum heat sinks offer better thermal conductivity than die cast products

For the production of these parts, a tool is made in the shape of a female impression of the component. In a high force press the selected material is forced to reflow into the tool by the sheer amount of pressure exerted by the press. As well as forming the heat sink shape the process re-aligns the grain structure of the base material leading to an improvement in thermal conductivity in the fin areas. The process ensures that no air bubbles or porosity (often found in die casting) will be present.

The most common shape is a series of pillars mounted onto a base section. Due to ease of tool manufacture the pillars are often a round section pin giving these heat sinks their common name of pin fin heat sinks.

By wire eroding the tooling, more elaborate shapes are possible such as squares, triangles, rhomboids and diamonds. At the design and tooling stage additional features such as larger bosses for screw threads or omissions of patterns can be incorporated into the tool, the pitch of the pins can be variable, and non-symmetrical if required.

Many aluminum alloys can be used for cold forging, starting from low alloy numbers in the 1100 range right up to aircraft grade alloys in the 7xxx range. The standard extrusion grades in the 606x range of alloys are also included, and offer thermal conductivities exceeding 170 W/m∙K.

In conclusion both die cast and cold forged heat sinks have their place. Both technologies have advantages and disadvantages. Cold forged components have a higher thermal conductivity and the finished material is structurally stronger than a die casing. However, the intricate features possible in a die cast part are simply not possible using cold forging.

Heat Pipe Assemblies

Die cast, cold forged and indeed extruded heat sinks can only function up to the limits imposed by the thermal characteristics of the material that they are made of. The thermal conductivity will eventually reach saturation and the only way to extend performance would then be by adding forced air that comes with its associated cost, noise, weight and reliability. The most flexible and often the most effective LED cooling solution is the Heat Pipe assembly.

Figure 3: Cold forged aluminum heat sinks offer better thermal conductivity than die cast productsFigure 4: Heat pipe assemblies are often used for cooling architectural lighting units because they offer the lowest thermal resistance of any passive cooling solution

Heat pipes can offer thermal conductivity many orders of magnitude more than the materials of the other solutions. Following furnace sintering the heat pipes are malleable, enabling 3D designs that can take the heat to be transported in any direction away from the light source. Passive solutions rely on surface area rather than airflow rate and heat pipe assemblies are no different. Heat pipes can be fitted with a large number of lightweight fins designed in such a way as to maximize performance for a given application. They can move large amounts of heat over distances not possible by conduction. Heat pipes are passive, two-phase based thermal super conductors. The working principle is that a small amount of working fluid, (mostly water) is evaporated in the heat input region. This vapor then travels at almost the speed of sound along the pipe towards the heat output region. Because the heat output area is cooler the vapor condenses. This condensed working fluid then gets returned to the heat input region in the wick structure of the heat pipe by capillary action. This wick structure is on the inside surface of the pipe and can be sintered copper particles or wire mesh. The return of the liquid to the heat input area is aided by gravity and as such this is a key component in the LED solution design.

The heat pipe has a Vacuum inside, therefore this evaporation process with water can start at temperatures as low as 4°C. It should be noted that a heat pipe is simply a highly efficient heat transport device and must be supported by adequate means of heat dissipation at the heat output area. This can be achieved by attaching the heat output or condenser area to an extruded aluminum heat sink or more commonly in LED coolers by attaching aluminum or copper fins directly to the heat pipe in appropriate numbers. Their number is largely determined by the thermal requirements of the particular application.

Due to the very high thermal conductivity of the heat pipes and the fact that through the provision of fins, a large amount of surface area can be provided without an excessive weight penalty, in high power applications, heat pipe based solutions can outperform both die cast and cold forged heat sinks. Heat pipe based solutions are largely used where high-powered LED spot lights are deployed into remote locations where space is less of an issue.

Material/ Process

Die Casting

Cold Forging

Heat Pipe Assemblies

Die Cast Alloys:
Si based AL Alloys (ADC 12 as most common example)

Density:
2740 kg/m³;
Thermal Conductivity:
96 W/m·k

 

 

Die Cast Alloys:
Zi based Alloys (Zamak 3 as most common example)

Density:
6600 kg/m³;
Thermal Conductivity:
113 W/m·k

 

 

Aluminum Alloys
1000 range

 

Density:
2700-2710 kg/m³;
Thermal Conductivity:
218-243 W/m·k

 

Aluminum Alloys
606x range
(AL6063 as example)

 

Density:
2700 kg/m³;
Thermal Conductivity:
200 W/m·k

Density:
2700 kg/m³;
Thermal Conductivity:
200 W/m·k

Aluminum Alloys
7xxx range

 

Density:
2720-2890 kg/m³;
Thermal Conductivity:
115-222 W/m·k

 

Copper (C101 as example)

 

 

Density:
8920 kg/m³;
Thermal Conductivity:
391 W/m·k

Table 1: Relative thermal properties of materials and their suitability for various manufacturing techniques

Surface Treatment

In some cases, particularly when the heat sink is visible to the end user, appearance can be very important. In other cases, protection of the solution from the elements or the environment is paramount. This aesthetic appearance or protection can be provided by the surface treatment applied to the finished solution. When considering surface treatments, the base material of the heat sink needs to be considered.

For aluminum based cold forged heat sinks, the most common surface treatment is anodizing. Anodizing is an electrochemical process that converts the metal surface into a decorative, durable, corrosion-resistant, anodic oxide finish. The process is available in a wide range of colors that can be specified from the RAL color chart. Often the color selected is black because this not only provides protection but it also changes the emissivity of the unit, thereby potentially improving its performance.

When looking at surface treatments for die cast heat sinks, and as an alternative to wet paint, powder costing is a very popular choice. It offers consistent finishes with a selection of RAL colors and surface textures, ranging from completely smooth to a more textured finish.

The powder coating process is based on charged powder particles being attracted to the components bearing the opposite charge.

This leads to a very good and even coverage of even the most complex surfaces, after particle coating the unit requires an oven curing process in which the powder particles are melted and form the finished coating.

A final alternative is the use of electrophoretic painting. This process was developed by the automotive industry and is also referred to as e-coating or electro-painting. It is very similar to powder coating, but liquid paint with an electric charge is used instead of powder. All remaining process steps are the same as when using powder coating. This process is widely used when coating many different types of heat exchangers, and is therefore ideal for the coating of heat pipe based LED coolers, utilizing the process’ main advantages of being able to cover even the smallest gaps and corners of the parts and providing a coating with unrivaled environmental protection. A wide range of different colors on the RAL chart are available.

For completeness, metal coatings such as Nickel plating should be mentioned. However, it has to be considered that due to their complexity and cost, they are not widely used as finishes for LED coolers.

Summary and Conclusions

Many different solutions are available and several manufacturing possibilities can be applied to LED cooling, but there is no one-fits-all solution. The actual application, the power, the physical size and the working environment would all need to be considered before a solution could be presented. The table below shows the relative thermal properties of materials and their suitability for various manufacturing techniques.

In conclusion, it can be said that a sound knowledge of material properties and process technologies is the key element to find the best, cost-effective solution for any application. It is therefore important to select a supplier that offers a full range of thermal design services, including CFD, thermal design, mechanical design and design for manufacture for a range of bespoke thermal products such as LED coolers, heat sinks, heat pipe assemblies, conduction cards and liquid cold plates as well as cooling systems.

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