Anodized Heatsink

Heat sinks play an important role in cooling electronic and mechanical devices. They act as passive heat exchangers, transferring heat from the hot components to a fluid medium (usually air).

There are many different types of heat sinks. Some heat sinks are made from copper and some are made from aluminum. But, most commonly, heat sinks are made through the process of aluminum extrusion. And these extruded heat sinks are also commonly anodized.

Why is this the case? What benefits does anodizing offer? Let’s first talk about how anodizing works and then the benefits it offers.

What is the Anodizing Process?

The most common method of surface finish in electronics cooling is the surface anodizing. Anodizing is an environmentally safe electrochemical process, that unlike painting or powder coating, there is nothing applied to the aluminum heat sink’s surface. Instead, anodizing thickens and toughens the naturally occurring protective oxide layer on the surface of metal parts. Anodizingcan changes the microscopic texture of the metal, making the surface durable, corrosion and weather resistant.

How does the anodizing process work?

Your aluminum extrusions heat sinks are placed in an electrolytic bath.

In the process, the extrusion heat sink metal forms the anode (the positive electrode) of an electrolytic circuit. By an acidic electrolytic solution, the electrical current releases hydrogen at the cathode (the negative electrode) and oxygen at the surface of the heat sink metal anode (the positive electrode), building up a deposit of metal oxide. The acid action is balanced with the oxidation rate to form a coating with some microscopic pores, 10-150 nm in diameter. The thickness of the heat sink metal oxide depends on the anodizing method and is between 0.5 and 115 microns. It is controlled by electrolyte concentration, acidity, temperature, and the process electrical current and voltage.

What are the Benefits of Anodized Heat Sink?

There are many benefits to anodized aluminum heatsink. Some of the key benefits of anodized heat sink are:

  1. 1. Improved Corrosion Resistance( The acetic salt spray test of ISO 9227 expects anodized aluminum to show no pitting after 24 hours)
  2. 2. Improved Wear Resistance ( Especially hard anodizing)
  3. 3. Increased Electrical Isolation ( Breakdown voltage varies from a few to several hundred volts, depending on the alloy and on the nature and thickness of the film)
  4. 4. Increased Surface emissivity ( Higher radiation effect in natural convection)
  5. 5. Improved Decorative appearance / colors
  6. Anodizing improves corrosion resistance, wear resistance, and electrical isolation, because of the thickness of the oxide layer.

    Emissivity improves as a result of its microscopic texturing and increased surface area.

    The unfinished surface of aluminum heat sink can be measured at an emissivity of approximately 0.05. The emissivity of an anodized aluminum heatsink surface can be measured at around 0.85.

    This is a significant difference. But the improved radiation heat transfer will have a more pronounced effect (percentage-wise) on heat sink that is smaller. It will also have less overall impact on active heat sink (which utilize a fan).

Does Anodizing Color Affect Heat Sink Performance?

Another benefit to Anodized Aluminum Heatsink is the ability to add colored dyes.

The porous oxide layer can be filled with dye, locked in, and resists fading.

Anodized heat sink can be dyed in colors like black, blue, green, and so on. It is important to note, however, that the only real impact of color is a visual one. Regardless of the color, heat transfer will not be affected. The clear anodized surface has the same emission characteristics as the black anodized surface.

The main benefit of these dyes is their visual appeal.

It’s also important to note that you should never paint or powder coat heat sink. These coatings will act as an insulator. They will negatively impact heat transfer.

Should You Choose Anodized Heat Sink for Your Project?

Anodizing improves the corrosion resistance, wear resistance, and electrical isolation of a heat sink.

It will increase surface emissivity as well. The increase in emissivity makes a more pronounced difference in small, passive heat sink.

If your heat sink is large, the improved radiation heat transfer makes less of a difference. In an active heat sink especially, the difference will be minimal.

Another reason to anodize is to improve the appearance of the heat sink with colored dyes. This can be useful for marketing purposes.

You should weigh the costs of  anodizing vs. the benefits it offers. And remember to never paint or powder coat your heat sink.

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