Which heatsink surface treatment to choose?

Abbreviations like “Natural Al“, “Natural elox“ and “Black elox“ marking various surface treatments of components made of aluminium alloys are probably familiar to you. However, is it only a “cosmetic” or aesthetic matter or is it a parameter, which has to be considered also from technical point of view?

The answer is simple – it is a key parameter for majority of applications. Surface treatment has a significant influence mainly on these three features:

1) Abrasion resistance – surface of common aluminium alloys (whether enclosures or heatsinks) has a typical “aluminium” colour (natural Al). After wiping with, for example a bare hand, usually you´ll find a dark mark on your hand. It is in fact a micro-layer of aluminium (and its oxides), which you´ve just removed from its surface. Aluminium is a relatively soft metal and thus it features low abrasion resistance (in a basic – untreated status). Much better from this point of view are electrolytically oxidized (anodized/elox) surfaces, where an oxide layer is much thicker than it would arise at common conditions. Deposited Al2O3 is a very hard material (Corundum) and therefore, this layer is very resistant to abrasion. If you scratch the anodized surface, it's a bit like "pulling" this layer of relatively soft aluminium with some hard and sharp object.

2) Chemical resistance – as we know, aluminium alloys can be categorized within “corrosion-resistant” materials - at common conditions and in a clean and dry environment. Aluminium itself, however, easily reacts with various acids and alkali and many times even a fingerprint is enough to be “baked in” after some time (if not removed on time). Even in this respect, an anodized surface is much more resistant and elox is effectively protecting aluminium surface even in harsh conditions. That´s also a reason why aluminium components intended for outdoor use are being always anodized (if an aesthetic aspect is important).

3) Cooling efficiency – at the end, perhaps the most important parameter. Untreated natural aluminium with a glossy surface features extremely low emissivity – simply said, a parameter defining heat radiation ability of a given surface. An emissivity of a vast majority of materials is approximately 0,95. Despite that, a glossy aluminium has emissivity of approximately only 0,05 (!), meaning that if a glossy heatsink gave off heat only by radiation, its efficiency would be dramatically low. Fortunately, convection (heat flow) also contributes to a heat transfer and it depends mainly on the size of the surface of the heatsink and the way the air heats up the radiator ribs. Even though it may not seem logic, even the bright natural anodized heatsink surface has significantly higher emissivity than a natural Al surface – approx. above 0,80. That´s why even seemingly similar surfaces (non-anodized vs. natural anodized) feature significantly different emissivity.

Influence of emissivity can be illustrated in two examples:
a) small glossy heatsink (non-anodized) for a transistor in TO220 package in a small closed enclosure (minimum air movement) will transfer a big portion of heat by radiation. That´s why in this case it would be very suitable to better use black-anodized surface.

b) heatsink directly fanned by fan gives off a vast heat majority by convection to the air flowing around, that´s why no surface treatment is necessary (as for heatsink efficiency).

If you also belong to designers, who control temperature by an infrared thermometer or camera, don´t forget to set the emissivity before measuring - according to the heatsink surface. Otherwise you might easily measure very misleading values.

Heatsinks with all three common surface treatments can be found in our iffer. Heatsinks from company Fischer elektronik are supplied with a natural- or black anodized surface, according to abbreviation in their name (SA = black anodized, ME = natural anodized, AL = raw degreased aluminium).
In case of interest in any heatsinks, please contact us at sales@soselectronic.com