The Heatsink Guide: Heatsink Information

THE HEASINK GUIDE: Information about heatsinks - part 2
Heatsink production methods

Extrusion

The most popular production method for heatsinks is extrusion. With the aid of high pressure and temperature, a flow of aluminum is forced through a shaped opening. This results in a long stick having the same form as the opening. Later, the aluminum is stretched, which straightens it and improves its mechanical properties (better strength through re-alignment on a molecular level). Finally, the long aluminum sticks are cut into heatsink-sized pieces, and possibly milled to improve flatness of the contact area. Even though the concept is simple, the machines involved are huge. If you are interested in how exactly extrusion works, I suggest that you read this excellent article about the extrusion process from the William L. Bonnell company. It provides much more details than this small introduction to extrusion.

The classic extruded heatsink has a base plate with fins on one side. If a fan is used, the direction of the air flow is orthogonal to the direction of the extrusion.

Extruded heatsink with classic design, made by FoxConn
This is the heatsink AMD used when they first presented their Opteron CPUs (codenamed "Hammer" back then).

With this classic extruded heatsink design, the air from the fan will at some point "hit" the base plate; it can escape only at two sides. A high pressure may occur within the heatsink, which is bad for air flow.

Modern CPUs have only a small contact area between die and heatsink. Therefore, it is possible to design extruded heatsinks where the direction of the air flow is identical to the direction of extrusion; they feature a thick core which leads the heat upwards. Air can more easily flow through the heatsink, and escape at all sides. The core is located below the fan motor, where little air flow occurs anyways.

Thermosonic Thermoengine
Direction of air flow same as direction of extrusion

Die-cast heatsinks

Another heatsink production method is die-casting. Unlike extrusion, it also suitable for producing copper heatsinks. It gives designers a lot of freedom as far as the form of the heatsink is concerned; however, height of the fins is limited, and fins cannot be made very thin.

Die-cast heatsinks made of aluminum and copper

Cold forging

Heatsinks with very fine and also high fins can be produced by cold forging. Cold forging uses impression dies as well, but the material is forced into the die (roughly) at room temperature. Obviously, very high pressure is required. Alpha was the cold-forged heatsink pioneer; by now, other companies, for example Taisol, also produce such heatsinks.

Taisol cold forged copper heatsink
Prototype as shown on CeBIT 2002

Milled/cut heatsinks

Heatsinks can also be milled or cut from a solid block of metal. This leaves a lot of freedom to the heatsink designer; however, such heatsinks are rather expensive to produce.

Milled HP/Agilent "PolarLogic" heatsink
Originally developed for HP PA-RISC CPUs, these heatsinks were, at the time, years ahead of the competition. Agilent later entered the market of PC CPU coolers, but they were never available in high volume, most likely due to high production costs. For a short time, surplus PolarLogic heatsinks were available on the market, and some retailers modified them to fit Socket 370 and Slot A CPUs.

JagWire "CoolJag" heatsink cut from a copper block
Notice the extremely fine fins; they are sometimes also referred to as "skiving" fins.

Bonded fin / folded fin

Instead of extruding, forging, or milling fins, it is also possible to simply use copper (or aluminum) plates as fins, and bond them on a base plate. If each fin is made of a separate plate, we refer to such heatsinks as bonded fin heatsinks. If all fins are made of one large plate that is folded to form the fins, we have a folded fin heatsink. Advantages of such heatsink designs include high surface, and low weight. However, the performance is only good if the bonding is done properly.

Thermalright "bonded fin" heatsink

Heatsink surface treatment

Aluminum heatsinks are usually anodized; a choice of many colors is available. One might think that black is good, because it is best for heat emission by radiation. This is wrong. Heatsinks get rid of heat by convection (that is, heat is transferred to the air molecules travelling along the heatsink surface - if a fan is involved, we call it "forced convection"). For convection, the color does not matter at all.

Copper heatsinks are sometimes plated with silver or even gold (seriously - Zalman produced such a heatsink, model CNPS3000GOLD). This is supposed to prevent corrosion and improve thermal characteristics. But actually, the plating is too thin to have any effect on thermal conductivity. It is true that the surface of a copper heatsink may slightly corrode; but since they are operating in a dry environment, and do not have to withstand extreme temperatures, corrosion is not really a problem. During the lifetime of a PC heatsink, the corrosion layer will not get thick enough to have any negative effect on cooling.

So, do not worry about the surface treatment of your heatsinks, and do not spend extra for some special surface treatment (such as silver plating). Technically, there is no justification for it.

THE HEASINK GUIDE: Information about heatsinks - part 2Heatsink production methods