Extruded heat sinks: fin geometry and airflow

What an extrusion offers a thermal design

An extruded heat sink is a single piece of aluminium with fins that run the full length. There is no joint between the fins and the base, so there is no bond line to resist heat or to fail in service. That is its main advantage over a bonded or a skived sink.

The process sets the geometry. A fin is a thin wall in an extrusion die, so it needs draft, a minimum thickness and a height that the die can support. A fin height to thickness ratio of about 6:1 is comfortable for a good die, and 10:1 is possible with care. Beyond that the fin may lean, the die wears quickly or the profile twists after quenching.

The base is part of the same profile. A thin base saves weight and material, but it spreads heat poorly, so the outer fins run cooler than the ones above the device. A thick base spreads better and adds mass, which slows a transient but does not help the steady state much. The base thickness is therefore a spreading decision, not a stiffness decision alone.

Fin spacing and the boundary layer

Air next to a fin is slowed by friction, and the layer grows along the fin. If two fins are close together, their boundary layers merge and the channel becomes a slow, warm duct. If they are far apart, each fin gets fresh air but the sink carries fewer fins for the same width. The best spacing is the point where the two effects balance for the airflow that is actually available.

For natural convection, an illustrative fin gap of 6 to 10 mm suits fins about 25 to 50 mm high. A narrower gap can work in a clean, short stack, but dust and paint soon close the margin. For forced convection, a gap of 2 to 4 mm is common because the fan pushes air through the channel and the boundary layer is thinner. The exact value depends on the air velocity, the fin height and the pressure drop the fan can overcome.

A dense stack is not automatically better. It has more surface area, but it also has more friction, so the fan delivers less air for the same power. At low airflow it can run hotter than an open sink.

Fin height, thickness and efficiency

A taller fin adds area, but its tip is cooler than its base, so the extra area is less effective. Fin efficiency falls as the fin gets taller, thinner or made of a lower conductivity alloy. At some height the added area no longer pays for the added weight and pressure drop. For an aluminium fin of 1.5 mm thickness, 40 to 60 mm is often a sensible band for natural convection.

Fin thickness is a compromise between conduction and flow. A thicker fin conducts heat to the tip better and is easier to extrude, but it blocks more of the channel and adds weight. A thinner fin gives more area but is fragile, hard to anodise evenly and may lean during extrusion. A thickness of 1.2 to 2 mm covers most electrical work.

The fin root needs a fillet. A sharp root is a stress raiser, a dead zone for the metal flow and a place where the die wears. A fillet of about half the fin thickness is a normal call, and a slightly larger fillet improves both the extrusion and the heat spread.

Natural, forced and ducted airflow

Natural convection relies on buoyancy. Air heats at the surface, rises through the channels and draws cooler air in at the bottom. The sink must be open at the bottom and the top, and the fins should run vertically. A sink mounted with horizontal fins under a shelf traps a layer of warm air and loses much of its rating. Orientation is part of the thermal specification, not an installation detail.

Forced convection uses a fan to push or pull air through the fins. A fan that blows across the top of the fins does little, because the air follows the path of least resistance over the tips. A duct that turns the flow into the channels is often worth more than a larger fan. The same applies to a card cage: air will bypass a restrictive fin stack unless a baffle forces it through.

Ducting has a cost. It adds a part, a pressure drop and an assembly step, and it can trap dust. A short, smooth entry and a clear exit are usually enough. A long duct with a sharp bend can add more pressure drop than the fins themselves.

Geometry bands for the two airflow cases

The table gives starting values for an aluminium extrusion in still air and in a ducted fan stream. Treat the numbers as illustrative, then confirm them with a thermal test on the actual enclosure, because the surroundings change the answer more than the fin shape does.

Pressure drop and the fan curve

A fan does not deliver its catalogue flow through a restrictive heat sink. It delivers the flow at the point where its pressure curve meets the system resistance. A dense fin stack has a steep resistance curve, so the operating point moves down and to the left. The result can be less air than an open sink would have received, even with the same fan.

Estimate the resistance before choosing the fan. Fin gap, fin length and entry and exit losses all contribute. A short stack with a smooth entry has a lower loss than a long stack with a sharp lip. If the operating point sits near the stall region of the fan curve, the flow can become unstable and the noise rises. A larger, slower fan is often quieter than a small, fast one against the same resistance.

The fan curve and the sink curve should be plotted together. A supplier who quotes a thermal resistance at a fixed velocity is giving a test condition, not a guarantee for the enclosure. Ask what airflow the figure assumes and how it was measured.

What to put in the heat sink specification

Specify the base size, the fin height, the fin thickness, the fin count or pitch, the base thickness, the alloy and temper, the finish and the flatness of the mounting face. Add the airflow assumption, the orientation and the ambient temperature. Without those, a supplier can meet the drawing and still miss the thermal target.

Say whether the sink will be anodised. Anodising improves corrosion resistance and emissivity, but it is an insulator at the mounting interface, so any earth bond needs a masked area. A black anodised sink also shows handling marks, so the cosmetic faces should be defined.

Finally, state the interface. A machined mounting face with a flatness call and a specified thermal pad gives a repeatable result. An as-extruded face is not flat enough for a thin pad.