Bonded fin vs extruded vs skived heat sinks

Three routes to the same shape

A heat sink needs a base, a set of fins and a path between them. There are three common ways to make that shape in aluminium: extrude the whole profile, bond separate fins into a base, or skive fins from a solid block. Each produces a sink that looks similar from the outside and behaves differently at the joint, at the fin root and at the volume break.

The differences are not cosmetic. An extruded sink is one piece of metal with no bond line. A bonded sink has a joint between every fin and the base, and that joint can be a thermal resistance or a fatigue location. A skived sink is also one piece, but the fins are cut and bent up from the base material rather than formed in a die.

The choice starts with the fin geometry the thermal design needs, then moves to the volume and the inspection you can afford. A design that needs a 20:1 fin aspect ratio cannot be extruded, and a design that needs a low cost sink at 2,000 pieces may not justify a die.

Extruded: one piece, die bound

Extrusion pushes hot aluminium through a die, so the base and the fins form in one piece. There is no joint, no adhesive and no secondary assembly. The process is fast per metre and the die is a one-off cost, which makes extrusion attractive from a few hundred kilograms upward.

The limits come from the die. A fin is a thin wall in the opening, so it needs thickness, draft and support. A fin height to thickness ratio of about 6:1 is comfortable, and 10:1 is possible with a good die and a forgiving alloy. Fin thickness is usually 1 to 2 mm. A wider base is limited by the press, and a very wide sink is often made as two extrusions rather than one.

Extrusion also gives a predictable surface and a well understood finish route. Anodising is common, and the fin roots are smooth because the metal flowed through the die rather than being cut. The trade-off is that the fin spacing and height are fixed by the die, so a change in the thermal requirement means a new die or a new profile.

Bonded: wide and thin, joint bound

A bonded fin sink starts with a base plate and separate fin stock. The fins are set into grooves or slots and joined with a thermal adhesive, a solder or a braze. The process allows a much higher fin aspect ratio than extrusion, because the fin is not formed in a die and does not need draft. A fin 0.5 mm thick and 50 mm tall is possible, and the base can be as wide as the plate.

The joint is the whole question. An adhesive joint has a bond line thickness and a thermal resistance that depend on the adhesive, the cure and the groove fit. A void or a missed groove is a hot spot that a visual inspection may not find. A brazed or soldered joint has a lower resistance and better fatigue behaviour, but it needs a compatible plating and a controlled furnace cycle.

Bonded sinks are often chosen for large, low profile units where a wide base is needed and the airflow is forced. They are less attractive where the sink sees repeated thermal cycling and the joint can work under differential expansion. The design should say whether the joint is structural or thermal only, because that changes the test.

Skived: shaved from solid

Skiving cuts a series of fins from a solid aluminium block. A tool lifts a thin layer of metal and bends it into a fin, leaving the base as part of the same piece. There is no joint and no die in the extrusion sense, so the fin can be very thin and very tall. A fin of 0.2 to 0.5 mm and an aspect ratio of 20:1 or more is achievable in a suitable alloy.

The process is material hungry. Most of the block becomes chips, so the buy to fly ratio is poor compared with extrusion. That is acceptable where the thermal performance justifies it and the volume is modest, and less acceptable where aluminium cost and scrap recovery dominate the piece price. The chips are recyclable, but the value does not fully return to the part.

Skiving also leaves a characteristic fin geometry. The fin thickness tapers from the root to the tip, and the root has a radius that depends on the tool. The base must be thick enough to support the cut and to spread heat. A skived sink with a thin base and tall fins can have a spreading problem even though the fins are excellent.

The comparison on the criteria that decide

The table compares the three routes on geometry, tooling, volume and joint. The values are typical bands, not a specification. A specialist supplier can move a limit, and the right question is always what the supplier can hold repeatedly rather than what is possible once.

Joint quality and inspection

A bonded sink needs a different inspection from a single piece sink, because the failure mode is hidden. A visual check can see a missing fin or a poor fillet, but it cannot see a void under a fin. A thermal test can show a hot fin, and a cross section can show the bond line, but both are destructive or sample based.

For an adhesive bond, specify the adhesive, the cure schedule, the groove fit and the bond line thickness. Ask for a pull test on a sample fin and a thermal test on a sample sink. For a brazed joint, specify the filler, the furnace atmosphere and the acceptance for fillet size and voids. A radiographic check may be justified for a high value unit, but it is rarely a production test for a low cost sink.

An extruded or skived sink needs a dimensional and visual check instead. Look for a lean or a bent fin, a torn fin root, a scratch across the base and a flatness error on the mounting face. A fin that is bent in transit closes the channel and raises the resistance, so packing is part of the thermal specification.

Selection rules

Start with the fin aspect ratio and the base width. If the design fits an extrusion, extrusion is usually the cheapest and most reliable at volume. A wide base or a high aspect ratio points to bonded or skived. A very thin fin at low volume points to skiving, which avoids a die and a joint.

Then check the joint. A bonded sink is only as good as its bond, so the supplier's process control and the test method matter more than the catalogue curve. A skived sink has no joint but a poor material yield, so the piece price reflects the block. An extruded sink has neither problem, but it cannot reach the same fin density.

Finally, check the finish and the flatness. An anodised sink is insulating at the mounting face unless it is masked. A machined base is flatter than an as-extruded or as-skived base, and a thin thermal pad needs that flatness.