Machined heat sink inspection: flatness, fin geometry and finish

Why a heat sink needs its own inspection plan

A heat sink is a thermal part, but most of its drawing is mechanical. The drawing shows an envelope, a base thickness, a fin count and a mounting hole pattern. The inspection plan has to connect those dimensions to the thermal job, because a part that measures inside tolerance can still perform badly.

The connection runs through the mounting face. That face carries the component, often through a thermal pad or a thin layer of paste. If the face is flat and smooth, the interface layer is thin and the resistance is low. If the face is bowed, the layer is thick in the middle or at the edges, and the thermal resistance rises.

The fins matter in a different way. Their pitch, thickness and height set the surface area. Their straightness and the finish between them set the airflow. A fin that is bent or blocked reduces the area that the fan can reach.

The inspection sequence

A practical sequence starts with the features that other measurements depend on, then moves to the functional checks. Measuring the fin pitch before the base is confirmed flat is wasted effort, because the base is the datum.

Flatness on the mounting face

Flatness is the first call because it sets the interface. A typical machined heat sink is specified flat within a few hundredths of a millimetre across the mounting face, but the correct value depends on the component footprint and the interface material. A small die on a large base does not need the whole face to be flat to the same value as a large module.

Measure flatness on the face that touches the component. It is common to see a drawing that calls flatness on the underside of the base because that is the face the machine tool sits on, while the actual thermal interface is the top of the base or the pedestal. Those are different surfaces and they can have different results.

A bowed base is usually a machining or a stress problem. Removing a lot of material from one side releases residual stress and the part curls. A roughing cut, a stress relief pause and a finishing cut reduce the risk. If the bow is consistent, the fixture may be clamping the part flat and releasing it after the cut.

Fin geometry: pitch, thickness and height

Fins are the expensive feature. A machined heat sink usually starts as an extrusion or a block, and the gaps between fins are cut with a slitting saw or a small end mill. The cutter width sets the minimum gap. The depth of cut and the rigidity of the fin set how straight the wall stays.

Fin pitch and fin thickness need a sample check at several places along the length. A single measurement at the end of the part can miss a drift caused by tool wear or a fixture that lifts. Record the values at the two ends and the middle, and note the trend. A pitch that grows along the part is a tool or feed problem, not a random variation.

Fin height is a stack of the base thickness and the cut depth. If the base is thick and the fins are shallow, the thermal resistance is dominated by the spreading resistance in the base. Checking the base thickness is therefore part of the fin check, not a separate item.

Surface finish where it matters

Surface finish is not uniform across a heat sink. The mounting face needs a controlled finish so the thermal interface material can fill the micro valleys. The fin faces need a finish that does not trap dust and does not add a thick coating. The outside edges need a finish that will not cut the person handling the part.

For the mounting face, a roughness in the range of 0.8 to 1.6 micrometres Ra is a common starting point for a machined aluminium interface. The exact value should come from the thermal material supplier and the assembly method. A very smooth face is not automatically better, because some pads need a rougher surface to key into.

Finish also interacts with coating. Anodising or chromate conversion adds a layer that can raise the thermal resistance of the interface. If the mounting face is coated, the coating thickness belongs on the drawing and in the inspection record. Many designs mask the mounting face and leave it bare.

The functional check

A dimensional result cannot prove the thermal path. The functional check puts the part under the condition it will see. That can be a thermal test with a known heat load and a recorded temperature rise, or a pressure test on a bonded or brazed assembly to prove the joint.

For a machined part that is assembled with a fan and a housing, the useful check is at the assembly level. Record the heat load, the ambient temperature, the airflow and the temperature at the component and at the base. A single temperature reading with no test condition is not a result that can be compared between batches.

If the part is a cold plate or a liquid cooled block, add a pressure decay or a helium leak check. A machined pocket that is later bonded or welded needs a joint check, because a void in the bond is a thermal gap that no external measurement will show reliably.

What to expect in the records

A heat sink shipment should arrive with a first article for the first batch and a dimensional record for the production lot. The record should name the datum, the method and the sample size.

For coated parts, add the coating thickness and the masked areas. For assembled parts, add the fan or the interface material lot. For a part with a thermal requirement, add the test condition and the result, even if it is a single sample from the batch.