Key takeaways
- The interface face is the part of a heat sink that most often decides the thermal result.
- Milling, fly cutting and skiving produce different flatness, roughness and stress patterns, so the process should follow the callout.
- Flatness needs a datum, a measurement area and a method, not just a number in a title block.
- Clamping and residual stress can make a part flat on the machine and bowed after it is released.
Why flatness decides the thermal result
Two flat surfaces still touch at a small number of high points. The gaps between them are filled by a thermal interface material, and the bond line thickness sets the interface resistance. A face that is flat within a tight band allows a thin, uniform bond line. A bowed face forces a thicker bond line in some areas and starves others.
The effect is larger than many design reviews assume. A thin paste layer can have a resistance that is several times lower than a thick pad. If the face is not flat, the designer is forced to choose a thicker material to bridge the gap, and the whole thermal path pays for it.
Flatness also affects the mechanical joint. A bowed heat sink can rock on the package, load one corner and leave another loose. The screws may feel tight while the contact pressure is uneven.
Milling, fly cutting and skiving
Milling with a face cutter leaves a pattern of overlapping tool marks. It is fast and suits a large face, but the flatness depends on the machine, the fixture and the cutter condition. A worn insert can leave a step between passes.
Fly cutting uses a single point tool on a large radius. It gives a fine, consistent surface and good flatness on a wide face. It is slower and needs a stiff setup, but it is a common choice for a thermal interface.
Skiving forms fins by shaving material from a solid block. The base stays attached to the fins, and the process can produce thin fins with a good thermal joint. The base can distort as the fins are cut, so the interface face is often machined after skiving.
Grinding is used when the flatness or the surface finish needs to be tighter than milling can hold. It adds cost and a process step, but it can rescue a face that would otherwise need a thicker interface material.
| Process | Typical flatness | Surface finish | Best use |
|---|---|---|---|
| Face milling | Moderate | Visible tool marks | General faces, large parts |
| Fly cutting | Good | Fine, consistent | Thermal interface on a wide face |
| Skiving | Good after base cut | Fine on the base | Thin fins from a solid block |
| Grinding | Tight | Very fine | High flux interfaces, tight bond line |
| Lapping | Very tight | Mirror | Small, high value interfaces |
Typical guidance only. The machine, fixture, tool and material decide the result. Agree the method with the supplier.
Datums and setup
A flatness callout does not need a datum, but the machining setup does. The face is cut relative to the fixture, and the fixture locates on something. If the part is held on the fin tips, the base can spring. If it is held on the base, the fins may interfere with the fixture.
A good setup supports the part close to the face being cut. For a heat sink, that usually means supporting the base on a flat plate or on a set of pads under the interface area, with clamps outside the cut.
The machined face should become the datum for the mounting holes and any pocket. If the holes are drilled first, the face may end up out of square to the hole pattern. If the face is cut first, the holes can be placed relative to it.
For a casting, the first cut also removes the casting skin. The depth of cut should be enough to clean up the face without breaking into porosity. A face that cleans up on one side and leaves a dark patch on the other is a warning sign.
Measuring flatness
A straight edge and a feeler gauge is a quick shop check, not a measurement. It finds a bow in one direction and misses a twist. A surface plate with a dial indicator is better, but it only measures the points the probe touches.
A coordinate measuring machine can map the face and fit a plane. The result depends on the number and position of the points and on the fitting method. Ask for the measurement area and the point grid, not just a number.
Optical methods such as interferometry or a laser flatness gauge can map a whole face quickly. They are common for high volume or high value parts, but they need a reflective surface and a controlled setup.
Whichever method is used, state it. A flatness value without a method is open to interpretation, and two labs can disagree without either being wrong.
Surface roughness and contact
Roughness affects how the interface material wets and how much it has to fill. A very rough face needs more material and can trap air. A very smooth face can be worse if the material is designed to key into a texture, although most pastes and pads do not need a rough surface.
A typical machined thermal face is often specified around Ra 1.6 to 3.2 micrometres, but the right value depends on the interface material. A phase change material may prefer a smoother face than a gap pad.
The roughness should be measured in the same direction as the thermal path and reported with the cutoff length. A single Ra number without a cutoff is incomplete.
Clamping distortion and residual stress
A part can be flat on the machine and bowed on the bench. Clamping force, cutting heat and residual stress from the casting or the extrusion all move the part when the clamps are released.
Light passes, sharp tools and a supported setup reduce the cutting stress. A stress relief step between roughing and finishing can help on a casting or a heavily machined block. On a thin base, a small change in clamping can move the face by more than the flatness tolerance.
Check the part after release, not only on the fixture. If the supplier measures on the machine, the result may not represent the shipped part.
For a large plate, the mounting method also matters. A plate that is bolted down at four corners can bow between the screws. The flatness callout should say whether it applies before or after mounting.
Flatness, pressure and the interface material
The interface material only works over the gap it was chosen for. A flat face allows a thin bond line. A bowed face needs a thicker material, which adds resistance and may need more pressure to compress.
The clamp load should be high enough to compress the material but not so high that it bends the package or the heat sink. Springs or belleville washers give a more predictable load than a torque value alone, especially when the stack includes a soft pad.
If the design uses a phase change material, the first power cycle melts it and the clamp load sets the final bond line. That means the flatness and the pressure have to be right before the first cycle, not after.
What the drawing should say
Give the flatness value, the area it applies to, the datum if one is needed, the surface roughness with a cutoff, and the measurement method. State whether the requirement applies before or after mounting. Add the interface material, the bond line and the clamp load.
If the face is machined after skiving or casting, say so. If the fins must not be used as a datum, say that too. A note that prevents a bad setup is worth more than a tighter tolerance.
Flatness is a measurement, not a claim
Name the area, the method and the point grid. A number without a method cannot be checked, and a claim without a record cannot be defended.
Frequently asked questions
What flatness should a heat sink interface have?
It depends on the interface material and the bond line. A thin paste can tolerate less shape error than a thick gap pad. Specify the flatness over the contact area and match it to the material and the clamp load.
Is fly cutting better than milling for a heat sink?
Fly cutting usually gives a finer, more consistent surface on a wide face, but it is slower. Face milling is faster and adequate for many parts. The choice depends on the flatness, roughness and volume.
Why does a heat sink bow after machining?
Clamping force, cutting heat and residual stress from the casting or extrusion can move the part when it is released. Light passes, a supported setup and a stress relief step can reduce the movement.
How is heat sink flatness measured?
Common methods are a surface plate with a dial indicator, a coordinate measuring machine, or an optical flatness gauge. The method, the measurement area and the point grid should be agreed and recorded.
Sources and further reading
- ISO 1101, Geometrical product specifications (GPS). Geometrical tolerancing
- ISO 4287, Geometrical product specifications (GPS). Surface texture. Profile method
- Qumek heat sink machining and inspection notes
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
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