Copper CNC machining for thermal parts

• 1
  Why copper CNC machining for thermal parts beats aluminum on heat flux
• 2
  C101, C103 or C110: picking the alloy for machined thermal parts
• 3
  Tooling and workholding for gummy copper
• 4
  Fins, microchannels and where the geometry stops working
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  Tolerances, flatness and plated interfaces
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  Alloy comparison for copper thermal parts

Copper wins on thermal conductivity, not on weight. C110 electrolytic tough pitch copper runs about 385 to 390 W/m·K. Aluminum 6061 sits near 167 W/m·K. That gap matters when a small cross-section has to carry a high heat flux, or when a spreader must hold a die at even temperature across its face.

The trade-off is density. Copper is 8.94 g/cm³ against 2.70 g/cm³ for aluminum. A copper cold plate weighs roughly 3.3 times its aluminum equivalent at the same volume. Below about 200 W of total load, an aluminum part with thicker fins often beats copper on cost and mass. Above that, the conductivity gain usually pays for itself.

Copper also helps in the transient case. Its volumetric heat capacity is higher, so temperature spikes spread out before they reach the die. For pulsed loads on inverters and RF modules, that spread matters more than the steady-state number on the datasheet.

One boundary to keep in mind. Conductivity only helps if heat can leave the part. A copper spreader bolted to a poor interface gains little over aluminum. Fix the interface before you pay for copper.

C110 is the default for machined thermal parts. It is 99.9% copper, widely stocked in plate from 3 mm to 100 mm, and carries the highest practical conductivity. It machines reasonably well and costs less than the oxygen-free grades.

C101 oxygen-free copper is the choice when the part will be brazed, welded, or run in a vacuum or hydrogen atmosphere. ETP copper contains copper-oxide inclusions. Heat those in a reducing atmosphere and the part can embrittle or outgas. C101 removes that risk at a modest price premium.

C103 is oxygen-free with a small silver addition. Conductivity holds up better after brazing or high-temperature service, and it resists softening above 200 °C where C110 starts to anneal. Use it for parts that see reflow cycles or repeated thermal excursions.

Beryllium copper is a different animal. It is chosen for spring contact and wear resistance, not conductivity. Its conductivity is well below C110. Do not specify it on a heat spreader unless the part also has to carry mechanical load.

Pure copper is gummy and ductile. It cold-welds to cutting edges, builds a built-up edge, and throws long stringy chips that wrap around the tool and the fixture. The chip is the first problem to solve, not the last.

Use carbide with sharp, high-positive rake geometry and polished flutes. Uncoated carbide usually beats TiAlN or AlTiN, because hard coatings spall under copper's adhesion. DLC coatings help on long runs. Keep the edge fresh; a dull tool rubs and work-hardens the surface.

On a 10 mm three-flute end mill in C110, start near 200 to 350 m/min surface speed with 0.05 to 0.10 mm feed per tooth. C145 tolerates 300 to 450 m/min. Rough with 30 to 50% radial engagement and axial depth up to 1×D when the setup is stiff. Flood coolant, high pressure, no pauses.

Workholding is the common failure point. Copper's modulus is about 117 GPa against 200 GPa for steel, so thin fins and walls deflect under clamp load and cutting force. Vacuum chucks on a dedicated fixture plate work for flat spreaders. Keep clamp pressure low and even. Below 6 mm plate thickness, use double-sided tape or a sacrificial backing plate with light finishing cuts.

Straight fins 0.5 mm wide at 1.0 mm pitch and 10 to 20 mm tall are achievable with 0.4 to 0.5 mm carbide slitting saws or small end mills. That is the practical envelope for milling. Past roughly 10:1 aspect ratio, tool deflection and chatter take over and the finish degrades.

When you need thinner or taller fins than milling allows, skiving is the alternative. A skiving tool peels fins from a solid block, reaching finer pitch and higher aspect ratio than an end mill. Above a few hundred parts, skiving is often the cheaper route as well.

Microchannels below 0.3 mm wide are a different process. Milling them with carbide is slow, and tool breakage drives cost. For those features, consider brazed or diffusion-bonded assemblies, or talk to us before the drawing is frozen.

Aspect ratio is the number to check on your own part. Measure fin height against fin width. If the result is above 8, expect to discuss skiving, EDM or a split-and-bond design.

Thermal parts usually need flatness and parallelism tighter than general machining. A typical spreader spec is 0.02 to 0.05 mm flatness over a 100 mm face, with parallelism of 0.03 mm between the die interface and the mounting face. We hold ±0.005 mm where the drawing calls for it.

Thermal interface surfaces often call for Ra 0.4 to 0.8 μm. Going below Ra 0.4 μm is rarely worth the cost, because thermal interface materials fill micro-roughness anyway. The paste, not the finish, sets the contact resistance at that point.

Bare copper tarnishes within days, and the oxide raises interface resistance. Nickel plating of 3 to 8 μm is the common finish for corrosion resistance and solderability. It costs a small conductivity penalty in the layer and moves tight dimensions slightly. Electroless nickel deposits evenly; electrolytic nickel needs careful racking.

Silver and gold plating are available for RF and low-resistance contacts. Specify the thickness and the allowed dimension change on any tight feature, because plating tolerance is not machining tolerance.