How to Design a Machined Copper Heat Sink
Updated September 26, 2026
A copper heat sink is one of the few parts where the design decisions and the machining cost are the same conversation. The thermal engineer wants more surface area in less space. The machinist wants a cutter that fits and survives. This guide sits between them.
Where the cost lives
On a busbar the metal dominates. On a heat sink the fin pack does. Everything else, the base, the mounting holes, the outline, is rounding.
Four numbers set the price.
Fin thickness. Thin fins mean a small cutter and a fragile one. They also deflect under cutting load, which means lighter passes.
Gap width. The gap decides the maximum cutter diameter. That single number cascades through feed rate, number of passes and tool life.
Fin height. Combined with the gap, this gives the aspect ratio, which is the real driver.
Fin count. Whatever a single fin costs, you pay it this many times, on every part.
A thermal model that adds twenty percent more fins in the same footprint can double the machining time. It is worth checking the cost before the geometry is frozen.
Aspect ratio is the number to watch
Fin height divided by gap width. It describes how far a cutter has to reach into a narrow slot without wandering.
- Up to about 5 to 1 is comfortable
- Around 10 to 1 is where shops start slowing down and taking more passes
- Past that you are into specialist territory, with long slender tools, very light cuts and a price that reflects it
If your thermal target needs a high aspect ratio, that is a legitimate requirement. Just know before you release the drawing that it is the expensive choice, and check whether a slightly wider gap with a taller fin gets you to the same thermal result.
The base
The mounting face is usually the tightest feature on the whole part, because it has to make thermal contact with a die, a module or a cold plate.
- Flatness on that face is what matters, more than general dimensional tolerance
- Surface finish matters because the thermal interface material has to wet it
- Machine the base after the fins where the operation sequence allows, because cutting a fin pack releases stress and can bow a thin base
Call out flatness and finish on that one face and leave the rest as machined. See copper surface finish for realistic values.
Grade
C110 is the normal choice. It has essentially the same thermal conductivity as C101 and it is cheaper and easier to source. C101 belongs in vacuum and hydrogen brazing applications, not on a standard heat sink.
Free-machining grades lose a few points of conductivity. On a part whose entire purpose is moving heat, that trade is usually not worth making, which is one of the few cases where staying on pure copper is clearly right.
When machining is the wrong process
Be honest about this early, because it saves everyone time.
Skived fins. A blade peels fins from a solid block. Very high fin density, no joints, one piece. Excellent for plain straight fin packs in volume.
Bonded or soldered fins. Separate fins joined to a base. Allows very thin fins and tall aspect ratios. Introduces a thermal joint and a reliability question.
Extrusion. Standard for aluminum, hard for copper because of the forces involved. Rarely the answer here.
Machining. Wins on prototypes and low volume, on integrated features such as internal channels, manifolds, threaded bosses and irregular outlines, and on anything where you need one solid piece with no joints.
A shop that tells you your production heat sink should be skived rather than machined is being useful, not lazy.
Cold plates and liquid cooling
If the part has internal channels, the conversation changes again. Channels in a machined copper block usually mean either a two-piece design that gets brazed or soldered together, or a deep drilled and plugged arrangement.
Both are real options. Both need decisions before quoting: how the joint is made, how it is pressure tested, what the leak criterion is and who owns it. Put those in the RFQ rather than leaving them for later. Joining machined copper parts covers the options.
A heat sink RFQ checklist
- Fin thickness, gap, height and count stated
- Aspect ratio checked against the cost curve
- Base flatness and finish called out on the mounting face only
- Grade justified, normally C110
- Any internal channels described with the joining method and leak criterion
- Real quantity, and whether it repeats
- Whether the part is plated, and if so where
What drives the cost of copper busbars and heat sinks breaks down where each line lands in the quote.
Frequently asked questions
Is copper worth it over aluminum for a heat sink?
Copper conducts heat roughly twice as well as aluminum, and weighs roughly three times as much and costs several times more per kilo. It wins where the heat flux is concentrated and space is tight. It loses where the sink is large and convection limited rather than conduction limited.
What fin aspect ratio is realistic?
Fin height divided by gap width is the number that matters. Past roughly 10 to 1 the cutter starts deflecting, the shop slows down and the price rises faster than the geometry suggests. Below that, machining is comfortable.
Can a machined heat sink compete with a skived one at volume?
Rarely on a plain fin pack. Machining wins on prototypes, on integrated features like coolant channels or mounting bosses, and on geometries that cannot be skived or extruded.
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