Soft Starter Heatsink Copper Machining

• 1
  Why copper wins inside a soft starter
• 2
  What makes copper hard to machine
• 3
  Fin geometry sets the machining limit
• 4
  Which copper grade to specify
• 5
  When copper is the wrong choice
• 6
  Machining sequence that holds flatness
• 7
  Copper vs aluminum for soft starter heatsinks
• 8
  Symptoms, causes, and what to change

A soft starter ramps motor voltage by phase-chopping the line through back-to-back thyristors or SCR pairs. Each device drops roughly 1–2 V while conducting, so a 200 A starter can shed several hundred watts as heat inside a sealed enclosure. That heat has one path out: the device base, the thermal interface, the heatsink base, and the fins.

Copper conducts at roughly 385–400 W/m·K, about 60% better than 6063 aluminum. In practice that means a copper heatsink can move the same thermal load with 30–50% less volume, or hold the same volume at a lower junction temperature. For retrofits into existing starter enclosures, the smaller volume is usually the deciding factor.

Copper also has a lower coefficient of thermal expansion, around 17 × 10⁻⁶ /K against 23 × 10⁻⁶ /K for aluminum. That matters when the heatsink sits under an AlN or Al₂O₃ ceramic substrate. Closer CTE match means less shear stress at the solder or sinter joint during power cycling, which is where field failures usually start.

The catch is that every one of these thermal advantages has to survive the machining process. Copper gives away heat so quickly that the tool edge runs hot, and it is soft enough to smear instead of shear. Flatness, fin straightness, and surface finish are what carry the thermal benefit from the drawing into the enclosure.

• 1
  ConductivityAbout 385–400 W/m·K for C110; roughly 200 W/m·K for 6063-T5.
• 2
  CTE match17 × 10⁻⁶ /K sits closer to ceramic substrates than aluminum.
• 3
  VolumeSame thermal load in 30–50% less heatsink volume.

Pure copper is gummy. It does not form a clean shear plane the way steel does, so the chip tends to pressure-weld to the cutting edge and form a built-up edge. That BUE breaks off, takes tool coating with it, and leaves a torn surface behind. A rough surface on a heatsink base is not cosmetic. It is an air gap under the interface material.

Copper also work-hardens fast. A light finishing pass over a surface already work-hardened by roughing cuts into harder material than the drawing suggests, and the tool deflects. Thin fins amplify this because there is almost no stiffness in the feature itself.

Chip control is the third problem. Copper produces long, stringy chips that wrap around the tool holder and pull into the cut. On an automated cell that means stopped spindles and scrapped parts. The fix is high-pressure through-coolant plus a chipbreaker geometry aggressive enough to break the chip at the source.

Finally, copper's thermal expansion is significant. A 300 mm heatsink base moving through a 40 °C temperature swing grows about 0.2 mm. If the machining sequence does not account for that, the final flatness measurement will not repeat from part to part.

Heatsink performance scales with surface area, so designers push fin count up and fin thickness down. On a soft starter module the usual target is a fin thickness of 1.5–3 mm with a height-to-thickness ratio between 8:1 and 15:1. Past roughly 15:1 on copper, the fin deflects under cutting force faster than any toolpath can compensate.

Aspect ratio is not the only constraint. Fin spacing has to stay wide enough for air to actually move through the channel. Below about 4 mm gap, boundary layers merge and the extra area does nothing. Forced-air designs usually settle between 5 mm and 8 mm pitch; natural convection wants more.

The base is where flatness gets specified. For a direct-bonded ceramic substrate we routinely hold 0.05 mm over the full base, and 0.02 mm under a pressed interface pad. Tighter than that is possible, but it only pays off if the enclosure mounting surface is machined to match.

One geometric trade the drawing rarely shows: a thicker base under the device footprint spreads heat laterally before it reaches the fins. Adding 2 mm of base thickness often beats adding three more fins, and it is far easier to machine.

C11000 (electrolytic tough pitch) is the default for heatsinks. It runs about 100% IACS conductivity and machines predictably once the tooling is set up for gummy material. It is the grade most thermal drawings call out.

C10100 (oxygen-free electronic) is worth the premium when the heatsink will be vacuum-brazed, sintered, or plated for a high-reliability assembly. The low oxygen content prevents hydrogen embrittlement and steam voids at brazing temperature. If your process never exceeds 300 °C, C110 is usually enough.

C10300 (oxygen-free, low phosphorus) sits between the two. It brazes well and still machines close to C110. For soft starter heatsinks that get a nickel or silver finish before assembly, C103 and C110 both work; the choice usually comes down to what the brazing house asks for.

Beryllium copper is a different animal. It offers high strength and good conductivity but the dust is a health hazard, so it needs controlled wet machining. We machine it, but we do not recommend it for a plain heatsink where C110 does the job.

Copper is about 3.3 times denser than aluminum. On a conveyor-mounted starter or any equipment where the heatsink weight loads a bracket, the thermal gain may not cover the structural penalty. Aluminum with a larger footprint is sometimes the better answer.

Cost is the second boundary. Copper stock runs several times the price of 6063 extrusion, and the machining time is longer because of chip control and slower finishing passes. If the thermal budget is comfortable with aluminum, copper adds cost for margin you do not need.

Corrosion is the third. Bare copper tarnishes and, in humid or sulfur-bearing plant air, forms a non-conductive oxide layer. Heatsinks that will be exposed usually get electroless nickel, silver, or a conductive anodize substitute. That adds a process step and a masking operation if the mounting face must stay bare.

Finally, if the design calls for a folded or extruded fin pack, copper is the wrong process. Extruded copper profiles exist but the die cost only makes sense at high volume. Below that, a machined copper base with a bonded or skived fin stack is usually the practical route.