SiC Module Pin-Fin Cooler: Skiving and CNC Machining of Cooling Fins
Introduction to Pin-Fin Coolers for SiC Modules
Pin-fin coolers are the most efficient single-phase liquid cooling solution for high-power SiC modules, offering significantly better thermal performance than conventional straight-fin or serpentine cold plates. The pin-fin geometry creates enhanced turbulence in the coolant flow, breaking up the thermal boundary layer and increasing the heat transfer coefficient by 2–4 times compared to unfinned channel flow.
The manufacturing of pin-fin coolers for SiC modules involves creating an array of small cylindrical or shaped pins on a baseplate that interfaces with the module baseplate. The pins extend into the coolant flow path, typically a liquid coolant such as water-glycol mixture flowing at 5–15 L/min. The heat from the SiC module conducts through the baseplate into the pins, where it is transferred to the flowing coolant through forced convection.
Skiving Process for Aluminum Pin-Fin Coolers
Skiving (also called cut-fin or extrusion-fin) is a high-productivity manufacturing process for creating pin fins from solid aluminum or copper billets. The process uses a specialized cutting tool that lifts and forms the fin material from the base material in a single pass, creating a monolithic structure with no thermal interface resistance between the fins and the baseplate.
The skiving tool consists of a cutting edge that penetrates the base material to a controlled depth, followed by a forming surface that curls the cut material upward to form the pin. The tool geometry determines the fin height, thickness, and angle. For aluminum pin fins, a typical skiving tool produces fins with a height of 5–20 mm, a thickness of 0.5–1.5 mm, and a spacing of 1.5–4.0 mm between adjacent fins.
The skiving process parameters for aluminum (6061-T6 or 6063-T5) include a cutting speed of 30–80 m/min with a feed rate of 0.1–0.3 mm per tooth. The depth of cut (fin height) is set in increments of 2–5 mm per pass, with multiple passes required for taller fins. Coolant is essential during skiving to remove heat from the cutting zone and prevent material welding to the tool.
| Parameter | Aluminum (6061) | Aluminum (6063) | Copper (C1100) |
|---|---|---|---|
| Cutting speed | 50–80 m/min | 60–100 m/min | 20–40 m/min |
| Feed per tooth | 0.15–0.30 mm | 0.20–0.35 mm | 0.08–0.15 mm |
| Fin height per pass | 2–5 mm | 2–5 mm | 1–3 mm |
| Max fin height | 20 mm | 20 mm | 12 mm |
| Fin thickness | 0.5–1.2 mm | 0.5–1.0 mm | 0.4–0.8 mm |
| Min fin spacing | 1.5 mm | 1.5 mm | 1.2 mm |
| Tool material | HSS or Carbide | HSS or Carbide | Carbide or PCD |
The skiving process offers several advantages for pin-fin cooler production. The monolithic construction eliminates the thermal contact resistance that would exist between separate fins and a baseplate, improving overall thermal performance by 10–20%. The process is also highly productive, with CNC skiving machines capable of producing 10–30 coolers per hour for typical SiC module sizes (100 × 100 mm to 200 × 200 mm).
Thermal Performance Comparison: Pin-Fin vs Straight-Fin Coolers
The pin-fin geometry provides substantial thermal performance advantages over conventional straight-fin cold plates, particularly at the flow rates and pressure drops available in typical SiC module cooling systems. The comparison between the two fin geometries must account for both the thermal resistance and the hydraulic performance, as the system pressure budget is often constrained by the pump capacity.
Pin-fin coolers create highly turbulent flow even at low Reynolds numbers (Re = 200–500), breaking up the thermal boundary layer effectively. Straight-fin designs require much higher flow rates to achieve the same level of boundary layer disruption. The thermal performance advantage of pin-fin designs is most pronounced at Reynolds numbers below 1,000, where straight-fin heat transfer is dominated by laminar boundary layer growth.
| Parameter | Pin-Fin (Staggered) | Straight-Fin | Unit |
|---|---|---|---|
| Heat transfer coefficient (at 5 L/min) | 8,000–12,000 | 3,000–5,000 | W/m²·K |
| Pressure drop (at 5 L/min) | 15–30 | 3–8 | kPa |
| Thermal resistance (Rth) | 0.010–0.020 | 0.025–0.040 | °C/W |
| Flow regime at 10 L/min | Turbulent | Transitional | — |
| Pump power required | 2.5–5.0 | 0.5–1.5 | W |
| Manufacturing cost | 1.5–2.5× | 1.0× | per area |
The pin-fin design achieves 2–3× higher heat transfer coefficients than straight-fin designs at the same flow rate, but at the cost of 3–5× higher pressure drop. For most SiC module cooling applications where pump power is available, the thermal performance advantage outweighs the pressure drop penalty.
CNC Machining of Copper Pin Fins
For the highest thermal performance requirements, copper pin-fin coolers offer approximately 2× the thermal conductivity of aluminum (390 W/m·K vs 170 W/m·K for 6061 aluminum). However, copper cannot be skived as effectively as aluminum due to its higher ductility and tendency to form built-up edge on cutting tools. CNC machining is therefore the preferred method for copper pin-fin coolers.
The CNC machining of copper pin fins typically uses a ball-end mill or a specialized fin-milling cutter in a 3-axis or 5-axis machining center. The tool path generates the pin geometry by plunging and interpolating between each pin position, creating the fin array through a series of slotting and profiling operations.
The feed rate for copper pin fin machining is slower than for skiving, reflecting the lower machinability of copper and the need to maintain dimensional accuracy. Typical parameters for copper pin fin machining include a cutting speed of 100–200 m/min (for carbide tools), feed per tooth of 0.02–0.05 mm, and a depth of cut of 0.5–2.0 mm per pass.
Pin Geometry Optimization
The geometry of the pin fins significantly affects the thermal-hydraulic performance of the cooler. The key geometric parameters are the pin diameter, height, spacing (both streamwise and spanwise), and arrangement pattern (in-line or staggered). Each of these parameters affects the heat transfer coefficient, pressure drop, and overall thermal resistance of the cooler.
Staggered pin arrangements provide 15–30% higher heat transfer coefficients than in-line arrangements for the same pin density, making them the preferred configuration for SiC module coolers where thermal performance is paramount. The optimal pin spacing in a staggered array is typically 2–3 times the pin diameter in both directions, minimizing the trade-off between heat transfer enhancement and pressure drop.
Coolant Channel Sealing and Leak Testing
The pin-fin cooler assembly must be hermetically sealed to prevent coolant leakage into the SiC module enclosure. The sealing system includes the baseplate-to-housing joint, the coolant inlet and outlet fittings, and any intermediate seals between the pin-fin insert and the housing body.
The sealing method depends on the cooler construction type. Monolithic skived or machined coolers use O-ring seals at the housing interface, while brazed or welded assemblies use a permanent joint. For bolted assemblies, the O-ring groove dimensions and surface finish must be controlled to ensure proper seal compression.
| Seal Type | Material | Test Pressure | Acceptable Leak Rate | Test Medium |
|---|---|---|---|---|
| O-ring (static) | EPDM or FKM | 5 bar | <1×10⁻⁶ mbar·L/s | Helium |
| Brazed joint | Cu-Ag braze | 10 bar | <1×10⁻⁹ mbar·L/s | Helium |
| Welded joint | Al or Cu weld | 10 bar | <1×10⁻⁹ mbar·L/s | Helium |
| Compression fitting | Metal gasket | 7 bar | Zero visible leak | Water + dye |
Production leak testing uses a pressure decay method with dry air or nitrogen at 5–7 bar, measuring the pressure drop over 30–60 seconds. For high-reliability assemblies, helium mass spectrometry is used on a sample basis to verify the absolute leak rate.
Baseplate Flatness and Surface Finish
The mounting surface of the pin-fin cooler must match the SiC module baseplate flatness to minimize thermal interface resistance. The flatness specification for the cooler mounting surface is typically 0.02 mm, matching the SiC module baseplate flatness requirement. This flatness is achieved through finish machining of the baseplate surface after the pin fin array has been created.
Thermal Performance Validation
The thermal performance of pin-fin coolers is validated through calorimetric testing using a heated test block that simulates the SiC module's heat flux and footprint. The test measures the thermal resistance from the heater block surface to the coolant inlet, expressed as Rth (°C/W or °C·cm²/W). The pressure drop across the cooler at the rated flow rate is also measured.
Conclusion
Pin-fin coolers manufactured through skiving or CNC machining provide the thermal performance needed for high-power SiC modules operating at heat fluxes exceeding 500 W/cm². Skived aluminum coolers offer the best cost-performance ratio for most applications, while CNC-machined copper coolers serve the highest performance requirements. The optimization of pin geometry combined with precision baseplate machining creates the thermal interface necessary for reliable SiC module operation.
