Heatsink designer and STEP generator
Size a plate-fin heatsink for a chip, with a fan on top, your own airflow or none, and download a STEP file and drawing that follow JLCPCB's CNC rules. Free, no sign-up.
- Air20.0 °C
- Base31.2 °C
- Under chip32.5 °C
- Case32.7 °C
- Junction36.7 °C
| Path | °C/W | K at 6.0 W | From |
|---|---|---|---|
| Junction to case | 0.67 | 4.0 | datasheet θJC |
| Interface | 0.026 | 0.2 | TIM over the contact |
| Spreading | 0.22 | 1.3 | Seri Lee 1998 |
| Heatsink to air | 1.86 | 11.2 | fins, base, radiation |
- Heatsink
- 40 x 40 x 15 mm
- Fins
- 8 x 2 mm, gap 3.43
- Fin height
- 12 mm on a 3 mm base
- Fin efficiency
- 0.98
- Airflow
- 7.16 CFM through the fins
- Pressure drop
- 14.7 Pa
- Channel Re* 220.6 is outside 0.1 to 175, the range Teertstra et al. checked.
- Fan on top: our channel model, about 35 % warm against the one published real-fan measurement (Kirchhofer et al. 2024). Expect the part to run a little cooler.
- Source smaller than the fan hub zone: air under a fan hub moves little. Kirchhofer et al. saw the base peak 12 K above its average with a small centered source at 70 W.
Solid: the fan's datasheet curve. Dashed: the heatsink's pressure drop.
Picked from 261 parts JLCCNC can cut in this space: fin heights in 1 mm steps, the thinnest fin the rules allow and two thicker, every fin count that fits.
STEP file, ready for JLCCNC
heatsink-rk3576-40x40x15-6061.step · one solid, millimeters · built in your browser, nothing uploaded.
JLCCNC cuts threads from the drawing, not the STEP: upload both and tick Threads YES. The holes in the STEP are the ⌀2.5 tap drill.
| Rule | This part | Limit | |
|---|---|---|---|
| Material | 6061-T6 | JLCCNC metals: 6061, 7075, T2, H59 | Pass |
| Fin thickness | 2.0 mm | ≥ 0.8 mm aluminum | Pass |
| Fin gap | 3.43 mm | ≥ 1.5 mm aluminum | Pass |
| Slot depth for the cutter | 12.0 mm deep, 3.43 mm gap | A 3.43 mm gap takes a ⌀3 cutter, which reaches 12 mm (15 at most) | Pass |
| Fin height to thickness | 6.0 : 1 | ≤ 6 : 1 aluminum | Pass |
| Inside corners from above | None | Cutter radius where there are any | Pass |
| Closed pockets | None | Every channel open at the top and an end | Pass |
| Post clearance | 3.2 mm | > cutter ⌀3, fins inside cut back | Pass |
| Fins cut back | 4 of 8 | More than half costs real fin area | Pass |
| Fan footprint | 40 mm fan on 40.0 x 40.0 | Heatsink at least the fan size | Pass |
| Fan holes | 4 x M3, through | Tap drill ⌀2.5 through, thread 6.0 mm from the drawing | Order note |
How this is calculated
T_J = T_A + P (θJC + R_interface + R_spreading + R_heatsink) R_interface = TIM area resistance / contact area R_heatsink = 1 / (Σ fins √(h P k A_c) tanh(mH) + h A_base + radiation) + t_b / (k W L) h from channel flow (your airflow), the fan-on-top model, or natural convection
- Fan on top
- The fan blows down into the fins and the air leaves through both ends of every channel. Our model: the Teertstra, Yovanovich and Culham (2000) channel correlation over two half-length channels, with no credit for the impingement turn, crossed with the fan's datasheet curve for the operating point. Against the one published measurement of a real axial fan on a plate-fin heatsink (Kirchhofer, Krieger and Hofer, 2024) it reads about 35 % warm, the safe direction for sizing. The pressure-drop term is fitted to Duan's (2003) measurements.
- Your airflow
- Air along the channels, from a fan at the end of the fins, a duct or the enclosure. h from Teertstra, Yovanovich and Culham (2000), valid for 0.1 < Re* < 100 (their numerical check ran to 175) and within 2.1 % RMS of their measurements; pressure drop from Culham and Muzychka (2001). Ducted CFM puts all of it through the fins. Approach velocity uses Simons' (1997) bypass model: air divides between the fins and the space round the sink by pressure balance.
- Natural
- Board upright, fins vertical: the Bar-Cohen and Rohsenow composite (taken from two secondary sources; the 1984 original was not obtained). Board flat, fins up: Tari and Mehrtash (2013), within about 20 % of most published data. Radiation from Shabany (2008), emissivity 0.85 anodized and 0.05 bare aluminum; copper's emissivity is not verified, so its radiation is left out.
- Spreading
- Seri Lee (1998), centered source; about 5 % while source and base stay under 2.5:1.
- Manufacturing
- JLCCNC: 6061-T6 or copper T2; fins at least 0.8 mm (1.0 copper), gaps at least 1.5 mm (1.8 copper), fin height at most 6:1 (4:1 copper) of the thickness; slot depth limited by the cutter's reach in their tool table. Fan screw posts stand free with a cutter-wide clearance round them, and fins inside it are cut back, so nothing is left thinner than a fin and no channel is closed. Holes are the tap drill; the thread is called out on the PDF drawing.
- Contact size
- Prefilled from the package body. Flip-chip parts conduct through an exposed die smaller than the body; enter it if you know it, since a smaller contact raises the interface and spreading terms.
- Power
- Never prefilled: the Rockchip datasheets give θJC and Tj max but no TDP. Measure your board under its real workload.
- Software
- The part is built in your browser by replicad (MIT license) on OpenCascade Technology, compiled to WebAssembly as replicad-opencascadejs (LGPL 2.1, served unmodified). Source: github.com/sgenoud/replicad and dev.opencascade.org.
- Teertstra, Yovanovich, Culham, Analytical Forced Convection Modeling of Plate Fin Heat SinksJ. Electronics Manufacturing 10(4), 2000
- Culham, Muzychka, Optimization of Plate Fin Heat Sinks Using Entropy Generation MinimizationIEEE Trans. CPT 24(2), June 2001
- Simons, A Simple Method to Estimate Heat Sink Air Flow BypassElectronics Cooling, May 1997
- Seri Lee, Calculating Spreading Resistance in Heat SinksElectronics Cooling, January 1998
- Shabany, Simplified Correlations for Radiation Heat Transfer Rate in Plate-Fin Heat SinksElectronics Cooling, August 2008
- Tari, Mehrtash, Natural convection heat transfer from horizontal and slightly inclined plate-fin heat sinksApplied Thermal Engineering 61, 2013
- Lienhard and Lienhard, A Heat Transfer Textbook, air properties Table A.66th ed., v6.00, 2024
- JLCCNC CNC Machining Design GuidelineUpdated 20 August 2025
- JLCCNC Threaded Hole GuidelineUpdated 14 June 2025
- Same Sky CFM-40V fan datasheet (P-Q curve); 25 to 80 mm from the same series12 September 2024
Questions
Yes. The part is built in your browser with OpenCascade; nothing is uploaded and there is no sign-up. Download the STEP and the PDF drawing and order from JLCPCB's CNC service or any machine shop.
The cutter has to fit between the fins and reach the base. In JLCCNC's tool table a 3 mm cutter reaches 12 mm (15 at most), so 12 mm fins need gaps of at least 3.2 mm (the cutter plus 0.2 mm). Fin height is also held to 6 times the fin thickness in aluminum. The fin count chart grays out the counts that cannot be cut.
Their ordering guide says threaded holes are made from the 2D drawing, with the hole modeled at the tap drill size (an M3 hole as 2.5 mm). Upload both files and choose Threads: YES.
Copper T2 conducts about 391 W/m·K against 167 for 6061-T6, which helps a small source on a large base. JLCCNC asks for thicker copper fins (1.0 mm) and wider gaps (1.8 mm), with height at most 4 times the thickness, so a copper part often fits fewer fins.
The channel-flow model is within 2.1 % RMS of measurements in its source, spreading within about 5 %, and the fan curves are read from datasheet graphs. The largest uncertainties are usually the real power, the airflow inside the enclosure and the interface. Treat the answer as a sizing estimate and measure the prototype.