Fin Spacing for Natural Convection: Finding the Optimum
Short answer: For natural convection in still air at sea level, the practical optimum fin gap is roughly 6 to 10 mm for vertical fins under about 50 mm tall, widening to 10 to 15 mm as fin height grows past 75 mm. The physics is simple: narrow gaps add surface area but strangle buoyancy-driven airflow, so thermal resistance stops improving and then rises. A common starting point is a gap of about 8 mm with 1.5 to 2 mm thick fins, then tune with a thermocouple. BQUQ, an ISO9001 factory in Dongguan, machines and extrudes heat sinks to ±0.005 mm on CNC features and quotes in 12 working hours.
Why fin spacing matters more in natural convection than in forced air
In a fan-cooled heat sink, air is pushed through the channels whether it wants to go or not. You can pack fins at 2 mm pitch and still move air, because the fan supplies the pressure. Natural convection is different. The only driving force is buoyancy: air near the hot fin surface expands, becomes lighter, and rises. That rising air drags cooler air in behind it, and the whole loop is driven by a pressure difference measured in fractions of a pascal.
That tiny driving pressure is the entire budget. Every millimetre you shave off the fin gap adds viscous drag and chokes the flow. At some point the extra surface area you gained is worth less than the airflow you lost, and total thermal resistance starts climbing. This is the classic optimum-spacing problem: there is a real minimum on the resistance-versus-gap curve, and it is not at the smallest gap you can manufacture.
A useful mental model is a chimney. A wide chimney moves a lot of air slowly; a narrow chimney moves a little air quickly but with high friction losses. Fin stacks behave the same way, and the sweet spot sits where the boundary layers from adjacent fins are just about to merge.
The boundary layer is the whole story
Each vertical fin grows a thermal boundary layer that thickens as air travels upward. If fins are far apart, each one has its own independent boundary layer and the air between them stays cool. If fins are too close, the boundary layers from both walls meet in the middle of the channel, the air core becomes hot, and the fin tips stop doing useful work.
The rule of thumb that falls out of this: the gap should be at least twice the boundary layer thickness at the fin exit. For air at typical electronics temperatures, that lands in the 6 to 12 mm range for fin heights between 25 and 75 mm. Very short fins can run tighter; very tall fins need to run wider.
What is the optimum fin spacing for natural convection?
There is no single number, because the optimum depends on fin height, air pressure, and the temperature rise you are willing to accept. But the table below gives practical starting points that hold up well in real enclosures at or near sea level.
| Fin height (vertical) | Recommended gap | Typical fin thickness | Fin pitch (gap + thickness) | Notes |
|---|---|---|---|---|
| 15–25 mm | 4–6 mm | 1.0–1.5 mm | 5–7.5 mm | Short fins tolerate tight gaps |
| 25–50 mm | 6–9 mm | 1.5–2.0 mm | 7.5–11 mm | Most common passive range |
| 50–75 mm | 8–12 mm | 2.0–2.5 mm | 10–14.5 mm | Boundary layers thicken fast |
| 75–120 mm | 12–18 mm | 2.5–3.0 mm | 14.5–21 mm | Chimney effect dominates |
| Above 120 mm | 15–25 mm | 3.0–4.0 mm | 18–29 mm | Consider a chimney duct |
These are indicative values for still air at roughly 20–25 °C ambient and a fin-to-ambient temperature difference of 40–60 K. If your device sits in a sealed plastic box, if it runs at 3000 m altitude, or if the fins are horizontal rather than vertical, the numbers shift and you should verify by measurement.
| Condition | Effect on optimum gap | Practical adjustment |
|---|---|---|
| Altitude 2000 m+ | Thinner air, weaker buoyancy | Widen gap 10–20% |
| Horizontal fin stack | Flow is obstructed by fins | Widen gap 20–30%, or rotate |
| Confined enclosure | Recirculation, warm intake air | Widen gap and add vents top and bottom |
| High emissivity black anodize | Radiation adds a parallel path | Gap can stay at the low end |
| Bare mill finish aluminium | Little radiative help | Bias toward the wider end |
| Nearby hot components | Preheated intake air | Widen gap, duct the intake |
Why the curve is flat near the optimum
One encouraging fact for designers: the thermal resistance curve near the optimum is shallow. Moving 20% away from the perfect gap typically costs only a few percent in performance. That means you do not need a CFD study to get a good result. You need to avoid the two failure modes — gaps so tight the channels stall, and gaps so wide you have wasted the volume you paid for.
What you should not do is copy a forced-convection extrusion into a passive design. A 2 mm-pitch profile that works beautifully with a 40 mm fan will underperform a 9 mm-pitch profile in still air, sometimes by 30% or more, because the narrow channels simply cannot sustain buoyancy-driven flow.
How do fin height, thickness and base spread interact?
Fin spacing is only one of four variables. The others are fin height, fin thickness, and how well heat spreads sideways through the base before it ever reaches a fin.
Base spreading resistance
If your heat source is a 10 mm square die sitting on a 100 mm wide heat sink, most of the base is doing nothing until heat spreads laterally. Thick bases spread better. For natural convection heat sinks, a base thickness of 5 to 8 mm is common, and it is often worth more than adding fins. A useful check: if the base is thinner than about one-tenth of the heat sink width, spreading resistance will dominate and extra fins will not help.
Fin efficiency
Tall, thin fins are less efficient at the tip than at the root, because the tip is cooler. Fin efficiency for aluminium in natural convection typically lands between 0.75 and 0.95. If your calculated efficiency drops below about 0.7, the fin is too tall or too thin for the job, and you should trade height for width or accept a shorter stack.
Thickness and pitch
Thicker fins conduct better but block more air. In natural convection the trade is usually resolved in favour of slightly thinner fins and wider gaps, because airflow is the scarce resource. A 1.5 to 2.5 mm fin thickness at 8 to 12 mm gap is a well-balanced starting envelope for aluminium extrusions.
Choosing a manufacturing route that holds the gap
Once the geometry is set, the gap has to survive manufacturing. Different processes hold different tolerances, and this matters because a nominal 8 mm gap that varies from 6 to 10 mm across the stack will not perform like a uniform one.
| Process | Typical gap tolerance | Best for | Notes |
|---|---|---|---|
| Aluminium extrusion | ±0.3–0.8 mm on pitch | Long, uniform passive profiles | Lowest cost per metre at volume |
| CNC machining from billet | ±0.05 mm or better | Prototypes, skived-fin-like geometry, small batches | Best for tight, verified gaps |
| Bonded fin assembly | ±0.2–0.5 mm | Very high aspect ratio, mixed materials | Epoxy or solder bond line matters |
| Die casting | ±0.3–0.5 mm | Complex 3D shapes, integrated bosses | Draft angles limit thin fins |
| Skiving | ±0.1 mm | Very thin, dense fins | Usually for forced air |
For a passive heat sink where the gap is the design variable, extrusion is normally the right answer at volume, and CNC machining is the right answer for prototypes, low volume, or any profile where you want to verify the exact gap before committing to a die. You can review the available families on the heat sinks product page and the extruded heat sink range, or look at CNC machined heat sinks if your geometry is not extrudable.
Alloy choice also matters, though less than people expect. Extruded 6063 has excellent thermal conductivity and extrudes cleanly; 6061 machines better but conducts slightly less. The differences are covered in more depth in our comparison of heat sink aluminium alloys.
How do you test and confirm the optimum?
Simulation gets you close; a thermocouple gets you the truth. The test is cheap and takes an afternoon.
1. Build two or three variants. Same envelope, different gaps — for example 6 mm, 9 mm, and 12 mm. CNC machining small batches makes this affordable.
2. Mount them identically. Same thermal interface material, same torque, same heater power. Interface variation can easily swamp a fin-spacing difference, so keep it constant.
3. Run in the real orientation. Vertical fins, in the real enclosure, with the real vents. A bench test in open air will flatter a tight-gap design.
4. Soak to steady state. Natural convection is slow. Allow 45 to 60 minutes, and log until the rise is under 0.2 K in 10 minutes.
5. Record ambient, base, and fin-tip temperatures. The base-to-ambient delta divided by input power gives you thermal resistance in K/W.
If the 9 mm variant wins, you have confirmed the model. If the 6 mm variant wins, your fins are short or your enclosure is unusually well vented. Either way you now have data, which is what belongs in the thermal spec sheet you hand to your supplier.
A note on radiation and finish
At natural convection temperatures, radiation is not a rounding error. A black anodized surface with emissivity around 0.85 can carry 15 to 25% of the total heat load at a 50 K rise. That parallel path reduces the penalty for a slightly tight gap, which is one reason black anodized passive heat sinks often run tighter than bare aluminium ones. If you are comparing finishes, run the test with the final surface treatment applied.
Design checklist before you send an RFQ
- Fix the fin height first; it sets the gap range.
- Start at 8 mm gap, 2 mm fin, 5 mm base for a typical 50 mm tall passive sink.
- Widen the gap for altitude, horizontal mounting, or a sealed enclosure.
- Keep fin efficiency above 0.7; otherwise shorten the fins.
- Confirm base spreading is not the bottleneck before adding fins.
- Specify the gap tolerance, not just the nominal, on the drawing.
- Test two or three gaps in the real enclosure before tooling.
BQUQ runs four production lines in one Dongguan factory covering CNC machining, metal stamping, custom springs, and heat sink production, with ISO9001 quality management and flexible MOQ. Send your geometry and target thermal resistance and we will return a quote in 12 working hours.
Frequently Asked Questions
Q: What is the best fin spacing for a natural convection heat sink?
A: For most passive electronics heat sinks with 25–75 mm tall vertical fins, 8–12 mm is the practical optimum gap in still air at sea level. Shorter fins can run 4–6 mm; taller stacks need 12–18 mm. The curve is flat near the optimum, so a few millimetres either way costs only a small percentage of performance.
Q: Why does adding more fins sometimes make a passive heat sink worse?
A: Because natural convection has almost no driving pressure. Narrow gaps increase viscous drag, the boundary layers from adjacent fins merge, and the air in the channel becomes hot and stops carrying heat away. Surface area rises but usable airflow falls faster, so thermal resistance climbs instead of dropping.
Q: Does fin spacing change with altitude?
A: Yes. Thinner air reduces buoyancy, so the driving pressure drops and channels stall more easily. As a rough guide, widen the gap by 10–20% for installations above 2000 m, and verify by test if the design is thermally marginal or the enclosure is sealed.
Q: Should passive heat sink fins be vertical or horizontal?
A: Vertical, always, if you have the choice. Vertical channels let buoyancy drive a clean chimney flow from bottom to top. Horizontal fins trap air against the fin surfaces and can cost 20–30% of performance. If the product orientation is fixed horizontally, widen the gap and add generous vents below and above.
Q: How do I specify fin spacing tolerance on a drawing?
A: Call out the gap or pitch with an explicit tolerance, not just a nominal. Extrusions typically hold ±0.3–0.8 mm on pitch; CNC machined profiles can hold ±0.05 mm or better. Tight tolerance matters most on prototypes and low-volume builds where you are validating a specific gap.
Related Resources
- About BQUQ and our Dongguan factory: /about/
- Heat sink product families: /heat-sinks/
- Extruded aluminium heat sink profiles: /extruded-heat-sinks/
- Industry trends in thermal management: /industry-dynamics/
- Technical articles and design guides: /bquq-blog/
- Frequently asked questions: /faq/
- Case studies: /case/
- Contact the engineering team: /contact/
Authored by the BQUQ Engineering Team. BQUQ (Dongguan) runs CNC machining (±0.005 mm), metal stamping, custom springs, and heat sink production in one ISO9001 factory. Source-direct from Dongguan, China — quote in 12 hours: sc@bquq.com | WhatsApp +86 13713157787 | www.bquq.com
