Heat Sink Sizing: A Worked Thermal Calculation for Your Power Budget
Size a heat sink by working backward from one number: the maximum thermal resistance allowed between the device case and the ambient air. Required sink-to-air resistance equals (maximum case temperature minus worst-case ambient) divided by dissipated power, minus the interface resistance between case and sink. If your budget comes out at 2.5 K/W, you need a sink rated 2.5 K/W or better at your airflow — anything else is guesswork dressed up as engineering.
The calculation is deliberately simple because it has to survive contact with reality: real ambient temperatures, real airflow, real interface materials. This guide walks one sizing exercise from watts to a finished part number, with the typical values you can plug into your own budget before you send anything to a factory.
The One Equation Behind Every Heat Sink
Every heat sink problem is a series of thermal resistances stacked between the heat source and the air. Junction-to-case is fixed by the component package. Case-to-sink is fixed by your interface material and mounting. Sink-to-air is the only term you can buy — it is the heat sink.
Required sink-to-air resistance: Rth(sa) = (Tcase,max − Tamb,max) / P − Rth(interface)
Worked in words: take the hottest temperature the component case may reach, subtract the hottest air the product will ever see, divide by the worst-case power, and what remains after subtracting the interface is the biggest heat sink resistance you can tolerate.
| Thermal link | Typical value | Notes |
|---|---|---|
| TO-220 junction→case | 2.5–4 K/W | Old workhorse package, needs a good sink |
| TO-247 junction→case | 0.3–0.6 K/W | Large tab, common for MOSFETs and diodes |
| Power LED junction→solder point | 1–3 K/W | Depends on die size and substrate |
| IGBT module junction→baseplate | 0.1–0.3 K/W | Wide DBC substrate spreads heat well |
| Grease, 25 × 25 mm interface | 0.1–0.3 K/W | Thin bond line, best value |
| Silicone pad, 25 × 25 mm | 0.4–1.0 K/W | Easy assembly, higher resistance |
| No TIM (dry metal contact) | 0.5–1.5 K/W | Air gap dominates; avoid in production |
Takeaway: the interface is rarely free. On a TO-247 dissipating 25 W, every 0.1 K/W of interface costs you 2.5 °C of case temperature — which is why the cheapest part of the cooling system is often the most profitable to fix. Full thermal resistance background is in our heat sink thermal resistance guide.
Worked Example: Sizing for a 25 W MOSFET in Free Air
Take a TO-247 MOSFET that dissipates 25 W worst case. The product sits indoors, so the worst-case ambient is 50 °C. The datasheet allows a 150 °C junction, and the package junction-to-case resistance is 0.4 K/W. You plan a 0.3 K/W grease interface. Allow 20% power margin so the sink is not sized exactly at the cliff edge.
Step 1 — junction budget: (150 − 50) °C = 100 K total allowed from junction to air. Step 2 — design power with margin: 25 W × 1.2 = 30 W. Step 3 — total allowed resistance: 100 K / 30 W = 3.33 K/W. Step 4 — subtract package and interface: 3.33 − 0.4 − 0.3 = 2.63 K/W for the sink in natural convection.
| Budget item | Value | Running total left for the sink |
|---|---|---|
| Max junction − ambient | 100 K | — |
| Design power (25 W × 1.2 margin) | 30 W | — |
| Total Rth junction→air | 100/30 = 3.33 K/W | 3.33 K/W |
| Junction→case (TO-247) | 0.40 K/W | 2.93 K/W |
| Grease interface | 0.30 K/W | 2.63 K/W |
| Required sink→air Rth | ≤ 2.63 K/W | target |
Takeaway: the sink now has a number — 2.63 K/W in still air. Nothing about the shape is decided yet, but every supplier conversation starts from this figure instead of from "a decent-size one."
Turning the Budget into a Real Sink
Sink-to-air resistance is set by exposed area, convective coefficient, and fin efficiency. A quick reality check uses Q = h × A × ΔT: for 30 W at a 50 K rise you need roughly h × A = 0.6 W/K. With an effective coefficient near 10 W/m²·K (typical for a black-anodized vertical fin array in free air, radiation included), that means about 0.06 m² of wetted fin surface — a 100 × 75 mm base with 8–10 fins around 35 mm tall gets you there.
| Cooling mode | Typical convective coefficient | Practical sink size for ~30 W at 50 K rise |
|---|---|---|
| Natural convection, bare aluminum | 5–8 W/m²·K | Large: 150 × 100 × 40 mm and up |
| Natural convection, black anodized | 9–14 W/m²·K | Moderate: 100 × 75 × 40 mm class |
| Forced air 1–2 m/s | 15–40 W/m²·K | Small: 75 × 60 × 25 mm with a 40–60 mm fan |
| Forced air 3–5 m/s, ducted | 40–90 W/m²·K | Compact: 60 × 50 × 20 mm pin fin |
Takeaway: radiation is not decoration in passive designs — black anodizing is worth roughly 30–50% more effective area equivalent in free air, which is why nearly every passive sink is dark. If the natural-convection sink does not fit your enclosure, the honest options are a small fan or a redesign of the power stage, not a taller fin stack that no longer fits.
Derating Factors Most Calculations Miss
The equation assumes clean vertical fins, sea level, an unobstructed sink, and a heat source that covers the base. Real products violate all four assumptions, so apply multipliers before finalizing the part number.
| Condition | Typical penalty | Fix |
|---|---|---|
| Fins horizontal instead of vertical | −20–30% natural convection | Rotate the sink or the product |
| Heat source smaller than ~25% of base area | −10–30% due to spreading | Thicken the base or add a heat spreader |
| Altitude 1,500 m | −5–10% air density effect | Oversize the sink |
| Altitude 3,000 m | −10–20% air density effect | Oversize or add airflow |
| Sink within 10 mm of an enclosure wall | −10–25% choked airflow | Keep clearance ≥ one fin pitch |
| Ducted airflow, unshrouded fan | −10–25% recirculation | Add a shroud or duct |
Takeaway: add the penalties that apply to your product, not all of them to every product. A sealed outdoor enclosure with a horizontally mounted LED driver gets three penalties at once; a ventilated indoor unit with a vertical board gets none. When in doubt, run the worst combination — then check it with a thermocouple on the first prototype.
From Calculation to Part Number
Once the budget and derating are fixed, geometry selection is a catalogue exercise: choose a profile whose published or measured natural-convection resistance meets the target, then confirm the base can be cut to length, milled flat, drilled, and finished to your drawing. That is where a source factory that runs both extruded heat sinks and CNC-machined heat sinks helps: extrusion wins at volume on standard profiles, machining wins when the base needs pockets, mounting bosses, or tight flatness that no die can give. Send watts, ambient range, case temperature limit, airflow (or "still air"), and envelope constraints together — with those five inputs a factory can size and price the sink in one pass. Our 8-step heat sink selection workflow covers the rest of the journey from this calculation to a released part number.
Email sc@bquq.com or WhatsApp +86 137 1315 7787 with your PDF/DXF/STEP file. An engineer reviews it and replies with price, lead time and DFM notes on working days.
Frequently Asked Questions
Q: What formula do I use to size a heat sink?
A: Required sink-to-air resistance equals (Tcase,max − Tamb,max) divided by dissipated power, minus interface resistance. Then select a sink whose rated resistance at your airflow is equal to or lower than that budget. The whole discipline is keeping that inequality true at worst case.
Q: What is a good thermal resistance for a natural convection heat sink?
A: A medium extruded aluminum sink around 100 × 75 × 40 mm typically lands at 2–3 K/W in still air when black anodized, and larger or finned profiles reach 1–1.5 K/W. Below roughly 1 K/W without a fan, the sink becomes impractically large — that is the crossover to forced air.
Q: How much power margin should I add when sizing?
A: Use 15–30% on top of measured worst-case dissipation, plus derating for orientation, altitude, or enclosure blockage. Margin is cheap at the drawing stage and very expensive after the enclosure, PCB, and thermal solution are all locked.
Q: Do I really need to include the interface material in the calculation?
A: Yes. A dry metal joint can add 0.5–1.5 K/W, which on a 25 W device is 12–37 °C of case temperature. Grease or a phase-change pad typically costs 0.1–0.5 K/W and is the lowest-cost performance upgrade in the whole thermal chain.
Q: Why does my prototype run hotter than the calculation predicted?
A: Usually one of four causes: the heat source is small and the base is spreading the heat poorly, the fins are not vertical, the sink sits too close to an enclosure wall, or the real ambient is higher than assumed. Measure case temperature with a thermocouple at several loads and compare against the budget line item by line item.
Related Articles
- heat-sink-thermal-resistance-guide — More from the BQUQ Thermal Management engineering series.
- heat-sink-selection-workflow — More from the BQUQ Thermal Management engineering series.
- heat-sink-fin-design-guidelines — More from the BQUQ Thermal Management engineering series.
Data Sources and Verification
Tolerances, cycle times and price ranges in this guide come from BQUQ production records at our Dongguan plant, where CNC machining (±0.005 mm), stamping, custom springs and heat sinks run under one roof. BQUQ is an ISO 9001:2015 certified factory; the certificate and batch inspection reports are available on request with every quotation.
Related Resources
- About BQUQ: an ISO9001-certified source factory in Dongguan running four production lines under one roof.
- Heat sinks and thermal parts: extruded, CNC-machined and stamped options from the thermal line — extruded heat sinks, CNC-machined heat sinks, stamped heat sinks.
- Industry trends: manufacturing, material market, and sourcing analysis for buyers.
- Technical articles: engineering guides and process comparisons — more where this article came from.
- FAQ hub: quick answers on CNC, stamping, springs, and heat sinks.
- Case studies: real parts and real numbers from projects we engineered and delivered.
- Contact us: send your drawing and get a quote within 12 working hours.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
