Heat sink thermal resistance (θ_SA) determines which heat sink will keep your component within safe operating temperature. The thermal circuit: junction → case → heat sink → ambient. Each interface adds resistance, and the total must stay below (T_J_max − T_ambient) / P_dissipated.
Thermal Parameters
From datasheet: typically 125°C (silicon), 150°C (SiC/GaN)
From component datasheet (thermal resistance junction to case)
Thermal paste: ~0.1–0.5. Dry contact: ~1–3°C/W
Enter component thermal specs to calculate required heat sink.
Typical Heat Sink Thermal Resistance Reference
How to Use the Heat Sink Thermal Resistance Calculator
This heat sink calculator finds the maximum allowable heat sink thermal resistance for your component's power dissipation. Look up θ_JC in the datasheet, enter ambient temperature and power, and see which heat sink rating you need.
Thermal Resistance Formula
Total path: T_J = T_ambient + P × (θ_JC + θ_CS + θ_SA). Rearranged: θ_SA = (T_J_max − T_ambient) / P − θ_JC − θ_CS. Example: 5W, T_J_max 125°C, ambient 40°C, θ_JC = 2°C/W, θ_CS = 0.5°C/W: θ_SA = (125−40)/5 − 2 − 0.5 = 17 − 2.5 = 14.5°C/W.
Safety Margin
Design to keep T_junction below 80–85% of the maximum rating. A component rated at 125°C should run below 100°C in practice. Use T_junction_target = 100°C in the calculator for 25°C headroom. This accounts for ambient temperature spikes and component aging effects.
Finding θ_JC in Datasheets
Look in the "Thermal Characteristics" table of the datasheet. Typical values: TO-220 linear regulator: 3–5°C/W. TO-247 MOSFET: 0.3–1°C/W. SMD D2PAK: 1–2°C/W. QFN package: 5–15°C/W. SMD SOT-23: 150–300°C/W. Higher package thermal resistance means more limited power dissipation.
FAQ
Is this heat sink calculator free?
Yes, completely free. Calculate thermal resistance requirements without any account or payment.
Is my data private?
All calculations run in your browser. No data is sent to any server.
What is thermal resistance (θ)?
Thermal resistance (theta, °C/W) measures how well a material resists heat flow. Lower values = better heat conduction. Total thermal resistance from junction to ambient = θ_JC (junction to case) + θ_CS (case to heat sink, from TIM) + θ_SA (heat sink to air).
How do I calculate required heat sink thermal resistance?
θ_SA = (T_J_max - T_ambient) / P_dissipation - θ_JC - θ_CS. Example: T_J_max = 125°C, T_ambient = 40°C, Power = 5W, θ_JC = 2°C/W, θ_CS = 0.5°C/W. θ_SA = (125-40)/5 - 2 - 0.5 = 17 - 2.5 = 14.5°C/W. Find a heat sink with θ_SA ≤ 14.5°C/W.
What is a TIM (Thermal Interface Material)?
TIM fills microscopic air gaps between the component and heat sink to improve thermal contact. Thermal paste/compound adds approximately 0.1–0.5°C/W (θ_CS). Without TIM, air gaps can create 2–5°C/W additional resistance. Always use TIM with heat sinks.
Do I always need a heat sink?
Not always. Small SMD components often rely on PCB copper pour for heat spreading. A heat sink is needed when the component case temperature exceeds its safe limit. As a rule of thumb, if T_junction > 85°C (for ICs) or the component is too hot to touch comfortably, add a heat sink.
What are typical heat sink thermal resistance values?
Small aluminum extrusion (40mm × 40mm): ~10–20°C/W. Medium finned heat sink (60mm × 60mm): ~5–10°C/W. Large heat sink with fan: ~1–3°C/W. Liquid cooling: 0.1–0.5°C/W. PCB copper pour only: ~50–100°C/W. A fan reduces θ_SA by 50–70%.