What happens if calculated θsa is a negative number?

If calculated θsa is negative or zero, the package internal resistance (θjc) and TIM insulator resistance (θcs) alone generate enough thermal drop to exceed max junction temperature, even if the heatsink were at absolute room temperature. You must reduce dissipated power, use multiple parallel transistors, or upgrade to a larger package (e.g. TO-247).

Why is thermal paste (grease) mandatory?

Microscopic air pockets between metal surfaces act as excellent thermal insulators (air conductivity is only 0.026 W/m·K). Thermal paste fills microscopic voids, lowering interface resistance from ~2.5 °C/W down to 0.4 - 0.5 °C/W.

What is a typical thermal resistance for natural convection vs fan-cooled heatsinks?

Small stamped PCB clip-on heatsinks have ratings of 10 to 25 °C/W. Medium extruded chassis heatsinks range from 2.0 to 5.0 °C/W. High-performance multi-fin heatsinks with forced fan airflow achieve 0.3 to 1.0 °C/W.

Free Heat Sink Thermal Resistance Calculator - Junction-to-Ambient Sizing Tool

❄️ Thermal Stack & Power Dissipation

Power Dissipated (P in Watts) MOSFET, BJT, linear reg, LED

Max Allowable Junction Temp (Tj °C) Standard silicon max: 150°C (derate to 125°C)

Ambient Air Temp (Ta °C) Inside enclosure ambient

Semiconductor Package

Thermal Interface Material (TIM - θcs)

📊 Required Heat Sink Rating

Max Allowable Heatsink θsa 4.17 °C/W Lower number = Larger heatsink

Heatsink Rise Above Ambient +62.5 °C Heatsink surface ~102.5°C

Total Allowable Thermal Budget: 5.67 °C/W (ΔT = 85°C)

Package Junction-to-Case (θjc): 1.00 °C/W

Case-to-Sink TIM Loss (θcs): 0.50 °C/W

Cooling Mechanism Needed: Moderate Natural Convection

✅ Feasible Heatsink: Sizing requires θsa ≤ 4.2 °C/W. A medium extruded aluminum heatsink (approx 50x50x40mm) will suffice under natural convection.

Recommended Tools & Equipment

Tested hardware and components for high reliability

100% Free Tool Zero Sign-Up

The Ohm's Law of Thermal Resistance

Thermal heat transfer follows an exact mathematical analog to electrical circuits. Temperature difference ((Delta T)) acts as voltage, thermal power dissipation ((P) in Watts) acts as current, and thermal resistance (( heta) in (^circ ext{C/W})) acts as electrical resistance.

T_junction = T_ambient + P × (θ_jc + θ_cs + θ_sa)

Solving for Required Heatsink Rating (( heta_{sa}))

To prevent semiconductor failure, rearrange the formula to find the maximum allowable thermal resistance of the heatsink to ambient air (( heta_{sa})):

θ_sa ≤ [ (T_j_max - T_ambient) / P ] - θ_jc - θ_cs

Note the inverse rule: A smaller ( heta_{sa}) number represents a larger, more efficient heatsink that can dump heat with less temperature rise.

Natural Convection vs Forced Air (CFM)

Natural convection heatsinks require massive fin surface area. Adding a small 40mm or 80mm cooling fan delivering 15 to 30 CFM (approx 200 to 400 LFM air velocity) reduces heatsink thermal resistance by 60% to 75%, allowing you to shrink the heatsink volume to a fraction of its natural size.

Free Heat Sink Thermal Resistance Calculator