Heat Sink Calculator
Calculate thermal requirements for power components, regulators, and semiconductors.
Thermal Resistances
Enter values to see results
Thermal Model
Tj
Tc
Ts
Ta
Thermal Interface Materials (θcs)
| Material | θcs (°C/W) | Notes |
|---|---|---|
| Thermal Paste (standard) | 0.5 - 1.0 | Most common, easy to apply |
| Thermal Paste (high-end) | 0.2 - 0.5 | Metal oxide or silver-based |
| Thermal Pad (silicone) | 1.0 - 3.0 | Easy handling, fills gaps |
| Thermal Pad (graphite) | 0.3 - 0.5 | Reusable, compressible |
| Mica + Paste | 0.4 - 0.9 | Electrical isolation |
| Direct Contact (no TIM) | 1.5 - 5.0+ | Not recommended |
Understanding Thermal Management
Heat flows from the semiconductor junction through thermal resistances to the ambient environment. Each interface adds resistance, limiting how much power can be dissipated safely.
The Thermal Equation
Junction temp equals ambient plus heat rise through all thermal resistances.
Thermal Budget
This is your total allowable thermal resistance from junction to ambient.
Heat Sink Selection Guide
Natural Convection
No fan required. Relies on rising hot air. Best for low to medium power.
- • θsa: 3-20 °C/W typical
- • Up to ~15W practical limit
- • Orientation matters (fins vertical)
Forced Air Cooling
Fan provides constant airflow. Much better thermal performance.
- • θsa: 0.5-3 °C/W typical
- • Can handle 50W+ easily
- • Fan noise and reliability concerns
Liquid Cooling
Water or coolant carries heat away. Best for high power density.
- • θsa: 0.05-0.5 °C/W typical
- • Can handle 100W+ per device
- • Complex, expensive, reliability
Heat Spreaders
Copper or aluminum plates spread heat over larger area.
- • Reduces hot spots
- • Often combined with other methods
- • PCB copper can act as spreader
Heat Pipes
Sealed tubes with phase-change fluid for efficient heat transport.
- • Move heat to remote sink
- • Very low thermal resistance
- • Gravity/orientation dependent
Thermoelectric (TEC)
Peltier coolers can actively pump heat. Needs heat sink on hot side.
- • Can cool below ambient
- • Low efficiency (30-60%)
- • Adds heat to system overall
Typical θjc Values by Package
| Package | θjc (°C/W) | θja (no sink) | Common Use |
|---|---|---|---|
| TO-220 | 0.5 - 2.0 | 50 - 70 | Power MOSFETs, regulators |
| TO-247 | 0.3 - 0.7 | 35 - 50 | High power MOSFETs, IGBTs |
| TO-263 (D2PAK) | 0.5 - 2.0 | 40 - 70 | SMD power devices |
| TO-252 (DPAK) | 1.0 - 3.0 | 50 - 100 | SMD regulators, MOSFETs |
| SOT-223 | 10 - 20 | 100 - 150 | Small regulators |
| SOT-23 | 50 - 100 | 200 - 350 | Small signal transistors |
| QFN (exposed pad) | 1 - 5 | 25 - 50 | ICs with thermal pad |
| BGA (with lid) | 0.1 - 0.5 | 15 - 30 | Processors, FPGAs |
Note: Actual values vary by manufacturer and specific device. Always check the datasheet.
Thermal Design Best Practices
PCB Thermal Management
- • Use thermal vias under hot components
- • Large copper pours act as heat spreaders
- • Thicker copper (2oz+) improves spreading
- • Keep hot components away from heat-sensitive parts
- • Consider internal copper planes for heat spreading
System Considerations
- • Account for worst-case ambient temperature
- • Allow for component aging (degraded θjc)
- • Include safety margin (10-20°C below max Tj)
- • Test at maximum load conditions
- • Consider altitude effects on air cooling