Thermal Resistance Calculator
Understanding Thermal Resistance
Thermal resistance represents the temperature difference per unit of heat flow across a structure. It is crucial for thermal management in electronic systems.
θtotal = θjc + θcs + θsa (Series)
1/θtotal = 1/θ1 + 1/θ2 + ... (Parallel)
ΔT = P × θtotal
Rcontact = t / (k × A)
Thermal Path
The thermal path represents the route that heat takes from source to ambient:
- Junction to case (internal)
- Case to heat sink (interface)
- Heat sink to ambient (external)
- Additional parallel paths
- PCB conduction path
Thermal Contact Resistance
Contact resistance occurs at the interface between two surfaces and can significantly impact overall thermal performance.
How to Minimize Contact Resistance
- Use thermal interface materials (TIM)
- Ensure proper surface flatness
- Apply appropriate mounting pressure
- Clean contact surfaces
- Choose compatible materials
| Interface Type | Resistance (°C/W) | Notes |
|---|---|---|
| Dry Contact | 0.5-1.0 | Poor thermal transfer |
| Thermal Paste | 0.2-0.3 | Good for uneven surfaces |
| Thermal Pad | 0.3-0.5 | Easy to apply |
| Liquid Metal | 0.1-0.2 | Excellent but conductive |
Thermal Resistance Network
Thermal networks can be analyzed similarly to electrical circuits:
| Type | Formula | Application |
|---|---|---|
| Series | Rtotal = R1 + R2 + R3 | Single path heat flow |
| Parallel | 1/Rtotal = 1/R1 + 1/R2 | Multiple heat paths |
| Complex | Mixed calculation | Real-world systems |
Design Considerations
Key factors to consider in thermal design:
- Power dissipation requirements
- Space constraints
- Cost limitations
- Reliability targets
- Environmental conditions
Heat Sink Design
Heat sink design involves optimizing multiple parameters:
Key Factors:
- Fin spacing and thickness
- Base thickness
- Surface area
- Material selection
- Airflow characteristics
| Type | Performance | Applications |
|---|---|---|
| Stamped | Basic | Low-power devices |
| Extruded | Good | Medium-power devices |
| Forged | Excellent | High-power devices |
Troubleshooting Guide
Common thermal problems and their solutions:
High Junction Temperature
Possible Causes:
- Poor thermal interface
- Inadequate heat sink
- High ambient temperature
Solutions:
- Reapply thermal paste
- Upgrade heat sink
- Improve ventilation
Thermal Cycling Issues
Possible Causes:
- Material expansion mismatch
- Poor mounting pressure
- TIM degradation
Solutions:
- Use compatible materials
- Adjust mounting pressure
- Replace TIM regularly
Preventive Measures:
- Regular maintenance
- Temperature monitoring
- Proper installation procedures
- Quality components
Applications
Thermal resistance analysis is essential in various electronic applications:
- Heat sink design and selection
- Semiconductor device cooling
- PCB thermal management
- Power electronics cooling
- LED thermal design
- Electronic packaging
Quick Reference
Package Thermal Resistance
TO-220: 3-5°C/W
DPAK: 5-8°C/W
QFN: 8-15°C/W
SOIC: 15-25°C/W
Design Tips
- • Minimize thermal interfaces
- • Use thermal paste/pad
- • Add thermal vias in PCB
- • Ensure good surface contact
- • Consider airflow path
Common Values
TIM Properties
Thermal Paste: 3-8 W/m·K
Thermal Pad: 1-5 W/m·K
Phase Change: 1-3 W/m·K
Thermal Grease: 0.7-3 W/m·K
Contact Resistance
Dry Contact: 0.5-1.0°C/W
With TIM: 0.1-0.3°C/W
Soldered: 0.05-0.1°C/W
Clamped: 0.2-0.5°C/W