Thermal Resistance Calculator
Understanding Thermal Resistance
1. Basic Principles
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
θparallel = 1 / (1/θ1 + 1/θ2 + ...)
ΔT = P × θtotal
Rcontact = t / (k × A)
Common Questions
What is Thermal Resistance?
Thermal resistance represents the temperature difference per unit of heat flow across a structure. It measures how well a material or system resists heat flow, similar to electrical resistance.
How to Calculate Thermal Resistance?
Thermal resistance can be calculated using:
- R = ΔT / Q (temperature difference / heat flow)
- R = L / (k × A) for conduction
- R = 1 / (h × A) for convection
- Rtotal = R1 + R2 + ... (series)
- 1/Rtotal = 1/R1 + 1/R2 + ... (parallel)
How to Calculate Heat Sink Thermal Resistance?
Heat sink thermal resistance calculation involves:
- Base plate resistance
- Spreading resistance
- Fin efficiency
- Convection resistance
- Interface resistance
Material Thermal Resistance
| Material | Thermal Resistance (K·m/W) | Applications |
|---|---|---|
| Copper | 0.0025 | Heat sinks, PCB |
| Aluminum | 0.0042 | Heat spreaders |
| FR4 | 0.25 | PCB substrate |
2. Thermal Path
Common thermal resistance paths:
- Junction to Case (θjc)
- Case to Heatsink (θcs)
- Heatsink to Ambient (θsa)
- Thermal Interface Material
- PCB Thermal Vias
- Contact Resistance
3. Applications
Thermal resistance analysis is used in:
- Heat Sink Design
- Package Selection
- TIM Selection
- PCB Design
- Cooling System
- Thermal Management
Thermal Contact Resistance
What is Thermal Contact Resistance?
Thermal contact resistance occurs at the interface between two materials in thermal contact. It depends on surface roughness, contact pressure, and interface materials.
How to Minimize Contact Resistance
Methods to reduce thermal contact resistance:
- Use thermal interface materials
- Increase contact pressure
- Improve surface finish
- Ensure proper mounting
- Select compatible materials
| Interface Type | Contact Resistance (°C/W) | Notes |
|---|---|---|
| Dry Contact | 0.5-1.0 | Poor thermal transfer |
| Thermal Paste | 0.1-0.3 | Common solution |
| Thermal Pad | 0.2-0.5 | Easy to apply |
Thermal Resistance Network
Network Analysis
Thermal resistance networks can be analyzed similar to electrical circuits:
- Series: Total resistance is sum of individual resistances
- Parallel: Total resistance follows reciprocal rule
- Complex networks can be simplified
- Temperature difference acts like voltage
- Heat flow acts like current
Network Examples
| Configuration | Formula | Application |
|---|---|---|
| Series Path | Rtotal = R1 + R2 + R3 | Single heat path |
| Parallel Paths | 1/Rtotal = 1/R1 + 1/R2 | Multiple paths |
| Mixed Network | Combine rules | Complex systems |
4. Design Considerations
Key factors in thermal design:
- Material Properties
- Surface Finish
- Contact Pressure
- Air Flow
- Mounting Method
- Space Constraints
Heat Sink Thermal Resistance
Heat Sink Design Factors
Key considerations for heat sink thermal resistance:
- Base plate thickness and material
- Fin geometry and spacing
- Surface area optimization
- Air flow characteristics
- Mounting pressure and interface
| Heat Sink Type | Thermal Resistance (°C/W) | Application |
|---|---|---|
| Stamped | 4-10 | Low power |
| Extruded | 1-4 | Medium power |
| Bonded Fin | 0.2-1.0 | High power |
Thermal Resistance in Different Materials
Common Materials
| Material | Thermal Resistance | Usage |
|---|---|---|
| Copper | Low | Heat sinks |
| Aluminum | Medium | Enclosures |
| Stainless Steel | High | Structural |
Material Selection Factors
- Thermal conductivity
- Cost considerations
- Manufacturing process
- Environmental conditions
- Weight constraints
Troubleshooting Guide
Common Problems
| Issue | Possible Causes | Solutions |
|---|---|---|
| High Thermal Resistance | • Poor contact • Insufficient TIM • Air gaps | • Check mounting pressure • Reapply TIM • Clean surfaces |
| Uneven Temperature | • Non-uniform pressure • Warped surfaces • Poor spreading | • Adjust mounting • Use thicker TIM • Add spreader |
| Degrading Performance | • TIM pump-out • Surface oxidation • Contamination | • Replace TIM • Clean surfaces • Check environment |
Diagnostic Steps
Follow these steps to identify thermal resistance issues:
- Measure temperatures at multiple points
- Calculate actual thermal resistance
- Compare with expected values
- Inspect thermal interfaces
- Check mounting pressure
- Verify airflow patterns
Prevention Tips
- Regular maintenance schedule
- Monitor temperature trends
- Document thermal performance
- Use quality thermal materials
- Follow mounting guidelines
- Consider environmental factors
When to Take Action
- Temperature exceeds limits
- Thermal resistance increases by >20%
- Uneven temperature distribution
- Visible TIM degradation
- Performance degradation
- After system modifications
Quick Reference
Package θjc
TO-220: 3-5°C/W
DPAK: 5-8°C/W
QFN: 8-15°C/W
SOIC: 15-25°C/W
Design Tips
- • Minimize interfaces
- • Use thermal paste
- • Add thermal vias
- • Ensure good contact
- • Consider airflow
Common Values
TIM Properties
Paste: 0.5-3.0 W/m·K
Pad: 1.0-5.0 W/m·K
Grease: 0.7-3.0 W/m·K
Phase Change: 1.0-5.0 W/m·K
Contact Resistance
Dry: 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