# Thermal Resistance Calculator

Calculate thermal resistance for heat sinks and material layers. Find junction temperature from power dissipation and thermal path. Free electronics tool.

## What this calculates

Thermal resistance describes how well a material or assembly resists the flow of heat, measured in degrees Celsius per watt (°C/W). Lower thermal resistance means better heat transfer. In electronics, the thermal path from a chip to ambient air is a series chain of resistances: junction-to-case, case-to-sink (thermal paste or pad), and sink-to-ambient (the heat sink itself). This calculator handles both the heat sink chain and single material layers.

## Inputs

- **Calculation Mode** — options: Heat Sink Chain (junction to ambient), Material Layer (conduction)
- **θ Junction-to-Case** (°C/W) — min 0 — Heat sink mode: thermal resistance from chip to case surface.
- **θ Case-to-Sink** (°C/W) — min 0 — Thermal pad or paste resistance (typically 0.1 to 1.0 °C/W).
- **θ Sink-to-Ambient** (°C/W) — min 0 — From heat sink datasheet (with airflow conditions).
- **Power Dissipation** (W) — min 0
- **Ambient Temperature** (°C)
- **Material Thickness** (mm) — min 0 — Material mode: layer thickness.
- **Cross-Section Area** (cm²) — min 0 — Material mode: heat flow area.
- **Thermal Conductivity (k)** (W/(m·K)) — min 0 — Aluminum: 200, Copper: 385, Steel: 50, FR4: 0.3.

## Outputs

- **Total Thermal Resistance** (°C/W) — Total resistance from heat source to ambient
- **Junction Temperature** (°C) — Temperature at the heat source (chip die)
- **Temperature Rise** (°C) — Temperature increase above ambient
- **Max Power (Tj = 150°C limit)** (W) — Maximum power if junction temp is limited to 150°C

## Details

The thermal model is an electrical analogy: heat flow (watts) is like current, temperature difference is like voltage, and thermal resistance is like electrical resistance. Just like Ohm's law:

Delta-T = P x R_thermal

And for series resistances in a heat sink assembly:

R_total = R_junction-case + R_case-sink + R_sink-ambient

For a material layer (conduction through a slab):

R = thickness / (k x A)

Where k is thermal conductivity in W/(m·K) and A is the cross-section area.

Typical thermal conductivities:

  - Copper: 385 W/(m·K)

  - Aluminum: 200 W/(m·K)

  - Steel: 50 W/(m·K)

  - Thermal paste: 1-10 W/(m·K)

  - FR4 PCB: 0.3 W/(m·K)

  - Air (still): 0.026 W/(m·K)

For electronics design, the junction temperature must stay below the component's rated maximum (often 125°C or 150°C for silicon). If your calculated junction temperature exceeds this limit, you need a better heat sink, forced airflow, or a lower-power design.

## Frequently Asked Questions

**Q: What is thermal resistance in electronics?**

A: Thermal resistance (θ or R_th, measured in °C/W) tells you how many degrees the temperature rises per watt of heat flowing through a thermal path. A heat sink rated at 5 °C/W means for every watt of heat it dissipates, its temperature rises 5°C above ambient. Lower thermal resistance means the heat sink keeps the component cooler.

**Q: Why does thermal interface material matter?**

A: The case-to-sink thermal resistance can be the bottleneck in your thermal path. Air gaps between the component case and the heat sink surface have extremely poor thermal conductivity (0.026 W/m·K). Thermal paste fills microscopic air gaps between the two surfaces, dropping the interface resistance from several °C/W to under 0.5 °C/W. Thermal pads are convenient but have higher resistance than paste.

**Q: How do I read thermal resistance from a datasheet?**

A: Component datasheets list θ_JC (junction to case) and sometimes θ_JA (junction to ambient without a heat sink). Heat sink datasheets list θ_SA (sink to ambient), usually at specific airflow conditions. For the case-to-sink resistance, check the thermal interface material datasheet or use a typical value of 0.2-1.0 °C/W depending on the material and mounting pressure.

**Q: Can I add thermal resistances in parallel?**

A: Yes, if heat flows through multiple paths simultaneously (like two heat sinks in contact with the same component), combine them in parallel: 1/R_total = 1/R1 + 1/R2. This is the same as parallel electrical resistors. However, most practical thermal paths are series (heat flows from junction through case through paste through sink to air), so you add them directly.

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Source: https://vastcalc.com/calculators/physics/thermal-resistance
Category: Physics
Last updated: 2026-04-08