2021.03.10

Points of this article

・Thermal resistance is a numerical expression of the difficulty of propagation of heat.

・As symbols, Rth and θ (theta) are used; units are ℃/W （K/W）.

・Thermal resistance can be considered to be roughly analogous to electrical resistance.

From here, we begin technical discussions relating to thermal design. The knowledge required for thermal design spans a broad range of fields. We begin by explaining the basics of thermal resistance and heat dissipation, an understanding of which is the bare minimum background knowledge needed for this subject.

Thermal resistance is a numerical representation of the difficulty of propagation of heat. It is the temperature difference between two arbitrary points, divided by the amount of heat flowing between the two points (the amount of heat that flows in a unit of time). The higher the thermal resistance, the harder it is for heat to propagate; if the thermal resistance is low, heat propagates easily.

As symbols, Rth and θ (theta) are used. Rth derives from the term “thermal resistance”.

Units are ℃/W （K/W）.

Thermal resistance can be regarded as more or less similar to electrical resistance, and the basic equations for thermal calculations can be handled similarly to Ohm’s law.

Electricity | Current I（A） | Voltage difference ⊿V（V） | Electrical resistance R（Ω） |
---|---|---|---|

Heat | Amount of heat flow P（W） | Temperature difference⊿T（℃） | Thermal resistance Rth（℃/W） |

Hence, just as a voltage difference ⊿V is calculated as R×I, so a temperature difference ⊿T can be calculated as Rth×P.

Downloadable materials, including lecture materials from ROHM-sponsored seminars and a selection guide for DC-DC converters, are now available.

- About Thermal Design
- Changes in Engineering Trends and Thermal Design
- A Mutual Understanding of Thermal Design
- Fundamentals of Thermal Resistance and Heat Dissipation: Heat Transmission and Heat Dissipation Paths
- Fundamentals of Thermal Resistance and Heat Dissipation : Thermal Resistance in Conduction
- Fundamentals of Thermal Resistance and Heat Dissipation : Thermal Resistance in Convection
- Fundamentals of Thermal Resistance and Heat Dissipation : Thermal Resistance in Emission
- Thermal Resistance Data: JEDEC Standards, Thermal Resistance Measurement Environments, and Circuit Boards
- Thermal Resistance Data: Actual Data Example
- Thermal Resistance Data: Definitions of Thermal Resistance, Thermal Characterization Parameters
- Thermal Resistance Data: θJA and ΨJT in Estimation of TJ: Part 1
- Thermal Resistance Data: θJA and ΨJT in Estimation of TJ: Part 2
- Estimating TJ: Basic Calculation Equations
- Estimating TJ: Calculation Example Using θJA
- Estimating TJ: Calculation Example Using ΨJT
- Estimating TJ: Calculation Example Using Transient Thermal Resistance
- Estimation of Heat Dissipation Area in Surface Mounting and Points to be Noted
- Surface Temperature Measurements: Thermocouple Types