Heat Transfer Fundamentals
Heat always flows from hot regions to cold regions. Master the three modes of heat transfer — conduction, convection, and radiation — that govern every thermal design decision, from a die attach layer to a heatsink's fin array.
Thermal energy is the kinetic energy of atoms and molecules. Higher temperature means faster molecular vibrations. When two objects at different temperatures are placed in contact, energy transfers from the hotter object to the cooler one until they reach thermal equilibrium — governed by the Second Law of Thermodynamics, which states that heat flows spontaneously only from hot to cold.
Click each card to expand its electronics-specific examples and key governing parameter.
Heat transfer through direct contact within a material.
Click to expand →Heat transfer between a surface and a moving fluid.
Click to expand →Heat transfer via electromagnetic waves — no medium required.
Click to expand →Conduction moves heat through a material by molecular vibration and free electron movement, with no bulk motion of the material. It is the primary mechanism carrying heat from a semiconductor die to the outside of its package.
Q = heat flow (W) · k = thermal conductivity (W/m·K) · A = cross-section area (m²) · L = length (m)
FPGA Package Thermal Path — Click Each Layer
Convection transfers heat between a solid surface and a moving fluid — it is the primary mechanism removing heat from a heatsink to the surrounding air.
h = 5-25 W/m²·K — buoyancy-driven
h = 25-250 W/m²·K — fan-driven, 5-10× better
Every object above absolute zero emits thermal radiation. Unlike conduction and convection, radiation needs no physical medium — it works even in vacuum.
σ = 5.67 × 10-8 W/m²·K⁴ · ε = surface emissivity (0 to 1)
| Property | Conduction | Convection | Radiation |
|---|---|---|---|
| Requires medium? | Yes (solid) | Yes (fluid) | No |
| Driving force | Temperature gradient | Temperature + flow | Temperature (T⁴) |
| Typical role | Die→Package→Heatsink | Heatsink→Air | Surface→Enclosure |
| Key parameter | k (W/m·K) | h (W/m²·K) | ε (0-1) |
| Contribution, forced air | ~30-40% | ~55-65% | ~5-10% |
| Contribution, natural conv. | ~25-30% | ~45-55% | ~15-25% |
Adjust power, airflow, material, heatsink area, and ambient temperature to see conduction, convection, and radiation working together in a simplified thermal model.
Module 2 Quiz
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