Thermal Conduction vs Convection: Heat Sink Resistance & Dissipation Mechanisms
Heat dissipation in power electronics requires two distinct thermodynamic mechanisms in series: conduction through semiconductor silicon, thermal interface materials (TIM), and heat sink bases, followed by convection into ambient air.
Technical Comparison & Conversion Matrix
| Parameter | Thermal Conduction | Thermal Convection |
|---|---|---|
| Governing Law | Fourier's Law: q = -k * A * (dT/dx) | Newton's Law of Cooling: q = h * A * (T_s - T_inf) |
| Key Material Metric | Thermal Conductivity k (Copper: 385 W/m-K) | Convective Heat Transfer Coefficient h (W/m^2-K) |
| Primary Bottlenecks | TIM voiding, contact pressure, base thickness | Fin spacing, air velocity (LFM), boundary layer |
| Natural vs Forced | Static material property | Natural (h = 5-25) vs Forced Air (h = 50-250) |
| Thermal Resistance Role | Theta_jc (junction-to-case) & Theta_cs (case-to-sink) | Theta_sa (sink-to-ambient airflow resistance) |
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Frequently Asked Questions
What is the biggest source of thermal resistance in power electronics?
The thermal interface material (TIM) between the transistor package and the heat sink base (Theta_cs), which often accounts for 30% to 50% of total junction-to-ambient thermal resistance unless high-conductivity phase-change materials or thermal pastes are used under adequate torque.
How does forced air velocity reduce convective thermal resistance?
Increasing airflow (e.g. from 100 LFM natural to 600 LFM forced) thins the stagnant thermal boundary layer clinging to heat sink fins, increasing the convective transfer coefficient h by 4x to 8x.