- Natural Convection: Heat transfer relies solely on the buoyancy of heated air. As air near the heat sink warms, it becomes less dense and rises, drawing in cooler air from below. It is silent, reliable (no fan to fail), and has zero power consumption. However, it has limited cooling capacity and requires larger, more openly spaced fins to allow for easy air movement.
- Forced Convection: Uses a fan or blower to actively move air across the heat sink. It provides much higher heat transfer coefficients, allowing for higher cooling capacity in a smaller volume. It allows for higher fin density. The trade-offs are added noise, power consumption, cost, and a potential point of failure (the fan).
The transition is often quantified by the Grashof Number (natural) vs. Reynolds Number (forced).