Heatsink Basics: Heat Transfer and Selection Parameters

This page delves into the fundamentals of heatsinks, covering the basics of heat transfer and outlining crucial parameters for choosing the right heatsink for your needs.

Understanding Heat Transfer

Heat transfer occurs whenever there’s a temperature difference between two surfaces. This temperature difference acts as a “driving force,” causing heat energy to move from the hotter surface to the colder one.

Think of it like this:

• Voltage is the force that drives electrical current.
• Temperature is the force that drives heat flow.

The amount of heat that flows depends directly on the magnitude of the temperature difference. There are three primary modes of heat transfer:

• Conduction: Heat transfer through or across a medium. Imagine the heat travelling along a metal rod.

• Convection: Heat transfer from a hot surface to a moving fluid (like air or water) at a lower temperature. Think of a fan blowing air across a hot surface.

• Radiation: Heat transfer in the form of electromagnetic waves between two surfaces with different temperatures. This doesn’t require any medium.

What is a Heatsink?

A heatsink is a hardware component, typically made of metal, designed to dissipate heat quickly. Its main purpose is to prevent excessive temperature increases within a circuit. Heatsink implementation is often considered early in the PCB design process.

Why are heatsinks necessary? Because semiconductor devices, like transistors and integrated circuits, inherently introduce some resistance into a circuit, similar to discrete resistors or coils. As these devices control the flow of current, they dissipate power in the form of heat. If this heat isn’t managed, it can damage the components.

Parameters for Selecting the Right Heatsink

When designing or selecting a heatsink, several key parameters need to be considered to ensure effective heat dissipation:

• Q: Amount of heat to be dissipated (measured in Watts, W). This is the most crucial factor.

• T_{j max}: Maximum allowable junction temperature (in degrees Celsius, °C). This is the maximum temperature the semiconductor device can withstand before damage occurs.

• T_a: Ambient fluid temperature (in degrees Celsius, °C). This is the temperature of the air or fluid surrounding the device and heatsink.

• R_Θjc: Thermal resistance of the device (junction-to-case). This measures how effectively heat can transfer from the device’s internal junction to its external case.

• R_Θcs: Thermal resistance of the interface material (case-to-sink). This measures the thermal resistance between the device case and the heatsink. Lower values are better.

• Thermal resistivity (�?), contact area (A), and thickness (t): These parameters influence the thermal resistance of the interface material. You ideally want low resistivity, a large contact area, and a thin interface layer. The relationship can be expressed as:

  $R = \rho \frac{t}{A}$

• Natural or forced convection cooling: Will the heatsink rely on natural air currents (natural convection) or will a fan be used to force air across it (forced convection)? Forced convection is more effective.

• Air flow (i.e., LFM): If forced convection is used, the amount of airflow (measured in Linear Feet per Minute, LFM) is a critical factor. Higher airflow generally means better cooling.