How to Dissipate Heat: Overview of Passive and Active PCB Heat Dissipation Techniques
As your printed circuit board operates, power dissipation in active components causes the junction temperature to increase, and heat begins flowing from your components into your conductors and your substrate. PCB substrate materials tend to have low thermal conductivity, causing thermal problems like hot spots and high temperature in your PCB. Keeping the temperature of your components below their rated maximum requires thermal heat dissipation techniques that help aid heat conduction away from hot areas of your printed circuit board.
Active and passive thermal heat dissipation techniques are designed with the same goal in mind: remove heat from components and dissipate it into the surrounding air, or dissipate it into a portion of the board with lower temperature. Passive heat management takes advantage of natural phenomenon where heat moves to cooler regions of space and does not require input energy to achieve heat transport. Active thermal management techniques provide more aggressive means of carrying heat away from a circuit board during operation. You often don’t need to choose between active and passive thermal design techniques; you can implement both in a design if you need a more aggressive cooling strategy.
Active Versus Passive Thermal Management
Each working component will act as a heat source, and it is unavoidable that the temperature of your board will increase during operation. As a result, you need to have a strategy to remove heat from active components and spread it throughout your circuit board in order to keep your component temperatures within acceptable limits.
Heat Sink
A heat sink is one of the most common passive cooling methods for active devices like high-speed processors, MCUs, FGPAs, and other devices. These components generate significant heat, and the low thermal conductivity of most circuit board substrate materials will cause heat to accumulate in the substrate below these components. As the substrate expands, conductive traces and vias in the substrate can experience significant stress. If a board is repeatedly cycled between extreme temperatures, thinner traces can delaminate from the substrate, and the next in vias can crack under extreme stress. This is particularly common in printed boards that use small diameter unfilled vias with a high aspect ratio.
Any heat sink should be mounted to a component with a thermal pad or thermal paste. Both mounting techniques take advantage of the low thermal resistance of the heat sink and the pad/paste material. This aids heat conduction away from the component and into the heat sink, where heat can be dissipated into the surrounding air. Placing a heat sink on active components that generate significant heat is an important part of any passive cooling strategy, although this may create problems with keeping a small footprint.
Thermal paste for a CPU heat sink
Thermal Vias
Judicious design and placement of thermal vias will help keep junction temperature in active components low during operation. Thermal vias are placed below active components and can be soldered to the die-attached pad on the bottom of an integrated circuit. These vias will span through the circuit board substrate, and they can be filled with a low thermal resistance epoxy to conduct heat away from the component in question. You can ground your thermal vias to provide a direct connection to an interior copper plane, which provides a low thermal resistance path for heat dissipation to cooler areas of the board.