Overview of Passive and Active Heat Dissipation Techniques

Zachariah Peterson
|  Created: March 22, 2021
Overview of Passive and Active 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 dissipation techniques that help aid heat conduction away from hot areas of your printed circuit board.

While you can’t compensate for the low thermal conductivity of most PCB substrate materials without using an exotic material or metal core, you can use some thermal design techniques to help transport heat around the board and ultimately dissipate it into the surrounding environment. Your thermal management strategy may require any number of thermal dissipation methods and components, ranging from something as simple to a heat sink on each active component, to cooling fans and exotic substrate materials.

With the right circuit board substrate material, PCB layout tools, and component choices, you can devise a complete thermal management strategy that includes active and/or passive thermal dissipation techniques. The goal is to direct heat away from active components and toward portions of the board that can dissipate heat to the surrounding environment. The strategy you need for each circuit board will be different, but you can implement the strategy that will work best for your system with the right design tools.

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Temperature rise in your circuit board during operation is unavoidable, but you can help remove heat from critical components with the right heat dissipation techniques. As part of a comprehensive thermal management strategy, you’ll need to determine the best component layout that prevents the formation of hot spots and implement some measures that allow heat conduction towards cooler areas of the board or the surrounding air. These thermal management methods can be broken into passive and active methods. With the right combination of active and passive methods, you can help combat unnecessary temperature rise in your circuit board.

Active Versus Passive Thermal Management

Active and passive thermal 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. Transporting heat to a cooler region of the board requires careful stackup design and PCB material selection as heat conduction occurs through the substrate and conductors on a printed circuit board. The only passive thermal management technique that dissipates heat to the air is conduction from a heat sink or from the substrate itself directly into the surrounding air.

Active thermal management techniques provide more aggressive means of carrying heat away from a circuit board during operation. This requires using a system that pulls cool air or liquid past hot components, or takes advantage of a thermal phase change to remove heat from active components. You don’t need to choose between active and passive thermal design techniques; you can implement both in your printed circuit board if you need a more aggressive cooling strategy.

Importance of PCB Heat Dissipation

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.

Thermal design and simulation software can help you validate your thermal strategy during design time. This requires some knowledge of the thermal properties of your circuit board substrate material and your components. Power dissipation ratings for your components and thermal resistance measurements for your substrate material can be used to create a comprehensive thermal simulation, allowing you to visualize the steady state temperature distribution in your circuit board during operation.

PDNA simulation results in Altium Designer

Thermal design and power integrity simulation results in Altium Designer

Active and Passive Techniques for PCB Heat Dissipation

There are a number of common active and passive techniques you can use to dissipate heat from your components and from your circuit board substrate. From a signal integrity perspective, using passive components is the best strategy as these components do not require any moving parts or power source for thermal dissipation. Active cooling methods are more powerful, but you need to consider how the component design affects power and signal integrity during operation.

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 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 paddle 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.

Layer Stack Design

Your layer stack plays an important role in providing passive heat conduction away from active components. Copper planes in the internal layers of your layer stack will provide a low thermal resistance path for heat away from active components. When thermal vias are combined with the proper layer stack design, you can increase the effective thermal of your PCB substrate near components and help keep component temperatures from exceeding your design limits.

Thermal design of a layer stack

Thermal design for your layer stack in Altium Designer

Exotic Substrate Materials

Ceramic and metallic substrates have higher thermal conductivity (i.e., lower thermal resistance) than FR4 and other laminates. Once heat does leave an active component and conduct into your substrate, a substrate with higher thermal conductivity will allow heat to quickly move to cooler regions of the circuit board. This helps ensure that your PCB has a more uniform temperature distribution and helps to eliminate thermal hot spots beneath active components.

Using ceramics is desirable in high-temperature applications as a ceramic PCB material can have a thermal conductivity value that is a factor 20 to 100 larger than that of standard FR4 and similar laminates. These boards are also mechanically stronger than FR4, making them useful for high-power electromechanical applications in high-temperature environments.

Metal Core PCB

A metal core PCB is especially useful in applications that run at high power, such as LED arrays and power electronics. The metal core provides the same function as a substrate material with a high thermal conductivity; it provides a low thermal resistance path for heat to conduct to regions of the board with low temperature. The metal core provides much lower thermal resistance than interior copper layers because the metal core is generally thicker. One great example is an aluminum core PCB, which offers high thermal conductivity at low cost.

Aluminum PCB with LEDs

Aluminum circuit board as part of thermal design for a multi-board system

https://www.shutterstock.com/image-photo/high-power-warm-white-smd-led-425954665

Active Cooling Systems

The simplest active cooling system is an electric fan that is mounted at the edge of the device’s enclosure or directly on top of important components. High power components like FPGAs and microprocessors typically combine a heat sink with a cooling fan. More aggressive active cooling methods include liquid cooling, where a fluid flows past hot components and conducts heat to a radiator. Even more aggressive is an evaporative heat exchanger, which dissipates heat by inducing a phase change in a fluid within a closed system. The latter method is used in highly overclocked PCs and is popular among gamers.

Electric cooling fan and heat exchanger with an electronic fan

Heat exchanger with electric cooling fan and heat sink

https://www.shutterstock.com/image-photo/pc-computer-mother-main-board-heat-524342161

Best Software for Managing PCB Heat Dissipation

The best software for controlling heat conduction around and away from your circuit board will include a complete set of layout, component management, simulation, and production planning features in a single program. You should have access to the components you need for thermal design alongside other important design and analysis features. Add to this a comprehensive PCB material library and stackup designer, and you can quickly implement a thermal management strategy for your next circuit board.

Integrated Thermal Design in Altium Designer

Altium Designer’s CAD and component layout tools are unique in that they are integrated with other important design and layout tools in a single application. You’ll have access to a powerful set of simulation and analysis features, as well as a unified component library for accessing the components you need for thermal design. The layer stack manager and materials library gives you a complete set of tools for thermal design and allows you to implement any thermal management strategy you can imagine.

Altium Designer includes many other design features beyond stackup design and component layout. You’ll have access to production planning tools that provide real supply chain visibility without having to incorporate a 3rd party application. You can also generate standard deliverables for your manufacturer while preparing your product for production.

Altium Designer has set a new standard in integrated circuit board design and analysis, and you’ll have access to the resources you need to be successful. Every user can access the AltiumLive forum, an extensive knowledge base, webinars and podcasts with industry experts, and design tutorials with plenty of design tips. No other PCB design software company is this invested in your success. Altium Designer can help you take your productivity to a new level.

About Author

About Author

Zachariah Peterson has an extensive technical background in academia and industry. He currently provides research, design, and marketing services to companies in the electronics industry. Prior to working in the PCB industry, he taught at Portland State University and conducted research on random laser theory, materials, and stability. His background in scientific research spans topics in nanoparticle lasers, electronic and optoelectronic semiconductor devices, environmental sensors, and stochastics. His work has been published in over a dozen peer-reviewed journals and conference proceedings, and he has written 1000+ technical blogs on PCB design for a number of companies. He is a member of IEEE Photonics Society, IEEE Electronics Packaging Society, and the American Physical Society, and he currently serves on the INCITS Quantum Computing Technical Advisory Committee.

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