Learn how to select appropriate board materials for high-heat scenarios.
Methods for component layout to maximize heat transfer away from your board.
Tips and tricks for optimizing your PCB’s routing and trace specifications for efficient thermal performance.
As circuits become faster and smaller, thermal issues are more prevalent than ever, making it important to design your board to deal with these high-heat scenarios
Recently, electronics have been getting faster, smaller, and are being used in more demanding environments. ICs and SMDs are smaller than ever, operate at faster frequencies, and require more power. As we demand more current from our devices, the voltage drop across resistive elements converts power into heat, causing temperature increases and hotspots. Over half of all electronic components now fail due to some stress stemming from heat. In order to develop high-quality circuit boards, it is now more important than ever to consider the safety and temperature of your system and design your PCB for efficient thermal performance. In this article, we will present some key strategies and solutions for PCB design for thermal performance improvement.
“Thermal conductivity” refers to the ability of a material to transfer heat. In the case of PCBs, this refers to the passive (or active) ability to move generated heat from the components to outside the system. If you are able to plan ahead and anticipate sources of high thermals before the production of your circuit board, you’ll be able to produce significantly more reliable devices. Especially with small or high-current devices, heat spots on your PCB can easily form. Knowing how to design for thermal performance is key in making the next generation of devices.
Examples of specific industries that have harsh operating environments and are especially susceptible to high thermal environments are the gas and oil, avionics, and automotive industries. Whether your board will be used in industrial equipment, power electronics, or embedded systems, designing circuit boards with high thermal performance requires special considerations. Specifically, we’ll look into methods for board manufacturing, layout, routing, and thermal analyses to keep your board performing cool.
PCB Materials and Board Operating Temperature
Allegro’s board stackup manager showing a four layer board. Changing the dielectric material can result in better thermal performance.
Before laying out your components, it’s essential to consider your board construction and material, as it plays a large role in distributing heat.
If your board is operating in a particularly hot environment, it is important to know about the material’s operating temperature, and specifically the TG rating (glass transition temperature). FR-4, the most common PCB material, is rated to operate up to 130 degrees before the glass begins to transform to a liquid state. If you plan on operating at higher temperatures, consider using a different material such as Shengyi S1000-2, ARLON 85N, or ITEQ IT-180A.
In cases where your board produces an especially large amount of heat, using an alternative substrate with higher thermal conductivity is your best bet. Ceramics, for example, have higher thermal conductivity and their mechanical properties can be tuned, which can accommodate stress that is introduced in thermal cycling.
Table outlining thermal conductivity for PCB materials including FR4, Alumina, Aluminum Nitride, and Beryllium Oxide
Compared to FR-4, the thermal expansion coefficient of ceramic is closer to that of silicon chips, thus not requiring an interface material. Other hybrid materials like PTFE and non-PTFE thermoset resin systems with ceramic fillers may offer lower dielectric losses as well.
The effective thermal conductivities of a PCB depend on the total thickness and the individual thicknesses of copper and glass-epoxy. Using a metal core for your FR4 or other board can help transport heat quicker throughout due to the high conductivity of metal. Aluminum or galvanized copper are commonly used–aluminum is a bit cheaper, but copper is more thermally efficient. To accelerate heat dissipation even more, consider using protective coatings or other encapsulating materials.
PCB Design for Thermal Performance
Knowing how to select specific components that are thermally efficient is crucial to keeping your board operating in high-heat conditions
Laying out your components while keeping thermal considerations in mind is incredibly important for the longevity and functionality of your board. Some key PCB design considerations for improved thermal performance include:
Components that are especially sensitive to temperature should be placed in the location with the lowest temperature, such as the bottom of the board. Make sure to keep them away from components with high heat productivity (e.g., frequency transistors, hyper-scale ICs, microprocessors). These heat-producing components, on the other hand, should be placed in locations with the best heat dissipation (wide copper traces and polygon fills, which we’ll delve into in the upcoming section). Place passive components that generate less heat and are more temperature-sensitive near the edges, with high-powered components in the center.
High-power components, such as high-powered resistors and voltage regulators, will create a lot of heat, and bunching them up can create hotspots. Distribute your high-power components throughout the board to avoid creating specific hotspots.
Over half of the heat generated is brought to the board through component leads. Consider using metal-plating holes and keeping leads short. Additionally, when selecting components with high power consumption, choose components with a larger cross-sectional area and high pin counts for better heat transfer. Using screws to mount the PCB can also allow for thermal access to the base of the system, distributing the heat more effectively.
When operating at high current, components like resistors, transforms, switching FETs, driver ICs, and control ICs create local temperature increases. Many of these components may not be designed to function in extreme heat and may break down at operating temperatures. Make sure to verify temperature ratings on your components when selecting them to prevent your board from overheating and failing.
Keep in mind that as components become smaller they are able to dissipate less heat. A MOSFET in an SOT-23 package, for example, has less surface area for heat dissipation compared to a TO-220 package MOSFET.
Routing and Trace Specifications for Thermal Performance
Your PCB traces won’t just serve as conducting elements for electrical current, they will also assist in transferring heat away from your hottest components.
Trace Length and Width
For power traces with high current, keep them as short as possible and away from other sensitive circuitry. Reducing the length that the current has to flow minimizes the trace inductance and reduces the noise that can be generated. Consider routing the corners at 45 degrees or even rounding them as opposed to using right angles. At the same time, increase your trace widths, as narrow traces can result in degraded performance. Wide traces reduce both inductance and resistance, thus reducing heat generation. Note that changing trace geometry can affect the trace impedance seen by AC signals. This can result in needing to change the stackup to keep impedances matched to the predefined values necessary for your source and load components.
Cadence’s IR Drop Vision tool can help create power and ground planes
Ground and Power Plane Construction
The construction of the ground and power plane is crucial. Large copper planes in a circuit board result in more surface area for heat dissipation, as they act as heat sinks. Therefore, making them as large as possible will help keep your board cooler. Connecting large planes to external layers of the board will create an even greater chance of transferring heat to the environment.
Thermal vias are additional holes placed in the board to intentionally dissipate heat. They can be especially useful for multi-layer boards with internal copper layers, providing a path for heat to flow out from the center of the board.
Thermal Standards for PCB Design
For cases with high AC and DC, other aspects of your PCB layout that need to be considered to produce thermally reliable boards are outlined in IPC, UL, and IEC standards for consumer and commercial products. For example, IPC compliance requires designing for current density in traces and spacing between conductors. Consult these standards to prevent excessive temperature rise in traces and components.
Thermal Analysis With PCB Design and Analysis Tools
Thermal analysis is a crucial method of evaluating the effects of thermals on your board. Based on the results of thermal analysis, thermal problems can be found quickly and taken care of, eliminating heat-dense areas, optimizing location for key components, and increasing heat transmission efficiency.
Before committing to manufacturing, there is a wide range of tools available for managing the thermal aspects of your design. Using a powerful finite element multiphysics simulator can help analyze how airflow and heat move throughout your board given your layout. A PDN analyzer can be used to detect hot spots in conductors that cannot always be identified from schematics. The design and analysis tools from Cadence provide everything you need to design circuit boards with efficient thermal properties.
For more information on PCB design for thermal performance, check out the E-Book for an introduction to electrical thermal co-design.
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