The thermal conductivity of a PCB substrate is often considered an afterthought. This important physical quantity measures the rate at which heat flows away from a hot object and towards cooler regions of the material. Many mass-manufactured boards will be placed on FR4, which has very low conductivity compared to a number of alternative materials. As a result, many designers use a number of thermal management methods and systems to keep the temperature of their boards and components within a safe range.
Thankfully, there are many materials available for use as PCB substrates that provide a number of other benefits beyond a higher thermal conductivity. Your PCB substrate material will also affect the thermal stress induced on conductors at high temperatures and losses at high speed/high frequency. There are a number of design tradeoffs that should be considered when choosing a substrate that spans beyond thermal conductivity, and you might be better off using a substrate with low thermal conductivity alongside some passive or active cooling methods.
How Thermal Conductivity Affects PCB Temperature
Choosing a substrate with an appropriate thermal conductivity is one of many important aspects of PCB design. Thermal management in PCBs becomes particularly important in smaller boards with active components that switch at high speed. It is also important in boards that will carry high current as resistive losses in a trace will generate heat, which then transfers into the substrate.
If you know that your board will generate a large amount of heat during operation, or if your board will operate in a high-temperature environment, then you may need to use a substrate with higher thermal conductivity. You may also need to implement passive cooling, active cooling, or both in order to keep the temperature of important components within their safe operating range. It is also important
Weighing Thermal Conductivity and Other Material Properties
It is well known that PCB substrate materials with high thermal conductivity also tend to have high electrical conductivity. This does not necessarily mean that the real part of the dielectric constant for the substrate will also take on a larger value. Rather, it means that the conductive losses in a substrate material tend to be larger in substrates with higher thermal conductivity.
This means that losses along a transmission line in a PCB will increase due to heating at the surface of a trace and due to leakage between the transmission line and its reference conductor. However, there are some alternative high speed/high frequency PCB substrate materials you can use that provide similar or lower electrical conductivity with comparable thermal conductivity. Ceramics provide much higher thermal conductivity with somewhat lower conductive losses than FR4. While these substrates tend to cost more than FR4 substrates, your board will have a longer expected lifetime, which becomes an important selling point for your product.
The other thermal aspect of your PCB substrate to consider is its volumetric expansion. Thermal conductivity does not tend to scale with volumetric expansion coefficient in all materials, although using a substrate with a higher thermal conductivity is desirable as it moves heat away from hot components, which produces a more uniform temperature distribution throughout the board. This combats localized volumetric expansion and formation of hotspots near active components or traces that carry high current.
These traces may be in danger if they carry very high current
Volumetric expansion becomes particularly important in PCBs that will be repeatedly cycled to high or low temperatures. As a PCB reaches a high temperature, the substrate and the conductors expand, which places stress on conductors. If the temperature reaches a very high level or the conductors are very thin (such as in HDI PCBs), stress can cause traces to start delaminating from the substrate. Unfilled vias with large aspect ratio can also crack near the center of the via or at the edge of the copper foil.
This is where it becomes important to match the volumetric expansion coefficients of your substrate and conductors as close as possible. Ceramic boards offer higher thermal conductivity than FR4 while their volumetric expansion coefficients match closer to the values for common PCB conductor materials. This is why ceramics tend to enjoy applications in environments with high temperatures or where the temperature is repeatedly cycled. Once the substrate temperature passes the glass transition temperature, the board will start expanding at a faster rate. Some Rogers materials have high glass transition temperatures, offering stable volumetric expansion over a broad temperature range.
Thermal Management in PCBs for Different Applications
Using a substrate with higher thermal conductivity aids in passive cooling and thermal management, which helps keep board temperatures low in smartphones, automotive applications, industrial electronics, and other areas. This ensures heat can be spread more evenly throughout the board, which produces a more uniform temperature distribution. There are some other simple passive cooling approaches or active cooling components that can be used to combat temperature rise, such as the use of thermal pads, heatsinks on important components, cooling fans, placing multiple planes in interior layers, and other methods.
Ceramic PCB Substrate Materials
Compared FR4, PTFE, and polyimide, ceramics offer significantly higher thermal conductivity, albeit at the tradeoff of higher manufacturing costs. These mechanically tough substrates are difficult to drill, both with lasers and mechanically, which makes multilayer fabrication difficult. Ceramic PCB substrates can also be easily used with sintered gold or silver conductors as nanoparticles of these materials will sinter at the same temperatures used to fire ceramic PCB substrates.
Note that other materials properties, such as the real and imaginary parts of the dielectric constant, will affect how a ceramic board operates in different applications. This should be considered alongside the other material properties mentioned above when determining whether you need to use a ceramic PCB substrate. If your application requires moderate analog frequencies or high-speed signaling at high temperature, then a ceramic substrate might be the right choice for you.
If you’d like to learn more, read about selecting PCB substrate materials.
Thermal conductivity of some alternative PCB substrate materials
Thermal Pads vs. Thermal Paste for Heatsinks
Thermal pads and thermal paste are two options for attaching heatsinks to active components, which act as a large reservoir to capture heat and transfer it to the surrounding environment. A heatsink must be attached to a component mechanically, or with a thermal pad or thermal paste. Different pastes provide different levels of heat dissipation, although thermal paste will outlast any thermal pad in terms of wear and tear. A heatsink that is attached to an active component with these materials can also be combined with a fan, providing serious cooling directly to active components like CPUs, GPUs, FPGAs, and any other component that switches at high speed.
If you’d like to learn more, read about working with heatsinks.
Thermal paste with high thermal conductivity for mounting a heatsink to a CPU
Ground and Power Planes in Multilayer PCBs
Your power and ground planes will both have high thermal conductivity and can dissipate heat throughout a board. However, they will not help reduce temperature to the same extent as ceramic boards as the temperature gradient between the board’s surface and the external environment tends to be lower than with ceramics. Including extra power and ground planes can provide some extra thermal dissipation, increasing the equivalent thermal conductivity of your PCB.
If you’re looking to provide better board reliability and thermal management without spending the extra money on a ceramic substrate, you could also consider using a metal core PCB. The internal metal core provides a region of high thermal conductivity that allows heat to distribute evenly throughout the board. This is a nice compromise when one considers the natural shielding from the metal core, the higher heat dissipation, and lower cost compared to ceramics.
If you’d like to learn more, read about PCB stackup design.
Cooling Fans for Thermal Management
As cooling fans must be driven with an analog signal or pulse wave modulation, there is an important element of signal integrity that must be considered when incorporating a cooling fan into any electronic system. These fans are notorious for producing EMI, which can interfere with other components in extreme cases. Proper stackup design, grounding, and mixed signal layout techniques may be needed to suppress noise in other areas of the board due to EMI problems. In some cases, judicious use of filtering or simply using shielding cans can help suppress noise from a cooling fan.
If you’d like to learn more, read about active and passive cooling options.
Do you know which of these components will need a cooling fan?
Dynamic Power Regulation
In devices that are too small to accommodate a cooling fan or heatsink, using dynamic power regulation with active components can temporarily reduce the amount of heat generated in a board. This is a standard strategy used in mobile devices, which are too small to incorporate active cooling. This involves actively switching different functional blocks in a device on or off as they are needed. This also involves putting components like microcontrollers into sleep mode in order to conserve power and eliminate a source of heat in the system. When used alongside some unique substrate material or metal core PCB, you can compensate for the low thermal conductivity of standard PCB substrates.
If you’d like to learn more, read about dynamic power regulation in mobile devices.
Thermal Management Techniques: Design, Build, and Test
In the past, designers had less freedom to choose the thermal conductivity of their PCB substrate materials, but new design methodologies and material options will aid standard approaches to thermal management. Working with the right design and simulation software can help you layout your board to prevent hotspot formation and examine how heat transfers throughout the system. Don’t forget the importance of measurement; you should always test a prototype board at full power when considering any required thermal management techniques to keep your boards cool.
Don’t let your next product get this hot
Cadence’s full suite of PCB design and analysis tools are adaptable to any application. The back-end board layout and routing features include the tools you need to implement any thermal management strategy for your next PCB. With Allegro PCB editor, you’ll have access to a complete electronics design and analysis solution when you work with Cadence’s industry-standard suite of design tools.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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