High Power PCBs and Thermal Conductivity: Keeping Your Boards on Ice
Anyone that has listened to the fan in their laptop spin up to a blazingly high speed knows that thermal management is extremely important in electronics. Thermal management is about more than just keeping components within a safe operating temperature. The circuit board itself can also be damaged by repeated thermal cycling and hot spots in many devices.
With these issues in mind, what are some ways that designers can improve their boards for effective thermal management? At some point, passive and active cooling techniques will be unable to compensate temperature rise without further scaling. Thankfully, there are some less-than-common board designs you can use for better thermal management.
Alternative Materials for High Power PCBs
FR4 is by far the most common substrate material for single and multilayer PCBs. Like other common electrical insulators, it has low thermal conductivity compared to other materials that are suitable for use as PCB substrates. Heat that accumulates in high speed devices, power electronics, and RF boards can be considerable. In addition, the harsh environments in which these systems are deployed can exacerbate thermal demands. Using an alternative substrate material with higher thermal conductivity may be a better choice than using FR4.
Ceramic materials offer some considerable advantages for thermal management in high power PCBs. In addition to the higher thermal conductivity of these materials, their mechanical properties materials can be tuned, helping compensate accumulated stress during repeated thermal cycling. The thermal expansion coefficient of ceramics for use in PCBs is closer to that of silicon chips compared to FR4, thus an interface material is not required.
A comparison of the thermal conductivity of some alternative substrate materials
With high power PCBs that operate at high frequencies, ceramics are an excellent choice as they have lower dielectric losses at frequencies ranging up to 10’s of GHz. At higher frequencies, some hybrid materials will offer similarly low losses with a modest reduction in thermal conductivity. One example is PTFE and non-PTFE thermoset resin systems with ceramic fillers.
Thermal Management with Metal-core and Metal-clad PCBs
Vias, pads, and interior power/ground planes in your PCB layer stack help move heat away from active devices in FR4, helping to prevent formation of major hot spots. If you aren’t working at very high frequencies, such as in power electronics applications, you will likely want to stick with FR4 due to the larger number of manufacturers that specialize in this material and competitive costs.
Using a metal core for your FR4 board helps quickly transport heat throughout the board thanks to the high thermal conductivity of metals. Some manufacturers also offer metal clad PCBs, where both surface layers are encased in metal. Aluminum and galvanized copper are two common metals used in these boards. From a cost per unit weight perspective, aluminum is the better choice, while copper offers higher thermal conductivity.
Using silver for traces, vias, pads, and the metal core also offers about 5% higher thermal conductivity than copper. If you are worried about thermal management, stay away from aluminum and gold as their thermal conductivity is about 40% and 15% lower than that of copper, respectively. If your board will be deployed in a humid environment with noxious gases (e.g., sulfur oxides or nitrogen oxides), using a silver plating finish on exposed copper traces and pads will help prevent corrosion in these environments.
Despite the lower thermal conductivity of aluminum, one common solution for thermal management is to use an aluminum base for your high power PCB. As an example, this is a common choice for boards supporting high power LED lighting. The reflectivity of aluminum across the visible range is particularly useful in LED lighting applications as it reflects light away from the substrate.
Infrared camera image showing hotspots in a server motherboard on FR4
PCB Layout Guidelines for Thermal Management
In addition to using the right substrate for high power PCBs, judiciously placing active components around the board can help prevent hot spots and ensure the temperature distribution throughout your board is more uniform. Including some basic passive or active cooling strategies can also help keep temperatures down.
Active components should be spread around your PCB in order to prevent hot spots from forming in a single area. Avoid placing active components right at the edge of your board as heat will have to conduct back towards the center of the board. Instead, active components that generate significant heat should be placed closer to the center of the board, while components that generate less heat can be placed closer to the edge.
Complementing your stackup with active or passive cooling (e.g., fans, evaporative cooling, or heatsinks) will help remove heat from the active components themselves rather than dissipating heat directly into the board. The right combination of thermal management strategies will depend on your application, packaging, budget, and manufacturer capabilities.
Combining a fan and heatsink helps remove heat from active components
When you need to design a board for a high power PCB with high thermal conductivity, you need the right PCB layout and design software with a full suite of design tools. Allegro PCB Designer and Cadence’s analysis tools can help you design the right stackup for your system and simulate thermal demands in your device.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.