Materials for High Temperature PCBs

PCBs operating at high power also tend to operate at high temperature, sometimes cycling between high and low temperatures repeatedly. Sometimes, a PCB will operate in a high ambient temperature environment, and the board needs to remove heat from components to prevent even higher temperatures from being reached in the system. Active cooling measures are important for aiding heat flow, but not all products can accommodate active cooling. Instead, designs can rely on passive cooling that relies on choosing the right PCB materials.

PCB materials selection for thermal designs involves almost always involves three main parameters:

• Coefficient of thermal expansion (CTE)
• Thermal conductivity
• Glass transition temperature

There are additional thermomechanical characteristics that matter for a good PCB material at high temperature, but for the most part these three characteristics will be most important. Here’s how these characteristics need to align with the thermal operating characteristics in high temperature PCBs.

Materials as Heat Sink Elements

The PCB material used to build a high temperature PCB can act as a heat sink and will be an important determinant of reliability in the final build.The laminate material allows the PCB to act like a heat spreader so that heat can be moved to an area where it can be dissipated away from the assembly. In order to use a PCB in this way, the right materials need to be selected in accordance with the system’s engineering requirements.

Here is a table outlining common materials that can be used as laminate materials or core materials for high-temperature boards.

The materials listed here are all standard and available from many manufacturers. Among the list above, the ceramics list contains the most specialized materials and not all manufacturers will offer builds on these materials (AlN and Al2O3 are most common).

Which Material Properties Matter Most?

The material properties that matter most depend on how the board will operate. For example, what is the expected peak temperature of the board, will this temperature be changing repeatedly, and what other cooling strategies are being used to help keep the temperature low? These factors should determine how the PCB materials are chosen. There are three common situations.

1. High Power, No Temperature Cycling

When components are running continuously at high power, and there are gentle changes between high and low temperature without repeated cycling, then the role of the PCB laminate materials should be as a heat spreader.The PCB laminate should spread Heat out to help bring down the temperature of the highest power components. Higher power dissipation could demand higher thermal conductivity in the substrate, such as ceramics or a metal core.

• Typically use metal core
• Higher power demands higher thermal conductivity

2. High Temperature Cycling

In these systems, it could be argued that the thermal conductivity is less important than the CTE value for the laminate materials. Repeated cycling of the temperature between high and low values can create stress on copper features when there is a CTE mismatch. If the CTE mismatch is kept low, then there will be less stress on copper features and lower chance of fatigue failure. When fatigue failure occurs in repeatedly temperature-cycled PCBs, it is most likely to occur around vias.

• Minimize CTE mismatch to copper
• Use engineered PTFE or specialized FR4

3. High Ambient Temperature

In these systems, the temperature could be high due to ambient temperatures being high, but that might not mean that components are running at high power and there may not be repeated cycling. In this case, PCB materials should also act as a heat spreader but high thermal conductivity insulators (like ceramics) may not be needed. Standard FR4 with heavy copper on relevant layers can be acceptable as the copper will help move heat. These systems probably also need an active cooling system, but this could be targeted to certain components.

• Heavy copper acceptable
• Standard FR4 acceptable

In all instances above, select the Tg value so that it exceeds the expected operating temperature. This is a basic requirement for any PCB expected to operate in a high temperature environment, or which will reach high temperature due to power requirements in components.

High temperature operating environments demand unique decisions, and you can implement the right strategy with the best PCB design features OrCAD from Cadence. If you’re ready to take even more control over net logic and board layout, you can graduate to Allegro PCB Designer for a more advanced toolset and additional simulation options for systems analysis. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.

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