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Smaller Circuits--Material Properties and Thermal Issues

Many PCB applications experience thermal cycling as part of their normal end-use operation. Some PCB assemblies used in outdoor enclosures may see changes from 75–160°F in a single day, and over the course of several months, the temperature swing can be much greater. If the circuit design is optimized at room temperature around 75°F, then at 160°F, the electrical performance can be very different depending on the design and the circuit material used.

Some RF PCB designs may be more sensitive to thermal changes than others. For example, designs with small circuit features can be more sensitive to performance change due to thermal issues as compared to circuits with larger features. Since high-frequency circuits have small wavelengths, the circuit features are also small. Also, with mmWave applications, a thinner circuit is needed in order to have proper wave propagation properties. Lastly, if a circuit has features which are tightly coupled, where a small space must be maintained between circuit features, that design can be more problematic for consistent electrical performance within a varying temperature environment.

Coefficient of thermal expansion (CTE) is typically considered for PCB reliability, but it can also have an impact on circuit performance for applications exposed to varying temperatures. Due to CTE, a circuit will change physical dimensions when the temperature changes. If the circuit has small features or tightly coupled features, the physical change of the circuit dimensions can cause a shift in electrical performance.

Another material property which further complicates designing for significant temperature variation is temperature coefficient of dielectric constant (TCDk). The TCDk is a property that all materials have and is a measure of how much the material will change dielectric constant (Dk) with a change in temperature. One example: If a circuit is using a material with a high TCDk, such as 400 ppm/°C, the dielectric constant of the material will change 0.020 with a 46°C change in temperature. This temperature change is equivalent to the temperature range previously mentioned from 75°F, then at 160°F. The Dk change will impact the impedance, phase response and other circuit attributes. This impact would be in addition to the impact of the physical dimensional change of small circuit features which would occur over the temperature range if the circuit was designed for mmWave applications.

There are also issues associated with circuit attenuation or RF losses, due to thermal changes. The analog of the TCDk to dissipation factor is the temperature coefficient of dissipation factor (TCDf). Again, every material has this property and it is a measure of how much the dissipation factor (Df) will change with a change in temperature. An RF circuit using a material with a high TCDf will have more RF losses with an increase in operating temperature.

To  read this article, which appeared in the August 2016 issue of The PCB Design Magazine, click here.