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74 The PCB Design Magazine • August 2016 Many PCB applications experience thermal cycling as part of their normal end-use opera- tion. 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 differ - ent depending on the design and the circuit material used. Some RF PCB designs may be more sensi- tive to thermal changes than others. For ex- ample, 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 tempera- tures. 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 di- mensions can cause a shift in electrical perfor- mance. by John Coonrod ROGERS CORPORATION Smaller Circuits: Material Properties and Thermal Issues LIGHTNING SPEED LAMINATES

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