Understanding the variables of dielectric constant for PCB materials used at microwave frequencies

PCB (Printed Circuit Board) materials are used extensively in the microwave industry. Understanding the details of electrical properties for these materials can improve the designer´s models and circuit performance. The ε_r (relative permittivity or dielectric constant) for PCB laminates is often assumed to be a fixed value; however there are times this can be wrong and be a costly assumption. An ε_r value of a laminate can vary with thickness or with the type of copper used. Some laminates can have issues with a higher or lower dielectric dispersion, which can cause the value to vary with frequency. Many designers are aware of the potential for PCB laminates to have anisotropic ε_r values, which is often assumed to be attributed to the glass fiber reinforcement layer/s, however some PCB materials with no glass reinforcement are highly anisotropic. The details of the variables associated with ε_r in many microwave PCB materials will be discussed and its supporting data will be given. The most common test methods which are used to characterize ε_r of these materials will be examined along with their capabilities and limits. Finally guidance will be given for improved use of ε_r values regarding common microwave PCB materials, which will enable the designer to have increased accuracy in models and circuit performance.