Extracting Out-Of-Plane Dielectric Tensor in Heterogeneous Packaging Core
Extracting out-of-plane permittivity requires guarded parallel-plate or re-entrant cavity tests to isolate vertical flux fields from in-plane glass weave effects.
Re-Entrant Cavity
Dielectric Testing
A specialized resonant structure featuring a central coaxial post with a small capacitive gap measures the dielectric properties of small material specimens at microwave frequencies. Inside a re-entrant cavity, the electromagnetic field is highly concentrated in the narrow gap between the top of the central post and the cavity ceiling, where the material specimen is placed. This concentrated electric field makes the resonant frequency and quality factor of the cavity highly sensitive to the permittivity and dissipation factor of the inserted dielectric specimen.
It is used to measure the dielectric properties of substrate materials with high accuracy.
Measurement Procedure
Executing a measurement involves first determining the resonant frequency and quality factor of the empty cavity, and then repeating the measurement with the material specimen inserted into the capacitive gap. The presence of the dielectric material shifts the resonant frequency downward and decreases the quality factor due to dielectric losses within the specimen. Mathematical formulas translate these measured shifts into the real and imaginary parts of the specimen’s complex permittivity, which gives the dielectric constant and loss tangent.
This method is highly valued because it accommodates small, thick specimens that would be difficult to measure in standard transmission line fixtures, and it minimizes the measurement error caused by conductor losses on the cavity walls.
Material Validation
Fabricators use this resonant technique to verify the consistency of high-frequency material properties across different raw material batches before beginning the circuit board layup process. By measuring small samples cut from the edges of laminate sheets, they can identify deviations in resin content or curing state that would affect the final impedance of the circuit. This pre-production validation reduces the risk of expensive scrap or rework in high-performance rf applications.