In-Situ Free-Space Dielectric Testing of Unclad Substrates
Non-destructive free-space characterization evaluates bulk dielectric properties of raw unclad laminates without etched coupon waste or copper interface errors.
Microwave Permittivity
Dielectric Characterization
Electromagnetic properties describing how a substrate interacts with high-frequency electric fields govern the propagation velocity and signal loss in transmission lines. The microwave permittivity of a circuit board material determines the characteristic impedance of printed conductors at gigahertz frequencies. This complex quantity consists of a real part representing energy storage and an imaginary part representing dielectric loss.
Accurate values are necessary to prevent timing delays and signal degradation in high-speed digital designs. Designing microstrip or stripline structures without verifying this parameter leads to severe impedance mismatches and failure of the high-speed interface.
Material Performance
Resin chemistry and reinforcement choices determine the effective dielectric properties of the final laminate. Because printed circuit boards use composite materials, the microwave permittivity varies with the glass-to-resin ratio across the board surface. Fluctuations in this property cause impedance mismatches that generate unwanted signal reflections.
Fab houses must control this composite mixture to ensure uniform signal propagation across all routing layers.
Measurement Method
Cavity perturbation techniques yield highly accurate measurements of dielectric behavior at specific test frequencies. This technique involves inserting a small sample of the laminate into a high-Q cavity and observing the frequency shift. The change in resonant frequency determines the microwave permittivity of the dielectric material.
This measurement prevents the assembly of high-speed boards using out-of-specification laminate materials.