Radio Frequency Interconnect Loss Driven by Substrate Dielectric Degradation Mechanisms
Substrate moisture absorption increases dielectric loss tangent, raising RF interconnect insertion loss and reducing long-term wireless link margins.
Effective Permittivity
Meaning
Electromagnetic propagation through a transmission line depends on the averaged dielectric constant of the inhomogeneous medium surrounding the conductor. Effective permittivity represents the value that a uniform dielectric would have to produce the same phase velocity as the actual mixed environment of air and substrate. In a microstrip, the field lines pass through both the board material and the air above it.
This value is always between the permittivity of the substrate and the permittivity of vacuum.
Field Distribution
Distribution of the electric field between the air and the dielectric determines the relative contribution of each medium. Narrow traces or thick substrates force more of the field into the air, reducing the effective permittivity of the structure. The ratio of the field in the dielectric to the total field is known as the filling factor.
Impedance Impact
Calculation of the characteristic impedance of a transmission line requires an accurate value for this property. High values for effective permittivity lead to lower phase velocities and shorter wavelengths for a given frequency. Tuning the line width allows designers to achieve the target impedance by compensating for the substrate properties.
Substrate Mix
Multilayer boards with different prepreg and core materials create a complex environment for signal travel. The effective permittivity varies depending on the specific layer stack and the proximity of ground planes. Manufacturers provide test coupons to verify these values after the lamination process.