Parallel Plate Capacitance Testing for Packaging Core Dielectrics
Parallel plate capacitance testing extracts packaging core permittivity from 100 Hz to 1 GHz by applying guarded electrodes to etched, preconditioned coupons.
Maxwell-Wagner Polarization
Dielectric Behavior
Interfacial phenomena occurring at the boundaries of different dielectric materials describe the accumulation of charges under the influence of an external electric field. Known as Maxwell-Wagner polarization, this effect occurs in heterogeneous materials where the constituent layers have different electrical conductivities and permittivities. Printed circuit board laminates, which are composed of glass fibers and epoxy resin, are susceptible to this polarization.
Charge Accumulation
When an electric field is applied to a heterogeneous substrate, charges migrate through the more conductive phase and accumulate at the boundary of the less conductive phase. In board laminates, Maxwell-Wagner polarization causes a build-up of charge at the interface between the glass fibers and the epoxy resin. This accumulation of charge alters the local electric field and increases the effective dielectric constant of the material.
The magnitude of this effect depends on the conductivity of the resin and the surface treatment applied to the glass fibers. Proper coupling agents on the glass fabric can reduce this charge accumulation and stabilize the electrical properties of the substrate.
Frequency Response
The contribution of this boundary effect to the overall dielectric properties of the board decreases as the operating frequency increases. Because Maxwell-Wagner polarization relies on the physical movement and accumulation of charge carriers, the process requires a finite amount of time to occur. At high gigahertz frequencies, the alternating electric field reverses too quickly for the charges to accumulate at the glass-resin boundaries.
This means that the dielectric loss associated with this polarization is primarily a concern for low-frequency and analog applications.