Thickness and conductivity of metallic layers from eddy current measurements

TLDR

A robust eddy‑current method for simultaneously determining conductivity and thickness of uniform conductive layers is presented. The technique measures electrical impedance with air‑core and ferrite‑core coils over 1 kHz–1 MHz, then fits the data to the exact Dodd‑Deeds solution via least‑squares to extract layer thickness and conductivity, validated on aluminum and copper foils of 20–500 µm on multiple substrates. The method yields absolute, calibration‑free estimates with typically <10 % error, accurately retrieving both thickness and conductivity when the layer thickness to coil radius ratio is 0.20–0.50, and allowing one parameter to be inferred from the other for thinner samples. Reference: Dodd and Deeds, J.

Abstract

A robust method that uses eddy current measurements to determine the conductivity and thickness of uniform conductive layers is described. The method was tested by estimating the conductivity and thickness of aluminum and copper layers on various substrate metals, and the thickness and conductivity of free-standing foils of aluminum. The electrical impedance was measured for air-core and ferrite-core coils in the presence and absence of the layer for frequencies ranging from 1 kHz to 1 MHz. The thickness and conductivity of the metal layers were inferred by comparing the data taken with air-core coils to the exact theoretical solution of Dodd and Deeds [J. Phys. 39, 2829 (1968)] using a least-squares norm. The inferences were absolute in the sense that no calibration was used. We report experimental tests for eight different thicknesses of aluminum (20–500 μm) in free space and on four different substrates: Ti-6Al-4V, 304 stainless steel, copper, and 7075 aluminum, and for five different thicknesses of copper (100–500 μm) on 304 stainless steel. Both the thickness and conductivity could be determined accurately (typically within 10%) and simultaneously if the ratio of the layer thickness to the coil radius was between 0.20 and 0.50. For thinner samples either the thickness could be found if the conductivity were known, or vice versa.