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.
Spectral Reflectometry
Thickness Measurement
An optical testing method determines the thickness of thin dielectric films and metallic coatings on circuit boards by analyzing the interference patterns of reflected light across a range of wavelengths. Spectral reflectometry measures the intensity of light reflected from the top surface of the layer and compares it to the light reflected from the bottom interface of that same layer. By measuring these interference fringes, the system can calculate the thickness of the film with nanometer-scale accuracy without making physical contact with the board.
It is widely used to monitor the thickness of photoresists, solder masks and ultra-thin conformal coatings during board fabrication.
System Operation
The measurement equipment directs a broadband light beam from a halogen or tungsten source through a focusing lens onto the surface of the specimen, where the different layers reflect the light back to a spectrophotometer. This device measures the intensity of each wavelength and generates a reflection spectrum that contains sinusoidal oscillations caused by constructive and destructive interference between the reflected beams. Analyzing the frequency and amplitude of these oscillations allows the measurement software to calculate both the film thickness and its refractive index simultaneously.
This process is very fast, taking only a fraction of a second per point, which allows high-speed automated mapping of layer thickness across an entire circuit board panel.
Application Boundaries
Measuring very thick or rough layers becomes difficult with this optical method because high surface roughness scatters the light, which reduces the contrast of the interference pattern and degrades measurement accuracy. For these thicker layers, such as standard prepreg sheets or thick solder mask coatings, mechanical or cross-sectional methods are often used instead. Despite this limitation, the non-contact nature of this optical technique makes it invaluable for measuring delicate, newly deposited polymer films before they are fully cured.