Material Characterization
A non-destructive measurement technique utilizes free-space electromagnetic waves in the millimeter-wave spectrum to determine the dielectric properties of unclad laminate materials without physical contact. Quasi-optical interferometry directs a focused beam of radiation through a flat material specimen and analyzes the resulting phase shift and transmission loss of the transmitted wave. This non-contact approach eliminates the need for pattern fabrication or copper etching, making it a highly efficient method for characterising material batches before they enter the assembly queue.
It is especially suited for high-frequency measurements above sixty gigahertz where traditional resonators become too small to handle.
Measurement Method
The experimental configuration consists of a millimeter-wave source, a series of focusing lenses and mirrors to shape the beam, and a sensitive receiver that measures the intensity and phase of the transmitted signal. When the dielectric specimen is placed in the path of the focused beam, the phase of the electromagnetic wave shifts in proportion to the material’s dielectric constant, while the amplitude of the wave decreases according to the material’s loss tangent. Analyzing these changes across a sweep of frequencies allows the system to extract the complex permittivity of the material with high precision.
This free-space measurement avoids the errors associated with conductor losses and electrode contact resistance that complicate contact-based measurement techniques.
Quality Verification
Using this non-contact technique allows laminators to perform continuous quality checks on raw material rolls during the manufacturing process. Because the measurement does not damage the laminate, every sheet can be verified for dielectric consistency before being shipped to board fabricators. This high-volume testing ensures that the material properties remain within tight specified tolerances across different production runs.
