Spectroscopic Method
Coherent electromagnetic pulse generation and sampling techniques enable contactless characterization of material complex dielectric properties across sub-millimeter wave and terahertz frequency bands. THz-TDS, or terahertz time-domain spectroscopy, measures the electric field vector of picosecond electromagnetic pulses after transmission through or reflection from a printed circuit board substrate. Ultrafast mode-locked lasers generate sub-picosecond terahertz pulses using photoconductive antennas or non-linear optical crystals.
By recording both amplitude attenuation and phase shift in the time domain, the system extracts real permittivity and dielectric loss tangent across broad gigahertz to terahertz bandwidths in a single measurement scan.
Permittivity Characterization
High-speed circuit designers require broadband material data extending into sub-millimeter wave regimes to model losses in next-generation radar and packaging substrates. THz-TDS provides non-destructive material evaluation without requiring copper cladding or electrical contact pads on the test sample. Fourier transformation of time-resolved electric field pulses converts raw time-domain data into frequency-dependent refractive index and absorption coefficient spectra.
Laminate manufacturers utilize this spectroscopic method to evaluate new low-loss polymer blends, ceramic filler distributions and glass fabric weaves prior to high-volume PCB manufacturing release.
Time Domain Resolution
High time resolution allows THz-TDS to resolve internal layer boundaries and detect sub-surface micro-voids inside multilayer panels. Time-gating signals eliminates unwanted internal reflections from sample surfaces during dielectric property extractions.
