Wave Propagation
Electrical resistance and reactive opposition oppose high-frequency alternating current signals travelling along conductor tracks over reference planes. High-speed board designs specify trace impedance to match driver output impedances with transmission line characteristics and receiver loads. Conductor width, copper thickness, dielectric height and substrate relative permittivity determine the characteristic impedance value of a signal channel.
Mismatches along signal paths cause energy reflections, signal distortion and electromagnetic interference in digital circuitry.
Geometry Control
Printed circuit board fabricators adjust line widths and dielectric spacing to achieve target ohmic values specified on design master prints. Variations in etched trace impedance stem from etching undercut, copper thickness variation and laminate thickness tolerances. Microstrip and stripline structures require tight etching controls to maintain signal integrity across long backplane routes.
Etch compensation factors applied during photo-tooling account for chemical lateral action during inner layer production.
Signal Verification
Coupon testing on panel borders verifies line geometry and dielectric performance prior to final board profiling. Time-domain reflectometry instruments measure trace impedance by launching fast voltage pulses into dedicated test coupons and analyzing reflected waveform amplitudes. Discontinuities in reflected signals indicate localized trace narrowing or dielectric thickness variation.
Acceptance of high-speed panels requires coupon test values to fall within specified percentage tolerances of nominal target values.
