Extracting In-Plane Dielectric Permittivity Values from Clamped Stripline Measurements
Clamped stripline measurements extract true in-plane dielectric permittivity when analytical models eliminate air gap capacitance errors.
Substrate Thickness Tolerance
Thickness Deviation
Permissible dimensional variations define the upper and lower thickness limits of fabricated printed circuit board substrates. Quality standards specify substrate thickness tolerance to ensure mechanical fit inside card enclosures and maintain target electrical properties. Fabrication parameters such as core thickness, copper weight, resin content, and prepreg compression dictate overall thickness variability.
Manufacturing Variation
Substrate thickness results from stacking dielectric cores, prepreg sheets, and outer copper foils prior to thermal lamination. Lamination press pressure, resin flow velocity, and mold plate uniformity influence final dielectric thickness across single panels. Dielectric height directly determines characteristic impedance in microstrip and stripline transmission lines, where thinner dielectrics reduce impedance values.
Variations in substrate thickness tolerance across a manufacturing lot alter signal propagation delay and create impedance mismatches along high-speed signal channels. Standard laminate specifications define tolerance classes ranging from loose commercial grades to tight precision classes for high-frequency applications. Fabricators utilize continuous optical and ultrasonic gauging systems during panel processing to track dielectric height variations across production runs.
Impedance Impact
Controlled impedance board designs require tight substrate thickness tolerance limits of plus or minus five percent. Exceeding substrate thickness tolerance limits alters transmission line impedance beyond acceptable design boundaries, causing signal reflection and data corruption in high-speed digital buses.