Split Post Cavity Resonance Corrections for Anisotropic Substrate Core Permittivity
Split post cavity resonance measures in-plane substrate permittivity; z-axis core corrections prevent multi-ohm stripline impedance errors on woven glass panels.
Microstrip Impedance Correction
Adjustment Calculation
Mathematical calculation modifies the physical dimensions of surface traces to compensate for real-world manufacturing deviations from nominal design values. This adjustment, known as microstrip impedance correction, is applied during the pre-production tooling stage to ensure the fabricated traces achieve the target impedance. It accounts for factors like the trapezoidal cross-section of etched copper lines and the effects of solder mask coverage.
Without these corrections, traces designed with ideal square dimensions would exhibit lower impedance than intended once manufactured. Front-end engineers use specialized field solvers to run these corrective models before photolithographic artwork is generated.
Geometrical Variable
Liquid solder mask application increases the localized capacitance of the microstrip, dropping its characteristic impedance by several ohms. Corrective algorithms reduce the target trace width slightly to offset this mask-induced impedance drop. The correction factor depends on whether the solder mask is applied using a dry film or a liquid photoimageable process, which yields different coat profiles.
Thickness variation of the underlying laminate also dictates the magnitude of this trace width adjustment.
Etching Profile
Chemical etching creates a trapezoidal trace shape rather than a perfect rectangle because the etchant removes more copper from the top of the line than the bottom. Software compensation increases the line width on the film artwork to ensure the average width of the finished trace is correct. This etching factor is determined by copper weight.
High copper weights require larger correction offsets.