Non-destructive PCB Substrate Height Extraction with Multi-Measurement Technique

Abstract

This paper introduces a non-destructive measurement technique that extracts the as-fabricated substrate height of printed circuit boards. The as-fabricated substrate height is a crucial parameter for dielectric constant extraction and impedance prediction of any fabricated printed circuit board. Multi-measurement and line fitting techniques were introduced to convert measured S-parameters of shorted transmission lines to frequency-dependent per-unit-length inductance. A computation routine used analytical PUL inductance equations to generate frequency-dependent PUL inductance curve with given substrate height. By changing the input substrate height and minimizing the difference between the calculated and measured PUL inductance results, the as-fabricated height was extracted. The obtained height value with associated uncertainty was shown to be consistent with substrate height measured with direct cross-section inspection.