Measurement and correlation-based methodology for estimating worst-case skew due to glass weave effect

Skew is unintentionally introduced within a differential pair, through misalignment of conductors and glass fiber bundles in Printed Circuit Board (PCB) dielectric layers. Manufacturers do not control interposition of specific glass bundles to supplied board design (artwork). Therefore, an unknown and random factor is added to each produced PCB. Current paper describes a method that utilizes a set of measurements and numerical models to estimate worst-case skew for the aforementioned effect. First, test vehicles are built, and then cross-sections are analyzed with Scanning Electron Microscope (SEM) for precise measurements. Numerical models are constructed to correlate with real DUT; after correlation is achieved, relative location of conductors to glass bundles is swept to obtain best and worst case skew.