Comprehensive design guidance for PTH via stub in board-level high speed differential interconnects

A plated-through-hole (PTH) via is commonly used to route vertical signal transitions between layers of a multi-layered PCB. It is well known that a via stub, the portion of the PTH via not directly involved in the layer transition, can have a significant impact on signal integrity, effectively acting as a bandpass filter, degrading the design margin or even potentially causing failures. This paper discusses via stub design issues and provides comprehensive design guidance based on both frequency and time domain results. The design guidance results were generated using a commercially available 3D FEM simulation tool and results. Data showing good correlation between simulation and measurement are provided to lend credence to the simulated results. A simple rule of thumb for estimating an operating frequency range safe from via stub effects is presented.