Impact of differential vias on high-speed connection design

Vias can introduce impedance discontinuity, slow down signal edge rate, add jitter and reduce eye openings, and cause data misinterpretation by the receiver in high-speed channel. This paper presents the design and optimization of differential vias for high-speed serial ATA connection at the data rate of 6 Gb/s. First, differential vias on a specific stackup were modeled in the 3D full-wave solver HFSS. Second, via mechanical design parameters, including via stub length, antipad radius, capture pad radius, barrel radius, and spacing, were swept to study their impacts on impedance discontinuity using TDR impedance determined by simulation. Third, importance of differential via optimization was highlighted by case studies of two PCB transmission line connections with nominal and optimized differential vias respectively.