Definition

A via stub is the unused portion of a through-hole via extending beyond the signal layers it connects. For example, if a via connects layer 1 to layer 4 of an 8-layer board, the portion from layer 4 to layer 8 is a stub. Stubs act as unterminated transmission line branches, causing resonances at frequencies where stub length equals odd multiples of quarter-wavelength. At multi-GHz frequencies, stub effects degrade signal integrity. Solutions include back-drilling (removing stubs), HDI with blind vias, or designing signal layers near the outer layers.