Definition

Broadside-coupled differential pairs stack one trace directly above its pair mate on adjacent signal layers, rather than placing them side-by-side. This enables high-density routing through BGA fanouts and single-track-width routing where horizontal space is limited. However, this apparently simple construction is actually one of the most difficult to fabricate with consistent impedance results. Layer-to-layer registration tolerances and etch differences between the two signal layers both affect coupling symmetry. Broadside coupling is used in USB Type-C designs where pairs must cross over and in tight BGA breakout routing.