In part three of the Ultra-Fine Line Design Guide series, previously introduced methodology is used to explore the design space for a differential pair when the return plane is far enough away that coupling to it is negligible. Read on.
With the recent introduction of the Averatek Semi-Additive Process (A-SAP) process, linewidths under 1 mil are possible using the same fabrication processing line as for traditional 4 mil wide lines. This opens up the possibility of using narrower traces in the BGA escape region than in long-path routing regions. However, using this routing architecture means the narrower traces in the BGA escape field are at a higher impedance than the wider, 50 ohm traces in the routing region. So, how long can these traces be before the impedance mismatch is a problem? The authors of this piece propose an analysis methodology to find out.