Issue link: https://resources.pcb.cadence.com/i/1541046
29. Eliminating Stubs and Dead-End Tees How To Implement 1. Avoid branching ("T" junctions) in schematic and layout: f Route signals point-to-point, not point-to-multiple points. For buses, use daisy-chain or fly-by topologies according to interface requirements. f Place signal terminations (e.g., series resistors) at or near the driver or load, not in the middle of a trace. 2. Place test points at endpoints, not mid-trace: f If test points or probe pads are required, connect them at the signal source or destination, not as branches from the main route. f For high-speed lines, use dedicated probe fixtures or high-impedance microstrip pads designed to minimize stub effect. 3. Control via stubs: f Minimize unused via barrel length, deep vias left after layer transitions form hidden stubs. f Use backdrilling (removing unused via length) on very fast lines (PCIe, SerDes, high-speed clocks) or prefer blind/ buried/microvias where supported. f For via-in-pad structures, ensure vias are filled and capped to prevent voids and stub formation. 4. Remove leftover copper or routing artifacts: f After all routing and changes, inspect for unused trace segments, unused pads, or unconnected stubs left from edits. f Use your EDA tool's DRC, "clean up", or "dangling traces" check before final signoff. 5. Check manufacturer guidance and simulate: f Review interface specifications (e.g., JEDEC DDR, PCIe) for maximum allowable stub lengths, typically less than one-tenth the signal's rise time travel length (for one ns rise time in FR-4, keep stubs <15 mm; shorter is better). f Simulate with signal integrity tools to identify any resonances or reflection artifacts due to stubs. Common Pitfalls, Their Impact, and How to Avoid Them Common Pitfall How to Avoid it Leaving probe/test pads as mid-trace branches Creates significant stubs and degrades fast edges – Place test pads at source or destination. Ignoring via stubs in high-speed/ fine-pitch designs Leads to difficult-to-diagnose eye closure and bit errors – Use back-drilling or blind vias to eliminate or minimize stubs. Accidental "dead copper" from edits or rework Unused trace segments left after moving or deleting parts – Run a DRC to check for unconnected copper. Not checking for stubs after auto-routing Automated tools often leave small stubs behind unless specifically instructed – Use back-drilling to remove stubs if necessary.