Why are notches and narrow channels risky?

Short Answer

A narrow passage between macros can be geometrically "open" and still be effectively useless if every entry into it is blocked — an open channel with no legal access is not usable routing/placement space. These regions concentrate routes and pins into a small area, which fragments standard-cell rows, complicates power/ground continuity, and often leaves dead space that neither cells nor wires end up using well.

Technical Explanation

• The places to specifically inspect are concave corners, two macro edges facing each other, pins that face into the channel from both sides, leftover row remnants, the preferred routing-layer direction through the channel, and whether vias actually have room to land there.
• Think of it like a hallway that's wide enough on a floor plan but has support pillars right at both doorways — the raw width number doesn't tell you it's usable.
• Treat notch/channel review as a dedicated floorplan check, not something that "will probably route fine" — this is one of the more common late-stage congestion surprises.

Formula Or Decision Rule

Decision rule: remove or justify every notch and prove capacity with the post-reservation routing model.

What To Check

• Warning sign: A notch survives legality checks but produces a persistent local hot spot.
• Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
• Correct: Move or align macros, widen the corridor, redirect pins, or reserve it explicitly, then rerun.

Healthy, Suspicious & Hard-stop Results

• Healthy: They concentrate routes and pins, fragment rows, complicate PG continuity, and can create dead space that neither cells nor wires use well.
• Hard stop: required legality or physical feasibility is missing or unexplained.

What The Interviewer Is Testing

” to measurable physical evidence and an owned correction.

Follow-up Question & Model Response

"* Candidate Model Response: Model response: “I would change it if the same controlled rerun shows that a notch survives legality checks but produces a persistent local hot spot. ”