Why are notches and narrow channels risky?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
They concentrate routes and pins, fragment rows, complicate PG continuity, and can create dead space that neither cells nor wires use well.
Technical Explanation
A narrow passage can be open yet useless if every entry is blocked. Inspect concave corners, parallel macro edges, facing pins, row remnants, layer direction, and required via access.
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.
Command Checks & Actions
• check_floorplan_rules: reports configured floorplan-rule violations
• create_channel_congestion_map -boundary <coordinates>: estimates congestion in channel regions
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.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. • Suspicious: A notch survives legality checks but produces a persistent local hot spot. • Hard stop: required legality or physical feasibility is missing or unexplained. Move or align macros, widen the corridor, redirect pins, or reserve it explicitly, then rerun.
Common Mistake
Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change. A notch survives legality checks but produces a persistent local hot spot.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “Why are notches and narrow channels risky?” to measurable physical evidence and an owned correction.
Follow-up Question & Model Response
Follow-up probe: What would make you change your decision? 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. I would use the named objects and reports to decide whether to iterate, waive, or return the issue.”
Practical Example
Symptom: A notch survives legality checks but produces a persistent local hot spot. Corrective path: Move or align macros, widen the corridor, redirect pins, or reserve it explicitly, then rerun.
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