All floorplan legality checks pass, but congestion is unacceptable. What do you do?

Short Answer

Legality and quality are two separate questions, and passing one tells you nothing about the other — a legal street map can still route you into an impossible traffic jam at one intersection. Keep the legal baseline as-is — don't start ripping up the whole floorplan just because congestion is bad in one area; isolate the problem first.

Technical Explanation

• Identify the specific hot spot and separate the two things that drive congestion there: demand (how many nets need to cross) and capacity (how many tracks are actually available).
• Work through the standard hypothesis list for that hot spot: macro pin walls forcing detours, narrow routing channels between macros, high local cell density, over-generous PG reservation eating routing resource, a blockage that's wider than necessary, bus concentration (many parallel wide buses through one corridor), or simply not enough routing layers available in that region.
• Use the congestion map plus named nets/cells at the hot spot to discriminate between these hypotheses — don't guess; trace which specific nets are contributing to the overflow and why they're routed through that exact spot.
• Change the smallest responsible physical choice — reorient one macro, widen one channel, shrink one blockage — and re-run the same congestion estimate on the same basis to confirm the fix actually worked, rather than assuming it did.

Formula Or Decision Rule

Decision rule: iterate until local capacity is credible; a legality pass is not a waiver for QoR risk.

What To Check

• Warning sign: The review stops because check_floorplan_rules is clean.
• Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
• Correct: Inspect hot spots and layers, assign the owning macro/pin/PG decision, repair, and compare before/after.

Healthy, Suspicious & Hard-stop Results

• Healthy: Treat legality and quality separately. Keep the legal baseline, identify the hot spot’s demand and capacity causes, change the smallest responsible physical choice, and rerun the same estimate.
• Hard stop: required legality or physical feasibility is missing or unexplained.

What The Interviewer Is Testing

The interviewer is testing whether you can connect “All floorplan legality checks pass, but congestion is unacceptable. ” 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 the review stops because check_floorplan_rules is clean. ”