One macro has a huge halo; another has pin-access failure from a small halo. How do you size them?

Short Answer

A single uniform halo number applied to every macro is lazy and wrong — macros aren't symmetric, and a halo is a personal-space bubble that should only be as big as each side actually needs. Look at each macro edge independently: where are the pins concentrated, how many routing tracks/vias does that side need, are optimization cells (buffers, tap cells) expected to land there, and is there PG-shape traffic hugging that edge?

Technical Explanation

• A side with dense pins and heavy routing needs real margin — via ladders, redundant vias, and buffer insertion all consume space right at the macro boundary. A quiet side with no pins and no nearby congestion wastes area if it gets the same halo.
• For the oversized-halo macro: check whether one or two sides actually need the large margin and shrink the rest — create_keepout_margin -outer {left bottom right top} takes independent values per side, so there's no reason to keep it uniform.
• 5) which scales with actual pin density instead of a flat guess.
• Also separate "placement keepout" (where standard cells can't sit) from "routing blockage" (where wires can't run) — sometimes the fix isn't a bigger halo at all, it's converting part of it from a hard placement blockage to a routing-only constraint.

Formula Or Decision Rule

Decision rule: each side’s margin protects a named resource without creating unusable area.

What To Check

• Warning sign: The same halo is copied to every macro.
• Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
• Correct: Resize by side and type, then compare access, rows, and congestion.

Healthy, Suspicious & Hard-stop Results

• Healthy: Use asymmetric, evidence-based margins where supported: reserve space where pins, vias, routing, or optimisation cells need it, and avoid wasting quiet sides.
• Suspicious: The same halo is copied to every macro.
• Hard stop: required legality or physical feasibility is missing or unexplained.

What The Interviewer Is Testing

The interviewer is testing whether you can connect “One macro has a huge halo; another has pin-access failure from a small halo. ” 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 same halo is copied to every macro. ”