Average utilisation is good, but one region is locally impossible. How do you prove it?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Measure usable local area after rows, halos, blockages, voltage areas, and fixed objects, then compare with the cell and routing demand assigned there.
Technical Explanation
Global whitespace cannot help logic trapped behind a macro wall. Competing causes include hierarchy assignment, pin affinity, macro placement, fragmented rows, or an over-restrictive blockage. Examine one region and one demand source.
Formula Or Decision Rule
Local utilisation = counted local cell area / usable local placeable area.
What To Check
• Warning sign: The team lowers global target utilisation without opening the trapped region. • Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause. • Correct: Change the local geometry or assignment, then recompute and rerun congestion.
Command Checks & Actions
• report_design -floorplan: reports the floorplan state understood by the tool
• report_congestion -mode hot_spot: reports routing demand, capacity, overflow, or hot spots
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.Healthy, Suspicious & Hard-stop Results
• Healthy: Measure usable local area after rows, halos, blockages, voltage areas, and fixed objects, then compare with the cell and routing demand assigned there. • Suspicious: The team lowers global target utilisation without opening the trapped region. • Hard stop: required legality or physical feasibility is missing or unexplained. Change the local geometry or assignment, then recompute and rerun congestion.
Common Mistake
Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change. The team lowers global target utilisation without opening the trapped region.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “Average utilisation is good, but one region is locally impossible. How do you prove it?” 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 the team lowers global target utilisation without opening the trapped region. I would use the named objects and reports to decide whether to iterate, waive, or return the issue.”
Practical Example
Symptom: The team lowers global target utilisation without opening the trapped region. Corrective path: Change the local geometry or assignment, then recompute and rerun congestion.
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