Congestion becomes severe only after PG-resource reservation. What does that prove?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
It proves your earlier "signal-only" congestion estimate was optimistic, not that power planning is somehow the enemy — power/ground routing consumes real tracks too, and any congestion estimate that ignores PG reservation is only looking at part of the picture. Once PG straps, rings, and rails claim their share of routing resources on certain layers, the remaining capacity for signal routing shrinks — sometimes dramatically in specific directions or on specific layers — and that's exactly when previously-invisible congestion hotspots surface.
Technical Explanation
- Don't jump to "we need a bigger die" as the first move — first go rebalance what you can: check whether PG straps can shift layers or spacing, whether signal routing can be redirected to underused layers/directions, or whether the floorplan geometry itself (macro placement, channel widths) is forcing PG and signal routing into the same tight corridor.
- Compare demand vs. capacity with the PG reservation actually present — layer by layer, direction by direction — around macro pin escapes, channel widths, and via corridors, since that's where PG and signal routing usually compete hardest.
- Also sanity-check the PG plan itself before blaming the floorplan: is the PG strap width/pitch actually approved and necessary at that density, or was it set conservatively "to be safe" and could be relaxed slightly without violating IR-drop/EM requirements?
- The real lesson: power and signal feasibility have to be co-planned, not sequenced — treating PG as an afterthought after signal routing looks clean is how teams get blindsided late in the flow.
Formula Or Decision Rule
Decision rule: no floorplan passes on a model that omits required power resources.
What To Check
- Warning sign: The team reports the earlier prettier map.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Keep the correct reservation, fix the shared-resource conflict, and archive both maps as evidence.
Command Checks & Actions
check_design-checks {dp_pre_power_insertion}: runs named design-planning check groupsreport_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: It proves the earlier signal-only model was incomplete, not that power planning should be ignored. Rebalance shared routing resources and floorplan geometry.
- Suspicious: The team reports the earlier prettier map.
- Hard stop: required legality or physical feasibility is missing or unexplained.
Common Mistake
The Trap: Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change. The team reports the earlier prettier map.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “Congestion becomes severe only after PG-resource reservation. ” 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 team reports the earlier prettier map. ”
Practical Example
Tapeout Scenario: Symptom: The team reports the earlier prettier map.
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