DenseChip

Physical Design · All levels

PD Closure Cheat Sheet

One-page reference: closure chain, fix order, signoff gates, report checklist, escalation.

Closure chain (say first in any interview)

diagram
METRIC → HYPOTHESIS → EXPERIMENT → FIX → REGRESSION

  METRIC:     WNS/TNS, corner, mode, path pin names
  HYPOTHESIS: ordered causes (no random tweaks)
  EXPERIMENT: one report/map before changing layout
  FIX:        minimal physical/constraint/RTL change
  REGRESSION: corners, modes, DRV, LVS, IR re-run

Fix order — setup (post-route)

diagram
1. Verify SDC tag + corner + mode + SI on
2. Path report: net delay vs cell delay
3. RTL pipeline (if allowed)
4. Layer up / spread placement / CTS latency
5. Size / VT swap / buffer / useful skew (hold risk)
6. ECO metal/cell — document tag

Fix order — hold (often post-CTS)

diagram
1. Hold corner report (often BC fast)
2. Short data paths — insert delay/buffer
3. Skew adjust if budget (check setup regression)
4. Hold-specific opt pass — cluster by clock domain

Congestion fix order

diagram
1. Map hotspot ↔ macro channel / pin escape
2. Macro rotate / widen channel / soft blockage
3. Spread placement density
4. Layer promotion / routing preference
5. Drop util 3–5% or die bump

Signoff hard gates

  • LVS clean (connectivity)

  • DRC clean or approved waivers only

  • STA matrix: setup @ WC + hold @ BC (all modes)

  • IR/EM @ agreed vectors

  • Revision tag matches GDS + SDC + netlist

Reports to name in interviews

diagram
Timing:  report_timing -max/-min, path slack, clock latency
Route:   congestion map, DRV summary
CTS:     skew/latency histogram
Power:   IR map, EM ratio report
PV:      DRC count by rule, LVS compare
Signoff: closure dashboard one slide

Escalation — when to stop tweaking

  • LVS fail → stop tapeout timeline until clean

  • Wrong revision tag → stop all exports

  • IR > spec with timing margin thin → escalate to architect + power lead

  • Systematic DRC >100 → cluster fix meeting, not hero edits

Related topics

Key takeaways

  • Name corner before fix — every time.

  • Congestion before timing opt on unroutable nets.

How to use this sheet in interviews

Do not recite this sheet linearly. Use it as a mental index. When the interviewer gives a failure, jump to the corresponding fix order and narrate the evidence you need before making a change.

Self-check

  • Timing failure: corner, mode, WNS/TNS, path report.

  • Physical failure: DRC/LVS rule, cluster, owner, waiver policy.

  • Power failure: IR/EM metric, vector, hotspot, grid fix.

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