A 60 ps useful-skew tweak fixed setup but created 140 new hold violations. What went wrong?
CLOSURE CHAIN
1. METRIC — name the failing report line (WNS, DRV, DRC count, IR %)
2. HYPOTHESIS — 2–3 likely causes ordered by probability
3. EXPERIMENT — one cheap check (corner, clock, report, map)
4. FIX — minimal physical or constraint change
5. REGRESSION — what you re-run and what must not regress1. Confirm stage, corner/mode, and database tag
2. Open the primary report and capture worst metric
3. Correlate metric with layout region or netlist hierarchy
4. Apply smallest fix with documented hypothesis
5. Re-run only required regressionsSeparate symptom from root cause
Fix systematic clusters before one-offs
Random optimization without metric
Skipping regression after local fix
For Useful Skew in Timing Closure, senior debug is branch-and-bound: reduce the search space quickly, keep experiments reversible, and avoid hiding a systematic issue behind one local fix.
Reproduce the failure on the tagged database and exact analysis setup.
Classify the failure as data issue, constraint issue, physical implementation issue, tool/methodology issue, or true design limitation.
Run one cheap experiment that can falsify the leading hypothesis.
Prefer a fix that improves a cluster over one that only hides the worst line.
After the fix, re-check clock path latency/skew delta report with hold impact and the likely regression surface: timing, routing, power, PV, DFT, package, or tapeout signoff.
The failure crosses team ownership boundaries: RTL, synthesis, CAD, IP, package, or foundry.
The local fix consumes margin that another signoff domain needs.
The issue repeats across blocks, suggesting methodology or library root cause.
The remaining risk is silicon-facing: late-stage schedule slip, silicon risk, or cross-stage regression.
Timing closure is a signoff matrix problem, not a single worst-path problem.
report_timing -max and -min with corner, mode, SI, and OCV enabled
path group WNS/TNS: which clock domain is actually blocking signoff
net delay percentage vs cell delay percentage
exception audit: false paths, multicycle paths, generated clocks
A -90 ps setup path has 75% net delay and crosses a macro channel. Sizing the launch flop is weak. The stronger response is to layer-promote or shorten the route, then re-run hold at the fast corner.
If net delay dominates, look for physical fixes before cell sizing.
If cell delay dominates, consider VT swap, sizing, or RTL micro-architecture.
If only one mode fails, inspect mode-specific SDC overlays and case analysis.
Ask yourself: what single report line would prove this page's concept is either passing or failing?
You are expected to predict what your fix can break before running it.
You should recognize when the issue is methodology, not one block's implementation.
You should communicate risk in tapeout language: owner, evidence, impact, mitigation, and decision date.
Deep subpage pages in this course should be read like real closure review material. For a 10+ year PD engineer, the bar is not remembering terminology; it is making a release-quality decision under ambiguity.
Names the failing metric, corner/mode, database tag, and analysis switches before proposing a fix.
Separates data, constraint, physical, tool, and methodology root causes instead of treating all failures as optimization problems.
Chooses experiments by information gain and reversibility, not by habit.
States regression blast radius across timing, route, power, PV, DFT, package, and tapeout manifest.
Turns recurring failures into methodology guardrails, dashboards, or checklist items.
STAFF / PRINCIPAL CLOSURE NOTE
Context:
stage: <pre-CTS | post-CTS | post-route | post-fill | signoff>
tag: <database / netlist / SDC / library stack>
failing metric: <exact report line>
affected scope: <block / hierarchy / path group / power domain / region>
Hypotheses:
H1: <most likely physical or constraint mechanism>
H2: <competing explanation>
H3: <methodology or input-data issue>
Decision:
next experiment: <cheap check that can falsify H1>
fix candidate: <minimal reversible change>
rollback trigger: <metric that says the fix is wrong>
regression set: <timing / route / power / PV / DFT / package>
escalation owner: <team or reviewer>Timing closure is a signoff matrix problem, not a single worst-path problem. Explain not only the preferred fix, but what margin or schedule you are spending to get it.
What must RTL, synthesis, CAD, STA, DFT, package, IP, or foundry agree to before this decision is final?
Which artifact becomes the source of truth after the decision: report, waiver, manifest, ECO script, or methodology deck?
What is the cost of being wrong: one rerun, ECO churn, mask risk, performance loss, or silicon escape?
"The current blocker is <metric> in <corner/mode/stage>. The leading cause is <mechanism>. I recommend <fix> because it is bounded and reversible. The regression surface is <domains>. If it fails, we escalate to <owner> with <evidence>."Always connect the concept back to a measurable signoff artifact.
A fix is not complete until you can name the regression checks.
Optimizing by habit instead of reading the current report.
Forgetting that a local fix can regress timing, routing, power, or PV elsewhere.