Scenario — Timing Closure Storm (60 min)

Timed scenario (60 minutes)

Post-route: WC setup -120 ps on CPU paths; BC hold -40 ps after CTS; scan mode worse.

Timer breakdown

1. 0–10 min: Confirm SDC tag, corners, modes — dashboard slide

2. 10–25 min: Setup — top 3 paths, net vs cell, fix order

3. 25–40 min: Hold — post-CTS short paths, skew vs delay

4. 40–50 min: Scan mode overlay — separate clock constraints

5. 50–60 min: ECO vs RTL pipeline — regression matrix signoff

RUBRIC (score 1–5 each)

  Metric clarity — names correct report and corner
  Hypothesis quality — ordered, plausible causes
  Fix discipline — minimal change, not random tweaks
  Regression mindset — states what to re-run

STRONG HIRE: METRIC → hypothesis → experiment without prompting
NO HIRE: tweaks buffers without naming WNS/TNS corner

Interviewer follow-up traps

• Fix setup at WC only — hold at BC?

• Useful skew without hold check?

• False path to green WNS?

• SI off to pass?

• Sign off scan mode later?

Strong hire signals

• States METRIC and corner before any fix

• Orders hypotheses — does not random tweak

• Names regression (corners/modes/tools) after fix

• Asks clarifying questions on spec early

No hire signals

• Silent tool clicking narrative

• Fix timing before route/congestion clean

• Waive DRC/LVS to meet date unprompted

• Cannot explain one report line

Related topics

• Related lesson

Key takeaways

• Complete scenario in 60 min wall clock.

• Self-score rubric honestly — redo weakest segment tomorrow.

Scenario debrief guide

After the timer ends, do a second pass without changing your answer. Mark where you named the metric, where you formed hypotheses, and where you stated regression checks.

Debrief prompts

1. What was the first metric you named?

2. Which hypothesis was cheapest to test?

3. Which fix carried the largest regression risk?

4. What would you escalate to a lead or architecture owner?

Common recovery line

"Before I change the design, I want to confirm the failing metric, stage, corner/mode, and whether this is a hard gate or tracked risk."

Principal-level review bar

Scenario 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.

What excellent looks like

• 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.

Closure note template

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>

Tradeoffs a senior engineer must discuss

Technical tradeoff

Interview performance is structured closure thinking under time pressure. Explain not only the preferred fix, but what margin or schedule you are spending to get it.

Cross-team tradeoff

• 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?

Leadership communication

"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>."

Key takeaways

• Always connect the concept back to a measurable signoff artifact.

• A fix is not complete until you can name the regression checks.

Common pitfalls

• Optimizing by habit instead of reading the current report.

• Forgetting that a local fix can regress timing, routing, power, or PV elsewhere.