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Signoff Regression Hygiene: Interview Drills

Interview Drills for Signoff Regression Hygiene.

Interview drills

Interview Drills for Signoff Regression Hygiene focuses on run-to-run WNS drift, database tag consistency, closure memo completeness. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

diagram

PROMPT
You see run-to-run WNS drift, database tag consistency, closure memo completeness on Signoff Regression Hygiene. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains Signoff quality depends on reproducible runs: same lib, SPEF, SDC, scenarios, and tool versions — tagged and archived.
3. Requests run manifest, git/DB tag log, signoff checklist.
4. Proposes one reduced sequence and one system regression.

WEAK ANSWER
Jumps to upsizing cells, widening routes, or blaming tools without reading constraints and path delay.

Diagram to draw on the whiteboard

Run manifest

diagram

STA RUN MANIFEST
design: chip_top tag v2.4.1
sdc:    constraints_v18.git
spef:   cworst_v3
libs:   ss_125 / ff_m40 / ...
tool:   PrimeTime Q-2020.03
scenarios: 36 (full matrix)

Root-cause tree to narrate

diagram

ROOT-CAUSE TREE — Signoff Regression Hygiene

run-to-run WNS drift, database tag consistency, closure memo completeness looks wrong
        |
   same netlist tag?
     /        \
   no          yes
   |            |
 view/SDC     real path or
 mismatch     constraint bug
 /    \           |
lib   SPEF    delay breakdown
diff  diff    cell vs net

STA deep dive

Debug hygiene separates senior STA leads from report readers.

Concept diagram

diagram

DEBUG ORDER

1. same tags?  2. constraints?  3. view?  4. real path?  5. ECO

Metric graph

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ROOT CAUSE MIX (typical)

constraint bugs  ████████  35%
view mismatch    ██████    25%
real path        ██████    25%
methodology      ████      15%

Reports and artifacts

run manifest
SDC diff
SPEF tag
scenario isolation

Mini case study

Mystery regression: SPEF corner tag typo in run script — not a design bug.

Debug branches

Manifest every run
War room by path group

Senior review question

Ask: what corner/mode/view proves this topic is closed or failing?

Key takeaways

State corner, mode, view, and database tag with every slack claim.
Run setup and hold plus MMMC regression after every ECO.

Common pitfalls

Comparing STA runs with different SPEF or SDC tags.
Broad false_path to green-wash violations.
Setup-only ECO without hold check.

Principal STA review addendum

Signoff quality depends on reproducible runs: same lib, SPEF, SDC, scenarios, and tool versions — tagged and archived.

Metric: run-to-run WNS drift, database tag consistency, closure memo completeness

Practice this lesson