Signoff Regression Hygiene: Theory Deep Dive

Foundational theory

Signoff Regression Hygiene is central to STA Debug Playbook. Signoff quality depends on reproducible runs: same lib, SPEF, SDC, scenarios, and tool versions — tagged and archived. Senior STA engineers always tie slack movement to corner, mode, constraint set, and parasitic view — not just a single report line.

Core concepts explained

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

• Primary metric: run-to-run WNS drift, database tag consistency, closure memo completeness

• Primary artifact: run manifest, git/DB tag log, signoff checklist

• Owners: STA lead, CAD owner, program management

• Setup: max delay path; Hold: min delay path

• WNS/TNS summarize tail risk across endpoints

Why this matters at signoff

At signoff, Signoff Regression Hygiene failures block tapeout or force risky ECO. Most STA emergencies are constraint, view, or process hygiene issues. Wrong analysis setup wastes weeks of PD effort.

Mental model

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)

Worked intuition

1. Name corner, mode, and view for the failing run.

2. Open run-to-run WNS drift, database tag consistency, closure memo completeness and identify worst path group.

3. Read critical path: cell vs net, launch/capture clocks.

4. Check constraints on that path (exceptions, generated clocks).

5. Collect run manifest, git/DB tag log, signoff checklist and tag database versions.

6. Classify: constraint bug, view mismatch, or real path.

7. Propose minimal ECO and list MMMC+SI regression.

Common misconceptions

• One typical corner is enough for signoff.

• False paths can be applied broadly to green-wash violations.

• Implementation WNS equals signoff WNS without view mapping.

• Setup fix automatically preserves hold.

Visual reinforcement

Run manifest

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)

Layer responsibilities

STA OWNERSHIP LAYERS — Signoff Regression Hygiene

layer              owns                         failure mode
----------------   --------------------------   ---------------------
constraints        clocks, IO, exceptions       false violations
implementation     cells, nets, placement       real path delay
extraction         SPEF/RC corners              view mismatch
signoff policy     derate, MMMC matrix          margin disputes
closure           ECO order, regression        fix breaks other corner

STA deep dive

Debug hygiene separates senior STA leads from report readers.

Concept diagram

DEBUG ORDER

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

Metric graph

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.

Theory reinforcement

Most STA emergencies are constraint, view, or process hygiene issues.