SI Signoff Reports: Theory Deep Dive

Foundational theory

SI Signoff Reports is central to SI Timing & Noise. SI signoff aggregates delta delay and noise against foundry rules and product margin; reports must map to fix ownership. Senior STA engineers always tie slack movement to corner, mode, constraint set, and parasitic view — not just a single report line.

Core concepts explained

• SI signoff aggregates delta delay and noise against foundry rules and product margin; reports must map to fix ownership.

• Primary metric: SI-clean path %, remaining aggressor violations, signoff waiver count

• Primary artifact: SI signoff summary, foundry SI checklist, waiver log

• Owners: SI lead, STA lead, signoff owner

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

• WNS/TNS summarize tail risk across endpoints

Why this matters at signoff

At signoff, SI Signoff Reports failures block tapeout or force risky ECO. Coupling makes delay context-dependent — aggressors matter. Wrong analysis setup wastes weeks of PD effort.

Mental model

SI SIGNOFF DASHBOARD

delta delay violations : 12  (owner: PD)
noise violations       : 3   (owner: SI)
waivers pending        : 1   (owner: STA lead)

Worked intuition

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

2. Open SI-clean path %, remaining aggressor violations, signoff waiver count 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 SI signoff summary, foundry SI checklist, waiver log 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

SI signoff summary

SI SIGNOFF DASHBOARD

delta delay violations : 12  (owner: PD)
noise violations       : 3   (owner: SI)
waivers pending        : 1   (owner: STA lead)

Layer responsibilities

STA OWNERSHIP LAYERS — SI Signoff Reports

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

Coupling makes delay a function of neighbors, not just wire RC.

Concept diagram

SI ON VICTIM

victim net delay = base_RC + coupling_from_aggressors

Metric graph

DELTA DELAY ON CRITICAL PATHS

no SI   ████████  1.2ns
with SI ██████████  1.35ns  (+150ps)

Reports and artifacts

• delta delay report

• aggressor list

• noise peaks

• SI WNS delta

Mini case study

Setup fixed by upsize but SI worsened — spacing ECO on bus bits recovered margin.

Debug branches

• Re-run SI after every timing ECO

• Shield wide switching buses

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

Coupling makes delay context-dependent — aggressors matter.