Derate Policy & Approval: Silicon PPA Impact

Silicon PPA and closure impact

Derate policy is a business decision with technical evidence.

Area drivers

• Upsize cells

• Buffer insertion

• Shield wires

Power drivers

• VT swap leakage

• Clock tree power for skew

Timing impact

• Net delay on critical routes

• SI delta delay

PD consequences

• Congestion limits sizing

• Layer promotion for nets

Verification burden

• ECO equivalence

• SI re-signoff

PPA — Derate Policy & Approval
area/power/timing/closure-time tradeoff

PPA takeaways

• ECO has PPA cost

• Signoff is multi-corner

Design option PPA snapshot

BEFORE / AFTER ECO — Derate Policy & Approval

WNS (ps)
  0 |                    --- target
-50 |    ● regressed
-80 |         ● before ECO
-20 |              ● after ECO
    +-------------------------------> ECO iteration
Prove setup AND hold moved safely.

STA deep dive

Variation policy is negotiated margin, not a tool checkbox.

Concept diagram

VARIATION STACK

nominal delay + OCV/AOCV/POCV margin = signoff delay

Metric graph

DERATE IMPACT ON WNS

OCV only     WNS -120ps
AOCV         WNS -45ps
POCV         WNS -60ps (policy dependent)

Reports and artifacts

• derate table

• AOCV depth report

• POCV sigma

• hold/setup split

Mini case study

Switched to AOCV without re-baselining hold — short paths failed min analysis.

Debug branches

• Short paths for hold

• Policy approval for derate changes

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

Derating policy is a contractual margin between implementation and signoff; changes need documented approval and regression.

Metric: approved derate set, waiver count, signoff pessimism budget