STA Mastery · All levels

Derate Policy & Approval: Interview Drills

Interview Drills for Derate Policy & Approval.

Interview drills

Interview Drills for Derate Policy & Approval focuses on approved derate set, waiver count, signoff pessimism budget. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

diagram

PROMPT
You see approved derate set, waiver count, signoff pessimism budget on Derate Policy & Approval. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains Derating policy is a contractual margin between implementation and signoff; changes need documented approval and regression.
3. Requests derate approval memo, signoff policy PDF, before/after WNS comparison.
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

Derate approval flow

diagram

DERATE CHANGE REQUEST
  -> STA lead review
  -> PD director sign
  -> foundry policy check
  -> re-run full MMMC matrix
  -> archive before/after WNS

Root-cause tree to narrate

diagram

ROOT-CAUSE TREE — Derate Policy & Approval

approved derate set, waiver count, signoff pessimism budget 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

Variation policy is negotiated margin, not a tool checkbox.

Concept diagram

diagram

VARIATION STACK

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

Metric graph

diagram

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

Practice this lesson