SI ECO Fix Order: Interview Drills

Interview drills

Interview Drills for SI ECO Fix Order focuses on post-ECO SI WNS recovery, reroute count, spacing violations cleared. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

PROMPT
You see post-ECO SI WNS recovery, reroute count, spacing violations cleared on SI ECO Fix Order. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains SI fixes use spacing, shielding, sizing, and reroute; order matters because a setup fix can worsen hold or noise elsewhere.
3. Requests ECO change list, before/after SI report, regression matrix.
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

SI fix priority

SI ECO ORDER
1. spacing / shield on worst aggressors
2. victim upsize (if delta delay)
3. reroute victim to quiet layer
4. re-run SI + setup + hold matrix

Root-cause tree to narrate

ROOT-CAUSE TREE — SI ECO Fix Order

post-ECO SI WNS recovery, reroute count, spacing violations cleared 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

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.

Principal STA review addendum

SI fixes use spacing, shielding, sizing, and reroute; order matters because a setup fix can worsen hold or noise elsewhere.

Metric: post-ECO SI WNS recovery, reroute count, spacing violations cleared