Interview drills

Interview Drills for Chain Stitching Rules focuses on chain balance spread, max chain length, scan shift time. The goal is to convert metric movement into mechanism, owner, and release decision.

PROMPT
You observe chain balance spread, max chain length, scan shift time on Chain Stitching Rules. Walk through diagnosis and release decision.

STRONG ANSWER
1. States context and run tags.
2. Explains mechanism: Scan flops are stitched into chains based on clock, power, and physical constraints; poor stitching increases test time and routing pain.
3. Requests chain map, balance histogram, stitch rule deck.
4. Proposes minimal fix and regression.

WEAK ANSWER
Jumps to tool switches without evidence quality, ownership, or regression plan.

Diagram to draw on the whiteboard

Chain stitching constraints

SCAN CHAINS

clock_domain_A: FF1 -> FF2 -> FF3 -> SO
clock_domain_B: FF4 -> FF5 -> FF6 -> SO

Rules:
1) keep domains legal
2) balance chain length
3) avoid long physical detours

Root-cause narrative

ROOT-CAUSE TREE - Chain Stitching Rules

chain balance spread, max chain length, scan shift time regresses
        |
  setup changed?
    /        \
  yes         no
  |            |
constraint    silicon or
or ATPG       physical/test path
 /    \          |
SDC   model    chain/clock/power/diagnosis
diff  diff     isolate first failing signature

DFT deep dive

Scan architecture quality determines whether ATPG can control and observe real silicon state.

Concept diagram

SCAN INSERTION FLOW

scan-ready RTL -> scan replacement -> stitch chains -> DRC -> ATPG handoff

Metric graph

CHAIN BALANCE

chain length spread
low spread   = better shift time
high spread  = routing + hold risk

Reports and artifacts

• scan insertion summary

• chain balance report

• scan DRC log

• clocking legality report

Mini case study

Coverage plateau traced to non-scan flops in reset islands; RTL + DFT lint policy fixed root cause.

Debug branches

• Check scan replacement first

• Audit chain legality by domain

• Validate shift/capture clocks

Senior review question

Ask: what evidence proves this DFT decision is safe for production?

Key takeaways

• State metric, lot/corner context, and pattern tag with every claim.

• Treat timing, power, and quality as one signoff problem.

Common pitfalls

• Chasing coverage without legality checks.

• Ignoring test-power side effects of pattern changes.

• Debugging silicon without reproducible tags.

Principal DFT review addendum

Scan flops are stitched into chains based on clock, power, and physical constraints; poor stitching increases test time and routing pain.

Metric: chain balance spread, max chain length, scan shift time