LBIST Overview: Interview Drills

Interview drills

Interview Drills for LBIST Overview focuses on LBIST signature quality, random-pattern-resistant fault count, runtime budget. The goal is to convert metric movement into mechanism, owner, and release decision.

PROMPT
You observe LBIST signature quality, random-pattern-resistant fault count, runtime budget on LBIST Overview. Walk through diagnosis and release decision.

STRONG ANSWER
1. States context and run tags.
2. Explains mechanism: LBIST uses on-chip pattern generation and signature compression for in-system logic test where external ATPG access is limited.
3. Requests LBIST architecture doc, MISR signature report, resistant fault list.
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

LBIST data path

PRPG -> logic under test -> MISR

pros: in-system test, no heavy tester channels
risk: random-pattern-resistant faults

Root-cause narrative

ROOT-CAUSE TREE - LBIST Overview

LBIST signature quality, random-pattern-resistant fault count, runtime budget 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

BIST value is realized only when insertion, diagnosis, and repair are tied to production flow.

Concept diagram

BIST FLOW

insert MBIST/LBIST -> execute -> collect signatures -> diagnose/repair -> signoff

Metric graph

REPAIR EFFECT

yield
  ^
  |      o pre-repair
  |             o post-repair
  +---------------------> lot

Reports and artifacts

• MBIST insertion coverage

• repair signature report

• LBIST resistant fault list

• BIST release checklist

Mini case study

Fuse programming mismatch blocked repair activation; corrected bring-up script recovered expected yield uplift.

Debug branches

• Validate BIST reachability

• Correlate fail maps to repair signatures

• Audit in-field boot test budget

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

LBIST uses on-chip pattern generation and signature compression for in-system logic test where external ATPG access is limited.

Metric: LBIST signature quality, random-pattern-resistant fault count, runtime budget