Extended case study

Release review: LBIST signature quality, random-pattern-resistant fault count, runtime budget regresses after a test-flow update touching LBIST Overview.

Background

Team had prior signoff, then a new program/config introduced regressions in selected buckets.

Symptoms observed

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

• Mismatch between simulation and tester

• Escalation without clear owner

Investigation timeline

1. Hour 0: freeze pattern set, constraints, and tester program tags

2. Hour 1: isolate first failing bucket by mode/lot

3. Hour 2: verify legality and constraints assumptions

4. Hour 3: correlate with physical/timing/power context

5. Hour 4: choose minimal reversible fix

6. Hour 5: run full signoff regression matrix

7. Hour 6: publish decision memo and owners

Root cause

Root cause tied to LBIST Overview: LBIST uses on-chip pattern generation and signature compression for in-system logic test where external ATPG access is limited.

Fix and validation

• Apply bounded fix with owner

• Re-run LBIST architecture doc, MISR signature report, resistant fault list

• Re-validate quality, timing, and test power

Lessons learned

• Tag every run artifact

• Mechanism first, command second

• Close with explicit release decision

CASE STUDY - LBIST Overview
baseline metric / regressed metric / post-fix metric

Sequence under stress

DFT FLOW - LBIST Overview

scan insertion -> chain stitch -> compression map -> ATPG -> tester apply -> diagnosis
      |                |                |            |             |
 controllability   shift balance    channel use   coverage     silicon correlation

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

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