LBIST Overview: Worked Example

Worked example

Worked Example 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.

A release review shows regression on LBIST signature quality, random-pattern-resistant fault count, runtime budget. The strongest first move is to freeze evidence, isolate one failing bucket, and prove mechanism before changing flow knobs.

Sequence under inspection

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

LBIST data path

PRPG -> logic under test -> MISR

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

1. Capture failing report and exact run tags.

2. Tag scenario (mode, lot/corner, pattern class).

3. Trace first dependency that changed.

4. Compare against LBIST architecture doc, MISR signature report, resistant fault list.

5. Choose one reversible fix and predefine regression checks.

Did the fix work?

BEFORE / AFTER - LBIST Overview

metric quality
  ^
  |                    --- release target
  |      o regressed
  |           o baseline
  |                o after fix
  +-------------------------------> closure iteration

Prove quality, timing, and test-power all moved safely.

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