Fault Models: Expanded Case Study

Extended case study

Release review: stuck-at / transition / bridge coverage, untestable fault ratio regresses after a test-flow update touching Fault Models.

Background

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

Symptoms observed

• stuck-at / transition / bridge coverage, untestable fault ratio 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 Fault Models: Fault models define what manufacturing defects ATPG targets; model mix drives pattern count, runtime, and outgoing quality.

Fix and validation

• Apply bounded fix with owner

• Re-run fault coverage report, fault class breakdown, untestable 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 - Fault Models
baseline metric / regressed metric / post-fix metric

Sequence under stress

DFT FLOW - Fault Models

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

Primary metric: stuck-at / transition / bridge coverage, untestable fault ratio

DFT deep dive

ATPG quality comes from fault model choice plus legal constraints, not raw pattern volume alone.

Concept diagram

ATPG FLOW

fault model -> constraints -> generation -> simulation -> coverage closure -> signoff

Metric graph

COVERAGE GAP

target coverage
  ^
  |      o before closure
  |          o after fixes
  +---------------------> iteration

Reports and artifacts

• fault model coverage

• untestable class report

• constraint legality errors

• pattern signoff memo

Mini case study

Transition coverage stalled due to clock constraints mismatch; updated at-speed capture definitions recovered target.

Debug branches

• Classify untestable faults

• Diff ATPG constraints each run

• Pair coverage with pattern 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

Fault models define what manufacturing defects ATPG targets; model mix drives pattern count, runtime, and outgoing quality.

Metric: stuck-at / transition / bridge coverage, untestable fault ratio