Coverage Closure: Expanded Case Study

Extended case study

Release review: coverage gap to target, dominant undetected classes, closure iteration count regresses after a test-flow update touching Coverage Closure.

Background

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

Symptoms observed

• coverage gap to target, dominant undetected classes, closure iteration count 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

Coverage gap driven by stale constraints and high-X blocks; targeted cleanup plus model tuning closed gap without large pattern growth.

Fix and validation

• Apply bounded fix with owner

• Re-run coverage gap report, undetected fault buckets, closure action tracker

• Re-validate quality, timing, and test power

Lessons learned

• Tag every run artifact

• Mechanism first, command second

• Close with explicit release decision

CASE STUDY - Coverage Closure
baseline metric / regressed metric / post-fix metric

Sequence under stress

DFT FLOW - Coverage Closure

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

Primary metric: coverage gap to target, dominant undetected classes, closure iteration count

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

Coverage closure uses targeted analysis of undetected faults, constraints, and design-for-test hooks to close quality gaps without exploding pattern volume.

Metric: coverage gap to target, dominant undetected classes, closure iteration count