Worked example

Worked Example for Scenario: Coverage Storm focuses on coverage shortfall under schedule pressure, pattern growth slope. The goal is to convert metric movement into mechanism, owner, and release decision.

A release review shows regression on coverage shortfall under schedule pressure, pattern growth slope. The strongest first move is to freeze evidence, isolate one failing bucket, and prove mechanism before changing flow knobs.

Sequence under inspection

DFT FLOW - Scenario: Coverage Storm

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

Primary metric: coverage shortfall under schedule pressure, pattern growth slope

Coverage crisis triage

coverage gap
  -> untestable? constraint? model?
  -> targeted fixes
  -> pattern budget check

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 coverage gap snapshot, fault bucket analysis, closure plan.

5. Choose one reversible fix and predefine regression checks.

Did the fix work?

BEFORE / AFTER - Scenario: Coverage Storm

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

Senior DFT interviews test tradeoff judgment under imperfect data.

Concept diagram

INTERVIEW ANSWER LOOP

state metric -> explain mechanism -> request evidence -> propose fix -> list regression

Metric graph

ANSWER QUALITY

mechanism clarity + ownership clarity + regression discipline

Reports and artifacts

• whiteboard rubric

• scenario scorecard

• trap checklist

• follow-up depth index

Mini case study

Candidate improved from tool-only answers to mechanism-first narratives by using metric->artifact->decision template.

Debug branches

• Always state assumptions

• Name cross-team owners

• Discuss risk and fallback plan

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 crises require separating true untestable faults from constraint/model issues and choosing the smallest high-impact closure plan.

Metric: coverage shortfall under schedule pressure, pattern growth slope