Scenario: Coverage Storm: Design Space

Design space exploration

For Scenario: Coverage Storm, release options trade quality, tester cost, and schedule risk.

Option A - conservative

• Conservative quality: helps debug confidence

• Risk: higher test cost

• Validate with: first production ramps

Option B - balanced

• Balanced release: helps quality and cost

• Risk: needs discipline

• Validate with: mainstream products

Option C - aggressive

• Aggressive schedule: helps faster closure

• Risk: escape risk

• Validate with: late tapeout pressure

Option D - architecture change

• Architecture change: helps long-term quality

• Risk: schedule hit

• Validate with: chronic recurring failures

DESIGN SPACE - Scenario: Coverage Storm
quality <-> tester cost <-> schedule

Design pitfalls

• No ownership for quality gap

• Fixing one metric while regressing another

Tradeoff curve

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