Scenario: Coverage Storm: Mechanism

Mechanism to understand

Mechanism 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.

Coverage crises require separating true untestable faults from constraint/model issues and choosing the smallest high-impact closure plan. Think of DFT as a quality pipeline where setup quality determines what silicon evidence means.

• Identify where controllability/observability is introduced.

• Identify legal constraints and mode assumptions.

• Identify failure class: architecture, constraints, physical, or silicon.

Layered view

DFT CLOSURE FLOW - Scenario: Coverage Storm

scan/test architecture
        |
        v
ATPG constraints + fault models
        |
        v
pattern generation + compression
        |
        v
timing/power/physical validation
        |
        v
silicon diagnosis and release signoff

Debug rule: always state metric, run tags, and owning team with any claim.

Coverage crisis triage

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

Layer responsibilities

DFT OWNERSHIP LAYERS - Scenario: Coverage Storm

layer              owns                         failure mode
----------------   --------------------------   -------------------------
rtl/architecture   scanability hooks            uncontrollable logic
atpg/constraints   legal pattern intent         aborts, low coverage
physical/clocking  chain route + test clocks    shift hold/timing escapes
tester/program     pattern apply integrity      false binning / bad fails
quality signoff    release criteria             escapes or schedule slip

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.

Mechanism deep dive

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