Design space exploration

For ATPG Constraints, 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 - ATPG Constraints
quality <-> tester cost <-> schedule

Design pitfalls

• No ownership for quality gap

• Fixing one metric while regressing another

Tradeoff curve

BEFORE / AFTER - ATPG Constraints

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

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

ATPG constraints encode legal test-mode behavior, clocks, resets, and masks so generated patterns are both executable and meaningful.

Metric: constraint consistency errors, illegal state count, pattern abort rate