ATPG Constraints: Worked Example

Worked example

Worked Example for ATPG Constraints focuses on constraint consistency errors, illegal state count, pattern abort rate. The goal is to convert metric movement into mechanism, owner, and release decision.

A release review shows regression on constraint consistency errors, illegal state count, pattern abort rate. The strongest first move is to freeze evidence, isolate one failing bucket, and prove mechanism before changing flow knobs.

Sequence under inspection

DFT FLOW - ATPG Constraints

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

Primary metric: constraint consistency errors, illegal state count, pattern abort rate

ATPG legality envelope

constraints define legal test states:
clocks + resets + masks + test enables + safe unknown handling

invalid envelope -> great-looking but unusable patterns.

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 ATPG constraint file, legality checker log, constraint diff report.

5. Choose one reversible fix and predefine regression checks.

Did the fix work?

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