DFT / ATPG · All levels
Fault Models: Comparison Matrix
Comparison Matrix for Fault Models.
Comparison matrix
Fault model and constraint policy shape runtime and outgoing quality.
+------------------+----------------+----------------+----------------+
| Approach | Strength | Weakness | Best when |
+------------------+----------------+----------------+----------------+
| Conservative | safe | higher cost | ramp-sensitive |
| Balanced | practical | needs rigor | production |
| Aggressive | fast | escape risk | schedule pressure |
| Architecture | durable | slow | repeated pain |
+------------------+----------------+----------------+----------------+When to choose each approach
Tie approach to product risk and tester constraints
Interview traps
Optimizing one metric only
No regression ownership
Evidence comparison
DFT EVIDENCE MATRIX - Fault Models
+-------------------+------------------------+--------------------------+-------------------------+
| Evidence | Tells you | Does not prove | Next action |
+-------------------+------------------------+--------------------------+-------------------------+
| coverage report | fault detect trends | silicon root cause | inspect bucket details |
| pattern diff | generation changes | architecture quality | replay suspect patterns |
| timing/power report| test closure margin | diagnosis confidence | correlate fail logs |
| diagnosis summary | likely defect classes | ownership decision | assign action owner |
| signoff checklist | process completeness | correctness of evidence | peer review artifacts |
+-------------------+------------------------+--------------------------+-------------------------+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 -> signoffMetric graph
COVERAGE GAP
target coverage
^
| o before closure
| o after fixes
+---------------------> iterationReports 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
Fault models define what manufacturing defects ATPG targets; model mix drives pattern count, runtime, and outgoing quality.
Metric: stuck-at / transition / bridge coverage, untestable fault ratio