Interview drills

Interview Drills for Compression Debug focuses on unknown/X inflation, compactor overflow incidents, pattern replay success rate. The goal is to convert metric movement into mechanism, owner, and release decision.

PROMPT
You observe unknown/X inflation, compactor overflow incidents, pattern replay success rate on Compression Debug. Walk through diagnosis and release decision.

STRONG ANSWER
1. States context and run tags.
2. Explains mechanism: Compression debug isolates whether failures come from EDT wiring, X-masking policy, clocking, or ATPG assumptions before blaming silicon defects.
3. Requests X-source trace, compactor status report, pattern replay logs.
4. Proposes minimal fix and regression.

WEAK ANSWER
Jumps to tool switches without evidence quality, ownership, or regression plan.

Diagram to draw on the whiteboard

Compression flow

compression architecture -> ATPG patterns -> fail logs -> diagnosis -> yield learning

Root-cause narrative

ROOT-CAUSE TREE - Compression Debug

unknown/X inflation, compactor overflow incidents, pattern replay success rate regresses
        |
  setup changed?
    /        \
  yes         no
  |            |
constraint    silicon or
or ATPG       physical/test path
 /    \          |
SDC   model    chain/clock/power/diagnosis
diff  diff     isolate first failing signature

DFT deep dive

Compression saves tester time only when diagnosis observability remains credible.

Concept diagram

COMPRESSION LOOP

EDT/decompressor -> compressed patterns -> compactor responses -> diagnosis

Metric graph

PATTERN vs COVERAGE

coverage up   -> pattern count up
compression up -> pattern count down (until aliasing risk)

Reports and artifacts

• compression ratio dashboard

• pattern count trend

• X-source report

• diagnosis bucket summary

Mini case study

Compactor overflow plus unknown inflation caused false diagnosis; masking policy and channel map corrected.

Debug branches

• Separate X issues from silicon defects

• Replay failing patterns uncompressed

• Track tester memory 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

Compression debug isolates whether failures come from EDT wiring, X-masking policy, clocking, or ATPG assumptions before blaming silicon defects.

Metric: unknown/X inflation, compactor overflow incidents, pattern replay success rate