Compression Debug: Worked Example

Worked example

Worked Example 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.

A release review shows regression on unknown/X inflation, compactor overflow incidents, pattern replay success 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 - Compression Debug

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

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

Compression flow

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

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 X-source trace, compactor status report, pattern replay logs.

5. Choose one reversible fix and predefine regression checks.

Did the fix work?

BEFORE / AFTER - Compression Debug

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

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