Chain Stitching Rules is central to Scan Fundamentals. Scan flops are stitched into chains based on clock, power, and physical constraints; poor stitching increases test time and routing pain. Senior DFT engineers tie metric movement to architecture assumptions, constraints, and silicon evidence rather than isolated tool output.
Scan flops are stitched into chains based on clock, power, and physical constraints; poor stitching increases test time and routing pain.
Primary metric: chain balance spread, max chain length, scan shift time
Primary artifact: chain map, balance histogram, stitch rule deck
Owners: DFT owner, PD owner, implementation owner
Controllability and observability must be explicit
Production-quality requires reproducible pattern and tester tags
At release, Chain Stitching Rules issues can create coverage escapes, unstable production bins, or long debug loops. Scan architecture determines controllability and observability quality.
CHAIN STITCHING (example)
clock/power legal groups -> balanced chains -> routed scan orderName failing metric and scenario context (mode, lot/corner, program).
Open chain balance spread, max chain length, scan shift time trend and isolate dominant failing bucket.
Trace architecture assumptions and legality constraints.
Check compression, clocking, and unknown handling dependencies.
Collect chain map, balance histogram, stitch rule deck and confirm run tags.
Classify issue: model/constraint, physical/test setup, or real defect signal.
Propose minimal fix and list timing/power/quality regression checks.
Coverage percent alone proves release readiness.
More compression always means better outcome.
Silicon mismatch can be debugged without pattern/tester traceability.
Shift timing and test power can be signed independently.
SCAN CHAINS
clock_domain_A: FF1 -> FF2 -> FF3 -> SO
clock_domain_B: FF4 -> FF5 -> FF6 -> SO
Rules:
1) keep domains legal
2) balance chain length
3) avoid long physical detoursDFT OWNERSHIP LAYERS - Chain Stitching Rules
layer owns failure mode
---------------- -------------------------- -------------------------
rtl/architecture scanability hooks uncontrollable logic
atpg/constraints legal pattern intent aborts, low coverage
physical/clocking chain route + test clocks shift hold/timing escapes
tester/program pattern apply integrity false binning / bad fails
quality signoff release criteria escapes or schedule slipScan architecture quality determines whether ATPG can control and observe real silicon state.
SCAN INSERTION FLOW
scan-ready RTL -> scan replacement -> stitch chains -> DRC -> ATPG handoffCHAIN BALANCE
chain length spread
low spread = better shift time
high spread = routing + hold riskscan insertion summary
chain balance report
scan DRC log
clocking legality report
Coverage plateau traced to non-scan flops in reset islands; RTL + DFT lint policy fixed root cause.
Check scan replacement first
Audit chain legality by domain
Validate shift/capture clocks
Ask: what evidence proves this DFT decision is safe for production?
State metric, lot/corner context, and pattern tag with every claim.
Treat timing, power, and quality as one signoff problem.
Chasing coverage without legality checks.
Ignoring test-power side effects of pattern changes.
Debugging silicon without reproducible tags.
Scan architecture determines controllability and observability quality.
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