Scan Cell Architecture is central to Scan Fundamentals. Scan insertion replaces functional flops with scan-capable cells so internal state becomes controllable and observable in test mode. Senior DFT engineers tie metric movement to architecture assumptions, constraints, and silicon evidence rather than isolated tool output.
Scan insertion replaces functional flops with scan-capable cells so internal state becomes controllable and observable in test mode.
Primary metric: scan cell legality rate, scan replacement ratio, scan DRC count
Primary artifact: scan insertion report, scan DRC log, scan replacement summary
Owners: DFT owner, RTL owner, synthesis owner
Controllability and observability must be explicit
Production-quality requires reproducible pattern and tester tags
At release, Scan Cell Architecture issues can create coverage escapes, unstable production bins, or long debug loops. Scan architecture determines controllability and observability quality.
SCAN CELL
functional path: D ------> Q
^
scan path: SI -> mux --|
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SE=1 (shift), SE=0 (functional)
Shift mode exposes internal state through SI/SO.Name failing metric and scenario context (mode, lot/corner, program).
Open scan cell legality rate, scan replacement ratio, scan DRC count trend and isolate dominant failing bucket.
Trace architecture assumptions and legality constraints.
Check compression, clocking, and unknown handling dependencies.
Collect scan insertion report, scan DRC log, scan replacement summary 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 CELL
functional path: D ------> Q
^
scan path: SI -> mux --|
|
SE=1 (shift), SE=0 (functional)
Shift mode exposes internal state through SI/SO.DFT OWNERSHIP LAYERS - Scan Cell Architecture
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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