Shift vs Capture Timing

What this topic teaches

Shift vs Capture Timing turns DFT intent into measurable release confidence. Shift mode is hold-sensitive while capture mode is setup-sensitive; closure must preserve both under test clocking assumptions. The senior challenge is proving whether a metric move came from real quality gain, setup drift, or hidden regression.

The senior-engineer question

When shift hold violations, capture setup failures, at-speed pattern pass rate moves, can you identify mechanism, evidence quality, owner, and the minimum safe next action?

DFT CLOSURE FLOW - Shift vs Capture Timing

scan/test architecture
        |
        v
ATPG constraints + fault models
        |
        v
pattern generation + compression
        |
        v
timing/power/physical validation
        |
        v
silicon diagnosis and release signoff

Debug rule: always state metric, run tags, and owning team with any claim.

Picture the closure flow

Draw the causal flow before opening tools. Use these diagrams to anchor architecture, constraints, and silicon behavior discussions.

Timing split

shift mode: hold-dominant
capture mode: setup-dominant

Do not sign off one without the other.

Process sequence

DFT FLOW - Shift vs Capture Timing

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

Primary metric: shift hold violations, capture setup failures, at-speed pattern pass rate

Ownership layers

DFT OWNERSHIP LAYERS - Shift vs Capture Timing

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 slip

Evidence to collect

• Primary metric: shift hold violations, capture setup failures, at-speed pattern pass rate.

• Primary artifact: shift/capture timing report, at-speed validation logs, path-group summary.

• Owners to bring into review: STA owner, DFT owner, PD owner.

• One failing signature and one reduced reproduction path.

• Exact run tags for constraints, patterns, and tester program.

Ownership map

OWNERSHIP MAP - Shift vs Capture Timing

artifact              owner
----------------      -----------------
architecture/report STA owner
constraints/setup   DFT owner
physical/test       PD owner

Name an owner for each failing metric cluster.

Subpages in this topic

Each topic includes mechanism, inputs/outputs, reports, debug, worked example, pitfalls, interview, checklist, theory, design space, expanded case study, walkthrough, comparison matrix, software view, and silicon impact.

Key takeaways

• State metric and run tags with every claim.

• Connect every fix to a regression matrix.

• Treat quality, timing, and power as coupled.

Common pitfalls

• Coverage-centric decisions without legality checks.

• Pattern changes without tester correlation.

• Release calls without owner signoff.

DFT deep dive

Test signoff fails when shift/capture timing and test power are treated independently.

Concept diagram

TEST SIGNOFF LOOP

test SDC -> shift/capture timing -> power-aware ATPG -> IR validation -> release

Metric graph

TEST CURRENT

functional current  baseline
scan shift current  peak-risk zone

Reports and artifacts

• test-mode STA report

• shift/capture split

• test power IR map

• waiver tracker

Mini case study

At-speed patterns passed timing but failed in production due to peak shift IR; staggered capture and power-aware fill resolved.

Debug branches

• Tag test and functional corners separately

• Check hold in shift mode

• Correlate fail bins with power hotspots

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.