scan chain design
**Scan Chain Design** is the **DFT technique of connecting flip-flops into serial shift-register chains enabling controllability and observability of internal states**, allowing ATPG tools to achieve >99% stuck-at fault coverage for manufacturing defect detection.
**Scan Insertion**: Each flip-flop replaced with a scan FF having: functional data (D), scan input (SI), scan enable (SE), and scan output (SO). When SE=1, flops form shift registers through scan I/O pins. When SE=0, normal operation.
**Architecture Decisions**:
| Parameter | Options | Tradeoff |
|-----------|---------|----------|
| Chain count | 8-2000+ | More = faster shift but more I/O pins |
| Chain length | Equal-balanced | Shorter = less shift time |
| Scan ordering | Physical proximity | Minimizes routing wirelength |
| Compression | 10x-100x | Higher = less data/time but more logic |
| Clock domains | Per-domain chains | Avoids CDC during shift |
**Test Compression**: EDT/Tessent/DFTMAX uses: **decompressor** (expands few external channels into many internal chains) and **compactor** (compresses chain outputs). 50-100x compression reduces test data from terabits to gigabits.
**Scan Chain Reordering**: Post-placement, chains reordered for physical adjacency. Constraints: equal chain lengths, clock-domain separation, lockup latches for domain crossings.
**ATPG**: Tools generate patterns that: **shift in** a pattern, **launch** via functional clocks, **capture** response in flops, **shift out** for comparison. Fault models: **stuck-at** (SA0/SA1), **transition** (slow-to-rise/fall), **path delay**, **bridge** (shorts).
**Advanced**: **Routing congestion** from scan connections — insert scan before routing for scan-aware routing; **power during shift** — all flops toggling causes 3-5x normal power (requires segmentation or reduced shift frequency); **at-speed testing** — launch-on-shift and launch-on-capture techniques.
**Scan design is the backbone of manufacturing test — without it, the internal state of a billion-transistor chip would be a black box, making defect detection impossible at production volumes.**