Digital Systems Testing | And Testable Design Solution High Quality

Problem: 50K flip-flops, 500K gates, 1M stuck-at faults, target 99.5% coverage.

Solution implemented:

Result:

| Technique | Problem Solved | Quality Metric | | :--- | :--- | :--- | | Logic BIST with MISR | At-speed testing without ATE | <1 ppm aliasing | | At-speed scan (OCC) | Delay faults | Launch-off-shift (LOS) or capture (LOC) | | Test points (control/observe) | Random-resistant faults | +5–10% coverage | | Memory BIST | Embedded memories | 100% stuck-at & retention | | Analog DFT (loopback) | Mixed-signal SoCs | ≤1dB SNR loss | Problem: 50K flip-flops, 500K gates, 1M stuck-at faults,

| Aspect | Low Quality | High Quality | | :--- | :--- | :--- | | Fault model | Stuck-at only | Stuck-at, delay, bridging, open | | DFT | None / ad hoc | Full scan + BIST + JTAG | | ATPG | Random patterns | Deterministic + fault simulation | | Coverage | <95% | ≥99% stuck-at, ≥95% timing | | Test time | >10 sec | <100 ms | | Diagnosis | Fail/pass only | Silicon debug support (scan dump) |

Final Principle: A high-quality testable design is not an afterthought — it is architected from RTL, validated with realistic fault models, and measured by defect level, not just fault coverage.

Traditional transition delay tests miss very slow defects that only appear under specific thermal or voltage conditions. High-quality solutions require: Result: | Technique | Problem Solved | Quality

One of the biggest hurdles to high-quality testing is time. To achieve 99%+ fault coverage, test patterns can number in the hundreds of thousands. Test Compression solutions (such as Linear Feedback Shift Registers and Stimulus Decompressors) bridge this gap.

By compressing test data on-chip and decompressing responses before sending them off-chip, engineers can apply significantly more test patterns without increasing ATE memory requirements. This allows for higher fault coverage (higher quality) without inflating test costs.

Modern DFT integrates test compression to reduce data volume. A decompressor expands a small number of input channels into many internal scan chains, while a compactor reduces output pins. Final Principle: A high-quality testable design is not

Result: 10–100× reduction in test data volume and test time.

Consider an ADAS controller chip (16nm, 200M gates, 500MB memory). The requirement: Zero Defect ( < 1 DPPM).

The High-Quality DFT Solution implemented:

Result: The chip passed AEC-Q100 Grade 1 (-40°C to +125°C) qualification. The test cost per device dropped by 40% due to compression, while DPPM remained under 2 for 100 million shipped units.