Scan insertion is one of the most important foundations of Design for Testability (DFT) in modern VLSI. As chips become more complex, with billions of transistors, multiple clock domains, and heterogeneous blocks, ensuring test coverage for manufacturing defects is critical. With advanced nodes like 3nm, 2nm, and beyond, and increased demands for functional safety and high yield, understanding scan insertion is essential for any aspiring VLSI or DFT engineer.

This blog breaks down how scan insertion works, why it’s needed, what modern EDA tools do, and how engineers implement it in real workflows. We’ll also include practical tips, common challenges, and suggestions for freshers.

What Is Scan Insertion?

At its core, scan insertion is a DFT technique that transforms sequential logic (flip-flops, latches) into a structured, testable chain (scan chain) so that internal states can be controlled and observed during testing.

Without scan chains, internal nodes of a chip are very difficult to test using automated test equipment (ATE). Scan insertion makes it possible to:

• Shift in test patterns

• Capture internal responses

• Shift out captured results for comparison

• Detect stuck-at and other structural faults

Because scan chains expose internal storage elements to external control and observation, they improve fault coverage dramatically and reduce re-spin costs.

Why Scan Insertion Matters

These industry trends make scan insertion even more critical:

1. Advanced Nodes Require Higher Test Coverage

At 3nm and 2nm, defects may reside deep in the layout. Traditional random patterns are insufficient to cover all possible fault sites.

2. Multiple Power Domains

Modern SoCs have multiple power states and domain crossings. Scan test structures help isolate and validate individual domains.

3. Functional Safety Requirements

Automotive and Avionics encourage higher test coverage with traceable test logs. Scan paths help generate digital signatures of tested logic blocks.

4. Low-Power Designs

Power-aware scan insertion (with DFT power constraints) ensures tests don’t violate power shut-off schemes like UPF or power gating.

The Conceptual Flow of Scan Insertion

Here’s a simplified overview of how scan insertion fits into the DFT flow:

1. Netlist Ready: The RTL is synthesized to a gate-level netlist.

2. Identify Sequential Cells: All sequential elements (flip-flops/latches) are identified for scan conversion.

3. Convert to Scan Cells: Standard flip-flops are replaced or augmented with scan-enabled flip-flops (scan cells).

4. Partition Scan Chains: Create scan chains that connect scan cells in a controllable sequence.

5. Insert Test Access Ports: Insert Test Data In/Out (TDI/TDO) and Scan Enable (SE) pins.

6. Re-Optimize Scan Chains: Optimize for length, power, and test coverage.

7. Verify Scan Functionality: Run ATPG (Automatic Test Pattern Generation) to validate stuck-at and transition faults.

Scan vs. Sequential Logic

In normal operation, a flip-flop updates its output on clock edges based on input. In scan mode, these same flip-flops act like cells in a shift register:

Normal Mode:

   D --> FF --> Q

Scan Mode:

   (Scan In) --> [Scan Cell] --> (Scan Out)

Scan chains essentially string many flip-flops into a large shift register that can be externally controlled.

Inside Scan Insertion: Key Concepts

Let’s walk through the major components and ideas that make scan insertion tick.

1. Scan Cells

A scan cell is a special version of a flip-flop that supports normal mode and scan mode:

• Normal Mode: Operates as a regular flip-flop for chip functionality.

• Scan Mode: The flip-flop’s input/output are redirected to chain connections.
  
  

Scan cells usually have extra pins:

• SI (Scan In)

• SO (Scan Out)

• SE (Scan Enable)
  
  

2. Scan Chains

All scan cells are grouped into chains. Designers choose:

• Number of chains (often multiple to balance test times)

• Lengths (preferably balanced for shorter shift cycles)

• Order (optimized for routing and power)
  
  

Example: A design with 10k flip-flops might have 10 chains of ~1k each.

3. Clocking the Scan Chain

Scan operation needs:

• A test clock for shifting in/out test vectors

• A functional clock for normal operation
  
  

During manufacturing test, test vectors are shifted into the scan chain via test clock and scan enable.

4. Test Access Mechanisms (TAM)

On large SoCs with multiple cores and domains, TAMs help route test data to/from internal cores efficiently. IEEE standards PAC and TAP are modern approaches to manage access.

How EDA Tools Perform Scan Insertion

Modern DFT tools (e.g., Synopsys DFTMAX Ultra, Cadence Modus, Siemens Tessent Scan) integrate scan insertion with ATPG and power-aware flows.

Typical Tool Process:

1. Read Gate-Level Netlist

2. Generate Test Points (Optional)

3. Insert Scan Cells

4. Partition Chains

5. Optimize Chains (length/power)

6. Generate Test Pins and Modes

7. Report Scan Coverage & Statistics
   
   

Python or TCL scripts are commonly used to automate batch runs across multiple designs or variants.

Test Coverage and Validation

Good scan insertion aims to maximize fault coverage:

• Stuck-at faults

• Transition faults

• Path delay faults

• Bridging faults
  
  

Tools generate coverage reports after ATPG runs. Designers aim for coverage goals often above 95% in manufacturing targets.

Power-Aware Scan Insertion

With aggressive power gating and low-power modes, designers must handle scan insertion such that test modes don’t violate:

• Power domain shut-off

• Clock gating constraints

• Safe isolation of registers

Industry workflows now use UPF (Unified Power Format) along with scan tools to perform power-aware scan insertion.

Practical Steps for Beginners

Here’s a recommended learning path:

Step 1: Understand Sequential Logic

Know how flip-flops and latches work.

Step 2: Learn Synthesis Flows

Scan insertion happens post-synthesis.

Step 3: Explore DFT Tools

Get hands-on with tools like Synopsys DFTMAX or Cadence Modus.

Step 4: Work on Small Projects

Insert scan into a small RTL, generate ATPG patterns, and measure coverage.

Step 5: Learn Power-Aware DFT

Incorporate power domains using UPF during scan insertion.

Common Scan Insertion Challenges (and Solutions)

Companies hiring DFT engineers expect familiarity with:

• Scan insertion flows

• ATPG pattern generation

• Power-aware DFT

• Test coverage analysis

• Scripts to automate DFT flows (Python/TCL)

DFT roles are among the high-demand positions in chip design, especially in automotive, AI, security, and high-reliability domains.

Conclusion

Scan insertion bridges the gap between design and manufacturability. It enables effective automatic testing, increases yield, and helps detect manufacturing defects early. In a world of multi-domain SoCs and stringent safety requirements, understanding and applying scan techniques distinguishes good designers from expert DFT engineers.

Embrace the concepts, practice with real tools, and you’ll find that scan insertion is not just a step, it’s a gateway to mastering DFT and physical design.