An Overview of VLSI Testing Techniques: DFT, BIST, and ATPG

  • January 10, 2024

    author: Ramya

Maintaining the proper functionality of VLSI (Very Large Scale Integration) circuits gets increasingly difficult as their complexity rises. VLSI testing methods are essential for ensuring the accuracy and dependability of integrated circuits. Three important testing methodologies will be briefly discussed in this blog: Design for Testability (DFT), Built-In Self-Test (BIST), and Automatic Test Pattern Generation (ATPG). For VLSI designs to be of the highest caliber and perform at their best, it is essential to comprehend these techniques.

Design for Testability (DFT)

Design for Testability (DFT) is a methodology that places a focus on the incorporation of test features during the design process in order to support effective and efficient testing. Enhancing the controllability and observability of internal nodes in a circuit is the fundamental objective of DFT since it makes it simpler to use test patterns and gather response data. Typical DFT strategies include:

Scan chains enable for the serial loading and capturing of test patterns and replies by adding extra flip-flops to a shift register. This method makes it easier to apply test patterns and makes it possible to observe internal node values.

Boundary Scan (JTAG): Boundary scan methods are standardized by the Joint Test Action Group (JTAG). It increases specialized test logic near the chip's edge, making it possible to test connections between chips and PCB components. Debugging and in-system programming are made easier by JTAG.

By enabling effective fault identification and lowering testing time and expense, DFT approaches improve the testability of VLSI designs.

Design for Testability Principles:

Controllability: The ability to apply test patterns to certain nodes or components within a VLSI circuit is referred to as controllability. Design strategies like scan chains and integrated test circuitry improve controllability by offering quick ways to input test patterns into the circuit.

Observability: During the testing process, the ability to record and examine the responses of internal nodes or components is referred to as observability. Design methods that enable the extraction of circuit output data for analysis and fault detection, such as scan chains and boundary scan, improve observability.

Testability Metrics: The effectiveness and efficiency of the testing process are gauged using testability metrics. Fault coverage, test application time, and the quantity of necessary test patterns are a few examples of popular metrics. Designing for testability seeks to maximize these criteria to enable thorough testing while using the least amount of resources and effort.

Design for Testability Advantages:

DFT approaches greatly improve the capacity to locate and characterize errors in VLSI circuits. DFT allows thorough test coverage, ensuring that a large number of errors can be found during the testing process by enhancing controllability and observability.

Reduced Test Application Time: DFT methods, such as scan chains, allow for the concurrent testing of several chip circuit blocks. This parallelism shortens the time needed to implement test patterns and record results, increasing testing effectiveness overall.

Effective Diagnosis and Debugging: DFT approaches offer improved observability, making defects easier to identify and troubleshoot. Faults can be more precisely targeted and detected, accelerating the debugging process, by recording internal node values and contrasting them with anticipated outcomes.

Cost and Time Savings: Potential problems can be discovered and rectified early by adopting DFT techniques during the design phase, which will ultimately save a lot of time and money. Additionally, the capacity for thorough testing and diagnosis contributes to the marketability and excellence of VLSI products.

Built-In Self-Test (BIST)

Built-In Self-Test (BIST) method embeds self-test circuits into the VLSI architecture to enable automatic testing without the need for additional test tools. The chip uses BIST techniques to create and use test patterns, apply them, record answers, and perform problem diagnostics. BIST has a number of benefits, including:

BIST is excellent for testing chips integrated in bigger systems since it lowers the dependence on external test equipment.

Increased Test Coverage: BIST can offer high test coverage since it can focus on particular chip regions that are challenging to examine using more conventional techniques.

Reduced Testing Time: By enabling the parallel testing of numerous circuit blocks inside the chip, BIST approaches can dramatically cut testing time.

BIST can be used in a variety of ways, such as embedded core test controllers in systems or built-in self-test circuits at the circuit or system level. It offers a quick and affordable method for testing intricate VLSI designs.

Built-In Self-Test Concepts:

Self-Contained Testing: BIST enables a VLSI circuit to create and use test patterns, record responses, and carry out fault diagnostics without the use of additional test tools. Because the test circuitry is integrated into the chip, it is self-contained and capable of independent testing.

On-Chip Test Pattern Generation: BIST strategies include tools for producing test patterns on the actual chip. These patterns are intended to identify potential issues and guarantee thorough circuit testing. Internally generated test patterns lessen dependency on external test apparatus and streamline the testing procedure.

Fault identification: By recording answers and examining them to find potential defects, BIST makes fault identification easier. The self-contained test circuitry contrasts the recorded answers with anticipated outcomes to identify circuit defects. This aids in problem localization and diagnosis, enabling effective debugging and fault-finding.

Automatic Test Pattern Generation (ATPG)

Automatic Test Pattern Generation (ATPG) is a technique for creating test patterns that are automatically generated and capable of identifying and diagnosing problems in VLSI circuits. The circuit's structure is analyzed using ATPG algorithms to find any potential flaws, which subsequently produce test patterns to find those flaws. With a minimum of patterns and application time, the created patterns seek to achieve excellent fault coverage. The following steps are involved in ATPG:

Fault Modeling: In order to illustrate potential circuit flaws like stuck-at or bridging faults, fault models are used.

Test Generation: To create test patterns that identify the specified faults, ATPG algorithms examine the circuit's structure and use a variety of techniques (such as Boolean satisfiability, fault simulation, etc.).

Test Application: To identify whether any faults are present, the generated test patterns are applied to the circuit under test. The results are then recorded and examined.


Conclusion:

The three significant VLSI testing methods covered in this blog are DFT (Design for Testability), BIST (Built-In Self-Test), and ATPG (Automatic Test Pattern Generation).DFT focuses on developing semiconductors that are testable, including elements and features that make testing effective. It makes defect detection and diagnosis simpler, which lowers testing's overall cost and duration.Contrarily, BIST entails building self-testing capabilities right into the semiconductor. This method gets rid of the need for external test apparatus and enables the chip to run tests independently, increasing test coverage and lowering reliance on pricey external testers.By automating the creation of test patterns for VLSI chips, ATPG does exactly what its name implies. It employs algorithms to create input patterns that consistently identify circuit failures. Complex designs where manual test pattern creation would be unfeasible are where ATPG is very helpful.Each method covered in this blog has benefits and drawbacks. DFT improves testability but might necessitate more design work. BIST provides the ability to self-test, however it can use up critical chip resources. Even while ATPG automates test pattern generation, it might not offer as much coverage as manually created patterns.The choice of VLSI testing method ultimately depends on a number of variables, including the complexity of the design, the amount of time available, the cost, and the desired test coverage. Designers may ensure the quality and dependability of VLSI chips while minimizing testing labor and costs by understanding these methodologies and making informed judgments.