A test vector is a specific set of input signals and expected output responses used to verify that a semiconductor device functions correctly during testing. Test vectors form the foundation of digital IC testing — they are the "questions" asked of the chip, with the expected answers used to determine pass or fail.

Key Concepts

• Structure: Each vector typically specifies the logic state (0, 1, or don't-care) for every input pin of the device at a particular clock cycle, along with the expected output values.
• Vector Sets: A complete test program may contain millions of vectors covering functional modes, corner cases, timing checks, and stress conditions.
• Coverage: The quality of a test is often measured by its fault coverage — the percentage of possible manufacturing defects that the vector set can detect.

Types of Test Vectors

• Functional Vectors: Exercise the chip's intended operations (instruction execution, data processing, I/O protocols).
• Structural Vectors: Generated by ATPG (Automatic Test Pattern Generation) tools targeting specific fault models like stuck-at, transition, and path delay faults.
• Parametric Vectors: Focus on measuring analog characteristics like voltage thresholds and timing margins rather than pure logic correctness.

Why It Matters

Generating efficient test vectors is a major engineering effort. The goal is achieving maximum fault coverage with the minimum number of vectors to keep test time — and therefore test cost — as low as possible.