Design For Test

Design for testing or design for testability (DFT) consists of IC design techniques that add testability features to a hardware product design. The added features make it easier to develop and apply manufacturing tests to the designed hardware. The purpose of manufacturing tests is to validate that the product hardware contains no manufacturing defects that could adversely affect the product’s correct functioning.

Tests are applied at several steps in the hardware manufacturing flow and, for certain products, may also be used for hardware maintenance in the customer’s environment. The tests are generally driven by test programs that execute using automatic test equipment (ATE) or, in the case of system maintenance, inside the assembled system itself. In addition to finding and indicating the presence of defects (i.e., the test fails), tests may be able to log diagnostic information about the nature of the encountered test fails. The diagnostic information can be used to locate the source of the failure.

In other words, the response of vectors (patterns) from a good circuit is compared with the response of vectors (using the same patterns) from a DUT (device under test). If the response is the same or matches, the circuit is good. Otherwise, the circuit is not manufactured as it was intended.

DFT plays an important role in the development of test programs and as an interface for test application and diagnostics. Automatic test pattern generation, or ATPG, is much easier if appropriate DFT rules and suggestions have been implemented.

DFT techniques have been used at least since the early days of electric/electronic data processing equipment. Early examples from the 1940s/50s are the switches and instruments that allowed an engineer to “scan” (i.e., selectively probe) the voltage/current at some internal nodes in an analog computer [analog scan]. DFT often is associated with design modifications that provide improved access to internal circuit elements such that the local internal state can be controlled (controllability) and/or observed (observability) more easily. The design modifications can be strictly physical in nature (e.g., adding a physical probe point to a net) and/or add active circuit elements to facilitate controllability/observability (e.g., inserting a multiplexer into a net). While controllability and observability improvements for internal circuit elements definitely are important for test, they are not the only type of DFT. Other guidelines, for example, deal with the electromechanical characteristics of the interface between the product under test and the test equipment. Examples are guidelines for the size, shape, and spacing of probe points, or the suggestion to add a high-impedance state to drivers attached to probed nets such that the risk of damage from back-driving is mitigated.

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Wikipedia