14.1.1 What Is DfT?

Design-for-test (DfT, also DFT ^[*]) is a major topic of interest in the automated testing field. Any design methodology or circuit that results in a more easily or thoroughly testable product can be categorized as DfT. DfT, when properly implemented, can offer lower production costs and higher product quality. Extensive literature exists on the subject of DfT, ^{[1] [3]} though much of it pertains to purely digital circuits. Since so much digital DfT literature is already available elsewhere, this chapter will concentrate mostly on analog and mixed-signal DfT. Nevertheless, some of the more common digital concepts will be reviewed to give the reader a basic overview of digital DfT.

There are many types of DfT. Some DfT approaches are highly structured, using industry-defined standards. Other approaches are totally ad hoc, invented by the design engineer or test engineer to solve a specific test problem on a particular device or category of devices. Some DfT concepts are based on built-in circuits that allow easier or more complete testing. Other methodologies, such as increasing design margin to reduce test cost, are equally cost effective but may not be recognized as DfT concepts. In the end, the choice of DfT approach depends very much on the specifics of the device under test (DUT) and the demands placed on it by its system-level application.

In the past, design engineers were sometimes reluctant to add testability features to a device, since DfT added design cycle time,...