38 VLSI Test Principles and Architectures to how the manufactured device was to be tested. Once the functionality was imple-mented, the design information was transferred to test engineers. A test engineer’s job was to determine how to efficiently test each manufactured device within a reasonable amount of time, in order to screen out the parts that may contain man-ufacturing defects and ship all defect-free devices to customers. The final quality of the test was determined by keeping track of the number of defective parts shipped to the customers, based on customer returns. This product quality, measured in terms of defective parts per million (PPM) shipped, was a final test score for quantifying the effectiveness of the developed test. While this approach worked well for small-scale integrated circuits that mainly consisted of combinational logic or simple finite-state machines, it was unable to keep up with the circuit complexity as designs moved from small-scale integra-tion (SSI) to
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