Faster-than-at-speed Test Research Articles

Built-In Test for Hidden Delay Faults

Marginal hardware introduces severe reliability threats throughout the life cycle of a system. Although marginalities may not affect the functionality of a circuit immediately after manufacturing, they can degrade into hard failures and must be screened out during manufacturing test to prevent early life failures. Furthermore, their evolution in the field must be proactively monitored by periodic tests before actual failures occur. In recent years, small delay faults (SDFs) have gained increasing attention as possible indicators of marginal hardware. However, SDFs on short paths may be undetectable even with advanced timing aware ATPG. Faster-than-at-speed test (FAST) can detect such hidden delay faults (HDFs), but so far FAST has mainly been restricted to manufacturing test. This paper presents a fully autonomous built-in self-test approach for FAST, which supports in-field testing by appropriate strategies for test generation and response compaction. In particular, the required test frequencies for HDF detection are selected, such that hardware overhead and test time are minimized. Furthermore, test response compaction handles the large number of unknowns (X-values) on long paths by storing intermediate MISR-signatures in a small on-chip memory for later analysis using X-canceling transformations. A comprehensive experimental study demonstrates the effectiveness of the presented approach. In particular, the impact of the considered fault size is studied in detail.

Design for Small Delay Test - A Simulation Study

Early Life Failures (ELFs) are becoming an important reliability issue in state-of-the-art technologies. ELFs can be indicated by Small Delay Faults (SDFs), however, some SDFs may not be detectable even with modern at-speed tests. For these hidden SDFs, Faster-than-at-Speed Test (FAST) provides a solution. However, FAST imposes new challenges on the test method. Unknown logic values (X-values) are a major challenge in FAST, due to the increased frequencies. A special Design-for-FAST architecture relying on an accordingly adjusted scan configuration and a simple, but efficient X-masking scheme can support X-tolerant compaction in the context of FAST. This work analyzes the trade-offs of this concept within the framework of a standard industrial workflow and presents a comprehensive case study. Simulation results indicate that for some designs, the conventional synthesis workflow does not produce optimal circuit behavior under FAST. In these cases, the Design-for-FAST approach can increase the fault efficiency, while at the same time reducing the amount of X-values in the test responses considerably.