Tailoring Tests for Functional Binning of Integrated Circuits

In recent years, a number of high level applications have been reported to be tolerant to errors resulting from a sizable fraction of all single stuck-at faults in hardware. Production testing of devices targeted towards such applications calls for a test vector set that is tailored to maximize the coverage of faults that lead to functionally malignant errors while minimizing the coverage of faults that produce functionally benign errors. Given a partitioning of the fault set as benign and malignant, and a complete test vector set that covers all faults, in this paper, we formulate an integer linear programming (ILP) problem to find an optimal test vector set that ensures 100% coverage of malignant faults and minimizes coverage of benign faults.We also propose a test strategy based on selectively masking appropriate outputs of the circuit to partition the circuits at production test into three bins - malignant, benign, and fault-free. As a case study, we demonstrate the proposed ILP based test optimization and functional binning on three adder circuits: 16-bit ripple carry adder, 16-bit carry lookahead adder, and 16-bit carry select adder. We find that the proposed ILP based optimization gives a reduction of about 90% in fault coverage of benign faults while ensuring 100% coverage of malignant faults. This typically translates to an (early manufacturing) yield improvement of over 20% over what would have been the yield if both malignant and benign faults are targeted without distinction by the test vectorset.