Abstract
SQUID and MR magnetic sensors have separately been used for fault isolation of shorts and resistive opens in integrated circuits and packages. These two technologies were once considered to be mutually exclusive, although recent studies [1] rather pointed to their complementary character. This paper shows, for the first time, the use of these two sensors together to isolate a low resistance short in a Quad-NAND gate microcircuit. Electrical test confirmed low resistance shorts between three of the device pins. However, internal optical inspection found no evidence of failure. The low resistance of the shorts was deemed insufficient for liquid crystal analysis. Magnetic current imaging with a SQUID sensor confirmed current flow through the package lead frame and isolated the defect to the microcircuit. Due to package design and the resulting distance of the scan plane, the SQUID was unable to resolve the current path on the microcircuit. In parallel with the SQUID, a magnetoresistive (MR) probe was employed to fit inside the device cavity, make direct contact with the microcircuit, and map high-resolution current images. Two sites with high-current density were accurately identified by MCI in input transistors. Subsequent deprocessing revealed that the defects were located under a broad sheet of aluminum metallization which blocked optical detection, and rendered detection by thermal emission difficult.
