Investigation of Loopback Test Development for MMDC Test Coverage Improvement

The Multi-Mode DDR Controller (MMDC) module is a DDR controller designed to support several DDR type memories, as well as 2ch x32 and x64 memory widths. The MMDC supports various debug and profiling modes. The MMDC is a configurable, high performance, and optimized DDR controller that supports DDR3 x16/x32/x64, and LPDDR2 2-channels x16/x32 memory types. MMDC as the DDR controller are widely used at recent years. The main products of major chip manufactures all support it. The performance of MMDC is very critical for chip quality. In the recent years, there are many effort spend on MMDC test coverage improvement to guarantee the DDR quality, improve DDR performance, and reduce the DDR defect. The traditional Scan test can not meet the requirement of MMDC since the Scan coverage is limited. The loopback test as a function test is developed for MMDC test. A loopback test is the process of sending digital data streams from a source back to the same point without any intentional modifications. It's generally performed to determine whether a device works properly and whether there are failing nodes in a network. The loopback test is intended to check the data path at speed. In this paper, the study work mainly focus on loopback test and its toggle coverage improvement. Analysis of customer returns revealed several failure modes due to inadequate loopback coverage with original pattern, which only covered lower 32bits (CH0). The test only runs in 32bit LPDDR2 mode so it must be run twice to cover both channels (CH0/1). For toggle coverage improvement, the original test contained only 4 data patterns. Developed new test pattern to add coverage on data path. The 64bit extended loopback pattern screened out customer return units. Failure Analysis(FA) is performed for the MMDC rejects. Shmoo tool, light emission and electro optical probe(EOP) are used in the study. The abnormal output of register was found. Physical failure analysis found that the high resistance was observed in IV curve between register net to Vss. Abnormal Via 5 Cu loss was found by TEM. The loopback test can screen out MMDC real failures. It is concluded that the loopback test is a good solution for MMDC coverage improvement.