High-speed fixture interconnects for mixed-signal IC testing

The authors present a coaxial interconnect scheme that allows analog signals to cross the boundary between test instrumentation and a fixture-mounted DUT (device under test) without being corrupted by digital crosstalk and system noise. A compressible coaxial probe that maintains an uninterrupted 50-Ω environment and is compact and easy to service has been developed. The use of the coaxial test probe necessitates a redesign of existing DUT fixture board technology. A blind via is used to mate the coaxial probes to the DUT board. This technology provides the mechanical characteristics required for reliable contact over the long term and minimizes the length of unshielded signal path. The blind via also allows a board geometry that provides maximum separation of digital and analog signals. The combined effect of the coaxial probe and the blind via is improved adherence to 50-Ω signal path requirements and appreciably reduced crosstalk and noise in mixed-signal test systems. The resulting analog signal environment exhibited a noise floor below the -120-dB target