The resolution of conventional time-domain reflectometry (TDR) is limited when applied to fault localization for defects smaller than 100 μm, especially in high-density advanced packaging. The principle of electro-optical terahertz pulse reflectometry (EOTPR) is introduced, which uses femtosecond laser excitation to generate stable, fast pulses in the terahertz frequency range. These pulses are injected into the device under test, and the reflected signals are rapidly sampled using asynchronous electro-optic sampling, achieving a resolution of sub-65 μm for failure localization. By utilizing EOTPR, rapid and accurate localization of failures, such as buried via fractures in wire-bonded BGAs, UBM fractures in FCBGAs, and through-hole fractures in MCM packages, has been achieved. This method provides the potential for three-dimensional failure localization in advanced complex packaging.