Time domain reflectometry (TDR) is increasingly being utilized as a failure analysis tool in the semiconductor industry. TDR can provide fast, nondestructive analysis of packaged integrated circuits. With one measurement, TDR analysis can provide instant identification of the general region of the fail site, whether it exists in the substrate, interconnect region, or in the die. A digital sampling oscilloscope with a TDR module supplies a fast risetime voltage edge to the signal pin of interest, and then records the subsequent voltage edge reflections. The time delay between the incident and reflected electrical signals are analyzed to characterize the electrical path of the signal pin. Initial implementation of TDR as an FA tool by the authors was in a comparative analysis method, where the TDR measurement from an unassembled substrate is used as the basis of comparison for the TDR measurement from the failing unit. Current development focuses on signature analysis of the TDR waveform for a given substrate path (including BGA substrate, flip-chip solder bump and pads, and flip-chip die). Interpretation of the TDR measurements for both comparative analysis and signature analysis of the TDR waveform will be discussed in this paper.