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This paper presents time-domain reflectometry (TDR) as a nondestructive sensing method for interconnect failure mechanisms. Two competing interconnect failure mechanisms of electronics were considered: solder joint cracking and solder pad cratering. A simple theoretical analysis is presented to explain the effect of each failure mechanism on the TDR reflection coefficient. Mechanical fatigue tests have been conducted to confirm the theoretical analysis. The test results consistently demonstrated that the TDR reflection coefficient gradually decreased as the solder pad separated from the circuit board, whereas it increased during solder joint cracking. Traditional test methods based on electrical resistance monitoring cannot distinguish between failure mechanisms and do not detect degradation until an open circuit has been created. In contrast, the TDR reflection coefficient can be used as a sensing method for the determination of interconnect failure mechanisms as well as for early detection of the degradation associated with those mechanisms.