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Deals with a method for the automatic analysis and characterization of defects due to encapsulation and/or surface mount processes of microelectronic devices. This method is based on digital signal processing of ultrasonic signals in the 10-100-MHz frequency range. In particular, it is used for automatic evaluation of time-of-flight between echoes received by the acoustic transducer. The signals are first pre-processed by a new algorithm, based on Wiener filtering, and then by a numeric algorithm, based on the wavelet transform, already applied successfully to this problem. The pre-processing phase increases the sensitivity of the successive numeric algorithm. The theory underlying the pre-processor and the chosen procedure to implement it are described in detail in this paper. Furthermore, experimental results obtained by applying the proposed method on acoustic signals from an electronic structure acquired through an ultrasonic scanning system are given and discussed.