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