Inclusions detection using Lamb waves in flexible printed circuits

The materials used for the manufacture of flexible printed circuits are selected according to various characteristics: thermal and electrical behavior, moisture absorption, flexibility... Those are determined by the basic materials of the three components of the circuit, which are the conducting layer, the adhesive layer and the dielectric film. Such circuits have a typical thickness of about 200 microm and are therefore an interesting solution for a great number of electronic applications. However, these circuits can present various defects like inclusions, delaminations, cracks... In this work, we are interested in the detection of inclusions using guided waves propagation in such structures. These waves also called Lamb waves have the advantage of propagating over long distances while informing us about the totality of the inspected volume. According to the range of frequencies considered and the method used for their generation, it is possible to make profitable use of different propagation modes. To serve this purpose, laser-induced thermoelastic excitation of the first antisymmetric Lamb waves mode is studied. The results obtained are analysed using signal processing methods and then compared in order to clearly highlight the potentialities of these guided waves for the detection of inclusions in such samples.