Application of 3-D FFT in laser ultrasonic NDT technique

Abstract

Laser ultrasonic NDT technique has drawn a lot of interests recently in various application areas. All advantages of traditional ultrasonic technique are inherited with additional non-contact and broadband generation characteristics. The principle of this NDT technique is using laser to generate broadband ultrasounds and then investigating the effect of any structural feature under inspection on the ultrasonic wave propagation. However, there are various structural features present in test samples, which will cause reflections of ultrasounds from different directions. Another important source of reflection is the sample boundaries. All these reflections usually are not directly related to the structural feature under inspection and will obscure the interactions with the real structural feature of interest. In order to highlight the real informative components in the received ultrasonic signals, it is desirable to remove useless reflection components and other signal components from received signals. This work proposed a 3-D fast Fourier transform(FFT) based algorithm to effectively remove useless signal components in laser ultrasonic signals. The implementation of the algorithm was demonstrated by processing an imaging signal from inspecting a ball grid array(BGA) chip using laser ultrasonic technique. The small chip dimension led to quick and strong reflections from four boundaries in acquired signals. The proposed method was proven to be able to effectively remove reflections and useless signal components and leave only the direct incident waves, which would make it easy to investigate the interaction between ultrasonic wave propagation and potential solder bump defects. The proposed method is a general signal processing method, which is not limited to the laser ultrasonic NDT technique.