Nondestructive defect detection in multilayer ceramic capacitors using an improved digital speckle correlation method with wavelet packet noise reduction processing

The nondestructive detection of defects in multilayer ceramic capacitors (MLCs) in-surface mount printed circuit board assemblies has been demonstrated by using an improved digital speckle correlation method (DSCM). The internal cracks in MLCs that contribute to the thermal displacements on the MLC surface after dc electrical loading may be uniquely identified using this improved DSCM combined with double lens optical arrangement. However, it is found that Joule heating of the MLC sample takes time, and therefore the thermal displacements on the MLC surface are not obvious at the beginning of the dc electrical loading. In order to shorten the detection time and increase the resolution of the DSCM, a wavelet packet noise reduction process is introduced into the DSCM technique. This new algorithm is used to reduce the background noise in the signal so as to improve the accuracy of detection of defect locations and reduce the detection time. By introducing wavelet packet noise reduction processing, the DSCM is found to be more sensitive to and faster at detecting defects in MLC samples. Furthermore, the DSCM with wavelet packet noise reduction process can cope with the problems of edge effect, rough and warped surface, which are the limitations of the scanning acoustic microscope (SAM).