Enhancing Defect Detection Using Lock In Thermography

The paper establishes foundational work for lock-in thermography as an in-line quality inspection tool during the production of power electronic devices. It focuses thereby on the fast, reliable, and unequivocal detection of production defects, which reveal themselves through a characteristic transient thermal signature and the analysis of phase and amplitude images. The experimental and finite element analyses were performed at several excitation frequencies and different numbers of cycles for excitation. A finite element model was used to make a comprehensive comparison between simulated outcomes and experimental results done with lock-in thermography (LIT). Different postprocessing techniques are applied to the thermal images to extract the amplitude and phase, such as the Fast Fourier Transform (FFT), the Digital Lock-In Correlation Method (DLCM), and the Four Points Method (4PM). The finite element analysis and experimental results both successfully detected the subsurface defects. The amplitude and phase values were found to be remain constant at specific values without increasing after a number of cycles.