Lock-in Thermography in combination with spectral phase shift analysis provides a capability for non-destructive 3D localization of resistive defects in packaged and multi stacked die devices. In this paper a novel post processing approach will be presented allowing a significant reduction of measurement time by factor >5 in comparison to the standard measurement routine. The feasibility of the approach is demonstrated on a specific test specimen made from ideal homogenous and opaque material and furthermore on a packaged hall sensor device. Within the case studies the results of multiple single LIT measurements were compared with the new multi harmonics data analysis approach.