It has been shown that microscopic Lock-in-Thermography (LiT) can be used for localization of electrical active defects like shorts and resistive opens in integrated circuits. This paper deals with the application of LiT for non-destructive failure analysis of fully packaged single and multi chip devices. In this case inner hot spots generated by the electrical defects typically can not be imaged directly because the mold compound or adhesives above are not IR transparent. Inner hot spots can only be detected by measuring the corresponded temperature field at the device surface. By means of failed and test devices will be shown, that LiT is sensitive enough to measure such temperature fields. In addition to the lateral localization of inner hot spots its depth can also be determined by measuring the phase shift between the electrical excitation and the thermal response at the device surface. Furthermore, the influence of the lock-in-frequency and mold compound thickness to lateral resolution and signal to noise ratio will be discussed. Using real failed single chip and stacked die devices two analysis flows were demonstrated to locate inner defects.