With the growing variety, complexity and market share of 3D packaged devices, package level FA is also facing new challenges and higher demand. This paper presents Lock-In Thermography (LIT) for fully non-destructive 3D defect localization of electrical active defects. After a short introduction of the basic LIT theory, two slightly different approaches of LIT based 3D localization will be discussed based on two case studies. The first approach relies on package internal reference heat sources (e.g. I/O-diodes) on different die levels. The second approach makes use of calibrated 3D simulation software to yield the differentiation between die levels in 8 die µSD technology.