The paper will present an approach for non-destructive localization of thermal active defects at multi chip devices combining the Lock-in Thermography and following local X-Ray inspection. In combination of both methods inner defects in inter chip connections of complex device built ups can be found in a non-destructive way before opening the device. The methods were demonstrated at defective flip chip devices with a high ohmic daisy chain with lots of chip to chip contacts. Subsequently, cross section analysis at located high ohmic contacts was performed in order to find the root cause of the failure.