Infrared thermography has been recognized for its ability to investigate integrated circuits in a non destructive way. Coupled to lock-in correlation it has proven efficient in detecting thermal hot spots. Most of the state of the Art measurement systems are based on amplitude analysis. In this paper we propose to investigate weak thermal hot spots using the phase of infrared signals. We demonstrate that phase analysis is a formidable alternative to amplitude to detect small heat signatures. Finally, we apply our measurement platform and its detection method to the identification of stealthy hardware Trojans.