Many functional faults in integrated circuits (ICs), like oxide breakdowns or shorts generated by conducting particles, are associated with local heat sources, when the supply voltage is applied to the IC. Lock-in thermography allows a highly sensitive imaging of these faults with a spatial resolution down to 5 microns. Compared to previous microthermal investigation methods, like liquid crystal imaging and fluorescent microthermal imaging, the detection limit of lock-in thermography of bout 100 ľK is improved by a factor of 100. This high sensitivity of lock-in thermography may lead to a significant extension of the application of microthermal investigations in the failure analysis of ICs. The image shows local heat sources generated by an electrostatic discharge (ESD) test in a CMOS IC.