Pulse IR thermography has been used for the detection of hidden flaws and cracks in macroscopic structures (e.g. turbine blades, propellers) for several years, but as IR cameras are becoming increasingly refined in their time, space and temperature resolution capabilities it is possible to transfer the method on to failure detection in microelectronic devices. It has been successfully employed for short circuit analysis in integrated circuits or for detection of voids and delaminations in BEOL and packaging materials and interconnects. The measurement principle is hereby always the same: The structure under test is excited by a transient thermal pulse before its thermal response over time is recorded by the IR camera. Different excitation modes exist to impart the energy pulse to the structure: One distinguishes between electrical, flashlight, laser-flash, eddy-current, ultrasonic and IR-beam excitation mode, where the method of choice depends on the structures on component, board or system level. The signal-to-noise ratio can be enhanced considerably by using a lock-in amplifier, allowing the detection of very small temperature differences generated actively or indirectly by the defects in the structure.

This paper focuses on the evaluation of best applicability and resolution of the different excitation modes for the detection of certain flaws prominent in microelectronic packages and devices. We have exemplified the method of pulse thermography on cracks in vias of organic boards, delaminations in adhesive die attach materials and in molding compounds of encapsulated packages. By trying various excitation modes, we have found that cracks in thermal vias can best be monitored by electrical stimulation, whereas large scale screening of imperfections on board or wafer level can be traced by single flashlight excitation. Flaws in materials with higher diffusivity can be found by laser excitation due to the possibility to introduce higher energy pulses. This method can be used in reflection and transmission mode, the latter being also capable of measuring thermal conductivity contactless at different temperatures without having to take a contact resistance into account (laser-flash method). Lock-in thermography was demonstrated on small hot-spots on die-level and periodically loaded mechanical structures exploiting the thermo-elastic effect. Thus mechanical stresses can be detected optically and evaluated quantitatively. Here, first measurement results will be presented. The measurements obtained by non-distructive IR thermography are compared to other failure-analytical techniques and finite element simulations in order to verify and explain the results where necessary.

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