Laser Stimulated Transient Thermal Analysis of Semiconductors

In this paper a new measurement method for the transient thermal analysis (TTA), used to determine the transient thermal impedance of power semiconductors such as MOSFETs and LEDs, is proposed. The standard TTA measurement method is generally utilized for assessment of semiconductor interconnects, but shows deficiencies in resolving defects located at layers close to the junction, like the die attach of a LED. This is due to electrical disturbances of the pulsed thermal excitation occurring in the relevant time domain below 10µs. For this reason the proposed method of the laser stimulated transient thermal analysis (LTTA) relies on an external heating process of the device under test via a pulsed laser, allowing for undisturbed and immediate transitions of the thermal excitation. For experimental validation of the measurement method, a test-group of three high power LEDs differing in their internal structures are soldered with two different lead free pastes (SnAgCu305 and SnAgCu107) onto Aluminum Insulated Metal Substrate (Al-IMS) boards and thermal cycled. After 100 temperature cycles with an extended range from -55°C to + 150°C, the time resolved thermal impedances obtained from the standard TTA and the new LTTA method are compared. The results of both methods show good compliance for the thermal impedance in the time domain above 50µs, whereas only the laser stimulated method is able to also provide information even below 1µs.