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 10 mu s. 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 degrees C to +150 degrees 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 50 mu s, whereas only the laser stimulated method is able to also provide
information even below 1 mu s.
