Transient Thermal Analysis as In-Situ Method in Accelerated Stress Tests to Access Package Integrity of LEDs

A combined and accelerated test system (high/low temperature operation live combined with temperature cycle) for In-Situ Transient Thermal Analysis (TTA) was developed. The transient thermal impedance Z _th (t) is measured at low and high temperature during temperature cycling and high current operation. The test system is demonstrated using high power LEDs on Al-IMS as devices. Spontaneous changes and slow degradation processes in the thermal path of the LEDs due to delamination and crack formation can be observed and separated. The signal quality of the In-Situ tester is benchmarked with standard TTA equipment and is able to measure the thermal signal accurately after 20μs. Due to the temperature dependency of the thermal capacity of the sapphire (Al ₂ O ₃ ceramic) the rise of the Z _th (t) curves shifts with increased case temperature. The effect is reproduced by transient thermal finite element simulations. Also changes in the thermal path are analysed by the FE simulations.