Investigation of Thermomechanical Local Stress Induced in Assembled GaN LEDs

Bonding of LEDs on substrates by soldering or sintering induce stresses in the LEDs. Raman spectroscopy is employed to investigate the local stress. Nine blue emitting gallium nitride (GaN) LEDs were first investigated at room temperature before the bonding process. Then, two LEDs were reflow soldered on Al-IMS with ENIG metallization and four LEDs were soldered on Cu substrate using 25 µm AuSn preform. Three LEDs were sintered under pressureless sintering conditions on Cu substrate using an Ag sinter paste. All assemblies were investigated between -50 °C and 180 °C. The change in position of the GaN Raman band E2(high) at ca. 568 cm-1 was used to characterise the thermomechanical stress. The obtained values are in the MPa order of magnitude and allow us to conclude that the dielectric layer of the Al-IMS relaxes the stress very efficiently. Moreover, the samples prepared via Ag-sintering show a reduced stress, which indicate that the stress is relaxed by the soft and ductile property of silver.