Failure mode analysis for the solder joints for the flip-chip light emitting diodes

This paper investigated the shear performance and failure modes of the flip-chip LED packages by soldering method. The results of the experimental tests demonstrate that the solder joints on the ENIG surface finish show higher bonding strength than that on the Cu-OSP surface finish after reflow and isothermal aging. For the solder joints for the anodes on ENIG and Cu-OSP surface finishes, the LED chip and the solder interface near the LED chip are the weakest part. In contrast, the solder bulk showed the poorest strength and become the failure position for the solder joints for the cathodes. The Au layers at the diodes dissolved into the molten solder during reflow and led to the broken of the insulation layer and the interface between the insulation layer and the solder layer are prone to become the crack initiation position. An Au-rich layer can be observed near the cathode/solder layer interface after reflow. After long time aging, the Au-rich layer transformed into an interface.