Chip-on-Flexible Packaging for High-Power Flip-Chip Light-Emitting Diode by AuSn and SAC Soldering

This paper presents a new chip-on-flexible (COF) packaging structure for flip-chip (FC) light-emitting diode (LED). Two bonding methods, AuSn and Sn-Ag-Cu (SAC) soldering, were compared from thermal, mechanical, and optical performances of the packages. Compared with the package by AuSn soldering method, the package by SAC soldering method showed lower chip temperature due to the avoiding of the large unbounded area. In addition, the SAC soldering package showed a much lower residue stress than the AuSn soldering package. Thus, the possibility for the defects, such as the crack on the reflecting layer and the warping of the pads formation decreased significantly. The reflectivity of the flexible substrate showed a sharp decrease because of the high temperature of AuSn soldering process. Based on the comprehensive evaluation of the thermal properties, residue stress, and optical performances of the LED packages, SAC soldering method is suggested to be more suitable for high-power LED packaging on COF substrates.