Analysis of new direct on PCB board attached high power flip-chip LEDs

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Abstract
Recent progress in wafer level packaging technologies has enabled high power Flip Chip (FC-LEDs), Chip Scale Package (CSP-LEDs) and Wafer Level LED Packages (WLP-LEDs) for direct assembly on printed circuit boards (PCB) for lighting application. The LEDs can be soldered with a SMT compatible reflow process on PCB. Packaging simplicity and flexibility, increased position accuracy, low thermal resistance and reduced cost has been the main motivation for these developments. In addition, the small chip sized LED package can enable on board level closer placement of the light emitting surfaces without the disturbing wire bond. This is a benefit, e.g. for optical design of LED matrix applications, in which multiple high power light pixels are switched individually and sharp contrast is required, as for automotive front lighting for instance. One major concern and bottleneck for high power FCLED attachment directly on PCB had been the mismatch between the coefficient of thermal expansion of the semiconductor material (GaN, InGaN) of the chip and the PCB. Adequate overall mechanical design of the FC-LED had to be developed. Also the electrical redistribution layer on the FC-LED is crucial for performance and reliability of FCLEDs. The reduction of one package level, i.e. the submount which functions also as heat spreader impacts the thermal performance. In the paper we evaluate potential of architectures based on the new WLP-LEDs and benchmark them with standard chip on board concepts and modules based on ceramic and lead frame packages. The requirements for the printed circuit boards (PCB) are investigated regarding electrical routing and thermal performance. Prototypes are assembled and the thermo-mechanical reliability of the FC-LED is investigated by temperature cycle tests.

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