Self alignment of flip chip LEDs on PCB for high position accuracy

Abstract

Recent progress in wafer level packaging technologies has enabled high power flip chip (FC-LEDs) for direct assembly on printed circuit boards (PCB) for general and automotive lighting applications. Based on LEDs with a light emitting area of 0.5mm2 arrays for miniaturized modules are realized. The individual LEDs are single addressable, the pitch is solely 790µm and the gap between the light emitting areas down to 90µm. This is challenging the PCB design but due to progress of PCB manufacturing technologies realistic designs can be realized. The primary and/or secondary optics for the LEDs are assembled using mechanical features like drilled holes. To enable passive alignment between the LED surfaces and optical elements the position tolerance between the LED light emitting area with respect to mechanical alignment structures like drilled holes requires an accuracy around 50–100µm (application dependent). In the field of optoelectronics self alignment of laser and photo diodes utilizing the surface tension of the liquid solder on silicon or ceramic substrates is standard and alignment accuracy in the range of 1µm can be reached. However on PCB self-alignment evan in the range of 10µm gets challenging. In the paper, the post soldering tolerance of the LEDs with respect to the solder mask, the electrical layer and drilling holes is investigated. Solder pad design, stencil thickness and design (solder volume) was varied and the post soldering accuracy measured. Standard type 5 paste and special is used. The accuracy in plane (xy) and tilt was measured using a 3D digital microscope before and after reflow. The tilt of the FC-LED was observed to be an important root cause for xy-tolerances of the LEDs. The solder volume was reduced until the amount of solder was too small for effective self alignment. Using well designed solder pads and solder volume the tilt of the components is reduced to acceptable 1–2° and a position accuracy (xy) in the range of 5µm (standard derivation) was reached.