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Accurate design of solder joints for passive alignment of a vertical emitting laser to a planar polymeric waveguide via a 45-degree mirror

We investigate a self-aligning method used to couple a vertical emitting laser (flip-chip) to a planar single-mode waveguide through a 45° mirror based upon solder self-alignment. The alignment tolerances to achieve targeted coupling loss of 3.5dB or better were determined for all axes by modeling the optical behavior of the vertical waveguide/45° mirror interconnect. Simulation models for optical design are carried out using commercial software "FullWAVE" from Rsoft, Inc. The design and optimization of the joint's parameters are performed using a public-domain software "Surface Evolver" for surface energy minimization in conjunction with GE proprietary Six Sigma regression and optimization tools. The parameters that were considered for the models included the misalignment along all axes, solder volume, the height of the joints, the radius of the metallization pads, the initial placement error, and the solder reflow time. It has been assumed that intermetallic growth at the solder/metallization/air triple line does not couple with the self-alignment process in order to simplify this problem. The study shows that, given good control over noise parameters such as vibration and reflow temperature fluctuations, solder self-alignment can be harnessed to achieve the targeted coupled efficiency.