Solder self-alignment is one of the most important technologies for cost effective optoelectronics assembly. In this study, the wetting of Sn-rich solder to the metal pads of chip and substrate was identified as a critical factor significantly affecting self-alignment accuracy during the assembly. Insufficient wetting of solder to the metallization pads was responsible for large chip-to-substrate misalignment post-assembly, while fabrication deviations, such as solder volume variation and pad diameter deviation, only account for misalignments in the range of submicrons. To aid the design of flip-chip assemblies requiring high alignment accuracy, a force optimization model was developed and validated experimentally. With the input parameters of design and manufacturing process for optoelectronics flip-chip assembly using solders, such as insufficient solder metallurgical wetting areas, positions and diameters of metallization pads, volume of individual solder bump, coefficient of solder surface tension, mass of chip, external forces acting on chip, and initial pick-and-place position of chip before assembly, the model predicts the assembled position of the chip in terms of the misalignments in the X-Y planes and the rotation angles along the Z axis. The model further confirmed that insufficient wetting of solder is the most critical modulator among the undesirable factors affecting solder self-alignment accuracy.

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