Solder self-alignment is an important phenomenon enabling cost-effective optoelectronics assembly. In this study, the wetting of Sn-rich solder to under bump metallization (UBM) pads is identified as a critical factor affecting self-alignment accuracy. Incomplete wetting of solder to the metallization pads is responsible for chip-to-substrate misalignment larger than 1 μm, while fabrication tolerances, such as solder volume variation and pad diameter deviation, only account for misalignments in the submicron range. To quantitatively investigate the effect of incomplete wetting on self-alignment accuracy, a three-dimensional (3D) model based on a force optimization method was developed. With the input parameters of incomplete solder metallurgical wetting area, position and diameter of metallization pad, volume of individual solder bumps, coefficient of solder surface tension, mass of the chip, external forces acting on the chip, and initial pick-and-place position of the chip before assembly, the model predicts the assembled position of the chip in terms of the misalignments in the X-Y plane and the rotation angle along the Z axis. The model further confirmed that incomplete wetting of solder is the most critical modulator among the undesirable factors affecting solder self-alignment accuracy.