Sub-Resolution Assist Features (SRAFs) have been extensively used to improve the process margin for isolated and semi-isolated features. It has been shown that compared to rule-based SRAFs, model-based placement of SRAFs can result in better overall process window. Various model-based approaches have been reported to affect SRAF placements. Even with model-based solutions, the complexity of two-dimensional layouts results in SRAF placement conflicts, producing numerous challenges to optimal SRAF placement for each pattern configuration. Recently, pixelated source in optical lithography has become the subject of increased exploration to enable 22/20 nm technology nodes and beyond. Optimization of the illumination shape, including free-form pixelated sources, has shown performance gains, compared to standard source shapes. This paper will demonstrate the influence of such different free-form sources as well as conventional sources on model-based SRAF placement.