What are some possible defects specific to underfills?
Filler settling solder ball cracking, streaking or fingering, and haloing are defects associated with underfills. Filler settling is separation within the underfill in which the smaller particles or resin-rich areas are closer to the die side. Solder ball cracking typically occurs to relieve stresses induced by CTE mismatch of the components in a flip chip package. Solder ball cracks are seen as missing or faded bumps on acoustic microscopy imaging equipment. Streaking or fingering is the separation of the filler during flow. Haloing is the separation of filler from the resin system around the solder bump.
