Special Solder Mask/Stencil Issues
Bumps are attached to the openings defined by the UBM mask. However because we are "slicing" the UBM geometries at the boundary, there will be many instances of a partial round opening. We do not wish to try to attach a bump to such an opening as it may come off. So when we build our solder stencil, we want to make sure that no solder paste is screened over any partial UBM opening.
We achieve this in HEXtract by using the -drop_partial_poly command line option when producing the solder mask. Below you can see an overlay of the solder bump layer and the UBM layer. Notice that there are no solder bumps (blue) over the partial UBM openings (magenta)