You maintain assembly scrap in the material master record, not in BOM maintenance.
Assembly scrap takes into account all faults that occur when the individual components of an assembly are assembled (in production).
This entry is ignored in BOMs if the material is not an assembly in the current BOM. The system automatically sets the Assembly indicator in a BOM (component overview) for components that have their own BOM.
Calculating Scrap
You can control how scrap is calculated as follows:
See also:
Assembly Scrap: Example Maintaining Assembly Scrap Overriding Assembly Scrap