After downloading Craig Thierren’s SOLIDWORKS World presentation on “Understanding Large Assemblies and Drawings“, you may have decided to create more sub-assemblies in your large assembly file for the purpose of improving performance on your computer. The performance improves because the mates in a sub-assembly are not solved during a rebuild unless necessary, unlike mates for top-level components that get solved every time the assembly is rebuilt. Less mates to solve upon rebuild = less time to rebuild = more happiness in your life.
However, when creating the Bill of Materials (BOM) for the top-level assembly, you are not really interested in listing the sub-assembly in the BOM, since it was only created to improve performance and not as part of your manufacturing workflow. You have 2 options to remove the sub-assembly and promote the components in the BOM:
1) After creating the BOM in the drawing, display the assembly structure column by clicking on the left-side handle of the BOM (has the three arrows), right-click in the sub-assembly cell and choose ‘Component Options’. Under ‘Child Component’, you can Promote the child components of the sub-assembly, which removes the sub-assembly structure from the BOM table and promotes the child components to the parent level.
2) In the sub-assembly itself, pull up the properties of the configuration you will use in the drawing and choose to promote the child components of the assembly when it is used as a sub-assembly. The benefit of this option is that it will automatically promote the child components in the BOM and it will behave this way in any drawing.
See the help file for more details and enjoy!