The Slot Cleaning (Desmear)
After the creation of a slot it must be cleaned.
This is to remove any particles that were left in them by the drilling process.
Cleaning is a chemical process, known as desmear, it also smooths the hole walls ready for the black hole treatment
During the cleaning a very small amount (less than a micron) of the hole wall is removed, thus increasing the slot size.
If the cleaning is not performed correctly then the black hole process may fail and so would the plating process.