Milling or routing is a technique performed on a CNC machine with a routing tool. Routing is used to form the contour of the PCB at the very end of the production process when we separate the individual PCBs from the production panel.
What we here mean with “milling” is a form of routing performed inside the PCB. This can be required to cut out the contour of a component, or to make room for any mechanical connection or for making slots, either NPTH or PTH in the board.
The standard router is 2mm in diameter. Smaller diameters up to 0.5mm are possible but have cost implications.
How this is done in our production can be viewed here.