The Multilayer Printed Circuits as produced at Elite Circuits Ltd usually require:
(1) Inner layers to be printed and etched.
(2) Remaining copper to be chemically blackened to enhance adhesion during bonding.
(3) Layers to be bonded together with outer layer copper.
(4) Bonded panels to be drilled.
(5) Drilled panels to be treated to desmear copper connections in holes.
(6) Desmeared panels to be processed as per normal D/S plated through hole.
There are exceptions to this sequence, most notably multilayer designs requiring buried vias.
The picture top right shows a cross section of a one mm hole through a four layer multilayer with the top and bottom pad annulus, and the bottom inner layer layer connection. Points to note include the evenness and thickness of the through hole plate and the excellence of the inner layer connection.