Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

I've done some home-brew 4-layer boards, but without plated-through
holes. It's tricky, you need to design for it, leaving plenty of room
on the outside layers where you want to connect to an inner layer, so
you can drill a hole big enough to stick your soldering iron down into.
It's not the most reliable results, either, but for some throw-away
boards it's the best solution.

For professional boards, fabrication involves at least one drill step
that drills all the way through all the layers, then a chemical bath
removes the smeared epoxy from the drill bit, exposing the edges of the
copper for the inner layers. When you then electroplate it, it makes
electrical connections between the layers.