Just as an FYI – I do not represent Sierra Circuits, nor have I ever been in their shop. The “laser” example I was referring to is from a shop that used to be in Southern California about 30 minutes from my house.
It looks like Lucy is going to ask and find out for you. Good luck!
Maybe what I posted here will help Optimizing VIA Drill Pairs for Multilayer PCB - #4 by steve.carney
Hey Jim – they’ll often not check via to via – it’ll more be net to net.
“Worst case is that one stitching via out of 20 is a stub, rather than a connection.”
Let’s talk a buried via for a moment. It’ll be mechanically drilled. When it comes out of the drill/route process, it’ll have a bunch of holes in it. But how many nets can be spotted at this point? The answer is none. It’s just two sheets of copper surrounding some fiberglass with some holes in it. It will be imaged and then all the nets will be plated up and then the entire panel goes into an acid bath where everything is etched away. The nets and vias lose material during etching too – but since the nets and vias were plated up, they survive the etching process.
Off on the edge of the board, the fabricators made some extra vias that they’ll cut out of the board, encapsulate in epoxy, and then sand down and inspect. If that coupon passes, and the flying probe, and AOI test pass, then the board passes. Unless you’re paying for x-ray inspection, there’s no way to really know if a single via failed. Either the board passes all the tests, or it’s rejected and “X” out of the panel. Individual vias aren’t inspected, individual nets are.
“And how is “do this to the two-layer board, then later to the full 4 layers” any worse than having both plated and non-plated through-holes?”
When you put in buried vias, you add a lot more steps that don’t exist when you don’t put in buried vias. You pay more for that.
If you’re near a board shop – give 'em a call and see if you can schedule a visit!
Apologies for my misunderstanding.