The key to understanding blind and buried vias is the lamination stackup. Careful consideration must be given during the design layout phase, as to how the board will be laminated, drilled and plated, and in particular, in what sequence. Other critical factors include: via hole size, via pad size, trace and space line widths and copper weights, both on the inner layer surface and inside the barrel of the via hole.
A blind via is a hole on a circuit board that is only visible on one side of the circuit board (as opposed to a thru-hole via which goes all the way through the board). Because it only appears on one side of the circuit board, it must be laminated, drilled and plated in separate steps. This is why blind and buried vias cost extra.
A buried via is like a blind via, however, it is NOT visible on either the top or bottom of the circuit board. Again, this requires multiple steps in lamination, drilling and plating. In the example below, the buried vias hooking up layers 2 & 3, requires that inner layer core to be drilled and plated, prior to lamination into the final stack. The same applies for layers 4 and 5.
As you can see from these simple examples, the complexity of the design grows exponentially with the number of layers and number of blind and/or buried vias you add to your design. There are definite physical limits to how many blind vias can be used to hookup different layers, especially if you are using fine traces and heavy copper weights.
PLEASE CONSULT WITH YOUR BOARD FABRICATOR BEFORE, DURING AND AFTER THE DESIGN IS COMPLETE!
You may save yourself countless hours of design time!
We have seen many customers paint themselves into a corner, designing a board that is virtually un-manufacturable or one that is so costly to manufacture that it renders the design useless. Don't fall into this trap!
If you have any questions, please give us a call! We would be happy to help with your next blind & buried via circuit board!