Several advanced fabrication technologies can be used to reduce the number of layers required to route a design, although each of these technologies increase fabrications costs of the board:
Blind Vias (+20% to +40% fabrication cost)
As opposed to a through-hole via, a blind via does not travel from the top layer to the bottom layer. A blind via travels either from the top or bottom layer to an inner signal layer, freeing up room above or below for other routing.
Buried Vias (+25% to +60% fabrication cost)
A buried via is located entirely inside the printed circuit board and does not touch the top or bottom layers.
Via-in-Pad (+10% to +20% fabrication cost)
Via-in-pad technology is where a via is placed directly over a BGA pad, reducing the need todog-bone the signal trace on to the top or bottom layer. This allows for easier escape routing under the BGA as the signal can travel directly down to another layer from the pad. In addition, the impedance of the signal is better improved by not having any portion of it on the top or bottom routing layers. The following figure illustrates the mechanical design of a Via-in-pad.