But via holes, because of the need for metallization and for structural strength, have to be made quite massive in size.
Their thickness is of course measured in millimeters, but for high-density boards a few millimeters makes a huge difference, which is why hidden and deaf vias are now used in such cards.
Deaf vias are links between the outer layer of a multilayer printed circuit board and several layers below. In a sandwich analogy - imagine that in a sandwich with three loaves, a skewer would pass from the middle slice of bread and come out through the top one to the outside of the sandwich. There is access through one of the outer layers.
Plugged, deaf, or as they are also called "buried" and "hidden" holes are the via-holes between the inner layers of the printed circuit board. They are not accessible after installation.
The use of both types of holes is justifiable in cases where very dense wired interconnections are involved or for devices with very dense packaging of planar components on the outer layers of the board. But the more sophisticated the PCB and the device, the greater the need for such vias.