In PCBs with two or more than two layers the signals are path-switched from one layer to another by using s a via. A via is a small round conductive object connected to at least two conductive layers. A via starting from one layer has circular conductive ring, called annular ring, that provides support to the vias as well as connects it to the respective layer. Both ends of the via are connected through a thin long cylindrical ring of copper which calls via barrel. A via that starts from-and-ends to the outer layers is called a through hole via. A through hole via with conductive barrel is called a plated through hole (PTH) via, and without conductive barrel is called non-plated through hole (NPTH) via. An NPTH via should filled with a thin wire and then soldered on both ends to the respective layers. In PCBs which have large number of components, high pin count and signals density other types of vias are also used, namely the blind via, microvia and buried via. A via that starts from any of the outer layer, top or bottom layer, and ends on any of the inner layer of the PCB is called a blind via. A via which starts and ends on any of the inner layers is called a buried via. In figure 1 different types of vias are shown in a PCB, 1 is a through hole via, 2 is a blind via and 3 is a buried via.
Figure 1: Through hole, blind and buried via types
In figure 2 the vias size definition are explained.
The diameter of a via as shown in figure 2 i.e. diameter of the barrel does matter from placement and routing point of view for very dense PCBs as well as for current carrying capability and also for manufacturing facility of the manufacturers. However, on an average, a 0.15mm via hole size and 0.5mm annular ring diameter are common. The via hole diameter with less than 01.5 mm diameter hole is called a microvia. The stitching vias which are used to make multiple parallel paths for high current, in ground or power supply rails, which are not different in terms of geometry or manufacturing but they are different in usage. They also help to conduct heat from one layer to another. By manufacturing point of view, there are some starting and ending layer limitations in buried and blind vias formation. It means it is not possible to start a buried and blind via from any of the layer due to technological limitation. So, before stating a PCB layout the manufacturer should be consulted for manufacturing capabilities of via diameter and annular ring size, type, start and end layer of via and PTH/ NPTH etc.