When you want to move a signal that travels across a trace from one side of a PCB to another (say, from front to back), you can create a via. A via is a hole with its sides covered with copper or gold (or other conductive material), that allows a trace to continue its route across layers. Figure 12.6: Vias allow a trace to continue between layers. In Figure 12.6, you can see the two sides of the same PCB. On the left, the arrows point to two vias in the front of the PCB, and on the right, the circles indicate the same vias on the back of the PCB. Vias are very similar to through-hole pads, except that they don’t have any exposed copper (they are covered by the solder mask), and they don’t have a pad (so you can’t solder a component). In simple circuits with only a few components, it is possible to create all of the traces on one layer of the PCB. When a PCB gets busy with more components it quickly becomes impossible to do the routing on a single layer. When multiple layers are needed, vias provide the simplest method of allowing a trace to use the available board real estate. In the Figure 12.7 you can see the types of interconnections between layers that are possible.

Figure 12.7: Types of interconnections between layers.

For through-hole components, you would design a hole that connects the top and bottom copper layers. This hole is implemented using a drill. It is wide enough to allow for the pin of the component to go through it. Vias are smaller than holes in terms of their diameter. They are not wide enough for pins to go through them, but they are plated, like holes, and they allow for electrical connection between layers to take place. A 'through via' is like a hole, but narrower. It connects the top and bottom layers. A buried via is a via that connects any two internal layers. In the four- layer example of Figure 12.7, the buried via connects the In1.Cu and In2.Cu. A 'blind via', also connects two layers, but has one end exposed on to the outside of the board, either top or bottom. In high-density boards, another option for interconnecting layers is to use a 'micro via' (‘uvia’). A micro via is made using high-powered lasers, instead of a mechanical drill; the use of lasers makes it possible to dramatically reduce the diameter of the via.