Via

A board carries copper on several layers, and a track lives on exactly one of them. A via is the hole that lets a net cross between them. It is drilled through the board, plated on the inside with copper, and landed on a pad on each layer it joins.

Three numbers describe one, and each is a separate promise. The drill is the hole the fabricator makes. The pad is the copper ring, and the drill goes inside it. The annular ring is the copper left of that pad once the drill is subtracted, half the pad diameter minus half the drill. A fourth fact is its span, which says whether it runs the whole thickness of the board or only between two of the inner layers.

A via is more likely than any other feature to be built differently from how it was drawn, so those numbers get checked. Drills wander inside their tolerance, and a ring too thin to absorb that wander lets the barrel break out of the edge of its pad. The board still passes visual inspection and the connection still works on the bench, and then it goes intermittent in the field. A drill smaller than the fab’s smallest bit fails more quietly. The order is either rejected outright or the hole is silently upsized, and every clearance around it moves with it.

Three rules read a via, and they answer to two different authorities. hole-size and annular-width compare the drill and the ring against the loosest mainstream fabrication floor, so a finding means the board may not be manufacturable at all. netclass-via-drill compares the same drill against the size declared by the net’s own class, so a finding there means the board is buildable and is not what the design asked for. The same distinction runs through the geometric rules generally. track-width and copper-clearance ask what a process can hold, while netclass-track-width asks what the project said it wanted.

A component’s through-hole is a related but separate feature. A resistor’s leg sits in a hole drilled and plated the same way. The difference is that a via carries no part and exists only to change layers. hole-size covers vias alone today, because checking a component pad’s drill needs facts that the geometry sidecar does not carry yet.

None of this is visible in a netlist. Two revisions can agree on every connection and disagree on every via, because the graph does not know the copper has a position or a width.

The course teaches it in chapter 12, and the whole chapter is about it. What geometry can answer runs the three geometric rules over a board. A fourth kind of authority covers the fab-versus-net-class distinction on its own.