4) A Hole is Not Just a Hole

A hole is not just a hole. You may remember that vias are holes between layers where electrons can move.

Looking closer, you can see that the hole is not just a hole, but the walls are plated with copper for electrons to move in.

Some of these holes need to have thick copper inside the walls to transport many electrons at the same time. It is always important that the copper thickness is even, otherwise we get bottlenecks where electrons have to slow down to go through, like passengers trying to get on to a busy escalator.

Let’s look at vias in an 8-layer PCB, seen from the side – you can see the 8 copper layers. Between them are grey layers of insulation, and the top and bottom are protected with green soldermask.

Vias come in different sizes. A via that goes through all layers, to the other side, is called a through hole.  A through hole takes up a lot of space on every layer, so usually a via will connect fewer layers. These are some versions of shorter vias.

• A blind via is drilled from the surface, but not all the way through.
• A buried via is a hole that never sees the air – it is drilled between inner layers only.
• A micro via is usually between 2 layers only, and very small, with a diameter of less than zero point fifteen millimeters (here’s a toothpick to compare, it’s much wider, 2 millimeters).

You can imagine that it’s difficult to plate a 0.15 mm via with copper, evenly. So microvias are kind of V-shaped, which makes it – not easy – but possible. It’s still hard though, and we have high demands on our factories.

Micro vias can go a bit deeper, but due to the slight V-shape we need them to be wider on top. Or we can stack them, or have them in stairs (called staggered via) to reach several layers. But we have to be careful; micro via can be vulnerable if we stack them on top of each other. So why not just drill all the way through? Because a through hole must be wider than a micro via or it’s impossible to plate good quality. And if it’s wider. it takes up more space. Also, a long thin hole is vulnerable – the slightest movement in one of the layers could break the plating. Remember, our PCBs must be able to function perfectly, even under quite unfriendly conditions.

One of a PCBs first challenges is to survive the manufacturing process, which is the subject of the next lesson.