PCB - ML (Multi layer PCB)

A multi layer PCB is much more expensive and difficult to manufacture than a single or double sided board, but it really does give you a lot of extra density to route power and signal tracks. By having your signals running on the inside of your board, you can pack your components more tightly on your board to give you a more compact design.

Deciding to go from double sided to multi layer can be a big decision, so make sure that a multi layer board is warranted on the grounds of board size and complexity. You can forget about making multi layer boards yourself, it requires a commercial manufacturer. Most of the hobby board suppliers will not do multi layer boards. Multi layer boards come in even number of layers. With 4, 6, and 8 layer being the most common. You can go many layers above this, but now you are in the realm of the very specialised. Technically you can get an odd number of layers manufactured, like a 3 layer board for instance. But it really won’t save you any cost over a 4 layer board. In fact a 3 layer board might even be more expensive than a 4 layer board because it calls for a non-standard manufacturing process.

If you decide to go multi layer then make sure you make use all of your layers, there is no point leaving one completely blank. With a multi layer board, you would typically dedicate one complete layer to a ground plane, and another to your power. With perhaps a few signal tracks thrown on the power layer if you need to. If you have a digital only board, then you’d often dedicate the entire power layer also. If you have room on the top or bottom layer, you can route any additional power rail tracks on there. Power layers are almost always in the middle of the board, with the ground closer to the top layer.

Once you have your power taken care of on the inner layers, you’ll be surprised at the room you now have available for your signal tracks. It really does open up a whole new dimension to routing.

If power planes are vital, and you have a lot of connections to route, then you may have to move from 4 to 6 layers. Six layers will give you four full signal routing layers and two layers dedicated to power. You can really do some advanced routing with 6 layers. Eight layers and above is basically more of the same.

With multi layer design comes the options of using different types of vias to improve your routing density. There are three types of vias - standard, blind, and buried. Standard vias go through the whole board, and can connect any of the top, bottom or inner layers. These can be wasteful of space on layers which aren’t connected. "Blind" vias go from the outside surface to one of the inner layers only. The hole does not protrude through the other side of the board. The via is in effect "blind" from the other side of the board. "Buried" vias only connect two or more inner layers, with no hole being visible on the outside of the board. So the hole is completely buried inside your board. Blind and buried vias cost more to manufacture than standard vias. But they are very useful, and almost mandatory for very high density designs like those involving Ball Grid Array (BGA) components.