MultiLayer PCBs

Multilayer PCBs (also known as ML-4, ML-6, ML-8, ML-10, ML-12 etc) have one or more conductor pattern inside the board, to increase the area available for the wiring. This is achieved by gluing (laminating) several double-sided circuit boards together with insulating layers in between.

The number of layers is referred to as the number of separate conductor patterns - usually even and including the two outer layers. Most boards have between 4 and 12 layers, but PCBs with almost 100 layers can be made.

Since the layers in a PCB are laminated together, it's often difficult to tell how many there are, but if you inspect the side of the board closely you might be able to count them.

Large super computers often contain boards with plenty of layers, but since it's becoming more efficient to replace these computers with clusters of ordinary PCs, high layer counts are now less common.

Vias in multilayer PCBs

The vias described in Double-sided PCBs always penetrate the whole board. When there are multiple layers of conductor patterns and you only want to connect some of them, vias waste space that could be used to route other wires. 'Buried ' and 'blind ' vias avoid this problem because they only penetrate as many layers as is necessary.

Blind vias connect one or more of the inner layers with one of the surface layers, without penetrating the whole board. Buried vias only connect inner layers so you can’t see them by looking at the surface of the PCB.

Classifying layers

In multi-layer PCBs whole layers are almost always dedicated to Ground and Power. So we classify the layers as Signal, Power or Ground planes. Sometimes there is more than one of both Power and Ground planes, especially if the different components on the PCB require different supply voltages.

Related Links:

• Single-sided/layer PCB
• Double-sided/layer PCB