A Via is simply a copper plated hole that connects two or more copper layers. Vias are very useful for efficiently making complex multilayer PCBs, highly populated with electronic components over a small PCB area. There are different types of vias: Through via, Stub via, Blind via and Buried via. A Through via runs through all the copper layers (top to bottom) but does not connect to any of the internal layers. A Stub via is similar to a Through via but can connect to any internal layers. In this post, we shall briefly discuss Buried and Blind vias.
A Blind via connects an even number of layers. It connects top or bottom layer to a number of internal layers. Just as the name suggests (blind), it never goes all through the board and always have to start from either the bottom or the top layer.
Like a Blind via, Buried via also connects two or more even number of layers. The difference between Blind and Buried via is that Buried via always connects internal layers and never connects top or bottom layers. As its name suggests, it is always ‘buried’ inside the PCB layers.
There are various rules that apply to blind vias:
Blind vias must start from either the top or bottom layer.
They must cover an even number of layers. That is 2,4,6,…
Blind vias can never run through the whole PCB.
Blind vias cannot start or end at the core or the center of the PCB substrate.
Blind vias are made by first making the vias in the center cores. The via is then progressed to the outward PCB layers. The PCB Layers are then stacked together with the help of a prepreg.
How to make Buried Vias
Buried vias are similar to blind vias only that, buried vias should remain buried. They do not touch any external layer. Blind vias are only possible for a PCB with 6 or more PCB Layers.
Vias may significantly increase the cost of a PCB. They are only used when it is absolutely necessary. To make a Buried via, the central layers involved are stacked up as usual after the required traces are made. The vias are then made and tested. After this step, the outer PCB layers are then stacked up to make a complete PCB with the Buried vias sandwiched in between.