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Vias in PCB boards

  • 2019-01-25 11:17:03

For a complicated Printed Circuit Board (PCB) such as the motherboard of a computer, you are likely to find several tracks going nowhere, and terminated rather abruptly. In fact, in PCB terminology, such an arrangement that allows tracks to continue, but on a different layer, are known as a PCB Vias. This blog is about to tell you the types of the Vias.



Fig.1: Close-Up of a Via

Types of Via in PCB

Multilayer PCBs use different types of Vias for various purposes, which is classified by the layers of origin and termination.



Fig.2: Typical types of Vias


1.    Through-hole via: a via originating from the outermost layer, traveling through the board, and terminating at the other outermost layer. In its passage through the layers of the board, it may or may not connect to intermediate layers, depending on the necessity of the electrical circuit.
2.    Blind via: connects the external layer to one or more inner layers of the PCB and is responsible for the interconnection between that top layer and the inner layers.
3.    Buried via: only the inner layers of the board are connected by the via. It is "buried" inside the board and not visible from the outside.
Among the three types Vias, through-hole Vias are the typical type we use in the regular multilayer PCBs. But for the high density interconnect or HDI boards, it is different. For the HDI boards, Vias are required to increasing the layout density and reduce the size or the number of layers on a multilayer PCB. This is what we can’t do with a through-hole.
Therefore, we usually use blind and buried Vias for the special design of the HDI boards. And in some situation, we will recommend using laser drilled micro Vias or stacked Vias though it might increase the cost compared with the mechanical drilling.

1.Laser drilling: extremely fine holes (sub 0.004”) which vaporizes the top copper layer, burns through the substrate dielectric layer beneath it, and stops when it touches the bottom copper layer.



Fig.3: Laser-Drilled Micro Via


2.    Stacked Vias: Vias are directly on top of each other. A stacked via is a way to further improve size and density considerations when manufacturing printed circuit boards — factors that are extremely important with today's miniaturization and high-signal transmission speed requirements in many applications.

Fig4: stacked via

Conclusion
The above types of Vias are the typical used in the pcb board design. The special requirement for the typical usage of the board decides the types of the Vias. So for the designer, before they start to layout, they have to get know of these basic requirement.

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