Blind vias and Buried Vias

A printed circuit board (PCB) is made up of layers of copper foil circuits stacked on top of each other, and the connections between different circuit layers rely on vias. The principle behind this is similar to the connectivity of multi-layer underground water channels. Friends who have played Super Mario Bros. may understand the road of pipe connectivity, but the difference is that pipes allow water to flow, while the purpose of PCB connectivity is for electrical conductivity, hence the name "via." However, if the holes drilled by a drilling machine or laser are left as is, they will not conduct electricity, because the surface of the original drilled hole is only resin, which is not conductive. Therefore, a layer of conductive material (usually copper) must be plated on the surface of the drilled hole. In this way, electrons can move between different copper foil layers.

Now that we have a general understanding of the purpose and principles of vias, let's take a look at several different types of vias commonly seen on PCBs. 

There are three types of through-hole vias that we often see, as follows:

• Plating Through Hole, referred to as PTH

This is the most common type of via, and you can identify it by holding the PCB up to a light source,the holes that allow light to pass through are the plating through holes. This is also the simplest type of via to produce, as it can be made by drilling or laser-cutting holes directly into the circuit board, and is relatively inexpensive. However, in some cases, certain layers of the circuit board may not require connections through these PTHs. For example, if I bought the third and fourth floors of a six-story building and wanted to design an internal staircase connecting only the third and fourth floors, the space on the fourth floor would be wasted by the staircase connecting the first floor to the sixth floor. Therefore, while PTHs are inexpensive, they can sometimes take up additional space on the PCB.

Vias are also divided into PTH and NPTH.

• Blind Via Hole, referred to as BVH

Blind via are used to connect the outer layer of PCB circuits to adjacent inner layers via electroplated holes. Because the opposite side cannot be seen, it is called a blind via. This manufacturing process is used to increase the utilization of space for PCB circuit layers. However, special attention must be paid to the depth (Z-axis) of the drilling, as it can often cause difficulties with electroplating the inner space of holes and thus is almost no longer used by manufacturers. It is also possible to drill the necessary connecting holes in individual circuit layers during the manufacturing process and then bond them together at the end. For example, in a 2+4+2 layer board,you can drill the outermost two layers first,or the 2+4 layers can be drilled simultaneously, but this requires more precise positioning and alignment equipment.Take the example of buying a building above as an example. In a six-story house, there are only stairs connecting the first floor to the second floor, or connecting the fifth floor to the sixth floor, which are called blind via.

• Buried Via Hole,referred to as BVH

Buried via holes connect two or more inner layers without reaching the outer layer. This process cannot be achieved by drilling after bonding, and requires partial bonding of the inner layers, followed by electroplating treatment, before final bonding can be completed. It is more time-consuming and expensive than making plating through holes or blind via, and is typically used for high-density (HDI) circuit boards to increase available space for other circuit layers. Using the analogy of buying a building, when only the stairs between the third and fourth floors are connected in a six-story building, it is called buried via holes. Buried via holes cannot be seen from the appearance of the board and are actually located in the inner layers of the circuit board. 

IPC standards recommend that the diameter of blind vias and buried vias should not exceed 6 mils (150μm).

How to Manufacture Blind/Buried Via Hole

Blind and buried vias can be manufactured in two ways: before or after multilayer lamination process. When creating blind and buried vias,one or more cores are drilled and plated through, and then the stack-up is built and compressed. More information about the manufacturing process can be found in the IPC-2221B handbook.

When creating blind vias, attention must be paid to the drilling depth.Holes that are too deep or too shallow can both have negative impacts.If the holes are too deep, signal distortion or attenuation may occur, while if they are too shallow, poor connections may result.

It is best to consult with your PCB manufacturer in advance for advice on the best practices for designing the circuit board to avoid unnecessary costs.

The Advantage of Blind/Buried Via

They can help you meet the high-density constraints of typical designs for traces and pads without increasing the total number of layers or the board size.

The Disadvantage of Blind/Buried Via Hole

The main issue with blind and buried vias is cost. Both two processing methods can significantly increase the cost of the circuit board. The reasons for cost increase are more complex board designs, more manufacturing steps, and more frequent testing and precision checks.

Considerations When Design Blind/Buried Via

When placing vias in PCB design, please pay attention to the following:

• These vias must always span an even number of copper layers.

• These vias cannot terminate on the top side of a core.

• These vias cannot start on the bottom side of a core.

• Blind or buried vias cannot start or end inside another blind or buried via unless one is completely enclosed within the other via. (This will increase additional costs as extra printing cycles are required).

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