Guideline

Ultimate Guide to Understanding Buried Vias in PCB

Buried Vias

PCBs are usually found in single-sided, double-sided and multilayers. Every layer of PCB is made from copper and connects one component to another component. In multilayer PCBs, there are a bunch of copper layers and every copper layer is separated from the dielectric material most commonly used is FR4. So, once the PCB uses dielectric material every layer of PCBs gets separated from each other. So, if you want to connect any layer to other layers you need to use vias. Vias are used to connect one layer to another layer. Vias are classified as through vias, micro vias, blind vias, buried vias., etc. Micro vias are divided into stacked vias and staggered vias. We will get the exposure of buried vias specifically in this article.

What are Buried Vias

Any vias which connect copper layers in multilayer PCB from the inner layer to the inner layer are called buried vias. For example, if we consider a 10-layer PCB board, the vias from Layer-4 to Layer-8 are called buried vias. In the buried vias, the holes are hidden inside the PCB, for this reason, it is called buried vias.

Connection of blind vias made from any outer layer to one of the inner layers. So the other side of blind vias is empty and used to hold the other component. Due to this process, the space on the opposite side of blind vias remains empty and board space is saved. But here in buried vias, the vias do not use the outer layer space as buried vias fall from the inner layer to the inner layer. So, in this case, buried vias save more space than blind vias.

The buried vias are used in more complex circuits than blind vias. Also buried vias require more time in manufacturing and are more expensive than regular PCBs. Buried vias are not produced at every manufacturing place as this process requires modern technology and too much attention in manufacturing. Generally, big players in PCB manufacturing are making buried vias PCBs. It also requires more time in production so customers have to keep some days to make it. It is not possible to manufacture buried vias in 1 to 2 days, it requires a minimum of 5 to 15 days to manufacture.

The Main Benefits of Buried Vias

Buried vias have many advantages in modern electronics. Some of them are stated below.

  1. Buried vias are not located on the outer body of the PCB, but underneath the PCB, for that reason buried vias consume less space on the PCB surface.
  2. The connection trace length of buried vias is very short compared to through vias. Since the connection trace length of the buried vias is shorter, the conductivity is higher than the regular through vias.
  3. Routing of buried via PCB is done in less space, so the space is increased and the layer count of PCB can also be increased.
  4. HDI (high-density interconnect) PCBs have many advantages of buried vias as high-speed signals work well in buried vias due to the shorter length of vias.

Buried Vias PCB

The Production Process of Buried Vias

Before understanding buried vias manufacturing we will understand the regular PCB manufacturing process in short as below.

  1. Create a circuitry image: After the designing of the PCB it transfers to manufacturing in Gerber format. The manufacturer first transfers the Gerber data to the board, there are two different processes used to do such tasks. 1. photo tooling and 2. direct imaging. In photo tooling, the image of the circuit is drawn on film and plotted on the board. Direct imaging is the laser imaging process. which directly draws circuitry onto the board.
  2. Creating an inner layer circuitry: After completion of photoresist. The film layer lies down to the core and passes through the ultraviolet rays or direct imaging onto the copper by laser. In both processes the circuitry area hardens and other areas remain soft. Then the soft area which is not protected by the photoresist will be removed in the etching process and all the inner layers are prepared by this method.
  3. Lamination of copper layers and drilling: After completing the etching process, lamination of all copper layers is stacked and made into a sandwich. After completing the lamination of the copper layer the drilling process runs on lamination. After drilling the holes are cleansed by chemicals and then the extra layer of copper forms into circuitry including the inner wall of the holes.
  4. Top and bottom layer circuitry: The same process of the inner layer repeats on the outer copper layers for creating a circuitry. Then an extra layer of copper forms onto the outer layers.
  5. Solder mask, silkscreen and finish: A solder mask layer will be applied on the board which protects the unwanted exposed area by nonconductive material. Then the reference designator is applied on the PCB surface in the form of ink which is called silkscreen. After that, the finish is applied to the exposed copper to protect the metal and assist the soldering during assembly.

These are the basic things of PCB production. More time-consuming production processes are required for special processes. Buried vias are also a special process of production that’s why it requires more time in production.

Now we will talk about the production process of buried vias.

Buried Vias Layers 2 to 6

As we discuss copper lamination in the above points, the blind vias require more lamination cycles to manufacture, which increases the time of production. If we want to get exposure to the buried via in the production process, we should make an example of any PCB which has buried vias. Here we use an 8-layer board where L2-6 and L1-2 are buried and blind vias as an example.

To produce this board, manufacturers make a multiple stack up of multiple layers, in this example. The laminate and drill sequences are as below.

  1. Lamination of layers 2 to 6: The manufacturers create a stack up of L2-6 and perform all the processes of PCB manufacturing.
  2. Drill the buried vias between layers 2 to 6: These buried vias are always done with a mechanical drill, it is better to plug these buried vias with conductive or non-conductive materials.
  3. Lamination of layers 1 to 8: Laminate all copper layers and again repeat all the production processes except masking and silk printing.
  4. Drill blind vias from layer 1 to layer 2: After lamination of all layers, we should drill the holes between layer 1 to layer 2, it is always done with a laser drill. Then, the PCBs have both blind and buried vias, so the blind vias L1 to L2 are drilled by laser drilling (laser drilling is usually done on lesser conductive layers and lesser PCB thickness) and after laser drilling through hole drilling performed on PCB, the buried and blind vias are formed in lamination cycles.

More complex boards have many sets of blind and buried vias. So, in those cases the lamination or layup count increases and if the lamination increases they require more time in production. Every bunch of lamination increases the time of production. That is the only reason the manufacturer gave the longer time for the production of buried or blind vias.

Application of Buried Vias

  1. Compact devices need PCBs with limited area.
  2. High-tech HDI boards generally use buried vias.
  3. PCBs with high signal integration.
  4. Very high-tech PCBs are used as the main control units of electronic devices.

Conclusion

If we compare blind vias to the buried vias then they both are very similar. The only difference is the blind vias connect the layers from outer to inner and the buried vias connect layer from inner to inner it is hidden into the PCB. The making cost of both vias is almost the same. So, as per the above content, we will understand the buried vias process and its manufacturing as well as its advantages and disadvantages.

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