Guideline

The Ultimate Guide to Understanding Blind Vias in Printed Circuit Board

There are various types of vias in PCB, like Through vias, Blind vias, micro vias, buried vias, stacked vias, staggered vias, etc. All of these vias play an essential part in printed circuit board production. In this article, we will discuss blind vias specifically, including blind vias definition, classification, production processes, benefits, applications, etc.

What is Blind vias

Blind vias are the type of vias which allow connection from the top or bottom layer to any inner layer. It is used to connect the outer layer of the PCB to any inner layer of the PCB through plated holes. It is not visible to the other side of the printed circuit board. That’s why it is called blind vias. Mainly, blind via is used to minimize the board area as the vias are only on the top or bottom side and do not go to the other side of the PCB, so those areas are free from vias and minimize the board space as those places are used to place other components. The blind vias are used in complex and expensive boards as the cost of blind vias manufacturing is very high. The blind vias is one of the most complex processes in PCB manufacturing. Generally, it takes longer to manufacture than standard through-hole printed circuit boards.

Classifications of Blind Vias

There are different types of blind vias according to their production techniques: mechanical blind vias, stacked blind vais and stagged blind vias. Mechanical blind vias are drilled by normal drill machines, but stacked and staggered blind vias need to be drilled by laser. They always exist on high-density interconnect PCBs.

Mechanical blind vias: mechanical blind vias are blind vias drilled by mechanical machines rather than lasers. For example, a 6-layer PCB may have mechanical blind vias from layer 1 to layer 2, layer 1 to layer 3, layer 1 to layer 4, and layer 1 to layer 5.

Stacked blind vias: Stacked blind vias means the blind vias in different layers are connecting and stacked on each other. For example, a 12-layer HDI circuit board with blind vias from top layer to inner layer 1 and inner layer 1 to inner layer 2, and the blind vias in these two layers are connecting and stacking on each other, which formed blind vias from top layer to inner layer 2.

Staggered blind vias: Staggered blind vias always exist on HDI circuit boards with laser drills. For example, an 8-layer HDI printed circuit board with blind vias from layer 1 to 2 and layer 2 to layer 3. The blind vias in these two different layers are not connected with each other.

PCB Blind Vias

Production Process of Blind Vias

Production of any PCB requires many stages. In a PCB production factory, there are many stages that printed circuit board go through to complete production. Please refer to the following PCB production process from start to finish.

  1. Material Cut: First the material like FR4, 370HR, Rogers, Megtron, etc are cut into smaller dimensions, which are easier in production.
  2. Inner Layer Chemical Cleaning: Where the inner layer cleaning is done.
  3. Inner Layer Dry Film and Expose: Where the thin film is applied on the copper surface passes through the light and exposes the required copper area.
  4. Etching: Etching is the process where unwanted copper is removed from the PCB; once all the excess copper is removed only the required circuit remains on the printed circuit board.
  5. Layup and Lamination: After etching, all the layers joint each other and make a bunch of copper layers, and then press those layers and joints. As for blind vias production, several times of layup and lamnination may be needed.
  6. Panel Cutting: Once the lamination of the copper layer is ready, the router cuts the lamination panel as per the manufacturers’ standard panel sizes: 16 x 18, 18 x 24, 21 x 24, 21 x 26 inches, etc.
  7. Final drilling: After the layup is done with panel size, they go through the drilling process, and then drilled holes are inspected by the manufacturer. Please note: in blind vias PCB production, a mechanical drill or laser drill needs to be done to get the blind vias. The drill sequences are different in every printed circuit board.
  8. Out Layer Dry Film and Expose: After drilling the outer layer, dry film are applied on the surface, and the same process as the inner layer is repeated on the outer layer for exposure.
  9. Plating: Plating is the process where the panel is immersed into the fluid tank and creates a redox reaction, and the surface of the form will create a layer of 20 to 40 micro inches of chemical copper so that the hole wall creates electrical conductivity to connect with inner layers.
  10. Etching: exposed copper gets etched and unwanted copper is removed from the top and bottom copper layers.
  11. Solder Mask: Solder mask or solder resist is a thin lacquer like layer of polymer that is normally applied to the copper traces of printed circuit board for protection against oxidation and to prevent solder bridges to closely situated pads.
  12. Silk Screen: The most top layer of the PCB is the silk screen layer. It is used for indicating component designator. It required ink whose standard colour is white. But the other colours like black, yellow and red are also used in silkscreen.
  13. Surface Finish: The PCB surface finish is a protective layer of bare copper. It protects the circuitry from corrosion. It also provides a solderable surface for your component. There are different surface finish types like HASL, ENIG, Silver, ENEPIG, soft gold, Hard gold etc. All finishes are used as per requirement from the customer and board application.
  14. Routing: Routing is the last printed circuit board manufacturing process after completing all the manufacturing processes. The PCB board will be cut as per its shape and delivered to the customer.

All the above processes are regular PCB production processes with basic knowledge of printed circuit board manufacturing. More steps are applied to PCB production when preparing more complex boards and special processes like via fill, blind vias, buried vias, etc.

Blind Vias PCB

Example: 10-Layer Blind vias PCB Production Process

Blind vias production requires more lamination (layup) than regular PCB manufacturing. To understand the blind vias printed circuit board production process, we will consider the 10-layer board as an example where Layer-1 and Layer-5 are blind vias, and other holes are regular through holes. Now, as we discussed in the above PCB manufacturing process, first, all inner layers are exposed and etched, then create a bunch of all layers that is layup or lamination. So, here in blind vias, PCB is processed on different lamination cycles.

First, lamination 1 is created from L-1 to L-5 and the drilling process of blind vias is done on lamination 1. Then, blind vias are filled with epoxy (blind vias are always filled with epoxy). Then, another lamination is formed by L6 to L10. After completing both lamination cycles, they will merge and do hole drilling, then plating and etching and apply finish on it. So, the main purpose of explaining the whole PCB manufacturing process is to explain the different lamination cycles. In different lamination cycles, the manufacturing consumes more time than regular printed circuit board manufacturing. Generally, blind vias PCB manufacturing process requires double the time of regular PCB manufacturing. That is why many manufacturers provide long lead times for blind vias printed circuit board manufacturing.

10 Layers Blind vias PCB

Benefits of Blind vias

Compared to through via, blind vias has many advantages.

  • Minimize the board area.
  • Electrical connection is found to be good compared to through-hole vias as it is short in length.
  • Blind vias reduce layer counts as the routing density is very high. They reduce the consumption of space and layer count.
  • High-density interconnect PCBs found great benefit from blind vias. The high-density interconnect board has high-speed signals and a complex design in less space.

Application of Blind vias

  • Used in the PCBs which need to be small in size.
  • Used in high density circuit boards.
  • Used in high layer count circuit boards.
  • Used in PCB with High-speed signal integration.
  • Used in Electronic devices with complex functions.

Comparison between blind vias and buried vias

  • Blind vias connect the outer layer of boards to one of the inner layers, while buried vias connect the inner layer to the inner layers.
  • Blind vias are not shown on the other side of the PCB; that is why they are called blind vias. Buried vias are not shown on the printed circuit board; that is why they are called buried vias.
  • Blind vias take some part of the outer layer from the PCB, but buried vias are totally hidden under the PCB and save space from the outer layers.
  • The manufacturing costs of blind and buried vias are similar. The costs depend on the structure of the printed circuit boards.
Summary

So, as per the above content, we will understand the blind vias, its manufacturing process as well as its advantages and disadvantages. This will be helpful for you to make the correct decision when you want to design or fabricate your circuit boards with blind vias.

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