Guideline

What are Blind Via, Buried Via, and PTH Via?

vias

There are different kinds of plated through holes in PCBs, such as blind vias, buried vias and PTH vias. All these vias have different functions and production processes, they are used as a combination to achieve multifunction of a printed circuit board. 

blind vias

What are Blind Vias?

When the vias connection is used between the outer layer and the inner layer, that is called blind vias. It is seen on one side of the PCB and is not seen on the other side of the PCB. It is a plated hole that is drilled into a certain thickness of the board but it stops at a given layer (inner layer). This type of vias provides the electrical connection between inner and outer layers where it is used. This does not affect the layer where these vias are not used.

Blind vias technology is used to improve signal integrity or reduce the PCB size. It also creates extra space for routing which helps in the HDI boards.

There are several production production methods for the Blind Vias, as below: 

  1. Controlled Depth Drilling: Drill bit drilled up to the required depth called controlled depth drilling. Then it is plated through holes. This method is used only when blind via holes are more than 6mil.
  2. Laser Drill: It is laser drilled generally blind via hole size is less than or equal to 6mil. It is a very expensive method and is used in HDI boards.
  3. Cap Construction Technology (Sequential Laminate): Sequential laminate adds extra process to laminate the two layers individually and drill. This is also called sub-laminate. As this method adds extra lamination process, this also adds extra cost in PCB.

Every time you add buried and blind vias you increase the number of lamination cycles, drill operations and plating operations all of which increase the costs.

sequential lamination

What are Buried Vias?

Vias which start and end in the middle of the boards are called buried vias. It is completely hidden vias in the PCB not shown on the top or bottom of the board. It connects two or more internal copper layers and no outer layers. This type of vias is used in the HDI board where the space is very important.

Buried vias are not difficult. You drill a core or series of cores, laminate together and process it like a double-sided board. With the increased density of semiconductor packaging demanding more interconnections, the number of designs using blind and buried vias is increasing. As the width of the conductor decreases and the hole size shrinks, it allows us to fit more circuitry in the same size package.

What are Plated-Through Vias? 

Plated-through vias provide the electrical connection from the top to the bottom layer. It is the most common method used in the PCB. It is created by drilling and adding the copper in the holes. It can be vias or component holes for soldering. Hole to thickness aspect ratio must be important while considering the smallest hole in the boards. You may refer to the following plated through vias drilling standards. 

SpecificationStandard ProductionAdvanced
Min. drilled hole diameter0.008”0.006”
Max. drilled hole diameter0.250”0.250”
Aspect ratio10:115:1
PTH Diameter tolerance+/-0.002+/-0.002
NPTH Diameter Tolerance+/-0.002+/-0.001
Hole location tolerance+/-0.003”+/-0.003” Hole to Edge

 

Via Filling in Blind and Buried Vias

Most of the advanced technology boards using HDI board designs always add blind-buried vias which are common today. This technology uses blind vias in fine pitch areas like BGA. Blind vias are used in BGA connection which is also called the micro blind vias. These vias are generally less than 6 mil holes. This type of blind is copper filled and plated over. Blind vias and buried vias are usually needed to be filled or plugged, by con-duction or non-conductive materials. Here are some rules: 

  • Microvias hole 🡪 Less than 6 mil copper filled
  • Blind vias (>6mil) 🡪 Filled and plated over
  • Buried vias 🡪  Non-conductive via filled
  • VIA in PAD  🡪  Filled and plated over

normal BGA via in pad

Designers have to keep in touch with the fabricator while designing the HDI board with blind buried via stackup. It helps the designer and fabricator both to manufacture the board cost-effectively and easily. 

Avoid below for cost effective:

  1. Whenever designing the stackup, the designer has to keep the blind vias at an even layer for an easily manufacturable and lower cost.
  2. Use buried vias like the fabricator can use sequential laminate. The fabricator can sub-laminate and drill it. This avoids controlled depth or laser drill process which lowers the cost.
  3. Stay in touch with the fabricator for blind buried via stackup.

Advanced Blind and Buried Vias: Staggered and Stacked Vias

Ultra HDI board uses staggered vias or stack vias technology which is more advanced today. As the following image shows the difference between staggered vias and stacked vias.

staggered and stacked vias

Example of Staggered Vias

In an 8-layer board, blind vias connect the top layer to layer 2, and other buried vias set connected to layers 2 to 3 are called staggered vias (Micro vias that are electrically connected and offset to one another through various layers of PCB).

Example of Stacked vias

Micro vias are electrically connected and stacked vertically on each other through the various layers of the PCB. As the 8-layer PCB board, vias in layer 1-2, 2-3 and 3-4 are stacked vias. 

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