Buried Via PCB： Here is what you need to know about this PCB type.

There is increased complexity and requirements in circuit board designs, especially in high-density circuit boards, Buried Via PCB, and other special PCB vias. These circuits are a standard part of multifunctional devices like smartphones and laptops. Hence, the technology promotes efficiency on little board space.

It gets more interesting as you read about the Buried Via PCB. As a design engineer, you learn the engineering design principles and how to make buried via PCBs. You also discover the advantages and disadvantages, PCB Via types, and more.

What is the Buried Via PCB?

Buried Via PCB is a PCB type with copper-plated holes that allow a connection of layers through the conductive holes. In Surface Mount Technology, a standard via example is the Through-hole via. But due to the many demerits, a buried via or blind via becomes a preferable option. There are also several measures for processing buried via PCB. These include plugged solder mask via, plugged copper mask via, staggered via, and plated via.

For buried vias, their corresponding parts are the inner layers. However, it is not visible from the outer layers because it remains buried in the board's interior.

Buried Via Advantages and Disadvantages

PCB boards with buried vias and blind vias have their advantages and disadvantages.

Advantages

Buried and blind vias are useful in HDI layer PCBs.

• In particular, they improve board density without enlarging the layer count and board size.
• Also, it enhances the spacing of board components by saving the board area.
• It allows the usage of small pitch devices.
• Efficient power delivery.

Disadvantages

Alternatively, the PCB via type has a few concerns.

• The buried via technology has a higher manufacturing cost than similar multilayer PCBs.
• Also, they are difficult to fabricate due to their small size and complexity.
• These boards also require special machinery for micro via drilling.

Five Types of PCB Vias

There are five main types of PCB vias based on their drilling patterns. Each of them is unique and specific for certain functions.

VIA:

This type of PCB uses plated-through holes (PTH) in its PCB design. They help insert the lead points of components. And they are ideal for inner layer electrical connections.

BLIND VIA:

The blind hole vias have a particular hole depth that does not surpass the aperture. In this case, the hole vias do not connect throughout the entire board. Therefore, it is a "blind hole" because you cannot see through from one end to the other.

BURIED VIA:

The Buried Via uses holes with copper plating that connects more than one internal layer. However, accessing the board via its external layers is impossible. This is because the via is underneath the outer layers.

THROUGH VIA:

These types of layer boards use through holes instead. The through-holes extend from one outer PCB layer to another.

(A printed circuit board showing printed traces along the through-holes)

COMPONENT HOLE:

The component hole type of PCB has slots that interconnect. Hence, components installed on it connect electrically in conductive patterns.

Blind Via vs. Buried Via

The blind via and Buried via are both useful types of PCBs. They both enable layer connections on a PCB. But the ideal choice depends on the unique requirements of your board.

Blind Via

On the one hand, the blind uses an internal connection of the outer layers of a multilayer PCB with the single/multiple inner layers.

Also, blind via uses lasers for manufacturing the blind holes. This technology allows small and accurate diameter holes between 4-8mil. Therefore, blind vias are expensive.

Buried Via

On the other hand, the buried via enables interconnection for only the internal layers because the content is unseen from the surface layer. Also, after drilling the inner layers, it requires hole-plating. Sometimes, you even need multilayer laminations in the blind via processing. So, the cost is comparatively higher than in blind vias. They are most common in HDI circuit board applications like cell phones, medical devices, multi-function digital electronics, etc.

(A multilayer printed circuit board with buried vias and other visa types).

How to make Blind and Buried Vias

Not every blind and buried vias undergo the laser-drilling process. Sometimes, you drill through the board and the different cores and then stack them up.

However, you need to follow these design rules in the via-board manufacturing process:

1. Always cut a via through even numbers of copper layers.
2. A via does not finish the topmost part of a core.
3. Still, it does not start from the underside of the body.
4. Also, buried vias or blind vias do not begin or complete the inner/outer sides of a similar buried/blind PCB. Hence, you need more press cycles. The only exception is when you enclose them within another.

Different methods to make a Blind Hole PCB

PCB designers use the following methods for making blind hole PCBs:

Mechanical Controlled Depth Drilling

In the conventional process of multilayer board manufacturing, the driller determines the Z-axis of the depth drilled. Some concerns, however, include:

• Output is low because it only allows sequential drilling of pieces.
• The machine for drilling needs to be level with the spindle's drill depth and aspect ratio. Else, controlling the hole size and hole depth is tricky.
• Electroplating is almost impossible due to the tiny hole diameter.

Therefore, these drawbacks make mechanical controlled depth drilling less common.

(A collection of milling cutter drills with different diameter measurements).

Sequential Lamination

Sequential lamination makes it easy to make buried and blind holes consecutively. You need to create the circuit, the four inner PTH layers, and laminate the four pieces of layer PCBs altogether. Afterward, drill in a through-hole. Although effective, this method is time-consuming.

The Build-up Process and Non-machine Drilling Method

It is the most famous method among several PCB and domestic manufacturers. More so, it exceeds sequential lamination constraints, which allows adding of extra external layers. It will enable non-machine drilling of blind holes to interconnect the adjacent layers. Instead, it uses three main methods: photo-defined, laser ablation, and plasma etching.

PCB Engineering Design Principles for Blind and Buried Vias

There are certain principles involved in making blind and buried vias.

• Laser drilling is the most suitable for boards with ≤ 100um dielectric layer thickness and ≤ 0.13mm aperture.
• The laser-drilled pads of an HDI PCB should have a solder ring of at least 3.5mil on one end.
• The standard RCC specifications are 65um, 100um, and a copper thickness of 12um.
• The minimum hole to conductor distance for one lamination is 9mil. Meanwhile, for two or more laminations, it is 10mil.
• In terms of dielectric layer thickness, laser drilling should be processed at ≤ 65T RCC for a 0.1mm aperture. At the same time, a 0.13mm laser aperture requires ≤ 100T RCC.

(A large PCB showing different via apertures).

Conclusion

In short, the buried via PCB and blind via board is a specially printed circuit board. They are instrumental in interconnection designs of multilayer boards. They have specially drilled holes, often copper-plated, to enhance functionality. But, because they are unique PCB technologies, they are costly.

Notwithstanding, there exist reliable and quality PCB manufacturers with reasonable pricing. So, for all PCB assembly needs and assistance, contact us.