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

What is a Buried Via PCB?

A buried via PCB, also known as a sub-surface mount PCB, is a type of printed circuit board that contains vias (electrical connections) that are “buried” within the inner layers of the board, rather than being exposed on the surface like in traditional PCBs. This allows for higher density connections and more compact board designs.

The vias in a buried via PCB are created by drilling holes through some, but not all, of the layers of the PCB. Conductive material is then deposited into these holes to create the electrical connections between layers. The vias are “buried” because they are covered by additional layers of the PCB material on both the top and bottom surfaces.

Advantages of Buried Via PCBs

Buried via PCBs offer several key advantages over traditional PCBs with surface-mounted vias:

1. Increased Density: By burying the vias within the inner layers, more space is available on the outer layers for components and traces. This enables higher density designs in a smaller footprint.

2. Improved Signal Integrity: Surface-mounted vias can act as tiny antennas that radiate electromagnetic interference (EMI). By burying the vias, this EMI is contained within the board, leading to cleaner signals.

3. Enhanced Mechanical Strength: Vias that go through all layers of the board create points of mechanical weakness. Buried vias distribute this weakness, leading to a stronger board overall.

4. Better Heat Dissipation: Buried vias can be used to transfer heat from hot components on the surface to the inner layers of the board, improving thermal management.

Disadvantages of Buried Via PCBs

Despite their advantages, buried via PCBs also come with some drawbacks:

1. Higher Cost: The additional processing steps required to create buried vias increase the manufacturing cost compared to traditional PCBs.

2. Longer Lead Times: The complexity of buried via PCBs means they typically take longer to design and manufacture than simpler boards.

3. Reduced Repairability: If a buried via fails, it is much harder to repair than a surface-mounted via, often requiring replacement of the entire board.

How Are Buried Via PCBs Manufactured?

The manufacturing process for buried via PCBs is more complex than for traditional PCBs. Here is a simplified overview:

1. The inner layers of the PCB are printed and etched as in a standard multilayer PCB process.

2. Holes are drilled in the inner layers where the buried vias will be located.

3. The holes are plated with a conductive material, typically copper, to create the electrical connection.

4. The inner layers are laminated together with insulating material.

5. Additional layers are added to the top and bottom of the board, covering the buried vias.

6. Holes are drilled for through-hole components and surface-mounted vias.

7. The outer layers are printed and etched.

8. The board is plated, typically with copper, nickel, and gold, to protect the copper and prevent oxidation.

9. Components are mounted on the board.

Types of Buried Via PCBs

There are several types of buried via PCBs, each with its own characteristics and manufacturing process.

Blind Via PCBs

In a blind via PCB, the vias are visible on one side of the board but not the other. These are typically used to connect an outer layer to an inner layer.

Backdrilled Via PCBs

Backdrilling is a process where a portion of a through-hole via is removed to improve signal integrity. This is done by drilling away the unused portion of the via from the back side of the board.

Stacked Via PCBs

In a stacked via PCB, vias are stacked on top of each other to connect multiple layers. This allows for even higher density designs but is more complex to manufacture.

Designing with Buried Via PCBs

Designing a PCB with buried vias requires careful consideration of several factors:

Via Size and Spacing

The size of the vias and the spacing between them will impact the density of the board and the signal integrity. Smaller vias allow for higher density but are more difficult to manufacture reliably.

Layer Stack-Up

The arrangement of the layers in the board, known as the stack-up, is critical in a buried via PCB. The stack-up determines which layers the vias will connect and impacts the signal integrity and EMI of the board.

Thermal Management

Buried vias can be used to transfer heat, but this must be carefully designed. The size, number, and placement of thermal vias will impact the effectiveness of the thermal management.

Signal Integrity

Buried vias can improve signal integrity by reducing EMI, but they can also create signal reflections if not designed properly. The via size, spacing, and layer transitions must be carefully considered to maintain signal integrity.

Applications of Buried Via PCBs

Buried via PCBs are used in a variety of applications where high density, high performance, and reliability are critical.

Aerospace and Defense

In aerospace and defense applications, space is at a premium and reliability is critical. Buried via PCBs allow for dense, robust designs that can withstand harsh environments.

Medical Devices

Medical devices, particularly implantable devices, require very small, highly reliable PCBs. Buried via technology enables these compact, high-performance designs.

High-Speed Communications

In high-speed digital communications, signal integrity is paramount. Buried via PCBs can provide the density and performance needed for these demanding applications.

Consumer Electronics

As consumer electronics become smaller and more powerful, buried via PCBs are increasingly used to enable compact, high-performance designs.

FAQs

1. Are buried via PCBs more expensive than traditional PCBs?

Yes, buried via PCBs are typically more expensive due to the additional processing steps required in their manufacture. However, they can enable designs that would not be possible with traditional PCBs, so the higher cost is often justified.

2. Can buried vias be used for thermal management?

Yes, buried vias can be used to transfer heat from hot components on the surface of the board to the inner layers, improving thermal management. However, this must be carefully designed to be effective.

3. What is the difference between a blind via and a buried via?

A blind via is a via that is visible on one side of the board but not the other, connecting an outer layer to an inner layer. A buried via is completely hidden within the inner layers of the board.

4. How small can buried vias be?

The size of buried vias is limited by the manufacturing process. Currently, buried vias as small as 0.1mm in diameter are possible, but this requires advanced manufacturing capabilities.

5. Are buried via PCBs repairable?

Buried via PCBs are more difficult to repair than traditional PCBs. If a buried via fails, it is often necessary to replace the entire board rather than repairing the individual via.

Conclusion

Buried via PCBs offer a powerful tool for creating high-density, high-performance electronic designs. By burying vias within the inner layers of the board, designers can increase density, improve signal integrity, and enhance mechanical strength and thermal management.

However, these benefits come with increased cost and complexity. Designing and manufacturing buried via PCBs requires careful consideration of via size and spacing, layer stack-up, thermal management, and signal integrity.

Despite these challenges, buried via PCBs are increasingly used in demanding applications such as aerospace, medical devices, high-speed communications, and consumer electronics. As electronic devices continue to shrink and increase in performance, buried via technology will likely become even more prevalent.

Understanding the capabilities and limitations of buried via PCBs is essential for any electronics designer or manufacturer looking to push the boundaries of what is possible with electronic packaging.