Understanding Blind and Buried Vias in PCB Design

Blind vias and buried vias optimize space and functionality in multilayer PCBs, essential for high-density designs that enable complex and efficient layouts. This article explains their definitions, benefits, construction, and the importance of working with experienced PCB manufacturers to correctly integrate blind and buried vias into your designs.

What are Blind and Buried Vias?

Blind Vias

Blind vias connect an outer layer of a PCB to one or more inner layers but do not go through the entire board. These vias are typically used to save space on the outer layers and are beneficial for high-density interconnect (HDI) designs.

Buried Vias

Buried vias also called hidden vias, connect inner layers of a PCB without reaching the outer layers. These vias are entirely enclosed within the board and are used to create more complex multilayer connections without occupying space on the outer layers.

Benefits of Using Blind and Buried Vias in PCB Design

Space Optimization

Blind and buried vias significantly reduce the need for through-hole vias, freeing up valuable space on the outer layers of PCBs. This optimization allows for more routing space and component placement, which is especially beneficial in high-density interconnect (HDI) designs.

For instance, buried vias help to free up surface space without affecting surface components or traces on the top or bottom layers. Similarly, blind vias provide additional routing options by connecting outer layers to inner layers without passing through the entire board. This is particularly useful for fine pitch BGA components, as it helps in reducing the complexity and congestion on the board's surface.

Improved Signal Integrity

Using blind and buried vias can improve signal integrity by shortening signal paths, thus reducing the chances of signal degradation and interference. Since blind vias only traverse a portion of the board, they minimize the creation of signal stubs, which can cause reflection and signal loss.

This benefit is crucial in high-speed PCB designs where maintaining signal integrity is paramount. By optimizing the signal paths, designers can ensure that high-frequency signals are transmitted more reliably.

Enhanced Design Flexibility

Blind and buried vias offer enhanced design flexibility, allowing designers to create more intricate and compact PCB layouts. This greater layer interconnectivity supports the development of advanced electronic devices that require dense and complex circuit arrangements.

In high-density interconnect PCBs, these vias enable better power delivery and increased layer density. As a result, PCBs can be made smaller and lighter, which is advantageous for miniaturized electronics such as smartphones, tablets, laptops, and medical devices. The ability to maintain a compact design without compromising functionality is a significant advantage in modern electronic design.

Specific Applications and Examples

• High-Density Interconnect PCBs: Both blind and buried vias are integral to HDI designs, providing the necessary space and signal integrity for complex circuit layouts.

• Miniaturized Electronics: Devices like smartphones, tablets, and wearables rely on these vias to achieve their compact and efficient designs. The use of hidden vias helps keep the overall size and weight of these devices down, making them more user-friendly.

• Medical Devices: In medical technology, where precision and reliability are critical, blind and buried vias contribute to the creation of small, yet highly functional devices.

How Are Blind and Buried Vias Built?

The manufacturing process of blind and buried vias involves several precise steps to ensure reliable connections and high performance in multilayer PCBs:

• Drilling: Laser or mechanical drilling is used to create the vias. Blind vias are drilled from the outer layer to one or more inner layers, while buried vias are drilled between inner layers without reaching the surface. It's crucial that the builder accurately controls the depth of the drill to avoid issues. If the hole is not deep enough, it may not provide a good connection. Conversely, if the hole is too deep, it could degrade the signal or cause distortion.

• Plating: After drilling, the vias are plated with copper to ensure electrical connectivity. This step is vital for creating a reliable connection between the layers. Proper plating ensures that there is no air trapped in the PCB, which could otherwise affect the board's performance.

• Lamination: Layers are laminated together to form the final multilayer board. During lamination, the layers with drilled vias are pressed together with heat and pressure, solidifying the connections. The vias can be made either before or after this multilayer lamination, depending on the specific design requirements.

• Inspection and Testing: Post-lamination, the PCB undergoes rigorous inspection and testing to ensure the vias are correctly formed and electrically functional. This step is essential to guarantee the quality and reliability of the finished board.

Technical Considerations

In a blind via, the hole needs to be defined using a separate drill file, with a hole diameter to drill diameter ratio of one or less. For buried vias, each hole is made with a separate drill file, ensuring the hole depth to drill diameter ratio does not exceed 12 to avoid touching other connections within the board.

Design Considerations for Blind and Buried Vias

When designing PCBs with blind and buried vias, several factors need to be considered:

• Design Rules: Adhere to the design rules provided by the PCB manufacturer, including minimum hole sizes and spacing.

• Layer Stack-Up: Plan the layer stack-up to effectively utilize blind and buried vias without compromising the integrity of the board.

• Thermal Management: Ensure that the vias do not adversely affect the thermal management of the PCB.

• Signal Integrity: Design the vias to maintain signal integrity, especially for high-speed signals.

• Cost-Benefit Analysis: Weigh the benefits of using these vias against the increased manufacturing costs.

Cost Considerations

While the benefits of blind and buried vias are substantial, they do come with increased manufacturing complexity and costs. The additional steps required to create these vias, along with the necessary testing and quality control, can add to the overall expense of PCB production.

However, the trade-off is often justified in high-performance applications where space optimization and signal integrity are critical. It is essential to perform a cost-benefit analysis to determine if the advantages of using blind and buried vias outweigh the additional costs for a particular project.

Comparison between Blind and Buried Vias

Here's a table that succinctly summarizes the comparison between blind vias and buried vias:

What layers can you go to and from with blind/buried vias?

The layers accessible with blind and buried vias depend more on the manufacturer's capabilities and policies than on electrical performance. The key difference between through-vias and these types is that blind vias do not penetrate the entire board, while buried vias are not visible from either outer layer because they only connect inner layers.

Blind vias can be drilled mechanically or with a laser from the outer copper to the first inner copper layer. Buried vias are typically used within an internal core or a core+prepreg stack that has been cured into a new core before the outer layers are added.

Testing these vias is more challenging due to their hidden nature, and the multi-stage plating process required can increase manufacturing costs. Some manufacturers may not have the capability to produce these vias or may charge a premium for boards that include them. Therefore, it is advisable to consult with your manufacturer for their suggested stack-ups and capabilities to meet your specific needs​.

Choosing Right Printed Circuit Board Manufacturer

Choosing the right PCB manufacturer is critical to ensure that blind and buried vias are added correctly and efficiently. At VictoryPCB, we offer reliable PCB manufacturing and assembly services that meet your specifications. Our state-of-the-art facility and expert team ensure quality and consistency in every order.

We provide a free engineering file review for all orders to ensure your boards are created correctly and function as intended. With fast turnaround times and shipping, you can trust us to deliver high-quality PCBs with blind and buried vias on time.

Contact us today via sales@victorypcb.com for an instant PCB quote and to learn more about how we can help with your PCB manufacturing needs. Ensure your next project’s success with VictoryPCB.