Blind and buried vias: An HDI technology

High-Density Interconnect (HDI) technology has transformed the electronics industry, enabling smaller, faster, and more powerful devices. One of the key innovations within HDI PCB design is the use of blind and buried vias, which enhance routing capabilities by freeing up additional routing resources, increasing the amount of routing channels while requiring fewer signal layers, and optimizing board space. This technology offers significant advantages in terms of routing density and electrical performance, but it comes with added complexity and cost. In this blog, we will explore what blind and buried vias are, their advantages, manufacturing processes, and their role in today’s complex PCB design structures.

What are blind and buried vias?

Vias are the conductive pathways that connect different layers of a PCB. Traditional through-hole vias extend from the top (Primary side) to the bottom (Secondary side) of the PCB, but as PCB designs have become very complex, and or significantly smaller, more compact and very dense, designers have adopted blind and buried vias to maximize space utilization.

• Blind vias – These vias connect an outer layer to one or more inner layers without going through the entire board. They are “blind” because they do not pass through the entire PCB thickness.
• Buried vias – These vias connect internal layers without reaching the outer layers. They remain “buried” within the PCB and are completely invisible from the outside.

Both types of vias help improve circuit density, meet complex requirements, and allow for the creation and manufacturing of densely compact, high-performance PCB designs.

Per IPC-2226A (Sectional Design Standard for High Density Interconnect (HDI) Printed Boards), a microvia (Build-Up Via) is defined as a blind or subsequently buried hole that is <0.15 mm (0.00591 in) in diameter having a pad diameter that is <0.35 mm (0.0138 in) and formed either through laser or mechanical drilling, wet/dry etching, photo imaging or conductive ink-formation followed by a plating operation.

A microvia has a max. aspect ratio of <1/1 terminating on or penetrating a Target pad (bottom), total length of no more than 0.25 mm (9.84mils) measured from the structure’s Capture Pad (top) foil to the Target Pad.  Typical aspect ratio is about 0.8/1 (4mils dielectric with a 5mils drill). Drill size range 3 to 10mils, with annular ring about 2.5 to 4mils.

Advantages of blind and buried vias

The adoption and implementation of blind and buried vias in HDI PCBs comes with several benefits:

• Increased routing density – By reducing the number of through-hole vias, designers can fit more traces and components into a smaller area.
• Smaller form factor – Devices using HDI technology can be more compact, which is essential for modern electronics such as smartphones, tablets, and wearable devices.
• Improved electrical performance – Shorter signal paths reduce inductance, capacitance, and signal loss, improving overall circuit performance.
• Enhanced reliability – Buried vias eliminate unnecessary drilling through multiple layers, reducing stress points and potential failure areas.
• Greater design flexibility – Engineers can optimize layer stacking and routing paths to meet specific performance and design requirements.

Cost considerations

Although there are significant advantages gained by implementing blind and buried vias, these HDI via structures, when utilized in a design, typically cost more than using a standard thru hole (TH) via structure in a standard PCB. The cost increase is influenced by:

• Number of HDI layers – Fewer HDI layers translate to lower costs.
• Density of microvias and other HDI features – Higher density increases complexity and manufacturing cost.
• Design complexity – Materials, laser drilling, and other specialized processes add to manufacturing costs.

On average, expect a 10–30% cost increase compared to a standard PCB, depending on the specific design and requirements. Blind and buried vias require fabrication to be done in stages. These stages are referred to as multiple laminations, which is where layers connected with blind and buried vias are pressed together and drilled, then other layers added with additional drilling.

It’s extremely important and an industry best practice to collaborate with your fabricator from the very beginning of the project to avoid designing a PCB layer stack-up that cannot be manufactured. This includes HDI via structures as well.

Manufacturing process of blind and buried vias

The fabrication of blind and buried vias involves advanced manufacturing techniques, making it a more complex process than traditional PCB production.

• Laser drilling – Since blind and buried vias require precise hole depth, laser drilling is often used instead of mechanical drilling for accuracy.
• Sequential lamination – Multiple lamination cycles are needed to build up the PCB layers, incorporating buried vias first before outer layers and blind vias are added.
• Plating and filling – The vias are plated with copper to ensure reliable electrical connectivity. Some may also be filled with conductive or non-conductive epoxy for mechanical strength.
• Etching and layer alignment – Advanced etching and imaging techniques ensure proper alignment and connectivity across all PCB layers.

Manufacturing challenges and design for manufacturing (DFM) considerations

Because HDI technology involves thinner copper and dielectric layers manufacturers must address:

• Tighter tolerances – Smaller hole sizes and trace widths increase fabrication difficulty.
• Laser drilling requirements – Precision is needed for accurate microvia formation.
• Higher defect risks – Misalignment, delamination, or via reliability issues can arise, especially if the manufacturer lacks HDI expertise.
• Microvias (µVias) – Can be either a buried via, blind via, stacked vias, or staggered via

To mitigate these risks, designers should know the capabilities and limitations of their PCB manufacturer and collaborate closely with experienced PCB manufacturers who understand the nuances of HDI technology. Because of this close collaboration, it’s very important to establish a good relationship with your PCB suppliers.

Implementing advanced technologies such as blind and buried vias structures are essential components in HDI PCB designs, enabling the miniaturization and performance enhancement of modern electronics. While they require advanced fabrication techniques, the benefits they provide in terms of space savings, signal integrity, and design flexibility make them a crucial aspect of high-tech device manufacturing. As electronic devices continue to evolve, HDI technology and innovative via solutions will remain at the forefront of PCB advancements.