High‑density Blind/Buried Via PCB for HDI mobile device interconnects

With the development of high-density PCB, Blind and Buried vias are often used in PCB design to manage space and signal integrity, when traditional through-hole vias (which go from top to bottom) become too bulky or electrically inefficient.It can be save space, and frees up "real estate" on the layers the via doesn't pass through, allowing for more traces or components. And often used in HDI designs, which is essential for fine-pitch BGA (Ball Grid Array) components where there isn't enough room to "fan out" using through-holes. Adding these vias can help reduce the total layer count by making routing more efficient. It is also eliminates via stubs (the unused portion of a through-hole via that acts as an antenna), which is critical for high-speed signals (>5Gbps).
The main disadvantage of using blind and buried vias is the cost higher than normal design. These processing methods make the circuit board much more expensive because they involve more complex designs, extra manufacturing time, and more frequent precision checks.

1.What is blind vias and burid vias?

◈ Blind Vias
A blind via serves as a connection between an outer layer and one or more internal layers without penetrating the entire board. Consequently, it is visible from only one side, appearing as a hole with a terminated depth.It is only visible from one side of the PCB.Designing with these vias is not as simple as placing a through-hole.
About Aspect Ratio, Blind Vias require an AR of 1:1 or less (e.g., if the hole is 0.15mm deep, it should be at least 0.15mm wide) to ensure the plating chemicals can reach the bottom.
◈ Buried Vias
A buried via is a 'hidden' connection that stays strictly inside the inner layers of a PCB. Because it never touches the outer layers, it is completely contained within the core and cannot be seen once the board is fully assembled.It is invisible from the outside of the finished board.
About Aspect Ratio, the burid vias can be higher (up to 1:12), as they are treated like small through-holes during their sub-lamination stage.

2. Design considerations:

◈ Even Layer Spans: it is highly recommended that blind/buried vias span an even number of layers (e.g., Layer 1 to 2, or 3 to 6) to maintain board symmetry and prevent warping.
◈ Annular Rings: Because of registration tolerances during sequential lamination, you often need larger annular rings compared to standard through-holes to ensure the drill hits the pad accurately.
◈ Via Filling: Buried vias are usually plugged/filled with epoxy (conductive or non-conductive) and then plated over to create a flat surface for the next lamination step.
◈ Drill Files: You must provide separate drill files for every unique via span (e.g., one file for L1-L2, one for L2-L5).
◈ Pro Tip: Always follow the pcb factory’s Stack-up Design Guide" before start routing. Adding blind/buried vias can increase fabrication costs by 30% to 50%, so use them only where space or signal speed makes them necessary.

3.Technology & Fabrication

The pcb manufacturing of the blind and burid via holes is more complex than standard through-holes because it requires sequential lamination.
◈ Drilling Methods:
● Mechanical Drilling: Used for larger blind/buried vias. The depth is precisely controlled (Depth-Controlled Drilling).
● Laser Drilling: Used for "Microvias" (typically <0.15mm). It is more precise and common in high-density (HDI) designs.
● Sequential Lamination: To create a buried via, the PCB manufacturer must drill and plate the inner "core" layers first. Then, they laminate additional outer layers on top. This process repeats depending on how many "steps" or "sub-stacks" the design has.