Blind, Buried and Through Vias Compared

Three Ways to Cross a Stack

A via exists to connect two layers, and the three types differ in which layers they reach. A through via passes through the entire board and is visible on both outer surfaces. A blind via starts on an outer layer and ends on an inner layer, so it is visible from one side only. A buried via connects two inner layers and is never visible from the outside, because subsequent lamination covers it. The choice between them is driven by routing density, by the layer count and by how much the fabricator has to work to build the result.

Through Vias

The through via is drilled after lamination with a mechanical bit, plated, and it occupies every layer in the stack whether or not it connects to them. Its advantages are cost and simplicity: one drill cycle, one plating cycle, no extra lamination and a well-understood aspect ratio limit typically in the region of 10:1 for a controlled process. Its disadvantage is that it blocks routing on every layer it passes through. On a dense board the through vias alone can consume the routing channels, which is exactly the problem that HDI was invented to solve.

Blind Vias

A blind via is drilled to a controlled depth, either mechanically with a depth-controlled bit or, more commonly now, by laser from the outside after each build-up lamination. Because it does not pass through the board, it frees the layers below it for other routing. The depth has to be controlled so that the via lands on the target pad without punching through it, and the aspect ratio of a laser-drilled blind via is limited by the dielectric thickness the beam can penetrate, typically around 0.75 to 1.0. A blind via that is stacked directly on another blind via creates the small high-density structure that gives HDI its name.

Buried Vias

A buried via is formed by drilling and plating before the outer layers are laminated. That means the core has to be processed as a sub-assembly, and the outer layers are added afterwards, which requires the plating to survive a further lamination cycle. Buried vias give the greatest routing freedom because they are invisible to the rest of the stack, but they add lamination cycles to the build, and each extra cycle reduces yield. They are usually reserved for designs where the layer count is high and the routing would otherwise not close.

Aspect Ratio and What It Restricts

Aspect ratio is the ratio of the via depth to its diameter, and it limits the plating process more than the drilling process. A deep, narrow via is difficult to plate uniformly, so the barrel wall thins toward the middle and the via becomes the weakest point in the structure. A mechanical through via in a thick backplane may reach 10:1; a laser blind via is usually below 1:1 because the depth is only one dielectric layer; a buried via in a laminated sub-assembly sits somewhere between and depends on the drill size the shop can plate reliably. Whenever the design pushes the aspect ratio up, the answer is usually to increase the drill diameter or to reduce the board thickness, not to ask for a thicker plating.

Build Sequence and Cost

The build sequence is what the designer is really choosing. Through vias need one lamination and one drill cycle. Blind vias need at least one additional lamination and a laser step. Buried vias need the core drilled and plated before the outer layers are applied, so the panel passes through the plating line more than once. Every additional cycle adds handling, adds registration tolerance stack-up and multiplies the probability of scrapping the whole panel at the last step. The cost difference between a design with through vias and the same layer count with buried vias is therefore much larger than the difference in drill count suggests.

Which to Use

Use through vias until the routing stops closing, and remember that selecting the right layer assignment often removes the need for anything more complex. When density demands more, add blind vias on the outer layers first, since they give most of the routing benefit for one extra lamination. Reserve buried vias for the inner structure of a high layer count design where the routing genuinely cannot be resolved any other way. In every case, confirm the maximum aspect ratio and the allowed via-to-via arrangements with the fabricator before the layout is frozen.

Vias and Signal Integrity

The via is not an ideal connection. A through via that passes unused layers leaves a stub below the layer where the signal is routed, and that stub behaves as an open transmission line that resonates at a frequency set by its length. At high data rates the stub produces a dip in the insertion loss that can close the eye, which is why backdrilling exists and why the layer assignment matters as much as the via type. A blind via has no stub by construction, which is one of its less obvious advantages, and a buried via has a short stub only if the signal continues past the connection point. The other effect is the antipad: the clearance in the reference plane around the via barrel has to be large enough to avoid shorting and small enough that the plane stays continuous in the return path. Both are layout decisions made on the same drawing as the via.

FAQ

What is the difference between a blind and a buried via? A blind via reaches an outer surface on one side; a buried via connects inner layers only and is covered by later lamination.

Which is cheapest? The through via, because it needs one drill and one plating cycle with no extra lamination.

What aspect ratio can be plated? Typically up to about 10:1 on a mechanical through via, with laser blind vias far below that because they are only one dielectric deep.

Why do buried vias cost so much? Because the inner core is drilled and plated as a separate sub-assembly before the outer layers are laminated onto it.

Can blind vias be stacked? Usually up to a limit set by the shop, but stacking concentrates stress and staggering is more reliable in thermal cycling.

Conclusion

Pick the simplest via that lets the routing close, and only then add build-up layers. Through vias cost least, blind vias buy routing freedom for one extra lamination, and buried vias are the last resort for high layer counts. Confirm the aspect ratio and the allowed stacking before the layout is released. The structures available are listed under PCB capabilities, the drilling and lamination sequence is covered in PCB manufacturing, and the layer assignment that decides how many vias are needed is part of PCB design and layout. High layer count prototypes are normally built through prototype PCB assembly in 2026.